T H E R O L E O F A D D E D N O N F A T DRY M I L K SOL IDS AND T H E P R E S S U R E O F H O M O G E N I Z A T I O N ON TH E S T A B I L I T Y , V IS C O SI TY , A N D O T H E R P R O P E R T I E S O F H A L F - A N D - H A L F H O M O G ENIZED MILK By Eile r Shirley H u m b e rt AN A B S T R A C T S u b m i t t e d to th e S c h o o l of G r a d u a t e S t u d i e s of M i c h i g a n S t a t e C o l l e g e of A g r i c u l t u r e a n d A p p l i e d S c i e n c e i n p a r t i a l f u l f i l l m e n t of the r e q u i r e m e n t s f o r th e d e g r e e of DOCTOR O F PHILOSOPHY D e p a r t m e n t of D a i r y Y e a r 1954 App r o v e d Lf ~ - <£ Cs> 1 EL L E R S H I R L E Y H U M B E R T ABSTRACT I n v e s t i g a t i o n s e n c o m p a s s i n g 5 to 7 s e r i e s a n d i n v o l v i n g 105 d i f f e r e n t s a m p l e s w e r e c o n d u c t e d on v a r i o u s p h y s i c a l , c h e m i c a l , a n d o r g a n o l e p t i c p r o p e r t i e s of h a l f - a n d - h a l f h o m o g e n i z e d m i l k p r o c e s s e d u n d er v a rio u s conditions. D a t a o b t a i n e d i n d i c a t e d t h a t the v i s c o s i t y a n d h e a t s t a b i l i t y of 10.5 p e r c e n t h a l f - a n d - h a l f , w i t h o u t a d d e d n o n f a t d ry m ilk solids, w ere s i m i l a r to t h o s e p r e v i o u s l y r e p o r t e d f o r h o m o g ­ enized m ilk. A s th e f a t p e r c e n t a g e i n c r e a s e d , th e p r o d u c t b e c a m e i n c r e a s ingly m o r e s e n s i t i v e to h o m o g e n i z a t i o n so f a r a s the r e s u l t a n t s t a ­ b i l i t y of the s a m p l e s w a s c o n c e r n e d . Extensive tria ls show ed th a t h a l f - a n d - h a l f w a s s tab le in coffee ) m a d e w i t h h a r d , ta p w a t e r . H o w ev e r, w hen the coffee w a s m a d e w ith d i s t i l l e d w a t e r , the h a l f - a n d - h a l f o c c a s i o n a l l y f e a t h e r e d . A l t h o u g h the a d d i t i o n of n o n f a t d r y m i l k s o l i d s to h a l f - a n d - h a l f p r o v i d e d s o m e b u f f e r i n g a c t i o n a g a i n s t t h e e f f e c t s of the d e s t a b i l i z i n g i o n s , f o r t i f i ­ c ation w ith th e se s o l i d s did n o t i m p r o v e the s t a b i l i t y of h a l f - a n d - h a l f a g a i n s t f e a t h e r i n g in h o t , d i s t i l l e d - w a t e r c o f f e e . T h e r a t e of m i g r a t i o n of the f a t g l o b u l e s in an e l e c t r i c a l f i e l d i n c r e a s e d a s th e r e s u l t of h o m o g e n i z a t i o n . z EL LE R S H I R L E Y H U M B E R T ABSTRACT W h e n h o m o g e n i z e d - i n d u c e d f a t c l a m p i n g w a s e v i d e n t , the a d d i ­ t i o n of the e m u l s i f i e r , s o r b i t a n m o n o s t e a r a t e , r e d u c e d the v i s c o s i t y a n d i n c r e a s e d th e h e a t s t a b i l i t y of h a l f - a n d - h a l f a n d c r e a m . T h e d r y - w e i g h t a n a l y s i s of the f e a t h e r e d m a t e r i a l f r o m h a l f a n d - h a l f a n d c r e a m i n d i c a t e d t h a t , in a d d i t i o n to 81 to 94 p e r c e n t f a t , o v e r 40 p e r c e n t of the o r i g i n a l c o l l o i d a l c a s e i n w a s p r e s e n t in the p r e c i p i t a t e . H o w e v e r , only a s m a l l p e r c e n t a g e of th e h e a t - c o a g u l a b l e p r o t e i n f r a c t i o n w a s f o u n d in t h i s f e a t h e r e d m a t e r i a l . I t m a y b e c o n c l u d e d t h a t h o m o g e n i z a t i o n of h a l f - a n d - h a l f t e s t i n g 10 to 11 p e r c e n t f a t d o e s n o t p r o d u c e th e i n s t a b i l i t y a s h a s b e e n e x p e r ie n c e d w ith table c r e a m . If f e a t h e r i n g of h a l f - a n d - h a l f d o e s o c c u r i n c o f f e e , th e f a c t o r s w h i c h s h o u l d b e c o n s i d e r e d , in o r d e r of i m p o r t a n c e , a r e : and-half; zation. (1) q u a l i t y of i n g r e d i e n t s i n t h e h a l f - (2) p H of the c o f f e e b r e w ; a n d (3) p r e s s u r e s of h o m o g e n i ­ T H E R O L E O F A D D E D N O N F A T DRY M I L K SOLIDS AND T H E P R E S S U R E O F H O M O G E N I Z A T I O N ON T H E S T A B I L I T Y , V IS C O S I T Y , AND O T H E R P R O P E R T I E S O F H A L F A N D -H A L F HOM OGENIZED M ILK By E IL E R SHIRLEY H U M B E R T A THE SIS S u b m i t t e d to th e S c h o o l of G r a d u a t e S t u d i e s of M i c h i g a n S ta te C o l l e g e of A g r i c u l t u r e a n d A p p l i e d S c i e n c e i n p a r t i a l f u l f i l l m e n t of th e r e q u i r e m e n t s f o r the d e g r e e of DOCTOR O F PHILOSOPHY D e p a r t m e n t of D a i r y 1954 ProQuest Number: 10008337 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10008337 Published by ProQuest LLC (2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 1 0 6 - 1346 ACKNOWLEDGMENTS T h e a u t h o r w i s h e s to e x p r e s s h i s s i n c e r e s t a p p r e c i a t i o n to D r . G . M. T r o u t , R e s e a r c h P r o f e s s o r of D a i r y i n g , f o r h i s v a l u a b l e d i r e c ­ t i o n a n d e n c o u r a g e m e n t d u r i n g the c o u r s e of t h i s s t u d y , a n d f o r hi s a s s i s t a n c e in the p r e p a r a t i o n of t h i s m a n u s c r i p t . Th e a u t h o r i s d e e p l y i n d e b t e d to D r . J . R. B r u n n e r , A s s i s t a n t P r o f e s s o r of D a i r y i n g , f o r h i s h e l p f u l s u g g e s t i o n s a n d a s s i s t a n c e i n m a n y of t h e t e c h n i q u e s u s e d in t h i s s t u d y . G r a t e f u l a c k n o w l e d g m e n t i s due D r . E a r l W e a v e r , H e a d of the D a iry D e p a rtm e n t, f o r m aking available a g rad u ate a s s is ta n t s h ip and the f a c i l i t i e s t h a t w e r e n e c e s s a r y i n th e f u l f i l l m e n t of t h i s i n v e s t i g a ­ ti o n . T h e a u t h o r a l s o w i s h e s to e x p r e s s h i s d e e p e s t g r a t i t u d e to his wife, J e a n , f o r h e r e n c o u r a g e m e n t, a s s i s t a n c e , and m a n y s a c r i f i c e s th a t she h a s m a d e , w ithout w hich this th e s i s m ig h t n e v e r have be en pos s ib le . ii TABLE OF CONTENTS Page I N T R O D U C T I O N .................................................................................................... 1 REVIEW O F L IT E R A T U R E ......................................................................... 4 V i s c o s i t y .............................................................................................................. 4 Definition ................................................................................................ . V i s c o s i t y of h o m o g e n i z e d m i l k a n d c r e a m .............................. F a c t o r s A f f e c t i n g the S t a b i l i t y 4 4 of M i l k a n d C r e a m ................. 7 I n t r o d u c t i o n ................................................................................................. 7 Hom ogenization . ................................................................................... 8 S o l i d s - n o t - f a t ............................................................................................. 9 P r o c e s s i n g t e m p e r a t u r e s ................................................................... 10 E f f e c t of S a l t s on S t a b i l i t y of M il k a n d C r e a m S ta te of c o m p o n e n t s in m i l k ....................... ............................................................ . 12 12 Effect of h e a t i n g on s t a t e of c o m p o n e n t s ................................. 16 Effect of s a l t b a l a n c e ........................................................................ 17 Effect of a c i d on s t a t e of c o m p o n e n t s ...................................... 19 Effect of m i l k e v a p o r a t i o n .............................................................. 20 F eath erin g C haracteristics 23 ....................................................................................... iii 23 Page .................................................. 24 a n d h o m o g e n i z a t i o n ............... 24 ........................................................................ 27 A b i l i t y of C r e a m in C o f f e e ............................................... 29 Effect of q u a l i t y of i n g r e d i e n t s Effect of pasteurization Effect of coffee Coloring H a lf - a n d - H a lf H o m o g e n iz e d M ilk ..................................................... 29 P r e p a r a t i o n ................................................................................................. 29 T h e o r i e s R e g a r d in g F a t C lum ping inM ilk and C r e a m . . . 30 I n t r o d u c t i o n ............................................................................ 30 Agglutinin th e o ry ................................................................................... 32 .................................................................................................... 34 Stickiness T h e o r y of e l e c t r o k i n e t i c s ................................................................ 34 ............................................................................. 35 ................................................................................ 36 S E C T I O N I: T H E I N F L U E N C E O F V A R I A T I O N IN C O M P O S I T I O N AND P R E S S U R E S O F H O M O G E N I Z A T I O N U P O N S O M E C H E M I C A L A ND P H Y S I C A L P R O P E R T I E S O F H A L F - A N D - H A L F H O M O G E N I Z E D M I L K .................................... 38 E le c tro s ta tic theory In te rfa c ia l tension Experim ental P ro ced u re ..................................................... 39 F a c t o r s a f f e c t i n g the p h y s i c a l , c h e m i c a l , a n d .............................. o r g a n o l e p t i c p r o p e r t i e s of h a l f - a n d - h a l f 39 I n f l u e n c e of a d d e d n o n f a t d r y m i l k s o l i d s a nd p r e s s u r e s of h o m o g e n i z a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f ....................................................................................... 43 iv Page E x p e r i m e n . t a l . R e s u l t s ...............................................................„ . . . . . . 44 V i s c o s i t y ....................................................................................................... 44 H e a t s t a b i l i t y ............................................................................................. 46 T itra ta b le a cid ity and pH ............................................................... 47 ....................................................................................... 47 Specific g ra v ity F e a t h e r i n g of h a l f - a n d - h a l f in c o f f e e .............................. 47 The i n f l u e n c e of a d d e d n o n f a t d r y m i l k s o l i d s a n d p r e s s u r e of h o m o g e n i z a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f ...................................................................................... 49 D i s c u s s i o n ........................................................................................................... 76 V i s c o s i t y ....................................................................................................... 76 H e a t s t a b i l i t y ............................................................................................. 80 T i t r a t a b l e a c i d i t y , pH , s p e c i f i c g r a v i t y , a n d .............................. o r g a n o l e p t i c p r o p e r t i e s of h a l f - a n d - h a l f 83 T h e s t a b i l i t y of h a l f - a n d - h a l f in c o f f e e .................................. 85 T h e i n f l u e n c e of a d d e d n o n f a t d r y m i l k s o l i d s a n d p r e s s u r e s of h o m o g e n i z a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f ....................................................................................... 86 S E C T I O N H: (1) T H E E F F E C T O F D E C R E A S I N G R A T I O S O F S O L I D S - N O T - F A T TO F A T ON SO M E C H E M I C A L AND P H Y S I C A L P R O P E R T I E S O F H A L F - A N D - H A L F AND C R E A M ; (2) A N A LY SI S O F F E A T H E R E D H A L F - A N D - H A L F AND C R E A M .............................. 93 E xperim ental P ro ced u re ..................................................... v 94 Page T he e f f e c t of d e c r e a s i n g r a t i o s of s o l i d s - n o t - f a t to f a t on s o m e c h e m i c a l a n d p h y s i c a l p r o p e r t i e s of h a l f - a n d - h a l f a n d c r e a m ............................................................ 94 P r e p a r a t i o n a n d a n a l y s i s of f e a t h e r e d h a l f - a n d ....................................................................................... half and c r e a m 96 E x p erim en tal R esults ................................................................................ 97 V i s c o s i t y ............................. 98 H eat stability .......................................................................................... 99 E f f e c t of a n e m u l s i f i e r on f a t c l u m p i n g a s m e a s u r e d b y v i s c o s i t y a n d h e a t s t a b i l i t y .............................. 100 The m i g r a t i o n of f a t g l o b u l e s a s a f f e c t e d b y h o m o g e n i z a t i o n a n d c o m p o s i t i o n ............................................ 101 A n a l y s i s of f e a t h e r e d h a l f - a n d - h a l f a n d c r e a m ................. 102 D iscussion . .................................................................................................... 117 V i s c o s i t y ....................... 117 H e a t s t a b i l i t y ............................................................................................. 118 E f f e c t of a n e m u l s i f i e r on the f a t - c l u m p i n g p h e n o m e n o n ................................................................................................. 120 M i g r a t i o n of f a t g l o b u l e s a s a f f e c t e d b y h o m o g e n i z a t i o n a n d c o m p o s i t i o n .................................................. 122 A n a l y s i s of f e a t h e r e d h a l f - a n d - h a l f a n d c r e a m ................. 123 S U M M A R Y .............................................................................- ................................. 125 C O N C L U S I O N S ........................................................................................................ 129 L IT E R A T U R E CITED 131 ....................................................................................... vi LIST OF TABLES Table Page 1. . T h e i n f l u e n c e of a d d e d s o l i d s - n o t - f a t on the c o m p o s i t i o n of h a l f - a n d - h a l f h o m o g e n i z e d m i l k ...................................................................................................... 2. 3. 4. 5. 6. 7. 8. , 52 T h e i n f l u e n c e of a d d e d s o l i d s - n o t - f a t on the v i s c o s i t y of h a l f - a n d - h a l f w h e n h o m o g e n i z e d at various p r e s s u r e s ........................................................................ 53 T h e i n f l u e n c e of l e n g t h of s t o r a g e a n d a d d e d s o l i d s - n o t - f a t on t h e v i s c o s i t y of h a l f - a n d h a l f h o m o g e n i z e d a t v a r i o u s p r e s s u r e s .................................. 54 T h e i n f l u e n c e of a d d e d s o l i d s - n o t - f a t on the h e a t s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k w h e n s u b j e c t e d to v a r i o u s p r e s s u r e s .................... 55 T h e i n f l u e n c e of a d d e d s o l i d s - n o t - f a t on th e specific g r a v ity , pH, and titr a ta b le a cidity of h a l f - a n d - h a l f h o m o g e n i z e d m i l k ..................................... 56 T h e i n f l u e n c e of c a l c i u m - i o n c o n c e n t r a t i o n , c o m p o s i t i o n , a n d p r e s s u r e s of h o m o g e n i z a t i o n on f e a t h e r i n g of h a l f - a n d - h a l f i n c o f f e e ........................... 57 T h e i n f l u e n c e of v a r i o u s c o n c e n t r a t i o n s of i o n ic s o l u t i o n s of c a l c i u m on the s t a b i l i t y of h a l f a n d -h a lf h o m o g e n iz ed m ilk, as m e a s u r e d by a c i d p r e c i p i t a t i o n ............................................................................. 58 T h e i n f l u e n c e of v a r i o u s c o n c e n t r a t i o n s of i o n i c s o l u t i o n s of m a g n e s i u m on the s t a b i l i t y of h a lf-a n d -h a lf hom ogenized m ilk, as m e a s u re d b y a c i d p r e c i p i t a t i o n ...................................................................... 60 V II Table 9. 10. 11. 12. 13. 14. 15. 16. 17. Page T h e i n f l u e n c e of v a r i o u s c o n c e n t r a t i o n s of i o n i c s o l u t i o n s of c i t r a t e on the s t a b i l i t y of h a l f a nd-half hom ogenized m ilk, as m e a s u r e d by a c i d p r e c i p i t a t i o n ............................................................................. 62 T h e i n f l u e n c e of a d d e d s o l i d s - n o t - f a t a n d i n c r e a s i n g p e r c e n t a g e s of f a t o n the c o m p o ­ s i t i o n of h a l f - a n d - h a l f h o m o g e n i z e d m i l k a n d c r e a m ............................................................... 105 T h e i n f l u e n c e of v a r i a t i o n s of t e m p e r a t u r e a n d c o m p o s i t i o n on the v i s c o s i t y of h a l f and-half and c r e a m when hom ogenized ......................................................... at various p r e s s u r e s 106 The influence on th e h e a t and c r e a m pressures 107 of v a r i a t i o n s in c o m p o s i t i o n s t a b i l i t y of h a l f - a n d - h a l f when hom ogenized at various ............................................................................................. T h e i n f l u e n c e of s o r b i t a n m o n o s t e a r a t e up on th e v i s c o s i t y of h a l f - a n d - h a l f a n d c r e a m when h o m o g en ized at v a rio u s p r e s s u r e s ................... T h e i n f l u e n c e of s o r b i t a n m o n o s t e a r a t e up on the h e a t s t a b i l i t y of h a l f - a n d - h a l f a n d c r e a m w hen h o m o g e n iz e d atv a rio u s p r e s s u r e s ... 108 110 T h e e f f e c t of v a r i o u s p r e s s u r e s of h o m o g e n i z a ­ t i o n a n d c o m p o s i t i o n of h a l f - a n d - h a l f a n d c r e a m o n th e m o v e m e n t of f a t g l o b u l e s in an e le c tr i c a l field ................ Ill T h e c o m p o s i t i o n of h a l f - a n d - h a l f a n d c r e a m , a s c o m p a r e d to th e r e s u l t a n t f e a t h e r e d m a t e r i a l ................................................................................................. 112 T h e n i t r o g e n d i s t r i b u t i o n of h a l f - a n d - h a l f a nd c r e a m , a s c o m p a r e d to t h e r e s u l t a n t f e a t h e r e d m a t e r i a l .......................................................................... 113 viii LIST OF FIGURES F igure 1. 2. 3. 4. 5. 6. Page The e f f e c t of s i n g l e - a n d d o u b l e - s t a g e h o m o g e n i z a t i o n o n the v i s c o s i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k .............................................. 64 The e f f e c t of s i n g l e - a n d d o u b l e - s t a g e h o m o g e n i z a t i o n on the h e a t s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k .......................... 65 T h e i n f l u e n c e of a d d e d i n c r e m e n t s of low -heat, sp ra y -p ro c e s s skim m ilk p o w d e r o n the t i t r a t a b l e a c i d i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k ........................................ 66 Th e i n f l u e n c e of i n c r e a s i n g a m o u n t s of c a l c i u m - i o n c o n c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a ­ s u r e d b y a c i d p r e c i p i t a t i o n ( T r i a l 1, J a n u a r y ) ............................................................................................. 67 T h e i n f l u e n c e of i n c r e a s i n g a m o u n t s of c a l c i u m - i o n c o n c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a ­ s u r e d b y a c i d p r e c i p i t a t i o n ( T r i a l 2, M a r c h ) .............................................. 68 The i n f l u e n c e of i n c r e a s i n g a m o u n t s of c a l c i u m - i o n c o n c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a ­ s u r e d b y a c i d p r e c i p i t a t i o n ( T r i a l 3, M a y ) ........................................................................................................ 69 ix Figure 7. 8. 9. 10. 11. 12. P a ge Th e i n f l u e n c e of i n c r e a s i n g a m o u n t s of m a g n e s i u m - i o n c o n c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a ­ s u r e d b y a c i d p r e c i p i t a t i o n ( T r i a l 1, J a n u a r y ) ............................................................................................. 70 The i n f l u e n c e of i n c r e a s i n g a m o u n t s of m a g n e s i u m - i o n c o n c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a ­ s u r e d b y a c i d p r e c i p i t a t i o n ( T r i a l 2, M a r c h ) ................................................................................................. 71 Th e i n f l u e n c e of i n c r e a s i n g a m o u n t s of m a g n e s i u m - i o n c o n c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a ­ s u r e d b y a c i d p r e c i p i t a t i o n ( T r i a l 3, M a y ) ....................................................................................................... 72 Th e i n f l u e n c e of i n c r e a s i n g a m o u n t s of c i t r a t e - i o n c o n c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a ­ s u r e d b y a c i d p r e c i p i t a t i o n ( T r i a l 1, January) ............................................................................................. 73 The i n f l u e n c e of i n c r e a s i n g a m o u n t s of c i t r a t e - i o n c o n c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a ­ s u r e d b y a c i d p r e c i p i t a t i o n ( T r i a l 2, M arch) ................................................................................................. 74 The i n f l u e n c e of i n c r e a s i n g a m o u n t s of c i t r a t e - i o n c o n c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a ­ s u r e d b y a c i d p r e c i p i t a t i o n ( T r i a l 3, M a y ) ........................................................................................................ 75 x F igure 13. 14, Page Th e e f f e c t of i n c r e a s i n g p e r c e n t a g e s of f a t on the v i s c o s i t y of h a l f - a n d - h a l f and c r e a m when hom ogenized at ................................................... variousp r e s s u r e s The e f f e c t of i n c r e a s i n g p e r c e n t a g e s of f a t on the h e a t s t a b i l i t y of h a l f - a n d - h a l f and c r e a m when hom ogenized at variousp r e s s u re s .............................................................................. xi 115 116 INTRODUCTION H a l f - a n d - h a l f h o m o g e n i z e d m i l k , a p r o d u c t t e s t i n g b e t w e e n 10 a n d 13 p e r c e n t f a t a n d 8 to 10 p e r c e n t s o l i d s - n o t - f a t , w a s i n t r o d u c e d in t o the S t a t e of M i c h i g a n in 1952. H a l f - a n d - h a l f w a s d e f i n e d b y the M i c h i g a n D e p a r t m e n t of A g r i c u l t u r e (1952) a s T,a h o m o g e n i z e d p r o d u c t r e s u ltin g f r o m com b in in g o r m ixing m ilk and c r e a m , with o r without the a d d i t i o n of n o n f a t m i l k s o l i d s , a n d s h a l l c o n t a i n n o t l e s s t h a n 10.0 p e r c e n t b u t t e r f a t a n d s h a l l h a v e a s p e c i f i c g r a v i t y a t 60 d e g r e e s F a h r e n ­ h e i t n o t g r e a t e r t h a n 1.0 3 3 . " W h i l e t h i s p r o d u c t i s s t i l l r e l a t i v e l y n e w in M i c h i g a n , it h a s b e e n d i s t r i b u t e d in m a n y o t h e r s t a t e s f o r s o m e t i m e . It has been sold in th e s e a r e a s u n d e r such n a m e s as " b r e a k f a s t c r e a m , " cream ," " c e r e a l t r e a t , " and " c e r e a l m i l k . " m only a s s o c ia te d w ith this p ro d u c t a r e R ich" ( T r o u t a n d W e i n s t e i n , 1950). "cereal T rade n am es also c o m ­ "M el-O -R ich" and " T a s t y - N e v e rth e le s s , this p ro d u c t was n o t r e c o g n i z e d b y t h e U n i t e d S t a t e s P u b l i c H e a l t h S e r v i c e u n t i l 1953, w hen it w as defined a s " a p r o d u c t c o n s i s t i n g of a m i x t u r e of m i l k a n d c r e a m w h i c h c o n t a i n s n o t l e s s t h a n 11.5 p e r c e n t f a t . " T h e i m p o r t a n c e of t h i s p r o d u c t in th e d a i r y i n d u s t r y c a n n o t be m in im iz e d . A lthough only a few r e p o r t s a re a v a ila b le (C olebank, 1 2 1953; C o l e s , 1 9 5 2 a , 1952b, 1 9 52 c ), t h e s e h a v e e m p h a s i z e d i t s i n c r e a s ­ in g i m p o r t a n c e d a r i n g th e p a s t f e w y e a r s . A m o n g the a d v a n t a g e s s u g g e s t e d , th e c h i e f one s e e m s to b e i t s p alatability. D u e to t h e l o w e r b u t t e r f a t c o n t e n t , the p r o d u c t d o e s n o t h a v e too r i c h a t a s t e ; h e n c e , m o r e of the p u b l i c s e e m s to p r e f e r i t s u s e on c e r e a l . H a lf-a n d -h a lf, when p r o p e r l y p r o c e s s e d , also c o m ­ p e te s fa v o r a b ly w ith table c r e a m is c o n c e r n e d . so f a r a s c o l o r i n g a b i l i t y in c o f f e e In a d d i t i o n , m a n y c r e a m e r y o p e r a t o r s f e e l t h a t t h i s p r o d u c t h a s m e r i t in its lo w e r c o st, thus opening new f ie ld s f o r s e l l ­ ing. cream A t the s a m e t i m e , the p r o d u c t c a n s t i l l t a k e the p l a c e of t a b l e successfully. O n th e o t h e r h a n d , s o m e p o s s i b l e p o t e n t i a l d i s a d v a n t a g e s h a v e been m entioned. M a n y c r e a m e r y o p e r a t o r s f e a r t h a t , t h r o u g h the i n t r o d u c t i o n of h a l f - a n d - h a l f , a d e c l i n e in t o t a l b u t t e r f a t s a l e s m i g h t occur. These o p e r a t o r s fe e l a lso that they a r e m e r e l y adding a new p r o d u c t to t h e i r a l r e a d y l o n g l i s t of s m a l l o p e r a t i o n s in t h e i r p l a n t s . Some c o n s u m e r s d islik e this l o w e r - f a t p ro d u c t b e c a u s e it la c k s r i c h ­ n e s s i n t a s t e , a n d d o e s n o t c o l o r th e c o ff e e i n the s a m e m a n n e r a s table c r e a m . A l t h o u g h h a l f - a n d - h a l f h a s b e e n on the m a r k e t i n v a r i o u s a r e a s f o r s o m e t i m e , w h i l e b e i n g i n t r o d u c e d o n l y r e c e n t l y in o t h e r s , l i m i t e d r e s e a r c h h a s b e e n u n d e r t a k e n in th is field . Volum inous stu d ie s, how ­ e v e r , h a v e b e e n c o n d u c t e d w i t h th e 1 8 - p e r c e n t - f a t p r o d u c t , t a b l e c r e a m . U n f o r t u n a t e l y , the r e s u l t s o b t a i n e d on t h e l a t t e r p r o d u c t s o m e t i m e s h a v e b e e n a p p l i e d d i r e c t l y to h a l f - a n d - h a l f . W ith s u c h a d i f f e r e n c e in c o m p o s i t i o n b e t w e e n th e two p r o d u c t s , t h i s w o u l d c e r t a i n l y n o t s e e m a d v is a b le in all c a s e s . T h e m a n u f a c t u r e a n d i n t r o d u c t i o n of h a l f - a n d - h a l f h a v e n o t b e e n w i t h o u t t h e i r p r o b l e m s , on e of w h i c h h a s b e e n f e a t h e r i n g in c o f f e e . T h e p h y s i c o - c h e m i c a l p r o p e r t i e s of t h i s r e a c t i o n , a s a p p l i e d to a h i g h s o l i d s - n o t - f a t to f a t p r o d u c t , s u c h a s h a l f - a n d - h a l f , h a v e n o t b e e n s a tisfa c to rily explained. In a d d i t i o n , t h e i n f l u e n c e of v a r i a t i o n s