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University

 

This is to certify that the

thesis entitled
STUDY OF FACTORS INFLUENCING
THE PHYSICAL PROPERTIES OF
FREEZE-DRIED DIRECT SET YOGURT

presented by

 

Katsuyuki Shirohata

has been accepted towards fulfillment
of the requirements for

MASTER dpgmin FOOD SCIENCE ‘
and HUMAN NUTRITION

We

Major professor

 

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r.~ — m._ _.._,_-_._V

STUDY OF FACTORS INFLUENCING THE PHYSICAL PROPERTIES
OF FREEZE-DRIED DIRECT SET YOGURT

By
Katsuyuki Shirohata

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

Department Of Food Science and Human Nutrition

T987

 

ABSTRACT

STUDY OF FACTORS INFLUENCING THE PHYSICAL PROPERTIES
OF FREEZE-DRIED DIRECT SET YOGURT

By
Katsuyuki Shirohata

Some process factors influencing the physical properties of
freeze-dried direct set yogurts were studied. An acceptable, fresh
direct set yogurt having good body and texture was obtained without
cultures by using glucono delta lactone as an acidogen, but freeze-drying
process caused the loss of typical body and texture.

In order to investigate this problem, many yogurts were prepared by
varying parameters such as fat and solids not fat (SNF) content,
stabilizers, heat treatment and pH. Physical properties of the various
yogurts so prepared were examined by use Of the penetrometer and the
Brookfield viscometer.

The effects of these variables on the characteristics Of the
reconstituted products were generally similar to the effects Observed
with fresh fermented yogurts. However, some additional fortification
Of the yogurt with milk solids and stabilizers was indispensable to
compensate for the physical loss of structure due to freezing and

dehydration.

 

ACKNOWLEDGMENTS

The author would like to express his sincere thanks to his adviser,
Dr. C.M. Stine, for his advice and guidance throughout this study and in
the preparation Of this manuscript.

Appreciation is extended to his committee members: Dr. E.K. Beneke,
Dr. P. Markakis, Dr. J.A. Partridge, for their critical review Of this
manuscript. Sincere thanks are extended to Dr. J.R. Brunner and
Dr. M.A. Filadelfi for their critical advice.

The author also wishes to express his thanks to Mr. K. Nakrani and
Mr. S.T. Hong as well as Mrs. Colleen Wood and to many graduate students
for their help.

Appreciation is also extended to many colleagues at Snow Brand
Milk Products CO., Ltd. for providing the Opportunity to carry out this
research work.

Lastly, he wishes to thank his wife for her help and encouragement

during the course Of his graduate studies.

ii

 

TABLE OF CONTENTS

LIST OF TABLES ..........................
LIST OF FIGURES ...................

INTRODUCTION ...........................
REVIEW OF LITERATURE .......................

Yogurt ................... . . . . . . . .
Nutritional Value Of Yogurt .................
Direct Acidification of Dairy Products . . . . .......
Glucono Delta Lactone ....................
Freezing Process ......................
Dehydration ...................... . . .
Freeze-drying ........................
pH of Yogurt ........................
Fat Content Of Yogurt ....................
Total Solids Content of Yogurt ...............
Milk Protein as a Source of Additional Solids ........
Stabilizer .........................
Homogenization in Yogurt Manufacture .......... . .
Heat Treatment Of Yogurt Milk ................
Flavor Of Yogurt ......................
Rheological Analysis Of Dairy Products . . . ........
Commercial Instant Yogurt Powder ..........

MATERIALS AND METHODS ..... . . . . ...........

Preparation of Fresh Direct Set Yogurt .......... .
Standardizing ......................
Heat Treatment (Pasteurization) .............
Homogenization .....................
Acidification .................... . .

Freeze-drying ........................

Preparation Of Samples for Determining Physical Properties .

Analytical Methods .....................
Penetrometer Value (PV) .................
Consistency Using the Brookfield Viscometer .......
Moisture ........................
Fat ...........................
Total Solids ......................

Page
v
vi

l

(A)

kDLOCDmVOWO‘U‘I-hww

Page

Sensory Evaluation ..................... 20
RESULTS AND DISCUSSION ..................... 22
Study of Glucono Delta Lactone ............... 22
The Effect of GDL Content ............... 22
The Effect Of Total Solids Content of the Mix ..... 22
The Effect Of Temperature ............... 25
The Time Required to Attain Desired pH ......... 25
Arrangement of Process Conditions ........... 25
Effect of Stabilizer on Physical Properties of Freeze-Dried
Direct Set Yogurt .................... 27
Effect of Total Solids Content on Physical Properties of
Freeze-Dried Direct Set Yogurt .............. 35
Effect of Fat Content on Physical Properties of Freeze-
Dried Direct Set Yogurt ................. 38
Effect of Heat Treatment on Physical PrOperties of Freeze-
Dried Direct Set Yogurt ................. 46
Effect of pH on Physical Properties of Freeze-Dried Direct
Set Yogurt ........................ 50
Effect Of Cold Storage on Physical Properties Of Freeze-
Dried Direct Set Yogurt ................. 54
Effect of Mechanical Force on Physical Properties Of
Freeze-Dried Direct Set Yogurt .............. 57
Effect of Moisture Content of Freeze-Dried Powder on
Physical Properties of Reconstituted Direct Set Yogurt . . 58
Sensory Evaluation ..................... 66
Study of Correlation on Data Obtained by Different Methods. 68
SUMMARY AND CONCLUSIONS ..................... 72
APPENDIX . . ............. . . . . ....... . . 75
REFERENCES ....................... 76

iv

 

LIST OF TABLES

Table Page
l Some physical properties Of different types of freeze-dried

direct set yogurt ................... . . . 4O

2 Sensory evaluation of plain yogurt ............. 67

3 Comparison of two analytical methods on physical properties
of freeze—dried direct set plain yogurt ........... 69

4 Comparison of physical analysis and sensory evaluation . . . 7l

LIST OF FIGURES

Figure Page
1 Preparation of samples ................... l6

2 The effect Of varying glucono delta lactone (GDL) content
on the pH of standardized milk as a function Of time. . . . 23

3 The effect of varying solids not fat_(SNF) content on the
pH Of standardized milk as a function Of time ....... 24

4 The effect Of varying mix temperature on the pH of
standardized milk as a function of time .......... 26

5 The effect Of varying pectin content of mix on the gel
firmness of fresh direct set yogurt ............ 28

6 The effect Of varying pectin content of mix on the gel
firmness of freeze-dried direct set yogurt ......... 29

7 The effect of varying pectin content Of mix on the
consistency of freeze-dried direct set yogurt ....... 3l

8 The effect of stabilizer blends on the gel firmness of
fresh direct set yogurt ................... 32

9 The effect Of stabilizer blends on the gel firmness of
freeze-dried direct set yogurt ............... 33

lO The effect of stabilizer blends on the consistency of
freeze-dried direct set yogurt ............... 34

ll The loss Of gel firmness Of direct set yogurt by freeze-
drying ........................... 36

12 The effect Of varying total solids content on the gel
firmness of fresh direct set yogurt .......... . . 37

l3 The effect of varying total solids content on the gel
firmness of freeze-dried direct set yogurt ......... 39

l4 The effect Of blending NFDM and yogurt powder on the gel
firmness of freeze—dried direct set yogurt ......... 41

vi

 

Figure
l5 The effect of varying fat content of mix on the gel
firmness of fresh direct set yogurt .............
l6 The effect of varying fat content of mix on the gel
firmness Of freeze—dried direct set yogurt . . . . .....
17 The effect Of varying fat content of mix on the consistency
of freeze-dried direct set yogurt ..............
18 The effect of varying heat treatment of mix on the gel
firmness Of fresh direct set yogurt .............
19 The effect Of varying heat treatment of mix on the gel
firmness Of freeze-dried direct set yogurt . . .. ......
2D The effect of varying heat treatment Of mix on the
consistency Of freeze-dried direct set yogurt ........
21 The effect of varying pH (GDL content) Of mix on the gel
firmness Of fresh direct set yogurt. . ...........
22 The effect of varying pH (GDL content) of mix on the gel
firmness Of freeze-dried direct set yogurt .........
23 The effect of varying pH (GDL content) Of mix on the
consistency Of freeze-dried direct set yogurt ........
24 The effect Of blending GDL powder and yogurt powder on the
gel firmness of freeze-dried direct set yogurt .......
25 The effect Of cold storage (5°C, 41°F) on the gel firmness
Of freeze-dried direct set yogurt ..............
26 The effect Of mechanical force on the gel firmness of direct
set yogurt; pH 4.2 .................
27 The effect Of mechanical force on the gel firmness Of direct
set yogurt; pH 4.0 .....................
28 The effect Of mechanical force on the gel firmness of direct
set yogurt; pH 3.9 .....................
29 The effect Of mechanical force on the gel firmness of direct
set yogurt; pH 3.8 .....................
30 The loss of gel firmness by mechanical force for different
pH direct set yogurt ....................
3l The effect of varying moisture content Of freeze-dried

powder on the gel firmness Of freeze-dried direct set
yogurt . . . ........................

Page

43

45

47

48

49

ST

52

53

55

56

59

60

6T

62

63

INTRODUCTION

Yogurt is one Of the most appealing dairy products because Of its
pleasant taste and high nutritional value.

A wide range Of flavored yogurts, particularly fruit yogurt, has
stimulated increased consumption Of this fermented food product. In
addition, yogurt-based products such as yogurt-like desserts, yogurt
drink and salad dressing, have appeared on the market as a result Of
technological improvements and better understanding of the factors
affecting the physical and chemical properties of yogurt.

One concern in yogurt production is the problem Of uniform
production from day to day because of difficulties in controlling rate
of bacterial growth and in protecting the culture from contamination.
The use of a chemical acidogen to attain the desired pH obviates this
problem.

