Food Biochemistry and Food Processing

20 Biochemistry of Milk Processing 461

uoles are dispersed. In products containing a very
high level of lactose (e.g., whey powders), the lactose
may be precrystallized to avoid the formation of an
amorphous glass. Usually, small lactose crystals are
added to milk or whey concentrate prior to drying to
promote crystallization under relatively mild condi-
tions (i.e., in the liquid rather than the powder form);
the added crystals act as nuclei for crystallization.

CHEESE AND FERMENTED
MILKS

INTRODUCTION

About 35% of total world milk production is used to
produce cheese (
15  106 metric tons per year
[mt/yr]), mainly in Europe and North America.
Cheese manufacture essentially involves coagulat-
ing the casein micelles to form a gel that entraps the
fat globules, if present; when the gel is cut or bro-
ken, the casein network contracts (syneresis), ex-
pelling whey. The resulting curds may be consumed
fresh as mild-flavored products, or ripened for a
period ranging from 2 weeks (e.g., for Mozzarella)
to 2 years (e.g., for Parmagiano-Reggiano).
Cheese production is the most biochemically sig-
nificant dairy process. However, since the action of
rennet on milk and dairy fermentations are de-
scribed in Chapters 10 and 26, respectively, only a
brief summary is given here. For more detailed dis-
cussions of the science and technology of cheese
making, see also Robinson and Wilbey (1998), Law
(1999), Eck and Gillis (2000), and Fox et al. (2004).
The coagulation of milk for cheese is achieved by
one of three methods:

1. Rennet-induced coagulation, which is used for
   most ripened cheeses and accounts for approxi-
   mately 75% of total cheese production.


2. Acidification to pH of approximately 4.6 at 30–
   36°C by in situ production of acid by fermenta-
   tion of lactose to lactic acid by lactic acid bac-
   teria (Lactococcus, Lactobacillus,or
   Streptococcus)or direct acidification with acid
   or acidogen (e.g., gluconic acid-delta-lactone).
   Most acid-coagulated cheeses are consumed
   fresh and represent about 25% of total cheese
   production. Major examples of acid-coagulated
   cheeses are cottage, quark, and cream cheeses.


3. Acidification of milk, whey, or mixtures there-
   of to a pH of approximately 5.2 and heating to

approximately 90°C. These cheeses are usually

consumed fresh; common examples include

ricotta and variants thereof, manouri, and some

forms of queso blanco.

ACID-COAGULATEDCHEESES

Acid-coagulated cheeses are at one end of the spec-

trum of fermented dairy products, the production of

which is summarized in Figure 20.2a. Depending on

the desired fat content of the final product, the start-

ing material may be low-fat cream, whole milk, semi-

skimmed milk, or skimmed milk. The milk for

cottage-type cheese is subjected to a mild heat treat-

ment so that the synergetic properties of the coagu-

lum are not impaired.

FERMENTEDMILKS

In addition to acid-coagulated cheeses, a wide range

of fermented milk products are produced worldwide

today (Surono and Hasono 2002a), including

• Products of fermentation by mesophilic or
  thermophilic lactic acid bacteria (e.g., yogurt)
  and


• Products obtained by alcohol-lactic fermentation
  by lactic acid bacteria and yeast (e.g., kefir).

For yogurt, as for acid-coagulated cheese, the

milk base (with a fat content depending on the final

product) is usually supplemented with milk solids

to enhance the viscosity and rigidity of the final

coagulum, in some cases to simulate the viscosity

of yogurt made from sheep’s milk, which has a high-

er total solids content than cow’s milk. The milk

solids added may be whole or skim powder or whey

powders, depending on the type of product required

(González-Martínez et al. 2002, Remeuf et al.

2003).

Syneresis, which causes free whey in the product,

is very undesirable in fermented milk products.

Therefore, the milk or starting mix is subjected to a

severe heat treatment (e.g., 90°C for 10 minutes) to

denature the whey proteins, increase their water-

binding capacity, and produce a firmer gel network,

consisting of casein-whey protein complexes. The

heat treatment also kills non-spore-forming patho-

genic bacteria, rendering the product safe (Robinson

2002a). Homogenization of the mix also increases

the firmness of the gel because the fat globules in