4. Directed Interesterification

A random distribution is not always the most desirable. Interesterification can be directed away from randomness if the oil is maintained at a temperature below the melting point of some triacylglycerols which might result. This results in selective crystallization of the trisaturated glycerides, with the effect of removing them from the reaction mixture as they crystallize upon formation and the fatty acid composition of the liquid phase keeps changing. Interesterification thus proceeds with the formation of more trisaturated glycerides than would have otherwise occured. The process continues until most of the saturated fatty acids in the oil have precipitated. If the original oil is a liquid one containing a substancial amount of saturated acids, it is possible, by this method, to convert the oil into a mixture of products, one of them a very unsaturate oil, the other a solid fat with the consistency of shortening, obtained without resorting to hydrogenation or blending with a hard fat. The procedure is relatively slow due to the low temperature used, the time required for crystallization, and the tendency of the catalyst to become coated. A dispersion of liquid sodium-potassium alloy is commonly used to slough off the coating as it forms.

Rearrangement can also be selectively controlled during interesterification by adding excess fatty acids and continuously distilling out the liberated acids that are highly volatile. This impoverishes the fat of its acids of lower molecular weight. The content of certain acids in fat also can be reduced by using suitable solvents to extract appropriate acids during the interesterification process.