Composition and Structure of Bovine Milk Lipids

The lipid (fat) fraction of milk is composed mainly of triacylglycerols, with minor contributions from diacylglycerols, free (unesterified) fatty acids, phospholipids and sterols. Trace amounts of fat-soluble vitamins, β-carotene and fat-soluble flavouring compounds are also present. These lipids exist in milk in the form of small globules surrounded by a membrane, which maintains the integrity of the globule. Although the vast bulk of the lipids are in the core of the globule, some fat is found in the milk fat globule membrane (MFGM), for example the majority of the phospholipids and small amounts of triacylglycerols and sterols. The triacylglycerols are relatively simple molecules consisting of a glycerol backbone to which are esterified three fatty acid molecules. However, the resultant milk fat is a complex mixture of triacylglycerol molecules due to the large variety of constituent fatty acids. The physical properties of the milk fat, for instance, the wide melting range of −40 °C to 40 °C, are in part a consequence of the composition of the triacylglycerol. This can markedly affect the functional properties of dairy products. For this reason, much research has gone into determining the constituent fatty acids and the way they are arranged on the glycerol backbone, and the rationale of variations in the composition. The minor components are also important in the overall attributes of milk fat, for instance the desirable flavour; hence, it is important to understand not only what is in milk lipids, but how they interact.