in s p e c i f i c p r o c e s s i n g t e c h n i q u e s a l s o s e e m e d to r e q u i r e f u r t h e r e x p l o r ­ ation. To tl^ese e n d s the p r e s e n t s t u d y w a s m a d e . REVIEW OF L IT E R A T U R E L i t t l e a v a i l a b l e i n f o r m a t i o n r e g a r d i n g th e p r o p e r t i e s of h a l f a n d - h a l f h o m o g e n iz e d m i l k a p p e a r s in the l i t e r a t u r e at this tim e . H ow ever, extensive s t u d i e s h a v e b e e n c o n d u c t e d on b o t h the h i g h e r - f a t p r o d u c t , t a b l e c r e a m , a n d up on r e g u l a r - a n d h i g h - f a t m i l k s . g e n e r a l f i n d i n g s of t h e s e The s t u d i e s w h i c h m i g h t p e r t a i n to h a l f - a n d - h a l f h a v e b e e n i n c l u d e d so t h a t the r e v i e w o n th e s u b j e c t m i g h t b e a s thorough as po ssib le. V iscosity D efinition. D avies (19 39) d e f i n e d th e v i s c o s i t y of a l i q u i d a s follows: Th e i n t e r n a l f r i c t i o n o r i t s r e s i s t a n c e to s h e a r , a g i t a t i o n o r f l o w , m e a s u r a b l e in a b s o l u t e u n i t s - - t h e p o i s e - - w h i c h m a y be d e f i n e d a s the f o r c e r e q u i r e d to p r o d u c e a d i f f e r e n c e i n the v e ­ l o c i t y of f l o w of a l i q u i d 1 c m . p e r s e c . w h e n t h i s f o r c e is e x ­ e r t e d on 1 s q . c m . b e t w e e n two p a r a l l e l p l a n e s , e a c h 1 sq. c m . in a r e a a n d 1 c m . a p a r t . The v i s c o s i t y i s u s u a l l y e x p r e s s e d in c e n t i p o i s e s , the s t a n d a r d b e i n g t h a t of w a t e r a t 20° C. (= 1.005 centipoise s ). V i s c o s i t y of h o m o g e n i z e d m i l k a n d c r e a m . Bishop and M urphy ( 1911 ) c o n d u c t e d one of t h e e a r l i e s t o r g a n i z e d i n v e s t i g a t i o n s o n h o m o g ­ e n iz e d m ilk and c r e a m . U s i n g a g l a s s p l a t e h e l d a t an a n g l e , t h e y 4 o b s e r v e d g r e a t e r v i s c o s i t i e s in c r e a m s h o m o g e n i z e d a t 140° F . t h a n a t 70 F. B a b c o c k a n d R u s s e l l (1896) h a d p r e v i o u s l y s u g g e s t e d t h a t c a s e i n e x e r t e d a g r e a t e r i n f l u e n c e on c o n s i s t e n c y t h a n f a t , a n d , in m i l k , w a s t h e c h i e f f a c t o r , w h i l e in c r e a m , th e f a t a s s u m e d a m o r e i m p o r t a n t r o l e b e c a u s e of i t s h i g h p e r c e n t a g e . D o a n (19 38) s t a t e d t h a t the v i s c o s i t y of h o m o g e n i z e d m i l k w a s h ig h er than unhom ogenized. T h i s w a s a t t r i b u t e d to the c l u m p i n g p h e ­ n o m e n o n ' (D o a n a n d S w o p e , 1927), b u t it w a s n o t e d t h a t t h i s d i d n o t b e c o m e a p p a r e n t u n t i l a b o u t 4 to 6 p e r c e n t f a t w a s p r e s e n t . Doan (1927) f u r t h e r e x p l a i n e d t h a t w h e n f l u i d m i l k o r c r e a m c o n t a i n i n g 8 p e r c e n t o r m o r e f a t w a s h o m o g e n i z e d a t a p r e s s u r e of 2,000 to 3,000 p o u n d s , t h e g l o b u l e s w e r e g r e a t l y r e d u c e d i n s i z e a n d t e n d e d to c l u m p t o g e t h e r w i t h th e c l u m p s s h o w i n g a t e n d e n c y to a r r a n g e t h e m s e l v e s i n s u c h a f a s h i o n t h a t t h e y o c c u p i e d the g r e a t e s t p o s s i b l e s p a c e . Doan (1929a) a d d e d t h a t t h i s c l u m p i n g of the s m a l l f a t g l o b u l e s i n h o m o g ­ e n iz e d m i x t u r e s w as g r e a t l y s tim u la te d by an i n c r e a s e d fa t content, and also by in c r e a s e d hom ogenization p r e s s u r e s . I n v e s t i g a t i n g the i n f l u e n c e of t e m p e r a t u r e on h o m o g e n i z a t i o n , T r o u t a n d S c h e i d (1941) s t a t e d t h a t l i t t l e o r no d i s p e r s i o n of the f a t g l o b u l e s o c c u r r e d u n t i l the f a t a p p r o a c h e d a l i q u i d s t a t e . M oreover, T r o u t , H a l l o r a n , a n d G o u l d (1935) f o u n d t h a t the r e d u c t i o n in s i z e of 6 th e f a t g l o b u l e s w a s g r e a t e r w h e n th e h o m o g e n i z a t i o n w a s do n e a t 145° F . t h a n a t 90° F . a t a n y g i v e n p r e s s u r e of h o m o g e n i z a t i o n . F r o m th e r e s u l t s of s i m i l a r s t u d i e s , W h i t a k e r a n d H i l k e r (1937) i n d i c a t e d t h a t no c l u m p i n g of f a t g l o b u l e s w a s n o t e d in a n y of the h o m o g e n i z e d m i l k s p r o c e s s e d a t 90° F . o r l e s s , w i t h v e r y s l i g h t clu m p in g being n o te d above 100° F . D o a n a n d M i n s t e r (1933) o b ­ t a i n e d th e g r e a t e s t d e g r e e of f a t c l u m p i n g i n v e r y - h i g h - f a t h o m o g e n ­ i z e d m i l k , w h e n the s a m p l e w a s p r e h e a t e d a t 145° F . a n d p r o c e s s e d a t 100° F . o r l o w e r . T h i s l e d to th e c o n c l u s i o n t h a t , in g e n e r a l , the g r e a t e r th e d r o p i n t e m p e r a t u r e b e t w e e n p r e h e a t i n g a n d h o m o g e n i z i n g , the g r e a t e r w a s the c l u m p i n g t e n d e n c y . H o w e v e r , w h e n the p r e h e a t ­ ing a n d h o m o g e n i z i n g t e m p e r a t u r e s w e r e i n c r e a s e d to 180° F . , no f a t c l u m p i n g w a s f o u n d in the s a m p l e s c o n t a i n i n g up to 10 p e r c e n t fat. D o a n ( 1 9 2 9 a ) , i n s t u d y i n g t h e e f f e c t of p l a s m a s o l i d s in c l u m p ­ i n g , p o s t u l a t e d t h a t a l i m i t i n g f a c t o r i n the f a t c l u m p i n g p h e n o m e n o n w a s th e r a t i o of the a m o u n t of p l a s m a s o l i d s to t h e a m o u n t of f a t in the m i x t u r e s p r o c e s s e d . A c r i ti c a l ratio w as s u g g e s te d above which no c l u m p i n g w a s o b t a i n e d , b u t b e l o w w h i c h c l u m p i n g w a s e v i d e n t o r even pronounced. 7 T r o u t a n d W e i n s t e i n (1952) n o t e d t h a t s i n g l e - s t a g e h o m o g e n i z a ­ t i o n i n d u c e d v a r y i n g d e g r e e s of f a t c l u m p i n g w h e n the f a t w a s 10 p e r ­ c e n t o r m o r e , a n d t h a t a p r o n o u n c e d i n c r e a s e in v i s c o s i t y w a s p r e s e n t a f t e r 48 h o u r s of s t o r a g e a t 40° F . W ith tw o -s ta g e h o m o g e n iz atio n , h o w e v e r , f a t c l u m p i n g w a s r e d u c e d to a m i n i m u m a n d v e r y l i t t l e c h a n g e i n v i s c o s i t y o c c u r r e d a f t e r the s a m e l e n g t h of s t o r a g e . Doan (1931a) a l s o p o i n t e d o u t t h a t , w h i l e d u a l h o m o g e n i z a t i o n m a d e i t p o s ­ s i b l e to u s e a m u c h h i g h e r t o t a l p r e s s u r e , t h u s i n c r e a s i n g the e f f i ­ c i e n c y of h o m o g e n i z a t i o n , i t a l s o r e d u c e d the v i s c o s i t y . F a c t o r s A f f e c t i n g the S t a b i l i t y of M i l k a n d C r e a m Introductio n . T h e v a r i a b i l i t y of m i l k t o w a r d s h e a t c o a g u l a t i o n w a s n o t e d b y H o l m , W e b b , a n d D e y s h e r (1932). F r o m an in v e s tig a ­ t i o n of m i l k f r o m i n d i v i d u a l c o w s d u r i n g t h e i r l a c t a t i o n p e r i o d s , wide v a ria tio n was o b serv ed . T h e s e a u t h o r s e m p h a s i z e d t h a t a w id e d i f ­ f e r e n c e e x i s t e d in the p r o p e r t i e s c o r r e l a t i o n in t h e i r d a ta. th e sam e results. of m i l k a s i n d i c a t e d b y the l a c k of C o le a n d T a r a s s u k (1946) r e p o r t e d m u c h T h e y f o u n d c o n s i d e r a b l e v a r i a t i o n b e t w e e n the s a m p l e s of c o w s ' m i l k s o f a r a s c o a g u l a t i o n t i m e a t a g i v e n t e m p e r a ­ ture was concerned. 8 Hom ogenizatio n . W h i t t i e r a n d W ebb (1950) s u g g e s t e d t h a t the h e a t s t a b i l i t y of m i l k a n d c r e a m w a s a f f e c t e d b y t h e t e m p e r a t u r e , pressure of h o m o g e n i z a t i o n , a n d b y the f a t a n d s o l i d s - n o t - f a t c o n t e n t . A. r e p o r t b y D o a n (1929a) n o t e d t h a t p r o t e i n s t a b i l i t y in h o m o g ­ e nized m i x tu r e s d e c r e a s e d with in c re a s in g p r e s s u r e s tion. of h o m o g e n i z a ­ He a d d e d t h a t th e f a t - c l u m p i n g t e n d e n c y w a s f o u n d to p a r a l l e l v e r y c l o s e l y th e s t a b i l i t y of th e p r o t e i n s , w i t h a d e c r e a s e b e i n g n o t e d a s th e c l u m p i n g i n c r e a s e d . This w as v e rifie d by a su b seq u en t r e p o r t of D o a n ( 1 9 3 1 a ) , w h e n he c o n c l u d e d t h a t the d e g r e e of f a t c l u m p i n g in n o r m a l , s w e e t , h o m o g e n i z e d c r e a m w a s p r o b a b l y th e m o s t i m p o r ­ t a n t s i n g l e f a c t o r a f f e c t i n g th e s t a b i l i t y of t h e c r e a m t o w a r d s h e a t . W e b b (1931) e x p l a i n e d t h a t c o a g u l a t i o n of s u c h a h y d r o p h i l i c c o l l o i d a s c a s e i n d e p e n d e d u p o n th e d e s t r u c t i o n of i t s w a t e r - h o l d i n g c a p a c i t y a n d u p o n the f l o c c u l a t i o n of the d e n a t u r e d p a r t i c l e s . Using a c r y o s c o p e to d e t e r m i n e b o u n d w a t e r , P y e n s o n a n d D a h l e (1938) s h o w e d t h a t h o m o g e n i z a t i o n of ic e c r e a m m i x e s , w i t h one p r e s s u r e , p r o d u c e d a n i n c r e a s e i n v i s c o s i t y , b u t d e c r e a s e d the b o u n d - w a t e r content. D u a l h o m o g e n i z a t i o n , t h e y p o i n t e d o u t, i n c r e a s e d b o t h b o u n d w a ter and viscosity. An e n d o t h e r m i c r e a c t i o n was shown by L e ig h to n and Mudge (1923) to h a v e a c c o m p a n i e d the c o a g u l a t i o n of c a s e i n of m i l k , w h e n 9 this co ag u la tio n w as b ro u g h t about by h e at. This p h en o m en o n w as a t t r i b u t e d to t h e p r e c i p i t a t i o n of c a l c i u m a n d m a g n e s i u m a s c i t r a t e s and p h o s p h a te s . H o w e v e r , t e s t s c o n d u c t e d b y S u l l a m (1942) w i t h a l c o h o l , a c e ­ t o n e , a n d n e u t r a l s a l t s i n d i c a t e d t h a t h o m o g e n i z a t i o n i n c r e a s e d the s t a b i l i t y of m i l k . Solids- n ot- f a t. Th e s o l i d s - n o t - f a t a p p a r e n t l y p l a y an i m p o r ­ t a n t r o l e in th e h e a t s t a b i l i t y of m i l k a n d c r e a m . (1930) Doan and M in s te r r e p o r t e d that, u n d er som e conditions, added solids i n c r e a s e d the s t a b i l i t y of c r e a m a s c o m p a r e d w i t h the p r o d u c t h o m o g e n i z e d a t a norm al solids content. H o w e v e r , W ebb (1931) s t a t e d t h a t a n i n ­ c r e a s e i n th e c o n c e n t r a t i o n of m i l k - s o l i d s - n o t - f a t g r e a t l y d e c r e a s e d the c o a g u l a t i o n t i m e , e x p r e s s e d a s h e a t s t a b i l i t y , of b o t h the u n ­ tr e a t e d and h o m o g e n iz ed c r e a m s . The c o n c l u s i o n w a s d r a w n f r o m t h i s i n v e s t i g a t i o n t h a t a s the s o l i d s c o n t e n t i n c r e a s e d , the s t a b i l i t y of the h o m o g e n i z e d s a m p l e t e n d e d to a p p r o a c h t h a t of th e u n t r e a t e d cream . F u r t h e r m o r e , D o a n (1931a) c a u t i o n e d a g a i n s t u s i n g e x c e s s i v e a m o u n t s of c o n d e n s e d s k i m m i l k o r s k i m m i l k p o w d e r , a s t h i s c o u l d c a u s e a l o s s of s t a b i l i t y . 10 P r o c e s sing t e m p e r a t u r e s . W h i t t i e r a n d W eb b (1950) f e l t t h a t the a c t u a l t e m p e r a t u r e of h o m o g e n i z a t i o n w a s m o r e i m p o r t a n t t h a n the p r e h o m o g e n i z a t i o n h e a t t r e a t m e n t w h e n n o n c o n c e n t r a t e d , h i g h - f a t p ro d u c ts w ere used. T r o u t , H a l l o r a n , a n d G o u l d (1935) r e p o r t e d t h a t , w h i l e h o m o g ­ e n i z a t i o n d e c r e a s e d the p r o t e i n s t a b i l i t y of m i l k t o w a r d a l c o h o l , the m ilk p a ste u riz e d before hom ogenization was considerably m ore stable to a l c o h o l t h a n h o m o g e n i z e d r a w m i l k . D o a n (1929b) a t t r i b u t e d t h i s h e a t i n g e f f e c t p o s s i b l y to the r e d u c e d a m o u n t of s o l u b l e c a l c i u m , t h u s m a k i n g the p r o t e i n m o r e s t a b l e to th e e f f e c t of h o m o g e n i z a t i o n . W orking with h o m o g e n iz ed m i l k s r a n g i n g f r o m 0 to 10 p e r c e n t , D o a n a n d M i n s t e r (1931) f o u n d t h a t a h i g h p r e h e a t i n g t e m p e r a t u r e t e n d e d to i n h i b i t o r l i m i t f a t c l u m p i n g w h i c h c o n s e q u e n t l y r e d u c e d th e l o s s of s t a b i l i t y u p o n hom ogenization. T his, they continued, would not e lim in a te fat c lu m p ­ in g in a n y of the m i x t u r e s c o n t a i n i n g o v e r 7 p e r c e n t f a t , if the m i x ­ ture was s u b s e q u e n t l y p r o c e s s e d a t the l o w t e m p e r a t u r e of 100° to 120° F . T h i s i n d i c a t e d to t h e s e i n v e s t i g a t o r s t h a t the p r e c i p i t a t i o n of c a l c i u m i o n s w a s p r o b a b l y n o t a s i m p o r t a n t a s h a d b e e n t h o u g h t . W e b b a n d H o l m (1928) s u g g e s t e d t h a t the m a x i m u m h e a t s t a ­ b i l i t y f o r a n y p e r c e n t a g e of b u t t e r f a t a s h i g h a s 30 p e r c e n t c o u l d be 11 a t t a i n e d b y h a v i n g c r e a m p r e h e a t e d to 80° C . , w i t h the e x c e p t i o n of th o se w ith low fa t c o n ten t and low h o m o g enizing p r e s s u r e s , w h e re 90° C , s l i g h t l y i n c r e a s e d the s t a b i l i t y . In g e n e r a l , D o a n (1931a) t h o u g h t t h a t c r e a m s h o u l d n o t be h o m o g e n iz e d a t a t e m p e r a t u r e lo w e r than that a t w hich it w as p r e ­ h e ate d u n le ss this w as (8 2.2° C.) f l a s h . 170° F . (7 6.6° C .) f o r 30 m i n u t e s , o r 180° F . In o r d e r t h a t t h e h e a t s t a b i l i t i e s of c r e a m s c o n t a i n ­ ing m o r e t h a n 15 p e r c e n t f a t b e i n c r e a s e d , W e b b (1931) s t a t e d t h a t d o u b l e h o m o g e n i z a t i o n a t 80° C . (176° F .), w i t h the s e c o n d s t a g e in the v i c i n i t y of 500 p o u n d s , w o u l d be b e n e f i c i a l . These re s u lts w ere v e r i f i e d b y t h e w o r k of W h i t t i e r a n d W ebb (1950), who c o n t e n d e d t h a t if th e c r e a m s w e r e h e a t e d to t h e o p t i m u m h o m o g e n i z i n g t e m p e r a t u r e , 176° F . , a n d t h e n c o o l e d a n d h o m o g e n i z e d a t a l o w e r t e m p e r a t u r e , such as 140° F . , t h e i r h e a t s t a b i l i t i e s w o u l d b e a l m o s t a s l o w a s if b o t h h e a t i n g a n d h o m o g e n i z i n g w e r e done a t 140° F . T h e r e s u l t s of P y e n s o n a n d D a h l e (1938) i n d i c a t e d t h a t the h e a t i n g of m i l k to h i g h t e m p e r a t u r e s d e c r e a s e d the b o u n d - w a t e r c o n t e n t , b u t i n c r e a s e d the s t a b i l i t y ' t o w a r d s a l c o h o l . 12 E f f e c t of S a l t s on S t a b i l i t y of M i l k a n d C r e a m S t a t e of c o m p o n e n t s in m i l k . c a s e in w as e n tir e ly in s u sp e n sio n . R o g e r s (1935) r e p o r t e d t h a t T h i s a u t h o r p o i n t e d o u t, h o w e v e r , t h a t w h e t h e r c a s e i n e x i s t e d in c o m b i n a t i o n w i t h c a l c i u m a l o n e , w i t h c a l c i u m p h o s p h a t e , w h e t h e r th e a g g r e g a t e w a s a c o m b i n a t i o n of c a l ­ c iu m c a s e in a te w ith c a lc iu m p h o sp h ate , o r w h e th e r sodium w as in- ' v o l v e d , w a s a q u e s t i o n s t i l l in d o u b t a t t h a t t i m e . In a n e a r l y s t u d y , V a n Slyke a n d B o s w o r t h (1914) s t a t e d t h a t the p r o t e i n c o m b i n e d w i t h c a l c i u m a m o u n t e d to 3.20 p e r c e n t . 1915, t h e s e In s a m e i n v e s t i g a t o r s n o t e d t h a t the b e l i e f a t t h a t t i m e s u g g e s t e d t h a t m i l k c a s e i n w a s c o m b i n e d w i t h c a l c i u m ( a b o u t 1.07 p e r c e n t ) , w h i c h f o r m e d a c a l c i u m c a s e i n a t e s a l t , a n d in a d d i t i o n , the c a s e i n w a s b e l i e v e d to be c h e m i c a l l y c o m b i n e d d i r e c t l y w i t h c a l c i u m p h o s p h a t e a n d t h a t t h i s p r o t e i n w a s a d o u b l e c o m p o u n d of calc iu m c ase in ate co m b in ed with c alc iu m phosphate. This c a lc iu m c a s e in a te -c a lc i u m phosphate c o m p le x was i s o ­ l a t e d f r o m m i l k b y R a m s d e l l a n d W h i t t i e r (194 4), t h r o u g h th e u s e of an u ltr a c e n trifu g e . On t h i s b a s i s , the c o m p l e x w a s f o u n d to c o n t a i n 4.80 p e r c e n t t r i c a l c i u m p h o s p h a t e a n d 95.20 p e r c e n t c a l c i u m c a s e i n a t e . T h e c a l c i u m c a s e i n a t e f r a c t i o n of the c o m p l e x w a s r e p o r t e d to c o n ­ t a i n 1.18 p e r c e n t c a l c i u m . 13 P reviously, Pyne (1932) h a d s u g g e s t e d t h a t c a l c i u m c a s e i n o g e - n a t e a n d t h e c o l l o i d a l p h o s p h a t e of m i l k e x i s t e d i n s o m e f o r m of ch em ica l com bination. S u b s e q u e n t r e s u l t s of P y n e (1934) s h o w e d t h a t t h i s d o u b l e s a l t c o n s i s t e d of c a l c i u m c a s e i n o g e n a t e a n d t r i c a l c i u m phosphate. T e r H o r s t (1947) a d d e d t h a t th e c a l c i u m c o m b i n e d w i t h the p h o s p h a t e i n m i l k w a s i n a q u a n t i t y e x c e e d i n g the s o l u b i l i t y p r o d u c t of t r i c a l c i u m p h o s p h a t e . V a n d e r B u r g (1947) p o s t u l a t e d t h a t t h i s c a l c i u m c a s e i n a t e k e p t in s o l u t i o n a n a m o u n t of c o l l o i d a l c a l c i u m a n d p h o s p h a t e w i t h the m o l e c u l a r r a t i o of t h i s c a l c i u m a n d p h o s p h a t e v a r y i n g , b e i n g a b o u t 1 to 50 o n the a v e r a g e . I t w a s a l s o b e l i e v e d t h a t the c a l c i u m , u n i t e d w i t h the o r g a n i c p h o s p h a t e of the c a s e i n , f o r m e d the c a l c i u m caseinate. R e c e n t l y , H a u r o w i t z (1950) re p o rte d that p ro te in s com bined p a r t i c u l a r l y with c alc iu m , m a g n e s iu m , p h o sphate, and b icarb o n ate i o n s a n d t h e l i n k a g e t h e y f o r m e d w a s so s t a b l e t h a t the i o n s w e r e re m o v e d n e i t h e r by d ia ly s is n o r by e l e c t r o p h o r e s i s , although they could be p o s s ib ly r e m o v e d by e l e c t r o d i a l y s i s . In a d d i t i o n , t h i s a u t h o r e x p l a i n e d t h a t the s t a b i l i t y of the b o n d f o r m e d w a s due to the h i g h e l e c t r o s t a t i c a t t r a c t i o n of the b i v a l e n t i n o r g a n i c i o n s , w h e r e a s un ivalent ions w e re l e s s f i r m l y bound. H ence, it w as r e a s o n e d that 14 th e c a t i o n s w e r e a t t r a c t e d to the n e g a t i v e l y c h a r g e d c a r b o x y l g r o u p s , w h i l e the a n i o n s w e r e b o u n d to the p o s i t i v e l y c h a r g e d g r o u p of the p rotein. V a n d e r B u r g (1947) a d d e d t h a t th e p o s i t i v e a m i n o g r o u p of l y s i n e t o o k p a r t in the b i n d i n g of the n e g a t i v e p h o s p h a t e g r o u p s ; c o n s e q u e n t l y o n l y th e n e g a t i v e c a r b o x y l g r o u p s w e r e l e f t f o r the c o l ­ loidal c alc iu m . E a r l y i n v e s t i g a t i o n s b y V a n S ly k e a n d B o s w o r t h (1916) s u g ­ g e s t e d t h a t the i n s o l u b l e c a l c i u m in f r e s h m i l k w a s i n c o m b i n a t i o n with p h o s p h o ric a cid as C a H P O , and with c a s e in as C a , c a s e in a te . 4 4 L i n g (1936) r e p o r t e d t h a t i t w a s g e n e r a l l y a g r e e d t h a t th e m a j o r p a r t of the c o l l o i d a l i n o r g a n i c p h a s e c o n s i s t e d of c a l c i u m p h o s p h a t e , b u t h e n o t e d t h a t o p i n i o n s w e r e d i v i d e d a s to w h e t h e r i t w a s th e d i - o r tric a lc iu m phosphate. T h e r e s u l t s of h i s i n v e s t i g a t i o n s s h o w e d t h a t t r i c a l c i u m p h o s p h a t e a c c o u n t e d f o r a l m o s t a l l the c o l l o i d a l i n o r g a n i c p h o s p h a t e of m i l k . P y n e a n d R y a n (1950) c o n c u r r e d b y s t a t i n g t h a t a b o u t 88 p e r c e n t of th e c o l l o i d a l p h o s p h a t e a p p e a r e d to b e t r i c a l c i u m p h o s p h a t e w i t h th e r e m a i n d e r t h o u g h t to be i n the d i c a l c i c f o r m . A s L e i g h t o n a n d D e y s h e r (1923) n o t e d , th e c a l c i u m a n d m a g ­ n e s i u m w e r e p r o b a b l y p r e s e n t in m i l k in c o m b i n a t i o n w i t h p r o t e i n s , a s i o n s , o r i n u n - i o n i z e d m o l e c u l e s a n d in a n i n s o l u b l e f o r m . (u n d a te d ) p i c t u r e d th e p r e s e n c e of c a l c i u m in m i l k a s f o l l o w s : Seekles 15 Ca++ S e e k l e s a n d S m e e t s (1947) e x p l a i n e d th e p r e s e n c e of c a l c i u m o n th e b a s i s of i t s u l t r a f i l t e r a b i l i t y ; t h a t i s , a n e q u i l i b r i u m existed b e t w e e n (1) u l t r a f i l t e r a b l e c a l c i u m b o u n d to p r o t e i n , (2) u l t r a f i l t e r a b l e c a l c i u m i o n s , a n d (3) c a l c i u m b o u n d to u l t r a f i l t e r a b l e o r g a n i c a n d inorganic com plexes such as c itr a t e s and p h o sp h ate s. N o r d b o (1939) c a l c u l a t e d t h a t o n l y a b o u t 20 p e r c e n t of the c a l ­ c iu m w a s p r e s e n t in ionic f o r m . As fo r m a g n e s iu m , it w as r e p o r t e d t h a t a t o t a l of 0 . 4 to 0 .5 m M . of m a g n e s i u m i o n s w e r e p r e s e n t in the u l t r a f i l t r a t e s of f r e s h m a r k e t m i l k , w h i l e th e t o t a l d i f f u s a b l e m a g n e s i u m a m o u n t e d to a b o u t 2.5 m M . p e r l i t e r . T h e s t a t e of c a l c i u m i n m i l k w a s r e p r e s e n t e d b y L i n g (1937) to b e a p p r o x i m a t e l y a s f o l l o w s : total c alciu m I--------------------s o l u b l e (1 /3 ) 1 c o l l o i d a l (2/3) calcium as C a P O (4 /9 ) 3 Z o caseinogen c a l c i u m (2/9) 16 E ff e c t of h e a t i n g o n s t a t e of c o m p o n e n t s . h e a t e d to 6 2 ° , 7 7 ° , a n d 100° When sk im m ilk was C. f o r 30 m i n u t e s , a n d to 100° F . f o r 30, 60, a n d 90 m i n u t e s , H a r m o n a n d S l a t t e r (1950) s h o w e d t h a t in a l l c a s e s t h e r a t e of d i f f u s i o n of t h e c a l c i u m , p h o s p h o r u s , a n d m a g n e s i u m w a s d e c r e a s e d m a r k e d l y , w h e r e a s the c i t r i c a c i d w a s a p p a r e n t l y u n ­ affected. These r e s u l t s s u b s t a n t i a l l y a g r e e d w i t h t h o s e of B e l l (1925), wh o h a d p r e v i o u s l y r e p o r t e d t h a t , w i t h a n i n c r e a s e in t e m p e r a t u r e , t h e r e w a s a c o r r e s p o n d i n g l o s s of the s o l u b l e c a l c i u m a n d p h o s p h o r u s c o n t e n t of s k i m m i l k . He n o t e d a l s o t h a t w h e n the t e m p e r a t u r e of 170° F . w a s r e a c h e d , no m e a s u r a b l e a m o u n t s of t h e s e s u b s t a n c e s w e r e r e ­ m o v e d f r o m solution. V e r m a a n d S o m m e r (1950) f o u n d a d e c r e a s e in the a m o u n t of s o l u b l e c a l c i u m a f t e r p a s t e u r i z a t i o n , b u t t h i s t e n d e d to c o m e to n o r ­ m a l on c o o l a g i n g , a n d a t the e n d of 24 h o u r s of s t o r a g e a t 45° F . , the a m o u n t of s o l u b l e c a l c i u m in m i l k w a s h i g h e r t h a n t h a t c o n t a i n e d in r a w m i l k . S im ila r r e s u l t s w e re noted by H ilg em an and J e n n e s s (195 1), w h o , w h e n u s i n g a h e a t t r e a t m e n t of 80° C. f o r 30 m i n u t e s , r e p o r t e d a 12 to 18 p e r c e n t d e c r e a s e in th e d i s s o l v e d c a l c i u m a n d i n o r g a n i c p h o s p h o r u s of s k i m m i l k . w a s s h o w n to o c c u r on co ol s t o r a g e . A s l o w r e v e r s a l of the e f f e c t 17 V a n d e r B u r g (1947) e x p l a i n e d t h a t b y h e a t i n g m i l k th e c o l l o i d a l c a lc iu m and phosphate w ere s p l i t off f r o m the c a s e i n . these salts w e re p re c ip ita te d as tric a lc iu m phosphate. g a to r also In w h e y s e r u m This in v e s t i­ su g g e s te d th a t u n d e r these conditions, they w e re readsorbed b y th e c a s e i n . As e a rly as 1918, S o m m e r a n d H a r t (1918) s h o w e d t h a t c i t r i c a c i d w a s n o t d e s t r o y e d on th e h e a t i n g of m i l k , e v e n i n a n a u t o c l a v e a t 15 p o u n d s p r e s s u r e f o r on e h o u r . H ence, it w as concluded that t h e c i t r i c a c i d s a l t s w e r e n o t c o n v e r t e d to th e i n s o l u b l e t r i c a l c i u m c it r a t e on heating. E f f e c t of s a l t b a l a n c e . t i o n s c o n c e r n i n g th e by S o m m e r. S o m e of the f i r s t s i g n i f i c a n t i n v e s t i g a ­ s a l t b a l a n c e in d a i r y p r o d u c t s w e r e a c c o m p l i s h e d T h i s i n v e s t i g a t o r (1929a) c o n s i d e r e d the m a i n f a c t o r in the a l c o h o l c o a g u l a t i o n t e s t w i t h m i l k to b e t h e s a l t b a l a n c e ; t h a t i s , th e a m o u n t of c a l c i u m a n d m a g n e s i u m s a l t s i n th e m i l k , a s o p p o s e d to the a m o u n t of c i t r a t e s a n d p h o s p h a t e s . E a r l i e r , S o m m er and H art (1919) h a d e x p l a i n e d t h a t c a s e i n a p p a r e n t l y r e q u i r e d a n o p t i m u m c a l ­ c i u m c o n t e n t f o r i t s m a x i m u m s t a b i l i t y , a n d , in a d d i t i o n , the c a l c i u m d i s t r i b u t e d i t s e l f c h i e f l y to th e c i t r a t e s a n d p h o s p h a t e s . H ence, they p o s t u l a t e d t h a t if m i l k w e r e h i g h in c i t r a t e s a n d p h o s p h a t e s , m o r e c a l c i u m w a s n e c e s s a r y in o r d e r t h a t c a s e i n c o u l d r e t a i n i t s o p t i m u m 18 c a lc iu m content. In a d d i t i o n , t h e y f e l t t h a t if m i l k w a s h i g h i n c a l ­ c i u m , t h e r e p o s s i b l y w o u l d n o t be e n o u g h c i t r a t e s a n d p h o s p h a t e s to c o m p e t e w i t h the c a s e i n i n o r d e r to l o w e r the c a l c i u m to th e o p t i m u m . T h e f u n c t i o n of m a g n e s i u m w a s e x p l a i n e d a s a r e p l a c e m e n t of c a l c i u m in the c i t r a t e s a n d p h o s p h a t e s . W e b b a n d H o l m (1932) c o n s i d e r e d m i l k to h a v e a c e r t a i n c r i t ­ i c a l s a l t c o n c e n t r a t i o n , b e y o n d w h i c h r a p i d s e n s i t i z a t i o n of the s ol to heat occurred. T h e y u n d e r s t o o d t h a t the v a l u e of the c r i t i c a l s a l t b a l a n c e d e p e n d e d on the s o l i d s c o n t e n t of m i l k a n d th e p r e c i p i t a t i n g ion. T h e a c t i o n of a d d e d c a l c i u m w a s n o t c o n s i d e r e d to be th e r e s u l t of c a l c i u m i t s e l f , b u t r a t h e r t h a t of the p o s i t i v e c h a r g e i t c a r r i e d . F r o m t h e i r r e s u l t s it w as c o n c lu d e d th a t the c r i t e r i o n fo r m a x i m u m s t a b i l i t y w a s a c r i t i c a l b a l a n c e of s t r o n g l y p o s i t i v e c a t i o n s a s o p p o s e d to th e a c t i o n of h e a v y a n i o n s . D o a n (1929b) i n d i c a t e d t h a t the c a l c i u m i o n c o n c e n t r a t i o n , o r a m o u n t of s o l u b l e c a l c i u m , w a s a p r i m e f a c t o r w h i c h a f f e c t e d b o t h fat clum ping and p ro te in stability. F o r s t a b i l i z a t i o n p u r p o s e s , D oa n (1929b) s h o w e d t h a t a d d i t i o n s of s o d i u m c i t r a t e , d i s o d i u m p h o s p h a t e , and sodium carb o n ate s t a b i l i z e d th e p r o t e i n s in b o t h th e h o m o g e n i z e d a n d n o n h o m o g e n i z e d s a m p l e s , w i t h th e g r e a t e s t i n c r e a s e in s t a b i l i t y b e i n g s h o w n i n the h o m o g e n i z e d p o r t i o n s . He t h o u g h t it r e a s o n a b l e 19 to s u p p o s e t h a t s o m e of th e e f f e c t of t h e s e s a l t s in s t a b i l i z i n g the p r o t e i n w a s d u e to th e f a c t t h a t t h e y p r e c i p i t a t e d s o l u b l e c a l c i u m . D o a n (1931b) s u g g e s t e d t h a t t h e s e s a l t s s h o u l d be a d d e d p r i o r to hom ogenization. S o m m e r a n d B i n n e y (1923) n o t e d t h a t a s l i g h t i n c r e a s e in the m a g n e s i u m o r c a l c i u m c o n te n t would cau se a p o sitiv e alco h o l te s t , while i n c r e a s e s in p o t a s s i u m , s o d iu m , c h l o r i d e s , c i t r a t e s , and p h o s ­ p h a te s w o u ld not give a p o s itiv e t e s t . A. p h o s p h a t e t e s t , d e v e l o p e d b y R a m s d e l l , J o h n s o n , a n d E v a n s ( 19 31) , s h o w e d t h a t if m i l k , w h e n m i x e d w i t h a p h o s p h a t e s o l u t i o n , c o a g u la te d upon i m m e r s i o n in boiling w a t e r , then th is p r o d u c t w as f o u n d to b e of l o w h e a t s t a b i l i t y . E f f e c t of ac_id on s t a t e of c o m p o n e n t s . D o a n (1929c) reported t h a t th e s t a b i l i t y of t h e p r o t e i n s w a s d e c r e a s e d b y l a r g e r a m o u n t s of soluble c a l c iu m and h ig h e r h y d ro g e n -io n c o n c e n tr a tio n s . However, a r e p o r t b y S o m m e r a n d H a r t (1919) h a d s t a t e d t h a t i n f r e s h m i l k , t h e r e w a s no r e l a t i o n b e t w e e n t i t r a t a b l e a c i d i t y a n d h e a t c o a g u l a t i o n , a n d i t w a s c o n c l u d e d t h a t th e h y d r o g e n - i o n c o n c e n t r a t i o n w a s n o t th e d e t e r m i n i n g f a c t o r in f r e s h - m i l k c o a g u l a t i o n . W e b b a n d H o l m (1932) s u g g e s t e d t h a t two d i f f e r e n t t y p e s of m i l k e x i s t e d w i t h r e s p e c t to the e f f e c t of the h y d r o g e n - i o n c o n c e n t r a t i o n 20 upon its h e a t stab ility . T hese in v e s tig a to r s explained th a t an addition of a c i d , o r a b a c t e r i a l d e v e l o p m e n t of a c i d , i n a m i l k e i t h e r c o u l d in c r e a s e o r d e c r e a s e its stability. The f i n a l e f f e c t , t h e y e x p l a i n e d , c o u l d b e d e p e n d e n t on s e c u r i n g i n th e m i l k the o p t i m u m pH f o r the p a r t i c u l a r sam ple in question. M i l l e r a n d S o m m e r (1940) f o u n d m i l k to b e v e r y s e n s i t i v e to p H c h a n g e s , e s p e c i a l l y w i t h i n two r a n g e s ; n a m e l y , f r o m a p p r o x i m a t e l y 6.4 to 6.2 a n d to a l e s s e r e x t e n t b e t w e e n p H 5.4 a n d 5.2 . p H 6.6 a n d 6 . 