The Composition and Properties of Bovine, Caprine and Ovine Milk Proteins

Law, Andrew J. R (1995) The Composition and Properties of Bovine, Caprine and Ovine Milk Proteins. PhD thesis, University of Glasgow.

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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 alphas1-, beta-, alphas2-, kappa- and gamma-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 kappa-casein. The relative amount of kappa-casein in whole casein varied with phenotype in the order kappa-casein BB > AB > AA; on average there was about 25 % more kappa-casein in whole casein containing the BB phenotype than in that containing the AA phenotype. The presence of beta-casein A1, A2 or B genetic variants had no significant effect on casein composition. Compared with bovine casein, caprine casein contained much less alphas1-casein, a similar amount of alphas2, and much more beta- and kappa-caseins. Caprine casein was also much more variable in composition, due to the occurrence of an unusual, quantitative genetic polymorphism of alphas1-casein. Polymorphism of caprine kappa-casein was also studied and it was found that although the amount of kappa-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 beta-casein. Compared with bovine casein, ovine casein contained much less alphas-casein, considerably more beta-casein and approximately the same amount of kappa-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 beta-lactoglobulin and more alpha-lactalbumin, whereas ovine whey protein contained slightly more beta-lactoglobulin and less alpha-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 kappa-casein dissociating from the micelles. The change in distribution of micellar and serum casein was completely reversed on re-equilibration at 20

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: J Leaver
Keywords: Animal sciences
Date of Award: 1995
Depositing User: Enlighten Team
Unique ID: glathesis:1995-75555
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 19:28
Last Modified: 19 Nov 2019 19:28

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