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ABSTRACT

A STUDY OF SOME PHYSICAL AND CHEMICAL
PROPERTIES OF DEHYDRATED
SOUR CREAM

by Rohini J. Desai

A study was made of the effects of foam-spray drying
and freeze drying on some physical, chemical and organolep-
tic properties of cultured (sour) cream. Samples of sweet
cream varying in fat content from 10 to 18% were individ-
ually pasteurized, homogenized and inoculated with a lyoph-
ilized mixed strain starter-culture. A portion of the
ripened cream was frozen for use as a control; the remaining
portion was divided into lots one of which was freeze dried
and the other foam-spray dried. Samples from each dehy-
drated sour cream were stored at 40 F and 72 F.

Suhstantial losses in amounts of flavor and aroma
constituents occurred on dehydration of the sour cream.
Compared to the control, both foam-Spray dried and freeze-
dried sour cream had less volatile acids, lower titratable
acidity and smaller amounts of acetoin-plus-diacetyl. In
general, the retention of these volatile compounds was bet-

ter in the freeze dried than in the foam-spray dried sour

cream. Diacetyl on the other hand, increased following

Rohini J. Desai

drying of the sour cream by both methods. The free fat
values were consistently higher in the freeze—dried cream,
conferring on the product a distinct yellowness of appear—
ance while the foam-spray dried counterpart was a light
cream in color. Though the ease of dispersion of both pow-
ders decreased with increasing fat content, the results
could not be correlated to the corresponding amounts of
free fat° Organoleptic evaluations also demonstrated the
superiority of the freeze-dried sour cream when compared to

the foam-spray dried product.

A STUDY OF SOME PHYSICAL AND CHEMICAL
PROPERTIES OF DEHYDRATED

SOUR CREAM

BY

Rohini J. Desai

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Food Science

1966

Dedicated

to

Mr. and Mrs. Jayantilal B. Desai

ii

ACIQIOWLEDGMENTS

The author is greatly indebted to her major profes-
sor, Dr. C. M. Stine, for his guidance and encouragement
throughout this study, his gift of understanding and for his
efforts in the preparation of the manuscript.

Grateful thanks are extended to Dr. H. A. Lillevik
and Dr. J. R. Brunner, who served on the examining committee
and reviewed the manuscript, to Dr. G. M. Trout for his help
in the preparation of the manuscript and to Dr. B. S.
Schweigert for providing financial assistance.

Grateful appreciation is also extended to Mr. Lee
Blakely, Mr. Don Wallace, Mr. Jaswant Singh and Mr. Jim Kirk
for their help in various ways.

Last but not the least, the author thanks her par-
ents, Mr. and Mrs. Jayantilal B. Desai, for their sacrifices
and constant inspiration for a higher education, and to whom

this thesis is respectfully dedicated.

iii

TABLE OF CONTENTS

Page

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . 1

REVIEW OF LITERATURE . . . . . . . . . . . . . . . . . 3

Body, Texture and Flavor of Cultured Cream . . . . 3

Starter Culture . . . . . . . . . . . . . . . . . 7
Production of Diacetyl and Acetoin by Aroma

Bacteria . . . . . . . . . . . . . . . . . . . . lO

Breakdown and Interconversion of Diacetyl,

Acetoin and 2,3-Butanediol . . . . . . . . . . . 17

Culture Preservation . . . . . . . . . . . . . . . 19

Liquid Cultures . . . . . . . . . . . . . . . 20

Frozen Cultures . . . . . . . . . . . . . . . 21

Dried Cultures . . . . . . . . . . . . . . . . 23

EXPERIMENTAL PROCEDURES . . . . . . . . . . . . . . . 26

Sources of Cultures . . . . . . . . . . . . . . . 26
Preparation of Fresh Sour Cream . . . . . . . . . 26
Method of Storage . . . . . . . . . . . . . . . . 27
Preparation of Samples for Analyses . . . . . . . 28
Control . . . . . . . . . . . . . . . . . . . 28
Reconstituted Foam-spray Dried and Freeze-
dried Sour Cream . . . . . . . . . . . . . . 28
Analytical Methods . . . . . . . . . . . . . . . . 28
Moisture . . . . . . . . . . . . . . . . . . . 28

pH . . . . . . . . . . . . . . . . . . . . . . 29
Titratable Acidity . . . . . . . . . . . . . . 29
Volatile Acidity . . . . . . . . . . . . . . . 29

Diacetyl Determination . . . . . . . . . . . . 3O
Diacetyl and Acetoin Determination . . . . . . 31
Total Fat . . . . . . . . . . . . . . . . . . 31
Free Fat . . . . . . . . . . . . . . . . . . . 3l
Dispersibility . . . . . . . . . . . . . . . . 32
Organoleptic Evaluation . . . . . . . . . . . 32

RESULTS . . . . . . . . . . . . . . . . . . . . . . . 33

iv

Page

Some Physical and Chemical Characteristics
of Dehydrated Sour Cream . . . . . . . . . . . . 33
Effect of Drying on the Volatile Acidity

of Sour Cream . . . . . . . . . . . . . . . . . 35
Effect of Drying on the Diacetyl Content

of Sour Cream . . . . . . . . . . . . . . . . . 35
Effect of Drying on the Diacetyl-plus-

Acetoin Content of Sour Cream . . . . . . . . . 38

Effect of the Method of Drying on the

Free Fat of Dehydrated Sour Cream . . . . . . . 38
Effect of the Method of Drying on the

Dispersibility of Dehydrated Sour Cream . . . . 41
Effect of Storage at 40 F and 72 F on the

Flavor Scores of Foam~spray Dried and

Freeze-dried Sour Cream . . . . . . . . . . . . 41

DISCUSSION . . . . . . . . . . . . . . . . . . . . . . 44
SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . 53

LITERAWRE CI'TED O O O O O O O O O O O O O O O O O O O 55

LIST OF TABLES

Some physical and chemical characteristics
of dehydrated sour cream . . . . . . . .

Effect of
of sour

Effect of
of sour

Effect of
acetoin

Effect of

drying on the volatile acidity
cream . . . . . . . . . . . . .

drying on the diacetyl content
cream 0 O O O O O C O O O O O O

drying on the diacetyl—plus-
content of sour cream . . . . .

the method of drying on the free

fat of dehydrated sour cream . . . . . .

Effect of

the method of drying on the

dispersibility of dehydrated sour cream

Effect of

storage at 40 F and 72 F on the

flavor scores of foam-spray dried and
freeze-dried sour cream . . . . . . . .

vi

Page

34

36

37

39

4O

42

43

IN TRODUC TION

Cultured (sour) cream is a ripened cream with a
pleasant acid flavor, distinctive aroma, smooth texture and
moderately heavy body. This fine food is made by inoculat-
ing sweet pasteurized cream with a culture of acid and fla-
vor producing organisms and allowing the fermentation to
proceed until the desirable qualities of the product are
developed.

Until fairly recently, the market for commercial
cultured cream was somewhat restricted to the metropolitan
areas of New York and other major cities. Today however it
is a food commonly enjoyed throughout the United States.
Per capita consumption of fresh sour cream in the United
States averaged 0.7 pounds in 1965.

In addition to direct consumption as a food, cul-
tured cream finds increasing acceptance on salads, as a
dressing for vegetables, in fillings for cakes and as a
replacement for buttermilk or sweet cream in many exotic
recipes.

All foods are subject to deterioration sooner or
later, depending on the particular food and conditions of
storage. Cultured cream keeps well for 2 weeks at ordinary

refrigerator temperatures of 40 F. Though storage for 4

l

THE

 

weeks or longer is possible under such conditions, bitter-
ness resulting from the growth of psychrophilic organisms
eventually sets in, often accompanied by undesirable yeast
and mold growth on the surface. Such defects render any
product unacceptable for consumption. Hence, in order to
prolong shelf life, improved and economical methods of
preservation are needed. Storage at sub—zero temperatures
was early recognized to be quite effective in inhibiting
microbial spoilage for extended periods but such conditions
also proved to be detrimental to the body and texture of
thawed sour cream.

The advent of many new and improved drying methods
and their widespread application to the food industry has
been an ultimate boon to the American homemaker in a multi-
tude of ways. Easily prepared food products that can be
stored at room temperature for many months are being uti-
lized in increasing numbers by today's modern housewife.
However, many fresh foods remain unexploited and continue to
be consumed in the fresh state. Cultured cream might well
be utilized in numerous convenience foods if dehydrated sour
cream of superior quality could be developed.