4 , th e c o a g u l a t i o n t e m p e r a t u r e Between showed v e r y little change w ith pH. L a m p i t t a n d B u s h i l l (1934) r e p o r t e d t h a t a n i n c r e a s e in a c i d i t y b y b a c t e r i a l a c t i o n m a t e r i a l l y i n c r e a s e d the p r o p o r t i o n of d i a l y z a b l e calciu m and inorganic phosp h o ru s. B e n t o n a n d A l b e r y (1926), u s i n g the a l c o h o l t e s t , s u g g e s t e d t h a t w h e t h e r the a l c o h o l t e s t w a s p o s i t i v e o r n e g a t i v e d i d n o t p r o v e a n y ­ th in g a s to th e h e a t s t a b i l i t y of t h a t m i l k . They ex p lain ed th at by t u r n i n g a n a l c o h o l - p o s i t i v e m i l k n e g a t i v e b y the a d d i t i o n of c i t r a t e s o r o t h e r b u f f e r s d i d n o t i n c r e a s e the s t a b i l i t y , u n l e s s th e o p t i m a l c o m b i n a t i o n of p H a n d s a l t b a l a n c e w a s a p p r o a c h e d . E f f e c t of m i l k e v a p o r a t i o n. A s t u d y of v a r i o u s p r o c e s s i n g t e c h n i q u e s a n d t h e i r s i g n i f i c a n c e i n e v a p o r a t e d m i l k w o u l d b e of 21 definite i m p o r t a n c e in the s t u d y of s a l t s a n d h e a t s t a b i l i t y in m i l k . D e y s h e r , W e b b , a n d H o l m (1929) c o n s i d e r e d th e h e a t s t a b i l i t y of a n e v a p o r a t e d m i l k to be d e p e n d e n t upo n s e v e r a l f a c t o r s , the m o s t i m p o r ­ t a n t of w h i c h w a s the t e m p e r a t u r e of f o r e w a r m i n g , w h il e h o m o g e n i z a ­ t i o n a f f e c t e d s t a b i l i t y to a l e s s e r d e g r e e . F o r e w a r m i n g of a n o r m a l m i l k , a c c o r d i n g to L e i g h t o n a n d D e y s h e r (19 23), m a d e th e n o n c o n c e n ­ tra te d m ilk le ss s t a b l e t o w a r d s h e a t , b u t , if the m i l k h a d b e e n c o n c e n ­ t r a t e d a f te r having b e e n f o r e w a r m e d , a m u c h m o r e stable p ro d u c t w ould r e s u lt. R o g e r s , D e y s h e r , a n d E v a n s (1921) u n d e r s t o o d the e f f e c t of h i g h f o r e w a r m i n g in r a i s i n g the c o a g u l a t i o n t e m p e r a t u r e of e v a p o r a t e d m i l k a s e v i d e n t l y b e i n g due to th e p r e c i p i t a t i o n of p a r t of th e c a l c i u m . If, h o w e v e r , t h e f o r e w a r m i n g t e m p e r a t u r e s w e r e m u c h b e l o w 95° C. (203° F . ) , l i t t l e e f f e c t w a s n o t e d on the c o a g u l a t i o n t e m p e r a t u r e . The r e s u l t s of W e b b a n d B e l l (1942) w e r e in a g r e e m e n t , i n t h a t the s t a ­ b i l i t y of t h e i r t e s t s a m p l e s , w h i c h w e r e f o r e w a r m e d o v e r a t e m p e r a t u r e r a n g e of 101° C. (21 3.8 ° F . ) to 165° C. (32 9.0° F . ) w a s i n c r e a s e d a s m u ch as s i x t i m e s o v e r t h a t of the c o n t r o l s a m p l e s f o r e w a r m e d a t 95° C . (203° F . ) . A s D e y s h e r , W e b b , a n d H o l m (1929) h a d p r e v i o u s l y m e n t i o n e d , h o m o g e n i z a t i o n a p p e a r e d to be of s e c o n d a r y i m p o r t a n c e in the h e a t 22 s t a b i l i t y of e v a p o r a t e d m i l k . These in v e stig ato rs hom ogenized e v a p ­ o r a t e d m i l k c o n t a i n i n g 18 p e r c e n t s o l i d s - n o t - f a t a n d 8 p e r c e n t f a t a t p r e s s u r e s up to 4,000 p o u n d s , a n d f o u n d t h a t t h i s t r e a t m e n t o n l y s l i g h t l y a f f e c t e d the h e a t s t a b i l i t y . H o w e v e r , S o m m e r a n d H a r t (1922) c o n s i d e r e d the m a i n f a c t o r in th e h e a t c o a g u l a t i o n of e v a p o r a t e d m i l k to be the s a l t c o m p o s i t i o n in th e m i l k . O t h e r f a c t o r s in o r d e r of i m p o r t a n c e a c c o r d i n g to S o m ­ m e r (192 3) w e r e a l b u m i n c o n t e n t , a c i d i t y , a n d r e n n e t - f o r m i n g o r g a n ­ ism s. This in v e stig a to r sug g ested that h eat coagulation was m o re p r e v a l e n t i n w i n t e r b e c a u s e m o s t c o w s w e r e f r e s h e n i n g o r d r y i n g off a t t h a t t i m e , a n d a s a r e s u l t , th e a l b u m i n a n d c a l c i u m c o n t e n t of the m ilk w ere higher. T h e l o w e r c i t r i c a c i d c o n t e n t of th e m i l k in the w i n t e r w a s a t t r i b u t e d to t h e l a c k of g r e e n f e e d . S o m m e r and H a rt (1926) p o s t u l a t e d t h a t t h e h i g h f o r e w a r m i n g t r e a t m e n t n o t o n l y p r e ­ c i p i t a t e d a l a r g e r a m o u n t of c a l c i u m s a l t s , b u t a l s o c o a g u l a t e d the a l b u m i n to a g r e a t e r e x t e n t . W e b b (1928) su g g e ste d that f o r e v e r y m ilk th e re was an o p ti­ m u m p H , t h e a t t a i n m e n t of w h i c h w o u l d r e s u l t i n m a x i m u m h e a t s ta b ility f o r the s y s t e m in volved. R o g e r s , D e y s h e r , a n d E v a n s (1921) p o i n t e d o u t , h o w e v e r , t h a t t h e r e w a s no v e r y d e f i n i t e r e l a t i o n b e t w e e n the c o a g u l a t i o n of e v a p o r a t e d m i l k a n d the t r u e a c i d i t y , a l t h o u g h if a 23 s m a l l i n c r e a s e i n a c i d i t y o c c u r r e d , a d i s t i n c t l o w e r i n g of t h e c o a g u ­ l a t i n g t e m p e r a t u r e of t h e e v a p o r a t e d m i l k w a s n o t e d . A c c o r d i n g to S o m m e r a n d H a r t ( 1 9 ^ 6 ) , th e e f f e c t of a s l i g h t i n c r e a s e in a c i d i t y on the c o a g u l a t i o n of e v a p o r a t e d m i l k w a s n o t due to the h y d r o g e n - i o n c o n c e n t r a t i o n a s a d i r e c t f a c t o r , b u t r a t h e r to the i n d i r e c t e f f e c t of the c o n v e r s i o n of t h e s e c o n d a r y to p r i m a r y p h o s p h a t e . T a k i n g a l l f a c t o r s in to c o n s i d e r a t i o n , W ebb a n d B e l l (1942) i n d i c a t e d t h a t the u s e of o p t i m u m h i g h - f o r e w a r m i n g t e m p e r a t u r e s b r o u g h t a b o u t , in the m i l k t e s t e d , a g r e a t e r i n c r e a s e in h e a t s t a b i l i t y i n the e v a p o r a t e d p r o d u c t t h a n c o u l d b e a t t a i n e d b y the a d d i t i o n of an o p t i m u m q u a n t i t y of s t a b i l i z i n g s a l t s to a n o r m a l l y f o r e w a r m e d m i l k . F e athe rin g C haracteristics. D o a n (1931b) r e p o r t e d t h a t f e a t h e r i n g w a s c h a r a c t e r i z e d b y the a p p e a r a n c e of a f l o c c u l e n t , f e a t h e r - l i k e p r e c i p i ­ t a t i o n u p o n th e a d m i x t u r e of u n s t a b l e c r e a m w i t h h o t c o f f e e . This p h e n o m e n o n ' o f f e a t h e r i n g of h o m o g e n i z e d t a b l e c r e a m a n d th e c o a g u ­ l a t i o n of s t e r i l i z e d c r e a m w e r e b o t h r e g a r d e d a s a h e a t c o a g u l a t i o n of the c a s e i n s i m i l a r to t h a t e n c o u n t e r e d i n t h e p r o c e s s i n g of e v a p ­ o ra te d m ilk. to t h e S o m m e r (1952) s u g g e s t e d t h a t the s c u m w h i c h f l o a t e d s u r f a c e of t h e c o f f e e , w h e n a d d e d c r e a m f e a t h e r e d , c o n s i s t e d 24 of ' ' c o a g u l a t e d p r o t e i n s w i t h e n o u g h o c c l u d e d f a t to f l o a t th e p a r ­ ticles r E f f e c t of q u a l i t y of i n g r e d i e n t s . F r o m the r e s u l t s of t h e i r s t u d i e s , A t h e r t o n a n d B r a d f i e l d (1952) t h o u g h t t h a t f e a t h e r i n g of c r e a m in c o f f e e a p p e a r e d a s a r e s u l t of a n i n t e r p l a y of s e v e r a l f a c t o r s , r a t h e r t h a n b y a n y one s i n g l e e f f e c t . W e b b a n d H o l m (1928) p a s ­ t e u r i z e d p u r e , f r e s h c r e a m , t e s t i n g 20 p e r c e n t f a t a n d 0 ,1 5 p e r c e n t acidity, f r o m 65° to 85° C . , a n d h o m o g e n i z e d a t p r e s s u r e s up to 3,000 p o u n d s w i t h th e g e n e r a l r e s u l t t h a t n o f e a t h e r i n g o c c u r r e d . The o p in ­ io n of t h e s e i n v e s t i g a t o r s w a s t h a t the b a s i c c r i t e r i o n a s to w h e t h e r o r not c r e a m would f e a t h e r w as quality. E f f e c t of p a s t e u r i z a t i o n a n d h o m o g e n i z a t i o n . D a h l b e r g (1944) c o n s i d e r e d h o m o g e n i z a t i o n of 1 8 - p e r c e n t c r e a m a d v a n t a g e o u s i n t h a t it i n c r e a s e d v i s c o s i t y , r i c h n e s s of t a s t e , a n d c o l o r i n g in c o f f e e . In addition, c r e a m plugs and sk im m ilk la y e r s w ere elim in a te d , and fat l o s s e s b y a d h e r e n c e to the b o t t l e w e r e r e d u c e d . The o n l y o b j e c t i o n v o i c e d w a s i n c r e a s e d f e a t h e r i n g in c o f f e e , a f a u l t w h i c h h e f e l t c o u l d be lim in a te d by h i g h - t e m p e r a t u r e p a s te u r iz a tio n , l o w - p r e s s u r e h o m o g ­ e n i z a t i o n , a n d the u s e of v e r y f r e s h c r e a m of f i n e q u a l i t y . 25 B u r g w a l d (19 23), h o w e v e r , h a d p r e v i o u s l y p o i n t e d out t h a t p a s t e u r i z a t i o n only h a d bu t little e f fe c t on f e a t h e r i n g , although t h e r e w a s a t e n d e n c y f o r p a s t e u r i z e d c r e a m to f e a t h e r a t a s l i g h t l y l o w e r a c id i ty th an the s a m e c r e a m n o t p a s t e u r i z e d . M a x i m u m s t a b i l i t y w a s o b t a i n e d w h e n W ebb a n d H o l m (1928) p r e h e a t e d c r e a m a t 80° C . f b u t w h e n the s a m p l e w a s p a s t e u r i z e d a t 70° to 74° C . f o r 30 m i n u t e s , p r e v e n t i o n of f e a t h e r i n g w a s o n ly p a r ­ tially successful. T r a c y a n d R u e h e ' s (1930) s u g g e s t i o n w a s t h a t c r e a m s h o u l d b e p a s t e u r i z e d a t 155° F . f o r 30 m i n u t e s , h e a t e d to 165° F . , a n d t h e n c o o l e d to 145° F . f o r h o m o g e n i z a t i o n . D o a n (1929a) c o n c l u d e d f r o m h i s w o r k t h a t a s a g e n e r a l r u l e th e t e n d e n c y f o r h o m o g e n i z e d c r e a m to f e a t h e r p a r a l l e l e d the d e g r e e of f a t c l u m p i n g , b u t in a n i n v e r s e f a s h i o n . H e n d e r s o n (1944) r e p o r t e d t h a t h o m o g e n i z e d c r e a m w o u l d f e a t h e r q u i c k e r t h a n n o n h o m o g e n i z e d c r e a m b e c a u s e of the d e c r e a s e d p ro te in stability. W ith 2 5 - p e r c e n t c r e a m , H e n d e rs o n su ggested an optim um hom ogenization p r e s s u r e B r a d f i e l d (1952) ferred of 1,000 p o u n d s . s u g g e s t e d 600 p o u n d s p r e s s u r e . A th e rto n and D o a n (1931b) p r e ­ 1,000 p o u n d s if a s i n g l e - s t a g e h o m o g e n i z e r w e r e u s e d ; b u t w i t h a t w o - s t a g e m a c h i n e , h e r e c o m m e n d e d t h a t the p r e s s u r e on the s e c o n d s t a g e s h o u l d n o t b e g r e a t e r t h a n t h a t p u t on th e f i r s t v a l v e . 26 F o r a s i n g l e - s t a g e h o m o g e n i z e r , T r a c y a n d R u e h e (1930) r e c o m m e n d e d a pressure of 500 to 1,000 p o u n d s . they s u g g e s te d a total p r e s s u r e If, h o w e v e r , two s t a g e s w e r e u s e d , of 1,500 to 2,500 p o u n d s , w i t h 500 to 1,000 p o u n d s b e i n g p l a c e d on t h e s e c o n d v a l v e . T r a c y a n d R u e h e (1928) t h o u g h t i t p o s s i b l e t h a t th e e f f e c t of h o m o g e n i z a t i o n w o u l d b e a n i n c r e a s e in t h e a m o u n t of f a t - a d s o r b i n g s u r f a c e s w i t h t h e r e s u l t t h a t an i n c r e a s e d a m o u n t of p r o t e i n w o u l d b e a d s o r b e d o n the f a t - g l o b u l e m e m b r a n e , w h i c h , a c c o r d i n g to T i t u s , S o m m e r , a n d H a r t (1 928 ), c o n s i s t e d m o s t l y of c a s e i n . T r a c y and R u e h e (1928) a l s o c o n s i d e r e d i t p r o b a b l e t h a t s o m e of the p h o s p h a t e s and c i tr a te s w e re r e m o v e d f r o m th e c o l l o i d a l s o l u t i o n a n d h a d b e c o m e f i x e d to the f a t g l o b u l e , w h i c h r e s u l t e d in a c a l c i u m e x c e s s in the rem aining se ru m . T r a c y a n d R u e h e (1930b) f u r t h e r t h e o r i z e d t h a t t h i s l o c a l i z i n g of the c a s e i n on th e f a t g l o b u l e m a d e th e c a s e i n m o r e s u s c e p t i b l e to h e a t c o ag u latio n , and they thought it p o s s ib le th at th ro u g h m o le c u la r o r i e n t a t i o n of t h e c a s e i n m o l e c u l e in the a d s o r b e d f i l m , the s t a b i l i t y of the p r o t e i n i n the p r e s e n c e These postulations w ere of s o l u b l e c a l c i u m w a s l e s s e n e d . s u b s t a n t i a t e d b y D o a n (19 38), who b e l i e v e d t h a t the l o s s of m o b i l i t y of t h e c a s e i n w a s one of the f i r s t s t a g e s of c o a g u l a t i o n . 27 K i n g (1953) n o t e d t h a t th e s t a b i l i t y of the p r o t e i n p h a s e d e ­ c r e a s e d w i t h i n c r e a s e d p r e s s u r e s of h o m o g e n i z a t i o n a n d f a t c o n t e n t with n o n s te r iliz e d c r e a m . The sa m e w as tru e with s t e r i l i z e d h o m o g ­ e n iz e d c r e a m , although th e se e ffe c ts w e r e not shown as c l e a r l y . E f f e c t of c o f f e e . A n o t h e r i m p o r t a n t f a c t o r in the f e a t h e r i n g of c r e a m i n c o f f e e i s t h a t of the c o ff e e i t s e l f . R u e h e (1948b) p o i n t e d o u t t h a t s o m e b r a n d s of c o f f e e w e r e m o r e l i k e l y to c a u s e f e a t h e r i n g t h a n o t h e r s t a s s o m e c o n t a i n e d a h i g h p e r c e n t a g e of s o l u b l e a c i d s . In a s t u d y of c u r i n g b e a n s a n d t h e i r e f f e c t s , T r a c y a n d C o r b e t t (1938) f o u n d t h a t t h e m e t h o d of c u r i n g the b e a n s h a d no r e l a t i o n to the pH of the b r e w on f e a t h e r i n g . H o w e v e r , a c c o r d i n g to t h e s e i n v e s t i g a t o r s , the d e g r e e of r o a s t to w h i c h th e b e a n w a s s u b j e c t e d d i d a f f e c t f e a t h ­ e r i n g ; a s m o r e r o a s t i n g o c c u r r e d , the h i g h e r w o u l d b e the pH of the b r e w , a n d , c o n s e q u e n t l y , l e s s e r th e d e g r e e of the f e a t h e r i n g . same In the s t u d y , i t w a s n o t e d t h a t t h e c o n c e n t r a t i o n in th e b r e w h a d no d e f i n i t e r e l a t i o n to f e a t h e r i n g . A f u r t h e r point was no te d by Ruehe (1948b) i n t h a t the b r e w m a d e f r o m a g e d c o f f e e h a d a h i g h e r a c i d i t y . S l i g h t l y m o r e f e a t h e r i n g w a s n o t e d b y T r a c y a n d C o r b e t t (1938) w i t h b r e w m a d e f r o m the d r i p o l a t o r m e t h o d t h a n f r o m t h a t of the p e r ­ c o l a t o r o r th e b o i l i n g m e t h o d . T he se in v e s t ig a to r s a lso p o inted out t h a t m o r e f e a t h e r i n g c o u l d r e s u l t w h e n the c o f f e e w a s a d d e d to the 28 c r e a m , r a t h e r t h a n w h e n th e c r e a m w a s a d d e d to th e c o f f e e . The s l o w , r a t h e r t h a n f a s t a d d i t i o n s of s m a l l q u a n t i t i e s of m i l k to the c o f f e e , a c c o r d i n g to W h i t a k e r (1931), e n h a n c e d the f e a t h e r i n g of e v a p ­ o r a t e d m i l k in h o t c o f f e e . I n i s o l a t e d c a s e s , D o a n (1931b) w a s a b l e to a t t r i b u t e f e a t h e r i n g to th e h a r d w a t e r u s e d in m a k i n g the c o f f e e . S o m m e r (1929b) e x ­ p l a i n e d t h a t if the c r e a m w a s of g o o d q u a l i t y , p r o p e r l y p a s t e u r i z e d , h o m o g e n iz e d a t a low p r e s s u r e i n g , the c a u s e w a s and yet showed trouble with f e a t h e r ­ a l m o s t c e r t a i n to be in the h a r d n e s s of the w a t e r a n d t h e s a l t c o n t e n t of the c r e a m . A n o t h e r m e t h o d f o r the p r e v e n t i o n of f e a t h e r i n g of c r e a m in c o f f e e w o u l d b e the a d d i t i o n of c e r t a i n s a l t s . D o a n (1931b) a n d M u l d e r (1949) s u g g e s t e d t h a t if o t h e r p r o c e d u r e s f a i l e d to h e l p , s a l t stabilizers such as p h osphate would be sodium c itra te , sodium b ic arb o n ate , or disodium b e n e f i c i a l , w i t h th e f o r m e r i n v e s t i g a t o r t h e r a t e of 0 . 0 2 5 to 0.1 p e r c e n t . T r a c y a n d R u e h e (1930a) suggesting recom ­ m e n d e d u s i n g t h e s e s a l t s a t the r a t e of 8 to 20 p o u n c e s p e r 1,000 p o u n d s of c r e a m . This in v e stig a to r added that these sodium salts should be added b e fo re any p r o c e s s in g o c c u r r e d . To p r e v e n t d r y m i l k f r o m f e a t h e r i n g i n c o f f e e , K e m p f a n d B l a n c h a r d (195 3) a d d e d d i s o d i u m p h o s p h a t e to th e m i l k j u s t p r i o r to condensing. 29 C o l o r i n g A b i l i t y of C r e a m i n C o ff e e T h e c o l o r i n g a b i l i t y of th e c r e a m i n c o f f e e , a c c o r d i n g to M a c k (193 9), d e p e n d e d u p o n (1) s o l i d s - n o t - f a t , a n d (2) s i z e of the f a t g l o b ­ ules. To i n c r e a s e t h i s c o l o r i n g p o w e r , he s u g g e s t e d t h a t th e s e r u m s o l i d s of th e c r e a m s h o u l d be b u i l t up a n d f a t g l o b u l e s s u b d i v i d e d by h o m o g e n i z a t i o n , w h i c h H e n d e r s o n (1944) e x p l a i n e d i n c r e a s e d the s u r ­ f a c e of t h e f a t , h e n c e m o r e l i g h t w a s r e f l e c t e d . N e i t h e r a g i n g the c r e a m n o r th e d e v e l o p m e n t of a c i d i t y , a c ­ c o r d i n g to W h i t a k e r (19 34), h a d a n y i n f l u e n c e on ftie q u a n t i t y of c r e a m r e q u i r e d to c o l o r c o f f e e . In a d d i t i o n , he p o i n t e d o u t t h a t the p a s t e u r ­ i z a t i o n of c r e a m b y t h e u s u a l h o l d i n g m e t h o d a l s o h a d no i n f l u e n c e o n th e c o l o r of the c o f f e e - c r e a m m i x t u r e . H a lf - a n d - H a lf H o m o g e n iz e d Milk A s this s t u d y i s p r i m a r i l y c o n c e r n e d w i t h th e p r e p a r a t i o n a n d t e s t i n g of d i f f e r e n t t y p e s of h a l f - a n d - h a l f h o m o g e n i z e d m i l k , a r e v i e w of t h e p r o c e s s i n g t e c h n i q u e s a s u s e d b y o t h e r i n v e s t i g a t o r s w o u l d , at t h i s p o i n t , be a d v a n t a g e o u s . P reparation. cream R u e h e (19 41, 1948a) should be s ta n d a r d iz e d f r o m r e c o m m e n d e d t h a t m i l k a nd 10 to 10.5 p e r c e n t f a t . In a d d i t i o n , 30 h e s u g g e s t e d t h a t the s o l i d s - n o t - f a t c o n t e n t s h o u l d b e i n c r e a s e d b y about 2 p e r c e n t by adding e i t h e r h ig h -q u a lity s k im m ilk p o w d e r or c o n cen trated skim m ilk. R o a d h o u s e a n d H e n d e r s o n (1941) p r o p o s e d t h a t h a l f - a n d - h a l f s h o u l d b e p a s t e u r i z e d a n d h o m o g e n i z e d a t 145° F . R u e h e (1941), h o w e v e r , h a d r e c o m m e n d e d c o o l i n g to 115° to 125° F . f o r h o m o g e n ­ i z i n g , b u t l a t e r (1948a) r a i s e d t h i s t e m p e r a t u r e to 145° to 150° F . f o r 30 m i n u t e s . This p r o d u c t , h a lf - a n d - h a l f , should be h o m o g e n iz e d at a p r e s ­ s u r e u n d e r 2 ,50 0 p o u n d s , a c c o r d i n g to R u e h e (19 41, 1 9 48 a ). However, R o a d h o u s e a n d H e n d e r s o n (1941) f e l t t h a t p r e s s u r e s of 1,000 to 1,500 p o u n d s w ould be sufficient. I n th e s t u d y of T r o u t a n d W e i n s t e i n (1952), both sin g le and double s ta g e s w e r e u tiliz e d . single stage, a p r e s s u r e W h e n u s i n g o n ly the of 2,500 p o u n d s w a s u s e d ; a n d f o r d u a l h o m o g e n i z a t i o n , 500 p o u n d s p r e s s u r e w a s p l a c e d on t h e s e c o n d v a l v e . T h e y s u g g e s t e d t h a t two s t a g e s s h o u l d be u s e d i n o r d e r t h a t a p r o d u c t of u n i f o r m v i s c o s i t y m i g h t b e o b t a i n e d . T h e o r i e s R e g a rd in g F a t C lum ping in M ilk and C r e a m Introduction. S o m m e r (1952) r e p o r t e d t h a t w h e n m i l k l e f t the c o w ' s u d d e r , t h e f a t g l o b u l e s w e r e in s u s p e n s i o n a s separate, 31 i n d i v i d u a l g l o b u l e s , b u t on s t a n d i n g f o r a s h o r t t i m e t h e s e g l o b u l e s g a t h e r e d in t o c l u s t e r s , w h i l e still having re ta in e d t h e i r identity. This c l u s t e r i n g of f a t g l o b u l e s w a s n o t i c e d b y B a b c o c k (1889), a n d W ol l, B a b c o c k , a n d R u s s e l l (190 3), w i t h th e l a t t e r a t t r i b u t i n g t h e h o l d i n g t o g e t h e r of t h e c l u s t e r s to s o m e i n v i s i b l e m a t e r i a l . R u s s e l l (1896) B abcock and s u g g e s t e d t h a t t h e s e a g g r e g a t i o n s of f a t gl o b u le s h a d a v e r y i m p o r t a n t i n f l u e n c e i n d e t e r m i n i n g the c o n s i s t e n c y of m i l k a n d , especially, cream . I n c r e a s e d v i s c o s i t y i n c r e a m s w h e n a g e d w e r e a t t r i b u t e d to c l u m p i n g of f a t g l o b u l e s b y S h e r w o o d a n d S m a l l f i e l d (1926) a n d D a h l b e r g a n d H e n i n g (1 925) . This v is c o s ity , h o w e v e r, w as shown by N a ir a n d M o o k (1933) to h a v e b e e n r e d u c e d b y a g i t a t i o n . P a l m e r (1923) n o t e d t h a t th e r i s e of f a t g l o b u l e s w a s du e p r i ­ m a r i l y to the d i f f e r e n c e b e t w e e n t h e s p e c i f i c g r a v i t y of m i l k f a t a n d th e p l a s m a i n w h i c h it w a s d i s p e r s e d . H ow ever, it w as in d icated by P a l m e r , H e n i n g , a n d A n d e r s o n (192 6) t h a t the d i f f e r e n c e s in d e n s i t i e s of f a t a n d p l a s m a d i d n o t d e t e r m i n e the v o l u m e of c r e a m n o r the r i s e of f a t , b u t w a s d e t e r m i n e d l a r g e l y by the p l a s m a c o l l o i d s . P alm er a n d A n d e r s o n (1926) a n d v a n D a m a n d S i r k s (1922) e s s e n t i a l l y a g r e e d t h a t t h e p r o p e r t i e s w h i c h i n f l u e n c e d the s e p a r a t i o n a n d v o l u m e of the c r e a m w e r e d e t e r m i n e d l a r g e l y b y th e p l a s m a , a n d n o t b y the f a t . 