Food Biochemistry and Food Processing

464 Part IV: Milk

the reduced availability of calf stomachs (due to the
birth of fewer calves and the slaughter of calves at
an older age). This shortage has led to a search for
alternative coagulants (rennet substitutes).
Many, perhaps most, proteinases can coagulate
milk under certain conditions, but almost all are un-
suitable as rennets because they are too proteolytic,
resulting in a reduced yield of cheese curd and off-
flavored cheese. Only five successful rennet substi-
tutes have been identified: bovine and porcine pep-
sins and acid proteinases from Rhizomucor mehei,
R. pusillus,and Cryphonectria parasitica. Recently,
the calf chymosin gene has been cloned in Kluyvero-
myces lactis, E. coli,and Aspergillus niger,and
fermentation-produced chymosin is now used widely.
Chymosin and most of the other commercially
successful rennets hydrolyze -casein specifically at
the Phe 105 -Met 106 bond; C. parasitica proteinase
cleaves -casein at Ser 104 -Phe 105. The liberated hy-
drophilic C-terminal segment, known as the (glyco)
caseinomacropeptide (CMP), diffuses into the sur-
rounding aqueous phase, and the stability of the mi-
celles is destroyed. When about 85% of the -casein
has been hydrolyzed, the rennet-altered micelles
aggregate to form a gel in the presence of a critical
concentration of Ca^2
and at a temperature greater
than about 18°C.

Acidification

The second characteristic step in cheese making is
acidification of the milk and curd from a pH of
approximately 6.7 to a value in the range 4.6–5.2,
depending on the variety. Until relatively recently,
and for some minor artisanal varieties still, acidifica-
tion was due to the production of lactic acid from
lactose by adventitious lactic acid bacteria (LAB).
Today, cheese milk is inoculated with a culture
(starter) of selected LAB for more controlled and
reproducible acidification. If the cheese curds are
cooked to 40°C, a culture of Lactococcus lactis
and/or cremorisis used, but for high-cooked cheese
(whose curds are cooked to a high temperature; 50–
55°C), a culture of Lactobacillus delbrueckiisubsp.
bulgaricus,possibly in combination with Strepto-
coccus thermophilus,is used. Cheese starters have
been refined progressively over the years, especially
with respect to the rate of acidification, resistance to
bacteriophages, and cheese-ripening characteristics.
Today, mixtures of highly selected defined strains of
LAB are used widely.

Acidification at the correct rate and time is essen-

tial for succesful cheese making; it affects at least

the following aspects:

• Activity and stability of the coagulant during
  renneting;


• Strength of the rennet-induced gel;


• Rate and extent of syneresis of the gel when cut,
  and hence the composition of the cheese;


• Retention of rennet in the curd;


• Dissolution of colloidal calcium phosphate (the
  concentration of calcium in the cheese has a
  major effect on the texture and functionality of
  cheese);


• Inhibition of growth of undesirable micro-
  organisms, especially pathogenic bacteria; and


• Activity of various enzymes in the cheese during
  ripening.

Postcoagulation Operations

If left undisturbed, a rennet-coagulated milk gel is

quite stable, but if cut or broken, it contracts, pro-

ducing curds and whey. By controlling the rate and

extent of syneresis, the cheese maker controls the

moisture content of cheese and thereby the rate and

pattern of ripening and the quality and stability of

the cheese. Syneresis is affected by

• Concentrations of fat, protein, and calcium;


• pH;


• Size of curd particles;


• Temperature of cooking;


• Stirring the curd-whey mixture and the curds
  after whey drainage;


• Pressing of the curd; and


• Salting (2 kg H 2 O lost for each kilogram of NaCl
  taken up; NaCl should not be used to control
  moisture content).

When the desired degree of syneresis has oc-

curred, as judged subjectively by the cheese maker,

the curds and whey are separated, usually on some

form of perforated metal screen. The curds are sub-

jected to various treatments, which are more or less

variety specific. These include cheddaring, kneading-

stretching, molding, pressing, and salting.

Ripening

Rennet-coagulated cheese curd may be consumed at

the end of the manufacturing process, and some is