Milk fat and primary fractions obtained by dry fractionation: 1. Chemical composition and crystallisation properties

The chemical composition and crystallisation properties of milk fat and its primary fractions, obtained by dry fractionation at 21 °C, were investigated. The solid fraction (stearin) and the liquid fraction (olein) displayed a different triacylglycerol (TG) composition. Stearin fraction was enriched in long-chain fatty acids, whereas olein fraction was enriched in short-chain and unsaturated fatty acids. Crystallisation properties of milk fat, and both the stearin and olein fractions were studied on cooling at |dT/dt| = 1 °C min−1 by differential scanning calorimetry and time-resolved synchrotron X-ray diffraction (XRD) at small and wide angles. Two main types of crystals corresponding to double chain length structures were characterised in the stearin fraction: α 2L1 (47.5 Å) and β′ 2L2 (41.7 Å). A triple chain length structure was formed in the olein fraction: α 3L (72.1 Å). Crystallisaton of milk fat showed the formation of two 2L (47.3 and 41.6 Å) and one 3L (72.1 Å) lamellar structures with an hexagonal packing (α form). A schematic representation of the 3L packing of olein fraction was proposed to explain how a wide diversity of TG can accommodate to form a lamellar structure with a thickness of 72 Å. Furthermore, the sharpness of the small-angle XRD lines associated to the α form was explained by the formation of liquid crystals of smectic type.