Food Biochemistry and Food Processing

26 Dairy Products 599

ach, is most commonly recognized as the enzyme
for coaguluating casein. However, proteases from
microorganisms and those produced through recom-
binant DNA technologies have been successfully
adapted as alternatives to calf rennet (Banks 1998).
Chymosin, the major enzyme present in rennet,
cleaves the peptide bond between Phe 105 and Met 106
of -casein, releasing the hydrophilic, charged ca-
sein macropeptide, while the para--casein remains
associated with the casein micelle. The loss of the
charged macropeptide reduces the surface charge of
the casein micelle and results in the aggregation of
the casein micelles to form a gel network stabil-
ized by hydrophobic interactions. Temperature in-
fluences both the rate of the enzymatic reaction and
the aggregation of the casein proteins; the optimal
temperature for casein coagulation is 40–42°C.
Rennet is used in the manufacture of most ripened
cheeses to hydrolyze the peptide bond and cause
aggregation of the casein micelles.

HOMOGENIZATION

Milk fat globules have a tendency to coalesce and
separate upon standing. Homogenization reduces
the diameter of the fat globules from 1–10 m to
less than 2 m and increases the total fat globule
surface area. The physical change in the fat globule
occurs through forcing the milk through a small ori-
fice under high pressure. The decrease in the size of
the milk fat globules reduces the tendency of the fat
globules to aggregate during the gelation period. In
addition, denaturation of the whey proteins and in-
teractions of the whey proteins with casein or the fat
globules can alter the physical and chemical proper-
ties of the milk proteins to result in a firmer gel with
reduced syneresis (Tamime and Robinson 1999b,
Fox et al. 2000). Milk to be used to process yogurt,
cultured buttermilk, and unripened cheeses is com-
monly homogenized to improve the quality of the
final product.

PASTEURIZATION

The original fermented dairy products relied on the
native microorganisms in the milk for the fermenta-
tion process. Current commercial methods for all
cultured dairy products include a pasteurization
treatment to kill the native microorganisms, fol-
lowed by inoculation with starter cultures to pro-

duce the desired product. The heat process, which

must be sufficient to inactivate alkaline phosphatase,

also destroys many pathogenic and spoilage micro-

organisms and enzymes that may have a negative

impact on the quality of the finished products. The

time-temperature treatments for fluid milk pasteur-

ization have been adapted for milk to be used in the

processing of cultured dairy products (62.8°C for 30

minutes or 71.1°C for 15 seconds). More severe heat

treatments than are characteristic of pasteurization

cause denaturation of whey proteins and interactions

between -lactoglobulin and -casein. In cheeses,

this interaction decreases the ability of chymosin to

hydrolyze the casein molecule and initiate curd pre-

cipitation and formation.

COOLING

The processing of cultured dairy products relies on

the metabolic activity of the starter cultures to con-

tribute to acid formation and flavor and texture

development. Once the desired pH or titratable acid-

ity is reached for these products, the products are

cooled to 5–10°C to slow the growth of the bacteria

and limit further acid production and other biologi-

cal reactions.

PROCESSING OF CULTURED

DAIRY PRODUCTS

The following discussion highlights the unique pro-

cessing steps that are involved in the production of

cultured dairy products. These processing steps con-

tribute to the unique flavor, texture, and overall sen-

sory characteristics of these products.

CHEESE

Over 400 different varieties of cheese have been rec-

ognized throughout the world. The wide diversity in

the flavor, texture, and appearance of these cheeses

is attributed to differences in the milk source, starter

cultures, ripening conditions, and chemical compo-

sition. Cheeses are frequently classified based on

moisture content, method of precipitation of the

cheese proteins, and the ripening process. Table 26.2

compares processing methods and compositions of

selected cheeses.

The coagulation of the casein proteins, separation

of the curds from the whey, and ripening of the curd