Thermal and structural changes of pasteurized milk fat globules during storage

Pasteurization, homogenization and storage are essential steps in the processing of commercial milk. However, their effects on thermal and structural properties of milk fat globules (MFG) remain poorly understood. MFG were extracted from raw bovine milk (RM), pasteurized milk (PM), and homogenized-pasteurized milk (HPM) that were stored at 4 degrees C for 24, 48 and 72 h. Crystallization and melting properties of the MFG were studied using differential scanning calorimetry, and the size distributions were analyzed using laser light scattering. Structural characteristics were observed using confocal laser scanning method and polarized light microscopy, and characterization of crystal polymorphism was done using X-ray diffraction (XRD). Briefly, the bovine milk fat contained about 65% saturated fatty acids. The fat globules from fresh RM and PM had a significantly higher mean diameter than the fresh HPM (2.9, 3.0 and 2.1 mu m, respectively). Notably, the crystallization and melting temperatures of the PM fat globules increased by about 4 degrees C after 72 h storage. Furthermore, the milk fat globule membrane (MFGM) of PM was pierced by fat crystals after 48 h storage. With HPM, the integrity of the membrane was more easily maintained due to finer crystals and thicker MFGM after high pressure treatment. XRD patterns showed a transformation trend of alpha -> beta' -> beta polymorphs in MFG from both processed milks. Thermal and morphology properties of MFG were altered during low temperature storage as a result of the formation of more thermodynamically stable and larger-sized beta crystals.