596 Part VI: Fermented Foods
the fat, protein, and calcium content and overall
composition and quality of the milk. On the average,
cow milk consists of 3.7% fat, 3.4% protein, 4.8%
lactose, and 0.7% ash and has a pH of 6.6 (Fox et al.
2000). Good microbiological quality of the raw milk
is also essential to the production of safe, high qual-
ity, cultured dairy products.
The predominant sugar in milk is lactose, a disac-
charide of glucose and galactose. The fermentation
of lactose by lactic acid bacteria in cultured dairy
products provides the flavor and textural attributes
that are desirable in cultured dairy products.
Triacylglycerols are the predominant lipid frac-
tion in milk, accounting for 98% of the total lip-
ids. Diacylglycerols, monoacylglycerols, fatty acids,
phospholipids, and sterols account for the remaining
lipid fraction. Approximately 65% of the fatty acids
in milk fat are saturated, including 26% palmitic
acid and 15% stearic acid. A significant amount of
short- and medium-chain fatty acids, including 3.3%
butyric acid are present. These fatty acids and the
breakdown products of these fatty acids are impor-
tant contributors to the flavor of many cultured dairy
products.
Two major classes of proteins, the caseins and the
whey proteins, are present in milk. The caseins,
which make up 80% of the total protein in cow milk,
are insoluble at a pH of 4.6, but are stable to heating.
The whey proteins remain soluble at pH 4.6 and are
heat sensitive. The caseins are the major protein
present in cultured dairy products.
The casein micelles exist in milk as a colloidal
dispersion, with a diameter ranging from 40 to 300
nm and containing approximately 10,000 casein mo-
lecules. The principal casein proteins, s1-, s2-, -,
and -casein, present in the ratio 40:10:35:12, vary
in the number of phosphate residues, calcium sensi-
tivity, and hydrophobicity. Within the casein
micelle, the more hydrophobic proteins, such as -
casein are located on the interior of the micelle,
while the more hydrophilic proteins, such as -
casein, are located on the surface of the micelle. The
carboxyl end of -casein is dominated by glutamic
acid residues and glycoside groups. These hydro-
philic carboxyl ends are represented as “hairy” re-
gions in the model of the casein micelle and pro-
mote the stability of the casein micelle in solution
(Walstra and Jenness 1984). Calcium phosphate fur-
ther facilitates the association of individual calcium-
sensitive casein proteins (s1-, s2-, and -casein),
within the casein micelle. Processing treatments
applied during the formation of fermented dairy
products, such as the addition of acid or enzymes,
destabilize the casein micelle causing the casein
proteins to precipitate.
The whey proteins consist of four major proteins,
-lactoglobulin (50%),-lactalbumin (20%), blood
serum albumin (10%), and immunoglobulins (10%).
These proteins have a significant number of cysteine
and cystine residues and are able to form disulfide
linkages with other proteins following heat treatment.LACTICACIDBACTERIAThe lactic acid bacteria used in the development of
fermented dairy products include Streptococcus,
Lactococcus, Leuconostoc, and Lactobacillusgen-
era. These bacteria are gram-positive bacteria and
belong to either the Streptococcaceae or Lacto-
bacillaceae families, depending on the morphology
of the bacteria as cocci or rods, respectively. These
bacteria also differ in their optimal temperature for
growth: 20–30°C is the optimal temperature for me-
sophilic bacteria, and 35–45°C is the optimal temper-
ature for thermophilic bacteria. Although the lactic
acid bacteria are quite diverse in growth require-
ments, morphology, and physiology, they all have
the ability to metabolize lactose to lactic acid and
reduce the pH of the milk to produce specific cul-
tured dairy products. The choice of lactic acid bacte-
ria is in part dictated by the heat treatment the cul-
tured dairy products undergo following inoculation.
Table 26.1 summarizes the growth characteristics of
common lactic acid bacteria.KEY PROCESSING STEPS IN
CULTURED DAIRY PRODUCTSMany of the processing steps important in the pro-
duction of cultured dairy products are not unique to
a specific product. Therefore, the following discus-
sion will provide an overview of the key processing
steps that are used in the production of several cul-
tured dairy products. Specific processing treatments
and concerns will be highlighted within the discus-
sion of the processing of the specific cultured dairy
products.LACTOSEFERMENTATIONLactic acid bacteria use the lactose in milk to pro-
duce lactic acid and other important flavor com-