602 Part VI: Fermented Foods
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 tex-
ture of the cheese by preventing the casein mole-
cules from associating to form a tough structure. In
general, most cheeses can be classified as natural or
processed cheeses. The natural cheeses include both
ripened and unripened cheeses. The stages involved
in processing these different types of cheeses and
their unique characteristics will be discussed further.
Natural Cheeses
A simplified overview of the steps involved in pro-
cessing fresh and ripened natural cheeses is present-
ed in Figure 26.1. Fresh cheeses are consumed im-
mediately after processing and are characterized as
having a high moisture content and mild flavor. In
most cases, lactic acid produced by the starter cul-
tures cause the precipitation of the caseins. The final
pH of the acid-coagulated cheeses is 4.6. Ripened
cheeses undergo a ripening period, ranging from 3
weeks to more than 2 years, following processing,
which 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. Rennet is primarily used for
the coagulation of the casein proteins and curd for-
mation. Starter cultures are added to produce acid
and contribute enzymes for flavor and texture devel-
opment during ripening.
Standardization of the Milk The casein and fat
content of the milk are standardized to minimize
variations in the quality of the cheese due to season-
al 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 vary for
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 casein micelles to form a three-dimensional gel
protein network is initiated through the addition of
rennet or other proteolytic enzymes 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 treat-
ment results in gels with different characteristics.
In most natural, aged cheeses, coagulation of the
casein proteins by the addition of rennet is most
common. This process is temperature dependent,
with no coagulation occurring below 10°C, and an
increase in coagulation rate accompanying an in-
crease in temperature until the optimal temperature
for coagulation (40–45°C) is reached. Above 65°C,
the enzyme is inactivated (Fox 1969, Brulé et al.
2000). The aggregation of the casein micelles is
influenced by temperature (Q 10 12) to a greater
degree than the enzymatic hydrolysis of -casein
(Q 10 2) (Cheryan et al. 1975). Aggregation of the
micelles begins when approximately 70–85% of the
-casein molecules are hydrolyzed, which reduces
the steric hindrance between the micelles. Reducing
the pH or increasing the temperature reduces the
degree of casein hydrolysis necessary for coagula-
tion (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 resulting gel has an irregular net-
work, 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 aggregation 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 coagulation is not stabilized by calcium ions.
The acid-coagulated gels are less cohesive and ex-
hibit less syneresis than enzyme-coagulated cheeses.
These cheeses generally have a high moisture con-
tent and a 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 coagulation of whey or a blend of whey and