32 D o a n (1926) a d d e d s o l i d s - n o t - f a t i n th e f o r m of p l a i n c o n d e n s e d s k i m ­ m i l k a n d s k i m m i l k p o w d e r , b u t f o u n d t h e y g a v e no i n c r e a s e d c r e a m i n g p o w e r e i t h e r to r a w o r to p a s t e u r i z e d m i l k . Th e r e s u l t s of S k e l t o n a n d E l l e n b e r g e r (1939) t e n d e d to m i n i m i z e the i m p o r t a n c e h i t h e r t o p la c e d upon fa t-g lo b u le clum ping as a m a j o r f a c to r influencing c r e a m viscosity. T r o y a n d S h a r p (1928) c a l c u l a t e d the r i s e of f a t g l o b u l e s a n d r e p o r t e d th a t th e y w e re in a c c o rd a n c e with S toke's Law. F ro m their r e s u l t s , t h e y c o n c l u d e d t h a t th e i n d i v i d u a l f a t g l o b u l e s r o s e so s l o w l y t h a t i t w o u l d h a v e r e q u i r e d m a n y t i m e s the n o r m a l c r e a m i n g t i m e of m i l k f o r t h e m to r e a c h the c r e a m l a y e r . c l u s t e r s did r i s e H o w ev e r, they showed that r a p i d l y e n o u g h to a c c o u n t f o r the n o r m a l c r e a m i n g tim e. H a m m e r (1916) h a d p o s t u l a t e d t h a t the s i z e of the f a t g l o b u l e s , the p e r c e n t a g e of th e t o t a l a m o u n t of f a t p r e s e n t i n g l o b u l e s of v a r y ­ in g s i z e , a n d the c o n s t i t u t i o n of th e f a t - g l o b u l e c l u s t e r s m u s t a l l be of i m p o r t a n c e i n t h e c r e a m i n g a b i l i t y of m i l k . No s i m p l e relation b e t w e e n th e v i s c o s i t y of m i l k a n d the r a t e of f o r m a t i o n of c r e a m w a s f o u n d b y R a h n (1921). Agglutinin th e o ry . B a b c o c k (1889) a t t r i b u t e d the c l u s t e r i n g of f a t g l o b u l e s to Ma p r i n c i p a l in m i l k w h e n i s a n a l a g o u s o r i d e n t i c a l 33 w ith blo o d f i b r i n , ” which, th rough spontaneous coagulation, clotted and thus e n ta n g le d the fa t globules, w h ic h con se q u e n tly p r e v e n te d efficient c re am in g . H o w e v e r , H e k m a (1922) p o i n t e d o u t t h a t f i b r i n w a s n o t a n o r m a l c o n s t i t u e n t of m i l k . M u l d e r (1949) n o t e d t h a t c r e a m i n g w a s s t i m u l a t e d b y the a g g l u t i n a t i o n of th e f a t g l o b u l e s , w h i c h c o m b i n e d in to c l u s t e r s . Sharp a n d K r u k o v s k y (1939) h a d p r e v i o u s l y c o n c l u d e d t h a t t h i s a g g l u t i n a t i n g s u b s t a n c e w a s a d s o r b e d on t h e s u r f a c e of th e s o l i d o r s o l i d i f y i n g f a t g l o b u l e s , a n d n o t on the s u r f a c e of th e l i q u i d f a t g l o b u l e s . evidence was F urther s u p p l i e d b y D u n k l e y a n d S o m m e r (1944), who p o i n t e d o u t th e s i m i l a r i t y of th e m e c h a n i s m s i n v o l v e d i n the a g g l u t i n a t i o n of b a c t e r i a a n d t h e c l u s t e r i n g of f a t g l o b u l e s i n m i l k . R o w l a n d (1933) r e p o r t e d t h a t th e h e a t i n g of n o r m a l m i l k c a u s e d a d e n a t u r a t i o n of t h e a l b u m i n a n d g l o b u l i n , b u t n o t t h e i r c o a g u l a t i o n . L a t e r , R o w l a n d (1937) w a s a b l e to p o s t u l a t e t h a t g l o b u l i n p r o b a b l y w a s the m a j o r f a c t o r i n t h e r e d u c t i o n of t h e c r e a m i n g p o w e r of h e a t e d m ilk. T h e a m o u n t of a l b u m i n a n d g l o b u l i n c o a g u l a t e d a t a p a s t e u r i z a ­ t i o n t e m p e r a t u r e of 143° F . f o r 30 m i n u t e s w a s c a l c u l a t e d b y S h a h a n i a n d S o m m e r (1951) to b e 9 a n d 5 p e r c e n t , r e s p e c t i v e l y . B r o u w e r (1 924) , m e a n w h i l e , h a d f r a c t i o n a t e d g l o b u l i n into e u g lo b u lin an d p s e u d o g lo b u lin , and d e m o n s t r a t e d th a t the euglobulin 34 w a s v e r y e f f e c t i v e i n i m p r o v i n g the c r e a m i n g p r o p e r t i e s of m i l k , while p s e u d o globulin had v e r y little effect. Stickines s . W h e n c r e a m w a s c o o l e d , K in g (1950) e x p l a i n e d t h a t the f a t g l o b u l e s b e c a m e c l u m p e d a s a c e r t a i n a m o u n t of the liq u id f a t w a s s q u e e z e d out f r o m the globules and s e r v e d as a d h esiv e m a t e r i a l b e t w e e n th e g l o b u l e s . K in g (1951) a l s o s u g g e s t e d t h a t a t v e r y l o w t e m p e r a t u r e s the f a t g l o b u l e s f a i l e d to c l u m p s i n c e the a m o u n t of t h e l i q u i d f a t f r a c t i o n i n the g l o b u l e s w a s too s m a l l f o r clum p fo rm a tio n . H o w e v e r , B r u n n e r a n d J a c k (1950) p o i n t e d o u t t h a t e v e n a t t e m p e r a t u r e s of 0° to 10° C. f o r 18 h o u r s , the d e g r e e of s o lid ific a tio n in c r e a m r a n g e d f r o m 48 to 68 p e r c e n t s o l i d i f i e d f a t . T h e o r y of e l e c t r o k i n e t i c s . N o r t h a n d S o m m e r (1935) t h e o r i z e d t h a t t h e p o t e n t i a l a t t h e f a t - l i q u i d i n t e r f a c e w a s the n e t r e s u l t of r e ­ v e r s i b l e a n d i r r e v e r s i b l e i o n s w i t h the f i l m of i r r e v e r s i b l e adsorbed m i l k c o l l o i d s p l a y i n g t h e d o m i n a n t r o l e i n d e t e r m i n i n g the p o t e n t i a l . P rev io u sly , Sirks (19 23), who h a d a s s u m e d t h a t the e l e c t r o p h o r e t i c m o b i l i t y of th e f a t g l o b u l e s w a s p r o p o r t i o n a l to t h e i r s u r f a c e p o t e n ­ t i a l a n d c h a r g e , f o u n d t h a t t h i s m o b i l i t y s h o w e d no r e l a t i o n s h i p to th e e x t e n t of c l u s t e r i n g of the f a t . H e n c e , t h i s i n v e s t i g a t o r w a s a b le to c o n c l u d e t h a t the c h a r g e on the f a t g l o b u l e w a s n o t i m p o r t a n t in 35 d e t e r m i n i n g th e e x t e n t to w h i c h f a t g l o b u l e s c l u s t e r e d . Somewhat d i f f e r e n t r e s u l t s w e r e o b t a i n e d b y N o r t h , C o u r t n e y , a n d S o m m e r (1935), who o b s e r v e d a g e n e r a l r e l a t i o n s h i p b e t w e e n the c h a r g e , a n d b o t h c l u s t e r f o r m a tio n and c r e a m volum e. J a c k a n d D a h l e (1937a) w e r e u n a b l e to f i n d a n y s i g n i f i c a n t d i f f e r e n c e i n th e m o b i l i t y of f a t g l o b u l e s f r o m c o w s of d i f f e r e n t breeds. T h e e l e c t r o p h o r e t i c m o b i l i t i e s of f a t g lo bu le s w e r e s h o w n to b e c o n s t a n t up to a f a t c o n t e n t of 65 p e r c e n t b y t h e s e same in v e s­ t i g a t o r s (1 9 3 7 b ) , b u t a b o v e t h i s , t h e r e w a s a r a p i d i n c r e a s e . By in ­ c r e a s i n g th e t e m p e r a t u r e a b o v e the p a s t e u r i z a t i o n p o i n t , D a h le a n d J a c k (1937) fa t globules. s t a t e d t h a t t h e r e w a s a n i n c r e a s e in the m o b i l i t y of the This phen o m en o n had b e en p re v io u sly explained by N o r t h , C o u r t n e y , a n d S o m m e r (1935) to b e a n i n c r e a s e i n the c h a r g e on the f a t g lo b u le s . E le c tro static th eo ry. The m o s t p l a u s i b l e r e a s o n f o r the c r e a m i n g of r a w a n d p a s t e u r i z e d m i l k , a c c o r d i n g to D a h l b e r g a n d M a r q u a r d t (1 92 9, 1931), w a s the a c t i o n of the c a l c i u m i o n s on the c l u s t e r i n g of f a t g l o b u l e s . These investigators p o s itiv e ly c h a r g e d c a lc iu m ions w e re s u g g e s t e d t h a t the a t t r a c t e d b y the n e g a t i v e c h a r g e of the f a t g l o b u l e s i n s u f f i c i e n t a m o u n t s so a s to c a u s e an e l e c t r i c a t t r a c t i o n of on e f a t g l o b u l e f o r a n o t h e r , h e n c e the f o r m a t i o n of 36 clusters. D o a n (1932) t h o u g h t t h i s t h e o r y to be l o g i c a l w i t h th e e x ­ c e p t i o n t h a t i t d i d n o t e x p l a i n t h e e f f e c t s of h o m o g e n i z a t i o n . In te rfacial te n sio n . F a t c l u m p i n g s e e m e d to b e b e s t e x p l a i n e d a c c o r d i n g to D o a n (1929a) on t h e b a s i s of a n i n t e r f a c i a l t e n s i o n e f f e c t . He t h e o r i z e d t h a t th e f a t a n d w a t e r i n t e r f a c e g a v e r i s e to r e l a t i v e l y high i n t e r f a c i a l t e n s i o n s , and sin ce th is f r e e e n e r g y could be d im in ­ i s h e d b y a d e c r e a s e in s u r f a c e a r e a , c o a l e s c e n c e o c c u r r e d . However, he c o n t i n u e d , c o a l e s c e n c e w o u l d n o t t a k e p l a c e p r o b a b l y b e c a u s e a n a d s o r b e d l a y e r p r e v e n t e d a n i n t i m a t e c o n t a c t of the g l o b u l e s . Still t h e i n v e s t i g a t o r r e a l i z e d t h a t the c o m p l e t e e x p l a n a t i o n of s u c h a c o m p l e x p h e n o m e n o n w o u l d p r o b a b l y b e b a s e d on m o r e t h a n one e f ­ fect. N o r t h , C o u r t n e y , a n d S o m m e r (1935) a n d D u n k l e y a n d S o m m e r (1944) d i s a g r e e d w i t h t h i s h y p o t h e s i s , a s t h e y w e r e u n a b l e to f i n d any r e la t io n s h ip b e tw e e n in t e r f a c i a l te n s io n a t the f a t - s e r u m in te r fa c e a n d th e e x t e n t of f a t - g l o b u l e c l u s t e r i n g . The l a t t e r i n v e s t i g a t o r s did s ta te , h o w e v e r , th a t th is f r e e e n e r g y , while n o t b ein g a c a u s e in c l u s t e r i n g , c o u l d p l a y a p a r t t h r o u g h i t s i n f l u e n c e on the a d s o r p t i o n b y the f a t . T h e p r e s e n t - d a y k n o w l e d g e of the r e l a t i o n s h i p b e t w e e n f a t g l o b u l e c l u s t e r i n g a n d v i s c o s i t y in c r e a m w a s u t i l i z e d b y H e n i n g a n d D a h l b e r g (1932); W i e s e , N a i r , a n d F l e m i n g (1939); S m i t h a n d D e a n (1948); a n d S n y d e r (19 48), a s a m e t h o d of i n c r e a s i n g c r e a m v i s c o s i t y through various s e p a ra tin g and heating techniques. SECTION I T H E I N F L U E N C E O F V A R I A T I O N IN C O M P O S I T I O N AND P R E S S U R E S O F H O M O G E N I Z A T I O N U P O N SOME C H E M I C A L AND P H Y S I C A L P R O P E R T I E S O F H A L F - A N D - H A L F H OM OGENIZED MILK 38 39 E xperim ental P rocedure F a c t o r s a f f e c t i n g t h e p h y s i c a l , c h e m i c a l, a n d o r g a n o l e p t ic p r o p e r t i e s of h a l f - a n d - h a l f . F r e s h r a w m i l k w a s o b t a i n e d f r o m the M ic h ig a n State College C r e a m e r y . and s e p a ra te d . Th e m i l k w a s w a r m e d to 90° F . , T h e r e s u l t a n t s k i m m i l k w a s p l a c e d i m m e d i a t e l y into th e m i l k - s t o r a g e room , p la c e d in cold, running held at 37° F . w a t e r (5 3°F . ) . M e a n w h i l e the c r e a m w a s A p e r i o d of a p p r o x i m a t e l y 30 m i n u t e s w a s r e q u i r e d f r o m the t i m e t h e s e p a r a t i o n w a s c o m p l e t e d u n t i l th e b e g i n n i n g of the p a s t e u r i z i n g p r o c e s s . Three separate s e r i e s w e r e c o n d u c t e d w i t h e a c h v a r y i n g i n c o m p o s i t i o n a s f o ll o w s : Series 1. S k im m ilk and c r e a m s t a n d a r d i z e d to 10.5 p e r c e n t fat. S e r i e s 2. S k i m m i l k a n d c r e a m s t a n d a r d i z e d to 10.5 p e r c e n t f a t p l u s the a d d i t i o n of 2 p e r c e n t l o w - h e a t , s p r a y - p r o c e s s skim m ilk powde r . Series 3. S k im m ilk and c r e a m s t a n d a r d i z e d to 10.5 p e r c e n t f a t p l u s the a d d i t i o n of 5 p e r c e n t l o w - h e a t , s p r a y - p r o c e s s skim m ilk powde r . E a c h s e rie s was h e a te d in a 30-gallon, stainle s s - s t e e l univat a n d p a s t e u r i z e d a t 150° F . f o r 30 m inutes. The p r o d u c t w a s t h e n 40 h o m o g e n i z e d a t 150° F . i n a M a n t o n - G a u l i n (M odel E) t w o - s t a g e h o m o g e n i z e r w i t h a c a p a c i t y r a t e d a t 75 g a l l o n s p e r h o u r . The h o m o g e n i z a t i o n p r e s s u r e s u s e d w i t h th e 2,5 00 a n d 5 , 0 0 0 p o u n d s p e r s q u a r e i n c h . w ere single stage w e re H o w e v e r , w h e n th e s a m p l e s s u b j e c t e d to d u a l h o m o g e n i z a t i o n , in a d d i t i o n to the a b o v e p r e s ­ s u r e s on the f i r s t s t a g e , 500 p o u n d s p e r s q u a r e i n c h p r e s s u r e w a s p l a c e d on the s e c o n d v a lv e. This gave h o m ogenization p r e s s u r e s of 0, 2 , 5 0 0 , 5 , 0 0 0 , 2,500 + 500, a n d 5,000 + 500 p o u n d s p e r s q u a r e i n c h for each s e rie s . T h e s a m p l e s w e r e c o o l e d to 60° F . w i t h r u n n i n g w a t e r , b o t t l e d b y h a n d in to h a l f - p i n t b o t t l e s , a n d p l a c e d i n c o l d s t o r a g e a t a t e m ­ p e r a t u r e of 37° to 38° F . In o r d e r to d e t e r m i n e the v i s c o s i t y of the v a r i o u s B rookfield S y n c h ro -L e c tric v is c o m e te r was em ployed. series, a The v i s c o s i t i e s w e r e m e a s u r e d a t t e m p e r a t u r e s of 20° C. (68° F . ) a n d 3° C. (37.4° F . ) , w i t h the r e s u l t s b e i n g e x p r e s s e d in c e n t i p o i s e s . W h e n th e r e a d ­ in g s w e r e t a k e n a t 20° G., t h e h a l f - p i n t s a m p l e s w e r e rem oved from c o l d s t o r a g e a n d p l a c e d in a w a t e r b a t h , h e l d a t 20° ± 1 ° m i n u t e s p r i o r to the d e t e r m i n a t i o n of the v i s c o s i t y . C. f o r 30 The s a m p l e s e x a m i n e d a t 3° C . w e r e t a k e n d i r e c t l y f r o m c o l d s t o r a g e a n d r e a d ­ ings w e re m ad e im m e d ia te ly . All s a m p l e s w e r e h e l d in c o l d s t o r a g e 41 f o r a p e r i o d of s i x h o u r s p r i o r to die i n i t i a l d e t e r m i n a t i o n s . O ther r e a d i n g s w e r e o b t a i n e d a t i n t e r v a l s of t h r e e a n d s e v e n d a y s . T h e h e a t s t a b i l i t y of th e s a m p l e s w a s d e t e r m i n e d a t the s a m e t i m e the v i s c o s i t y r e s u l t s w e r e o b t a i n e d . were s e a l e d in 8 - m i l l i l i t e r P y r e x g l a s s t u b i n g a n d i m m e r s e d in an oil b a t h h e l d a t a t e m p e r a t u r e tubes w e re ute. O n e-m illiliter sam ples of 120° C. (248° F . ) . The s u b m e r g e d r o t a t e d a t a c o n s t a n t r a t e of s e v e n r e v o l u t i o n s p e r m i n ­ T h e t i m e i n t e r v a l b e t w e e n i m m e r s i o n of the s a m p l e s a n d the a p p e a r a n c e of the f i r s t s i g n s of p r e c i p i t a t i o n in the g l a s s t u b e s w a s r e c o r d e d , i n m i n u t e s , a s the m e a s u r e m e n t of h e a t s t a b i l i t y . T i t r a t a b l e a c i d i t y a n d p H d e t e r m i n a t i o n s w e r e c o n d u c t e d on all s a m p le s . ing th e p H . A B e c k m a n ( M o d e l G) p H m e t e r w a s u s e d f o r r e c o r d ­ T itra ta b le acidity was e x p r e s s e d as p e rc e n ta g e la ctic acid. The p e r c e n t a g e to ta l solids and fa t w as d e t e r m i n e d as outlined b y M o j o n n i e r a n d T r o y (1925) f o r c r e a m t e s t i n g 25 p e r c e n t a n d u n d e r . The p r o c e d u r e gen. of B r u n n e r (1952) w a s u s e d to d e t e r m i n e t o t a l n i t r o ­ A f a c t o r of 6 .3 8 w a s u s e d to c o n v e r t t o t a l n i t r o g e n i n t o t o t a l pro tein . T h e s p e c i f i c g r a v i t y w a s d e t e r m i n e d b y m e a n s of a Q u e v e n n e l a c t o m e t e r a t 60° F . , the s a m p l e s b e i n g t e m p e r e d f o r 30 m i n u t e s in w ate r . 42 T h e p e r c e n t a g e of a s h w a s d e t e r m i n e d by e v a p o r a t i n g 10 g r a m s of h a l f - a n d - h a l f to d r y n e s s on a s t e a m b a t h . The d r i e d p r o d u c t t h e n w a s p l a c e d in a m uffle f u r n a c e , h e ld at a t e m p e r a t u r e no h ig h e r than 550° C. (1 ,0 22 ° F .) u n t i l f r e e of c a r b o n . The a s h e d s a m p le was c o o l e d f i n a l l y in a d e s i c c a t o r , w e i g h e d , a n d th e p e r c e n t a g e of a s h then c a lc u la te d . I n o r d e r t h a t the s t a b i l i t y of h a l f - a n d - h a l f i n c o f f e e m i g h t be d e te r m in e d , s e v e r a l t r i a l s w e re conducted. F i v e m i l l i l i t e r s of the p r o d u c t w a s p i p e t t e d f i r s t in t o a b e a k e r to w h i c h 50 m i l l i l i t e r s of h o t c o f f e e , r a n g i n g in t e r m p e r a t u r e f r o m 195° to 205° F . , w a s a d d e d . The m i x t u r e w a s a l l o w e d to r e m a i n q u i e s c e n t f o r two m i n u t e s , a f t e r w h i c h t h e f e a t h e r i n g a b i l i t y of the s a m p l e s w a s o b s e r v e d a n d r e c o r d e d . T h e c o f f e e u s e d i n the t r i a l s f o r the d e t e c t i o n of f e a t h e r i n g w a s p r e ­ p a r e d b y a d d i n g 3 g r a m s of r e g u l a r - g r i n d c o f f e e to 50 m i l l i l i t e r s of b o i l i n g w a t e r c o n t a i n i n g th e v a r i o u s c o n c e n t r a t i o n s of c a l c i u m i o n s . The b r e w w a s a l l o w e d t o b o i l f o r one m i n u t e , a f t e r w h i c h i t w a s f i l t e r e d t h r o u g h a p i e c e of c h e e s e c l o t h a n d t h e n a d d e d to a b e a k e r containing the 5 - m i l l i l i t e r s a m p le . T rials w ere coffee m a d e b y the p e r c o l a t o r m e th o d . r u n a l s o w i t h the H e re, directions as r e c o m ­ m e n d e d b y the m a n u f a c t u r e r s w e r e f o l l o w e d f o r the p r e p a r a t i o n of th e c o f f e e . The p r o p o r t i o n s u s e d w e r e 15 g r a m s of r e g u l a r - g r i n d 43 c o f f e e p e r c u p (150 m l . ) of w a t e r . Both d istilled and h a rd w a te r w e r e u s e d i n t h i s m e t h o d of c o f f e e m a k i n g . T h e o r g a n o l e p t i c q u a l i t i e s of h a l f - a n d - h a l f w e r e d e t e r m i n e d by a p a n e l of e x p e r i e n c e d d a i r y j u d g e s . The s a m p l e s w e r e j u d g e d w h e n t h e y w e r e f r o m t h r e e to f i v e d a y s o ld . o n the b o d y a n d f l a v o r of I n f l u e n c e of a d d e d Special e m p h a s is was placed the p r o d u c t s . nonfat d ry m ilk solids and p r e s s u r e s h o m o g e n i z a t i o n on t h e s t a b i l i t y of h a l f - a n d - h a l f . of W it h the v i e w of s t u d y i n g the e f f e c t of a d d e d s o l i d s - n o t - f a t a n d p r e s s u r e s of h o m o g ­ e n i z a t i o n o n the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k , io n i c s o l u t i o n s of c a l c i u m , m a g n e s i u m , a n d c i t r a t e w e r e p r e p a r e d i n d i s ­ tilled w a te r. C o n c e n t r a t i o n s of e a c h g r o u p of i o n s ranged fro m 0 to 500 p a r t s p e r m i l l i o n , i n g r a d u a t i o n s of 100 p a r t s p e r m i l l i o n . F i v e m i l l i l i t e r s of e a c h s p e c i f i c c o n c e n t r a t i o n w a s p i p e t t e d in to a t e s t t u b e . To t h i s , 1 m i l l i l i t e r of th e s a m p l e of h a l f - a n d - h a l f under study was added. The c o n t e n t s w e r e i n v e r t e d fi v e t i m e s . To t h i s m i x t u r e , v a r i o u s i n c r e m e n t s of N / 1 0 0 HC1 w e r e a d d e d f r o m a b u r e t t e , a f t e r w h ich the tube w as i n v e r te d five t i m e s and o b s e r v e d . T h i s t e c h n i q u e w a s c o n t i n u e d u n t i l the f i r s t s i g n s of a f i n e p r e c i p i ­ t a t e i n th e t e s t tu b e w e r e n o t e d , a t w h i c h p o i n t the n u m b e r of m i l l i ­ l i t e r s of N / 1 0 0 HC1 w e r e r e c o r d e d a s the r e l a t i v e s t a b i l i t y of the s a m p l e . 44 R esults T h e r e s u l t s , o b t a i n e d f r o m th e f i r s t s e c t i o n of t h i s s t u d y , w e r e s e c u r e d f r o m t h r e e s e r i e s of h a l f - a n d - h a l f h o m o g e n i z e d m i l k . to s e v e n t r i a l s w e r e c o n d u c t e d on e a c h s e r i e s . Five These sam ples w ere s t a n d a r d i z e d a t 10.5 p e r c e n t f a t , w i t h th e m i l k - s o l i d s - n o t - f a t c o n t e n t ranging f r o m 8.7 3 to 1 2 .3 8 p e r c e n t ( T a b l e heat, s p r a y - p r o c e s s T ables skim m ilk pow der. 1 to 9> i n c l u s i v e , a n d a r e 1) b y th e a d d i t i o n of l o w - T h e d a t a a r e p r e s e n t e d in s h o w n g r a p h i c a l l y in F i g u r e s 1 to T a b l e 2, i l l u s t r a t e d i n F i g u r e 1, 12, i n c l u s i v e . V iscosity. The d a ta f r o m m a y b e d i v i d e d in to two m a j o r c a t e g o r i e s : (1) th e e f f e c t of s i n g l e a n d d o u b l e h o m o g e n i z a t i o n ; a n d (2) the e f f e c t of a d d e d m i l k - s o l i d s n o t - f a t o n t h e v i s c o s i t y of h a l f - a n d - h a l f . Th e v i s c o s i t y m e a s u r e m e n t s r e c o r d e d in t h i s t a b l e w e r e o b t a i n e d a f t e r th e s a m p l e s h a d b e e n h e l d in s t o r a g e f o r s i x h o u r s . In g e n e r a l , t h e v i s c o s i t y d i d n o t e x c e e d 17.8 c e n t i p o i s e s , e v e n w h e n h o m o g e n i z e d a t 5,0 00 p o u n d s p e r s q u a r e i n c h p r e s s u r e w i t h one stage. T h e g r e a t e s t in c r e a se in v i s c o s i t y o c c u r r e d w h e n o n l y one s t a g e w a s u s e d , th e g r e a t e s t b e i n g 11.2 c e n t i p o i s e s a t 5,000 p o u n d s p e r sq u are inch p r e s s u r e . W he n 500 p o u n d s p e r s q u a r e i n c h p r e s s u r e 45 w a s a p p l i e d to the s e c o n d v a l v e , t h e v i s c o s i t y i n c r e a s e d o n l y s l i g h t l y a s t h e p r e s s u r e of h o m o g e n i z a t i o n i n c r e a s e d . W i t h th e two s t a g e s in o p e r a t i o n , th e g r e a t e s t i n c r e a s e i n v i s c o s i t y w a s o nl y 1.5 c e n t i p o i s e s o v e r t h a t of t h e n o n h o m o g e n i z e d c o n t r o l s a m p l e ( T a b l e 2). F u r t h e r i n s p e c t i o n of T a b l e 2 s h o w s t h a t t h e a d d i t i o n of n o n ­ f a t d r y m i l k s o l i d s d i d i n c r e a s e th e v i s c o s i t y s l i g h t l y . W i t h one e x c e p t i o n , t h e r e a p p e a r e d to be a s m a l l b u t d e f i n i t e i n c r e a s e in v i s ­ c o s i t y a s a r e s u l t of a d d i n g s k i m m i l k p o w d e r . th e s a m p l e s w e r e h o m o g e n i z e d a t a p r e s s u r e This o c c u r r e d when of 5,000 p o u n d s p e r s q u a r e i n c h , s i n g l e s t a g e , r e s u l t i n g i n a n i n c r e a s e i n v i s c o s i t y of 4 .3 c e n t i p o i s e s . from T h e r e m a i n i n g s a m p l e s s h o w e d o n l y a n i n c r e a s e of 1.0 to 1.8 c e n t i p o i s e s . Only slig h t i n c r e a s e s in v i s c o s i t i e s w e re n o te d , e v e n a f t e r th e s a m p l e s w e r e h e l d i n s t o r a g e f o r s e v e n d a y s . i l l u s t r a t e d in T able 3. in v i s c o s i t y w a s v e r y This is c le a r ly A s t h e s e d a t a i n d i c a t e , the g e n e r a l i n c r e a s e s m a l l , a n d in m o s t c a s e s w o u l d n o t b e c o n s i d ­ e r e d as being significant. This w as tru e w h e th er nonfat d ry m ilk solids w e re added o r not. W i t h o n l y two e x c e p t i o n s , the l a r g e s t i n c r e a s e i n v i s c o s i t y n o t e d i n s e v e n d a y s w a s o n ly 2.5 c e n t i p o i s e s . T h e r e a s o n f o r the a b n o r m a l l y h i g h v i s c o s i t y s h o w n i n t h e s e c o n d t r i a l in S e r i e s 1 ( T a b l e 3) c a n n o t be e x p l a i n e d . The s a m p le w as 46 n o r m a l in a ll o t h e r r e s p e c t s . T h i s w a s the o n l y t i m e t h i s d e v i a t i o n o c c u r r e d in a l l the t r i a l s c o n d u c t e d on h a l f - a n d - h a l f i n t h e c u r r e n t study. H eat stability. The e f f e c t of v a r i o u s a d d i t i o n s of n o n f a t d r y m i l k s o l i d s on the h e a t s t a b i l i t y of h a l f - a n d - h a l f i s p r e s e n t e d in T a b l e 4 and F ig u re noted. 2. F r o m th e se r e s u l t s , s e v e r a l d ifferen t fa c ts m a y be T h e f i r s t i s t h a t th e m a x i m u m h e a t s t a b i l i t y o c c u r r e d i n the n o n h o m o g e n i z e d s a m p l e s to w h i c h no s k i m m i l k s o l i d s h a d b e e n a d d e d . A s t h e p e r c e n t a g e of m i l k - s o l i d s - n o t - f a t i n c r e a s e d i n t h e s e n o n h o m o g ­ e n i z e d s a m p l e s , t h e r e a p p e a r e d a d e f i n i t e d e c r e a s e in h e a t s t a b i l i t y . In a l l c a s e s , h o m o g e n i z a t i o n , w h e t h e r s i n g l e o r d o u b le s t a g e , a l s o d e c r e a s e d t h e h e a t s t a b i l i t y of th e h a l f - a n d - h a l f h o m o g e n i z e d m i l k s . H o w ev er, a d is tin c t differen ce a p p e a re d b etw een single- and dou ble-stag e hom ogenization. square inch p r e s s u r e The r e s u l t of p l a c i n g 500 p o u n d s p e r on flie s e c o n d v a l v e m o r e t h a n d o u b l e d th e h e a t s t a b i l i t y o v e r the s a m p l e s w h i c h w e r e stage p r e s s u r e s . s u b j e c t e d to t h e s a m e single- I t m i g h t w e l l be p o i n t e d o u t t h a t in th e s e r i e s w h e r e th e s k i m m i l k p o w d e r w a s a d d e d , the h e a t s t a b i l i t y a t the two d i f f e r e n t p r e s s u r e s w i t h the s i n g l e - s t a g e p r o c e s s w a s i d e n t i c a l . r e s u l t s o b t a i n e d f r o m the d u a l - h o m o g e n i z e d s a m p l e s , a l t h o u g h n o t id en tical with e a c h o th e r, w e re v e ry c lo se . Th e s e r i e s w h e r e no Th e p o w d e r w a s a d d e d s h o w e d a g r a d u a l d e c r e a s e in s t a b i l i t y a s the p r e s s u r e i n c r e a s e d , w i t h th e s i n g l e - s t a g e h o m o g e n i z e d s a m p l e s b e i n g th e le a s t stable. T i t r a t a b l e a cid ity a nd p H . 5 and F ig u re The i n f o r m a t i o n s u p p l i e d in T a b l e 3 i s of c o n s i d e r a b l e i m p o r t a n c e w h e n c o n s i d e r i n g u s e of a d d e d m i l k - s o l i d s - n o t - f a t i n h a l f - a n d - h a l f . P e r h a p s the m o s t s i g ­ n i f i c a n t i n f o r m a t i o n t h a t i s s h o w n by t h e s e d a t a i s th e d e f i n i t e i n ­ c r e a s e i n t i t r a t a b l e a c i d i t y a s the p e r c e n t a g e of m i l k - s o l i d s - n o t - f a t increased. A s s h o w n in F i g u r e 3, f o r e v e r y p e r c e n t a g e i n c r e a s e in the s o l i d s - n o t - f a t , up to 5 p e r c e n t , th e c a l c u l a t e d i n c r e a s e in t i t r a t ­ a b l e a c i d i t y a m o u n t e d to 0 . 0 1 8 9 p e r c e n t . H o w e v e r, adding m ilk - s o l i d s - n o t - f a t a p p e a r e d to p r o d u c e no s i g n i f i c a n t d i f f e r e n c e i n p H of the s a m p l e s u s e d in t h i s s t u d y . Specific g r a v i t y . The s p e c i f i c g r a v i t y w a s f o u n d to i n c r e a s e d i r e c t l y a s i n c r e m e n t s of m i l k - s o l i d s - n o t - f a t w e r e a d d e d to th e s a m ­ ple s . F e a t h e r i n g of h a l f - a n d - h a l f in c o f f e e . Th e d a t a s h o w i n g the s t a b i l i t y of the s a m p l e s of h a l f - a n d - h a l f in c o f f e e a r e c o m p i l e d in T a b l e 6. P e r h a p s th e m o s t s i g n i f i c a n t f a c t o r in t h i s s t u d y w a s t h a t , w i t h one e x c e p t i o n w h e r e the v i s c o s i t y w a s a b n o r m a l l y h i g h ( S e r i e s 1 48 T r i a l 2, T a b l e 6), n o s a m p l e of h a l f - a n d - h a l f p r e p a r e d i n t h i s s t u d y f e a t h e r e d w h e n t h e c o f f e e w a s m a d e w i t h h a r d ta p w a t e r . The h a r d ­ n e s s of t h i s w a t e r w a s c a l c u l a t e d to be 419 .2 p a r t s p e r m i l l i o n c a l ­ cium carbonate. H o w e v e r , a s i n d i c a t e d in T a b l e 6, w h e n th e c o f f e e w as m ad e with d is tille d w a te r , d ifferen t r e s u lts w e re obtained. The n o n h o m o g e n i z e d s a m p l e s w e r e q u it e s t a b l e w h e n a d d e d to t h i s c o f f e e . W h e n the s a m p l e s w e r e h o m o g e n i z e d , t h i s s t a b i l i t y a g a i n s t f e a t h e r i n g dropped sharply. m ore s t a b i l i t y t h a n t h o s e t r e a t e d w i t h the s i n g l e c o n d i t i o n s of t h i s prove The d u a l-h o m o g e n iz e d s a m p le s showed slightly stage. U n d e r th e s t u d y t h e a d d i t i o n of s k i m m i l k p o w d e r d i d n o t i m ­ s i g n i f i c a n t l y t h e s t a b i l i t y of t h e h a l f - a n d - h a l f in c o f f e e w h e n m ade with d istille d w a ter. W hen coffee w as m a d e e i t h e r w ith d is tille d o r w ith tap w a t e r , a d i f f e r e n c e i n t h e p H of the b r e w s w a s n o t e d . Th e c o f f e e m a d e w i t h d i s t i l l e d w a t e r h a d a p H r a n g e f r o m 5 . 1 8 to 5 .6 5, w hile t h a t m a d e w ith h a r d w a te r w as m u ch h ig h e r, ranging f r o m 6.00 to 6 .3 2. A p a n e l of e x p e r i e n c e d d a i r y - p r o d u c t j u d g e s c o n s i d e r e d the s a m p l e s f r o m a n o r g a n o l e p t i c p o i n t of v i e w . The n o n h o m o g e n i z e d p r o d u c t s w e r e c r i t i c i z e d b e c a u s e of t h e i r s a l v y type of c r e a m p l u g , w hich f o r m e d upon s to r a g e . In a d d i t i o n , t h e s e s a m p l e s w e r e c o n s i d ­ e r e d to l a c k the f l a v o r a n d body w h i c h is d e s i r e d i n h a l f - a n d - h a l f . 