Hence, the intent underlying this undertaking was to
make a comparative study of the flavor properties and chem—
ical and physical properties of fresh, cultured cream and of

dehydrated sour cream prepared by spray and freeze drying.

 

 

Tassv

 

REVIEW OF LITERATURE

Body, Texture and Flavor gf_Cultured Cream

From the marketing standpoint, cultured cream
should have a smooth, rather heavy body with the moisture
homogeneously incorporated. For consumer acceptability most
markets require a product which yields a plummet reading of
7.5 or 8 as determined by the Hilker—Guthrie method. In the
majority of states and cities the same quality regulations
are applied to cultured cream as to sweet cream. In general,
the product must contain at least 18% fat and be made from
inspected creams (Guthrie, 1952).

An acid gel, accompanied by a delicate flavor result-
ing from the growth and activity of lactic acid streptococci
and flavor producing leuconostoc bacteria, characterizes
cultured cream. Thus, excellent cultured cream possesses a
mild, subtle, aromatic acid flavor reminiscent of the flavor
of a 93 score or AA grade ripened cream butter (Kosikowski,
1966).

Various factors influence and contribute to the
overall excellence of cultured cream. Certainly, the
starter culture employed is one of foremost importance in
the production of a good body and the desired flavor char-

acteristics (Guthrie, 1963).

1143.99

 

 

Cream separated from milk at 42 F had a higher
viscosity as determined by a Borden flow meter than corre-
sponding cream separated at 90 F, irrespective of whether
pasteurization was before or after separation (Roberts,
Blanton and Marley, 1953). However, cream separated from
cold pasteurized milk was found satisfactory by Glazier £5
_31. (1954) from the standpoint of bacterial contamination,
in spite of the lower viscosity as compared to cold sepa-
rated cream from raw milk.

Guthrie (1952) found that "different makes of homog-
enizers are not important factors in the manufacture of cul-
tured cream." His results indicated, however, that the
final product made from cream which was homogenized twice
in either the first or the second stage with a total of 5000
psi was superior in body to that obtained if two-staged
homogenization were employed at a gauge pressure of 5000 psi.
Double homogenization at 2500 psi has been considered opti-
mum for obtaining desired body and smoothness in sour cream
(Guthrie, 1952).

The temperature of the cream at the time of homogeni-
zation also affects the final body of the sour cream. The
body was best with cream homogenized at 165 F and poorest
when homogenized at 120 F, according to Guthrie (1952).

Aule and Storgards (1958) also reported that viscosity and
stability of the cream on standing increased directly with

increasing homogenization temperatures. The adverse effects

produced by lower homogenization temperatures could not be
overcome by increasing the homogenization pressure to higher
than 300 kg/cm2. Homogenization of cream after, instead of
before, pasteurization, as is sometimes practiced, does not
affect the viscosity when all other conditions remain the
same. However, homogenization followed by pasteurization is
often preferred to avoid an increase in the coliform and the
standard plate counts due to recontamination (Savage and
Brown, 1953).

Hening and Dahlberg (1943) observed that cream
required a longer holding time than milk at 160 F to achieve
phOSphatase inactivation and the equivalent of 99.9% destruc-
tion of coliform organisms. 0n the basis of their study the
Optimum time-temperature relationships for pasteurization of
cream ranged from 145 F/30 mins to 170 F/3 sec. Guthrie
(1952) also noted that extending heat treatment at 165 F
beyond 30 minutes resulted in a noticeably weaker body in
the sour cream. Processing sweet cream at 165 F/30 min has
been demonstrated by Savage_ggial. (1953) to have the least
effect on changes in viscosity due to homogenization. Fur-
ther studies conducted by Guthrie (1963) to determine the
optimum time/temperature relationship for pasteurization of
raw sweet cream confirmed his earlier choice of 165 F/30
min. The body of the final sour cream was weak or weak and

grainy when pasteurization temperatures of 145 F/30 min and

180 F/30 min respectively were used. Heating cream to
excessively high temperatures precipitates the "casein"
and results in a thicker body, according to Guthrie (1952).

The relationship of fat content of the cream to body
characteristics of cultured cream was investigated by
Guthrie (1952). He found that 18% fat was optimal, with
progressive decreases in quality as the fat content was
raised or lowered from that value. The addition of milk-
solids-not-fat (MSNF) to "normal" cream did not cause an
improvement in physical characteristics of cultured cream.
However, Guthrie noted (1963) that MSNF improved the body
of low solids cream.

Although the main contributions to the body of sour
cream are made by the fat and casein, the addition of stabi-
lizers has also been found to be important in order to main-
tain uniform viscosity and plasticity from batch to batch,
day to day and from season to season. The use of various
stabilizers has been investigated (Guthrie, 1963), yet
rennet at levels of 0.5 ml/lO gals of cream has been the
stabilizing agent most commonly recommended. According to
Guthrie (1952), agitation of the warm cultured cream
eXpresses some moisture, creating graininess of texture.
This can be avoided by stirring the ripened cream only after

partial cooling.

THE-ZS

 

 

Savinovsky (1948) stored sour cream for 6 months at
—10, -15 and -25 C with very little decrease in acidity at
the end of that period. An increase in acidity was observed
when cultured cream was stored at 3 C. Freezing has a weak-
ening effect on the body of the cream (Guthrie, 1952).
Thawing of creams stored at ~10 and -15 C yielded a thin
liquid in which lumps of protein and fat granules were sus-
pended. Creams stored at -25 C appeared normal (Savinovsky,
1948). Addition of stabilizers to cream before frozen stor-
age considerably improved physical stability during storage

and subsequent thawing, according to Bell (1947).

Starter Culture

The first starters used in manufacturing cultured
dairy products were natural cultures obtained by allowing
milk to sour. The isolation of Streptococcus lactis in 1873
stimulated interest in the identification of various other
strains and their ultimate cultivation and laboratory prop-
agation ensued. By the turn of the 19th century, commercial
cultures for use in creameries were available all over
Europe and America.

During this period the lactic streptococci were con-
sidered synonymous with starter cultures and it was not
until 1919 that this rather restricted outlook was broadened

to accommodate a set of "associated" organisms, now known as

1H ES“

 

 

the citric acid fermenters. These organisms were simulta-
neously isolated by Bailey and Hammer in the United States
and Boekhout and Otto de Vries in Europe (Hales). Further
work by various other investigators led to a greater enumera-
tion of newer strains and today we have at least 12 distinct
types of starter cultures for milk fermentations, which have
marked differences in morphology and substrate utilization.
Milk is the substrate most widely used for the
growth of dairy cultures, but it is not the natural habitat
for all of them. Some appear to be of plant origin (Speck,
1964). However, the complement of nutrients in milk is
essentially adequate for the lactic streptococci though the
required nitrogen does not necessarily exist in forms that
are readily available. Milk as it is secreted contains very
little non protein nitrogen; hence, the nitrogen requirements
of the cultures have to be met by the hydrolysis of proteins
(Speck, 1964). Not all bacteria are endowed with equal pro-
teolytic activity and this limits the ability of cultures to
obtain their nitrogen in amounts or forms necessary for
maximum growth. In addition, analyses of individual milk
samples from various cows, as conducted by Anderson _£._l~
(1955), indicated a variation in the peptide content, provid—
ing evidence that the activity of starters could be corre-
lated to the amount of the peptide fraction in milk. Thus,
the quality of the original raw milk is important (Greene

and Jezeski, 1955) and the nature of its subsequent treatment

x .‘V: 03:“.

 

has been shown to affect its suitability as a starter medium.
Certain manufacturing processes, especially heating, were
found by Greene and Jezeski (1957) to alter the substrate,
rendering it stimulatory in some cases and inhibitory in
others. .Foster (1952) reported an improved growth of 6-8
species of homofermentative lactobacilli in autoclaved milk,
presumably due to the resulting partial hydrolysis of casein,
while grossly overheating the milk had a deleterious effect
on the same organisms. Gilliland and Olson (1963) observed
that acid production by lactic cultures incubated for 10-12
hours was more rapid in whole milk and buttermilk than in
skim milk. The use of fresh skim milk or reconstituted non—
fat dry milk (NFDM) is, however, widespread as a substrate
for culture prOpagation. Horral and Elliker (1950) reported
that reconstituted milk promoted more constant activity in
starters than did selected whole milk. 0n the other hand,
different lots of NFDM varied in their ability to provide a
satisfactory medium. In general, however, commercial high
heat powders supported starter activity more favorably than
did low heat powders, with the exception that those powders
with an excessively severe heat history exerted a deleteri-
ous effect.