Meanwhile the change of life-style has encouraged the fOOd
industry to produce "instant" and "convenience" type foods. The food
industry responds tO the demand for such products through product
development and constant research effort to improve quality.

Interest in freeze-drying products has been increasing, because
freeze-drying usually produces a superior product compared with
products dehydrated by other means (Stein, l966). Loss Of typical
body and texture, however, is Often Observed as a result of freeze-

drying.

 

The goal Of this research was to investigate the use of an acidogen
in preparing a direct set or imitation yogurt for subsequent freeze-
drying and evaluate means of preserving body and texture in the

reconstituted product.

 

REVIEW OF LITERATURE

Yogurt

Yogurt is a coagulated milk product produced by lactic acid

fermentation by Lactobacillus bulgaricus and Streptococcus thermo-

 

 

phjlu§_growing symbiotically. The advantage Of using mixed cultures
was well reviewed by Wong et al. (1982). Generally, more than one
culture is used for commercial yogurt production though Marshall et al.
(l982) indicated that single starter yogurts with some chemical
additive had acceptable acidities, acetaldehyde level and good quality.
There are two main types of yogurt, set and stirred yogurt, which
depend on the process method and on the physical structure of the
coagulum. Set yogurt, which is fermented in the retail container, is
in a continuous semi-solid mass. On the other hand, the stirred yogurt
is coagulated in bulk and its gel structure is broken before cooling
and packaging. Another method to be used to differentiate various
types of yogurt is flavoring difference. There are three categories:

plain, fruit and flavor yogurt.

Nutritional Value of Yogurt

 

According to Rasic et al. (l978), the chemical changes of milk
constituents during fermentation result in reducing the content Of
lactose and the formation of lactic acid, in increasing the content Of

free peptides, amino acids and fatty acids and in considerable changes

of content of some vitamins. There are some fennentative changes in
milk constituents that may give yogurt nutritional superiority over
milk due to superior digestibility (Rasic and Kurmann, 1978; Tamime
and Deeth, 1980; and Broussalian and Westhoff, 1983). However, Wong
(1983) suggested the growth stimulating factor Of rats that were fed

yogurt was within the cells of Streptococcus thermophilus and did not

 

depend upon fermentation Of milk.

Direct Acidification Of Dairy Products

 

For the production of good fermented dairy products such as yogurt
and cheese, a correct rate of acidification and degree of acidity is
essential. Therefore problems involving the starter organisms may
cause severe consequences. This is why alternative methods Of
acidification are attractive to consider. A successful method would,
at one stroke, eliminate the problems of bacteriophage, antibiotics,
winter slowness and other difficulties.

Trop (1984) studied pH decrease in milk by various chemical
acidogens such as propionic anhydride, lactic acid, glucono delta
lactone (GDL), ethyl butyrate with its esterase and glucose with
oxidase. According to his research, GDL gave the best coagulation Of

milk without unfavorable taste.

Glucono Delta Lactone

 

 

 

 

 

O O \‘ OH
\C \(Iz/
1
H—C—OH H-C-OH
I H20 1
HO-C-+I O \\_ HO—C-Ji
I > I
H—C—OH ' H—C-OH
I l
H—C . lh-C—OH
l l
CHZOH CHZOH
GDL Gluconic Acid

Glucono delta lactone (GDL) hydrolyzes to the free acid at a
convenient rate (Mabbit et al., 1951). It is readily available,
non-toxic and free from other Objectionable properties such as
insolubility Of calcium salts and unpleasant taste. The study Of pH
decrease Of milk by GDL was followed by O'Keeffe et al. (1975). Their
data indicated the falling pH depends on the concentration of GDL and
the temperature of hydrolysis. pH decrease was relatively rapid and
continuous, and about 70% Of total pH decrease occurred within
90 minutes. They also reported the advantage Of using GDL compared
with acidification by starters from the viewpoint Of stable and rapid
fall in pH. They concluded the merit of yogurt acidified with GDL as
follows:

1. rapid pH decrease
relatively stability Of pH after 90 minutes

ease of handling

poem

no problems with contamination of a starter culture

Freezing‘Process

 

Tremendous amounts Of frozen foods can be seen in supermarkets
and frozen foods today are one Of the most popular food items. One
reason is the superiority of freezing as a means of preservation and
another is convenience since most Of them are preprocessed and
precooked. However, freezing process sometimes causes a serious
problem to dairy products. Dahle (1941) observed the disruption Of
the milk fat emulsion by freezing lent to the formation of a surface
layer Of fat aggregates in the thawed material. This phenomenon was
studied by Webb and Hall (1935) in products before being stored.
According to Desai et al. (1961), the loss of soluble lactose through
crystallization during freezing enhanced destabilization. Wildasin
and Doan (1951) studied the effect Of hemogenization and preheat
treatment on protein stability of frozen milk products.

It has been suggested that ice crystals exert pressure on the
suspended milk fat globules and disrupt the colloidal protein suspension

during freezing (Doan and Baldwin, 1936).

Dehydration

 

The storage life of yogurt refers to maintaining its characteristics
unaltered until the product is consumed. There are many different
methods to accomplish this objective, but dehydration is the most
unique and the best method from the viewpoint of storage energy cost,
shelf-life and ease of handling. Dried yogurt, which is essentially
free of enzymatic and microbial spoilage, can be produced by sun-drying,

spray-drying and freeze-drying. In industrial production Of dried

yogurt by spray-drying or freeze-drying a loss of some flavor compounds

was indicated by Tamime and Deeth (1980).

Freeze-Drying

 

In recent years there has been substantial processing of foods by
freeze—drying, and such products are used widely as convenient,
"instant" food materials. Freeze-drying is a process of sublimation
which removes moisture from frozen products by changing ice to vapor
without going through the liquid state, and thus, represents a
combination of freezing and drying processes. This combination has
great effect on the finished product and a lot of examples have shown tO
overcome disadvantages caused by using either process alone. The
superiority of freeze-drying was well described by Stein (1966) and
King (1970). Reconstituted freeze-dried foods are generally superior
in flavor and other quality attributes when compared to products
dehydrated by more conventional methods.

Desai (1966) studied the effects Of different drying methods on
some Of the physical, chemical and organoleptic properties of cultured
cream. She Observed the complete loss Of the typical body and texture
during freeze-drying. However, the retention of volatile flavor
compounds was better in the freeze-dried sour cream than in spray-dried
sour cream.

One of the big problems in freeze-drying is its high energy cost,
which was estimated to be approximately seven times greater than

spray-drying (Goldblith, 1965).

pH of Yogurt

 

pH Of yogurt is variable, but it must be below the isoelectric
point (pH 4.6), because low acidity in yogurt influences the consistency
unfavorably due to insufficient protein hydration and/or charge
stabilization Of the coacervate. Cooling of yogurt must be carried
out within a narrow pH interval, since too slow cooling causes over-
ripening, which results in a grainy and sandy structure. 0n the other
hand, cooling too rapidly results in changing in the structure of
coagulum with corresponding whey separation due to the very rapid
contractions of protein filaments and their disturbed hydration
(Kurmann et al., 1978).

Using an acidogen rather than bacterial cultures may help solve
the problem Of cooling, since pH is relatively stable after 90 minutes.
The common pH range Of final products is pH 3.7 - 4.3 (O'Neil et al.,
1979).

Fat Content Of Yogurt

 

Milk fat favorably affects the various prOperties of yogurt such
as flavor, aroma and viscosity.

Milk fat stabilizes contraction Of the protein gel and prevents
whey separation in the final product (Rasic and Kurmann, 1978).
Regarding the taste, it has been Observed that skim milk yogurt is more
acid, less mild and less aromatic than whole milk yogurt. Commercial
yogurt usually contains at least 1.5% milk fat, since low fat content

in the milk also encourages whey separation in yogurt.

Total Solids Content of Yogurt

 

The level of total solids in the milk is significant fOr both the
consistency and aroma Of yogurt. In general, an increase in total
solids will enhance these properties (Emmons and Tuckey, 1967).

Tamime and Robinson (1983) indicated that the recommended range is
14 - 16% and Kozhev et a1. (1972) concluded that the best yogurt was
made from milk containing 15.5 - 16.0% total solids since tOO high
fortification with skim milk powder (SMP) causes excessive acid
production and some off-flavors (Gennip, 1973).

For this reason the solids not fat (SNF) content of yogurt is
recommended in the range 10 - 13% (Jenson and Nielsen, 1982), which
means that whole milk is usually fortified with an additional 3 - 4%
serum solids.

Mickle (1966) studied the physical stability Of reconstituted
freeze-dried milk products. According to his research, solids not fat
(SNF) improved fat stability and prevented the problem of poor
dispersibility Of freeze-dried milk products due to the instability of

the milk fat emulsion.

Milk Protein as a Source of Additional Solid

 

Whey protein concentrate (WPC) produced by ultrafiltration or
reverse osmosis was used in the manufacture Of yogurt by Modler et a1.
(1983). Adding WPC produced by ultrafiltration Of whey resulted in a
high quality yogurt.

Caseinate possesses good functional properties such as stabilizing

effects in yogurt manufacture. Sodium caseinate is effective in

lO

increasing gel strength and reducing syneresis. The effect of adding
sodium caseinate and skim milk powder on the viscosity of stirred
yogurt was studied by Gennip (1973). According to his research, sodium
caseinate demonstrated a much greater effect on the viscosity of yogurt
so that three parts of skim milk powder could be replaced by one part
of sodium caseinate. Modler et a1. (1983), however, indicated that
yogurt containing sodium caseinate was less smooth in texture than

yogurt containing only serum solids.

Stabilizer

 

Stabilizers have a multitude of functional properties. They are
used as crystallization and syneresis inhibitors, adhesives, viscosity
modifiers and as agents for binding, gelling and thickening. They are
polymeric substances which gradually hydrate, when dispersed in water,
whereby a large number of water molecules are bound, primarily by
hydrogen bonding.

