The chemical and physical properties of interesterified milk fat fractions

Five fractions of anhydrous milk fat were obtained by fractional crystallization in the absence of solvent. The fractions were characterized, physically and chemically, before and after interesterification. Cholesterol tended to fractionate into the lowest‐melting fraction. Slipping points of 38.5C, 32C, 28C, 22.5C, and 17C, respectively, were reflected in widely varying micropenetration curves, while differences in fatty acid composition were relatively small. The three higher‐melting fractions, with very similar fatty acid compositions, had very similar physical properties after interesterification. Slipping points of 38C, 37C, 37C, 33C, and 32C, respectively, indicated that interesterification generally hardened the fractions. Interesterification altered the physical properties of lower‐melting fractions more than the properties of higher‐melting fractions. Interesterification caused some triglyceride degradation and some butyrate loss, but these factors could not fully explain the unequal physical property changes induced in the different fractions. Fractionation possibly tends to separate symmetrical triglycerides from their lower‐melting, unsymmetrical isomers, yielding fractions with apparent differences in the degree of randomness of their triglyceride structure. Positioning of fatty acids within the triglyceride molecule seems to be a principal determinant of the physical properties of milk fat fractions.