Increasing the MSNF content of reconstituted NFDM
was found to stimulate acid production by strains of orga-
nisms belonging to the lactobacilli and the streptococcus

genera (Yano gt 31., 1960), possibly because of the buffering

1H ssx

 

 

10

action of MSNF and a higher concentration of growth factor(s)
in the milk.

Many strains of streptococci, leuconostoc and lacto-
bacilli are known to require pantothenic acid, nicotinic
acid and biotin for maximum growth; riboflavin is necessary
for the growth of some strains and is stimulatory to others
(Nambudripad _£H_1., 1957; Anderson and Elliker, 1953).
Folic acid and pyridoxine requirements have been shown to
be variable. The presence of various amino acids such as
proline, valine, leucine, isoleucine, histidine and methio-
nine in the growth medium was also demonstrated by Anderson
and Elliker (1953) as being essential for growth of various
strains of S, cremoris and g. lactis. Cystine, tryptophan,
aspartic acid and serine were shown to be dispensable.
Similarly, all strains of Leuconostoc citrovorum studied
required arginine, histidine, isoleucine, leucine, lysine
and valine, while threonine, aSparagine, aspartic acid,

glycine and cystine were not essential by any of them

(Prouty, 1961).

Production 9£_Diacety1 and Acetoin

 

by_Aroma Bacteria

 

In manufacturing cultured or fermented dairy prod-
ucts, starter cultures are added to produce lactic acid or
to produce a desired aroma in the cultured food product.

Beginning in the late 19th century, there was much confusion

TH 25“

 

 

11

as to whether the desired aroma imparted to butter by
starters was the result of a single organism or a mixture
of organisms (Collins, 1962). Later interest was centered
around certain low acid producing organisms which produced
good butter aroma only when grown in association with S,
lactis. Further research by various workers led to the
establishment of their identity by several different names.
Hammer (1920) called them Streptococcus citrovorus and

Streptococcus paracitrovorus, Krishnaswamy and Babel (1951)

 

suggested S. lactis var. aromaticus, while Knudsen and

 

 

Sorensen (1929) named the organisms Betacoccus cremoris. A
year later the terms Leuconostoc citrovorum and Leuconostoc

paracitrovorum, were coined by Hucker and Pederson (1930).

 

The terms currently used (Breed, Murray and Smith, 1957) are

Leuconostoc citrovorum and Leuconostoc dextranicum.

 

For many years, little attention was given to the
fact that some single strain cultures had been found able
to produce good butter aroma in the absence of S. lactis or
S, cremoris. Shown to be variants of the lactic strepto-
cocci, many strains of organisms have been reported in the
past 35 years which are characterized by their ability to
ferment citrate actively with the production of carbon
dioxide, volatile acids and C4 compounds such as diacetyl,
acetoin and 2,3-buty1ene glycol. Matuszewski (1936) was the
first to isolate and identify the organisms as Strgptococcus

diacetilactis. At about the same time, van Beynum and Pette

 

THES

 

 

12

(1936) described two citrate utilizing organisms capable of
producing lactic acid and diacetyl in milk and suggested for

them the name Streptococcus citrgphilus. Swartling (1951)

 

isolated 35 strains of acetoin—producing lactic streptococci
from raw milk starter cultures and dairy products identical
to the strains accounted for earlier and concluded that the

name S. diacetilactis, rather than the other names prOposed,

 

should be retained. Czulak (1953) characterized S. diaceti-
lactis strains isolated from Australian Cheddar cheese
starters.

Present day starter cultures employ either_S. citro-

vorum or S. diacetilactis or both for the production of

 

aroma compounds desirable in certain fermented milk foods.
The leuconostocs grow best in association with any of a

variety of strains of S, lactis or S, cremoris, which pro-

 

duce lactic acid from lactose. The presence of either S.

lactis or S, cremoris is beneficial to sufficiently reduce

 

the pH of the medium and thereby initiate leuconostoc
activity.

Flavor has long been recognized as a major factor
in the quality and acceptability of foods. The value of
selected cultures of bacteria for the development of a
desirable flavor and aroma in many dairy products has been
thoroughly established. Rapid deve10pment in analytical
techniques and instrumentation over the past two decades has

enabled the elucidation of the complex flavor chemistry of

13

many foods. Some families of flavor compounds have been
studied more thoroughly than others. In dairy products,

the aliphatic carbonyls are one of the most important of the
various groups of flavor compounds encountered and they are
important as contributors to the flavor spectrum of most
dairy products (Day, 1965).

Diacetyl is one of the more important of these but
other compounds such as the volatile acids are also signif—
icant. Generally, no single carbonyl compound can be
implicated as the sole source of a typical flavor; rather
the flavor appears to result from a composite of many com-
pounds (Day, 1965). Wong and Patton (1962) indicated the
presence of formaldehyde, acetaldehyde, methyl sulfide,
acetone, butanone, pentanone-2 and hexanone-2 in milk and
cream. Most carbonyls produced as a result of lipid oxida-
tion are objectionable; however, a recent paper by Begemann
and Koster (1964) has identified cis-4-heptenal as an impor-
tant component of the "cream-like" flavor.

The importance of acetoin and diacetyl was first
emphasized by Michaelian _£._;. (1933) who found that butter
cultures with a desirable flavor and aroma contained rela-
tively large amounts of these compounds while those lacking
in flavor were quite low in acetoin and diacetyl. Other
investigations have confirmed this observation (Hoecker and

Hammer, 1941; Dolazalek, 1952; Calbert and Price, 1949).

TH EST-HS

 

 

 

 

14

The source of these C4 compounds remained a highly
speculative and controversial issue for many years. The
available literature on the subject is replete with contra-
dicting reports stemming from individual eXperimentation.

In the early stages of flavor research, workers believed
that diacetyl and acetoin were metabolites resulting from
the fermentation of lactose (Virtanen _£'Sl., 1941; Coppens,
1954). Others viewed citrate as the source (DeMan, 1956;
Pette, 1949; Bang, 1945; Glenn and Prouty, 1955: Federov and
Kruglova, 1955), and some felt that both compounds are in—
volved (Mizuno and Jezeski, 1959; van Beynum and Pette, 1939;
Andersen, 1959; Taufel and Krusen, 1952). Storgards (1941)
on the other hand stated that neither glucose nor citrate,
alone or in combination, supported production of acetoin and
believed the presence of barium or calcium salts were essen-
tial to initiate the reaction. He further propounded the
involvement of pyruvic acid in the synthesizing mechanism
and much evidence is now available (Bang, 1943; van Beynum
and Pette, 1939: Mizuno, 1956; Harvey and Collins, 1961;
Juni, 1952a; Taufel and Behnke, 1960) which confirms his
early observation. The pyruvate is derived from citrate by
reversal of the condensing enzyme and decarboxylation of the
oxaloacetate formed (Andersen, 1959). Various studies have
succeeded in isolating and characterizing the citritase

enzyme implicated in catalyzing the cleavage of citric acid

15

into oxaloacetate and acetic acid (Taufel and Behnke, 1960;
Harvey and Collins, 1963; Sandine _£.Sl., 1961; Seitz _E.§£~:
1963).

van Beynum and Pette (1939) and Federov and Kruglova
(1955) discussed possibilities for the pathway between pyr-
uvate and acetoin, postulating acetaldehyde as a likely
intermediate. Acetaldehyde in turn is thought to polymerize
to acetoin directly (van Beynum and Pette, 1939) or condense
with pyruvic acid to form alpha acetolactate (Andersen,

1959). DeMan (1956) detected alpha acetolactic acid in the

formation of acetoin by S. citrovorum while Juni (1952a)

 

demonstrated a similar phenomenon in organisms of the genus

Aerobacter. Thus S, citrovorum appears to form acetoin from

 

 

pyruvate by the pathway most generally used by acetoin-pro-
ducing bacteria, namely, the formation of active acetate
from pyruvate and reaction of active acetate with pyruvate
to give alpha acetolactate which is subsequently decarbox-
ylated to acetoin. These observations are in agreement with
those of Andersen (1959) and Taufel and Behnke (1960). The
same scheme is valid for S. diacetilactis, according to
Seitz _E.§l- (1963). They isolated the various enzymes

involved and presented the following schematic for the

mechanism of acetoin and diacetyl synthesis by bacteria:

l6

.mumnmmonmon>m mcHEmflnu mucmmmummu mme
k.

