Dairy Chemistry And Biochemistry

382 DAIRY CHEMISTRY AND BIOCHEMISTRY

Clostridium tyrobutyricum (an anaerobic spore-former) causes late gas

blowing (through the production of H, and CO,) and off-flavours (butanoic

acid) in many hard ripened cheeses; Cheddar-type cheeses are major

exceptions. Contamination of cheese milk with clostridial spores can be

avoided or kept to a very low level by good hygienic practices (soil and

silage are the principal sources of clostridia) but they are usually prevented

from growing through the use of sodium nitrate (NaNO,) or, less frequently,

lysozyme, and/or removed by bactofugation (centrifugation) or microfiltra-

tion.

10.2.2

Typically, five steps, or groups of steps, are involved in the conversion of

milk to cheese curd: coagulation, acidification, syneresis (expulsion of whey),

moulding/shaping and salting. These steps, which partly overlap, enable the

cheesemaker to control the composition of cheese, which, in turn, has a

major influence on cheese ripening and quality.

Conversion of milk to cheese curd

Enzymatic coagulation of milk. The enzymatic coagulation of milk involves

modification of the casein micelles via limited proteolysis by selected

proteinases, called rennets, followed by calcium-induced aggregation of the

rennet-altered micelles:

Casein

Rennet

____.* Para-casein + Macropeptides

Ca?', - 30°C

Gel

If present, the fat globules are occluded in the gel but do not participate in
the formation of a gel matrix.
As discussed in Chapter 4, the casein micelles are stabilized by ti-casein,
which represents 12-15% of the total casein and is located mainly on the
surface of the micelles such that its hydrophobic N-terminal region reacts
hydrophobically with the calcium-sensitive clsl-, cts2- and 0-caseins while its
hydrophilic C-terminal region protrudes into the surrounding aqueous
environment, stabilizing the micelles by a negative surface charge and steric
stabilization.
Following its isolation in 1956, it was found that ti-casein is the only
casein hydrolysed during the rennet coagulation of milk and that it was
hydrolysed specifically at the Phe,,,-Met,,, bond, producing para-lc-
casein (K-CN fl- 105) and macropeptides (f106- 169; also called glycomac-
ropeptides since they contain most or all of the sugar groups attached to
ti-casein) (Figure 10.1). The hydrophilic macropeptides diffuse into the
surrounding medium while the para-#-casein remains attached to the