Since the addition Of stabilizers increases the consistency Of a
coagulum due to increased water binding, some stabilizers are used in
yogurt.

A variety of gums is used in dairy products to control viscosity.
Guar gum, which is derived from the seed Of the guar plant, is a
polygalactomannan gum. It is a good stabilizer for many dairy products
due to high solubility at low temperature. Carageenan, which is
extracted from seaweeds, is an anionic polysaccharide gum having
hydrocolloidal properties. It is commercially employed for increasing

the viscosity and stabilizing various aqueous systems.

ll

Pectin, which is obtained from the water-insoluble protopectin Of
fruit, is widely used in making jam and jellies. It is also useful as
a stabilizer and thickener in various kinds of dairy products with
particular application in products Of low pH such as ices, sherbets
and fenmented foods.

The microstructure Of yogurt with thickening agents such as
gelatin, carageenan and starch was studied by Kalab et a1. (1975).
Scanning electron microscopic observation indicated the arrangement of
the micelles and the formation of'a protein skeleton which created
large free spaces inside the network depending on the type of
stabilizers and the mechanical treatments employed.

Stabilizers such as propylene glycol alginate, sodium carboxy-
methyl cellulose and pectin were used for cultured milk and their
effects were studied by Towler (1984). These studies have indicated
the usefulness and importance of using various stabilizers in yogurts

as an agent which performs gelling and water-binding.

Homogenization in Yogurt Manufacture

 

Homogenization of yogurt milk has the following advantages (Rasic
et al., 1978):
1. no rising Of cream during incubation of yogurt milk and uniform
distribution Of fat
2. improvement of the consistency and viscosity Of yogurt
3. greater stability Of the coagulum against whey separation
According to Abrahamsen and Holmen (1981), some whey separation and a

thin cream layer at the surface was observed in non-homogenized yogurt.

12

In addition, flavor and physical properties such as curd tension were
also inferior to yogurt prepared from homogenized milk. The study of
the effect of homogenization and heat treatment on the firmness and
viscosity of yogurt by Galesloot (1958) coincided with these observa-
tions. Storgards (1964) reported that best results Of yogurt were
achieved with homogenization of 100 - 200 kg/cmz (1420 - 2840 psi) at
50 - 60°C (122 - 140°F).

Heat Treatment Of Yogurt Milk

 

Heat treatment Of yogurt milk is extremely important from the
vieWpOint of improving consistency as well as killing organisms and
inactivating naturally occurring enzymes.

In general, milk for yogurt manufacture is preheated at 80 - 85°C
(176 - 185°F) for 30 minutes or 90 - 95°C (194 - 2030F) for 5 - 10
minutes. Changes in the protein fraction through heat treatment are
critical in determining the stability Of the yogurt gel. Davies et al.
(1978) and other scientists have considered that the heat treatment Of
milk resulting in denaturation of beta-lactoglobulin and the association
of this protein with micelle surface is an important determinant Of
micelle fusion and gel strength.

The effects of heat treatment on rheological properties Of yogurt
were well studied by Labmpouios et al. (1984). According to their
study, heat treatment had a definite effect on yogurt body and texture.
They suggested good rheological properties, measured by penetrometer
value and viscosity could be attributed to increased water-binding

capacity of the milk proteins subjected to proper temperature and time

13

relationships.

Flavor Of Yogurt

 

Lactic acid fermentation during yogurt manufacture provides lactic
acid as a main component with small quantities of carbonyl compounds,
volatile fatty acids and alcohol. Important components as sources of
flavor and aroma are carbonyl compounds such as acetaldehyde, diacetyl,
acetoin (acetylmetylcarbinol) and acetone. Marshall et a1. (1982) and
many others have reported the major flavor component of yogurt is
acetaldehyde. Typical yogurt flavor is obtained at 30 ppm acetaldehyde,
according to Gorner et a1. (1973). Fullness of flavor, however, also
depends on the presence Of other carbonyl components such as acetone
and diacetyl, which are produced at very low concentration (Bottazzi
et al., 1973). Flavor components can arise from fat, protein or
lactose, but those which have important relationships to flavor are

formed by microbial fermentation of lactose (Tamime and Deeth, 1980).

Rheological Analysis Of DairngrOducts

 

In order to study physical properties of products such as viscosity
and gel firmness, rheological analysis is indispensable. Various
techniques have been developed and used according to the nature Of
characteristics to be evaluated.

In the field of dairy products, O'Neil et a1. (1979) used the
“Curd—O-Meter", an apparatus for measuring curd tension (a second
generation version of the Submarine Signal curd tension meter produced

eventually by the Raytheon Corporation) and the Brookfield viscometer,

14

which was used to measure viscosity or consistency. A penetrometer
used by Hamilton (1970) and Labropoulos et al. (1984) is employed to
determine gel firmness by measuring the depth of penetration when a

standard cone is dropped into the specimen.

Commercial Instant Yogurt Powder

 

It has been difficult to product "powdered yogurt" as a commercial
product because of the loss of physical properties as well as the high
energy cost involved in its production. However, the first instant
yogurt including active cultures in the United States has recently been
manufactured and introduced by a commercial corporation.

The desirable characteristics of this instant yogurt are claimed
to be: extended shelf-life and content of active, viable organisms.

The powder, which is called "Instant culture" for its active cultures,
is promoted in different flavors. Active cultures may be used for a
nutritional contributer, not for a function Of coagulation. Mainly,
the stabilizer system has the responsibility to form the good body and
texture in this instant product. Instant yogurt powder may have the
possibility to impact to the dairy industry as "instant coffee” did

earlier to the coffee industry.

MATERIALS AND METHODS

Preparation Of Fresh Direct Set Yogurt

 

Some specific conditions were changed each time according to the
factors Of interest, but the basic procedure (Figure 1) was accomplished
as follows:

Standardizing

 

According to the desired formula, non-fat dry milk (NFDM) (Grade A
Low-Heat Spray Non Fat Dry Milk, Michigan Milk Producers Association)
and water were added to fresh raw whole milk which was Obtained from
the Michigan State University Dairy Plant or Michigan State University
Dairy Barn. Other additives such as stabilizers, if used, were added
and dispersed by means Of a blender (Lightnin Mixer 10X, Mixing
Equipment CO.) at 1725 RPM.

Heat Treatment (Pasteurization)

 

The standardized milk inside a container was put in a hot water
bath. The milk was heated to, and maintained at, 80°C (176°F) for
30 minutes, unless otherwise specified.

Homogenization

 

After cooling the milk to 60°C (140°F), the milk was homogenized
at 140 kg/cm2 (2000 psi) by a homogenizer (Type 75K, Manton Gaulin
Mfg. Co., Inc.). Although a two-stage homogenizer was used, the
second stage was kept at 0 pressure through this research since the

fat content Of the mix was relatively low and did not require two—stage

15

16

Basic Procedure of Direct Set Yogurt

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fresh Whole Milk NFDM Water Other Additives
L l l 1
4L
Standardizing by a blender at 1725 RPM
Heat Treatment
(Pasteurization) 80°C (176°F) for 30 min.
I
Homogenization 60°C (140°F) at 140 kg/cm2
\1/ (2000 psi)
Acidification by 3.0% GDL
I at 30°C (86°F)for 90 min.
4L J
Cold Storage at 0C Freeze-drying < -40°C (-40°F)
(39 F) < 0.6 mm Hg
11 LIL
Fresh Direct Freeze-dried
Set Yogurt Direct Set stored in
Yogurt Powder screw top jars
it
Reconstitution with distilled
w water
Cold Storage at 4°C (39°F)
ILL
Freeze-dried
Direct Set
Yogurt
J
Isak
Analysis

 

 

 

Figure 1.

Preparation Of samples

17

homogenization.

Acidification

 

The homogenized milk was held at 30°C (86°F), unless specified, in
a water bath and glucono delta lactone (F.C.C. Coarse Powder,
Pfizer INC.) was added to the milk. The mix was initially stirred well
and then held quiescent for 90 minutes. The condition of acidifica-
tion; temperature, amount Of glucono delta lactone and time were

reviewed early in this thesis.

Freeze-Drying

 

The fresh direct set yogurt prepared was moved to special stainless
trays and spread thinly in order to reduce the depth of yogurt to less
than 1 cm. The trays including yogurt were rapidly frozen in a
freeze-dryer (Virtis REPP Model FFD 42WS) by direct contact with the
platens which were cooled to less than -40°C (-40°F). After freezing
was accomplished, the drying process was initiated by lowering absolute
pressure to 0.6 mm Hg, or less, by means Of a Delco Model 2J4724-Z
vacuum pump.

The process Of freeze-drying was terminated when the temperature
of the platen was between 16°C and 20°C (62°F and 67°F). In order to
Obtain a powder with less than 5% moisture, twenty-four hours were
required for the dehydration process and the rise in temperature was not
Observed until the end of this process. The freeze-dried direct set
yogurt was transferred to screw tOp jars for storage and subsequent

analysis.

18

Preparation of Samples for Determining

 

Physical Properties

 

A small portion Of fresh direct set yogurt obtained was stored at
4°C (39°F) for at least 12 hours before analysis to eliminate the
temperature influence on the body and texture of yogurt.

The freeze-dried direct set yogurt was reconstituted to the
original total solids Of the fresh yogurt with distilled water at room
temperature. At that time the moisture content of each different
sample was taken into consideration when hydrating. The powder and
water were weighed into the same beaker on a top loading Mettler
balance and mixed by a hand mixer (Mixette Model 97-1, Hamilton Beach).
In order to reduce the physical damage to the body of reconstituted
yogurt and achieve constant condition, the best procedure was studied
early in this research. The mixer was Operated at the lowest speed for
one minute to achieve a homogeneous yogurt body.

The reconstituted yogurt was refrigerated at 4°C (39°F) for at
least 12 hours before determining physical properties to allow extensive

hydration and formation of gel structure and body.

