Melting and Crystallization of Milk Fat Triglycerides

The complex fatty acid composition of milk fat is reflected in its melting behavior. Melting begins at — 30°C and is complete only at 37°C. At any intermediate temperature, milk fat is a mixture of solid and liquid. To a large extent, the solid: liquid ration determines the rheological properties of the fat. For example, at refrigeration temperature, butter has a higher solids content than does a tub margarine. Hence the latter product is more easily spread (18).

As crystallization proceeds, the growing crystals impinge to form aggregates. A network results, in which both the solid and liquid phases may be regarded as continuous. Formation of the network greatly increases the firmness of the fat.

As a liquid fat is cooled, crystallization begins. There are two parts to the crystallization: (1) nucleation and (2) growth. In a bulk fat, nucleation occurs at the surfaces of impurities, a phenomenon described as heterogeneous nucleation. A considerable degree of supercooling is necessary to initiate nucleation. Subsequent growth of the nuclei tends to be slow in natural fats because of competitive inhibition. In materials of low molecular weight, impurities are rejected at the face of growing crystal. In fats, however, the various triglyceride species are so closely related that the term impurity tends to lose its meaning (19).

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