Crystallization kinetics and structure of milk fat fractions
The crystallization process of high (HMF), medium (MMF) and low (LMF) melting fractions of milk fat and their mixtures was studied at 5, 15, and 25°C. Solid fat content (SFC) was measured by pulsed nuclear magnetic resonance, melting behavior was characterized by differential scanning calorimetry, and polymorphism was studied by synchrotron x-ray diffraction. The microstructure was imaged by polarized light microscopy and fractal dimensions of the crystal networks were calculated from the micrographs by a box counting method. HMF was responsible for nucleation and the initial stage of crystal growth at all temperatures studied. MMF displayed an ambivalent behavior (i.e., as a high and low melting fraction) depending on the proportion at which it is present and the crystallization temperature. The presence of MMF and/or LMF resulted in a decrease of SFC, and the induction of a two-step crystallization due to secondary crystallization and polymorphic transformations. The morphology of the crystals, the formation of a fat network as well as the nucleation and growth mechanism were dictated by the composition of the mixtures and temperature of crystallization.