49 The s a m p l e s to w h i c h 2 p e r c e n t s k i m m i l k p o w d e r w e r e a d d e d w e r e c o n s i d e r e d to h a v e a g o o d b o d y a n d e x c e l l e n t f l a v o r . H owever, w h e n the a d d i t i o n s w e r e i n c r e a s e d to 5 p e r c e n t ( a l t h o u g h th e p r o d u c t s w e r e c o n s i d e r e d to h a v e a d e s i r a b l e , h e a v y b o d y ) , t h e y h a d a d e f i n i t e , u ndesirable salty taste. O c c a s i o n a l l y , w h e n th e s a m p l e s w e r e h o m o g e n i z e d a t the l o w e r p r e s s u r e s u s e d i n t h i s s t u d y , t h e y w e r e c r i t i c i z e d f o r the a p p e a r a n c e of a f a t r i n g a r o u n d th e to p of t h e b o t t l e . This was n o tic e ab le both w ith sin g le - and with d o u b le -stag e hom ogenization. When the h ig h e r p r e s s u r e s w e r e u s e d , t h i s d e f e c t d id n o t o c c u r . T h e i n f l u e n c e of a d d e d n o n f a t d r y m i l k s o l i d s an d p r e s s u r e of h orn o g e n i z a t i o n o n the s t a b i l i t y of h a l f - a n d - h a l f . The d a t a p r e ­ s e n t e d i n T a b l e s 7, 8, a n d 9, s h o w th e i n f l u e n c e of v a r i o u s c o n c e n ­ trations of c a l c i u m , m a g n e s i u m , a n d c i t r a t e i o n i c s o l u t i o n s on the s t a b i l i t y of d i f f e r e n t t y p e s of h o m o g e n i z e d m i l k s , a s m e a s u r e d by acid p recip itatio n . Three s e p a ra te tr i a ls w ere conducted. t r i a l w as m a d e in J a n u a r y ( F ig u r e s Th e f i r s t 4, 7, a n d 10), th e s e c o n d in M a r c h ( F i g u r e s 5, 8, a n d 11), a n d the t h i r d i n M a y ( F i g u r e s 6, 9, a n d 12). P e r h a p s t h e m o s t s i g n i f i c a n t p o i n t t h a t m i g h t be n o t i c e d f r o m t h e s e t h r e e t a b l e s i s t h a t a c i d w a s n o t r e q u i r e d to p r e c i p i t a t e the 50 n o n h o m o g e n iz e d s a m p le s when they w e re m ix e d w ith d is tille d w a te r . A l t h o u g h t h e t h r e e d i f f e r e n t t y p e s of i o n i c s o l u t i o n s of v a r y i n g c o n ­ c e n t r a t i o n s w e r e a d d e d to h a l f - a n d - h a l f , t h e s e s a m p l e s w e r e r e a d i l y p r e c i p i t a t e d up on m i x i n g . still H o w e v e r , w h e n th e s a m p l e s of h a l f - a n d - h a l f w e r e h o m o g e n i z e d , th e a d d i t i o n of a c i d w a s n e c e s s a r y to g iv e t h i s p r e c i p i t a t i o n , th u s s u g g e s t i n g i n c r e a s e d s t a b i l i t y . T h e g e n e r a l e f f e c t of a d d i n g m i l k - s o l i d s - n o t - f a t to h a l f - a n d h a l f w a s t h a t , a s the p e r c e n t a g e i n c r e a s e d , m o r e a c i d w a s r e q u i r e d to p r o d u c e a p r e c i p i t a t e , t h u s i n d i c a t i n g a g r e a t e r s t a b i l i t y . m o r e , when these F urther­ sam p les w ere hom ogenized at higher p r e s s u r e s , i n c l u d i n g b o t h s i n g l e a n d d o u b le s t a g e , a d d i t i o n a l a c i d u s u a l l y w a s n e c e s s a ry fo r precipitation. T h i s w a s e s p e c i a l l y n o t i c e a b l e in the f i r s t two t r i a l s . The e f f e c t s of t h e c a l c i u m , m a g n e s i u m , a n d c i t r a t e i o n s on the s t a b i l i t y of the d i f f e r e n t s e r i e s of h a l f - a n d - h a l f p r o v e d i n t e r e s t ­ ing. ures Th e e f f e c t of th e c a l c i u m i o n s w a s t h a t of d e s t a b i l i z a t i o n ( F i g ­ 4 , 5, a n d 6). M a g n e s i u m t e n d e d to h a v e b u t l i t t l e e f f e c t , f o r a s t h e c o n c e n t r a t i o n of t h e s e i o n s i n c r e a s e d , the a m o u n t of a c i d re q u ire d fo r precip itatio n re m a in ed fairly constant (F igures a n d 9). 7 , 8, The a d d i t i o n of c i t r a t e i o n s , h o w e v e r , t e n d e d to i n c r e a s e s l i g h t l y the s t a b i l i t y of the s a m p l e s a s the io n i c s t r e n g t h w a s 51 in c re a s e d (F igures 10, 11, a n d 1Z). The a b o v e e f f e c t s of t h e s e i o n s w e r e m o s t n o t i c e a b l e i n t h e two s e r i e s w h i c h c o n t a i n e d a d d e d i n c r e ­ m e n t s of 2 a n d 5 p e r c e n t m i l k - s o l i d s - n o t - f a t , e s p e c i a l l y w h e n t h e y w e r e h o m o g e n i z e d a t 5,0 00 a n d 5,000 + 500 p o u n d s p e r s q u a r e i n c h pressure s . Of i n t e r e s t a l s o i s a c o m p a r i s o n of the d a t a f r o m th e t h r e e separate tria ls . W h e n c o m p a r i n g the t r i a l s i n w h i c h n o s o l i d s w e r e a d d e d , i t m a y be s e e n t h a t t h e t r i a l c o n d u c t e d i n J a n u a r y a p p e a r e d to b e r a t h e r u n s t a b l e , a s l i t t l e a c i d w a s r e q u i r e d to g iv e a p r e c i p i ­ tate. T h e o t h e r two t r i a l s , w h i c h w e r e g iv e s o m e w h a t h i g h e r r e a d i n g s . r u n in M a r c h a n d M a y , did The a d d i t i o n of s k i m m i l k p o w d e r t e n d e d to m i n i m i z e the d i f f e r e n c e s i n t h e r e s u l t s of t h e t h r e e t r i a l s , a s w a s n o tic e d in the f i r s t s e r i e s . A. m a r k e d d i f f e r e n c e o c c u r r e d b e t w e e n th e d a t a o b t a i n e d f r o m the f i r s t two t r i a l s a n d t h o s e of th e t h i r d . The r e s u l t s f r o m t h i s l a s t t r i a l s h o w e d t h a t , a l t h o u g h th e d i f f e r e n t h o m o g e n i z a t i o n p r e s s u r e s w e r e u s e d i n e a c h s e r i e s , th e r e s u l t s w e r e a l m o s t i d e n t i c a l . How­ e v e r , in the f i r s t two t r i a l s , the d a t a w e r e m o r e e r r a t i c , t h e r e b e i n ' a m a r k e d d i f f e r e n c e in the f i n d i n g s , due to the d i f f e r e n t p r e s s u r e s of hom ogenization. A s in a l l t h r e e t r i a l s of e v e r y s e r i e s , l i t t l e signifi­ cant differen ce could be d e tec te d betw een single- and d o u b le-stag e hom ogenization. 52 Table Series No..* 1. T h e i n f l u e n c e of a d d e d s o l i d s - n o t - f a t on the c o m p o s i t i o n of h a lf-a n d -h a lf hom ogenized m ilk. A d de d Incre­ m ents of Solids N ot-F at The C o m p o s i t i o n of H a l f - a n d - H a l f H o m o g e n i z e d M i l k W h e n V a r i o u s A m o u n t s of S o l i d s - N o t - F a t W ere Added Fat P r o t e in Ash SolidsN ot-F at Total Solids (pet.) (pet.) (pet.) (pet.) (pet.) (pet.) 1 0 10.58 3,36 0 .6 9 6 7 8.73 19.20 2 2 10.47 4.18 0 .8353 10.05 20 .5 3 3 5 10 .2 8 4,66 1 .0 168 12.38 22. 57 * F i v e to s e v e n t r i a l s on e a c h s e r i e s . 53 T a b l e 2. Series N o.* 1 2 3 T h e i n f l u e n c e of a d d e d s o l i d s - n o t - f a t on the v i s c o s i t y of h a lf-a n d -h a lf when hom ogenized at various p r e s s u r e s . Added Increm ents of SolidsN ot-F at Tempe r a ture OI V is­ cosity M ea­ sure­ m ents (pet.) (°C.) (centip o i s e s) 0 20 4.9 6.7 3 6.7 20 2 5 Th e V i s c o s i t y of H a l f - a n d - H a l f W h e n H o m o g e n i z e d at: 2500 + 500 lb s ./ sq.in. 5000 lb s . / sq.in. 5000 + 500 lb s./ sq.in. (centip o i s e s) (centip o i s e s) 5.4 8.9 5.8 8.9 7.0 13.5 7.5 5.7 8.0 6.3 10.4 6.7 3 6.6 9.3 7.5 17.8 8.1 20 5.9 7.8 6.7 9.1 7.2 3 7.9 10.3 8.6 15.9 9-3 0 lb s./ sq.in. 2500 lbs./ sq.in. (centi(centip o i s e s)1 p o i s e s ) * F i v e to s e v e n t r i a l s on e a c h s e r i e s . 54 Table 3. T h e I n f l u e n c e of l e n g t h of s t o r a g e a n d a d d e d s o l i d s - n o t - f a t o n the v i s c o s i t y of h a l f - a n d - h a l f h o m o g e n i z e d a t v a r i o u s p re s s u r e s . Ho- V i s c o s i t y of H a l f - a n d - H a l f W h e n S t o r e d a t 3° C. f o r : eni0 days zation -----—— -------P r e s - Trial Trial Trial sures 1 2 3 3 days Trial 1 (centipoises)* Series 0 2500 2500+ 500 5000 5000+ 500 (centipoises) 1: 0 2500 2500+ 500 5000 5000+ 500 (centipoises) No S o l i d s - N o t - F a t A d d e d 4.6 7.4 4.7 7.5 4.9 6.8 4.8 7.0 5.1 - 5.3 5.6 7.6 5.9 7.8 5.7 7.2 5.1 9-2 39.3 8.6 5.4 9.5 5.5 10.0 7 8 .4 - 5.5 - 5.8 10-.4 82.6 8.9 5.5 10.0 5.0 79.8 5.7 6.3 16 9.8 5.1 6.8 20 9.6 5.6 S e r i e s 2: 0 2500 2500+ 500 5000 5000+ 500 7 days —----------Trial Trial Trial Trial Trial 2 3 1 2 3 A dde d S o l i d s - N o t - F a t (2%) 5.4 8.2 5.4 8.7 5.6 8.1 5.5 7.9 5.6 — 5.2 - 6.1 8.4 6.0 9.6 6.0 8.3 5.8 9.3 7.9 8.3 6.2 11.9 6.1 9.8 8.2 — 5.4 — 6.0 10.3 8.9 8.8 6.0 14.4 6.7 15.2 6.5 7.5 18.2 6.7 7.1 21.4 6.8 A d d e d S o l i d s - N o t - F a t (5%) S eries 3: 9.9 7.5 5.8 7.8 5.8 8.2 10.7 10.4 7.5 6.3 11.9 7.6 6.8 8.6 6.5 8.1 7.6 8.4 7.1 10.5 6.5 8.4 8.3 8.6 7.3 6.4 “ ” 9.2 8.6 7.6 12.0 7.2 8.9 7 .6 7.8 6.8 8.3 7.9 7.4 9.2 8.2 7.6 * R e a d i n g s m a d e a t 20° C. 55 T a b l e 4. ries T h e i n f l u e n c e of a d d e d s o l i d s - n o t - f a t on the h e a t s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k w h e n s u b j e c t e d to v arious p r e s s u re s . Adde d Incre­ m ents of SolidsN ot-F at Th e H e a t S t a b i l i t y of H a l f - a n d - H a lf W he n H o m o g e n i z e d at: 0 lb s./ sq.in. 2500 l b s ./ s q .in . 2500 + 500 lb s./ sq.in. 5000 lb s./ sq.in. 5000 +500 lb s ./ sq.in. (pet.) (m in­ ut e s) (m in­ ute s) (m in­ ute s) (m in­ ute s) (m in­ ute s) 1 0 97 28 80 15 76 2 2 92 11 59 11 58 3 5 76 15 33 15 37 ro.* * F i v e to s e v e n t r i a l s on e a c h s e r i e s . 56 T a b l e 5. T h e i n f l u e n c e of a d d e d s o l i d s - n o t - f a t on the s p e c i f i c g r a v ­ i t y , p H , a n d t i t r a t a b l e a c i d i t y of h a l f - a n d - h a l f h o m o g e n i z e d m ilk. T rial No. Specific G ravity Series 1 2 3 4 5 6 1: 7 8 9 10 11 12 13 6.55 6.67 6.52 6.65 6.67 6 .6 8 0.1 8 0 0 .1 5 5 0 .180 0.185 0.160 0.155 S o l i d s - N o t - F a t A d d e d (2%) 1 .0 31 9 1.0330 1.0320 1 .0 31 3 1.0310 1. 030 2 1. 0302 S eries 14 15 16 17 18 No S o l i d s - N o t - F a t A d d e d 1.0261 1. 0242 1.0255 1 .0 268 1 .0 263 1.0259 S e r i e s 2: T h e A c i d i t y of H a l f - a n d Half H o m o g e n iz e d M ilk a s M e a s u r e d by: -----------------------------------------Titratable pH Acidity 6 .7 8 6.58 6.49 6.5 3 6.6 2 6.59 6.58 0.205 0.205 0.210 0.195 0 .1 9 5 0.190 0.195 3:__ S o l i d s - N o t - F a t A d d e d (5%) 1.0405 1. 0422 1.0410 1 .0 4 3 8 1 .0 42 8 6 .4 3 6.50 6.57 6.59 6.64 0.275 0.270 0.245 0.235 0.265 57 T a b l e 6. T h e i n f l u e n c e of c a l c i u m - i o n c o n c e n t r a t i o n , c o m p o s i t i o n , a n d p r e s s u r e s of h o m o g e n i z a t i o n on f e a t h e r i n g of h a l f - a n d half in coffee. O c c u r r e n c e of F e a t h e r i n g i n H a l f - a n d H a l f w h e n H o m o g e n i z e d at: 25Z 50° ° +50° lb s . / sq.m . 0 lb s./ sq.m . 2500 lb s./ sq.m . Series 1* 2 3 Series 1 2 3 Series Series Series 1 2 3 1 2 3 1 2 3 + ? C alcium -Ion C oncentration 5000 lb s./ . sq.m . +50° lb s . / sq.m . (ppm) H ard** 0 _ _ _ 50 75 - 100 _ - _ _ 125 - + ? - + + + + _ + + + + ? + + + + + + ? + + + + + + + + + + + + + _ _ _ + + + + + + + + + + + + 200 - - + + + + + + + + + + + + 300 + ++ + + + + + + + + + + + + 400 + ++ + + + + + + + + + + + + 500 + ++ + + + + + + + + + + + + 150 - * Two t r i a l s c o n d u c t e d on e a c h s e r i e s . ** H a r d , t a p w a t e r , t e s t i n g 41 9.2 p p m c a l c i u m c a r b o n a t e . - No f e a t h e r i n g . ? Q uestionable. + Definite fe a th e rin g . 58 T a b l e 7. T h e i n f l u e n c e of v a r i o u s c o n c e n t r a t i o n s of i o n i c s o l u t i o n s c a l c i u m o n th e s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m ilk, as m e a s u r e d by acid p re c ip ita tio n . A c i d R e q u i r e d to P r e c i p i t a t e Trial 1 Trial 2 Trial 3 ( m l . N / 1 0 0 HC1) (ppm) Series 0 0 0 0 0 0 S e r i e s 2: 0 100 200 300 400 500 3: Trial 2 Trial 3 ( m l . N / 1 0 0 HC1) 0 0 0 0 0 0 0 0 0 0 0 0 1.305 0.875 0.930 0.750 0.825 0 .545 2.050 1.915 1.180 1.0 30 1.305 1.130 0.715 0.870 0.490 0.000 1.900 1.935 2.020 2 .220 1.780 1.610 3.520 3.435 2.380 1.915 1.375 1.845 1.935 1.930 1.665 2.4 15 2 .6 2 0 2.680 3.380 2. 9 2 0 2.725 2.455 4.375 4.1 35 3.140 2.3 70 0.715 0.500 0.195 0.210 0.135 0.000 A d d e d S o l i d s - N o t - F a t (2%) 0 0 0 0 0 0 Series Trial 1 No S o l i d s - N o t - F a t A d d e d 1: 0 100 200 300 400 500 0 100 200 300 400 500 2500 l b s . / s q . i n . 0 lbs./sq.in. C alcium C oncentration 0 0 0 0 0 0 0 0 0 0 0 0 Added S o lid s -N o t- F a t 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 59 T a b l e 7 (C o n tin u e d ) H a l f - a n d - H a l f w h e n H o m o g e n i z e d at: 2500+5 00 l b s . / s q . i n . Trial 1 T rial 2 5000 l b s . / s q . i n . Trial 3 Trial 1 ( m l . N / 1 0 0 HC1) 0.865 0.785 0.970 1.420 1.320 0.985 0.990 0.815 0 .5 3 0 0.555 1.790 1.815 1.890 2.290 2 .0 8 5 1.680 3.5 75 3.500 2.8 1 5 2.090 Se r i e s 3: 2.2 7 5 1.760 1.965 1.610 1.735 1.650 2.7 4 0 2.365 2.910 3.1 1 5 2.755 2.8 4 0 4.205 3.840 3.315 2.470 Trial 1 T rial 2 Trial 3 ( m l . N / 1 0 0 HC1) No S o l i d s -■ N o t - F a t A d d e d 0 .6 8 0 0.470 0 .4 5 0 0.425 0 .2 0 5 0.190 S e r i e s 2: 0.955 1.215 1.625 2.165 0.635 0.595 Trial 3 ( m l . N / 1 0 0 HC1) Se r i e s 1: 0.655 0.430 0.360 0 .4 0 0 0.120 0.000 Trial 2 5000+500 l b s ./ s q . i n . 0.800 0 .5 9 0 0.710 0.3 5 0 0.4 1 0 0.6 7 0 2.810 2 .4 00 1.710 1.290 1.130 0 .9 1 0 0 .7 4 5 0.6 0 0 0.630 0.5 5 0 1.305 1.890 1.915 1.455 1.490 1.460 1.515 2.5 45 2.0 05 1.430 3.435 3.225 3.225 2.730 2.495 2.4 20 3.650 2.805 3.130 2.410 4.60 5 4.340 3.795 3.140 2.950 2.7 20 4.520 4.110 3.220 2.5 75 A d d e d S o l i d s - N o t - F a t (2%) 3.240 2.980 2 . 50 0 1.985 1.620 1.225 3.240 2.900 2.5 85 2.000 1.460 1.495 3.760 3.015 2.6 4 5 2.100 3. 525 3.085 2.800 2.580 2.170 1.665 A d d e d S o l i d s - N o t - F a t (5%) 4.345 4.070 3.580 2.985 2.865 2.665 4.425 3.960 3.615 3.460 2.870 2.710 4.210 3.830 3.370 2.800 4.810 4.5 05 4.275 3.970 3.700 3.290 60 T a b l e 8. T h e i n f l u e n c e of v a r i o u s c o n c e n t r a t i o n s of i o n i c s o l u t i o n s m a g n e s i u m o n t h e s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m ilk, as m e a s u r e d by acid p recip itatio n . A c i d R e q u i r e d to P r e c i p i t a t e M agnesium c on ce n tr a ti o n 0 lbs./sq.in. T rial 1 T rial 3 ( m l . N / 1 0 0 HC1) (ppm) 0 100 200 300 400 500 0 0 0 0 0 0 S e r i e s 2: 0 100 200 300 400 500 3: T rial 2 T rial 3 • ( m l . N / 1 0 0 HC1) 0 0 0 0 0 0 0 0 0 0 0 0 0.790 0.560 0.370 0.280 0.160 0 . 1 8.0 1.480 1.610 1.710 1.920 1.4 50 1.4 85 2.170 2.115 2.220 2 .7 7 0 3.2 45 3. 3 4 5 3.240 3.3 10 3.575 3.130 - 1.820 - 1.870 A d d e d S o l i d s - N o t - F a t (2%) 0 0 0 0 0 0 S eries T rial 1 No S o l i d s -- N o t - F a t A d d e d Se r ie s 1: 0 100 200 300 400 500 T rial 2 2500 l b s . / s q . i n . 0 0 0 0 0 0 0 0 0 0 0 0 1.940 1.885 1.480 0.885 0.865 0.540 - 3.075 - 3.100 A d d e d S o l i d s - N o t - F a t (5%) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.970 1.890 2. 1 2 5 2. 4 2 5 2 .3 25 2.115 1.995 2.330 2. 4 2 0 3 .5 4 5 3.795 3.930 4.380 3.940 - 3.935 - 3.970 61 T a b le 8 (C o n tin u e d ) H a l f - a n d - H a l f w h e n H o m o g e n i z e d at: 25 00 +5 00 l b s . / s q . i n . Trial 1 T rial 2 5000 l b s . / s q . i n . Trial 3 Trial 1 ( m l . N / 1 0 0 HC1) 1.110 1.420 1.0 40 1.370 1.660 1.480 0.640 0.905 1.405 1.005 1.240 1.275 1.685 1.745 1.405 1.775 1.775 1.490 2 .0 3 5 1.995 1.910 2.305 2.165 2.135 4.5 5 5 4.4 1 0 3.930 3.9 85 3: Trial 2 T rial 3 ( m l . N / 1 0 0 HC1) 3.280 2.930 2.980 2.970 2.645 2.785 2.825 2 .6 65 2.300 2.415 0. 7 0 0 0.485 0.745 0.520 0.695 0.945 3.120 3.2 10 3.1 65 3.140 3.115 2 .9 95 2.900 2.6 00 2.6 8 5 2.7 45 3.245 3.855 3.880 3.820 3.600 3.6 85 3.800 3.450 3.190 3.280 3.805 4.400 4.205 4.020 4.260 4.315 4.725 4.290 4.1 1 5 4.120 A d d e d S o l i d s - N o t - F a t (2%) 3.890 3.760 3.695 3.660 3.455 3.560 3.655 3.600 3.0 25 3.3 55 Series 1.370 2.0 0 5 2.325 1.870 1.700 1.620 2: Trial 1 No S o l i d s - ■ N o t - F a t A d d e d 0.750 0.460 0 .2 7 5 0.370 0.370 0.270 1.650 1.975 2.135 2.5 8 0 Series Trial 3 ( m l . N / 1 0 0 HC1) Se r i e s 1: 0.640 0.535 0.175 0.135 0.130 0.0 60 Trial 2 5000+500 l b s . / s q . i n . 3.660 3.445 3.420 3.425 3.340 3.500 3.325 3.415 3.385 3.210 3.830 4.005 3.790 4.170 3.975 3.815 A d d e d S o l i d s - N o t - F a t (5%) 4.2 45 4.070 4.010 3.640 3.630 3.775 4.475 4.430 4.420 4.320 4.290 4.315 4.070 3.935 4.0 80 4.430 4.365 4.300 3.920 4.140 3.970 62 T a b l e 9. T h e i n f l u e n c e of v a r i o u s c o n c e n t r a t i o n s of i o n i c s o l u t i o n s of c i t r a t e on t h e s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m ilk , as m e a s u r e d by a c id p r e c ip ita tio n . A c i d R e q u i r e d to P r e c i p i t a t e C itrateIon C o n c e n tr a tio n 0 lbs./sq.in. Trial 1 Trial 3 ( m l . N / 1 0 0 HC1) ppm) Series 0 0 0 0 0 0 S e r i e s 2: Series 3: 0 0 0 0 0 0 0 0 0 0 0 0 0.495 0.715 0.385 0.365 0.290 0 .2 9 0 0.920 1 .3 45 1.5 85 1.80 5 1.690 1.740 1.955 2.745 - 2. 8 1 5 - 3.330 A d d e d S o l i d s - N o t - F a t (2%) 0 0 0 0 0 0 0 100 200 300 400 500 ( m l . N / 1 0 0 HC1) No S o l i d s - ■ N o t - F a t A d d e d 1: 0 100 200 300 400 500 0 100 200 300 400 500 Trial 2 2500 l b s . / s q . i n . ~ T rial Trial Trial 2 1 3 0 0 0 0 0 0 0 0 0 0 0 0 1.215 1.195 1.045 1.170 1.350 1.950 2.600 2.885 2.765 2.910 2.865 2.815 3.985 4.1 85 - 4.4 15 - 5.180 A d d e d S o l i d s - N o t - F a t (5%) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.800 1.5 80 1.620 1.780 2.430 2.370 2.045 2.155 2.220 2.415 2 .3 9 5 2 .3 3 5 4.300 4.8 05 - 5.320 - 5.590 63 T a b le 9 (C o n tin u e d ) H a l f - a n d - H a l f w h e n H o m o g e n i z e d at: 2 5 00 + 500 l b s . / s q . i n . T rial 1 T rial 2 5000 l b s . / s q . i n . Trial 3 Trial 1 ( m l . N / 1 0 0 HC1) Series 0.520 0.430 0.380 0.345 0.175 0.160 1.570 1.960 1.680 1.770 1.865 1.715 1.945 2.475 2.420 2. 590 2.265 2.385 1: 1.975 2.800 3.690 - 4.070 3.985 4.250 - 4.680 - 4. 7 9 5 Se r i e s 3: 1.835 1.780 1.785 2.455 2.495 2 .2 60 1.765 1.97 5 1.990 2 .2 2 5 2.435 2 .6 30 4. 1 8 5 4.5 50 - 4.990 - 5.2 50 Trial 3 T rial 1 ( m l . N / 1 0 0 HC1) S e r i e s 2: 1.190 1.425 1.240 1.840 1.375 1.760 Trial 2 5000+500 l b s . / s q . i n . Trial 2 Trial 3 ( m l . N / 1 0 0 HC1) No S o l i d s - N o t - F a t A d d e d 0 .780 1 .110 1 .270 1 .520 1 .620 1 .630 2.7 85 3.795 3.145 2.80 5 3.360 2.850 2 .9 60 3.040 1.130 1.295 1.655 1.330 2.015 2.220 - 3.200 - 3.710 3.47 5 3.275 3.200 3.135 3.130 3.295 2.9 80 3.1 85 - 3.460 - 3.625 A d d e d S o l i d s - N o t - F a t (2%) 3 .775 4 .210 4 .505 4 .280 4 .485 4 .655 3.610 3.855 4.455 4.500 4.350 4.165 3.510 3.8 1 5 - 4. 24 0 - 4.55 0 Added S olid s-N ot-F at 4 .475 4 .990 5 .310 5 .320 5 .455 5 .575 4.305 4.785 4.925 5.33 5 5.620 6.12 5 3.750 4.220 4.665 4.7 15 4.630 5.100 4.22 5 4.33 5 4.98 0 - 5.1 95 i 3.720 4.060 3.700 3.700 3.870 3.910 3.890 4.290 - 4.610 - 4.9 40 m 4.500 5.130 5.51 5 5.555 5.620 6.205 4 .445 5 .215 5 .3 5 5 5 .575 5.7 30 6.295 4.185 4.6 85 - 4.820 - 5. 2 8 5 64 20 o 0 rO 1 UJ (/> O CL UJ o > H (/) O O <0 > No M.S.N.F added 2% M.S.N.F added 5% M.S.N.F added 2 ,5 0 0 + 5 0 0 5 ,0 0 0 + 5 0 0 2,500 5 ,0 0 0 HOMOGENIZATION PRESSURE ( LBS. / SQ. I N. ) F igure 1. T h e e f f e c t of s i n g l e - a n d d o u b l e - s t a g e h o m o g e n i z a t i o n on the v i s c o s i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k . 65 HEAT STABILITY ( MI NUTES) 100 No M.S.N.F. added 2% M.S.N.F added 5% M.S.N.F added 80 60 40 20 0 2 ,5 0 0 + 5 0 0 5 ,0 0 0 + 5 0 0 2 ,5 0 0 5 ,0 0 0 HOMOGENIZATION PRESSURE ( LBS/ SQ. I N. ) F i g u r e 2. T h e e f f e c t of s i n g l e - a n d d o u b l e - s t a g e h o m o g e n i z a t i o n on the h e a t s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k . 66 TITRATABLE ACIDITY (% ) 0 .2 6 0 .2 4 0 .2 2 0.20 0.16 0 2 4 ADDED SOLIDS-NOT-FAT (%) F igure 3. T h e i n f l u e n c e of a d d e d i n c r e m e n t s of l o w - h e a t , s p r a y p r o c e s s s k i m m i l k p o w d e r on the t i t r a t a b l e a c i d i t y of h a lf-a n d -h a lf hom ogenized m ilk. 4.0 2.0 4.0 2 % M.S.N.F. •• 2.0 N/100 HCL (ML.) 5 % M.S.N.F 4.0 0 % M.S.N.F. L B S / S Q IN -2 5 0 0 -2 5 0 0 + 500 - 5000 - 5000+500 2.0 100 CALCIUM-ION ure 200 300 400 500 CONCENTRATION (P.RM.) T h e i n f l u e n c e of i n c r e a s i n g a m o u n t s of c a l c i u m - i o n c o n ­ c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a s u r e d by a c i d p r e ­ c i p i t a t i o n ( T r i a l 1, J a n u a r y ) . 4.0 2.0 2 % M.S.N.F. 4.0 2.0 N/100 HCL (ML.) 5 % M.S.N.F. 0 % M.S.N.F. 4.0 —0 L B S ./S Q .IN . 2500 2500 +500 5000 5000+500 2.0 0 100 200 CALCIUM-ION F igure 5. 300 400 500 CONCENTRATION (RPM.) T h e i n f l u e n c e of i n c r e a s i n g a m o u n t s of c a l c i u m - i o n c o n ­ c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a s u r e d by a c i d p r e ­ c i p i t a t i o n ( T r i a l 2, M a r c h ) . 4.0 5 % M.S.N.F. L B S . / S Q . IN. 2 .0 2500 2500 + 500 5000 5000 +500 J 4.0 2 _l O x 2.0 o o z 4.0 0 % M.S.N.F. 2.0 0 100 200 300 400 500 CALCIUM - ION CONCENTRATION (RRM.) F igure 6. T h e i n f l u e n c e of i n c r e a s i n g a m o u n t s of c a l c i u m - i o n c o n ­ c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d of v a r y i n g c o m p o s i t i o n , a s m e a s u r e d by a c i d p r e — c i p i t a t i o n ( T r i a l 3, M ay ). 4.0 2.0 2 % M.S.N.F. 4.0 2.0 N/100 HCL (ML.) 5% M.S.N.F. 0 % M.S.N.F 4.0 L B S . / S Q . IN. 2500 2500 + 500 5000 5000 +500 2.0 0 0 100 200 MAGNESIUM-ION F igure 7. T h e i n f l u e n c e of c o n c e n t r a t i o n on m i l k of v a r y i n g cipitation (T rial 300 400 500 CONCENTRATION (RP.M.) i n c r e a s i n g a m o u n t s of m a g n e s i u m - i o n the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d c o m p o s itio n , as m e a s u r e d by a c id p r e ­ 1, J a n u a r y ) . 4.0 2.0 4.0 HCL (ML.) 5 % M.S.N.F. N/100 2.0 2 % M.S.N.F. 0 4.0 0 % M.S.N.F. 5000 + 500 5000 2.0 L B S ./S Q .IN . ^ -2 5 0 0 L B S ./S Q .IN . L B S . / S Q . IN. 2500 + 500 L B S ./S Q .IN . f ~ 0 L B S . / S Q . IN. 0 0 100 200 MAGNESIUM - ION F igure 8. T h e i n f l u e n c e of c o n c e n t r a t i o n on m i l k of v a r y i n g cipitation (T rial 300 400 500 CONCENTRATION (P.P.M.) i n c r e a s i n g a m o u n t s of m a g n e s i u m - ion the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d c o m p o s itio n , as m e a s u r e d by a c id p r e ­ 2, M a r c h ) . 4 .0 (ML.) 2.0 4.0 HCL 4U L B S . / S Q . IN. 2.0 2500 N/100 2500 + 500 5000 5000 +500 0 % M.S.N.F 4.0 2.0 0 Figure 9. 400 500 300 200 MAGNESIUM-ION CONCENTRATION (RRM.) 100 T h e i n f l u e n c e of c o n c e n t r a t i o n on m i l k of v a r y i n g cipitation (T rial i n c r e a s i n g a m o u n t s of m a g n e s i u m - i o n the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d c o m p o s i t i o n , a s m e a s u r e d by a c i d p r e ­ 3, M a y ) . 5% M.S.N.F • • • 4.0 2.0 N/100 HCL (ML.) 0 60 2% M.S.N.F. 4.0 2.0 0 ----------1 1 t 6.0 — 4.0 — 0 r L B S ./S Q .IN . ^ 2500 — 2500 + 500 M " " n 5000 ■■ H i . . . — . 5 0 0 0 + 5 0 0 *• « •> »• « «i » 2.0 0 J_ _ _ _ _ _ _ _ _ _ I_ _ _ _ _ _ _ _ _ _ I 0 100 CITRATE-ION F igure 200 300 400 500 CONCENTRATION (P.P.M.) 10. T h e i n f l u e n c e of i n c r e a s i n g a m o u n t s of c i t r a t e - ^ o n c o n ­ c e n t r a t i o n ^n the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a s u r e d by a c i d p r e ­ cipitation (T rial 1, J a n u a r y ) . 60 4.0 i 6.0 : 2 % M.S.N.F. 4.0 ••• HCL (ML.) 2.0 N/100 2.0 6.0 0 % M.S.N.F. 4.0 ^ -5 0 0 0 L B S . / S Q . IN. 5 0 0 0 + 5 0 0 L B S . / S Q . IN. 2.0 < - 2 5 0 0 + 5 0 0 L B S ./ S Q .I N . — 2 5 0 0 L B S . / S Q . IN. 0 100 200 CITRATE - I O N F igure f ~ 0 L B S . / S Q . IN._________ 300 400 500 CONCENTRATION (P.P.M.) 11. T h e i n f l u e n c e of i n c r e a s i n g a m o u n t s of c i t r a t e - i o n c o n ­ c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a s u r e d b y a c i d p r e ­ c i p i t a t i o n ( T r i a l 2, M a r c h ) . 6.0 5 % M.S.N.F. 4.0 L B S . / S Q . IN 2500 2.0 2500 + 500 5000 5000 + 500 0 (ML.) 6.0 N/100 HCL 4.0 2.0 2 % M.S.N.F. 0 6.0 4.0 2.0 0 % M.S.N.F. 0 0 100 CIT R A T E -IO N F igure 200 300 400 500 CONCENTRATION (P.P.M.) 12. T h e i n f l u e n c e of i n c r e a s i n g a m o u n t s of c i t r a t e - i o n c o n ­ c e n t r a t i o n on the s t a b i l i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k of v a r y i n g c o m p o s i t i o n , a s m e a s u r e d b y a c i d p r e ­ c i p i t a t i o n ( T r i a l 3, M ay ). D iscussion T h e s t u d i e s u n d e r t a k e n in the f i r s t s e c t i o n of t h i s i n v e s t i g a t i o n were d e s i g n e d to g a t h e r i n f o r m a t i o n on s o m e of th e p h y s i c a l a n d c h e m ­ i c a l p r o p e r t i e s of h a l f - a n d - h a l f h o m o g e n i z e d m i l k . were F a c to rs considered (1) t h e e f f e c t of v a r y i n g th e c o m p o s i t i o n t h r o u g h the a d d i t i o n of low -heat, s p ra y -p ro c e s s s k i m m i l k p o w d e r , a n d (2) n o t i n g w h e t h e r h o m o g e n iz a tio n w a s a s ig n ific a n t f a c t o r in a p ro d u c t te stin g 10 to 11 p e rc e n t fat. These cent fat. s e r i e s of h a l f - a n d - h a l f w e r e s t a n d a r d i z e d to 10.5 p e r ­ T h i s w a s i n a c c o r d a n c e w i t h the l a w s of the S t a t e of M i c h ­ i g a n (1 9 5 2 ) , w h i c h s p e c i f i e d t h a t the a m o u n t of b u t t e r f a t s h o u l d n o t be l e s s t h a n 10 p e r c e n t . A s the p u r p o s e of i n t r o d u c i n g h a l f - a n d - h a l f w a s to p r e s e n t to th e p u b l i c a l o w - f a t , c r e a m - l i k e p r o d u c t , th e p e r ­ c e n t a g e of f a t w a s r e d u c e d to 10.5 p e r c e n t , t h e r e b y r e t a i n i n g a s a f e t y m a r g i n of 0 .5 p e r c e n t . V iscosity. In the o p i n i o n of t h i s i n v e s t i g a t o r , the m o s t s i g ­ n i f i c a n t f a c t n o t e d i n t h i s s t u d y w a s th e r e l a t i v e l y s m a l l i n c r e a s e in v i s c o s i t y , r e g a r d l e s s of h o m o g e n i z a t i o n o r a d d i t i o n s of n o n f a t d r y m ilk solids. N e v e r t h e l e s s , the e f f e c t of a d d i n g s k i m m i l k p o w d e r d i d i n c r e a s e th e v i s c o s i t y of the h a l f - a n d - h a l f s l i g h t l y . This in c re a s e 77 w a s c o n s i s t e n t , w h e t h e r the p r o d u c t w as h o m o g e n iz e d o r n o n h o m o g ­ enized. T h e s e o b s e r v a t i o n s w e r e e s s e n t i a l l y in a g r e e m e n t w ith those of T r o u t a n d W e i n s t e i n (1952) a n d B a b c o c k (1 931 ). H ow ever, these i n v e s t i g a t o r s , b y a d d i n g d i f f e r e n t t y p e s of n o n f a t d r y m i l k s o l i d s , did o b ta in a g r e a t e r i n c r e a s e in v is c o s i ty . O f c o n s i d e r a b l e i m p o r t a n c e a l s o w a s t h e c o n t r a s t i n g e f f e c t of s i n g l e - v e r s u s d o u b l e - s t a g e h o m o g e n i z a t i o n on th e h a l f - a n d - h a l f hom ogenized m ilk. T h e g r e a t e s t i n c r e a s e in v i s c o s i t y o c c u r r e d a s th e r e s u l t of s i n g l e - s t a g e h o m o g e n i z a t i o n . W h e n th e p r e s s u r e w a s i n c r e a s e d to 5 , 0 0 0 p o u n d s p e r s q u a r e i n c h , t h e i n c r e a s e i n v i s c o s i t y w a s e r r a t i c , t h e g r e a t e s t i n c r e a s e b e i n g 11.2 c e n t i p o i s e s . when a p r e s s u r e How ever, of 500 p o u n d s p e r s q u a r e i n c h w a s p l a c e d on the s e c o n d v a l v e , the g r e a t e s t i n c r e a s e w a s o nly 1.5 c e n t i p o i s e s . A l­ t h o u g h t h e v i s c o s i t y r e a d i n g s w e r e h i g h e r w h e n the d e t e r m i n a t i o n s w e r e m a d e a t 3° C . , no s i g n i f i c a n t d i f f e r e n c e in t h e i n c r e a s e i n v i s c o s i t y a s a r e s u l t of p r o c e s s i n g c o u l d b e d i s t i n g u i s h e d f r o m t h o s e readings r e c o r d e d a t 20° C. I n f o r m a t i o n c o n c e r n i n g v i s c o s i t i e s of h a l f - a n d - h a l f - t y p e p r o d ­ ucts a re som ewhat scarce. H o w e v e r , t h e s e r e s u l t s a r e in r e a s o n a b l e a g r e e m e n t w i t h t h o s e of T r o u t a n d W e i n s t e i n (1952), who h a d m a d e i n v e s t i g a t i o n s on s i m i l a r t y p e s of p r o d u c t s . Using m ilk te s tin g 3.7 78 to 4.1 p e r c e n t , D e m o t t (1954) o b t a i n e d a m a x i m u m v i s c o s i t y of 4 .3 c e n t i p o i s e s a t 6, 000 + 500 p o u n d s p e r s q u a r e i n c h p r e s s u r e w h e n u s i n g the s a m e h o m o g e n i z e r a s w a s u s e d i n t h i s s t u d y . In m i l k t e s t i n g up to 6 p e r c e n t f a t , B r u n n e r (19 52), u s i n g a h o m o g e n i z a t i o n p r e s s u r e of 3,500 p o u n d s p e r s q u a r e i n c h , o b t a i n e d v i s c o s i t y v a l u e s up to 6.2 centipoises. H e n c e , in t h i s l a t t e r c a s e , a l t h o u g h u s i n g a d i f f e r e n t h o m o g e n i z e r , th e r e s u l t s w e r e c l o s e to s o m e of th e r e a d i n g s o b s e r v e d in this in v e stig a tio n . T h e g e n e r a l c o n s e n s u s of o p i n i o n a m o n g i n v e s t i g a t o r s (D o a n , 1938; B a t e m a n a n d S h a r p , 1928; a n d T r o u t , H a l l o r a n , a n d G o u l d , 1935) i s t h a t , a l t h o u g h t h e r e m a y b e a s l i g h t i n c r e a s e in the v i s c o s i t y of m i l k a s a r e s u l t of h o m o g e n i z a t i o n , t h i s i n c r e a s e c o u l d n o t be c o n ­ s id e r e d as significant. T h e r e i s , h o w e v e r , s o m e d i s a g r e e m e n t a s to the p e r c e n t a g e of f a t n e c e s s a r y in o r d e r f o r c l u m p i n g to b e p r o d u c e d up on h o m o g e n i z a t i o n . D o a n a n d Swope (1927) s u g g e s t e d f r o m 4 to 6 p e r c e n t f a t; w h e r e a s D o a n (1927) b e l i e v e d 6 to 8 p e r c e n t f a t to be the c r i t i c a l p e r c e n t a g e to b r i n g a b o u t c l u m p i n g . M i n s t e r (1931) L a t e r , Doan and show ed that high p r e h e a tin g t e m p e r a t u r e s would e l i m ­ i n a t e c l u m p i n g w h e n t h e p r o d u c t c o n t a i n e d l e s s t h a n 7- p e r c e n t f a t . W h e n h o m o g e n i z i n g a t a t e m p e r a t u r e of 37° C. (100° F .), D o a n (1929) s u g g e s t e d t h a t the t h r e s h o l d p o i n t of c l u m p i n g o c c u r r e d 79 when a p ro d u c t containing f r o m 10.5 to 10 .6 p e r c e n t f a t w a s h o m o g ­ e n i z e d a t 3,5 00 p o u n d s p e r s q u a r e i n c h p r e s s u r e . U s i n g th e s a m e p r o c e s s i n g t e c h n i q u e , D o a n c o n s i d e r e d th e c r i t i c a l r a t i o of p l a s m a s o l i d s to f a t to b e 0 . 