Milk caseins exist in large colloidal particles called micelles. Many evidences from electron microscopy and other means suggest that the micelles are composed of smaller units called sub-micelles having diameters of from 10 to 20 nm. To date, there are several observations regarding the properties of casein micelles. However, none of the suggested micelle models could explain all the following observations:
Curding process caused by the proteolysis between phenylalaine 105 and methionine 106 of k casein.
At natural milk pH (6.7),the solubility of calcium and phosphorous of milk is much higher than the solubility of calcium phosphate at pH 6.7 solution. High speed centrifugation will precipitate out the calcium.
The formation of micelles requires the presence of calcium at concentrations greater than required to precipitate a s-casein.
Removal of Colloidal calcium phosphate (CCP) results in disintegration of micelles.
Casein micelles are reversible. At low temperatures, some of the b casein is able to leave the micelle and returns to the micelle when the temperature is increased.
The micelles can be destabilized by urea or SDS, and by alcohols, acetone, and similar solvents.
More k casein Milks have smaller average micelle diameters than do milks with less k casein.
Addition of extra k casein to a mixture of casein micelles decreases the average micelle diameter.
Casein micelles are highly solvated (contain about 3.7 g water per gram of protein).
Kappa casein will stabilize a s-casein from precipitation by calcium.