Dairy Chemistry And Biochemistry

184 DAIRY CHEMISTRY AND BIOCHEMISTRY

8.

9.

10.

11.

The micelles can be destabilized by alcohols, acetone and similar

solvents, suggesting an important role for electrostatic interactions in

micelle structure.

As the temperature is lowered, caseins, especially /?-casein, dissociate

from the micelles; depending on the method of measurement, 10-50% of

/?-casein is non-micellar at 4°C.

Electron microscopy shows that the interior of the micelles are not

uniformly electron dense.

The micelles have a surface (zeta) potential of about - 20 mV at pH 6.7.

Micelle structure. Various models of casein micelle structure have been
proposed and refined over the past 40years. Progress has been reviewed
regularly, including Schmidt (1982), McMahon and Brown (1984), Farrell
(1988), Holt (1992, 1994), Rollema (1992) and Visser (1992).
The proposed models fall into three general categories, although there is
some overlap:

1. core-coat;


2. internal structure;


3. subunit (submicelles); in many of the models in this category, it is
   proposed that the submicelles have a core-coat structure.
   For many years there has been strong support for the view that the
   micelles are composed of submicelles of mass -106Da and diameter
   10-15nm. This model was introduced in 1967 by Morr who proposed that
   the submicelles are linked together by CCP, giving the micelle an open
   porous structure. On removal of CCP, e.g. by acidification/dialysis, EDTA,
   citrate or oxalate, the micelles disintegrate. Disintegration may also be
   achieved by treatment with urea, SDS or at pH greater than 9; presumably,

Submicelle

F'rF'r;uding

0

• Calcium
  phosphate

Figure 4.20 Submicelle model of the casein micelle (from Walstra and Jenness, 1984).