Analytical Methods

 

Penetrometer Value (PV)

 

In order to determine the gel firmness of yogurt, a penetrometer
(“Precision" Penetrometer, 1>recision Scientific CO.) was used. The
chilled reconstituted yogurts and fresh yogurts which had been held
quiescently for at least 12 hours at 4°C (39°F) were tested in duplicate

by this equipment immediately following removal Of the sample from the

19

refrigerator. 550 g fresh direct set and reconstituted direct set
yogurts in 600 ml beakers were used for this determination. The tip Of
the cone was set to the center Of yogurt surface in the 600 m1 beaker
and the cone (weight 102.4 9, diameter 65 mm and height 45 mm) was
allowed to penetrate the surface Of yogurts for five seconds. The
depth Of penetration was measured in tenths Of millimeters. Readings
shown in figures are averages Of two determinations.

Consistency Using the Brookfield Viscometer

 

The consistency of chilled reconstituted yogurt was determined
with a viscometer (Brookfield SyncrO-lectric Viscometer Model RTV,
Brookfield Engineering Laboratories). A modified Operation of the
method used by O'Neil et a1. (1979) was used. The viscometer was
Operated at 2.5 RPM with a NO. 4 spindle which was set in the center Of
a sample Of 300 grams of undisturbed reconstituted yogurt in a 400 ml
beaker. Samples were analyzed in duplicate on yogurt equilibrated to
5°C (41°F) and recorded as centipoises.

Moisture

The percent moisture of the freeze-dried yogurt powder was
determined by a vacuum oven method using an oven (Model 524-A, Precision
Scientific CO.) attached to a vacuum pump (Duo-Seal) and an analytical
digital balance (Mettler, AE160). Approximately 2 9 samples were
accurately (10.1 mg) weighed into aluminum moisture dishes and held at
1000C (2120F) and less than 100 mm Hg absolute pressure for at least
five hours until constant weight was obtained. The weight was recorded
to four significant figures and averages Of triplicate determinations

were Obtained.. These values were then used to calculate the amount of

20

water required for reconstitution to the desired total solids content.
[at_

The fat content Of the standardized milk was determined in
duplicate by the Official Babcock method and the average of two results
was used to determine fat content of the standardized milk.

Total Solids

 

The percent total solids of the standardized milk was determined
by a vacuum oven method. Approximately 5 9 milk samples were examined
in triplicate using the same equipment described earlier. The average
of three determinations was used to calculate the total solids content
of the standardized milk.
pH_

The pH measurements were made with an analog pH meter (Model 301,
Orion Research) using a calomel half cell and glass electrode standardized
to read accurately in the range Of pH 4.0 - 7.0. The reading was made
to the nearest 0.1 unit after the system had equilibrated with the

electrodes.

Sensory Evaluation

 

Sensory evaluation Of four plain yogurts which were prepared by
four different procedures were accomplished to supplement the physical
analysis and confirm the sensory properties of these yogurts.

Approximately 2 oz (76 9) samples were placed in 3 oz (113 9) cups
at 4.40C (40°F) and evaluated by a selected panel Of 10 trained or
semi-trained judges among faculty and graduate students Of Michigan

State University. The evaluation Of the four yogurts was made in the

21

sensory evaluation room, using a hedonic scale with a range Of 1 - 7

(Appendix Figure l).

 

RESULTS AND DISCUSSION

Study Of Glucono Delta Lactone

 

As a source Of chemical acidogen for producing direct set yogurt,
glucono delta lactone (GDL) was selected since Trop (1984) concluded
GDL was the best acidifying agent among those he evaluated. It has
been widely used in the manufacture of acidified dairy foods.

Several factors which relate to the effect of acidogens were
examined early in this research work to select Optimal conditions for
lowering pH.

The Effect of GDL Content

 

The drop in pH of standardized milk after addition Of GDL
(Figure 2) follows a pattern similar to that reported by O'Keeffe
et a1. (1975). The change Of pH indicates that 2% or more Of GDL is
needed to prepare yogurt, since the pH of the final product should be
below the isoelectric point of caseins (pH 4.6) in order to form a good
firm coagulum.

The Effect of Total Solids Content of the Mix

 

The decline in pH as a function of total solids content is shown
in Figure 3. Due to the buffer capacity Of the proteins and salts in
the mix greater concentrations of GDL are needed to lower pH as solids
not fat (SNF) content increases. It is concluded that larger amounts

Of GDL should be used as a chemical acidogen in direct set yogurt

22

23

 

 

 

 

 

 

7
.0. +0.57. cm. 1.57 m
1. +1.07. (:01. 10.07. s00
6.55% D +2.07 GDL at 306
‘33.: I +3.07 cm.
V\
6‘ \\.\\‘ CA
\11. “A,
l “S ‘xc
AL ‘\\\ ‘~we~_‘~H‘{LH~hh
5.5. ‘~ '1
:g— \\\.‘ \B\ \K\
5- "\ (LN ‘ 1 M
\\ ‘\_8\
IL \\\
4.5. \ \0\
Ifi\\ iixMMTT‘i}--—-N
4‘ \kN—K
3.5
0 l I I 1
Time (Hour)
Figure 2. The effect of varying glucono delta lactone (GDL) content

on the pH of standardized milk as a function of time;
temperature 30°C (Milk standardized to 1.5% fat, 10.0%

SNF)

24

   
   

 

O 15.37. sur 1.57. m
I 12.77. 310-“ +3.07. cm.

6. g 10.07. 310" at 30c
| 0.27. snr

 

 

 

 

 

Time (Hour)

Figure 3. The effect Of varying solids not fat (SNF) content on the
pH Of standardized milk as a function Of time; 3.0% GDL,
temperature 30°C (Milk standardized to 1.5% fat)

25

since most yogurt mix is fortified with varying levels Of SNF.

The Effect Of Temperature

 

The fall in pH Of fortified yogurt mix containing 3% GDL was
examined at 25°C (77°F), 30°C (86°F), 35°C (95°F) and 40°C (104°F)
(Figure 4). The higher temperature seems to stimulate the hydrolysis Of
GDL more between 25°C and 35°C, but there is no significant difference
between 35°C and 40°C.

In addition, the body and texture of yogurt tends to deteriorate
at higher temperature and severe whey separation can be seen in the
yogurt treated at 40°C. It is possible that denaturation Of protein in
the mix is responsible for this phenomenon.

The Time Required to Attain Desiredng

 

For yogurt mix containing 3% GDL, the decrease in pH is relatively
rapid during the initial 90 minutes and then slows down during the
ensuing time to 120 minutes (Figure 3 and Figure 4). In addition,
keeping the mix for a long time at higher temperatures is not desirable
from the viewpoint Of effect on body and texture and perhaps undesired
microbial growth. For these reasons, incubation for 90 minutes is
considered to be reasonable as an acidification time.

Arrangement of Process Conditions

 

From the above experiments the basic process conditions for
acidification with GDL were selected as follows:

1. amount of GDL 3%

2. mix temperature 30°C

3. acidification time 90 minutes

26

 

at 25C 1.52 FAT
at 30C 10.07. SNF
at 356 +3.87. GDL
at 40C

 

 

 

 

”4 “a \.
\x, W
4. .
3.5
9

 

Time (Hour)

Figure 4. The effect Of varying mix temperature on the pH Of
standardized milk as a function of time; 3.0% GDL (Milk
standardized to 1.5% fat, 10.0% SNF)

 

27

By employing these conditions any problem on physical properties

in fresh direct set yogurt was not Observed.

Effect Of Stabilizer on Physical Properties

 

Of Freeze-Dried Direct Set Yogurt

 

The loss of typical body and texture Of yogurt through freeze-
drying was Observed by the author early in this research, as was
earlier noted by Desai (1966) in freeze-dried cultured cream. This
phenomenon may be due to the destruction Of matrix structure during
freeze-drying process. Therefore, higher levels Of solids not fat
(SNF) and varying amounts Of pectin (Grade 150, Sunkist Growers) were
added to yogurt milk in order to compensate for this loss Of desirable
structure.

Nevertheless, the comparison Of data (Figure 5 and Figure 6)
indicates very clearly the loss Of gel firmness when direct set yogurt
is freeze-dried. Although there is no significant effect Of pectin on
gel firmness in fresh yogurts (Figure 5), there is a very significant
effect in reconstituted yogurts (Figure 6). It is interpreted that
the yogurt milk fortified with NFDM can create a firm gel by proper
acidification without stabilizers, but the freeze-dried powder requires
additional stabilization to exhibit the desired properties upon
reconstitution.

As a matter of fact, the body of reconstituted yogurt with 0.5%
pectin can be maintained within 100 Penetrometer Value (PV), but the
loss of gel firmness increases as pectin content decreases. The

complete loss of typical yogurt body can be Observed for reconstituted

 

28

399

 

£3 9 2 pectin
C) 9.17. pectin
379. D 9.37. pectin
I 9.5% pectin

399-

369-

359

349.

339-

329-

319.

 

399,

290,

Penetrometer Value (0.1 mm)

289.

279.

269,

259

 

 

 

9 5 19 1’5 2'9 25 39

Time (Sec)

Figure 5. The effect of varying pectin content Of mix on the gel
firmness of fresh direct set yogurt (Milk standardized
to 3.0% fat, 15.5% SNF)

 

29

 

 

 

 

 

 

 

 

 

500
D 9.3% pectin 3.97. FAT

489- I 9.57. pectin 15.57. SMI"

4ML
E 440.
o’
3 42°- F, L n 3_——e 0
r- U U U
(U
>' 400
t ‘ l—»——-—I-——-I—"——’—l I l
4..)
Q)
8
L 3mm
.p
(D
C
OJ
e- 360.

340.

320.