6 0 to 0 .8 5 w h e n th e f a t c o n c e n t r a t i o n w a s f r o m 8 to 18 p e r c e n t . T h e r a t i o s of p l a s m a s o l i d s to f a t u s e d i n t h i s 0 . 8 3 , 0 . 9 6 , a n d 1.2 f o r S e r i e s study w e r e 1, 2, a n d 3, r e s p e c t i v e l y . U n d e r the c o n d i t i o n s of t h i s i n v e s t i g a t i o n , w h e r e the h o m o g e n i z i n g t e m p e r a t u r e w a s 150° F . , s o m e f a t c l u m p i n g w a s e v i d e n t , e s p e c i a l l y i n th o s e s a m p l e s h o m o g e n i z e d a t 2,500 a n d 5,000 p o u n d s . ing in t h e s e l a t t e r c a s e s w a s r e l a t i v e l y l i g h t . H o w e v e r, the c l u m p ­ T h e re fo re , although a good h o m o g e n iz in g t e m p e r a t u r e w a s u s e d , and the p l a s m a s o l i d s - t o f a t r a t i o w a s a b o v e t h a t of D o a n ’s (1 9 2 9 a ), f a t c l u m p i n g w a s e v i d e n t at a p re s s u re of 2, 5 0 0 p o u n d s p e r s q u a r e i n c h . D o a n (1931a) a l s o t h o u g h t t h a t f a t c l u m p i n g c o u l d be r e d u c e d b y i n c r e a s i n g the a m o u n t of p l a s m a s o l i d s in h o m o g e n i z e d c r e a m , t h u s l o w e r i n g th e v i s c o s i t y . The d a t a p r e s e n t e d in t h i s s t u d y , h o w ­ e v e r , do n o t s h o w a n y d e c r e a s e i n v i s c o s i t y a s a r e s u l t of a d d i n g nonfat d ry m ilk solids. H e n c e , i t m i g h t b e a s s u m e d t h a t th e s a m p l e s in t h e f i r s t s e r i e s , w h e r e no s o l i d s w e r e a d d e d , h a d a s u f f i c i e n t a m o u n t of s o l i d s in th e p l a s m a f o r a d s o r p t i o n o nt o the n e w s u r f a c e s 80 of t h e h o m o g e n i z e d f a t g l o b u l e s . P r o b a b l y the m a i n e f f e c t of a d d i n g n o n f a t d r y m i l k s o l i d s to h a l f - a n d - h a l f w a s a s l i g h t i n c r e a s e i n the v i s c o s i t y due to t h e p r e s e n c e of a d d i t i o n a l s o l i d s i n t h e p l a s m a p h a s e . A s e v id e n c e d by the s m a l l d e g r e e of c l u m p i n g , the t h r e e studied had r e la tiv e ly low v is c o s itie s . series T h e r e f o r e , a s m i g h t be e x ­ p e c t e d , l i t t l e i n c r e a s e in v i s c o s i t y o c c u r r e d w h e n t h e s a m p l e s w e r e stored fo r 7 days. T h i s w a s i n a c c o r d a n c e w i t h t h e v i e w s of S h e r ­ w o o d a n d S m a l l f i e l d (1 926 ), w ho n o t e d t h a t w h e n v i s c o s i t y i n c r e a s e d u po n s t o r a g e , the c l u m p s of f a t g l o b u l e s w e r e l a r g e r . I t s h o u l d be n o t e d t h a t t h e i n i t i a l d e t e r m i n a t i o n s w e r e m a d e a f t e r the s a m p l e s w ere stored for 6 hours. He at s t a b i l i t y . T h e m a x i m u m h e a t s t a b i l i t y w a s f o u n d to be in the n o n h o m o g e n i z e d c o n t r o l s a m p l e s of S e r i e s m ilk po w d er w as not added. 1, in w h i c h s k i m ­ As the s o l i d s - n o t - f a t c ontent w as i n ­ c r e a s e d b y th e a d d i t i o n of t h i s p o w d e r ( S e r i e s 2 a n d 3), th e s t a b i l i t y of the n o n h o m o g e n i z e d s a m p l e s d e c r e a s e d . This d e s ta b iliz in g effect of s k i m m i l k p o w d e r o c c u r r e d a l s o in the h o m o g e n i z e d s a m p l e s . H o w e v e r , t h e a d d i t i o n of s k i m m i l k p o w d e r to the s a m p l e s h o m o g e n i z e d at a p re s su re of 5 ,000 p o u n d s p e r d e c r e a s e in the h e a t s t a b i l i t y . s q u a r e i n c h p r o d u c e d no f u r t h e r 81 In a l l c a s e s , h o m o g e n i z a t i o n of h a l f - a n d - h a l f d e c r e a s e d the heat stability. A d i s t i n c t d i f f e r e n c e c o u l d b e d e t e c t e d b e t w e e n the s t a b i l i t i e s of t h o s e stage. s a m p l e s p r o c e s s e d w ith a single an d w ith a double H o w e v e r , o n c e the s o l i d s h a d b e e n a d d e d , a n d t h e p r e s s u r e of 2,500 p o u n d s p e r s q u a r e i n c h w a s r e a c h e d , n o f u r t h e r d e c r e a s e in h e a t s ta b ility w as n o tic e a b le w ith i n c r e a s e d p r e s s u r e s . In a d d i t i o n , o n c e the s a m p l e s , w i t h the a d d e d s o l i d s , w e r e h o m o g e n i z e d a t p r e s ­ s u r e s of 2 ,500 + 500 p o u n d s p e r s q u a r e i n c h , a n y i n c r e a s e i n p r e s s u r e on th e f i r s t v a l v e d i d n o t p r o d u c e a n y f u r t h e r d e s t a b i l i z a t i o n . T h e h e a t s t a b i l i t y of th e n o n h o m o g e n i z e d p r o d u c t s to w h i c h no p o w d e r w a s a d d e d (97 m i n u t e s ) a p p e a r e d to b e w i t h i n r e a s o n a b l e a g r e e m e n t w i t h t h a t f o u n d on m i x e d - h e r d m i l k , w h e r e the h e a t s t a ­ bility v a rie d f r o m (O sborne, 195 3). 50 to 120 m i n u t e s , d e p e n d i n g up on the t i m e of y e a r D e m o t t (1 954) , i n d e t e r m i n i n g the e f f e c t of d u a l ­ h o m o g e n i z a t i o n o n m i l k , f o u n d t h a t b y u s i n g p r e s s u r e s up to 6,000 + 500 p o u n d s p e r s q u a r e i n c h , t h e h e a t s t a b i l i t y r a n g e d f r o m 154 m i n u t e s f o r p e rc e n t m ilk. 128 to 3.7 p e r c e n t m i l k , a n d 98 to 128 m i n u t e s f o r 4.1 In b o t h c a s e s , p r e s s u r e s a ffe c t the r e s u l t s significantly. of h o m o g e n i z a t i o n did n o t W e b b a n d H o l m (1928), who h a d p r e v i o u s l y r e p o r t e d d a t a f o r 5- a n d 1 0 - p e r c e n t p r o d u c t s , i n d i c a t e d th a t, while s o m e r e d u c t i o n i n h e a t s t a b i l i t y d id o c c u r in m i l k a s a 82 r e s u l t of h o m o g e n i z a t i o n , a g r e a t e r d e c r e a s e w a s f o u n d w h e n the f a t c o n t e n t w a s i n c r e a s e d to 10 p e r c e n t . H e n c e , the r e l a t i v e l y s m a l l d e c r e a s e i n h e a t s t a b i l i t y t h a t o c c u r r e d in S e r i e s 1 a s a r e s u l t of h o m o g e n i z a t i o n a p p e a r e d to be in r e a s o n a b l e a g r e e m e n t w i t h t h e v a l u e s o b t a i n e d f o r b o t h m i l k a n d th e 10- p e r c e n t - f a t p r o d u c t . Th e l o w e r i n g in h e a t s t a b i l i t y of p r o d u c t s c o n t a i n i n g a b n o r m a l l y l a r g e a m o u n t s of m i l k - s o l i d s - n o t - f a t w a s b o r n e out b y the r e s u l t s of W ebb (1931) a n d W h i t t i e r a n d W ebb (1950), alth o u g h the fin d in g s in th is in v e s tig a ti o n w e r e not in a g r e e m e n t w ith t h o s e of D o a n a n d M i n s t e r (1930). A s y e t , the r e a s o n w hy h o m o g e n i z e d p r o d u c t s h a v e a l o w e r h e a t s t a b i l i t y a p p e a r s to be n o t f u l l y u n d e r s t o o d . This in stab ility h a s b e e n e x p l a i n e d b y D o a n (1 9 3 1 a ) , T r a c y a n d R u e h e (19 30b) , a n d W e b b a n d H o l m (1928) a s due to t h e c l u m p i n g of th e f a t i n h o m o g ­ enized c r e a m . provide W e b b (1931) f e l t t h a t the f a t c l u m p s i n c r e a m c o u l d a su ita b le n u c le i a ro u n d w hich flo c c u latio n could p r o c e e d . D o a n (193 1a ) b e l i e v e d t h a t t h e c a s e i n a d s o r b e d on the f a t g l o b u l e s , a s a r e s u l t of h o m o g e n i z a t i o n , w o u l d l o s e i t s m o b i l i t y a n d h e n c e be m o r e s u s c e p t i b l e to a g e n t s of c o a g u l a t i o n . If t h i s w e r e t r u e , he p o s tu la te d , then the fa t c lu m p s could b rin g such e a s ily p re c ip ita te d c a s e i n into in tim a te c o n tac t, thus p r e s u m a b ly favoring coagulation. 83 T h a t the m e m b r a n e - p r o t e i n of the h o m o g e n i z e d f a t g l o b u l e s i s d i f f e r e n t f r o m th e n o n h o m o g e n i z e d m a t e r i a l h a s b e e n i n d i c a t e d byB r u n n e r , D u n c a n , a n d T r o u t (1952) a n d B r u n n e r , D u n c a n , T r o u t , a n d M a c K e n z i e (1 9 5 3 ) . In a d d i t i o n , 1he r e s u l t s of t h e i r w o r k s h o w t h a t th is m e m b r a n e - p r o t e i n m a t e r i a l on the h o m o g e n iz e d f a t globule dif­ f e r e d m a r k e d l y f r o m a n y of the r e c o g n i z e d p r o t e i n s . In l o w - f a t p r o d u c t s , w h e r e c l u m p i n g i s n o t so p r o n o u n c e d , o th e r f a c t o r s m a y be involved. A s a r e s u l t of h o m o g e n i z a t i o n , s o m e of the s e r u m p r o t e i n b e c o m e s a d s o r b e d onto the n e w f a t s u r f a c e s ( B r u n n e r , L i l l e v i k , T r o u t , a n d D u n c a n , 1953), t h e r e b y l e a v i n g a h i g h p r o p o r t i o n of s a l t s to th e p r o t e i n r e m a i n i n g i n t h e s e r u m . If t h i s l e f t a h i g h e r p r o p o r t i o n of c a l c i u m i n the s e r u m , t h e n i n s t a b i l i t y would r e s u l t . T his in s ta b ility would in c r e a s e as hom o g e n iz atio n becam e m o re efficient. T itra ta b le a c id ity , p H , specific g r a v ity , and o rg a n o le p tic p r o p e r t i e s of h a l f - a n d - h a l f . The a d d i t i o n of s k i m m i l k p o w d e r r e s u l t e d in a d e f i n i t e i n c r e a s e in t i t r a t a b l e a c i d i t y . When 5 - p e r c e n t solids w e r e a d d e d , t h e c a l c u l a t e d i n c r e a s e in a c i d i t y f o r e a c h p e r c e n t w a s 0.0189 p e r c e n t la c tic a cid. T h e r e w a s no s i g n i f i c a n t c h a n g e i n p H a s a r e s u l t of t h e s e a d d i t i o n s . The s o l i d s - n o t - f a t , p r e s e n t in m i l k , c o n ­ t a i n s the c o n s t i t u e n t s w h i c h a r e r e s p o n s i b l e f o r the a p p a r e n t a c i d i t y 84 of m i l k ( S o m m e r , 1952). H e n c e , the a d d i t i o n of n o n f a t d r y m i l k s o l i d s to h a l f - a n d - h a l f w o u l d i n c r e a s e the t i t r a t a b l e a c i d i t y . H ow ever, as t h e s e c o n s t i t u e n t s h a v e a l o w i o n i z a t i o n c o n s t a n t , l i t t l e e f f e c t on the pH would be e x p e c te d by adding s k i m m ilk p o w d e r. T h e i n c r e a s e in s p e c i f i c g r a v i t y w a s in p r o p o r t i o n to th e a m o u n t of n o n f a t d r y m i l k s o l i d s a d d e d . The m a x i m u m s p e c if ic g r a v i t y a l l o w e d i n h a l f - a n d - h a l f h o m o g e n i z e d m i l k in M i c h i g a n is 1.0 32 ( M i c h i g a n D e p a r t m e n t of A g r i c u l t u r e , 1952). ing to th e r e s u l t s of t h e s e t r i a l s Therefore, accord­ ( T a b l e 5), w i t h h a l f - a n d - h a l f c o n ­ t a i n i n g b e t w e e n 10 a n d 11 p e r c e n t f a t , t h e m a x i m u m a m o u n t of p o w d e r th a t m ig h t be a d d e d le g a l ly w ould be a p p r o x im a te ly 2 p e r ­ cent, f o r at this varied fro m s o l i d s - n o t - f a t to f a t r a t i o , th e s p e c i f i c g r a v i t y 1.0 302 to 1.0 3 3 0 . W h e n c o n s i d e r i n g the p r e f e r e n c e f o r th e p r o d u c t s of t h e t h r e e s e r i e s of h a l f - a n d - h a l f b a s e d o n t h e i r o r g a n o l e p t i c p r o p e r t i e s , the s e r i e s w h i c h c o n t a i n e d n o a d d e d i n c r e m e n t s of m i l k - s o l i d s - n o t - f a t w a s o m i t t e d f r o m f u r t h e r c o n s i d e r a t i o n due to the t h i n b o d y , a n d l a c k of f l a v o r . salvy c r e a m plug, T h e a d d i t i o n of s k i m m i l k p o w d e r y i e l d e d a h e a v i e r b o d y a n d e n h a n c e d the f l a v o r of the p r o d u c t . However, when 5 p e r c e n t s k i m m i l k p o w d e r w a s a d d e d , a l t h o u g h the b o d y w a s c o n s i d e r e d a s b e i n g v e r y d e s i r a b l e , the f l a v o r w a s 85 u n d e sira b le , being d istin ctly salty. C onsequently, f o r d e s ir a b le body and f la v o r , the s e r i e s containing 2 p e r c e n t added s k im m ilk po w d er was p re f e r r e d . T h e s t a b i l i t y of h a l f - a n d - h a l f i n c o f f e e . The m o s t u n e x p e c te d f a c t n o t e d w a s t h a t h a l f - a n d - h a l f d i d n o t f e a t h e r in c o f f e e m a d e w i t h h a r d , t a p w a t e r , w h i l e i n s o m e c a s e s f e a t h e r i n g d i d o c c u r in c offe e m ade with d istilled w a ter. W i t h t h i s p r o b l e m of f e a t h e i n g , t h e p H of th e b r e w a p p e a r s to b e of c o n s i d e r a b l e i m p o r t a n c e . Investigators s u c h a s R u e h e (1948b) a n d T r a c y a n d C o r b e t t (1938) w e r e c o n c e r n e d w i t h the m e t h o d s of p r e p a r a t i o n a n d th e u s e of d i f f e r e n t t y p e s of c o f f e e on the p H of the brew . T h e ir opinion w as th a t b r e w s w ith h ig h e r a c id itie s , or lo w e r p H v a l u e s , w o u l d h a v e a g r e a t e r t e n d e n c y to f e a t h e r . should be n o te d in this The p o i n t t h a t s t u d y w a s t h a t th e p H of the b r e w s m a d e f r o m h a r d w a t e r w e r e m u c h h ig h e r than those m a d e with d is tille d w a te r . The l a c k of a n y b u f f e r i n g a c t i o n of th e s a l t s , w h i c h a r e p r e s e n t in h a r d w a t e r , m a y be one e x p l a n a t i o n f o r t h e d e c r e a s e d p H in the b re w s m ade w ith d is tille d w a te r. T h e f a c t t h a t no f e a t h e r i n g o c c u r r e d i n c o f f e e m a d e w i t h h a r d w a t e r n e e d n o t s e e m too s t a r t l i n g . P r e v i o u s l y D o a n (1931b) w a s a b l e o n l y i n i s o l a t e d c a s e s to a t t r i b u t e f e a t h e r i n g to the h a r d w a t e r u s e d 86 in th e c o f f e e m a k i n g . S o m m e r (1929b) a g r e e d b y s t a t i n g t h a t if a l l o th e r p r o c e s s in g techniques w ere sa tisfa c to ry and feath erin g still r e ­ s u l t e d , t h e n the c a u s e w a s a l m o s t c e r t a i n to b e i n the h a r d n e s s of the w a t e r a n d t h e s a l t c o n t e n t of th e c r e a m . O ther investigators ( T r a c y a n d R u e h e , 1930a; B u r g w a l d , 1923; a n d A t h e r t o n a n d B r a d f i e l d , 1952) t e n d e d to a g r e e t h a t , a l t h o u g h th e h a r d n e s s of w a t e r m i g h t b e a f a c t o r in t h e f e a t h e r i n g of c r e a m i n c o f f e e , o t h e r f a c t o r s s u c h a s q u a l i t y of i n g r e d i e n t s i n t h e c r e a m , o c c u r r e n c e of f a t c l u m p ­ in g a s a r e s u l t of h o m o g e n i z a t i o n , a n d p H of the b r e w w e r e c o n s i d ­ e r e d to b e of g r e a t e r i m p o r t a n c e . T h e i n f l u e n c e of a d d e d n o n f a t d r y m i l k s o l i d s a n d p r e s s u r e s of h o m o g e n i z a t i o n on t h e s t a b i l i t y of h a l f - a n d - h a l f . T h e r e a s o n w hy a c i d w a s n o t r e q u i r e d to b r i n g a b o u t p r e c i p i t a t i o n of the n o n h o m o g ­ e n i z e d s a m p l e s , w h e n m i x e d w i t h d i s t i l l e d w a t e r , i s o p e n to c o n s i d e r ­ able s p e c u la tio n . It is r a t h e r im p ro b a b le that this p re c ip ita tio n was due to c h u r n i n g , s i n c e the a g i t a t i o n w a s l i m i t e d b e f o r e th e o b s e r v a ­ tion w as m ad e (the t u b e s w e r e i n v e r t e d o n l y f i v e t i m e s ) . F u rth er­ m o r e , the r e p o r t of P a l m e r a n d W i e s e (1 932 ), t h a t the l a y e r of adsorbed m a te ria l was cream s so f i r m l y h e l d a t the f a t s u r f a c e t h a t r i c h could be r e p e a t e d l y d ilu ted w ith d is tille d w a t e r and 87 r e s e p a r a t e d w i t h o u t b r e a k i n g the e m u l s i o n , w o u l d l e n d s u p p o r t a g a i n s t c h u r n i n g a s the p o s s i b l e c a u s e of p r e c i p i t a t i o n . O n e c o n j e c t u r e i n f a v o r of d i l u t i o n a s c o n t r i b u t i n g to p r e c i p i ­ t a t i o n i s t h a t of the d i l u t i o n of the s a l t s t h e m s e l v e s . As th e p r o t e i n r e q u i r e s a n o p t i m u m q u a n t i t y of s a l t s ( S o m m e r a n d H a r t , 1919; T r a c y a n d R u e h e , 193 0 b ), th e e f f e c t of d i l u t i o n w o u l d b e a d e s t a b i l i z i n g f a c t o r , du e to th e r e m o v a l of s o m e of th e s a l t s f r o m the p r o t e i n . T h i s d e s t a b i l i z a t i o n o c c u r r e d e v e n w h e n the v a r i o u s c o n c e n t r a t i o n s of th e t h r e e i o n s r c a l c i u m , m a g n e s i u m , a n d c i t r a t e , w e r e u s e d a s a diluent. H e n c e , i t m i g h t b e s u p p o s e d t h a t a c o n c e n t r a t i o n of 500 p a r t s p e r m i l l i o n of t h e s e i o n s w a s n o t a d e q u a t e f o r p r o p e r s t a b i l i z a t i o n . T h e a d d i t i o n of a c i d w a s n e c e s s a r y to p r o d u c e p r e c i p i t a t i o n in a l l the h o m o g e n i z e d s a m p l e s . A p o s s i b l e e x p l a n a t i o n of t h i s e f ­ f e c t m i g h t b e i n the f o r m a t i o n of n e w m e m b r a n e - p r o t e i n l a y e r s a s a r e s u l t of h o m o g e n i z a t i o n . T h i s p r o t e i n c o m p o n e n t of the f a t g l o b u l e m e m b r a n e of h o m o g e n i z e d m i l k w a s a l t e r e d due to th e a d d i t i o n of m i l k p l a s m a p r o t e i n s to th e n o r m a l c o m p l e m e n t of the m e m b r a n e p r o t e i n c o m p l e x ( B r u n n e r , D u n c a n , T r o u t , a n d M a c K e n z i e , 195 3; B ru n n e r, L illevik, T rout, and Duncan, 1953). D o a n (1931a) h a d p r e v io u s ly d e s c r ib e d this a l t e r e d - m e m b r a n e p ro te in as having lo st its m o b ilit y . T h e r e f o r e , u n d e r the c o n d i t i o n s of t h i s e x p e r i m e n t , 88 the a l t e r e d m e m b r a n e m i g h t p o s s i b l y be m o r e r e s i s t a n t to t h i s d i l u ­ tion effect. W h e n s k i m m i l k p o w d e r w a s a d d e d to h a l f - a n d - h a l f , m o r e a c i d w a s n e c e s s a r y to b r i n g a b o u t p r e c i p i t a t i o n ( T a b l e s 7, 8, a n d 9). This a m o u n t of a c i d i n c r e a s e d a s a d d i t i o n a l i n c r e m e n t s of the p o w d e r w e r e a d d e d to th e h a l f - a n d - h a l f . T h e s e a d d i t i o n s of s k i m m i l k p o w d e r w o u l d i n c r e a s e t h e c o n c e n t r a t i o n of the p l a s m a s o l i d s a n d h e n c e p r o v i d e a b u f f e r i n g a c t i o n , n o t o n l y f o r th e v a r i o u s i o n s , b u t a l s o a g a i n s t the a d d i t i o n s of a c i d . C o n se q u e n tly , the a s s u m p t i o n m ig h t be m a d e th a t a s m o r e n o n f a t d r y m i l k s o l i d s w e r e a d d e d to t h e s a m p l e s , a s i n S e r i e s 2 a n d 3, m o r e a c i d w o u l d b e r e q u i r e d to p r e c i p i t a t e the p r o ­ tein. In a l l t h e t r i a l s c a r r i e d o u t , t h i s w a s s h o w n to b e the c a s e . T h e r e f o r e , i t m a y be s a i d t h a t w h e n l o w - h e a t , s p r a y - p r o c e s s skim ­ m i l k p o w d e r i s a d d e d to h a l f - a n d - h a l f , i n c r e a s e d s t a b i l i t y of the p r o d ­ uct is in d ic a te d . W i t h the e x c e p t i o n of t h e M a y t r i a l , m o r e a c i d w a s r e q u i r e d to p r o d u c e p r e c i p i t a t i o n w h e n t h e h i g h e r p r e s s u r e s w ere used. of h o m o g e n i z a t i o n T h i s w a s e s p e c i a l l y n o t i c e a b l e in the s e r i e s w h e r e i n 2 and 5 p e r c e n t s k im m ilk p o w d e r w e re added. One p o s s i b l e e x p l a n a ­ t i o n of t h i s a p p a r e n t i n c r e a s e i n s t a b i l i t y m i g h t be a s s o c i a t e d w i t h th e f a t - g l o b u l e m e m b r a n e a s p r e v i o u s l y e x p l a i n e d f o r the d i f f e r e n c e 89 of s t a b i l i t y b e t w e e n h o m o g e n i z e d a n d n o n h o m o g e n i z e d s a m p l e s i n d i s ­ tilled w a te r. In t h i s i n s t a n c e , the i n c r e a s e i n p r e s s u r e w o u l d c r e a t e a g r e a t e r n u m b e r of f a t g l o b u l e s , w h i c h w o u l d r e s u l t i n a g r e a t e r a m o u n t of p r o t e i n b e i n g i n c o r p o r a t e d i n th e f a t - m e m b r a n e l a y e r . T h e r e f o r e , t h i s g r e a t e r p r o p o r t i o n of a l t e r e d p r o t e i n c o n c e i v a b l y c o u l d r e q u i r e m o r e a c i d f o r p r e c i p i t a t i o n t h a n t h a t of th e s a m p l e s h o m o g e n i z e d a t th e l o w e r p r e s s u r e s . L i t t l e d i f f e r e n c e w a s n o t e d in a c i d s t a b i l i t y of the s a m p l e s s u b j e c t e d to s i n g l e - a n d to d o u b l e - s t a g e h o m o g e n i z a t i o n . H o w e v e r , on c l o s e o b s e r v a t i o n of th e t r i a l s c o n d u c t e d i n J a n u a r y a n d M a r c h , the p r o d u c t s h o m o g e n i z e d a t p r e s s u r e s of 5 ,00 0 + 500 p o u n d s p e r s q u a r e i n c h g e n e r a l l y w e r e f o u n d to b e s l i g h t l y m o r e sam ples. s t a b l e t h a n the o t h e r No d i f f e r e n c e c o u l d b e d i s t i n g u i s h e d b e t w e e n th e two s a m ­ p l e s h o m o g e n i z e d a t th e l o w e r p r e s s u r e s of 2,500 a n d 2,500 + 500 pounds p e r sq u are inch. T h e a b o v e r e s u l t s m i g h t s e e m c o n t r a r y to t h o s e w h i c h h a v e b e e n p r e v i o u s l y r e p o r t e d b o t h i n t h i s a n d i n o th r investigations ( D o a n , 19 31 a ; W e b b , 1931). high-fat p roducts w ere T h e r e i n t h e s t a b i l i t y of re p o r te d as having b een d e c r e a s e d m a rk e d ly a s a r e s u l t of s i n g l e - s t a g e h o m o g e n i z a t i o n . The d e c r e a s e in s t a b i l i t y w a s a t t r i b u t e d to t h e c l u m p i n g of f a t g l o b u l e s . If, h o w e v e r , t h e s e c l u m p s w e r e b r o k e n up a s a r e s u l t of l o w - p r e s s u r e h o m o g e n i z a t i o n 90 o r a g i t a t i o n (D o a n , 1 9 2 9 a ) , a r e s u l t a n t i n c r e a s e in s t a b i l i t y o c c u r r e d . T h e r e f o r e , b e c a u s e of t h e d i l u t i o n a n d a g i t a t i o n f a c t o r s u s e d i n the p r o c e d u r e of t h i s part. s t u d y , t h e s e f a t c l u m p s w o u l d b e b r o k e ' up i n l a r g e C o n s e q u e n t l y , t h i s t e c h n i q u e w o u l d n o t s ho w a n y s i g n i f i c a n t d i f ­ f e r e n c e s in t h e s t a b i l i t i e s of the s i n g l e - a n d d u a l - h o m o g e n i z e d s a m p l e s . W h e n c o m p a r i n g th e e f f e c t s of th e c a l c i u m , m a g n e s i u m , a n d c i t r a t e i o n s on t h e s t a b i l i t y of the v a r i o u s t y p e s of h a l f - a n d - h a l f , som e definite tr e n d s w e re in d ic a ted . iliz e the p r o d u c t s . C a l c i u m i o n s t e n d e d to d e s t a b ­ T h e m a g n e s i u m i o n s , i n the c o n c e n t r a t i o n u s e d , a p p a r e n t l y d i d n o t a f f e c t th e s t a b i l i t y of th e s a m p l e s . On the o th e r h a n d , th e c i t r a t e i o n s t e n d e d to a c t a s a s t a b i l i z i n g i o n , f o r a s the io n i c c o n c e n t r a t i o n s w e r e i n c r e a s e d , m o r e a c i d w a s r e q u i r e d to produce a p re c ip ita te . These r e s u l t s w e r e in a g r e e m e n t w i t h t h o s e p r e v i o u s l y r e p o r t e d b y s u c h i n v e s t i g a t o r s a s D o a n (1 931b ), S e e k l e s and S m e e ts (1 947) , S o m m e r ( 1 9 2 9 b ) , a n d T r a c y a n d R u e h e (1930 b) , who n o t e d t h a t c a l c i u m i o n s h e l p e d d e s t a b i l i z e p r o d u c t s a n d the c i t r a t e s w ere used fo r stabilization p u rp o se s. Little in fo rm atio n has been r e p o r t e d o n th e s t a b i l i t y of p r o d u c t s , a s a f f e c t e d b y the m a g n e s i u m ion. Three sep arate tria ls w ere c o n d u c t e d i n t h i s p h a s e of the e x p e r i m e n t , th e f i r s t i n J a n u a r y , the s e c o n d in M a r c h , a n d the t h i r d 91 in M a y . Th e r e s u l t s o b t a i n e d f r o m th e f i r s t t r i a l , on t h e s e r i e s in w h i c h no s o l i d s w e r e a d d e d , i n d i c a t e d a n u n s t a b l e p r o d u c t ( F i g u r e s 4, 7, a n d 10). W h e th e r c a lc iu m , m a g n e s iu m , o r c itra te ions w e re added, v e ry little acid w as re q u ire d fo r p re c ip ita tio n . C o m p a rin g this sam e s e r i e s to T r i a l 2 ( F i g u r e s 5, 8, a n d 11) a n d T r i a l 3 ( F i g u r e s 6, 9, a n d 12), a g r a d u a l s t a b i l i z a t i o n of h a l f - a n d - h a l f w a s s e e n to o c c u r . T h i s w o u l d s e e m to a g r e e w i t h t h e w o r k of S o m m e r a n d H a r t (1926) a n d H u n z i k e r (1 949 ), w h o b e l i e v e d t h a t the c i t r i c a c i d c o n t e n t i n m i l k w a s i n c r e a s e d w h e n th e c o w s w e r e t u r n e d to g r e e n s p r i n g p a s t u r e . H o w e v e r , O s b o r n e (1953) s u g g e s t e d t h a t th e c i t r i c a c i d c o n t e n t of m i l k m i g h t i n c r e a s e o r d e c r e a s e in the s p r i n g , d e p e n d i n g on s u c h f a c t o r s a s th e c o w a n d t y p e of p a s t u r e . In a d d i t i o n , t h e s e r e s u l t s w e r e in a g r e e m e n t w i t h t h o s e of J a c o b s e n a n d W a l l i s (1939) a n d E l l e n b e r g e r , N ew lander, and Jones (19 50), w h o f o u n d t h e c a l c i u m a n d p h o s p h o r u s v a l u e s to b e l o w f o r m i l k d u r i n g the s u m m e r , b u t r e a c h i n g t h e i r h i g h e s t l e v e l i n t h e w i n t e r m o n t h s of N o v e m b e r t h r o u g h F e b r u a r y . In S e r i e s 2 a n d 3, w h e r e i n 2 a n d 5 p e r c e n t s k i m m i l k w e r e a d d e d , r e s p e c t i v e l y , t h i s d i f f e r e n c e i n s t a b i l i t y of the t h r e e t r i a l s was m inim ized. W hen 5 p e r c e n t p o w d e r w a s added, only a little d iff e r e n c e in the r e la tiv e s t a b i l i t i e s of the t h r e e t r i a l s w a s n o t e d , e s p e c i a l l y w h e n the s a m p l e s w e re h o m o g e n iz e d a t the h ig h e r p r e s ­ sures . In T r i a l 3, alth ou gh , t h e s a m p l e s w e r e h o m o g e n i z e d a t the u s u a l h i g h a n d l o w p r e s s u r e s , v e r y l i t t l e d i f f e r e n c e i n th e r e s u l t was o b serv ed . O ne p o s s i b l e e x p l a n a t i o n m a y b e t h a t a t t h e lo w e p r e s s u r e s , the n e c e s s a r y d i s p e r s i o n of the f a t g l o b u l e s to b r i n g a b o u t s t a b i l i t y a g a i n s t th e e f f e c t of a c i d w a s a t t a i n e d . S E C T I O N II 1. 