 

 

mmmcmmoupmnmo Hoflomcmusmlm.N .oa mmmumnucxm 02¢
mmmuosomn H>DOUMAQ .m UHumESNchcoz
mmmmeonnmomU mumuomHoumofi I m .m onwamxonnmump mum>5n>m
mmmpflxo mumuomaoumud I m .n mmmmeonumomp mumumumonxo
mmmumnuchm mumuomHoumod I m .m mmmufluuflo
NOU + HocflQHMUHmzumEH>umo<
0H \/ .m
HoflcmcmudmIm.N m . oflum OHDOMHODOO¢ I
N 5

oo + Hmumomflo

 

 

 

 

 

m

mmB + Uflom UHuUmHoumoa I m A o Uflom OH>SM>Q + mma I mphnmpamumoa
mas + Hocflnumoamnumsampmo¢_ A m omommo + Ems I momnmcamumum
mas + momnmcamumoa. A a mag I mwmnmoamumoa
NOON + mme I momnmoamumo¢ N A m «mmB N + oflom UH>summ N
NOO + oflom UH>Duwm A N Uflom UflumumonxO

 

UHOM Uflumom + UHOM oaumomonxO

A\
H

aflom unuuflo

 

 

mfluomHHumomHU .w.%n HOHUmQMDDQIm.N
pom Hocflnnmuawnumaamumom .HhumUMHw ou Uflom OHHDHU mo coamum>coo How mmm3£umm

 

1HESl

 

 

l7

Breakdown and Interconversion g: Diacetyl,

 

Acetoin and 2,3-Butanediol

 

As evident from the foregoing schematic, diacetyl,
acetoin and 2,3-butanediol are related through an oxidation-
reduction mechanism. The amount of oxidized or reduced
substances in the medium determines the corresponding pro-
portion of these compounds. Obviously, the presence of
oxygen or highly oxidized substances will favor the forma-
tion of diacetyl; on the other hand, a strongly reducing
potential would promote the predominance of acetoin or
butanediol, both of which are flavorless and odorless com-
pounds (Marshall, 1961). This interrelationship is of great
significance to industry due to the established importance
of diacetyl in many dairy products and the ease of its
destructive conversion into acetoin and butanediol, with an
accompanying loss of flavor.

The most potent diacetyl—producing organisms are
paradoxically, the ones most active in its subsequent
destruction. Thus, of the lactic streptococci, S. diace-
tilactis exhibits the strongest reducing potential favoring
the formation of butanediol (Sandine, 1964). This ability
is attributed to the presence of certain enzyme systems with
which the bacteria are endowed and which are activated under

favorable conditions.

18

Various investigations have provided an insight into
these mechanisms, shedding light on new theories to replace
the old. Strecker and Harary (1954) reported the isolation
and purification of two enzyme systems, one catalyzing the
reversible oxidation by DPN+ of butanediol to acetoin and
the other catalyzing an essentially irreversible reduction
by DPNH of diacetyl to acetoin. They named the enzymes
2,3-butylene glycol dehydrogenase and diacetyl reductase
respectively. This observation refutes the hitherto accepted
concept of acetoin being the immediate precursor of diacetyl.

A slightly different mode of diacetyl breakdown was
suggested by Green _£.El- (1947) in a study of a diphos-
phothiamine-dependent enzyme which catalyzed the conversion
of two molecules of diacetyl into two molecules of acetic
acid and one molecule of acetoin. They called this enzyme
diacetyl mutase. Strecker and Harary (1954) indicated that
the diacetyl reductase was possibly a component of the
diacetyl mutase reported since acetoin was not oxidized in
the presence of the reductase.

Recent studies by Juni and Heym (1956) revealed yet
another pathway for the reduction of diacetyl and acetoin to
butanediol, proceeding through the intermediate compounds
diacetylmethylcarbinol and acetylbutanediol, which is
dependent on the presence of diphosphothiamine and DPN+.
These mechanisms would serve to explain the increase in

butanediol content which parallels the decrease in diacetyl

19

and acetoin contents and causes a deterioration in the

flavor of cultured dairy products.

Culture Preservation

 

Interest in the preservation of starter cultures has
intensified during the past decade. The ideal method of
preservation would be to take the organisms at the peak of
their metabolic activity, hold them for days or months in a
state of arrested development and have them resume their
work immediately on restoration to a favorable environment
(Foster, 1962). Unfortunately, this ideal has never been
realized since it is virtually impossible to keep a living
organism in a completely inactive state. Hence, alternative
methods have had to be resorted to, based on one of two
principles involving either the reduction of the metabolic
rate of the organisms or the separation of the cells from
their metabolic waste products. The choice of a preserva-
tive method depends largely on the ultimate purpose for
which the culture is to be used and maybe any one of the
following:

(a) Refrigeration at low temperatures between trans-
fers, as often employed by many dairy plants and research
laboratories,

(b) Freezing, where extended storage is required,

(c) Freeze drying or lyophilizing, involving ini-
tial freezing of the cultures, subsequent drying by sublima-

tion and final storage at low temperatures,

THE-ZS

 

20

(d) Spray drying of the culture. Although not
commercially used, spray drying has been investigated as a
possible method of economically producing dehydrated starter

cultures (Foster, 1962).

Liquid Cultures

 

Normally, ripened cultures can usually be held at
4-8 C for several days without a serious change in activity.
Storage at higher temperatures, however, resulted in an
accelerated loss of activity. According to Swartling and
Lindgren (1960) the activity of cultures refrigerated imme-
diately after inoculation was better retained than that of
cultures permitted to ripen before storage.

The effect of the addition of various compounds on
prolonging storage activity has been investigated by many
workers. Heinemann (1958) was one of the first to show that
glycerol has a protective effect on starter bacteria. The
cultures under study remained active as long as two months
at 35 F and six months at 5 F and -20 F. Under similar con-
ditions, the activity of cultures without glycerol was appre—
ciably decreased. Olson (1959) added various insoluble
buffers to starter cultures and found that CaCO3 gave the
best protection of those investigated. The findings of
Lindgren and Swartling (1960), however, did not indicate
storage of cultures in "chalk milk" as a reliable method of

preservation. Certain concentrations of glycerol, salt and

21

sugar also aided in preserving the cultures, a combination
of 20% glycerol, 3% salt and 30% sugar being the most effec-

tive, with or without added CaCO3, according to Olson (1959).

Frozen Cultures

 

Freezing can be used to preserve many types of
microorganisms. Although the process kills some of the
cells, as many as 75-90% of the viable bacteria have been
recovered on thawing of the frozen cells. A further probe
into the matter has revealed that the infliction of greatest
injury to the bacteria occurs during the early part of stor—
age and injury increases further with time (Moss and Speck,
1962). A rise in death rate is thus continuous resulting
in a decrease of activity with length of storage (Rudnik and
Glenn, 1960). Though Foster (1962) could demonstrate no
effect on the rate of freezing and thawing on survival,

Moss and Speck (1962) have shown that some cultures survive
best when frozen rapidly. The converse has been demon-
strated for many other cultures. Addition of glycerol con-
fers protection from damage (Heinemann, 1958) while use of
fresh liquid skim or 2% dried skim milk was found definitely
superior to other media (Moss and Speck, 1962; Simmons and
Graham, 1959; Foster, 1962). Greatest destruction of cells
was found by Moss and Speck (1962) to occur when the cells
were frozen in distilled water. The acidity and physiolog-

ical age of cultures prior to freezing also influences their

TH

EAL-“H“ .~

 

 

22

survival and overall activity. Swartling and Lindgren

(1960) observed that concentrated suspensions of younger
cultures (15-18 hours old) were definitely more active on
thawing than cells from older cultures. They also recorded
an even better performance when inoculated milk, frozen with-
out prior incubation, was thawed and ripened.