300

l l 'T ‘1
0 i 10 15 20 2'5 30 33
Time (Sec)

Figure 6. The effect Of varying pectin content Of mix on the gel
firmness of freeze-dried direct set yogurt (Milk standardized
to 3.0% fat, 15.5% SNF)

 

30

yogurts with less than 0.3% pectin, making it impossible to determine
PV due to the very weak body. The penetrometer cone "bottoms out" due
to very low resistance of the body to the penetrating force.

Measurement of consistency in reconstituted yogurts (Figure 7)
reveals a non-Newtonian behavior of yogurt. There is a large difference
Of consistency between the yogurt containing 0.3% or more pectin and
the sample with less than 0.3% pectin. Yogurt containing no pectin or
0.1% pectin had extremely low consistency upon reconstitution.

These results show the importance of stabilizer performance in the
reconstituting process and the strong swelling and gelling capacity Of
pectin which may be as important as the effect of coagulation in such a
system. They also indicate that at least 0.3% pectin should be added to
yogurt mix to insure good body in the reconstituted product.

In the food industry proprietary blends of stabilizers are often
used for yogurt production, because various stabilizers in combination
may exhibit complementary effects.

One attempt to evaluate a blend in this research involved the
addition of guar gum (Celanese Jaguar A-20-B, Polymer Specialties
Company) with pectin in the yogurt mix, since guar gum performs as a
syneresis inhibitor.

An advantage in using the stabilizer combination was obvious,
but other desired effects were not noted. For example, the data in
Figure 8 show a decrease in gel strength when guar gum was added to the
0.5% pectin systems. Likewise (Figure 9) the gel strength Of the
reconstituted yogurt was lower in the products containing 0.5% pectin

plus additional guar gum and consistency (Figure 10) indicated weaker

 

31

 

 

 

 

 

250
I 9.5% pectin
g 0.37. pectin
0 9.17. pectin
233‘ lg .8 9 z pectin
13
U
8 150-
X
>5
U
C
(I)
+9
.33 100.
U)
C
O
L.)
59.
Et:2:ih::=$§=:=4k:==d&==:#&==-
-=£F===£k=::j§::::fi
9 7 l 7
0 .5 i 1.5 i 2.5 i 3.5 4 4.5
Time (Min)
Figure 7. The effect of varying pectin content Of mix on the

consistency of freeze—dried direct set yogurt (Milk
standardized to 3.0% fat, 15.5% SNF)

 

32

 

37B
.9. 9.57. pectin + 9.27. guar gun
368‘ I 9.5% pectin + 9.17. guar gun
359. [3 9.32 pectin + 9.2% guar gun
1 9.3% pectin + 9.17. guar- gun
349..
339.

A
329.. W

310. 4.———«—l
300. m

VHF-J—

Penetrometer Value (0.1 mm)

 

 

 

290. O/
280.
270.
260.
250
0 i 1‘0 15 210 2‘5 3'0 35

Time (Sec)

Figure 8. The effect of stabilizer blends on the gel firmness of
fresh direct set yogurt (Milk standardized to 2.9% fat,
15.5% SNF)

33

 

599
O. 9.57. pectin + 9.27. guar gun
489- I 9.57. pectin + 9.1;: guar gun
[1 9.37. pectin + 9.27. guar gun
469- l 9.37. pectin + 0.17 guar gun
449-

 

: W

 

 

Penetrometer Value (0.1 mm)

 

 

| .4L_. .-—1 .3 1 .41
389-
369-
349-
329-

3996 g l l l 1 l

19 15 29 25 39

Time (Sec)

Figure 9. The effect Of stabilizer blends on the gel firmness Of
freeze-dried direct set yogurt (Milk standardized to
2.9% fat, 15.5% SNF)

 

899

34

 

799-

699-

5991

Consistency x 100 (cp)

499-

399

D 9.32 pectin + 9.2% guar gun
I 9.5% pectin + 9.1% guar gun
.1 9.37 pectin + 9.1% guar gun
C: 9.57. pectin + 9.27. guar gun

 

 

Figure 10.

.1; l 135 I 215 I 31.5 1

Time (Min)

The effect of stabilizer blends on the consistency of
freeze-dried direct set yogurt (Milk standardized to
2.9% fat, 15.5% SNF)

 

35

body in such combinations. This may be due to reduction in coagulation
or matrix structure when high levels of stabilizer are used. Their
interaction with the milk proteins may actually weaken the tendency to
clot at the isoelectric point.

From the effect of stabilizer combinations on gel finmness and
consistency it appears that 0.3% pectin in combination with 0.1% or
0.2% guar gum yields better physical properties.

This conclusion is substantiated by observations Of the loss of
gel firmness through freeze-drying which indicates that 0.4% - 0.5%

stabilizers added to yogurt mix yielded superior properties (Figure 11).

Effect of Total Solids Content on Physical Properties

 

Of Freeze-Dried Direct Set Yogurt

 

Yogurt produced by a mix containing 10.3% SNF (13.4% TS) has a
very weak body even when fortified with stabilizer.

The solids not fat (SNF), and especially casein, is very important
in the formation Of a firm gel structure. This is true Of yogurt
reconstituted from freeze-dried powder. In such a system, SNF has an
important function as a structural contributor and as a water-binding
material for gelation when the powder is hydrated. For this reason
higher levels of SNF are necessary to Obtain Optimal body.

Sodium caseinate, which might be considered to have basically the
same function as SNF on the physical properties in yogurt except for
properties related to salt balance, was evaluated as a possible means
of avoiding high SNF and TS content. The effect of added sodium

caseinate on the body of direct set yogurt (Figure 12) is contrary to

36

no stabilizer

° 0.1% pectin

0.3% pectin

0.3% pectin + 0.1% guar gum
0.5% pectin

0.3% pectin + 0.2% guar gum
0.5% pectin + 0.1% guar gum
0.5% pectin + 0.2% guar gum

A:
B.
C:
D:
E:
F:
G:
H:

PV increase (0.1 mm) after 30 sec

 

A B C 0 ' E F G H
0 0.1 0.3 0.4 0.5 0.5 0.6 0.7

Total Stabilizer %

Figure 11. The loss Of gel firmness Of direct set yogurt by
freeze-drying (Milk standardized to 2.9 - 3.0% fat, 15.5%
SNF

37

499

 

0 12.07. SNF + 1.07. sodium caseinate
1 12.87. 310*
389- g 15.27. sup

399-

3?9.

369-

Acre/”##MéfflffllrflféFffl—

349-
339-
329-

319-

Penetrometer Value (0.1 mm)

399

299-

289-

279-

269

 

l l l 1 l
9 5 19 15 29 25 39

 

Time (Sec)

Figure 12. The effect of varying total solids content on the gel
firmness of fresh direct set yogurt (Milk standardized
to 3.1% fat containing 0.3% pectin and 0.1% guar gum)

 

38

Observations Of Gennip (1973). The data show no beneficial effect Of
fortification with sodium caseinate.

On the other hand, fortification with sodium caseinate results in
improved firmness of reconstituted yogurt (Figure 13) and the data
suggest that 2.4% SNF could be replaced by 1.0% of sodium caseinate.
However, the texture Of yogurt was found to be slightly impaired by
sodium caseinate (Table 1). This Observation was indicated earlier by
Modler and Kalab (1983).

Another possible approach to improved body in the reconstituted
yogurt would be dry blend solids in the dehydrated yogurt prior to
packaging. Experiments were designed wherein instantized non-fat dry
milk solids (Natural Non Fat Dry Milk, Carnation) were dry blended with
the yogurt powder. There was no significant difference (Figure 14)
between the fortified and non-fortified samples.

Possibly the freeze-dried powder has its own internal matrix as a
result of process conditions and the mere addition Of NFDM to the dried
yogurt failed to achieve the desired increase in firmness. It would
seem appropriate in future work to pursue this matter further possibly
using lower pH in the dried yogurt to compensate for buffer effects Of
the added NFDM, and also, to evaluate low vs high heat NFDM for

fortification.

Effect of Fat Content on Physical Properties

of FreezeQDried Direct Set Yogurt

 

There is a paucity of literature on the importance and role that

fat plays on the physical properties Of yogurt. In the data shown in

39

 

500
A 12.07. 010‘

400- 0 12.07. 0111" + 1.07. sodium caseinate
g 15.27. 0111"

400-

440.

420- M

5_ 4

 

499-

 

 

389- G —._.i__:fi_,__,:|_.——r—J3fiL—————fl

369-

Penetrometer Value (0.1 mm)

 

 

 

349-
329-
399
l 1 l
5 19 1‘5 29 215 39 351
Time (Sec)

Figure 13. The effect Of varying total solids content on the gel
firmness Of freeze-dried direct set yogurt (Milk standardized
to 3.1% fat containing 0.3% pectin and 0.1% guar gum)

40

Table 1. Some physical properties Of different types Of freeze-dried

direct set yogurt

 

 

 

 

 

 

 

Fat SNF Stabilizer % Process Body Texture Whey-
% % pectin guar Separation
3.0 15.5 0 0 normal* soupy smooth -
0 1 0 normal soupy smooth -
0 3 0 normal 51. weak smooth -
O 5 0 normal 51. weak smooth -
2.9 15.5 0.5 0.1 normal weak smooth -
0.5 0.2 normal weak smooth -
0.3 0.1 normal s1. weak smooth -
0.3 0.2 normal 51. weak smooth -
3.1 12.8 0.3 0.1 normal weak smooth -
12.8** normal 51. weak sl. rough —
10.3 normal liquid +++
10.3** normal liquid +++
3.1 15.7 0.3 0.05*** normal weak smooth +
2.0 15.7 normal ex. weak smooth +
1.0 15.6 normal soupy smooth -
0.5 15.4 normal liquid ++
3.2 14.8 0.3 0.1 90C 30M 51. weak smooth +****
80C 15M good smooth —
normal good smooth -
70C 30M 51. weak sl. rough ++
3.1 15.2 0.3 0.1 2.7% GDL s1. weak smooth +
normal 51. weak smooth -
3.3% GDL sl. weak smooth -
3.6% GDL sl. weak smooth -

