Figure 6. A unified mechanism for lipid autoxidation. Source: Ref. 32.

habit, emulsion, etc. Triglycerides exhibit multiple crystalline structures. For example, tristearin melts at 54.7, 64.0, and 73.3°C, representing the transition from the less stable a form to the more stable forms, /?' and fi. The a form has a hexagonal crystal subcell. The intermediate melting /?' form is orthorhombic and the highest melting polymorph; the fl form has a triclinic subcell. This multiplicity of molecular conformation and packing influences the fluidity, texture, and appearance of the product. Alpha crystals are fragile platelets of 5 fim in size. Beta crystals tend to be large and coarse with 25-50 jum in diameter. Fats in the /?' form appear as tiny needles of ~ 1 //m in length. The ft' crystals can incorporate large numbers of air bubbles providing a smooth texture to an oil product such as margarine and shortening. The fatty acid composition, as well as the position of a particular fatty acid in the glyceride, affects the crystal habit of a particular fat. Interesterifi-cation usually causes a /?' to /? conversion.

The current concept of emulsion stability came with the understanding of the factors controlling attractive/ repulsive forces and their interaction with distance between disperse particles. Flocculation and coalescence of oil droplets in an emulsion is dependent on the balance between van der Waals attraction and electrostatic repulsion. The combination of these two gives a net potential energy vs distance curve with a potential minimum at certain intermediate ranges, where repulsion may be greater than attraction and a measure of stability exists (34). At long and short ranges, the net potential is always attraction.

An emulsifier consists of both hydrophilic and hydrophobic segments in the same molecule. Food grade emul-sifiers are usually partial esters of fatty acids, polyols, and water-soluble organic acids. When an emulsifier is dispersed in water and heated, a liquid crystalline mesophase is formed (35). The mesophase assumes a lamellar, hexagonal, or cubic structure depending on the type of emulsifier and temperature. In a ternary system such as an emulsifier in an oil/water system, similar types of meso-phases are formed (36). The ordered layer of this liquid-crystalline phase stabilizes the oil-in-water emulsion by forming a film at the interface. It decreases the attractive forces between the oil droplets and provides a steric barrier against coalescence between the droplets.

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