The molecular phenomena that determine the viscosity and melting/crystallization temperatures of triacylglycerides (TG) in vegetable oils are discussed. These physico-chemical properties determine some of the functional properties of vegetable oils when used alone or in complex food system. For instance, when oils are cooled, the TG family with the highest melting temperature is the first to crystallize developing a solid in a liquid phase. These systems, generally known as plastic fats, have fractal organization with significant effect on the melting properties and spreadability of food systems like butter and margarine. On the other hand, in vegetable oils TG's are organized in bi-layer lamellar structures whose shape and size change with temperature and determine the magnitude of oil viscosity. In turn TG crystallization is affected by viscosity which determines, in a great extent, nucleation rate and crystal growth.Lipids are a heterogenous group of compounds mainly composed of carbon, hydrogen, and oxygen, and are soluble in non-polar solvents and insoluble in water. However, the term lipid has not a strict definition. Christie (7) indicated that: "Lipids are fatty acids and their dérivâtes and substances related biosynthetically or functionally to these compounds." This definition will be adopted throughout this chapter.From the physiological point of view lipids have basic functions such as supplying energy (9 Kcal/g, value traditionally assigned and derived from caloric measurements) or storing it for further utilization, providing essential fatty acids for human metabolism, and providing a transport medium for liposoluble vitamins. On the other hand, the particular functional properties of lipids provide foods with specific characteristics of texture, flavor, odor, and color. These functional properties result from a lipid's distinctive physical chemical characteristics, and from their interactions with other food components as affected by process variables. Such is the case of the interaction of lipids with starch (2-5), milk and meat proteins (6-70), water (77, 12 ), or even other lipid molecules (13-15).
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