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Analytical methods of ion-exchange and gel permeation chromatography were developed to determine the composition of the caseinate complex and the whey protein fraction in bovine, caprine and ovine milk. These methods were subsequently used to study natural variation in the composition of the milk proteins, casein micellar structure and stability, and changes in the proteins during milk processing. Initially traditional anion-exchange chromatography on DEAE-cellulose was used to examine the composition of bovine casein, but a more rapid method of anion-exchange FPLC (fast protein liquid chromatography) was introduced in which the main fractions, namely αs1-, β-, αs2-, κ- and γ-caseins, were separated under dissociating conditions at pH 7.0. A complementary method of cation-exchange FPLC, in which the caseins were separated under dissociating conditions at pH 5.0, was also developed to determine the compositions of bovine, caprine and ovine caseins. Both methods could also be used on a preparative scale. In detailed studies of natural variation in the composition of bovine casein, it was found that in creamery milk in South-West Scotland there was a pronounced seasonal variation in the concentrations of the total and individual proteins but that the relative amounts of the proteins did not vary markedly. The composition of whole casein from individual cows, however, was affected by genetic polymorphism of κ-casein. The relative amount of κ-casein in whole casein varied with phenotype in the order κ-casein BB > AB > AA; on average there was about 25% more κ-casein in whole casein containing the BB phenotype than in that containing the AA phenotype. The presence of β-casein A¹, A² or B genetic variants had no significant effect on casein composition. Compared with bovine casein, caprine casein contained much less αs1-casein, a similar amount of αs2, and much more β- and κ-caseins. Caprine casein was also much more variable in composition, due to the occurrence of an unusual, quantitative genetic polymorphism of αs1-casein. Polymorphism of caprine κ-casein was also studied and it was found that although the amount of κ-casein varied between 9 and 20% of the total casein, the variants were produced in approximately equal amounts in the heterozygotes. There was considerable variation in the composition of individual samples of ovine casein, mainly due to proteolysis of β-casein. Compared with bovine casein, ovine casein contained much less αs-casein, considerably more β-casein and approximately the same amount of κ-casein. In a study of the composition of bulk milk from a commercial dairy flock, it was found that there were considerable seasonal changes in the concentrations of total and individual proteins, but the relative amounts of the proteins remained fairly constant. The composition of the whey protein fraction of all three species was determined by gel permeation FPLC, and four main fractions were separated at pH 7.0. Compared with bovine whey protein, caprine whey protein contained less β-lactoglobulin and more α-lactalbumin, whereas ovine whey protein contained slightly more β-lactoglobulin and less α-lactalbumin. The methods of casein analysis were combined with ultracentrifugation to examine in detail the composition and stability of casein micelles, and in particular the effect of temperature and pH on the dissociation of the caseins and calcium phosphate from the micelles. On cooling milk there was a marked increase in the level of serum casein, which was due almost entirely to β-casein dissociating from the micelles. The change in distribution of micellar and serum casein was completely reversed on re-equilibration at 20°C for 18h. In a series of studies on controlled acidification of milk in the pH range 6.7-4.9 at 30, 20 and 4°C, changes were found in the level and composition of serum casein. These were attributed to a decrease in hydrophobic interaction at lower temperatures, removal of Ca and inorganic phosphate from the micelles as the pH was lowered, and isoelectric precipitation of the caseins at low pH. Results of an investigation using controlled dialysis to selectively remove Ca or inorganic phosphate from casein micelles showed that the binding strength of individual caseins was in the order αs2- > αs1- > β- > κ-casein, indicating that linkage within the micelle increases with the number of phosphate centres in the caseins. In two separate studies it was found that the relative amount of κ-casein in micellar casein increased markedly, and that of β-casein decreased, with decreasing micellar size. A number of other important changes which take place in the caseins and the whey proteins during processing were examined. Using gel permeation FPLC, detailed quantitative information was obtained on the extent of denaturation of the individual whey proteins of the cow, goat and sheep with increasing temperature, heating time, and pH. An increased association of the whey proteins with the caseins was observed, and there was a close correlation between the levels of denaturation of bovine β-lactoglobulin and α-lactalbumin and the amounts of these proteins retained in the curd during the pilot-scale production of Cheddar cheese. The effect of incorporation of the whey proteins into the curd on the proteolysis of the caseins during the ripening of Cheddar cheese was examined by ion-exchange FPLC. The caseins were found to be less sensitive to heat treatment than the whey proteins, but on severe heating, such as occurs during UHT treatment, changes were detected using alkaline and SDS PAGE and anion- and cation-exchange FPLC. These changes were consistent with loss of positive and negative charges on the proteins. Comparison of laboratory and commercially prepared sodium caseinates indicated that considerable heat damage and change in functional properties had occurred in the commercial products. In a study of acid gelation, it was found that on heating milk at functionalisation temperatures, the concentration of κ-casein in the serum increased. On controlled acidification of the heat-treated milk, Ca, inorganic phosphate and caseins dissociated from the micelles but, compared with unheated milk, maximum dissociation of the caseins at 4 and 20°C occurred at higher pH, and the overall extent of dissociation was reduced. On acidification at 30°C, the level of serum casein decreased, compared with the slight increase found for raw milk. Results showed that a detailed knowledge of the composition and interactions of the milk proteins could result in considerable improvements and increased efficiency in milk processing.