The influence of storage time and temperature on the distribution of individual milk proteins between the micellar and soluble phases has been examined. Storage at 4 or 7 °C is accompanied by a dissociation of micellar caseins, particularly /?-casein, into the soluble phase during the first 48 h, but on further storage there is a partial reversal of this process. At higher storage temperatures (10-20 °C) the contents of all the individual caseins in the soluble phase decrease throughout storage. During cheese-making, losses of fat and curd fines in whey were greater with increased soluble phase casein and clotting times were prolonged. Curd structure was weaker, curds were more moist and slightly lower cheese yields were obtained in stored milks with elevated soluble-phase casein. When milks were stored for up to 3 d at 4 °C cheese gradings were virtually unaffected by storage, but higher temperatures (10-15 °C) led to cheeses being downgraded, largely for body and texture defects but also for flavour after 3 d.

Whey protein preparations isolated from cheese whey form stable gels when their aqueous solutions are heated at 80°C. The gels consist of polypeptide chains crosslinked by disulphide bonds. They dissolve if a sulphydryl (-SH) reagent is added after heating; their formation is retarded if compounds that react with -SH groups are added before heating. Opaque gels are formed from powders containing relatively large amounts of total -SH; clear gels form from those with low total -SH. Gels form faster on heating in Nz instead of air. Gels form more slowly at alkaline pH; it is suggested that this effect is caused by electrostatic repulsion between protein molecules with like charges.

The molecular weights of milk-fat-globule-membrane proteins solubilized in sodium dodecyl sulphate were estimated by gradient gel electrophoresis. Standard curves were calibrated from both protein and glycoprotein markers of known molecular weight. Six major proteins were observed with Coomassie Blue staining and six with periodic acid-Schiff staining. The behaviour of the membrane proteins and the marker proteins was compared on several different single strength sodium dodecyl sulphate-polyacrylamide gels between 3 and 12% (w/v). The results were used to calculate the free electrophoretic mobility and retardation coefficient of each protein. Glycoprotein markers had a significantly lower mean free electrophoretic-mobility value than the protein markers. Three of the milk-fat-globule-membrane glycoproteins were shown to be independent of any of the Coomassie Blue-stained bands. On the basis of a comparison of the free electrophoretic-mobility and retardation- coefficient values of markers and unknown proteins the most appropriate standard curve for molecular-weight estimation was chosen.

The stability of homogenized ultra-heat-treated (UHT) aseptically-packed cream of 18 % fat content when added to hot coffee was indirectly monitored in 10 batches of cream, stored for 10 weeks at different temperatures. A tendency to coagulate under standard conditions (instability) is known as feathering. Stability was determined by a procedure in which acetate buffer instead of coffee solution was used as the test solution. The distribution of components between the fat and aqueous phases of the stored cream was also monitored. Feathering score (stability) decreased during storage but was not accompanied by any increase in non-protein nitrogen levels. The amount of casein and Ca in the fat phase of the cream increased with time and was inversely related to feathering score. Some batches of cream contained 0-1 % Na 2 CO 3 and 0-1 %Na citrate as stabilizers and these additions were found to improve feathering scores early in the storage period. Initially, the fat phase of these creams contained less casein and Ca than those without the additives, but after 10 weeks the differences in feathering score and in casein and Ca distribution, between the 2 types of cream diminished. Examination by electron microscopy showed that casein micelles were associated with fat globules, apparently linking several globules together, which was less in the creams containing additives, but after 10 weeks the appearance of all creams was similar. Electrophoretic separation of the fat globule membrane (FGM) proteins by SDS-polyacrylamide-gel electrophoresis showed that casein was the principal protein component, but that /Mactoglobulin and native FGM proteins were aslo present.When added to hot coffee homogenized cream sometimes shows an instability known as 'feathering' which results in the formation of an unpleasant curd floating on the surface of the coffee. Investigations with pasteurized homogenized creams have indicated that the likelihood of the freshly prepared product feathering is dependent on a number of factors including homogenization temperature and pressure (Doan, 1931), fat: protein ratio in the cream (Doan, 1929) and preheating temperature (Webb & Holm, 1928). Much of this early work has been reviewed by Glazier (1967).Creams which are stable when tested immediately after processing may, on storage, show changes in stability which eventually are sufficient to give rise to feathering when the creams are added to hot coffee. This problem is especially serious with homogenized UHT aseptically-packed creams whose potentially long shelf life is

Measurements of residual native protein remaining after heat treatment of cheese whey have been made using quantitative polyacrylamide gel electrophoresis. The results have been expressed in terms of kinetic constants. The effect of concentration was investigated up to 3 times the normal total solids content, dialysis treatments were used to study the effect of non-protein constituents, and the effect of pH was studied at pH 4, 6 and 9. The results indicated that increased total solids (TS) concentration slowed the denaturation of /9-lactoglobulin A and B (/?-lg A, yff-lg B) but hastened the denaturation of a-lactalbumin (a-la). However, increased lactose concentration slowed the denaturation of both a-la and /ff-lg, perhaps by preventing formation of heat-induced complexes. Increased Ca concentration, up to 0-4 mg/ml, tended to slow the denaturation of both proteins, but further increase in Ca up to 0-9 mg/ml produced little effect. The rate of denaturation of both a-la and /#-lg was slower at pH 4 than at pH 6 or 9, and was probably slowest at the isoelectric point. However, not all the changes associated with pH could be explained in terms of net molecular electrostatic charge. The genetic variants of '/?-lg showed different heat stabilities -below 90 °C /?-lgA was more stable than /MgB, but above 90 °C the situation was reversed at all TS concentrations, and pH 6. However, at pH 4 and 9, yff-lg A was less stable than /?-lgB over the entire temperature range at normal concentration.