The nature of the protein materials which adsorb to the fat/serum interface of homogenised milk : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand
The fat globules of milk range in diameter from below 0.5 microns to above 15 microns with an average diameter of about 3.1 microns though the average figure varies with breed of cows, stage of lactation and other factors. Upon homogenisation the average diameter of the fat globules is reduced to about 0.54 - 1.4 microns depending upon the effectiveness of homogenisation (Waletra, Oortwijn and de Graaf, 1969). There is an increase in globule surface area which can be calculated from the above figures as a change from 850 cm2/ml in normal milk to 4950 cm2/ml in homogenised milk which is an increase of 5.7 times (walstra, 1969c). Various authorities quote increases of 4 to 8 times (Jenness and Patton, 1959; Trout, 1950; Brunner, 1965). The fat/plasma interface that is created in this way is stabilised in some manner which results in an increase in the amount of nitrogenous material associated with the fat. Presumably protein from both milk plasma and the original fat globule memberane moves to the interface and acts as an emulsifying agent to statbilise the fat dispersion. The ability of milk proteins to act in this manner has been demonstrated in model systems containing butter oil, water, and pure milk proteins (Jackson and Pallansch, 1961). A soluble protein desorbed from the natural globule was very effective in reducing interfacial tension in the model system and the whey proteins were all shown to have a similar, though less extensive, effect. Micellar casein, however, does not depress free energy at the fat/serum interface and casein components would need to be activated in some manner if they were to adsorb to the interface.