2. THE E F F E C T O F DECREASING RATIOS O F S O L ID S -N O T -F A T TO F A T ON S O M E C H E M I C A L A N D P H Y S I C A L P R O P E R T I E S O F H A L F - A N D - H A L F AND C R E A M A N A L Y S I S O F F E A T H E R E D H A L F - A N D - H A L F AND C R E A M 93 Experim ental P rocedure T h e e f f e c t of d e c r e a s i n g r a t i o s of s o l i d s - n o t - f a t to^ f a t on s o m e c h e m i c a l a n d p h y s i c a l p r o p e r t i e s of h a l f - a n d - h a l f a n d c r e a m . F r e s h r a w m i l k w a s r e c e i v e d b y the M i c h i g a n S t a t e C o l l e g e C r e a m e r w h e r e i t w a s s e p a r a t e d a t 40° F . w i t h a c o l d - m i l k s e p a r a t o r . The r e s u l t a n t c r e a m a n d s k i m m i l k w e r e u t i l i z e d to s t a n d a r d i z e v a r i o u s s e r i e s of h a l f - a n d - h a l f a n d c r e a m s . was p r e p a r e d S eries A t o t a l of s i x d i f f e r e n t s e r i e s w ith e a c h v a r y in g in c o m p o s i tio n as follows: A. S k i m m i l k a n d c r e a m s t a n d a r d i z e d to 10.5 p e r c e n t fat. S eries B. Skim m ilk and c r e a m s t a n d a r d i z e d to 10.5 p e r c e n t f a t p l u s th e a d d i t i o n of 2 p e r c e n t l o w - h e a t , s p r a y - p r o c e s s skim m ilk pow der. S eries C. Skim m ilk and c r e a m s t a n d a r d i z e d to 10.5 p e r c e n t f a t p l u s th e a d d i t i o n of 5 p e r c e n t l o w - h e a t , s p r a y - p r o c e s s skim m ilk pow der. S e r i e s D. Skim m ilk and c r e a m s t a n d a r d i z e d to 13.0 p e r c e n t S eries E. S k i m m i l k a n d c r e a m s t a n d a r d i z e d to 18 p e r c e n t f a t S eries F. S k i m m i l k a n d c r e a m s t a n d a r d i z e d to 23 p e r c e n t f a t fat. 95 H e n c e , S e r i e s A to C w e r e p r e p a r e d i n the s a m e m a n n e r a s S eries 1, Z, a n d 3 i n S e c t i o n I, a n d S e r i e s D, E , a n d F w e r e s t a n d ­ a r d i z e d so a s to p r o d u c e s a m p l e s of i n c r e a s i n g f a t c o n t e n t . Each s e r i e s w a s p a s t e u r i z e d , h o m o g e n iz e d , and b o ttle d a s s ta te d in S e c ­ t i o n I. The v is c o s it y , h e a t s ta b ility , t i tr a t a b l e a cid ity , pH, p e r c e n ta g e t o t a l s o l i d s , a n d f a t a l s o w e r e d e t e r m i n e d a s o u t l i n e d in the E x p e r i ­ m ental P r o c e d u r e u n d e r S e c t i o n I. A n a t t e m p t w a s m a d e to n o t e w h e t h e r a n e m u l s i f y i n g a g e n t w o u l d h a v e a n y e f f e c t on th e c l u m p i n g p h e n o m e n o n of h o m o g e n i z e d h a lf-a n d -h a lf and c r e a m . was Th e e m u l s i f y i n g a g e n t u s e d f o r t h i s s t u d y so rb itan mono ste a ra te . T h i s a g e n t w a s a d d e d , a t the r a t e of 0.05 p e r c e n t , j u s t p r i o r t o t h e h o l d i n g t i m e of p a s t e u r i z a t i o n . o t h e r f a c t o r s i n th e p r e p a r a t i o n of th e to th e c o n t r o l a s p o s s i b l e . All s a m p le s w e re kept as s im ila r T h e e f f e c t of the e m u l s i f i e r w a s d e t e r ­ m ined by h e a t stab ility and viscosity. A p r e l i m i n a r y i n v e s t i g a t i o n w a s u n d e r t a k e n w i t h t h e v i e w of s t u d y i n g the m i g r a t i o n of f a t g l o b u l e s i n a n e l e c t r i c a l f i e l d a s a f ­ f e c t e d b y v a r i o u s p r e s s u r e s of h o m o g e n i z a t i o n a n d b y d i f f e r e n c e s in c o m p o s i t i o n of the s a m p l e s . 96 A. p h o s p h a t e b u f f e r , s t a n d a r d i z e d to a p H of 7. 6 a n d a n io n i c s t r e n g t h of 0 . 0 2 , w a s p r e p a r e d b y d i s s o l v i n g 0 . 3 0 9 g r a m of m o n o b a s i c s o d i u m p h o s p h a t e a n d 6 . 3 6 g r a m s of d i b a s i c s o d i u m p h o s p h a t e in a l i t e r of d i s t i l l e d w a t e r . T h e M u d d m o d i f i c a t i o n of the N o r t h r o p - K u n i t z C a t a p h o r e s i s A p p a r a t u s w a s u s e d in t h i s e x p e r i m e n t . T h e s a m p l e s w e r e p r e p a r e d b y f i r s t t e m p e r i n g the p r o d u c t a n d the b u f f e r to 70° F . , o r r o o m t e m p e r a t u r e , a n d t h e n s u s p e n d i n g 0.1 5 m i l l i l i t e r of th e p r o d u c t in 50 m i l l i l i t e r s of th e b u f f e r . Th e m i g r a ­ t i o n of t h e f a t g l o b u l e s w a s o b s e r v e d u n d e r a m i c r o s c o p e a t a f i x e d f o c u s d e p t h i n t h e c a t a p h o r e t i c c e l l , in a f i x e d e l e c t r i c a l f i e l d . t i m e f o r th e f a t g l o b u l e s to m i g r a t e 2 3 4 . 6p w a s r e c o r d e d , a n d r e s u l t s w e r e t h e n c a l c u l a t e d on q u i r e d f o r th e g l o b u l e s to m o v e The the the b a s i s of t i m e , i n s e c o n d s , r e ­ 1 O O jjl u n d e r th e a b o v e c o n d i t i o n s . F i v e to s i x r e a d i n g s w e r e m a d e f o r e a c h s a m p l e . P r e p a r a tio n and analysis A. s a m p l e of f e a t h e r e d m a t e r i a l of f e a t h e r e d h a l f - a n d - h a l f a n d c r e a m . w a s p r e p a r e d t h r o u g h the u s e of a 300 p a r t s p e r m i l l i o n c a l c i u m - i o n s o l u t i o n , a n d h a r d , ta p w a t e r w i t h the h a r d n e s s c a l c u l a t e d to b e 41 9 .2 p a r t s p e r m i l l i o n c a l c i u m c a r b o n ­ ate. The c a l c i u m - i o n s o l u t i o n w a s u s e d in th e f i r s t two t r i a l s , w i t h the h a r d w a t e r b e i n g e m p l o y e d f o r t h e t h i r d t r i a l . Fifty m illilite rs of the w a t e r w a s h e a t e d to b o i l i n g (200° to 205° F . ) , r e m o v e d f r o m 97 th e s o u r c e of the h e a t , a n d t h e n 5 m i l l i l i t e r s of the s a m p l e to be te sted was added. Th e p r e c i p i t a t e d m a t e r i a l w a s r e m o v e d b y f i l t e r i n g t h r o u g h a n u m b e r 42 f i l t e r p a p e r . The r e s u l t a n t f i l t r a t e s a n d the o r i g i n a l s a m p l e s w e r e a n a l y z e d fo r total n itro g e n , n o n c a s e in n itro g en , nonheat-co ag u lab le nitro g en , a n d n o n p r o t e i n n i t r o g e n a c c o r d i n g to th e p r o c e d u r e a s g i v e n b y B r u n n e r (1 95 2) . T h e p e r c e n t a g e of c a s e i n n i t r o g e n w a s o b t a i n e d b y s u b t r a c t i n g th e n o n c a s e i n n i t r o g e n f r o m t h e t o t a l n i t r o g e n , a n d th e he a t - c o a g u l a b l e n i t r o g e n w a s o b t a i n e d b y s u b t r a c t i n g the n o n h e a t - c o a g u l a b l e n i t r o g e n f r o m the n o n c a s e i n n i tr o g e n . T h e p e r c e n t a g e t o t a l s o l i d s a n d f a t w e r e d e t e r m i n e d b o t h on the p a r e n t s a m p l e a n d on t h e m a t e r i a l p r e c i p i t a t e d f r o m it. E x p erim en tal R esults A t o t a l of s i x s e r i e s of s a m p l e s w a s p r e p a r e d in t h i s p h a s e of the s t u d y . The f i r s t th re e a t 10.5 p e r c e n t f a t . series, A, B , a n d C, w ere I n c r e m e n t s of 2 a n d 5 p e r c e n t n o n f a t d r y m i l k s o l i d s w e r e a d d e d to S e r i e s B a n d C, r e s p e c t i v e l y . w ere standardized As these p ro d u cts s i m i l a r to t h o s e i n v e s t i g a t e d in S e c t i o n I, o n l y one t r i a l f o r e a c h s e r i e s was conducted. Th e r e m a i n i n g t h r e e s e r i e s , D, E , a n d F , 98 w e r e p r e p a r e d w i t h i n c r e a s i n g p e r c e n t a g e s of f a t , a n d w e r e i z e d o n the b a s i s of t h e i r s o l i d s - n o t - f a t to f a t r a t i o . the a n a l y s e s of t h e s e p r o d u c t s a r e i n c l u d e d i n T a b l e V iscosity. in F i g u r e The r e s u l t s , as standard­ T h e r e s u l t s of 10. s h o w n in T a b l e 11 a n d i l l u s t r a t e d 13, i n d i c a t e d t h a t , a s p r e v i o u s l y r e p o r t e d in S e c t i o n I, T a b l e 2, h o m o g e n i z a t i o n of the t h r e e s e r ie s testing 10.5 p e r c e n t f a t p r o d u c e d only a r e l a t i v e l y s m a l l i n c r e a s e in v is c o s ity , e v en w hen s u b ­ j e c t e d to v e r y h i g h p r e s s u r e s . F u r t h e r m o r e , a s a r e s u l t of s i n g l e - s t a g e h o m o g e n i z a t i o n , v i s c o s i t y of t h e s a m p l e s , t e s t i n g 13 p e r c e n t f a t , s t i l l d i d n o t e x c e e d 50 .9 c e n t i p o i s e s a t a p r e s s u r e of 5,0 00 p o u n d s p e r s q u a re inch. W h e n th e p e r c e n t a g e of f a t w a s i n c r e a s e d to 18, a s h a r p i n c r e a s e i n v i s c o s i t y a p p e a r e d a s a r e s u l t of s i n g l e - s t a g e hom ogenization. This in c r e a s e was e v en m o re 2 3 -p e rc en t c r e a m was hom ogenized. 5,000 p o u n d s p e r s i g n i f i c a n t w h e n the This c r e a m , h o m o g e n iz e d at s q u a r e i n c h p r e s s u r e , w a s n o t o n ly v i s c o u s , b u t a l m o s t s o l i d a t 3° C. W hen dual h o m o g e n iz a tio n was u s e d , v e r y little i n c r e a s e s in the v i s c o s i t y of th e p r o d u c t s t e s t i n g up to 18 p e r c e n t f a t w e r e n o t e d (T able 11, F i g u r e 13). a definite i n c r e a s e was pressure H o w e v e r , b y i n c r e a s i n g the f a t to 23 p e r c e n t , s h o w n a s a r e s u l t of h o m o g e n i z i n g a t the of 5,000 + 500 p o u n d s p e r s q u a re inch. 99 stability. ic a lly in F i g u r e D a ta p r e s e n t e d in Table 14 s h o w the e f f e c t of i n c r e a s i n g p e r c e n t a g e s of f a t o n the h e a t s t a b i l i t y of the s a m p l e s . three 12 a n d s h o w n g r a p h ­ A g a i n , the r e s u l t s of th e f i r s t s e r i e s w e r e f o u n d t o b e s i m i l a r to t h o s e r e c o r d e d in S e c t i o n I, T a b l e 4. T h i s c o n f i r m e d t h e f i n d i n g s t h a t (1) a d e c r e a s e in h e a t s t a b i l i t y o c c u r r e d i n the n o n h o m o g e n i z e d a n d i n the d u a l - h o m o g e n i z e d s a m p l e s , w h e n th e p e r c e n t a g e of s o l i d s - n o t - f a t w a s i n c r e a s e d , a n d (2) th e h e a t s t a b i l i t y of th e d u a l - h o m o g e n i z e d s a m p l e s w a s b e t w e e n t h a t of t h e r e l a t i v e l y s t a b l e n o n h o m o g e n i z e d p r o d u c t s a n d the u n s t a b l e , single-stage hom ogenized sam p les. A s t r i k i n g o b s e r v a t i o n m a d e on the f i n a l t h r e e s e r i e s , in w h i c h t h e f a t c o n t e n t w a s i n c r e a s e d , w a s t h e i n c r e a s e in the h e a t s t a b i l i t y of th e n o n h o m o g e n i z e d s a m p l e s ( T a b l e 12) . One f a c t o r w h i c h s h o u l d b e n o t e d i n t h i s c o n n e c t i o n i s t h a t the d a t a p r e s e n t e d in t h i s s e c o n d s e c t i o n w e r e o b s e r v e d in M a y a n d J u n e , w h i l e the r e s u lts in Section I w e r e r e c o r d e d m a in ly in D e c e m b e r , J a n u a ry , and F e b r u a r y . H o w e v e r, a s w as shown p r e v io u s ly , once these h ig h -fa t p r o d ­ u c t s w e r e h o m o g e n i z e d , a s i g n i f i c a n t d e c r e a s e in th e h e a t s t a b i l i t y occurred. This stability b e ca m e of f a t w a s i n c r e a s e d . m o r e a p p a r e n t a s the p e r c e n t a g e H e n c e , a r e d u c t i o n in the h e a t s t a b i l i t y o c c u r r e d 100 in the d u a l - h o m o g e n i z e d s a m p l e s a s th e r e s u l t of b o t h i n c r e a s i n g p r e s s u r e s a n d i n c r e a s i n g p e r c e n t a g e s of f a t . s u b j e c t e d to o n e p r e s s u r e W he n th e p r o d u c t s w e r e o n ly , th e s e r i e s t e s t i n g 13 p e r c e n t f a t a n d o v e r w e r e a l m o s t i m m e d i a t e l y c o a g u l a t e d b y the h e a t . f r o m the s e r i e s This d iffered c o n t a i n i n g 10.5 p e r c e n t f a t , w h i c h r e q u i r e d a t l e a s t 10 m i n u t e s f o r c o a g u l a t i o n . E f f e c t of a n e m u l s i f i e r on f a t c l u m p i n g a s m e a s u r e d b y v i s ­ cosity and h e a t s ta b ility . An e m u l s i f ie r , s o rb ita n m o n o s te a r a te , was a d d e d to v a r i o u s s a m p l e s of h a l f - a n d - h a l f a n d c r e a m to n o t e i t s e f ­ f e c t on the c lu m p in g p h e n o m e n o n . The f i r s t t r i a l was conducted, u s i n g e x t r e m e s i n h o m o g e n i z a t i o n p r e s s u r e s , to s e e w h a t d i f f e r e n c e s , if a n y , o c c u r r e d in the v i s c o s i t y a n d h e a t s t a b i l i t y ( T a b l e s The s e c o n d a n d t h i r d t r i a l s t h e n w e r e 13 a n d 14).. ru n , using h ig h e r - f a t p ro d u c ts in o r d e r t h a t a n y d i f f e r e n c e s a s a r e s u l t of a d d i n g th e e m u l s i f i e r c o u l d be m o r e readily detected. T hese data indicated little signifi­ c a n t d i f f e r e n c e s b e t w e e n the c o n t r o l a n d the t r e a t e d s a m p l e s i n n o n ­ hom ogenized and d u al-hom ogenized tria ls . H ow ever, when a high d e g r e e of c l u m p i n g o c c u r r e d , a s p r o d u c e d b y s i n g l e - s t a g e h o m o g e n i ­ z a t i o n , t h e a d d i t i o n of 0. 0 5 p e r c e n t s o r b i t a n m o n o s t e a r a t e p r o d u c e d a significant d e c r e a s e in v i s c o s i t y in a l l th e t r i a l s c o n d u c t e d . 101 T h e e f f e c t of t h i s e m u l s i f i e r on h e a t s t a b i l i t y i s r e c o r d e d in Table 14. These findings s h o w e d t h a t th e a d d i t i o n of the e m u l s i f i e r m a d e no d e f i n i t e c h a n g e s i n the h e a t s t a b i l i t y of the n o n h o m o g e n i z e d sam ples. F u r t h e r m o r e , no s i g n i f i c a n t d i f f e r e n c e s in t h i s p r o p e r t y w e r e n o t e d w i t h the s i n g l e - s t a g e h o m o g e n i z e d , 1 8 - p e r c e n t s a m p l e . T h i s w a s u n d o u b t e d l y due to t h e i r r a p i d c o a g u l a t i o n w h e n s u b j e c t e d to h e a t . H o w e v e r , w h e n th e p e r c e n t a g e of f a t w a s d e c r e a s e d to 10.5, a d iffe re n c e w a s d is c e r n ib l e w ith this s in g le -s ta g e t r e a tm e n t. W h e n t h e p r o d u c t s w e r e h o m o g e n i z e d a t 2,500 + 500 p o u n d s p e r s q u a r e i n c h p r e s s u r e , t h e a d d i t i o n of s o r b i t a n m o n o s t e a r a t e d e f i n i t e l y i n c r e a s e d t h e s t a b i l i t y of th e s a m p l e s . A t a p r e s s u r e of 5,00 0 + 500 p o u n d s p e r s q u a r e i n c h , one of th e t r i a l s d i d s h o w i n ­ c r e a s e d s t a b i l i t y , w h i l e th e o t h e r r e m a i n e d a p p a r e n t l y u n a f f e c t e d . T h e m i g r a t i o n of f a t g l o b u l e s a s a f f e c t e d b y h o m o g e n i z a t i o n and c o m p o s i t i o n . T h e d a t a s h o w i n g the m i g r a t i o n of f a t g l o b u l e s in a n e l e c t r i c a l f i e l d , u n d e r f i x e d c o n d i t i o n s , a r e p r e s e n t e d in T a b l e These 15. r e s u l t s w e r e o b t a i n e d a f t e r a p e r i o d of f r o m f o u r to f i v e d a y s of s t o r a g e , w h e n i t w a s a s s u m e d t h a t a m p l e o p p o r t u n i t y w a s p r o v i d e d f o r the s a m p l e s to r e a c h a s t a t e of e q u i l i b r i u m . d e t e r m i n e d u n d e r the s a m e T rials 6 and 7 w e re r e l a t i v e c o n d i t i o n s , b u t t h r o u g h the p r o c e s s 102 of r e c a l i b r a t i o n , p r o b a b l y a s l i g h t l y d i f f e r e n t f o c a l l e n g t h w a s o b t a i n e d in the M u d d C e l l . These r e s u l t s i n d i c a t e t h a t t h e f a t g l o b u l e s of the n o n h o m o g ­ e n i z e d s a m p l e s r e q u i r e a l o n g e r p e r i o d to m i g r a t e th e h o m o g e n i z e d p r o d u c t . lOOp t h a n t h o s e of In t h i s r e s p e c t no s i g n i f i c a n c e c o u l d be a t t a c h e d to a n y d i f f e r e n c e s i n p r e s s u r e s w h e t h e r w i t h a s i n g l e - o r with d o u b le -s ta g e t r e a t m e n t . In a d d i t i o n , t h e s e d a t a w o u l d i n d i c a t e t h a t c o m p o s i t i o n h a d n o e f f e c t on m i g r a t i o n of f a t g l o b u l e s . However, t h i s c o u l d n o t be c o n s i d e r e d a s c o n c l u s i v e . A n a l y s i s of f e a t h e r e d h a l f - a n d - h a l f a n d c r e a m . A pparently, l i t t l e i n f o r m a t i o n h a s b e e n s u p p l i e d r e g a r d i n g the c o m p o s i t i o n of the fe a th e r e d m a t e r i a l in coffee. In o r d e r t h a t t h i s p r o b l e m m a y be m o r e t h o r o u g h l y u n d e r s t o o d , v a r i o u s s a m p l e s of h a l f - a n d - h a l f a n d c r e a m were s u b j e c t e d to f e a t h e r i n g t r e a t m e n t , a n d t h i s p r e c i p i t a t e d m a t e r i a l , a s w e l l a s the o r i g i n a l p r o d u c t s , w e r e t h e n a n a l y z e d . The te ch n iq u es s e l e c t e d to f o r m th e p r e c i p i t a t e i n v o l v e d s u c h f a c t o r s a s h e a t , p r e s ­ e n c e of c a l c i u m i o n s , a n d i n s t a b i l i t y of the p r o d u c t due to h o m o g e n i ­ z a t i o n , s i n c e t h e s e f a c t o r s w e r e p r o b a b l y m o s t c o n d u c i v e to f e a t h e r i n g in c o f f e e . Four in T a b l e s s e p a ra te tr ia ls w ere conducted. 16 a n d 17. The d a t a a r e p r e s e n t e d T h e r e s u l t s , p r e s e n t e d f o r e a c h t r i a l in t h e s e 103 two t a b l e s , w e r e o b t a i n e d f r o m th e s a m e s e r i e s of s a m p l e s . The a n a l y s i s of t h e f e a t h e r e d m a t e r i a l s h o w e d t h a t , on a d r y - w e i g h t b a s i s , the p e r c e n t a g e of f a t r a n g e d f r o m 81.0 2 to 9 4 .1 1 , w i t h t h e i n c r e a s e c o r r e s p o n d i n g t o a s i m i l a r i n c r e a s e in the p e r c e n t a g e of f a t i n the original p ro d u cts. T h e d i s t r i b u t i o n of the n i t r o g e n in the f i l t r a t e a n d p a r e n t m a ­ t e r i a l is show n in Table 16. H e re th r e s u lts a re e x p r e s s e d as m i l l i g r a m s of n i t r o g e n p e r 100 m i l l i l i t e r s of p a r e n t p r o d u c t . Since 5 m i l l i l i t e r s of t h e p a r e n t p r o d u c t w a s a d d e d to 50 m i l l i l i t e r s of w a t e r , a d i l u t i o n f a c t o r of e l e v e n w a s h a d . T h e r e f o r e , the n i t r o g e n v a l u e s , o b t a i n e d i n the f i l t r a t e s , w e r e c o n v e r t e d onto the s a m e b a s i s a s t h a t of t h e p a r e n t m a t e r i a l , b y m u l t i p l y i n g b y t h i s d i l u t i o n f a c t o r . The n i t r o g e n m a y b e c o n v e r t e d in t o p r o t e i n s i m p l y t h r o u g h m u l t i p l y ­ ing b y 6 . 3 8 . W h i l e t h e r e s u l t s f r o m th e a n a l y s i s of the o r i g i n a l s a m p l e s a p p e a r e d to b e r a t h e r c o n s i s t e n t , t h o s e o b t a i n e d f r o m the f i l t r a t e a r e inconsistent. A s a r e s u l t of p r e c i p i t a t i n g the f e a t h e r e d m a t e r i a l w i t h w a t e r c o n t a i n i n g the c a l c i u m i o n s , a h i g h e r p e r c e n t a g e of the c a s e i n apparently was p re c ip ita te d . F o r T r i a l 2 ( T a b l e 17), a n e g a t i v e v a l u e w a s c a l c u l a t e d f o r t h e c a s e i n n i t r o g e n in the f i l t r a t e . The r e ­ s u l t s of T r i a l 3 s h o w e d t h a t a p p r o x i m a t e l y 91 p e r c e n t of the c a s e i n 104 was p re c ip ita te d . H o w e v e r , i n th e f o u r t h t r i a l , i n w h i c h h a r d w a t e r w a s u s e d , o n l y a b o u t 43 p e r c e n t of the c a s e i n w a s f e a t h e r e d . Un­ d o u b t e d l y t h i s w a s du e to th e h i g h d e g r e e of f a t c l u m p i n g i n the p a re n t product. W i t h t h e t e c h n i q u e u s e d i n t h i s s t u d y , l i t t l e of the h e a t - c o a g ulable f r a c t i o n w a s p r e c i p i t a t e d . Of the c a l c u l a t e d v a l u e s o b t a i n e d ( T r i a l s 2 a n d 3), o n l y a b o u t 19 to 31 p e r c e n t of the he a t - c o a g u l a b l e f r a c t i o n of th e p a r e n t m a t e r i a l w a s p r e s e n t i n th e f e a t h e r e d m a t e r i a l . F r o m t h e a b o v e r e s u l t s , s e v e r a l g e n e r a l t r e n d s m a y be n o t e d . When the s a m p l e s w e r e p r e c i p i t a t e d w ith c a lc iu m ions, a la r g e p r o ­ p o r t i o n of t h e c a s e i n w a s p r e s e n t i n th e p r e c i p i t a t e . Th e a d d i t i o n of a s a m p l e , h o m o g e n i z e d a t a p r e s s u r e of 2,5 00 p o u n d s p e r s q u a r e i n c h , to h o t , h a r d w a t e r p r o d u c e d a p r e c i p i t a t e w i t h a m u c h s m a l l e r a m o u n t of c a s e i n . In t h e sam ples s t u d i e d , l e s s of the h e a t - c o a g u l a b l e f r a c t i o n w a s p r e s e n t i n the p r e c i p i t a t e d m a t e r i a l t h a n in the o r i g i n a l sam ple. 105 Table Series No. 10. T h e i n f l u e n c e of a d d e d s o l i d s - n o t - f a t a n d i n c r e a s i n g p e r ­ c e n t a g e s of f a t o n the c o m p o s i t i o n of h a l f - a n d - h a l f h o m o g ­ e n ized m ilk and c r e a m . Added Increm ents of SolidsN ot-F at A n a l y s e s of H a l f - a n d - H a l f a n d C r e a m W hen T r e a t e d W ith V a rio u s I n c re m e n ts of S o l i d s - N o t - F a t a n d F a t — -----------------------------SolidsTotal S .N .F ./ Fat Solids N ot-F at Fat (p et.) (pet.) (pet.) (pet.). A* 5 10 .5 4 1.1.61 2 2 .1 5 1.101 B* 2 10.71 9 .6 8 2 0. 3 9 0.904 c* 0 10.50 8.0 8 18 .5 8 0.770 D ** 0 12.7 6 7.87 2 0 .6 3 0.617 E 0 18.4 4 7.50 2 5 .9 4 0.407 0 22 .7 4 7 .1 6 29 .9 0 0 .3 1 5 * One t r i a l . ** Two t r i a l s . 106 T a b le Series No. A B C D E F 11. T h e i n f l u e n c e of v a r i a t i o n s of t e m p e r a t u r e a n d c o m p o s i t i o n on th e v i s c o s i t y of h a l f - a n d - h a l f a n d c r e a m w h e n h o m o g ­ e n iz e d a t v a rio u s p r e s s u r e s . Adde d Incre­ m ents of SolidsN ot-F at Tem p. of V is­ cosity M ea­ sure­ m ents (pet.) 5 2 0 0 0 0 The V i s c o s i t y of H a l f - a n d - H a l f W h e n H o m o g e n i z e d at: 0 lb s./ sq.in. 2500 lb s./ sq.in. 2500 + 500 lb s./ sq.in. 5000 lb s./ sq.in. 5000 +500 lb s./ sq.in. (°C.) (centip o i s e s) (centip o i s e s) (centip o i s e s) (centipoises) (centipoises 20 6.7 7.0 6.1 8.6 7.0 3 8.1 8.9 8.1 14.6 9.3 20 5.9 7.1 5.5 9.3 6.3 3 6.9 8.1 6.8 14.8 8.1 20 5.3 6.5 5.0 8.1 5.7 3 6.5 7.9 6.3 11.0 7.1 20 6.1 19.6 5.7 37.9 6.8 3 7.2 22.5 7.1 50.9 8.2 20 7.6 114.7 7.5 231.0 8.9 3 9.3 20 2.3 8.7 331.0 11.1 20 11.2 172.4 9.9 1,593.0 26.0 3 18.0 1,,0 5 3 . 0 16.3 4 ,66 5.0 44.0 107 T a b le 12. T h e i n f l u e n c e of v a r i a t i o n s i n c o m p o s i t i o n on th e h e a t s t a b i l i t y of h a l f - a n d - h a l f a n d c r e a m w h e n h o m o g e n i z e d at v a rio u s p r e s s u r e s . Fat Adde d Incre­ m ents of SolidsN ot-F at (pet.) A. Series No. The H e a t S t a b i l i t y of H a l f - a n d - H a l f a n d C r e a m W h e n H o m o g e n i z e d at: 0 lb s./ sq.in. 2500 lb s ./ sq.in. 2500 + 500 lb s./ sq.in. 5000 lb s./ sq.in. 5000 + 500 lb s./ sq.in. (pet.) (m in­ ute s) (m in­ ute s) (m in­ ute s) (min­ u te s ) (m in­ ute s) 10.5 4 5 90 15 31 13 30 B 10.71 2 106 12 43 10 44 C 10.50 0 130 12 107 10 94 D 12. 76 0 156 3 55 2 48 E 18.4 4 0 148 1 38 1 6 F 22.74 0 166 1 10 _❖ 1 * S a m p l e too v i s c o u s f o r a d e t e r m i n a t i o n . 108 T a b le 13. The i n f l u e n c e of s o r b i t a n m o n o s t e a r a t e u p o n th e v i s c o s i t y of h a l f - a n d - h a l f a n d c r e a m w h e n h o m o g e n i z e d a t v a r i o u s p re s s u re s . Th e V i s c o s i t y io f H a l f - Trial No . Fat SolidsN ot-F at 0 lb s./sq .in . Con­ trol (pet.) (pet.) E m ul­ sifier Added (centipoises) 2500 l b s . / s q . i n . Con­ tro l E m ul­ sifier A dd e d (centip o ises) V i s c o s i t y M e a s u r e d a t 20° C. 1 10.87 7.50 5.9 6.0 2 18.20 7.39 7.9 7.8 47 .4 2 6 .7 3 18.67 7.61 7.2 8.0 180.0 33.8 - V i s c o s i t y M e a s u r e d a t 3° C. 1 10.87 7.50 7.0 6 .8 - - 2 18.20 7.39 8.4 9 .9 5 6 .6 26 .7 3 18.67 7.61 10.1 10.4 358.0 41 .4 * V a lu e s n o t d e t e r m i n e d . 109 T a b le 13 (C o n tin u e d ) a n d - H a l f a n d C r e a m W h e n H o m o g e n i z e d at: 2500+ 500 l b s . / s q . i n . Con­ trol E m u l­ sifier Added (centipoises) 5000 l b s . / s q . i n . Con­ trol E m ul­ sifier Added (centipoises) 5000+50 0 l b s . / s q . i n . Con­ trol E m ul­ sifier Added (centipoises) V i s c o s i t y M e a s u r e d a t 20° C , 12.9 8.5 7.6 7.2 194.0 120.0 9.1 8.5 7.5 7 .6 2 66 .0 74.9 8.7 8.6 V i s c o s i t y M e a s u r e d a t 3° C , 15.2 10.1 8.1 8.6 253.0 145.0 11.0 11.4 9.2 9.0 406.0 84.4 13.0 11.9 110 T a b le 14. T h e i n f l u e n c e of s o r b i t a n m o n o s t e a r a t e u p o n th e h e a t s t a b i l i t y of h a l f - a n d - h a l f a n d c r e a m w h e n h o m o g e n i z e d a t v a rio u s p r e s s u r e s . H e a t S t a b i l i t y of H a l f - a n d - H a l f a n d C r e a m W h e n H o m o g e n i z e d at: T rial No. Fat SolidsN ot-F at (p e t.) (pe t.) 0 lb s./ sq.in. 2500 lb s./ sq.in. 2500 + 500 l b s ./ sq.in. 5000 lb s ./ sq.in. 5000 +500 lbs./ s q .i n , (m in­ utes) (m in­ ut e s) (m in­ ut e s) (m in­ utes) (m in­ utes) No E m u l s i f i e r A d d e d 1 10.87 7.50 95 - - 3 - 2 18.20 7 .3 9 116 1 66 1 7 3 18 .6 7 7.61 190 2 10 1 5 E m u l s i f i e r A d d e d (0,05%) 1 10.87 7.50 105 - - 7 - 2 18.20 7 .3 9 111 2 78 1 18 3 18.67 7.61 190 2 74 1 5 Ill Table T rial No. 15. T h e e f f e c t of v a r i o u s p r e s s u r e s of h o m o g e n i z a t i o n a n d c o m p o s i t i o n of h a l f - a n d - h a l f a n d c r e a m on t h e m o v e m e n t of f a t g l o b u l e s in a n e l e c t r i c a l f i e l d . _ Added Increm ents of Solids N ot-F at T i m e f o r F a t G l o b u l e s in H a l f - a n d H a l f a n d C r e a m to M i g r a t e 10Op W h e n H o m o g e n i z e d a t: —-------- 2500 5000 0 2500 5000 +500 + 500 lb s./ lbs./ lb s./ lb s./ l b s ./ sq.in. sq.in. sq.in. sq.in. sq.in. (pet.) (pet.) (sec­ onds). (sec­ onds) (sec­ on ds ) (sec­ onds ) (sec­ onds ) 1 1 0 .5 4 5 3.51 3.41 3.36 3.37 3.41 2 10.71 2 3.43 2.85 2 .9 4 2 .9 3 3.09 3 12.70 0 3.70 3.5 6 3.58 3.57 3.45 4 18.20 0 3.70 3.58 3.29 3.41 3.51 5 22.74 0 3.60 3.39 3.50 -* - 6 12 .8 2 0 3 .2 8 2 .7 8 2 .9 2 2.90 3.09 7 18.6 7 0 3. 13 2.93 2. 75 2.81 2 .6 6 * V a lu es n o t d e te r m in e d . 112 Table 16. T h e c o m p o s i t i o n of h a l f - a n d - h a l f a n d c r e a m , a s c o m ­ p a r e d to t h e r e s u l t a n t f e a t h e r e d m a t e r i a l . C o m p a r i s o n i n C o m p o s i t i o n of th e P a r e n t P r o d u c t A g a i n s t the R e s u l t a n t F e a t h e r e d M a t e r i a l T rial No. Fat SolidsN ot-F at T otal Solids _ FeathFarent . ered _ FeathParent ered _ FeathParent ered (pet. ) (pet.) (pet.) F a t p e r 100 g r a m s Solids Parent Feath­ ered (grams) 1 10.68 28.21 7.70 5 .4 2 18 .3 8 33. 63 5 8 .1 0 83.88 2 10 .8 7 21.09 7.81 4. 9 4 1 8 .6 8 26.03 5 8 .1 9 81 .0 2 3 1 8. 6 7 67 .2 9 7.61 4.21 26.28 71.50 71.04 94.11 4 22.69 61. 65 6.55 6.17 29.24 67.83 7 7 .5 9 90.90 113 Table 17. T h e n i t r o g e n d i s t r i b u t i o n of h a l f - a n d - h a l f a n d c r e a m , a s c o m p a r e d to th e r e s u l t a n t f e a t h e r e d m a t e r i a l . C o m p a r i s o n in N i t r o g e n D i s t r i b u t i o n of th e T rial No. N oncasein N itrogen Total N itrogen Parent F iltrate (mg./lOO m l. p r o d u c t ) 5!1 Parent F iltrate (mg./lOO m l . product) N onheat-coagulable N itro g e n Parent F iltrate (mg./lOO m l, product) 1 480.50 10 8.4 6 8 6. 8 6 67.7 6 4 4 .6 6 40.70 2 522 .20 6 7 .7 6 87.50 1 0 8 .4 6 4 3 .1 2 44 .4 4 3 417.6 0 284.5 7 68.99 84.70 25.56 1 03 .2 9 4 434.30 141.90 _ >;<** * H e a d i n g s b a s e d on 100 m i l l i l i t e r s of p a r e n t p r o d u c t . ** V a l u e s c a l c u l a t e d w e r e n e g a t i v e . V alues not d e term in e d . 