Composition of Fats and Oils
Fats are made up primarily of TriAcylGlycerols (TAGs), approximately 98%, with the remainder of the fat being more polar lipids like DiacylGlycerols (DAGs), MonoacylGlycerols (MAGs), Free Fatty Acids (FFAs), phospholipids, glycolipids, sterols, and other minor components. In refined fats, these minor lipids are much lower in concentration than in unrefined fats.
Although the TAGs form the main crystalline phase, the minor components, or impurities, can often play a large role in how crystallization occurs and crystallization may be substantially different in a refined oil than in the unrefined starting material.
TAGs are composed of three fatty acids arranged on a glycerol molecule, and with variations in chain length and degree of saturation of the fatty acids, a wide range of components is possible. This range of composition leads to complexities in crystallization. Knowledge about the detailed structures of the triglycerides present in fats and oils is important because they define some of the physical characteristics of the oil. The melting points of triglycerides are dependent on the structures and position of the fatty acids on the glycerol backbone. They also affect the crystallization behavior of the oil.
Palm oil is composed of about 40 different types of TAGs. However, there are three important types of Triglycerides (e.g. POP, PPP and POO for the experts). The semisolid nature of palm oil for instance at room temperature has been attributed to the presence of the POP. Oil modifications process can lead to a product richer in POO (Palm Olein) and a product richer in PPP (Palm Stearin).
Complex Crystallization Process
The crystallization process is divided into nucleation and crystal growth phases. During cooling a crystalline nucleus forms and, it begins to grow by incorporating other molecules. Crystallization kinetics has a profound influence on fats’ final structures and is intrinsically related to their rheological and plasticity properties.
Fats’ tendency to crystallize is of fundamental concern to processing techniques. Triacylglycerols generally crystallize initially into the α polymorphic forms, which then transforms to the β′ and/or β forms which are more stable. This polymorphic transformation is an irreversible process from the less stable to the more stable form, and depends on the temperature and time involved.
Fats with crystals in the β′ form offer greater functionality, because they are softer, support aeration better, and offer creaming properties. The β′ form is thus generally the polymorph of greatest interest for producing high-fat foods, such as margarines and confectionary and bakery products.
The speed of the transformation of the α polymorphic form to the more stable form depends on which type of crystals are crystallizing. PPP type crystals will crystallize fast and transform rapidly to the stable form whereas POP type crystals will transform slowly to the more stable form.
The art of processing fat products in the case of paste products for instance is to control processing settings to allow the right consistency at packing (liquid or semi solid) and which in time will achieve a harder consistency which is solid and with a nice texture.