Lindgren and Swartling (1960) considered deep freez-
ing a very satisfactory method of preserving the activity of
a freshly inoculated culture for as long as one year. The
successful use of frozen cultures for direct inoculation in
the commercial manufacture of fermented products (Simmons
and Graham, 1959; Rudnik and Glenn, 1960) has served to con-
firm this observation. Simmons and Graham (1959) regularly
made good buttermilk with frozen culture stored as long as
three months; the activity of the thawed culture compared
favorably with that of fresh starters transferred daily.
Similarly, Rudnik and Glenn (1960) employed frozen culture
up to 5 months old to inoculate milk directly for cottage
cheese manufacture. All 39 lots of cheese so made were
salable.

These and similar investigations have so far been
encouraging enough to advocate freezing as a means of pre-

serving organisms for extended periods of time.

.n' . -‘?‘?k—fi. .

 

 

 

23

Dried Cultures

 

Lactic cultures can be dried by lyophilization or by
spray drying. The former is the less destructive of the two
processes and is readily adapted to the preservation of
small amounts of culture (Foster, 1962).

Freeze drying. This process, which has enjoyed wide-

 

spread use in the food industry for dehydrating foods, has
likewise been successfully employed in the preservation of
stock cultures. Freeze—dried cultures can be used for
months or years as the seed material for developing vigorous
starters. Such powdered cultures stored by Maxa and Teply
(1960) at refrigeration temperatures retained their activity
at almost the initial levels throughout the two-year experi-
mental period. The ability of organisms to endure the dry-
ing process varies with the species, according to Foster
(1962). Several other factors, including age of the culture
and nature of the suspending medium play influential roles
on activity of the culture. Watts (1955) for example,
lyophilized a milk culture at various stages in the growth
cycle up to 19 hours. Samples dried at 9 and 12 hours of
age, which represented the late logarithmic and early max-
imum stationary phases of growth respectively, showed the
highest survival values, namely 76 and 84%. On rehydration,
their acid—producing ability approached that of the undried
culture. No changes were observed by Morichi t al. (1961)

in the physiological characteristics of the freeze-dried

THESl

 

 

 

24

cultures. Death rates could be minimized by maintaining the
acidity of the cultures between pH 6-9. Hence, the benefi-
cial effect of diluting cultures with skim milk on the
survival rate can actually be attributed to the consequent
increase in pH. Once dried, lactic cultures must be stored
at low temperatures and be protected from moisture and light.

Spray drying. Several investigators have considered

 

the possibility of Spray drying large quantities of culture
since it offers considerable economic advantage by way of
lower processing costs over other methods of drying.

Mamaeva (1955) spray dried a mixture of lactobacilli and
yeasts used for koumiss culture, but recovered only a frac-
tion of 1% of the cells in a viable condition. Nonetheless,
these dried cultures after reconstitution with water exhib-
ited a high rate of acid production and retained their
activity for six months, depending on the storage conditions.
Attempts to spray dry ordinary milk cultures of lactic acid
bacteria were not very successful initially and early efforts
by Richardson (1960) were abandoned because the product, in
addition to being less active than the lyophilized culture,
was difficult to rehydrate. S, lactis in 5% reconstituted
skim milk dried to a 3.5% moisture level yielded 50-6G%
viable cells immediately after drying, as reported by
Lattuada and Foster (1963). Residual moisture, within the
2.4 to 4.4% range, did not affect stability during storage,

and low storage temperatures prolonged shelf life of the

25

dried culture. Extensive studies conducted by Sapp and
Hedrick (1960) show that with favorable conditions, appre-
ciable activity can be maintained in spray-dried cultures.
Outlet air temperatures of 135-165 F favored greater sur—
vival while neutralization of the acidity of the cultures
before drying decreased rather than increased the activity
of the dry product. Cultures dried at 12, 16 and 24 hours
of age showed practically the same activity but those dried
at 8 hours were less active.

Foster (1962) reported consistent differences

between the survival values of S. lactis and S. cremoris,

 

the former being more resistant both to Spray drying and to
storage in the dry state. Use of 5% NFDM as the suSpension
medium was recommended over others such as phosphate milk

or dextrin-ascorbic acid-thiourea diluent. Under the best
of conditions, dry cultures could be stored at least four
months without a loss of greater than 15%, even though spray
dried cultures have been shown by others to die rapidly if
stored at temperatures above freezing. Cultures stored at
40 F by Sapp and Hedrick (1960) were active after one week
but found unsatisfactory after three weeks. Retention of

initial activity was considerably extended at -15 F.

EXPERIMENTAL P ROCEDURES

Sources 2: Cultures

 

Commercial freeze-dried sour cream cultures were
obtained from the Michigan State University (MSU) Dairy
Plant and from a culture supply house. These cultures were
propagated in skim milk which had been heated in flowing
steam for one hour. The cultures were incubated at 72 F

and were transferred daily during the course of the research.

Preparation 9: Fresh Sour Cream

 

 

For the research reported herein, creams of three
different fat contents were prepared: 10, 14 and 18%. Each
lot of cream was standardized at the MSU Dairy Plant and was
processed in 10 gal stainless steel cans. The cream was
pasteurized at 165 F for 15 min under constant agitation,
homogenized twice at 2000 psi single stage using a Manton
Gaulin three plunger homogenizer and immediately cooled to
72 F. The cream was inoculated with 1% starter culture and
incubated until at least 0.70% titratable acidity, calculated
as lactic acid, was attained.

Approximately four gallons of the sour cream thus

obtained was then layered (% inch thick) in enamel trays,

26

27

covered with aluminum foil and quick frozen in a -10 F mov—
ing air hardening room. Half of this frozen sour cream was
broken up into small pieces and dried for 40 hr in a Stokes
freeze drier chamber evacuated to 100 microns of mercury, as
measured on a McLoed gauge. The temperature of the platens
was gradually raised from 28 C to 42 C within the first 24
hrs.

The remaining half of the ripened sour cream was
atomized into a Rogers cocurrent inverted tear drOp drier
using two Spraying Systems SX high pressure nozzles with
number 17 spinners and number 70 cores. The dryer was Oper—
ated at an exit air temperature of 165 F. Nitrogen was in—
jected into the feed at a rate of 2.0 ft3/gal cream in a
mixing cylinder located between the high pressure pump and

the atomizing nozzle.
Method 2E Storage

The foam-spray dried and freeze dried sour creams
were stored in cryovac plastic bags at 40 F. Small amounts
of these powders which were to be used for organoleptic

evaluations were bottled and stored at 40 F and 72 F.

28

Preparation 22 Samples for Analyses

 

 

Control

The frozen sour cream stored at -10 F was used as
the control. Each day as per requirement, portions of the
frozen cream were thawed at room temperature and homogenized
once in a stainless steel hand homogenizer.
Reconstituted Foam-spray Dried and
Freeze—dried Sour Cream

The powders were reconstituted to the total solids
content of the corresponding control by blending with dis-
tilled water. This mixture was stirred, allowed to stand at
ambient temperature for 15 min and was then homogenized in

the hand homogenizer.

Analytical Methods

Moisture

 

The moisture content of all foam—spray dried and
freeze-dried sour cream samples was determined by a standard
vacuum oven technique employing a Mojonnier milk tester. A
sample approximately 0.3 g in weight, accurately weighed
directly into a Mojonnier moisture dish, was spread evenly
over the entire bottom of the dish by adding 2 ml hot dis-
tilled water (ca. 100 C). The dish was kept in direct con—
tact upon the outside hot plate having a temperature of

180 C and heated until the first traces of brown began to

29

appear. The sample was then transferred to a 100 C vacuum
oven and kept for 10 min under 27 inches of vacuum and
thereafter placed in a cooling dessicator for 5 min with the
water circulating pump operating continuously. At the end
of this period, the dish was weighed rapidly and the mois-
ture content calculated and expressed to the nearest one-
tenth of one per cent.

The total solids of the fresh unfrozen sour cream
was similarly determined using approximately 1 g samples,

accurately weighed.

33
The pH measurements on the control and reconstituted
samples were made with a Beckman Zeromatic pH meter using a
calomel half cell and a glass electrode standardized to read
accurately in the range of pH 4.0 to 5.0. The results were

expressed to the nearest one-tenth of a pH unit.

Titratable Acidity

Nine gram aliquots of the control and the reconsti-
tuted samples were titrated with 0.1 N NaOH to the phenol-
phthalein endpoint. The acidity was reported as per cent

lactic acid.

