Molecular Structure and Organization

Chemically, triacylglycerols are esters of a glycerol molecule and three fatty acid molecules (Figure 4.1). Each of the fatty acids may contain different numbers of carbon atoms and may have different degrees of unsaturation and branching (Lawson 1995, Nawar 1996). Nevertheless, most naturally occurring fatty acids have an even number of carbon atoms (usually less than 24) and are straight chained. The fact that there are many different types of fatty acid molecules, and that these fatty acids can be located at different positions on the glycerol molecule, means that there are a huge number of possible triacylglycerol molecules present in foods. Indeed, edible fats and oils always contain a great many different types of triacylglycerol molecules, with the precise type and concentration depending on their origin (Weiss 1983).

Triacylglycerol molecules have a "tuning-fork" structure, with the two fatty acids at the ends of the glycerol molecule pointing in one direction and the fatty acid in the middle pointing in the opposite direction (Figure 4.1). Triacylglycerols are predominantly nonpolar molecules, and so the most important types of molecular interactions with their neighbors are

FIGURE 4.1 Chemical structure of a triacylglycerol molecule, which is assembled from three fatty acids and a glycerol molecule.

van der Waals attraction and steric overlap repulsion (Chapter 2). At a certain molecular separation, there is a minimum in the intermolecular pair potential whose depth is a measure of the strength of the attractive interactions which bind the molecules together in the solid and liquid states (Section 2.4). Whether a triacylglycerol exists as a gas, liquid, or solid at a particular temperature depends on a balance between these attractive interactions and the disorganizing influence of the thermal energy (Section 4.2.3).

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