240 Chapter 10
growth profi les during cheese manufacture to
improve the effi ciency as well as the fi nal
consistency of cheese manufactured on a
daily basis. Moreover, these starters provide
specifi c fermentation profi les to produce cus-
tomized cheeses with a variety of fl avor and
texture profi les, which has enhanced the use
of cheese as an ingredient in different appli-
cations. For example, exopolysaccharide -
producing bacteria enhance the texture of
reduced - and low - fat cheeses (Hassan, 2008 ),
salt - sensitive starters improve the fl avor and
texture of reduced - or low - sodium cheeses,
and probiotic microorganisms can be incor-
porated into cheeses (Ong et al., 2006 ; Dias
and Mix, 2008 ).Coagulation
Milk coagulation is a key step in cheese pro-
duction. Milk has a well - defi ned physical
equilibrium between various constituents
that mainly exists in three forms: emulsion,
colloidal solution, and true solution. Milk
lipids are present in an oil - in - water type of
emulsion in the form of microscopic glob-
ules, varying from 0.1 to 22 μ m in diameter.
The colloidal phase contains casein micelles,
calcium phosphate, and globular (serum/
whey) proteins. Whey proteins are in a col-
loidal solution (diameter 4 to 6 nm) and the
caseins are in colloidal suspension (diameter
50 to 300 nm). Lactose, acids, some inorganic
minerals, vitamins and enzymes are present
in true solution (diameter 0.5 nm).
The physical stability of milk is ascribed
to the negative charge on casein micelles,
which repel each other and do not aggregate
nor form clumps. However, if the charge is
neutralized by adding protons (H^ +^ ions) fur-
nished by the addition of acid, casein micelles
tend to stick to each other and milk begins to
coagulate. This is acid - coagulation; it accel-
erates as the temperature of milk increases.
The second type of coagulation is induced
by the highly specifi c proteolytic enzyme
chymosin (Andren, 2003 ). Rennet is thecheese starters. Bio - variant specie diactely-
lactis , also called Lactococcus lactis citrate + ,
produces CO 2 and a buttery fl avor compound
(diacetyl) from citrate, a normal constituent
of milk. A weak acid producer, Leuconostoc
mesentroides ssp. cremoris , also produces
diacetyl and CO 2. The fl avor compound
(diacetyl) is essential in fresh cheese produc-
tion. These are used in soft ripened, cheddar,
most washed, and fresh cheese varieties.
Thermophilic starters are traditionally
used in Swiss, gruyere, and some Italian
cheeses such as mozzarella. In addition to
lactic acid, these cultures characteristically
produce acetaldehyde. Thermophilic starters
consist of cultures capable of growth at tem-
peratures from 39 ° C to 50 ° C (102 ° F to
122 ° F). Minimum growth is at 20 ° C (68 ° F),
but they are partially inactivated at tempera-
tures below 20 ° C (68 ° F). They can survive
at 55 ° C (131 ° F). Thermophilic starters are
blends of cocci and rods. The cocci consist
of Streptococcus thermophilus (ST) and the
rods are Lactobacillus delbrueckii sub -
species bulgaricus (LB), lactis and helveti-
cus. The traditional cultures of yogurt, ST
and LB, grow together in a symbiotic rela-
tionship, which implies that together their
rate of acid production is faster than that of
either culture individually. The balance
between rods and cocci can be controlled by
temperature manipulation. Higher tempera-
tures favor rods and lower temperatures stim-
ulate cocci. Rods are more resistant to high
acid conditions. It is useful to know how the
temperature and salt level affect the growth
of a starter because they are important tools
in manipulating and controlling their effects
during cheese making.
There has been a major thrust in the study
of the microbial genetics of starter cultures
both in industry and academia over the past
25 years (Johnson and Lucey, 2006 ). New
starter strains of traditional starters that are
genetically engineered to provide a variety of
attributes are now available. These attributes
include better phage resistance and specifi c