Microstructure, permeability and appearance of acid gels made from heated skim milk

Abstract

The effects of a range of heat treatments on the microstructure and appearance of acid skim milk gels were investigated using confocal scanning laser microscopy, permeability measurements and photography. Gels were made from reconstituted skim milk heated at 75, 80, 85 and 90 °C for 15 or 30 min, by acidification with glucono-δ-lactone at 30 °C, with a few samples at 40 °C. Heating milks, at temperatures ≤80 °C, resulted in a gel microstructure that appeared to have thinner but more numerous branches, and had a higher ‘apparent interconnectivity’ (in the thin optical section of the x - y plane) of aggregates compared with unheated or mildly heated milks that had tortuous, bent or irregular clusters and strands making up the gel network and much less ‘apparent interConnectivity’ of strands and clusters. There were no major differences in the microstructure of acid milk gels formed from milk heated in the range 80–90 °C. Permeability measurements, which gave information on the size and number of the largest pores in the gel matrix, indicated that heat treatment had little effect on the overall porosity of the gels. It was proposed that the aggregation of denatured whey proteins during the acidification of heated milk altered gel formation and were responsible for the modified microstructure. Heating probably reduced the thickness and altered the orientation of the strands in the network. High treatment of milk also resulted in gels having large visible cracks and a rough surface appearance, but gels made from unheated milk had a smooth, unblemished appearance. It was observed that these structural rearrangements of the network, which were responsible for the large cracks and rough appearance of gels made from heated milk, occurred just after gel formation. It was proposed that a reduction in the shear deformation at fracture of gels made from heated milk may have contributed to the greater susceptibility of these gels to localized fracture