BLBS102-c23 BLBS102-Simpson March 21, 2012 13:44 Trim: 276mm X 219mm Printer Name: Yet to Come
428 Part 4: Milkquality of the raw milk is also essential to the production of safe,
high-quality cultured dairy products.
The predominant sugar in milk is lactose, a disaccharide
of glucose and galactose. The fermentation of lactose by lac-
tic acid bacteria in cultured dairy products provides the fla-
vor and textural attributes that are desirable in cultured dairy
products.
The fat is present in the milk in the form of fat globules,
which are surrounded by a polar milk fat globule membrane
(MFGM). Triacylglycerols are the predominant lipid fraction in
milk, accounting for 98% of the total lipids. Diacylglycerols,
monoacylglycerols, fatty acids, phospholipids, and sterols ac-
count for the remaining lipid fraction. The phospholipids are
integral components of the MFGM. 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
middle-chain fatty acids, including 3.3% butyric acid are present.
These fatty acids and the breakdown products of these fatty acids
are important contributors to the flavor of many cultured dairy
products.
Two major classes of milk proteins are caseins and whey
proteins. 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 casein micelles exist in milk as a colloidal dispersion,
with a diameter ranging from 40 to 300 nm and containing
approximately 10,000 casein molecules. The principal casein
proteins,αs1,αs2,β,andκ, present in the ratio 40:10:35:12,
vary in the number of phosphate residues, calcium-sensitivity,
and hydrophobicity. Within the casein micelle, the more hy-
drophobic proteins, such asβ-casein, are located on the interior
of the micelle, while the more hydrophilic proteins, such asκ-
casein, are located on the surface of the micelle. The carboxyl
end ofκ-casein is dominated by glutamic acid residues and gly-
coside groups. These hydrophilic carboxyl-ends are represented
as “hairy” regions in the model of the casein micelle and pro-
mote the stability of the casein micelle in solution (Walstra and
Jenness 1984). Calcium phosphate further facilitates the associ-
ation of individual calcium-sensitive casein proteins (αs1-,αs2-,
andβ-casein), within the casein micelle. Hydrogen bonds and
hydrophobic interactions also play a critical role in stabilizing
the casein micelle. Processing treatments applied during the for-
mation of cultured dairy products, such as the addition of acid
or enzymes destabilize the casein micelle causing the casein
proteins to precipitate (Lucey 2002).
The whey proteins consist of four major proteins, β-
lactoglobulin (50%),α-lactalbumin (20%), blood serum albu-
min (10%), and immunoglobulins (10%). These proteins have
a significant number of cysteine and cysteine residues and are
able to form disulfide linkages with other proteins following heat
treatment.
Fresh cow’s milk is characterized as having a distinctive sub-
tle flavor. Classes of volatile flavor compounds that have been
shown to have the greatest impact on milk flavor include nitro-
gen heterocyclics, linolenic acid oxidation products,γ-lactones,
phenolics and phytol derivatives; many of these compounds arefound in foods of plant origin. Differences in the milk flavor
from cows fed different diets have been attributed to concen-
tration differences of these flavor compounds rather than the
presence of different compounds (Friedrich and Acree 1998,
Bendall 2001). Although these flavor compounds are not signif-
icant contributors to the characteristic flavors of cultured dairy
products, they do contribute to the background flavors of these
products (Urbach 1995).Lactic Acid BacteriaThe lactic acid bacteria used in the development of cultured dairy
products includeStreptococcus, Lactococcus, Leuconostoc, and
Lactobacillusgenera. These bacteria are gram-positive bacteria
and belong to either theStreptococcaceaeorLactobacillaceae
families, depending on the morphology of the bacteria as cocci
or rods, respectively. These bacteria also differ in their optimal
temperature for growth, with 20–30◦C the optimal temperature
for mesophilic bacteria and 35–45◦C the optimal temperature
for thermophilic bacteria. Although the lactic acid bacteria are
quite diverse in growth requirements, morphology, and physi-
ology, they all have the ability to metabolize lactose to lactic
acid and reduce the pH of the milk to produce specific cultured
dairy products. The heat treatment the cultured dairy products re-
ceive following inoculation is one of the factors that influences
the selection of lactic acid bacteria for specific cultured dairy
products. Table 23.1 summarizes the growth characteristics of
common lactic acid bacteria.KEY PROCESSING STEPS IN CULTURED
DAIRY PRODUCTSMany of the processing steps important in the production of
cultured dairy products are not unique to a specific product.
Therefore, the following discussion will provide an overview
of the key processing steps that are used in the production of
several cultured dairy products. Specific processing treatments
and concerns will be highlighted within the discussion of the
processing of the specific cultured dairy products.Lactic Acid FermentationLactic acid bacteria use the lactose in the milk to produce lactic
acid and other important flavor compounds in the cultured dairy
products. Many of these bacteria have lactase activity and hy-
drolyze lactose to its monosaccharide units, glucose and galac-
tose, prior to further metabolism. The hydrolysis of lactose in
most cultured dairy products is significant for individuals who
are lactose intolerant, allowing them to consume dairy prod-
ucts without the undesirable effects of the inability to hydrolyze
lactose.
The homofermentative lactic acid bacteria produce lactic acid
from lactose according to the reactionLactose+4ADP+4H 3 PO 4 →4Lactic Acid+4ATP+3H 2 OThe glucose and galactose molecules are metabolized through
the glycolytic and tagatose pathways, respectively. Key steps in