Fractionation of fat 209 Feed oils/fats CRYSTALLISATION cooling utteroilAMF stirring Nuclei formation ightly hydrogenate soyabean oil, etc Further cooling Crystal growth (filtration) containing detergent is used to facilitate the separation of the crystals from olein (Lipofrac) by centrifugation (3) Solvent fractionation. The crystallisation is carried out in solvents followed by iltration. This process is not used widely due to its high operating costs except for the production of high value products such as cocoa butter replacer. It will therefore only receive a brief treatment in this chapter. 8.1.1 Crystallisation: nuclei formation and crystal growtH The controlled cooling of molten fat slows the thermal motion of the molecules, drawing them closer together through intermolecular forces, whilst simultaneously enabling parallel ordering of the fatty acid chains to take place. As a consequence nuclei form and crystallisation commences. Here, if the probability of a molecule being absorbed exceeds that of a molecule being liberated, then these molecular aggregates will grow into real crystals. Cooling aids the absorption of molecules by lowering the potential energy. The nucleation rate increases until a maximum is reached (Tammann, 1903)but further cool ng contributes to a reduction in nucleation rate because the viscosity of the melt is ncreased, which as a consequence reduces the rate of diffusion Mortensen(1983)reported that when formed, milk fat crystal nuclei grow thre position of successive single layers of molecules on an already ordered crystal The probability of the incorporation of these molecules into the crystal lattice as the material density and the temperature, which influences the rate of diffusion, all play a primary role in the rate of growth of the crystals. It is also evident from studies using milk fat that for a given reactor vessel with a fixed rate of agitation(stirring), the cooling rate, i.e. rate of temperature drop, determines final crystal composition( Rajah, 1988) (1) Rapid cooling rates resulted in high yields of crystals with low N20 values(solid fat lues at20°C) due to (2) Moderate cooling rates promoted the development of crystals with primarily high melting triacylglycerols and high N2o values, although the yields were somewhatFractionation of fat 209 Feed oildfats CRYSTALLISATION e.g. palm oil cooling butteroil/AMF * Nuclei formation hydrogenated fish oil stirring (nucleation) lightly hydrogenated soyabean oil, etc. Further cooling and gentle stirring Crystal I growth (morphology and polymorphism) Crystal 1-’ separation (filtration) Olein (soft fraction) Stearin (hard fraction) Fig. 8.1. The fractionation process. containing detergent is used to facilitate the separation of the crystals from olein (Lipofrac) by centrifugation. Solvent fractionation. The crystallisation is carried out in solvents followed by filtration. This process is not used widely due to its high operating costs except for the production of high value products such as cocoa butter replacer. It will therefore only receive a brief treatment in this chapter. (3) 8.1.1 Crystallisation: nuclei formation and crystal growth The controlled cooling of molten fat slows the thermal motion of the molecules, drawing them closer together through intermolecular forces, whilst simultaneously enabling parallel ordering of the fatty acid chains to take place. As a consequence nuclei form and crystallisation commences. Here, if the probability of a molecule being absorbed exceeds that of a molecule being liberated, then these molecular aggregates will grow into real crystals. Cooling aids the absorption of molecules by lowering the potential energy. The nucleation rate increases until a maximum is reached (Tammann, 1903) but further cool￾ing contributes to a reduction in nucleation rate because the viscosity of the melt is increased, which as a consequence reduces the rate of diffusion. Mortensen (1983) reported that when formed, milk fat crystal nuclei grow through the deposition of successive single layers of molecules on an already ordered crystal surface. The probability of the incorporation of these molecules into the crystal lattice as well as the material density and the temperature, which influences the rate of diffusion, all play a primary role in the rate of growth of the crystals. It is also evident from studies using milk fat that for a given reactor vessel with a fixed rate of agitation (stirring), the cooling rate, i.e. rate of temperature drop, determines final crystal composition (Rajah, 1988): (1) Rapid cooling rates resulted in high yields of crystals with low N20 values (solid fat index values at 20°C) due to entrapped olein. (2) Moderate cooling rates promoted the development of crystals with primarily high melting triacylglycerols and high N~o values, although the yields were somewhat reduced