 

*
**
***

80C 30M Heat Treatment, pH 4.0 - 4.1 (3.0% GDL)
fortified with 1.0% sodium caseinate
carageenan rather than guar gum
**** browning was observed

 

41

 

 

 

 

 

459
(g, 12.07. 005 + 2.57 new
440- A 12.07. 001‘
(3 15.27. 010*

439-
,\ J
E 420- _‘_,-_4

Jad—

8 410- W_P
(U
3
‘>° 400.
S.
(D
4.:
E's” 390..
E
5 n n 0
5 300. WW— ” u
CL

379.1

369

1
359
5 1‘9 1‘5 2‘9 25 319 35
Time (Sec)
Figure 14. The effect Of blending NFDM and yogurt powder on the

gel firmness of freeze-dried direct set yogurt (Milk
standardized to 3.1% fat containing 0.3% pectin and
0.1% guar gum)

42

Figure 15 one can see the major contribution fat makes to the body of
fresh yogurt. As fat content was increased from 0.5% to 3.1%, there
was a significant increase in firmness Of the yogurt. The contribution
to firmness is even more dramatic when one compares the PV of reconsti-
tuted yogurt (Figure 16). Yogurts produced by a mix containing less
than 2% fat yielded extremely weak body, making it impossible to
determine PV. In addition, the shape of PV curve Of yogurt produced by
a mix containing 2.0% fat is concave, which means low resistance of the
body to the penetrating force and the rate of penetrating was accelerated
after 45 mm penetration since the height Of cone is 45 NHL The definite
effect of milk fat can be Observed for the consistency of the freeze-
dried yogurt as well (Figure 17), as measured by viscometry.

Fat seems to be indispensable for desirable body in reconstituted
yogurt, since lower levels of fat yield body too weak to determine by
penetrometry or viscosimetrically. This may be due to the interaction
between the fat globules and the coagulated protein filaments which
affects the formation Of solid gel structure. In addition, in yogurt
Of lower fat content and particularly, in the case Of 0.5% fat yogurt,
whey separation was readily Observed on the surface Of the yogurt
(Table 1). This defect was noted earlier by Rasic and Kurmann (1978).

In this experiment about varying fat content of yogurt mix
(Figure 15 - 17), 0.05% carageenan was used rather than guar gum.
However, guar gum can be considered better from the viewpoint Of
solubility because carageenan is difficult to dissolve in milk and
causes very viscous milk, which does not necessarily produce firm body

of yogurt. For this reason, the blends of pectin and guar gum was used

43

 

 

 

 

 

 

450
459‘ 0 0.57. m (15.47. 310‘)
4 0 A 1.07. m (15.67. 307)
4 ‘ u 2.07. m (15.77. sun
439. | 3.17. m (15.77. SNF)
420.
E 410. Jy_F,__,_4yprrc"”°5’fJTd—Fé
3 400- Afr—re
S 390. H
'8
> 300.
3 1 -—5r—-—"‘
+5; 370- ”if/.3— é -
E 369- W3 4‘3 ° -—0
4.)
OJ
5 350.
Q
340- W
330. If
320.
310-
300
1 1 1 1
a) 3 10 15 20 2’5 30 35
Time (Sec)
Figure 15. The effect of varying fat content of mix on the gel

firmness Of fresh direct set yogurt (Milk standardized to
15.4 - 15.7% SNF containing 0.3% pectin and 0.05%
carageenan)

699

44

 

589-
569-
549-
529.
599-
489-
469-
449-

+120.

Penetrometer Value (0.1 mm)

499-

389-

369-

349.

329

g 2.07. m
| 3.17. m

 

 

 

Figure 16.

1 1
5 1'0 1‘5 2‘0 25 30
)

Time (Sec

The effect of varying fat content Of mix on the gel
firmness Of freeze-dried direct set yogurt (Milk
standardized to 15.7% SNF containing 0.3% pectin and
0.05% carageenan)

 

35

699

45

 

599-

Consistency x 100 (cp)
7:5

299.

199-

| 3.17. m (15.77. sun

n 2.07 m (15.77 sum
4 i 1.07. m (15.07 3115)
'1, 0 0.57. m (15.47. 31171

 

 

Figure 17.

 

Time (Min)

The effect Of varying fat content Of mix on the consistency
of freeze-dried direct set yogurt (Milk standardized to
15.4% - 15.7% SNF containing 0.3% pectin and 0.05%
carageenan)

46

for yogurt production in the following study.

Effect of Heat Treatment on Physical Properties

 

Of Freeze-Dried Direct Set Yogurt

 

Four different heat treatments were employed to study the effects
Of various heat treatments. The treatments used were 70°C/30 min;
80°C/3O min; 90°C/30 min; and 80°C/15 min. In regard to gel firmness,
there were no significant differences except that the 70°C/30 minute
treatment resulted in weaker body in both fresh yogurt (Figure 18) and
reconstituted yogurt (Figure 19).

Browning, however, was very apparent in the yogurt heated to 90°C
for 30 minutes (Table 1). This color Change is due primarily to
caramelization Of lactose and to a lesser extent, to the Maillard
reaction between protein amino groups and lactose carbonyl.

With reference to the consistency of reconstituted yogurt
(Figure 20), both 80°C heat treatment for 15 minutes and 30 minutes
produced good results. Although Gilles and Lawrence (1981) and others
have recommended heating milk to 80°C - 85°C for 30 minutes for yogurt
manufacture, good body and texture of reconstituted yogurt in this
study was obtained by heating milk to 800C for 15 minutes, as well as
for 30 minutes.

It is concluded that similar beneficial effects Of heat treatment
apply to the physical properties Of reconstituted yogurt as well as
those which have been reported for fresh yogurt (Schmidt et al., 1980).

Such effects may be related to the adsorption of denatured whey

proteins to casein micelles. The hydrophilic properties Of such

 

499

47

 

390-
300.
370-
369
350-
340-
330
320-
310-
300.
290-

Penetrometer Value (0.1 mm)

289-
279-
269-
259-
249

0 700 30111::
1 00c 30m
[1 90C 39Hin
| 000 15111:.

 

 

Figure 18.

 

5 1}9 1‘5 219 25 3’9 351

Time (Sec)

The effect Of varying heat treatment Of mix on the gel
firmness of fresh direct set yogurt (Milk standardized to
3.2% fat, 14.8% SNF containing 0.3% pectin and 0.1% guar
gum

48

 

 

 

 

450
0 700 3011111
440- 1 000 397110
[1 99C 39Hin
439_ I 890 159119
E 420-
9:
qua
CD
3
'5
> 400-
5..
OJ
+> W
CU H
E 390. EMA
S.
53 |
E 300. ‘_,_._.—A—-———-'-‘ 1 i
370-
369-
350
1 1 l 1 .
(l i 10 15 20 215 30 35
Time (Sec)
Figure 19. The effect of varying heat treatment of mix on the gel

firmness Of freeze-dried direct set yogurt (Milk
standardized to 3.2% fat, 14.8% SNF containing 0.3%
pectin and 0.1% guar gum)

 

49

 

 

 

 

000
1 00c 3011111
| 00c 15Hin
799. .
[1 90c 3011111
0 70C 3011111
500. A.
\
\.
A I \
a \
" SBB‘ \\ ‘\
8 it \\\1\L
x 400.
5 m
:7, \0\ \KL
5 300. M
C
O
200. \ “3
AKT“4LH_ g
190' “Mam
9 1 1* 1 1
.1 51151231351144
Time (Min)
Figure 20. The effect of varying heat treatment Of mix on the

consistency Of freeze-dried direct set yogurt (Milk
standardized to 3.2% fat, 14.8% SNF containing 0.3%
pectin and 0.1% guar gum)

50

aggregates may improve the gel finnness and consistency of yogurt.

Effect Of EH on Physical Properties of

 

Freeze-Dried Direct Set Yogurt

 

The pH range of commercial yogurts is generally 3.7 - 4.3
(Richmond, 1982). Other factors related to desired pH such as tartness
and the type Of yogurt (plain or flavored yogurt) are of importance,
but pH is functionally vital as a factor influencing the physical
properties of yogurt.

The final pH of a product can naturally be controlled by the
amount Of GDL added, as was evident from this, and earlier research
studies.

Within practical limits the lower the pH, the firmer will be the body
of fresh yogurt (Figure 21) and that of reconstituted yogurt (Figure 22).

The effect Of hydrogen ion concentration on consistency (Figure 23)
parallels the effects previously noted for gel firmness. pH has
definite effects on the physical properties Of freeze-dried direct set
yogurt between pH 3.8 - 4.2 and the lower pH results in better body Of
the yogurt.

Coagulation occurs near the isoelectric point Of caseins as a
result of minimal stabilizing charge on the casein micelles in this
range of pH. In addition, acidification to a pH Of 4.6 and below tends
to increase the hydration and solubility due to strong protonation of
the micelle Of protein. This may have a bearing on the physical

properties noted.

51

 

 

 

 

359
0 pH 4.2 (2.77 cm.)
340- 1 p11 4.0 (3.07 001.)
u pH 3.9 (3.37 GDL)

333‘ | pH 3.0 (3.67. 0111.)
E 320-
:5
v 310- .0— {H
g M
(U H
> 399- /
g pr’fflf ‘d—_f——F‘
4% M
g 290. J"
5..
4-2 /
G)
g 200- 'J’f,,~a4Ir-"“’4Id—Pffld—‘F_—__F_’l
D. /'

279- I“

260-

259

0 (1 1‘0 15 2111 25 3'0 351
Time (Sec)
Figure 21. The effect of varying pH (GDL content) of mix on the gel

firmness of fresh direct set yogurt (Milk standardized to
3.1% fat, 15.2% SNF containing 0.3% pectin and 0.1% guar
gum

459

52

 

449-

439-

429-

419-

499-

399-4

389-

Penetrometer Value (0.1 mm)

379-

350.

