Food Biochemistry and Food Processing (2 edition)

BLBS102-c24 BLBS102-Simpson March 21, 2012 13:47 Trim: 276mm X 219mm Printer Name: Yet to Come

456 Part 4: Milk

(A)

(C)

(B)

Figure 24.3.Models of casein micelle structure: (A) the submicelle model of Walstra (1999); (B) the Holt model (from Fox and McSweeney

1998); (C) the dual binding model, showing interactions betweenαs1-,β- andκ-caseins (Horne 2003).

dissociation of the micelles (O’Connell et al. 2001a, b). Micelle-

like particles reform on cooling and form a gel at∼ 4 ◦C. It is

not known if the sub-particles formed by any of these treatments

correspond to casein sub-micelles.

The turbidity of skim milk increases on addition of SDS up

to 21mM, at which concentration a gel is formed; however, at

≥28 mM (∼0.8%, w/v) the micelles disperse (see Lefebvre-

Cases et al. 2001a, b).

There have been few studies on variations in micelle size

throughout lactation and these have failed to show consistent

trends. No studies on variability in the micro-structure of the

casein micelle have been published. We are not aware of studies

on the effect of the nutritional or health status of the animal on

the structure of the casein micelles, although their stability and

behaviour are strongly dependent on pH, milk salts and whey

proteins, all of which are affected by the health and nutritional

status of the cow.

The milk of all species that have been studied is white, pre-

sumably due to light scattering by the casein micelles. However,

the micelles of only a few species have been studied in detail;

electron microscopy indicates that the micelles of all species are

spherical and range in size from approximately 50 nm in human

milk to∼600 nm in equine milk. Some of the large micelles in

equine milk settle out under gravity or low centrifugal forces;

the large micelles reflect the low level ofκ-casein in equine

milk. The milk of most species is supersaturated with calcium

phosphate and the insoluble part (CCP) is present in the casein

micelles, in which it acts as a cementing material; the micelles

disintegrate when the CCP is removed. Human milk lacks CCP

and its micelles have a rather porous structure.