A study of the structure of triacylglycerols of bovine milk fat : a thesis presented for the degree of Doctor of Philosophy in Biochemistry at Massey University

Abstract

Samples of milk fat obtained at different stages of the dairying season were investigated to determine the influence of the observed seasonal changes in the fatty acid (FA) compositions of New Zealand milk fats on the structure of milk triacylglycerols (TG1s) . Of the three milk fats studied the January and March samples had FA compositions typical of hard summer milk fat, while the September sample had a FA composition typical of soft spring milk fat . Each of the three selected samples of milk fat was effectively separated into TG fractions of high, medium and low molecular weight which had distinctly different FA and TG compositions. Stereo specific analyses o f these fractions showed that in TG fractions of similar average molecular weight the arrangement of FA 's within the TG ' s was similar and that in all TG fractions the FA 's were arranged within positions 1 , 2 and 3 of the TG1s in a highly selective manner. The TG fractions were separated into TG classes of different degrees of unsaturation. Corresponding TG c lasses of the three samples of milk fat had generally comparable FA compositions. However an important distinguishing feature was that each TG fraction of the September sample contained a higher proportion of unsaturated TG's than the corresponding TG fractions of the January and March samples. Thus in New Zealand milk fats of differing FA composition, the nature of the TG species is similar but there exist differences in the relative proportions of the TG species. The variation in the proportions of the constituent TG species of New Zealand milk fat would appear to be the overriding factor whic h influences the seasonal variation in its physical characteristics. The thermal properties of TG fractions of milk fat were examined with a view to determining the influence of TG structure on the physical characteristics of milk fat. It was found that the unsaturated TG's of low molecular weight , were largely responsible for the considerable proportion of milk fat which melted below 0º C and consequently for the wide melting range which is characteristic of milk fat. The structural difference between these TG's and the remaining TG' s of milk fat was found to be sufficiently large to prevent significant formation of solid solutions. Consequently the wide melting range of milk fat is due to both the large number of different TG species and to the large structural difference between the various TG species.