359

1‘. pH 4.2 (2.77. cm.)
1 1211 4.0 (3.07. cm.)
1] pH 3.9 (3.37. cm.)
I p11 3.0 (3.07. cm.)

 

 

 

Figure 22.

 

Time (Sec)

The effect Of varying pH (GDL content) of mix on the gel
firmness Of freeze-dried direct set yogurt (Milk
standardized to 3.1% fat, 15.2% SNF containing 0.3%
pectin and 0.1% guar gum)

1999

53

 

999-

899-

799-

699-

599-

499-

Consistency x 100 (cp)

399-

299-

199

| pH 3.0 (3.07. cm.)
[3 pH 3.9 (3.37. cm.)
1 pH 4.0 (3.07 cm.)
0 p11 4.2 (2.77. cm.)

 

 

Figure 23.

 

.5 l 135 1 215 I 315 1 4.5;

Time (Min)

The effect Of varying pH (GDL content) Of mix on the
consistency Of freeze-dried direct set yogurt (Milk
standardized to 3.1% fat, 15.2% SNF containing 0.3%
pectin and 0.1% guar gum)

54

An experiment was also designed to study the effect of dry
blending additional GDL to direct set freeze-dried yogurt. The intent
of this experiment was the possibility of compensating for loss Of
yogurt structure through freeze-drying by providing additional acidity
upon reconstitution. It was contemplated that GDL might have the
ability to contribute to reforming the matrix structure which was
partially destroyed during the process. However, no positive effect
was noted in the characteristics of blended yogurt powder and GDL
powder (Figure 24). Apparently the coagulum strength could not be
intensified when the mixture was hydrated with water. Additionally,
GDL is not as readily hydrolyzed at the low temperature used for

reconstitution.

Effect of Cold Storage on Physical Properties

 

Of Freeze-Dried Direct Set Yogurt

 

The storage Of yogurts in the refrigerator after reconstituting
the freeze-dried powder has definite influence on the physical
properties as well as sensory improvements. Apparently, the changing
temperature Of products during storage affects the firmness of coagulum
in yogurt (Figure 25). The low temperature and extended storage at
refrigerator temperature (5°C, 41°F) enhanced body, since it has long
been noted with proteins such as gelatin that hydration occurs to a
greater extent in the cold. This phenomenon may occur due to the
hydration Of milk protein and solidification of the gel structure of

yogurt during cold storage (Kurmann, 1978).

55

 

 

 

 

 

 

 

 

 

459
0 2.77 0111. (p11 4.2)
440- o 2.77 (:01. + 0.07 (:01. (pH 3.9)
g 2.77 (:01. + 0.97. (:01. (p11 3.0)
11301 (1 3.37 cm. (pH 3.9)
| 3.67 cm. (pH 3.0)
E 420-
c3
V 410-
CD
3
76
=’ 4mm
S.
(D
.p
O -(
4E A Jl n. M
C M V V U
3 389* W 40 0 I“ {1
370. P——~I—’—'—" .4 I———"4
369-
359
1 1 *7 1
5 19 15 2'9 25 39 35
Time (Sec)
Figure 24. The effect Of blending GDL powder and yogurt powder on

the gel firmness of freeze-dried direct set yogurt (Milk
standardized to 3.1% fat, 15.2% SNF containing 0.3%
pectin and 0.1% guar gum)

 

679

56

 

659-
639.
619-
599-
579-
559-
539.
519-
499-
479-
459-

Penetrometer Value (0.1 mm)

439-
419-
399-
379-
359

0. SH Storage (9.50) at SC
1 011 Storage (7.00)

g 1011 Storage (5.50 /J

/
I

 

c:

C:

CD
CD
d3
[:5

 

 

Figure 25.

 

1 1'9 1‘5 2'0 25 319 35

Time (Sec)

The effect Of cold storage (5°C, 41°F) on the gel firmness
of freeze-dried direct set yogurt (Milk standardized to
3.1% fat, 15.2% SNF containing 0.3% pectin and 0.1% guar
gum

57

The required storage time to achieve reasonable physical properties
are also dependent on other factors such as cooler capacity, total
solids of yogurt, the volume Of the vat or package and the heat
conductivity Of the vessel. Storage times up to 24 hours or longer are
Often empolyed.

It is concluded that a temperature Of less than 7°C is indispensable
for reconstituted yogurts from the viewpoint Of their body as well as

their taste (Figure 25).

Effect Of Mechanical Force on Physical

 

Properties Of Direct Set Yogurt

 

In yogurt manufacture, mechanical or physical forces which greatly
affect the physical properties of yogurt can not be avoided. This is
Obvious in the case Of stirred yogurt which has been coagulated in
bulk in tanks and is subsequently agitated or blended to incorporate
flavoring components prior to packaging. Processing steps resulting in
flow through the plate cooler, pumps and pipes and filling into retail
containers as well as incorporation of fruit can all be detrimental to
structure of the yogurt. Therefore, consideration of mechanical effects
is very important when the physical properties of the final yogurt is to
be considered.

In this research work, the mechanical force was derived by stirring
fresh or reconstituted yogurts slowly with a teaspoon for approximately
five seconds, at which time the samples were again stored in the

refrigerator before testing.

58

The comparison of gel firmness on quiescent yogurt and stirred
yogurt Of different pH (Figure 26 - Figure 29) indicates a large effect
of mechanical treatment in only fresh yogurt. The data in the bar
graph (Figure 30) show this tendency more clearly. The data suggest
that the firm coagulum structure in fresh yogurt is much stronger,
perhaps due to apolar bonding Of the micelles, whereas the coagulum
structure in reconstituted yogurt is weaker and elastic. The character-
istic difference comes mainly from the difference in process handling
with or without reconstituting process. Reconstituted yogurt which has
already received mechanical treatment while being hydrated has a
relative stability against additional mechanical force. This observa-
tion suggests that reconstituted yogurt would be suitable for stirred
yogurt products which are subjected to the additional mechanical force.

Regardless of the relative stability of reconstituted powders is
the fact that they are apparently weaker in structure than their fresh

counterparts.

Effect Of Moisture Content of Freeze—Dried Powder on Physical

 

Properties Of Reconstituted Direct Set Yogurt

 

The moisture content of freeze-dried powder must be considered
from the standpoint Of reconstitutibility as well as the effect of
moisture on storage stability and physical and chemical properties.

According to the study of storage stability on cultured cream
(Hamilton, 1970), the effect of level Of moisture content and storage
temperature are critical for some Of the physical and chemical

properties of the freeze—dried product.

59

 

 

 

 

 

 

 

449
430 0 Reconstituted Yogurt(stirred) pH 4.2 (2.77. GM.)
0 Fresh Yogurt (stirred)
429- 1 Reconstituted Yogurt(quiesc.)
419- I Fresh Yogurt (quiescent)
E 400-
0' 399-
7’ _1 A g 1
9 380- 33—37405:— 5 i? U "3
'5 ' I I 4-JF5 L
> 379- ‘—
S.
.8
a) 369-
5
+4 359-
w
5
0- 349-
339-
329_ _rgg_FrJ~Ir""7‘—"_°_—f_—°_°_°-__°—_l
310. W4
399
1 l 1 l .
0 5 10 15 20 25 30 35

 

Time (Sec)

Figure 26. The effect of mechanical force on the gel firmness of
direct set yogurt; pH 4.2 (Milk standardized to 3.1%
fat, 15.2% SNF containing 0.3% pectin and 0.1% guar gum)

 

60

 

 

 

 

 

 

 

 

459
440- 0 Reconstituted Yogurt(stirred) p11 4.0 (3.07. cm.)
439- I. Reconstituted Yogurt(quiesc.)
429- [1 Fresh Yogurt (stirred)
410, I Fresh Yogurt (quiescent)
A 400.
E 390.
'_. , A
0 39° 1. .5 .5 4 -. 4.1
V 379.
C1)
3 3‘9- g ___._1_-—4 4 .1 1
> 350- 0——'—‘—{LF {,L U m u
S.
13 397
“2’
o 330-
4‘3 0
g 32 -
g 310.
389‘ W
299-
289-
270.
269-
259
l 1 1 1
9 6 19 15 29 25 39 35’
Time (Sec)

Figure 27. The effect of mechanical force on the gel firmness of
direct set yogurt; pH 4.0 (Milk standardized to 3.1%
fat, 15.2% SNF containing 0.3% pectin and 0.1% guar gum)

61

 

 

 

 

 

 

 

 

 

 

 

 

459
448. 0 Reconstituted Yogurt(stirretl) pH 3.9 (3.37. cm.)
430- i Reconstituted Yogurt(quiesc.)
429- [1 Fresh Yogurt (stirred)
4181 I Fresh Yogurt (quiescent)
499-
A 399-
E A ,5, £9 A it .A
,— 389.
O
v 379- ‘— I I I A 1‘
0’ 369
3 1 n :1 If:1 {11 {A
r— at U [J
g 350-
3-, 340.
E3 330
g -(
g 329-
: 319..
g
300.
299-
200. [W l- H
279-
200.
259
1 1 l 1
9 6 19 15 29 25 339 351
Time (Sec)

Figure 28. The effect of mechanical force on the gel firmness Of
direct set yogurt; pH 3.9 (Milk standardized to 3.1% fat,
15.2% SNF containing 0.3% pectin and 0.1% guar gum)

62

 

 

 

 

 

 

 

 

