MICROSTRUCTURE, PERMEABILITY AND APPEARANCE OF ACID GELS MADE FROM HEATED SKIM MILK

The effects of a range of heat treatments on the microstructure and ap pearance of acid skim milk gels were investigated using confocal scann ing laser microscopy, permeability measurements and photography. Gels were made from reconstituted skim milk heated at 75, 80, 85 and 90 deg rees C for 15 or 30 min, by acidification with glucono-delta-lactone a t 30 degrees C, with a few samples at 40 degrees C. Heating milks, at temperatures less than or equal to 80 degrees C, resulted in a gel mic rostructure that appeared to have thinner but more numerous branches, and had a higher 'apparent interconnectivity' (in the thin optical sec tion of the x - y plane) of aggregates compared with unheated or mildl y heated milks that had tortuous, bent or irregular clusters and stran ds making up the gel network and much less 'apparent interconnectivity ' of strands and clusters. There were no major differences in the micr ostructure of acid milk gels formed from milk heated in the range 80-9 0 degrees 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 durin g the acidification of heated milk altered gel formation and were resp onsible for the modified microstructure. Heating probably reduced the thickness and altered the orientation of the strands in the network. H igh treatment of milk also resulted in gels having large visible crack s and a rough surface appearance, but gels made from unheated milk had a smooth, unblemished appearance. It was observed that these structur al rearrangements of the network, which were responsible for the large cracks and rough appearance of gels made from heated milk, occurred j ust after gel formation. It was proposed that a reduction in the shear deformation at fracture of gels made from heated milk may have contri buted to the greater susceptibility of these gels to localized fractur e. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.