Food Biochemistry and Food Processing (2 edition)

BLBS102-c25 BLBS102-Simpson March 21, 2012 13:23 Trim: 276mm X 219mm Printer Name: Yet to Come

472 Part 4: Milk

3. Acidification of milk, whey or mixtures thereof to pH ap-
   proximately 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.

Rennet-Coagulated Cheeses

These cheeses, which represent approximately 75% of total pro-

duction, are produced in a great diversity of shapes, flavours and

textures (∼1400 varieties worldwide). Their production can be

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

and (2) ripening of the curd (Fig. 25.2B).

Cheese may be made from milk of many species, with bovine,

ovine and caprine being most common globally. Cheese may be

made from raw or pasteurised milk, and in large-scale industrial

cheese production the latter is far more common, with some

exceptions. The heat treatment applied is normally conventional

HTST pasteurisation, as more severe treatments may impair

the coagulation properties of milk due to heat-induced interac-

tion of whey proteins and casein micelles. Other pre-treatments

that may be applied to cheese milk include adjustment of the

fat:protein or fat:casein ratio (i.e., standardisation) to achieve

certain targets for cheese composition, and addition of CaCl 2 ,

(A)

Milk Culture of LAB Coagulum Cooled, packaged

milk

Cut

Cooked

Drained

Washed

Acid-curd cheese

Optional:

Heat-treat

Various condiments

Package

Various products, e.g., petit filous

(B)

5-24 h 2 wks–2 yr

Milk Curds Cheese

Acidification

Coagulation

Dehydration cutting

Rate directly related

to moisture content

acidification

heating

stirring

Moulding

Pressing

Salting

Fermented

Figure 25.2.Principles of manufacture of (A) acid-coagulated and

(B) rennet-coagulated cheese.

which may be used to adjust for seasonal differences in mineral

balance of milk. For some varieties, milk pH may be reduced

(e.g., by adding gluconic acid-δ-lactone) prior to coagulation.

Coagulation

The coagulation of milk for the production of rennet-coagulated

cheese exploits a unique characteristic of the casein system. As

described in Chapter 24, the casein in milk exists as large (di-

ameter 50–600 nm, mean 150 nm) colloidal particles, known as

casein micelles. The micelles are stabilised byκ-casein, which

is concentrated on the surface, with its hydrophobicN-terminal

segment interacting with theαs1–,αs2– andβ-caseins, and its

hydrophilicC-terminal third protruding into the aqueous en-

vironment, forming a layer approximately 7 nm thick, which

stabilises the micelles by a zeta potential of about –20 mV and

by steric stabilisation. The stability of the micelles is lost when

the surfaceκ-casein layer is destroyed by heat, alcohol or pro-

teinases (known as rennets).

Several proteinases can coagulate milk but the traditional and

the most effective rennets are 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

chymosin decreases and is replaced by pepsin. The supply of

chymosin has been inadequate for about 50 years due to the

increased production of cheese and 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 unsuitable as rennets be-

cause they are too proteolytic, resulting in a reduced yield of

cheese curd and off-flavoured cheese. Only five successful ren-

net substitutes have been identified: (1) bovine and (2) porcine

pepsins and acid proteinases from (3)Rhizomucor mehei,(4)

Rhizomucor pusillusand (5)Cryphonectria parasitica.Plant

rennets (e.g., that fromCynara cardunculus) have also been

studied as possible coagulants, but have high proteolytic activity

relative to their milk-clotting activity, which may lead to bitter-

ness in cheese. Plant rennets have been most successfully used

for cheese varieties made in Spain and Portugal from ovine milk

(e.g., Torta del Casar and Serra Da Estrela), in which bitterness

does not seem to be a problem.

Rennet pastes, including those from kids or lambs, have been

traditionally used for some hard Italian and Greek cheese vari-

eties. These are less purified than conventional calf rennet, and

include a lipase, pregastric esterase (PGE), which contributes to

lipolysis during ripening of these varieties. The calf chymosin

gene has been cloned inKluyveromyces lactis, Escherichia coli

andAspergillus nigerand fermentation-produced chymosin is

now used widely.

Chymosin and most of the other commercially successful

rennets hydrolyseκ-casein specifically at the Phe 105 –Met 106

bond;C. parasiticaproteinase cleavesκ-casein at Ser 104 –Phe 105.

The liberated hydrophilicC-terminal segment, known as the

(glyco) caseinomacropeptide (CMP), diffuses into the surround-

ing aqueous phase and the stability of the micelles is destroyed.