Food Biochemistry and Food Processing (2 edition)

BLBS102-c23 BLBS102-Simpson March 21, 2012 13:44 Trim: 276mm X 219mm Printer Name: Yet to Come

438 Part 4: Milk

Figure 23.4.Processing scheme for sour cream.

formulation vary widely from country to country resulting in

products with a diversity of flavor and texture characteristics.

While in many Western societies, yogurt is produced from cows’

milk, other mammalian milk can be used to produce yogurt.

Figure 23.5 outlines the steps involved in the processing of

stirred-style and set-style yogurt.

The milk is initially standardized to the desired fat (0.5–3.5%

fat) and milk solids-not-fat (12.5%) contents. The increase in

the protein content is most commonly achieved through the

addition of nonfat milk powder and improves the body and de-

creases syneresis of the final product. In addition to decreasing

the size of the fat globule, homogenization of the milk alters the

milk proteins to reduce syneresis and increase firmness (Tamime

and Robinson 1999b). The subsequent heat treatment (85◦Cfor

30 minutes) eliminates pathogenic microorganisms and reduces

the oxygen in the milk to provide a good growth media for the

Figure 23.5.Processing scheme for set-style and stirred-style

yogurt.

starter cultures. Enzymes and the major whey proteins, including

β-lactoglobulin andα-lactalbumin, but not the casein proteins

are also denatured by the heat treatment. The denaturation of the

whey proteins and subsequent interactions between the whey

proteins and casein and/or fat globules improves the stability of

the gel and increases water-binding capacity of the caseins to

decrease syneresis (Tamime and Robinson 1999b).

Following heat treatment, the milk is cooled to 43–45◦Cfor

inoculation of the starter cultures. The fast acid-producing ther-

mophilic lactic acid bacteria,Streptococcus salivariusssp.ther-

mophilusandLactobacillus delbrueckiissp.bulgaricusare the

primary microorganisms used in the production of yogurt. These

bacteria have a synergistic effect on each others’ growth and

should be present in approximately equal numbers for optimal

flavor development.Lb. delbrueckiissp. bulgaricushas impor-

tant protease activity and hydrolyzes the milk proteins to small

peptides and amino acids. These peptides and amino acids en-

hance the growth ofS. thermophilus,which has limited pro-

teolytic activity.S. thermophilusmetabolizes pyruvic acid to

formic acid and carbon dioxide, which in turn, stimulates the