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 degrees 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 vertical bar dT/dt vertical bar = 1 degrees 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: alpha 2L(1) (47.5 angstrom) and beta' 2L(2) (41.7 angstrom). A triple chain length structure was formed in the olein fraction: alpha 3L (72.1 angstrom). Crystallisaton of milk fat showed the formation of two 2L (47.3 and 41.6 angstrom) and one 3L (72.1 angstrom) lamellar structures with an hexagonal packing (alpha form). A schematic representation of the 3L packing of olem fraction was proposed to explain how a wide diversity of TG can accommodate to form a lamellar structure with a thickness of 72 angstrom. Furthermore, the sharpness of the small-angle XRD lines associated to the alpha form was explained by the formation of liquid crystals of smectic type