Food Biochemistry and Food Processing (2 edition)

BLBS102-c26 BLBS102-Simpson March 21, 2012 13:51 Trim: 276mm X 219mm Printer Name: Yet to Come

26 Equid Milk: Chemistry, Biochemistry and Processing 501

but approximately 3:9 in bovine milk (Holt and Jenness 1984)

and might indicate that either a smaller proportion of micellar

calcium is incorporated into nanoclusters in equine milk, or that

equine nanoclusters contain a higher proportion of casein-bound

phosphate, which would imply smaller nanoclusters. However,

unlike bovineκ-casein, equineκ-casein does not have a dis-

tinctly hydrophilicC-terminal domain; thus, it is unclear if this

part of the protein is capable of protruding from the micellar

surface to sterically stabilise the micelles. Furthermore, given

that the size of casein micelles and the content ofκ-casein are

inversely related (Yoshikawa et al. 1982, Dalgleish 1998.), a

low level ofκ-casein would be expected in equine milk com-

pared to bovine milk. Further research is required to elucidate

the structure of equine and asinine casein micelles as destabili-

sation of the micelles is the basis for the successful conversion

of milk into a range of dairy products, for example cheese or

yoghurt.

Stability of Equid Casein Micelles

Coagulation of milk occurs when the colloidal stability of the

casein micelles is destroyed and may be desirable or undesir-

able. Coagulation is desirable in the manufacture of yoghurt

and cheese and is also important from a nutritional point of

view, as clotting of the caseins in the stomach, and the type

and structure of the resultant coagulum strongly affect di-

gestibility. In contrast, heat-induced coagulation of casein mi-

celles, which can occur at a temperature> 120 ◦C, is undesir-

able. In this section, common types of micellar instability are

described.

Bovine casein micelles are sterically stabilised by a brush of

predominantlyκ-casein (De Kruif and Zhulina 1996), which

protrudes from the micelle surface. Coagulation of casein mi-

celles can occur only following collapse of the brush, which

occurs on acidification of milk, that is in the manufacture of yo-

ghurt or on removal of the brush that occurs on rennet-induced

coagulation of milk. The combined process of enzyme- and

acid-induced coagulation is likely to contribute to coagulation

of casein micelles in the stomach.

Enzymatic Coagulation of Equid Milk

Enzymatic coagulation of milk is the first step in the manufac-

ture of most cheese varieties and also plays an important role

in the flocculation of casein micelles in the stomach. For cheese

manufacture, the process involves the addition of a milk-clotting

enzyme, for example chymosin, to the milk, followed by incu-

bation at a temperature≥ 30 ◦C. During the incubation of bovine

milk with rennet, chymosin hydrolyses the Phe 105 –Met 106 bond

ofκ-casein, leading to the formation of two fragments, the hy-

drophobicN-terminal fragment, f1–105, which remains attached

to the casein micelles and is referred to as para-κ-casein, and the

hydrophilicC-terminal fragment, f106–169, which is released

into the milk serum and is referred to as the CMP. As a result, the

micelles lose steric stabilisation and become susceptible to ag-

gregation, particularly in the presence of Ca^2 +(for reviews, see

Walstra and Jenness 1984, Wong et al. 1988, Walstra 1990, Fox

and McSweeney 1998, Walstra et al. 2006b). Equineκ-casein is

hydrolysed slowly by chymosin at the Phe 97 –Ile 98 bond (Kotts

and Jenness 1976, Egito et al. 2001), without gel formation,

and it appears that either the chymosin-sensitive bond of equine

κ-casein is located in the micelle in a manner that renders it inac-

cessible by chymosin, or that the equine casein micelle derives

colloidal stability from constituents other thanκ-casein. The

high degree of glycosylation may also affect the ability of chy-

mosin to hydrolyse equineκ-casein. Figure 26.2 illustrates the

coagulation of equine, asinine and bovine milk by calf chymosin

at 30◦C. While it is clear that no gel is formed from equine milk,

as judged by lack of an increase in storage modulus, G^1 , asinine

milk seems to form a gel, although it is very weak compared to

the gel formed from bovine milk. Further investigation is war-

ranted to determine if there are differences in the coagulation

properties of asinine and equine milk.

Acid-induced Coagulation of Equid Milk

When bovine milk is acidified to a pH below 5.0, flocculation

of casein micelles occurs, leading ultimately to gel formation.

This process is the basis of the manufacture of yoghurt, in which

acidification is induced by the production of lactic acid by lactic

acid bacteria and also occurs at the low pH of the stomach (for

0.1

1

10

100

0 20 40 60 80 100 120

0.0001

0.001

0.01

Time (min)

Log G' (Pa)

Figure 26.2.Rennet-induced coagulation of equine milk (----), asinine milk (---) and bovine milk ( )at30◦C.