Volatile Acidity
A rapid direct-distillation method of Kosikowski and
Dahlberg (1946) was adapted to quantitate the volatile acid

content of the control and the reconstituted samples of sour

3O

cream. To a 10 9 sample of cream was added 50 ml 10% H2804
(at 50 C) and 35 g MgSO4°7H20. The mixture was stirred,
refluxed for 5 minutes to drive off the C02 and cooled by
standing at ambient temperature for 30 min. Distillation
was then begun and continued for 60—70 min until the boiling
mixture reached a temperature of 116 C. The distillate so
collected contained the water soluble volatile acids; the
condenser was rinsed with 25 ml neutral alcohol to recover
the water insoluble volatile acids. Each fraction was then
titrated with 0.1 N NaOH to the phenolphthalein endpoint and
the sum of the titers reported as ml of volatile acid per

100 g sample.

Diacetyl Determination

 

The diacetyl content of the control and the recon-
stituted sour cream samples was determined by the method of
Prill and Hammer (1938) employing a 25 g aliquot weighed
into a 500 ml, two necked distillation flask. The flask was
connected to the distillation apparatus and a slow stream of
C02 was passed over the sample and through the apparatus for
5 min. Steam was then admitted under reflux to displace any
remaining air and the C02 from within the system. When
bubbles of gas ceased to appear in the collection trap, dis-
tillation was permitted to proceed at a slow rate for 25—30
min collecting 5.0 to 5.2 m1 distillate in 1 ml hydroxylamine

acetate solution. The absorbance of diacetyl (as ammono

‘r‘a
....

 

 

31

ferrous dimethylglyoxime) was measured at 530 mp. in a model
14 Coleman spectrophotometer. This value was then converted

to mg diacetyl by referring to a standard curve.

Diacegyl and Acetoin Determination

For the diacetyl—plus—acetoin determination, 15 ml
40% FeCl3 solution was added to 25 g of the control or recon-
stituted samples being analyzed and the mixture refluxed for
10 min before distillation was commenced. The development
and measurement of the colored ammono ferrous dimethylgly-

oxime complex were accomplished as previously described.

Total Fat

 

The total fat of the spray-dried and freeze-dried
powders was extracted by slightly modifying the standard
Roese-Gottlieb procedure to include addition of 3 ml NH4OH
instead of the suggested 1.5 ml, since the acidity of the
sour cream necessitates the use of additional alkali. The

results were eXpressed as per cent fat on a dry basis.

Free Fat

 

The freeze dried and foam-spray dried powders were
analyzed for free fat by the method of Thomas, Holgren,
Jokay and Bloch (1957) and the findings reported as mg free

fat/g total fat.

32

Dispersibility

 

The method outlined by Stone _£._l. (1954) was
modified to determine the dispersibility of the freeze-dried
and foam-spray dried cultured cream. A 10 g sample of the
powder was blended with 90 ml distilled water at 25 C for
30 sec in a high speed blender and immediately filtered
under vacuum using a medium porosity sintered glass funnel.
The resulting filtrate was transferred to a 100 ml volumet-
ric flask and filled to the mark with distilled water. The
solids content of a 10 ml aliquot of this filtrate was
determined by the vacuum oven technique employing a Mojonnier

milk tester and the dispersibility reported as g of powder

dispersed/100 g sample.

Organoleptic Evaluation

 

The flavor of the reconstituted freeze-dried and
foam-spray dried sour creams, stored at 40 F and 72 F for
8 weeks, was judged by a panel of 3-4 members at the end of
0, 4 and 8 week intervals. The frozen sour cream served as
the control. The hedonic preference scale with a range of
0 to 9 was used in evaluating the samples and the average

value for each sample was reported.

THES

 

RESULTS

Some Physical and Chemical Characteristics

9: Dehydrated Sour Cream

 

The data collected on selected physical and chemical
characteristics of dehydrated sour cream are presented in
Table l.

The moisture content of the foam—spray dried sour
cream ranged from 1.8 to 2.9% and of the freeze-dried sam-
ples varied from 2.2 to 2.6%. In four of the six pairs of
samples analyzed, the moisture content of the freeze—dried
sour cream exceeded those of the corresponding foam—spray
dried sour cream. The total solids of the control increased
from 17.9 to 25.8% with increasing fat content of the cream.

The pH of the dehydrated sour cream, in both trials,
was higher than the control. Results obtained for the
freeze-dried samples, ranging in value from 3.9 to 4.5,
were consistently lower than those of the corresponding
foam—spray dried cream, which varied from 4.1 to 4.7.

The titratable acidity of the dehydrated sour cream,
with the exception of the sample foam-Spray dried from 14.6%

fat cream, was lower than the control. The losses resulting

33

THE

 

34

 

 

 

 

 

 

 

mo mGOHDMCHEnmqu HMOHumamcm

05.0 m.¢ 0.N 05.0 0.0 0.N 55.0 N.¢ 0.¢N 0.5a

05.0 0.0 0.N 00.0 5.¢ 0.N 55.0 ¢.v 5.0N N.ma

55.0 0.0 N.N 00.0 m.v 0.N 05.0 m.v 0.5a H.0H
HH HMHHB

H0.0 v.0 m.N 05.0 0.0 0.N m0.0 m.¢ 0.mN 0.0a

05.0 0.0 m.N 05.0 H.¢ 0.N 0%.0 0.m m.NN 0.0a

55.0 0.m 0.N @5.0 H.¢ N.N 05.0 0.m H.0H 0.0a
H HMHHB

0x0 050 A80 $0 $0 0x; 0x0
xufloflo< mm manumfloz muflcfiod mm mHSDmHOE muflcflum mm mUHHom Emmuu mo
manmumuufle manmumuufle manmumnufla HMDOB ucmucou umm

Gmflua mummnm Umflua mmnmm Emom Houucoo

 

 

Emmuo usom Umumuownmo Mo mUHumHHmuomnmno HMUflEmSU 0cm HMUHmmnm 0800

.H mHQMB

35

from foam—spray drying the cream are relatively greater than

those incurred due to freeze drying.

Effect 2£_Drying‘gg the Volatile

 

Acidity 2E Sour Cream

 

The data in Table 2 illustrate the effect of drying
on the volatile acids content of sour cream. Although there
is an obvious decrease in volatile acidity on dehydration,
the quantity retained by the freeze-dried powders is consis-
tently higher than the corresponding foam-Spray dried sam-
ples. An exceptional 100% retention is observed in the

freeze—dried 14.6% fat sour cream analyzed in Trial I.

Effect 9§_Drying 2g_the Diacetyl

 

Content 2: Sour Cream

 

The changes in the diacetyl content of sour cream
as a consequence of drying are presented in Table 3. The
overall trend indicates an increase in diacetyl in the
resulting powders although, a decrease in the case of four
samples is also recorded. In Trial II, the diacetyl content

of the freeze-dried product is substantially greater than of

the corresponding foam-Spray dried counterparts. This obser-

vation, however, is not duplicated in Trial I.

 

Table 2. Effect of drying on the volatile acidity of sour

cream

36

 

 

Fat Content

Volatile Acidity
(m1 of 0.1N NaOH/lOO gm. sample)

 

 

 

of Cream
(%) Control Foam Spray Dried* Freeze Dried*

Trial I

10.0 25.0 13.4 16.0

14.6 20.0 9.8 20.5

18.0 23.8 17.3 21.5
Trial II

10.1 17.0 9.0 12.0

13.2 16.0 12.0 13.0

17.5 18.0 9.5 14.0

 

*Reconstituted.

Table 3. Effect of drying on the diacetyl content of sour

cream

37

 

 

Fat Content

Diacetyl (mgs/kg)

 

 

 

of Cream
(%) Control Foam Spray Dried* Freeze Dried*

Trial I

10.0 0.89 1.35 0.84

14.6 0.68 0.74 0.72

18.0 0.52 0.48 0.58
Trial II

10.1 0.49 0.55 0.89

13.2 0.52 0.41 0.81

17.5 0.56 0.44 0.77

 

*Reconstituted.

THES

 

38

Effect 9; Drying 22 the Diacetyl-plus-

 

acetoin Content g£_Sour Cream

 

 

Table 4 contains data which enumerate the effect of
drying on the diacetyl-plus—acetoin content of sour cream.
The results are significant in the drastic decreases caused
by foam-spray drying, from 122—185 ppm diacetyl-plus—acetoin
in the control to 4-8 ppm in the resulting powders. Losses
incurred by the freeze-dried samples are also substantial,
i.e., a decrease from 122-185 ppm in the controls to amounts
ranging from 29-66 ppm in the powders. However, retention
of diacetyl-plus-acetoin is substantially higher in freeze-

dried powders than in foam-spray dried powders.

