Milkfat is one of the most complex of all fats, consisting of numerous different triglycerides as well as complex fats, such as phospholipids and cerebrosides, and traces of mineral substances (Walstra and Jenness, 1984). It is liquid above 40°C and usually completely solidified below -40°C. At intermediate temperatures, it is a mixture of crystals and oil, the oil being the continuous phase. On cooling, a liquid fat usually forms small crystals at first (in rapid cooling, often in α modification) of needle to platelet shape, 0. 1-3.0μm long, with a ratio length:width:thickness of 4:2:1, (Jensen, 1967). As crystals grow, similar shaped but larger crystals in aggregate which can be as much as 50μm across, are formed. Crystallisation is initiated by the presence of suitable crystal nuclei (centres of crystallisation) in the liquid fat as a few molecules gather in molecular aggregates where the potential energy is reduced to the minimum. Lowering of temperature strongly influences the rate of nucleus formation. Homogeneous nucleation occurs when crystals nuclei are formed spontaneously from a pure melt. Usually a supercooling of 3-5°C is needed to start a nucleation, and a further temperature decrease by a few degrees can caused rapid nucleation. In practice, for fat in bulk, nucleation is always heterogeneous, i.e. crystallisation always starts at the surface of extraneous particles, often called catalytic impurities (Skoda el at, 1963).