479
4694 it Reconstituted Yogurfistirred) PH 3.8 (3.67. GDL)
459« I. Reconstituted Vogurfiquiesc.)
4494 D Fresh Yogurt (stirred)
439.4 I Fresh Yogurt (quiescent)
429.4
’5‘ 419.4
E 4991
9; 390+
a, 380‘
r: 379. A— i‘ A A A 9‘
>‘3sa. ‘d__’__P‘_,___,_4_______;_,___,—L———————A
$-
3 359,
a) 11 n B
s 340. n n U “
e W“ W
4) 339-:
w
5 3204
‘L 310.
3931
299.4
2804 W
279.
269‘
259
1 I l
9 g 19 115 29 215 39 33
Time (Sec)

Figure 29. The effect of mechanical force on the gel firmness of
direct set yogurt; pH 3.8 (Milk standardized to 3.1% fat,
15.2% SNF containing 0.3% pectin and 0.1% guar gum)

63

 

E Fresh Y9 wt
11 m Reconstl uted Yogurt

1

PV increase (0.1 mm) after 30 sec

PH 4.2 P“ 4.8 P" 3.9 P“ 3.8

Figure 30. The loss of gel firmness by mechanical force for different
pH direct set yogurt (Milk standardized to 3.1% fat,
15.2% SNF containing 0.3% pectin and 0.1% guar gum)

64

In this research, a comparison of the gel firmness on two
reconstituted yogurts prepared from powder of different moisture
content was carried out. One of the freeze-dried powder samples was
obtained through the normal process time (24 h) freeze-drying, on the
other hand, the other powder sample was obtained through the shorter
process time (approximately l9 h) freeze-drying. As a result, their
moisture content was 3.78% and 5.l0% respectively. Powder which was
stored in screw top jars at room temperature (24°C - 26°C) for less
than one month was used for this purpose to minimize stability loss
during storage.

The data in Figure 3l indicate no effect of moisture content (at
least in this range) on physical properties of freeze-dried reconstituted
yogurt. Although this suggests that moisture content of powder within
this range does not affect reconstitutibility of powder, one should
also recognize that these are high moisture levels for optimal storage
quality and additional data on samples of l - 3% moisture would be worth
investigating.

In addition, no browning was observed in even high moisture
freeze-dried powder during storage for four months at room temperature,
in contrast to data of Hamilton and other researchers. Obviously,
further investigation is necessary to study the effect of the moisture
content of powder stored for longer times on the physical properties

of reconstituted products.

458

65

 

448-

438-

429.

413.

488.

399.

389.

Penetrometer Value (0.l mm)

379.

368.

358

A- 540 7. noisture
l 3.78 z moisture

I>

 

 

Figure 3l.

% ib 1% 2% 6k 3%

Time (Sec)

The effect of varying moisture content of freeze-dried
powder on the gel firmness of freeze-dried direct set

yogurt (Milk standardized to 3.l% fat, 15.2% SNF
containing 0.3% pectin and 0.l% guar gum)

 

33

66

Sensornyvaluation

 

Fresh fermented yogurt (Dannon Plain Lowfat Yogurt), freeze-dried
fermented yogurt, fresh direct set yogurt and freeze-dried direct set
yogurt samples (all plain yogurts) were evaluated by the panel using a
hedonic scale with a range of l - 7. Mean values show a significant
difference between the fermented yogurt and the direct set yogurt
(Table 2). The difference can be attributed mainly to the difference
in flavor components of the two types of yogurt. The fermented yogurts
tend to be evaluated as having higher quality, particularly on their
freshness. This sensory tendency can be explained by the assumption
that one of the major characteristics of plain yogurts is their typical
fermented flavor and aroma due to acetaldehyde, acetone and diacetyl.

The loss of flavor components as a result of freeze-drying is not
critical in judging for Freshness and Overall evaluation even though
the score for Aroma decreases (Fresh vs Freeze-dried Fermented Yogurt).

The comparison of process difference between fresh and freeze-dried
yogurts reveals the change of their physical properties. The mean value
of the body evaluation on freeze-dried yogurts is similar to the results
by analytical methods described before. 0n the other hand, the mean
value of texture evaluation indicates the Opposite results. This is
due in part to psychological effect which causes people to evaluate
elastic type materials as smooth.

The average of standard deviation values is related to the grade
of sensory evaluation accuracy. According to the deviation values, the
evaluation on Tartness and Body shows fair correctness; on the other

hand, Freshness, Aroma and Overall evaluation results indicate a large

67

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A.o.mv cowpmw>mo vgmvcmpm *«
AF xvucmqa<v n - _ mpmum uwcovm: co comma mwmczw cm» we mmcoum :mwz e

 

 

FN.F mP.NHP.m Fo.mno.m mm._np.m mm.Pn¢.m :owumzfim>m Fpmcm>o
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oo.m oF.Nno.¢ mo.Nn_.¢ oo.mnm.¢ Nu.—um.m meog<
om._ m¢.mnm.m Fo.NHo.m mm.Fuv.m *kwm.~ukm.m mmmccmmcu
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mmwwm>< pom powers umpcwsgwm

 

“Lamox :wmra mo :owpmzpm>m

xgomcwm .m wpamh

68

variation, which suggests the necessity of training judges for
evaluating these categories.

The evaluation on physical properties such as Body and Texture,
which is relatively reliable, does not indicate any major problem
regarding physical properties of all of the yogurt samples, since
freeze-dried direct set yogurt was made from milk fortified with NFDM
and stabilizers.

None the less, sensory evaluation indicates the need to improve
flavor and aroma in direct set plain yogurts in order to insure consumer
acceptance. The success of food products in the market is greatly
dependent on the consumer preference and acceptance of a product.
Accordingly, direct set yogurts might be utilized better in flavored

yogurt or fruit yogurt, rather than plain yogurt.

Study of Correlation on Data Obtained

 

by Different Methods

 

The comparison of two sets of physical analysis data obtained by
different equipment, i.e. penetrometer and Brookfield viscometer, was
carried out by calculating correlation coefficient (R) and coefficient
of determination (R2) (Table 3).

Since it is unquestionable that the gel firmness of yogurt
increases as the consistency of the same yogurt increases, it is easy
to understand the reverse relationship between Penetrometer Value (PV)
and consistency. This relationship is shown as negative value of R.
The grade of R is in a range of 0.78 - 0.93, but two physical analyses

can be considered to be correlated with each other. This result brings

69

 

 

Table 3. Comparison of two analytical methods on physical properties
of freeze-dried direct set plain yogurt
Study l ‘ Study 2
Heat Treatment pH Difference
PV* Con.** PV Con.3
x 103 x 10
(cp) (CD)
Condition l 37.8 21.52 39.2 29.44
Condition 2 37.7 30.00 38.8 25.60
Condition 3 38.2 34.80 38.0 43.60
Condition 4 39.5 7.52 37.2 49.60

 

Correlation
Coefficient (R)

Coefficient of

Determination (R2)

-0.78

0.61

-0.93

0.87

 

* Penetrometer Value (mm) after 30 seconds

**Consistency (cp) after 4 minutes

70

about the reproducibility of analysis on physical properties of yogurt.

Furthermore, the investigation of correlation between physical
analysis and sensory evaluation for the same sample was carried out.
The results (Table 4) reveal the following tendency.

The body of yogurt is evaluated finner as PV decreases (gel
firmness increases) and consistency increases. 0n the other hand, the
texture of yogurt is evaluated smoother as PV increases and consistency
decreases. The grade of correlation is variable, but the body
evaluation and the determination of consistency is correlated very
well. This result indicates the appropriateness of the analytical

method to relate to the sensory evaluation of the yogurt body.

71

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SUMMARY AND CONCLUSIONS

An acceptable, fresh direct set yogurt which has good physical
properties can be prepared without cultures by using glucono

delta lactone (GDL) as an acidogen.

An acidification of 3% GDL to fortified milk for 90 minutes at
300C brings about a good final product.

The loss of typical body and texture of yogurts through the
freeze-drying process is readily observed.

The physical properties of freeze-dried reconstituted yogurt can
be improved by fortification with NFDM and some stabilizers.

A reasonable range of stabilizer content was found when stabilizers
were used for yogurt mix.

Higher SNF content of the yogurt mix is necessary to form a firm
gel in freeze-dried reconstituted yogurt. Therefore, yogurt mix
must be fortified with NFDM.

The milk fat content is an important factor for the formation of
apprOpriate gel structure. The milk fat contributes to good body
in reconstituted yogurt.

The addition of sodium caseinate results in an increase in gel
firmness of reconstituted yogurt to the extent that 2.4% SNF could
be replaced by l.0% of sodium caseinate. However, the texture of

final product seemed to be slightly rough.

72

10.

ll.

12.

l3.

14.

15.

l6.

17.

18.

73

The direct addition of instantized skim milk powder to the
freeze-dried powder failed to contribute a firm matrix structure.
Good physical properties of freeze-dried reconstituted yogurt can
be obtained with heat treatment of 80°C (l76oF) for 15 or 30
minutes.

Lower pH produced a better body of reconstituted yogurt in the
range of pH 3.8 - 4.2.

Direct supplementation of the dehydrated yogurt with dry GDL failed
to improve the physical properties of reconstituted yogurt.

The physical properties of reconstituted yogurt are greatly
affected by the temperature of the product. Lower temperature
favors a firm body as well as good sensory response.

Mechanical force did not adversely affect the loss of physical
properties of reconstituted yogurt as much as it did in fresh set
yogurt.

The moisture content of freeze-dried powder (3.78% - 5.l0%) did
not affect the reconstitutibility of the powder.

Naturally fermented yogurts tended to be evaluated higher by the
sensory panel than the direct set yogurts even though the physical
properties did not differ greatly.

An improvement of flavor and aroma is required for the direct set
yogurt to obtain consumer acceptance.

The two sets of physical analysis data obtained by different
instruments indicate high correlation, which enables reproducibility

of physical analysis.

19.

74

The comparison of physical analysis and sensory evaluation reveals
high correlation between the analytical determination and the
evaluation of body, which indicates appropriateness of analytical

methods on physical properties of yogurts.

APPENDIX

 

75

 

 

 

 

 

 

 

 

 

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