114 T a b le 17 (C o n tin u e d ) P a r e n t P r o d u c t A g a i n s t th e R e s u l t a n t F i l t r a t e N onprotein N itrogen Parent Filtrate (mg./lOO m l. product) C asein N itrogen Parent F iltrate H eat-coagulable N itrogen Parent (mg./lOO m l. product) 2 9 .4 2 3 0 .1 4 39 3.6 0 27.00 29.70 4 34 .7 0 28.6 5 23.76 3 48 .6 0 Filtrate (mg./lOO m l. product) 40.70 15.24 10.56 ** 16.12 14.74 _ 199-87 _ * * 79.53 115 350 - / / y 2 ,5 0 0 + 5 0 0 5 ,0 0 0 + 5 0 0 VjflS and Half 2 ,5 0 0 5 ,0 0 0 HOMOGENIZATION PRESSURE ( LBS./SQ. IN.) F igure 1 3. T h e e f f e c t of i n c r e a s i n g p e r c e n t a g e s of f a t on t h e v i s c o s i t y of h a l f - a n d - h a l f a n d c r e a m w h e n h o m o g e n i z e d a t v a r i o u s pre ssure s . 116 STABILITY (MINUTES) 180 160 140 120 100 10.5 °A 80 \ 60 HEAT » * > \ 40 20 0 0 2 ,5 0 0 + 5 0 0 5 ,0 0 0 + 5 0 0 2 ,5 0 0 5 ,0 0 0 HOMOGENIZATION PRESSURE ( LB S . / S O . IN.) F igure 14. T h e e f f e c t of i n c r e a s i n g p e r c e n t a g e s of f a t on th e h e a t s t a b i l i t y of h a l f - a n d - h a l f a n d c r e a m w h e n h o m o g e n i z e d at various p r e s s u r e s . 117 D is c u s sion T h e d a t a p r e s e n t e d f o r th e w ere not added, w ere o b t a i n e d f r o m S e c t i o n I. c o m p a r e d w i t h th e s e r i e s t e s t i n g 13 a n d 14). The l a t t e r t h r e e These re su lts w ere 13, 18, a n d 23 p e r c e n t f a t ( F i g u r e s s e r i e s w e r e p r e p a r e d in two s e p a r a t e t r i a l s , th e r e s u l t s of w h i c h w e r e V iscosity. 1 0 .5 - p e r c e n t s e r i e s , in w h ic h solids r e p o r te d in duplicate. A n i n c r e a s e o c c u r r e d in t h e v i s c o s i t y of h a l f - a n d - h a l f a n d c r e a m , n o n h o m o g e n i z e d , a n d h o m o g e n i z e d , a s th e p e r c e n t a g e of f a t i n c r e a s e d . W i t h d u a l - h o m o g e n i z a t i o n , the p r o d u c t s t e s t i n g up to 18 p e r c e n t f a t s h o w e d v e r y l i t t l e , if a n y , i n c r e a s e in v i s c o s i t y . A p p a r e n t l y , t h e a d d i t i o n of 500 p o u n d s p e r s q u a r e i n c h p r e s s u r e th e s e c o n d v a l v e w a s on s u f f i c i e n t to b r e a k d o w n a l a r g e p r o p o r t i o n of the v i s c o s i t y - c o n t r i b u t i n g c l u m p s of f a t g l o b u l e s w h i c h w e r e f o r m e d a s a r e s u l t of t h e h i g h e r p r e s s u r e s on th e f i r s t v a l v e . How ever, w h e n t h e a m o u n t of f a t w a s i n c r e a s e d to 23 p e r c e n t , the v i s c o s i t y in c r e a s e d r a t h e r s h a r p ly when a p r e s s u r e of 5,000 + 500 p o u n d s p e r sq u are inch was applied. W ith s in g le - s ta g e h o m o g e n iz atio n , i n c r e a s e d p r e s s u r e s in l i t t l e i n c r e a s e in v i s c o s i t y i n th e 13). 10.5-percent-fat series resulted (Figure W h e n the a m o u n t of f a t w a s i n c r e a s e d to 13 p e r c e n t , a r e l a t i v e l y 118 s l i g h t i n c r e a s e i n v i s c o s i t y w a s n o t e d a t a p r e s s u r e of 5,000 p o u n d s p e r s q u a re inch. H o w e v e r , on h o m o g e n i z a t i o n of t h e 1 8 - p e r c e n t s e r i e s , a s h a r p i n c r e a s e in v i s c o s i t y could be se e n w ith p r e s s u r e s of 2 , 5 0 0 a n d 5 , 0 0 0 p o u n d s p e r s q u a r e i n c h . This in c r e a s e b e c a m e s t i l l m o r e a p p a r e n t w h e n t h e f a t w a s i n c r e a s e d to 23 p e r c e n t . T h e r e f o r e , w i t h d o u b l e - s t a g e h o m o g e n i z a t i o n , the s h a r p i n ­ c r e a s e i n v i s c o s i t y a p p e a r e d w h e n the f a t w a s b e t w e e n 18 a n d 23 percent. W h e n c o n s i d e r i n g the s o l i d s - n o t - f a t to f a t r a t i o , th e a b o v e s h a r p i n c r e a s e i n v i s c o s i t y w o u l d be b e t w e e n th e r a t i o s of 0 . 4 0 7 and 0.315. W i t h s i n g l e - s t a g e h o m o g e n i z a t i o n , th e c r i t i c a l r a t i o f o r e x c e s s i v e c l u m p i n g of f a t g l o b u l e s w a s f o u n d to b e b e t w e e n 0 . 6 1 7 and 0.407. T h e s e v a l u e s a r e b e l o w th e c r i t i c a l r a t i o s of 0.6 0 to 0 . 8 5 , a s s u g g e s t e d b y D o a n (1 9 2 9 a ) . H ence, although d ifferen t h o ­ m o g e n i z i n g t e m p e r a t u r e s w e r e u s e d , p r o n o u n c e d c l u m p i n g of t h e f a t g l o b u l e s w o u l d be e x p e c t e d a t t h e s e l o w r a t i o s u s e d i n t h i s s t u d y . Heat stability . A g r a d u a l d e c l i n e o c c u r r e d in the h e a t s t a ­ b i l i t y of the h o m o g e n i z e d p r o d u c t s , a s the p e r c e n t a g e of f a t i n c r e a s e d (F igure 14). T h e h e a t s t a b i l i t y of the n o n h o m o g e n i z e d s a m p l e s a p ­ p a r e n t l y i n c r e a s e d a s th e p e r c e n t a g e of f a t i n c r e a s e d . Osborne (1953) n o t e d t h a t th e h i g h e s t h e a t s t a b i l i t y of m i x e d - h e r d m i l k o c c u r r e d in 119 June, w h ich m ig h t in d icate why th ese h ig h e r -f a t p ro d u c ts had a g r e a t e r d e g r e e of s t a b i l i t y t h a n t h e h a l f - a n d - h a l f s a m p l e s . In all t h e t r i a l s c o n d u c t e d , the h e a t s t a b i l i t y of the s a m p l e s w a s r e d u c e d a s a r e s u l t of h o m o g e n i z a t i o n . W it h d u a l - h o m o g e n i z a - t i o n , the h e a t s t a b i l i t y , a l t h o u g h s h a r p l y r e d u c e d , s t i l l w a s d e f i n i t e l y a b o v e t h e s t a b i l i t i e s a s f o u n d i n the s a m p l e s h o m o g e n i z e d w i t h a single p r e s s u r e . W i t h d u a l - h o m o g e n i z a t i o n t h e r e a p p e a r e d to b e no p o i n t w h e r e a d e f i n i t e b r e a k in t h e h e a t s t a b i l i t y of the s a m p l e s o c ­ c u r r e d a s a r e s u l t of i n c r e a s e d p e r c e n t a g e s of f a t . A distinct dif­ f e r e n c e in t h e h e a t s t a b i l i t y b e t w e e n e a c h s e r i e s w a s n o t e d , b u t t h i s d i f f e r e n c e w a s r e d u c e d w h e n the 18- a n d 2 3 - p e r c e n t c r e a m w a s h o ­ m o g e n i z e d a t p r e s s u r e s of 5,000 + 500 p o u n d s p e r s q u a r e i n c h . When these h ig h - f a t p ro d u c ts w e re h o m o g en ized with d u a l- s ta g e s , v e ry little t i m e w a s n e e d e d f o r th e s a m p l e s to c o a g u l a t e . W ith s i n g l e - s t a g e h o m o g e n iz a tio n , h o w e v e r , t h e r e did a p p e a r to b e a d e f i n i t e b r e a k i n t h e h e a t s t a b i l i t y o n c e the f a t c o n t e n t w a s i n c r e a s e d to 13 p e r c e n t ( F i g u r e 14). did n o t c o a g u la te u ntil the s a m p l e w as a t l e a s t 10 m i n u t e s . The s e r i e s t e s t i n g 10.5 p e r c e n t s u b m e r g e d in the oil b a t h f o r H o w e v e r , b y i n c r e a s i n g the f a t to 13 p e r c e n t , o n l y 2 to 3 m i n u t e s w e r e r e q u ir e d f o r coagulation. 120 When c o m p a rin g v isc o s ity with h e at stability, as has been n o t e d p r e v i o u s l y ( D o a n , 19 3 1 a ; W e b b , 1931), a n i n v e r s e r e l a t i o n s h i p w a s f o u n d to e x i s t , f o r , a s th e v i s c o s i t y i n c r e a s e d , a d e c r e a s e in the heat stability was o b serv ed . W h e n the s e c o n d s t a g e w a s u s e d , due to t h e b r e a k i n g up of t h e c l u m p s of f a t g l o b u l e s , th e v i s c o s i t y w a s d e c r e a s e d w hile the h e a t s ta b i lity w a s i n c r e a s e d . E f f e c t of a n e m u l s i f i e r on t h e f a t - c l u m p i n g p h e n o m e n o n . Th e a c t i o n of s o r b i t a n m o n o s t e a r a t e w a s d e t e r m i n e d b y the e f f e c t of t h i s e m u l s i f i e r on t h e v i s c o s i t y a n d h e a t s t a b i l i t y of the t r e a t e d s a m p l e s (T ables 13 a n d 14). T h i s e m u l s i f i e r h a d l i t t l e to no e f f e c t on the v i s c o s i t y of the nonhom ogenized and d u al-h o m o g en ized sa m p le s. If a n y t h i n g , a s the r e s u l t of a d d i n g t h i s e m u l s i f i e r , the v i s c o s i t y t e n d e d to i n c r e a s e in th e n o n h o m o g e n i z e d p r o d u c t s a t 3° C. w e re h o m o g e n iz e d w ith a single in g a p p e a r e d e v i d e n t . mono ste a ra te H ow ever, when these sam ples s t a g e , s u b s t a n t i a l r e d u c t i o n in c l u m p ­ H e r e the a d d i t i o n of 0 .0 5 p e r c e n t s o r b i t a n r e d u c e d the v i s c o s i t y 33.5 p e r c e n t w i t h the 1 0 . 5 - p e r c e n t p r o d u c t , w h i l e t h i s r e d u c t i o n a m o u n t e d to 88 .4 p e r c e n t w i t h t h e p e r c e n t table c r e a m . 18- T h i s e m u l s i f i e r h a d no a p p a r e n t e f f e c t on the s a m p l e s h o m o g e n i z e d a t p r e s s u r e s of 2,5 0 0 + 500 a n d 5,000 + 500 pounds p e r sq u a re inch. 121 W h e n c o n s i d e r i n g th e h e a t s t a b i l i t i e s of t h e s a m e inverse bility. s a m p l e s , an r e l a t i o n s h i p e x i s t e d b e t w e e n the v i s c o s i t y a n d th e h e a t s t a ­ O n c e m o r e , no s i g n i f i c a n t d i f f e r e n c e c o u l d b e a t t a c h e d to the n o n h o m o g e n i z e d s a m p l e s a s a r e s u l t of a d d i n g the e m u l s i f i e r . W ith h e a t s t a b i l i t y , t h e s i g n i f i c a n c e i s n o t w i t h the s a m p l e s h o m o g e n i z e d with a single s t a g e , a s w i t h v i s c o s i t y , b u t w i t h the d u a l - h o m o g e n i z e d p r o d u c t s w h e r e th e c l u m p i n g w a s n o t so h e a v y . The p r e s s u r e showing the g r e a t e s t e f f e c t of t h i s e m u l s i f i e r w a s a t 2,500 + 500 pounds p e r sq u are inch, fo r at this p r e s s u r e the h e a t s t a b i l i t y d e f i n i t e l y i n c r e a s e d a s a r e s u l t of a d d i n g t h e e m u l s i f i e r . O n c l o s e i n s p e c t i o n of t h e s e d a t a , th e a d d i t i o n of s o r b i t a n m o n o s t e a r a t e a p p e a r e d to e m u l s i f y th e f a t g l o b u l e s in s u c h a m a n n e r t h a t t h e y w e r e l e s s s u s c e p t i b l e to c l u m p i n g . This m ig h t indicate t h a t t h e p l a s m a s o l i d s , in n o r m a l c r e a m , d i d n o t c o n t r i b u t e a p p r e ­ c i a b l y to t h e s t a b i l i t y of the f a t g l o b u l e e m u l s i f i c a t i o n of the h o m o g ­ e n i z e d p r o d u c t s , s i n c e t h e y t e n d e d to f o r m c l u m p s . T h e re fo re , these o b s e r v a t i o n s a p p e a r to l e n d s u p p o r t to the t h e o r y t h a t o n c e c l u m p i n g of the f a t g l o b u l e s o c c u r s , a s th e r e s u l t of h o m o g e n i z a t i o n , t h e r e is a c o r r e s p o n d i n g d e c r e a s e in h e a t stability. If t h i s c l u m p i n g w e r e r e d u c e d , h o w e v e r , a r e s u l t a n t i n c r e a s e in the s t a b i l i t y of the p r o d u c t w o u l d o c c u r ( D o a n , 19 31a) . 122 M i g r a t i o n of f a t g l o b u l e s a s a f f e c t e d b y h o m o g e n i z a t i o n a n d com position. I n d i c a t i o n s a r e t h a t o n c e the sam p les w ere hom ogenized, l e s s t i m e w a s r e q u i r e d f o r th e f a t g l o b u l e s to m o v e a c e r t a i n d i s ­ t a n c e . t h a n t h a t r e q u i r e d f o r t h e f a t g l o b u l e s of the n o n h o m o g e n i z e d products (Table 15). T h i s , h o w e v e r , a p p e a r e d to b e c o n t r a r y to the f i n d i n g of J a c k a n d D a h l e (19 37b) , who r e p o r t e d t h a t n e i t h e r s i n g l e - n o r d o u b l e - s t a g e h o m o g e n i z a t i o n h a d a n y e f f e c t on the e l e c t r o p h o r e t i c m o b i l i t y of th e f a t g l o b u l e s . H o w e v e r , o n c e th e s a m p l e s w e r e h o m o g e n i z e d , i n c r e a s e d p r e s ­ s u r e s of h o m o g e n i z a t i o n p r o d u c e d no d i f f e r e n c e in the m i g r a t i o n r a t e of t h e f a t g l o b u l e s . A l t h o u g h c l u m p i n g w a s e v i d e n t in the s a m p l e s h o m o g e n iz e d w ith a single s t a g e , the m o v e m e n t of the c l u m p s w e r e s i m i l a r to t h o s e i n d i v i d u a l f a t g l o b u l e s p r e s e n t i n th e d u a l - h o m o g e n ­ ized s am p les . From t h i s p r e l i m i n a r y i n v e s t i g a t i o n , i n c r e a s e d p e r c e n t a g e s of f a t in h a l f - a n d - h a l f an d c r e a m r a t e of t h e f a t g l o b u l e s . d i d n o t a p p e a r to a f f e c t th e m i g r a t i o n This had b e en noted p r e v io u s ly by J a c k a n d D a h l e ( 1 9 3 7 a ) , who r e p o r t e d t h a t t h e e l e c t r o p h o r e t i c m o b i l i t i e s of f a t g l o b u l e s of n o n h o m o g e n i z e d s a m p l e s w e r e c o n s t a n t up to a f a t c o n t e n t of 65 p e r c e n t . 123 A n a l y s i s of f e a t h e r e d h a l f - a n d - h a l f a n d c r e a m . Th e r e d u c e d p r o p o r t i o n of s o l i d s - n o t - f a t f o u n d in th e p r e c i p i t a t e i n d i c a t e d t h a t s o m e of t h e s e s o l i d s - n o t - f a t w e r e l o s t i n the f i l t r a t e . The d e t e r ­ m i n a t i o n of th e d i s t r i b u t i o n of n i t r o g e n in the f e a t h e r e d m a t e r i a l s h o w e d t h i s to be t r u e . A s the m e t h o d s s e l e c t e d f o r o b t a i n i n g th e f e a t h e r e d m a t e r i a l w e r e a r b i t r a r y , n o d e f i n i t e r e f e r e n c e s w e r e f o u n d in the l i t e r a t u r e r e g a r d i n g t h e c o m p o s i t i o n of t h i s m a t e r i a l . H o w e v e r , the v a l u e s o b t a i n e d i n th e p a r e n t s a m p l e w e r e in r e a s o n a b l e a g r e e m e n t w i t h the f i n d i n g s of S h a h a n i a n d S o m m e r (1951) a n d M e n e f e e , O v e r m a n , a n d T r a c y (1 9 4 1 ) , a l t h o u g h th e h e a t - c o a g u l a b l e f r a c t i o n a p p e a r e d to be m u c h l o w e r than the v a lu e s o b ta in e d by th e se i n v e s t i g a t o r s . T h e a n a l y s i s of th e f i l t r a t e i n d i c a t e d t h a t w h e r e c a l c i u m i o n s w e r e a p r o m i n e n t f a c t o r in f e a t h e r i n g , a l a r g e r p r o p o r t i o n of c a s e i n a p p e a r e d in t h e p r e c i p i t a t e d m a t e r i a l . of the If, h o w e v e r , the i n s t a b i l i t y s a m p l e s w e r e due to a h i g h d e g r e e of f a t c l u m p i n g , a s p r o ­ d u c e d b y s i n g l e - s t a g e h o m o g e n i z a t i o n , t h e n the a m o u n t of p r e c i p i t a t e d c a s e i n w o u ld be som ew hat le ss. O n l y a s m a l l p r o p o r t i o n of the h e a t - c o a g u l a b l e f r a c t i o n of the p a r e n t s a m p l e a p p e a r e d in the f e a t h e r e d p r e c i p i t a t e . n i q u e u s e d in t h i s W i t h the t e c h ­ s t u d y , the w a t e r w a s h e a t e d to b o i l i n g (200° to 124 205° F . ) , r e m o v e d f r o m t h e h e a t , a t w h i c h p o i n t 5 m i l l i l i t e r s of the s a m p l e w a s a d d e d , a n d th e s a m p l e w a s t h e n f i l t e r e d w i t h i n 2 m i n ­ utes. From th e w o r k of R o w l a n d (1 93 3) , i t m i g h t be q u it e p o s s i b l e t h a t n o t to o m u c h of t h i s f r a c t i o n w a s p r e c i p i t a t e d . This in v e s t i­ g a t o r f o u n d t h a t 3 1 . 4 p e r c e n t of the t o t a l a l b u m i n a n d g l o b u l i n w a s d e n a t u r e d a t 75 .0 ° C. (167° F . ) in 8.2 m i n u t e s , w h e r e a s if the t i m e i n t e r v a l w a s d e c r e a s e d to 1.2 m i n u t e s , o n ly 13.7 p e r c e n t w a s d e ­ n a tu re d at this sa m e te m p e r a tu r e . w ere T h e re fo re , as these sam ples s u b j e c t e d to a h i g h t e m p e r a t u r e f o r o n l y a s h o r t t i m e , i n c o m ­ p l e t e p r e c i p i t a t i o n of t h e h e a t - c o a g u l a b l e f r a c t i o n m i g h t b e e x p e c t e d . SUMMARY I n v e s t i g a t i o n s w e r e c o n d u c t e d on the e f f e c t of v a r i a t i o n s in c o m p o s i t i o n of p r o d u c t a n d i n p r e s s u r e s of h o m o g e n i z a t i o n on the v i s c o s i t y , h e a t s t a b i l i t y , o c c u r r e n c e of f e a t h e r i n g i n c o f f e e , the a c t i o n of c a l c i u m , m a g n e s i u m , a n d c i t r a t e i o n s , a s m e a s u r e d b y a c i d p r e ­ c i p i t a t i o n , a n d t h e o r g a n o l e p t i c p r o p e r t i e s of h a l f - a n d - h a l f h o m o g ­ enized m ilk and c re a m . In a d d i t i o n , the e f f e c t of an e m u l s i f i e r on t h e c l u m p i n g of f a t g l o b u l e s , the e f f e c t of h o m o g e n i z a t i o n on the m i g r a t i o n of f a t g l o b u l e s in a n e l e c t r i c a l f i e l d , a n d th e c o m p o s i t i o n of f e a t h e r e d h a l f - a n d - h a l f a n d c r e a m w e r e studied. T h e a d d i t i o n of s k i m m i l k p o w d e r p r o d u c e d o n ly a s l i g h t i n ­ c r e a s e i n v i s c o s i t y of the n o n h o m o g e n i z e d a n d h o m o g e n i z e d s a m p l e s . T h e s e a d d i t i o n s of p o w d e r a p p a r e n t l y d i d n o t h i n d e r the f o r m a t i o n of the f a t c l u m p s . I n c r e a s e s in t h e p e r c e n t a g e s of f a t p r o d u c e d i n c r e a s e s i n the v i s c o s i t y of the n o n h o m o g e n i z e d a n d h o m o g e n i z e d s a m p l e s . H o m o g e n i z a t i o n , e i t h e r s i n g l e - o r d o u b l e - s t a g e , did n o t p r o d u c e a significant in c re a s e in th e v i s c o s i t y of 1 0 . 5 - p e r c e n t h a l f - a n d - h a l f . The f i r s t m a r k e d i n c r e a s e in v i s c o s i t y r e s u l t e d f r o m 125 single-stage 126 h o m o g e n i z a t i o n w h e n t h e f a t p e r c e n t a g e of t h e s a m p l e s w a s b e t w e e n 13 a n d 18 p e r c e n t . a pressure W hen 2 3 - p e r c e n t p ro d u c ts w e re h o m o g e n iz ed at of 5,0 0 0 p o u n d s p e r s q u a r e i n c h a n d s t o r e d a t 3° C ., a p la s tic - lik e p ro d u c t w as obtained. W i t h d u a l h o m o g e n i z a t i o n , no s i g - n i f i c a n t i n c r e a s e i n v i s c o s i t y w a s o b s e r v e d i n the p r o d u c t s t e s t i n g l e s s t h a n 18 p e r c e n t f a t . content above H o w e v e r , a s a r e s u l t of i n c r e a s i n g the f a t 18 p e r c e n t , t h e v i s c o s i t y i n c r e a s e d s h a r p l y a t a p r e s ­ s u r e of 5 , 0 0 0 + 500 p o u n d s p e r s q u a r e i n c h . T h e a d d i t i o n of n o n f a t d r y m i l k s o l i d s d e c r e a s e d th e h e a t s t a b i l i t y of b o t h th e n o n h o m o g e n i z e d a n d h o m o g e n i z e d s a m p l e s . h e a t s t a b i l i t y w a s f o u n d to i n c r e a s e The s l i g h t l y a s the p e r c e n t a g e of f a t in th e n o n h o m o g e n i z e d s a m p l e s i n c r e a s e d . On the o t h e r h and, h o m o g e n iz a tio n d e c r e a s e d the h e a t sta b ility of a l l t h e p r o d u c t s . T h e s a m p l e s s u b j e c t e d to s i n g l e - s t a g e h o m o g e n i ­ z a t i o n h a d the l e a s t s t a b i l i t y a g a i n s t h e a t . pressure H o w e v e r , w h e n 500 p o u n d s w a s p l a c e d o n t h e s e c o n d v a l v e , a n i m p r o v e m e n t in the h e a t stability w as o b serv ed . A s t h e p e r c e n t a g e of f a t i n c r e a s e d in the h o m o g e n i z e d s a m p l e s , a g r a d u a l d e c r e a s e i n the h e a t s t a b i l i t y o c ­ curred. W h e n t h e p e r c e n t a g e of f a t w a s i n c r e a s e d f r o m 10.5 to 13 p e r c e n t , a s h a r p d e c r e a s e in t h e h e a t s t a b i l i t y w a s o b s e r v e d in the sin g le-stag e hom ogenized sam p les. No s u c h d e f i n i t e d e c r e a s e w a s o b s e r v e d in th e h e a t s t a b i l i t y of the d u a l - h o m o g e n i z e d s a m p l e s . 127 T h e a d d i t i o n of n o n f a t d r y m i l k s o l i d s i n c r e a s e d the t i t r a t a b l e a c i d i t y of t h e s a m p l e s , w h e r e a s th e p H r e m a i n e d l a r g e l y u n a f f e c t e d . F o r h a l f - a n d - h a l f t e s t i n g b e t w e e n 10 a n d 11 p e r c e n t , th e a d d i ­ t i o n of 2 p e r c e n t s k i m m i l k p o w d e r w a s j u d g e d to be the m o s t d e s i r a b l e f r o m the v i e w p o i n t of b o d y a n d f l a v o r . C o f f e e m a d e w i t h h a r d , ta p w a t e r d i d n o t f e a t h e r h a l f - a n d h a l f , w h e r e a s t h a t m a d e w i t h d i s t i l l e d w a t e r o c c a s i o n a l l y d id s h o w feathering. T h e d i f f e r e n c e i n s t a b i l i t y w a s b e l i e v e d to be due to the l o w e r p H of th e b r e w m a d e w i t h d i s t i l l e d w a t e r . Th e a d d i t i o n of s k i m m i l k p o w d e r to h a l f - a n d - h a l f d i d n o t i m p r o v e s i g n i f i c a n t l y the s t a b i l i t y of t h e s a m p l e s a g a i n s t f e a t h e r i n g w h e n the c o f f e e w a s m a d e w ith d is tille d w a te r. G e n e r a l l y , h a l f - a n d - h a l f , h o m o g e n i z e d a t the h i g h e r p r e s s u r e s , a p p e a r e d to b e m o r e s t a b l e a g a i n s t t h e e f f e c t of a c i d t h a n t h a t h o ­ m o g e n i z e d a t the l o w e r p r e s s u r e s . This i n c r e a s e d stability ag ain st a c i d p r e c i p i t a t i o n w a s a t t r i b u t e d to th e a l t e r e d m e m b r a n e p r o t e i n of the h o m o g e n iz e d f a t globule. T h e a d d i t i o n of n o n f a t d r y m i l k s o l i d s a p p a r e n t l y p r o v i d e d a b u f f e r i n g a c t i o n n o t o n l y a g a i n s t the a c t i o n of the c a l c i u m , m a g n e s i u m , and c i t r a t e ions, b u t also a g a in s t p re c ip ita tio n by acid. i n d i c a t e i n c r e a s e d s t a b i l i t y of t h e s e p r o d u c t s . This would Th e c a l c i u m i o n s 128 t e n d e d to d e s t a b i l i z e t h e s a m p l e s ; the m a g n e s i u m io n s h a d l i t t l e to n o e f f e c t ; w h i l e t h e a c t i o n of t h e c i t r a t e i o n s w a s one of s t a b i l i z a ­ tion. W h e r e h o m o g e n i z e d - p r o d u c e d f a t c l u m p i n g w a s e v i d e n t , the a d d i t i o n of th e e m u l s i f i e r , s o r b i t a n m o n o s t e a r a t e , r e d u c e d th e v i s ­ c o s i t y a n d i n c r e a s e d th e h e a t s t a b i l i t y of h a l f - a n d - h a l f a n d c r e a m . T h e s e e f f e c t s w e r e n o t o b s e r v e d w i t h th e n o n h o m o g e n i z e d c o n t r o l sam ples. L e s s t i m e w a s r e q u i r e d f o r the f a t g l o b u l e s of h o m o g e n i z e d p r o d u c t s to m i g r a t e a c e r t a i n d i s t a n c e , i n a f i x e d e l e c t r i c a l f i e l d , th an f o r th o se in n o n h o m o g e n iz e d p r o d u c t s . O n c e the s a m p l e s w e r e h o m o g e n i z e d , no d i f f e r e n c e c o u l d be d e t e c t e d i n the m i g r a t i o n r a t e of the f a t g l o b u l e s e i t h e r due to i n c r e a s i n g p r e s s u r e s o r to s i n g l e v e rs u s double-stage hom ogenization. T h e f e a t h e r e d m a t e r i a l of h a l f - a n d - h a l f c r e a m w a s f o u n d to c o n t a i n a p p r o x i m a t e l y 81 to 94 p e r c e n t f a t , c a l c u l a t e d on a d r y - w e i g h t basis. O v e r 40 p e r c e n t of t h e o r i g i n a l c o l l o i d a l c a s e i n w a s p r e c i p i ­ t a t e d w i t h t h e a c t u a l a m o u n t d e p e n d i n g upo n th e m e t h o d of d e n a t u r a tion. O n l y a s m a l l p e r c e n t a g e of the h e a t - c o a g u l a b l e p r o t e i n f r a c t i o n a p p e a r e d in t h e f e a t h e r e d m a t e r i a l . CONCLUSIONS T h e v i s c o s i t y of h a l f - a n d - h a l f h o m o g e n i z e d m i l k i s a f f e c t e d f a v o r a b l y b y s u c h f a c t o r s a s (1) p r e s s u r e s of h o m o g e n i z a t i o n ; (2) s i n g l e - a s o p p o s e d to d o u b l e - s t a g e h o m o g e n i z a t i o n ; (3) p e r c e n t a g e of f a t , a n d (4) p e r c e n t a g e of s o l i d s - n o t - f a t . T h e h e a t s t a b i l i t y of h a l f - a n d - h a l f i s d e c r e a s e d b y (1) h o m o g ­ e n i z a t i o n ; (2) s i n g l e - a s o p p o s e d to d o u b l e - s t a g e h o m o g e n i z a t i o n ; a n d (3) a d d i t i o n s of n o n f a t d r y m i l k s o l i d s . I t is i n c r e a s e d by (1) l o w - p r e s s u r e h o m o g e n i z a t i o n ; (2) d u a l h o m o g e n i z a t i o n ; a n d (3) a d d i t i o n of an e m u lsify in g agent. E a c h p e r c e n t a g e of n o n f a t d r y m i l k s o l i d s a d d e d to h a l f - a n d h a l f i n c r e a s e s th e a c i d i t y , t i t r a t e d a s l a c t i c a c i d , 0 . 0 1 8 9 p e r c e n t . T h e s e a d d i t i o n s p r o d u c e d no s i g n i f i c a n t c h a n g e i n the p H of the p r o d ­ uct. T h e r a t e of m i g r a t i o n of the f a t g l o b u l e s in a f i x e d e l e c t r i c a l f i e l d is a c c e l e r a t e d b y h o m o g e n i z a t i o n . O n c e the s a m p l e s a r e h o m o g ­ e n i z e d , no f u r t h e r d i f f e r e n c e s i n th e m i g r a t i o n r a t e m a y be d e t e c t e d , e i t h e r a s a r e s u l t of i n c r e a s e d p r e s s u r e o r s i n g l e - v e r s u s d o u b l e ­ stage h o m o g e n iz atio n . 129 130 F e a t h e r e d m a t e r i a l f r o m h a lf-a n d -h a lf and c r e a m contained from 81 to 94 p e r c e n t f a t , c a l c u l a t e d on a d r y - w e i g h t b a s i s . This m a t e r i a l a l s o c o n t a i n e d a t l e a s t 40 p e r c e n t of the o r i g i n a l c o l l o i d a l c a s e i n , b u t o n l y a s m a l l p e r c e n t a g e of th e h e a t - c o a g u l a b l e p r o t e i n fraction. i H o m o g e n i z a t i o n of h a l f - a n d - h a l f , t e s t i n g 10 to 11 p e r c e n t f a t , d o e s n o t p r o d u c e the i n s t a b i l i t y a s h a s b e e n e x p e r i e n c e d w i t h t a b l e cream . If f e a t h e r i n g of h a l f - a n d - h a l f d o e s o c c u r i n c o f f e e , th e f a c ­ t o r s w h i c h s h o u l d b e c o n s i d e r e d in o r d e r of i m p o r t a n c e a r e i t y of i n g r e d i e n t s in th e h a l f - a n d - h a l f ; (1) q u a l ­ (2) p H of the c o f f e e b r e w ; a n d (3) p r e s s u r e s of h o m o g e n i z a t i o n , w i t h s p e c i a l r e f e r e n c e to s i n g l e versus double-stage. / L IT E R A T U R E CITED (1) (2) A t h e r t o n , H . F . , a n d B r a d f i e l d , A. 195 2. D o e s y o u r c r e a m f e a t h e r ? 18-23. A m e r . M ilk Rev. M ( l l ) : B a b c o c k , C. J . 1 93 1 . S o m e f a c t o r s a f f e c t i n g the v i s c o s i t y of c r e a m . D e p t . A g r . T e c h . B u i . 249. 20 p p . U. S. (3) B a b c o c k , S. M. 18 89 . T h e c o n s t i t u t i o n of m i l k , a n d s o m e of the c o n d i t i o n s w h i c h a f f e c t the s e p a r a t i o n of t h e c r e a m . W is. A g r. E x p t . S t a . B u i . 18. 35 p p . (4) B a b c o c k , S. M . , a n d R u s s e l l , H . L . I 8 9 6 . C o n d i t i o n s a f f e c t i n g the c o n s i s t e n c y of m i l k . A g r . E x p t . S t a . 13th A n n . R p t . :73-80. W is. (5) B a t e m a n , G. M., and Sharp, P . F . 1928. A s t u d y of t h e a p p a r e n t v i s c o s i t y of m i l k a s i n f l u ­ enced by some physical fa c to rs. Jo u r. A gr. Res. 36:647-674. (6) B e l l , R. W. 192 5. T h e e f f e c t of h e a t on th e s o l u b i l i t y of the c a l c i u m a n d p h o s p h o r u s c o m p o u n d s in m i l k . J o u r . B i o l . C h e m . . 6 L 391 - 400. (7) B e n t o n , A n n G . , a n d A l b e r t y , H. G. 1926. S t u d i e s on the s t a b i l i t y of e v a p o r a t e d m i l k d u r i n g s t e r i l i z a t i o n w i t h r e f e r e n c e to the h y d r o g e n i o n c o n c e n t r a t i o n , a l c o h o l t e s t a n d the a d d i t i o n of s p e ­ cific b u ff e rs . J o u r . B i o l . C h e m . 68: 25 1- 263. 1 31 132 (8) B i s h o p , J . L . , a n d M u r p h y , R. M . 191 1 . (9) T h e h o m o g e n i z a t i o n of m i l k a n d c r e a m . M aste r's T h e s i s , U n i v . of W i s . , M a d i s o n , W i s . (Original not seen. C i t e d b y M o o r e , A. V. , a n d T r o u t , G. M ., M i c h . A g r . E x p t . S ta . Q u a r t . B u i . 2 £ ( 3 ) : 1 7 7 - 1 8 7 . 1947). B rouw er, E. 1924. 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