Effect 9: the Method 2: Drying 2g the

 

Free Fat 9; Dehydrated Sour Cream

 

 

As evident from inspection of the data in Table 5,
the free fat content of freeze-dried sour cream is consider-
ably higher than of the corresponding foam-spray dried pow-
ders, by amounts varying from 138 mg/g total fat to as high
as 231 mg/g total fat. An increase in free fat values of
both foam-spray dried and freeze-dried powders with increas-
ing total fat content of the original cream is noticed in
Trial II. However, this trend is not evident in the powders
studied in Trial I and may be related to processing condi-

tions not studied.

THEf

 

Table 4.

39

content of sour cream

Effect of drying on the Diacetyl—plus-acetoin

 

 

Fat Content

Diacetyl-plus-acetoin (mgs/kg)

 

 

 

of Cream
(%) Control Foam Spray Dried* Freeze Dried*

Trial I

10.0 122.4 6.3 52.0

14.6 122.5 5.6 39.5

18.0 138.9 4.6 29.2
Trial II

10.1 153.5 7.0 57.3

13.2 185.0 7.6 65.3

17.5 151.3 6.4 53.0

 

*Reconstituted.

4O

 

 

 

0.0mm 0.0H0 5.00 0.000 0.00 0.50
0.Hmm 0.000 5.00 0.000 0.H0 0.0H
0.000 0.000 0.00 0.000 0.00 H.0H
HH HMHHB
0.000 0.000 0.05 0.005 0.05 0.00
0.00H 5.050 0.00 0.NH5 0.00 0.0a
0.000 0.000 0.00 0.005 5.00 0.0a
H Hmflue
000 H0009 A0\0sv “mm H0009 Am\0sv Axe 000 00009 Am\0sv 4&0 Axe
Emmuu H500 wmflna mmumm umm mmum 000 000 mmum umm Emmno mo
H0>O 000nm mummum Hmuoe H0008 ucmucoo 000

MO 005H0> umm wmum 00
mmozmumwMHQ UmmmmuocH

 

Umflua 0000mm

mo 005Hm> umm

 

000nm wmumm Emom

 

 

Edmno 0500 Umumuwmsmw mo umm mmum mnu 00 mcflhnc no 003005 030 m0 uommmm

.m magma

 

 

THE

 

41

Effect 9; the Method 2: Drying 22 the

Dispersibilityigfi Dehydrated Sour Cream

 

The dispersibility of dehydrated sour cream decreased
with increasing total fat content of the cream, as indicated
by the data enumerated in Table 6. Foam-spray dried powders,
possibly due to their lower free fat value, were more dis—
persible than the corresponding freeze-dried samples in four
analyses out of six. However, no definite correlation can
be established since many other factors, not taken into con-
sideration in this research project, are found to influence

the dispersibility of dehydrated samples.

Flavor Scores 2; Foam-gpray Dried

 

and Freeze-dried Sour Cream

Within a period of 8 weeks duration, there was no
appreciable difference in the effect of storage at 40 F or
72 F on the flavor scores of foam-spray dried and freeze-
dried sour cream. Evidence in support of this observation
is presented in Table 7. The freeze-dried powders of vary-
ing fat contents, scored much higher ratings ranging from
5.6 to 7.5, than the corresponding foam—spray dried samples
(2.0 to 4.6). The superiority of the freeze-dried powders
over their foam-spray dried counterparts was established at
the very outset and a continued preference sustained by all

the judges during the entire study.

42

Table 6. Effect of the method of drying on the dispersibil-
ity of dehydrated sour cream

 

 

Fat Content Dispersibility (g/10 9- powder) Of

 

 

 

of Cream
(%) Foam Spray Dried Freeze Dried
Trial I
10.0 2.56 2.19
14.6 1.59 1.82
18.0 1.64 1.61
Trial II
10.1 2.40 2.52
13.2 2.19 1.97

17.5 1.87 1.61

 

TH:

J—IT'QQ‘.‘

 

43

Table 7. The effect of storage at 40 F and 72 F on the
flavor scores of foam—spray dried and freeze-dried
sour cream

 

 

Hedonic Scores For

 

 

Period of Control Spray Dried* Freeze Dried*
Storage Stored at Stored at
(In Weeks) 40 F 72 F 40 F 72 F

 

10.1% fat cream

 

 

 

0 7.3 3.3 6.3

4 7.0 3,6 4.6 6.3 6.3

8 7.0 3.5 4.0 6.0 6.3
13.2%.fat cream

0 4.6 3.3 5.6

4 6.5 3.6 3.6 7.2 7.3

9 7.3 ... 2.0 ... 5.6
17.5% fat cream

0 8.2 3.7 6.7

4 8.3 2.3 3.2 7.5 7.5

8 7.0 2.0 2.0 6.0 4.0

 

*Reconstituted.

DISCUSSION

Six batches of sour cream, varying in total fat
content, were prepared during the entire course of study.
Cultures employed to inoculate the sweet creams listed under
Trial I and Trial II were from two separate sources. The

characteristics of a culture depend a great deal on the

-01112114 my -..-v

particular strain(s) of bacteria present. Differences in

 

.Pnis ._' ..

the activity of the culture, stemming from variations in the
mode and conditions of propagation are subsequently reflected
in the resulting cultured food product. The obvious differ-
ences in physical characteristics and chemical composition
of the controls of Trial I and Trial II (Tables 1—6) could
partly be accounted for on this premise. The aroma produc-
ing leuconostocs are activated only after the pH of the
medium is sufficiently lowered. Hence the production of
adequate amounts of acidity is an important aspect of sour
cream manufacture and contributes substantially to the over-
all flavor and desirable body characteristics of the result-
ing product. Values ranging from 0.6 to 0.8% titratable
acidity, eXpressed as lactic acid, are generally achieved;
the optimum level varies with such factors as the total
solids content of the cream, cultures employed and manufac-

turing procedures used, as well as the ultimate flavor

44

45

desired. In this study, the samples of cream were cultured
to an average acidity of 0.78%. As evident from Table 1,
the pH values and the corresponding titratable acidity could
not be correlated. Two batches of sour cream prepared from
13.2% and 17.5% fat creams with a titratable acidity of
0.77% recorded a pH of 4.4 and 4.2 respectively, while two
other samples of identical pH values registered a consider—
able difference in their respective titratable acidities.
Frozen sour cream was employed as the control. Its
body, on thawing, was considerably destabilized with much
fat clumping and protein flocculation. Such destabilization
defined by Favstova and Vlodavets (1955) as "a reduction of
fat dispersion due to fusing of fat particles," is a conse-
quence of the destructive action of freezing on the fat
emulsion of a product. Due to the expansion of water on
freezing and a possible denaturation of the membrane protein,
the fat globule membrane is ruptured, resulting in the
liberation and fusion of globular fat causing a subsequent
loss of viscosity (Kn00p and Wortmann, 1959). Rapid freez-
ing minimizes this effect if the rate of contraction of the
fat is the same at which the water eXpands. Slow freezing
inflicts greater damage on the structure of the cream.
Hence, destabilization is largely dependent on the rate and
magnitude of the temperature change (Lagoni and Peters, 1961)
and is favored by low temperatures of storage and a high fat

content of the product. The thawed cream therefore, had to

46

be homogenized using a hand homogenizer, to disperse the
fat and ensure homogeneity in composition of the samples
analyzed.

There was a distinct difference in color of the
dehydrated sour cream obtained by the drying methods em-
ployed in this study, the freeze—dried powder being lemon
yellow in contrast to the light cream appearance of the
corresponding foam-spray dried sample. The state of the fat
in the particles and the particle size itself may account
for the difference in color. The amount of fat liberated
as free fat and the state of its dispersion either on the
surface or throughout the entire mass of the powder, is
largely dependent on the method of dehydration employed.

