Food Biochemistry and Food Processing

20 Biochemistry of Milk Processing 463

The starter culture for yogurt contains Strepto-
coccus thermophilusand Lactobacillus delbrueckii
subsp. bulgaricus. In recent years, many fermented
milk products with added probiotic bacteria such as
Bifidobacteria spp. or Lactobacillus acidophilus
have been introduced,due to increasing consumer
interest in the health benefits of such bacteria, and
increasing clinical evidence of their efficacy (Sur-
ono and Hosono 2002b, Robinson 2002b). The tex-
ture and viscosity of yogurt can be modified by use
of an exopolysaccharide(EPS)-producing starter
culture (strains of both S. thermophilusand L. del-
brueckiisubsp. bulgaricushave been identified that
secrete capsular heteropolysaccharides; De Vuyst et
al. 2001, 2003; Hassan et al. 2003).
During fermentation, the production of lactic acid
destabilizes the casein micelles, resulting in their
coagulation (acid gelation) (Lucey and Singh 1998).
The main point of difference between the production
protocols for yogurt and acid-coagulated cheese is
that when the pH has reached 4.6 and the milk has
set into a coagulum, this coagulum is cut or broken
for the latter but not for the former.
Depending on the desired characteristics of the
finished product, the milk may be fermented in the
final package (to produce a firm set yogurt) or in
bulk tanks, followed by filling (to produce a more
liquid stirred yogurt) (Tamime and Robinson 1999,
Robinson 2002a). The rate of acidification for set or
stirred yogurt differs due to differences in the level
of inoculation and incubation temperature.
It is important to cool the yogurt rapidly when pH
4.6 is reached, to preserve the gel structure and pre-
vent further starter activity (Robinson, 2002a). In
some cases, the viscosity of the yogurt may be re-
duced by postfermentation homogenization to pro-
duce drinking yogurt.
Yogurt has a relatively short shelf life; common
changes during storage involve syneresis, with whey
expulsion, and further slow acidification by starter
bacteria. To prevent the latter, and thereby extend
the shelf life of yogurt, the final product may be pas-
teurized after acidification; to prevent damage to the
gel structure at this point, the role of hydrocolloid or
protein stabilizers is critical (Walstra et al. 1999).

ACID-HEATCOAGULATEDCHEESES

These cheeses were produced initially in southern
European countries from whey, as a means of recov-

ering nutritionally valuable whey proteins; their pro-

duction involves heating whey from rennet-coagu-

lated cheese to about 90°C to denature and coagu-

late the whey proteins. Well-known examples are

ricotta and manouri. Today, such cheeses are made

from blends of milk and whey, and in this case, it is

necessary to adjust the pH of the blend to about 5.2

using vinegar, citrus juice, or fermented milk.

Acid-heat coagulated cheese may also be pro-

duced from whole milk by acidifying to pH 5.2 and

heating to 90°C. An example is US-style queso blan-

co, which does not melt on heating and hence has

interesting functional properties for certain applica-

tions.

RENNET-COAGULATEDCHEESES

These cheeses, which represent about 75% of total

production, are produced in a great diversity of

shapes, flavors, and textures (approximately 1400

varieties worldwide). Their production can be divid-

ed into two phases: (1) conversion of milk to cheese

curd and (2) ripening of the curd (Fig. 20.2b).

Coagulation

The coagulation of milk for the production of rennet-

coagulated cheese exploits a unique characteristic of

the casein system. As described in Chapter 19, the

casein in milk exists as large (diameter 50–600 nm,

mean 150 nm) colloidal particles, known as casein

micelles. The micelles are stabilized by -casein,

which is concentrated on the surface, with its hydro-

phobic N-terminal segment interacting with thes1-,

s2- and 
-caseins, and its hydrophilic C-terminal

third protruding into the aqueous environment,

forming a layer approximately 7 nm thick, which

stabilizes the micelles by a zeta potential of about

20 mV and by steric stabilization. The stability of

the micelles is lost when the surface -casein layer

is destroyed by heat, alcohol, or proteinases (known

as rennets).

Several proteinases can coagulate milk but the

traditional, and most effective, rennets were NaCl

extracts of the stomachs of young, milk-fed calves,

kids, or lambs. The active enzyme in these rennets is

chymosin; as the animal ages, the secretion of chy-

mosin decreases and is replaced by pepsin. The sup-

ply of chymosin has been inadequate for about 50

years due to the increased production of cheese and