Food Biochemistry and Food Processing

26 Dairy Products 609

the desired fat content, the mixture is warmed and
homogenized to improve the consistency of the final
product. The starter cultures include Lc. lactisssp.
lactisand Lc. lactisbiovar. diacetylactisand Ln.
mesenteroidesssp. cremoris. As in buttermilk, these
cultures contribute to the acid and buttery flavor of

the sour cream. The sour cream may be packaged

prior to or following fermentation at 20°C until the

pH is reduced to 4.5. Packaging prior to fermenta-

tion results in a thicker product because the gel is

not disturbed. As the pH decreases, the fat clusters

aggregate to form a viscous cream. Rennet or thick-

ening agents are sometimes added to increase the

firmness of the sour cream.

YOGURT

The tremendous increase in the popularity of yogurt

in recent decades has been attributed to its health

food image and the wide diversity of flavors, com-

positions, and viscosities available to consumers.

The manufacturing methods, raw materials, and for-

mulations vary widely from country to country,

resulting in products with a diversity of flavor and

texture characteristics. While in many Western soci-

eties, yogurt is produced from cows’ milk, other

mammalian milks can be used to produce yogurt.

Figure 26.5 outlines the steps involved in the pro-

cessing of stirred- and set-style yogurts.

The milk is initially standardized to the desired

fat (0.5–3.5% fat) and milk solids-not-fat (12.5%)

content. Increase in the protein content is most com-

monly achieved through the addition of nonfat milk

powder, which improves the body and decreases

syneresis of the final product. In addition to decreas-

ing 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 heat treatment eliminates pathogenic microor-

ganisms and reduces the oxygen in the milk to pro-

vide a good growth medium for the starter cultures.

Enzymes and the major whey proteins, including -

lactoglobulin and 
-lactalbumin, but not the casein

proteins, are also denatured by the heat treatment.

The denaturation of the whey proteins and subse-

quent interactions between the whey proteins and

casein and/or fat globules improves the stability of

the gel and decreases syneresis (Tamime and

Robinson 1999b).

Following heat treatment, the milk is cooled to

43–45°C for inoculation of the starter cultures. The

fast acid-producing thermophilic LAB Streptococ-

cus salivariusssp. thermophilusand Lactobacillus

delbrueckiissp. bulgaricusare the primary micro-

organisms used in the production of yogurt. These

bacteria have a synergistic effect on each others’

Figure 26.4.Processing scheme for sour cream.