BLBS102-c23 BLBS102-Simpson March 21, 2012 13:44 Trim: 276mm X 219mm Printer Name: Yet to Come
23 Dairy Products 431PROCESSING OF CULTURED
DAIRY PRODUCTSThe following discussion highlights the unique processing steps
that are involved in the production of cultured dairy products.
These processing steps contribute to the unique flavor, texture,
and overall sensory characteristics of these products.CheeseOver 400 different varieties of cheese have been recognized
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
composition. Cheeses are frequently classified based on mois-
ture content, method of precipitation of the cheese proteins, and
the ripening process. Table 23.2 compares processing methods
and composition of selected cheeses.
The coagulation of the casein proteins, separation of the curds
from the whey, and ripening of the curd are the primary steps
involved in the processing of cheese. The resulting product is
a highly nutritious product in which the casein and fat from
the milk are concentrated. The fat plays a critical role in the
texture of the cheese by preventing the casein molecules from
associating to form a tough structure. In general, most cheeses
can be classified as natural or processed cheeses. The natural
cheeses include ripened or unripened cheeses. The stages in-
volved in processing these different types of cheeses and their
unique characteristics will be discussed further.Natural CheesesA simplified overview of the steps involved in processing fresh
and ripened natural cheeses is presented in Figure 23.1. Fresh
cheeses are consumed immediately after processing and are
characterized as having a high moisture content and mild fla-
vor. Most fresh cheeses are acid-coagulated cheeses, in which
lactic acid produced by the starter cultures causes the precipita-
tion of the caseins. The final pH of the acid-coagulated cheeses
is 4.6. Rennet is primarily used for the coagulation of the casein
proteins and curd formation in ripened cheeses. Starter cultures
are added to produce acid and contribute enzymes for flavor
and texture development during ripening. Ripened cheeses un-
dergo a ripening period, ranging from 3 weeks to more than
2 years, following processing that contributes to the development
of the flavor and texture of the cheese. The moisture content of
these cheeses ranges from 30% to 55% and the pH ranges from
5.0 to 5.3.Standardization of the Milk Casein and fat contents of the
milk are standardized to minimize variations in the quality of the
cheese due to seasonal effects and variation in the milk supply.
The casein-to-fat ratio can be adjusted by the addition of skim
milk, cream, milk powder, or evaporated milk or the removal
of fat. Calcium chloride (0.1%) may also be added to improve
coagulation of the milk by rennet and further processing of the
cheese. The actual casein and fat content of the milk will varyfor each cheese type and influence the curd formation, cheese
yield, fat content, and texture of the cheese (Banks 1998).Coagulation of the Milk Proteins Aggregation of the ca-
sein micelles to form a three-dimensional gel protein network is
initiated through the addition of rennet or other proteolytic en-
zymes or the addition of acid. Fat and water molecules are also
entrapped within this protein network. Enzymes and starter bac-
teria also tend to associate with the curds, and thus contribute to
a number of biochemical changes that occur during the ripening
process. The whey, which includes water, salts, lactose, and the
soluble whey proteins, is expelled from the gel. The aggregation
of the casein micelles by either enzyme or acid treatment results
in gels with different characteristics.
In most natural, aged cheeses, coagulation of the casein pro-
teins by the addition of rennet is most common. This process
is temperature dependent, with no coagulation occurring be-
low 10◦C, and an increase in coagulation rate accompanying an
increase in temperature until the optimal temperature for coagu-
lation at 40–45◦C. Above 65◦C, the enzyme is inactivated (Fox
1969, Brul ́e et al. 2000). The aggregation of the casein micelles
is influenced by temperature (Q 10 ∼12) to a greater degree than
the enzymatic hydrolysis ofκ-casein (Q 10 ∼2) (Cheryan et al.
1975). Aggregation of the micelles begins when approximately
70–85% of theκ-casein molecules are hydrolyzed, which re-
duces the steric hindrance between the micelles. Reducing the
pH or increasing the temperature reduces the degree of casein hy-
drolysis necessary for coagulation (Fox and McSweeney 1997).
The presence of Ca^2 +ions further facilitates the aggregation of
the casein micelles through the neutralization of the negative
charge on the micelle and the formation of ionic bonds. The re-
sulting gel has an irregular network, is highly elastic and porous,
and exhibits a high degree of syneresis.
The production of acid by lactic acid bacteria or the direct
addition of hydrochloric or lactic acid can also result in the ag-
gregation of the casein micelles and formation of clots. As the pH
of the milk is reduced, the casein micelles become insoluble and
begin to aggregate. Because calcium phosphate is solubilized as
the pH of the milk drops, the gel formed during acid coagula-
tion is not stabilized by Ca^2 +ions. The acid-coagulated gels are
less cohesive and exhibit less syneresis than enzyme-coagulated
cheeses. These cheeses generally have a high moisture content
and low mineral content. Acid coagulation is most frequently
used in the manufacture of cottage cheese and other unripened
cheeses.
A few unique types of cheese are prepared through acid coag-
ulation of whey or a blend of whey and skim milk in conjunction
with heat treatment. Ricotta cheese is the most common cheese
prepared in this manner.
Lactic acid bacteria cultures are added to the milk in con-
junction with the rennet in ripened cheeses. Although the lactic
acid bacteria cultures do not have a significant role in the co-
agulation of casein, they contribute to the changes that occur
during the ripening process. The different strains of starter cul-
tures differ in characteristics including growth rate, metabolic
rate, phage interactions, proteolytic activity, and flavor promo-
tion (Stanley 1998). Frequently, mixed-strain cultures, which