Firmness and crystallization of milk fat in relation to processing conditions.

We studied the influence of different processing conditions on the crystallization and firmness of milk fat. Liquid milk fat was quickly cooled with a scraped surface heat exchanger to several temperatures ranging from 10 to 20 degrees C to initiate crystallization, and in some cases subsequently passed through an agitated working unit, in which the crystallizing fat was subjected to high shear. Samples of the crystallizing fat were taken directly after either the scraped-surface heat exchanger or the agitated working unit. Crystallization and development of firmness were monitored by a number of techniques that were applied simultaneously. Crystallization was monitored by temperature measurement, nuclear magnetic resonance, differential scanning calorimetry, and microscopic imaging. Firmness was measured by penetrometry. The development of firmness of milk fat produced by a Votator line at temperatures encountered in practice, ranging from 10 to 20 degrees C, can be explained by the kinetics of crystallization into the beta'-form of high-melting and middle-melting triglyceride fractions of milk fat and subsequent recrystallizations. The new insights allow better control of the firmness by adjusting the processing conditions.