Factors Affecting the Composition of Milk and the Quality of Butterfat

Abstract

Milk is the food elaborated by the mother for the nourishment of her young until such time as they are able to digest and assimilate the normal food of their species. Milk is an emulsion of fat globules in a colloidal solution of proteins, lactose and mineral matter. At birth, mammalian young differ considerably because the extent of intra-uterine development varies with each species. Since milk is nature's food designed to meet the requirements of these varyingly developed offspring, the composition of the mother's milk must differ between species. These variations can affect the levels of any or all of its constituents. The word" milk" is now almost synonymous with" cow's milk", because this species has been selected and bred by man for increasingly big milk yields in order that cow's milk could play a larger and larger role in human dietetics. The study of factors affecting milk and butterfat compositions has therefore been made very largely on cow's milk although these same changes would be paralleled in the differently composed milks of other species. The following remarks are restricted to cow's milk and deal only with the variations in the fat and non-fatty solids. THE INFLUENCE OF BREED There is a marked difference between the breeds of cows in the fat content of their milks, eg the difference between Jersey and Friesian milk is well known. When such breeds as the Zebu and the Friesian (with their widely differing fat contents) are inter-bred the amount of fat in the milk of the offspring varies directly with the amount of Zebu blood in these grade animals. Not only is there a difference in the fat content between breeds but the size of the fat globule varies from the large globules in Guernsey milk to the small ones in Ayrshire or Friesian milks. The larger the globule the easier it is to separate the cream and to make butter, whereas milks with small globules are better suited for cheese production. The colour of butterfat varies enormously between the different breeds, and milks with a higl! ly coloured fat, such as that from the Channel Island breeds, are often mixed with the milks from other breeds (Ayrshire and Friesian), which have pale coloured fats, to improve the attractiveness of the cream layer. Because the colour of butterfat is derived very largely from the carotene of the food, the colour varies directly with the amount of green foods in the diet., Since carotene is a precursor of vitamin A, high fat colour is popularly associated with high vitamin A activity of the milk. This is not necessarily correct because vitamin A itself is colourless, and one can have pale milk possessing a greater total vitamin A activity than highly coloured Jersey milk. Both the vitamin A and the carotene contents are dependent on the supply eaten in the food and are liable to marked seasonal fluctuations. However, for each of these substances there is a" ceiling" value for each breed, above which the level cannot be raised. The chemical characteristics of pure butterfat do not vary significantly between breeds (Zebu butterfat is not distinguishable by the ordinary analytical procedures from Ayrshire butterfat).