As evident from Table 5, there is a substantial increase in
the free fat content of the freeze-dried over the foam-spray
dried powder. Berlin _E._l. (1964) observed by fluorescence
microscopy, that the free fat in foam dried whole milk pow-
ders exists on the surface of the powder particles in the
form of small fat globules, whereas the same is present on
the surface of the spray-dried powders as lakes or films.
Microscopic examinations by Nickerson _£._l- (1952) showed
that the fat globules of freeze-dried whole milk were dis-
persed throughout the mass of the particles. Particle size
also is governed by the method of dehydration. Since the

size and shape of the frozen particles do not alter during

freeze drying, the resulting powder particles are irregularly

47

shaped and porous in nature. Spray-dried particles, on the
other hand, tend to be uniform in size and were relatively
smaller as obtained in this study. Thus, a high free fat
content and larger particle size may be responsible for the
deepening of color in the freeze—dried over the spray—dried
samples.

The ease of dispersion of a dehydrated product is
greatly influenced by its lipid content. The dispersibility
of dried milks was observed by Ashworth (1955) to decrease
with increasing total fat in the samples. As evident from
the data in Table 6, a similar decrease in dispersibility
occurs in both foam-spray dried and freeze-dried sour cream
with increasing total fat content. However, a similar corre-
lation between free fat values and their effect on dispers-
ibility could not be established in this study since the
results of Trial I totally contradicted those of Trial II,
indicating that other unknown factors are involved. In
Trial II the dispersibility of the samples decreased with
increasing amounts of free fat, whereas, with the samples
reported in Trial I, dispersibility decreased with decreas-
ing amounts of free fat. Investigations by Tamsma _£._l.
(1958) indicated a positive relationship between the free
fat content of foam-spray dried whole milk and its dispers-

ibility, which was unaffected by amounts of free fat up to

40% but decreased as the levels increased from 40 to 95%.

48

Data reported by Reinke and Brunner (1959) failed to reveal
any such interdependence. Certain variations in their
processing procedures yielded greater amounts of free fat
in the resulting whole milk powder but did not diminish the
ease of dispersion of the product. Spraying of the feed
through large orifice nozzles at low pressures favored free
fat formation but also enhanced dispersibility while homog-
enization of the condensed milk prior to atomization de-
creased both. Reducing the total fat content on the other
hand, yielded smaller amounts of free fat and increased the
dispersibility.

The volatile acidity of sour cream was diminished on
dehydration; the losses incurred on foam—Spray drying were
greater than those due to freeze drying. According to
Bradley (1964), foam-spray drying of natural cheese slurries
permitted a greater retention of flavor volatiles than could
be achieved by conventional spray drying. This was attrib-
uted in part to the increased particle size of the powder,
as a consequence of gassing the feed, which entrapped greater
concentrations of the volatiles. In addition, sparging with
an inert gas increases the porosity of the droplets thereby
accelerating the evaporation of moisture. This, in turn,
promotes rapid cooling of the particles during the drying
period. As observed by Bradley (1964), the retention of
flavor volatiles is enhanced by lower powder temperatures.

On this premise, one might also account for the better

49

retention of the flavorful compounds in the freeze-dried
samples over the foam-spray dried counterpart (Table 2).
The freeze-dried powders obtained in this study were rela-
tively more porous and larger in size than the corresponding
foam-spray dried sour cream. Subjection to high tempera-
tures, another cause for losses induced on spray drying, is
totally absent in freeze drying; hence, losses in the vola-
tile acids content is considerably minimized and restricted
to a decrease in amounts of those water soluble, low molec-
ular weight compounds which are most easily removed during
sublimation of the ice.

This eXplanation can further be applied to account
for the substantial losses in acetoin-plus-diacetyl, encoun-
tered in both powders. The loss incurred by the foam-spray
dried samples is remarkably high. Both acetoin and diacetyl
are low molecular weight compounds (88.1 and 86.1 respec-
tively), very soluble in water and greatly prone to volatil-
ization on drying. The boiling points of diacetyl and
acetoin are 85 C and 144 C respectively. The acetoin con-
tent which always greatly exceeds the amount of diacetyl
present, is diminished either as a direct loss or via
oxidation to diacetyl due to air incorporation in the drying
chamber.

0n the other hand, contrary to eXpectations, the

diacetyl content of sour cream registered an increase on

 

50

dehydration (Table 3) in eight out of twelve analyses. The
additional amounts of diacetyl recorded for the two powders
over initial quantities of the control, very possibly result
due to conversion of precursor compounds such as acetoin to
diacetyl induced by incorporation of air during stirring of
the ripened cream prior to drying or occurring as a result
of oxidative changes during drying itself. Shaking the cul-
tures during ripening is conducive to increased yields of
diacetyl (Prill and Hammer, 1939) while slow churning of

the cream in preference to holding it for the same time with-
out churning resulted in a butter with improved flavor, pre-
sumably due to the aeration involved (Peterson, 1943). Con-
sequently, losses of original diacetyl incurred by the pow-
ders may be obscured by the concurrent oxidation of acetoin
during processing, resulting therefore in an apparent over—
all gain in diacetyl content of the powder.

The analytical results discussed heretofore, are
significant. However no study directed towards product
development in the food industry is complete without accom-
panying flavor scores since palatability of the commodity in
question largely influences its success on the consumer mar-
ket. The dehydrated sour creams employed in Trial II of
this study were also organoleptically evaluated using the
conventional hedonic scale (Table 7). The flavor scores for
the 10% fat cream are, in general, in good agreement. How-

ever, some discrepancies occur in the two succeeding samples;

51

for example, the ratings of the control containing 13.2% fat
increased from 4.6 to 6.5 in 4 weeks. This does not repre-
sent an amelioration of flavor on storage. The hedonic
scale is purely comparative in function and the figurative
range of preference varies with each evaluation conducted at
different times. Hence an implicit correlation cannot be
established. On a relative basis it is evident from the
data in Table 7 that the freeze-dried sour cream is dis-
tinctly superior to its foam-spray dried counterpart, this
preference being sustained by the judges during the entire
period. The original flavor and aroma of the cultured sour
cream was considerably retained in the freeze-dried product
for as long as 8 weeks. The foam—spray dried samples on the
other hand were repeatedly described as being chalky,
astringent and stale at the very outset; the stale flavors
intensified greatly during 8 weeks storage at both 40 F and
72 F. These stale flavors are associated with the tempera-
ture dependent deteriorative changes occurring in the milk
fat phase (Tarassuk and Jack, 1946; Tamsma _E.El-: 1963).
Studies conducted by Nawar g£_gi. (1963) indicate the pos-
sibility that the stale flavor sensation, observed in dried
whole milk, exists in one or both of two chemical forms
identified as being an aldehyde similar to heptaldehyde and
an unsaturated dicarbonyl or hydroxy carbonyl compound. The
amounts of these components of the stale fraction vary with

the time and temperature of storage. The stale flavor

52

development is most rapid in the initial months of storage
and is greatly accelerated by higher temperatures. Oxygen
promotes staling, hence deaeration and inert gas packing of
the powders results in beneficial effects of extending the
storage life. Another important factor involving subjection
to high heat treatment prior to drying greatly enhances the
quality of the dehydrated product by serving to minimize

staling (Christensen t 1., 1951).

SUMMARY AND CONCLUS IONS

Cultured (sour) cream ranging in fat content from 10 to
18% was dehydrated by foam—spray drying and freeze
drying.

The moisture content of the foam-spray dried sour cream
ranged from 1.8 to 2.9% and was generally lower than the
corresponding freeze-dried samples, the values for which
varied from 2.2 to 2.6%.

Compared to the control, both foam—spray dried and
freeze-dried sour cream contained less volatile acids,
lower titratable acidity and lower levels of acetoin-
plus—diacetyl. In general, losses of volatile constit-
uents were lower in the freeze-dried than in the foam-
spray dried sour cream.

An increase was recorded in the quantity of diacetyl in
the dehydrated samples over the controls, possibly due
to an incomplete volatilization of the additional diace-
tyl formed on oxidation of some of the acetoin during
drying.

Free fat values of the freeze—dried samples were consis-
tently higher than the amounts extracted from the corre-

sponding foam-spray dried creams.

53

54

The dispersibility of both the dehydrated sour creams
decreased with increasing fat content of the original
cream. However, no correlation could be established
from the results obtained in this study, between the
free fat value and its influence on the ease of disper-
sion of the powders.

The flavor and aroma of the freeze—dried sour cream was
far superior to the foam—spray dried equivalent. After
storage at 40 F and 72 F for 8 weeks the freeze-dried
sour cream was rated acceptable to good. The foam-Spray
dried samples were repeatedly described as being chalky,
astringent and stale. The stale flavors became more

pronounced during storage.

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