Effect of processing conditions on physical properties of a milk fat modelsystem: Microstructure

The effect of processing conditions on the microstructure of th ree blends of 30, 40, and 50% high-melting fraction [Mettler dropping point (MDP) = 47 .5 degreesC] in the low-melting fraction (MDP = 16.5 degreesC) of milk fat was studied. The effect of cooling and agitation rates, crystallization tem perature, chemical composition of the blends, and storage time on crystalli ne microstructure (number, size, distribution, etc.) was investigated by co nfocal laser scanning microscopy (CLSM). To improve resolution, a mix of Ni le blue and Nile red dyes was dissolved in the melted samples in proportion s that did not modify the nucleation kinetics. Samples were then crystalliz ed by cooling (0.2 or 5.5 degreesC/min) to crystallization temperature (25, 27.5. and 30 degreesC). After 2 h at crystallization temperature, a slurry was placed on a microscope slide and samples were stored 24 h at 10 degree sC. During this period, more material crystallized. Slowly crystallized sam ples (0.2 degreesC/min) formed different structures from rapidly crystalliz ed samples (5.3 degreesC/min). Crystals were sometimes diffuse and hard to distinguish from the liquid. Samples were darker as a result of this solid- mass distribution. However, rapidly crystallized samples had well-defined c rystals and seemed to be separated by a distinct liquid phase. These crysta ls were not in touch with each other as was the case for slowly crystallize d samples. Higher agitation rates led to smaller crystal size due to enhanc ed nucleation. Larger crystals were formed when crystallization occurred at higher temperatures. Storage time resulted in an increase of crystal size. Larger crystal size and structures with more evident links had a more elas tic behavior with higher elastic modulus E'.