Effects of heat treatment and whey protein addition on the rheological properties and structure of acid skim milk gels

In this study, the effects of heat treatment of milk on the rheological properties and microstructure of acid milk gels formed by the hydrolysis of glucono-delta-lactone were investigated. Gels were formed from reconstituted skim milk, with or without added whey protein concentrate (WPC), which had been heated at temperatures in the range 75-90 degrees C for 15 or 30 min. The rheological properties of these gels were determined using dynamic low-amplitude oscillation in the Bohlin rheometer. The large deformation and fracture properties were studied using a low, constant shear rate method with gels formed in the rheometer. Microstructure was determined using confocal scanning laser microscopy and permeability measurements. Heating milk above 80 degrees C resulted in an increase in the pH of gelation, a reduction in the gelation time and a marked increase in the storage modulus compare to unheated milk. Addition of WPC to milk followed by heat treatment at 80 degrees C caused further increase in pH of gelation, reduction in gelation time and increase in storage modulus. Heat treatment resulted in a large decrease in shear deformation at fracture. The microstructure of gels made from unheated milk appeared to be irregular with areas of dense protein clusters. In contrast, the gels made from heated milk had a microstructure that appeared more regular with clearly defined pores and thinner, straighter strands compared to unheated milk. The large differences in the rheological properties at small and large deformations also suggested that the microstructure of gels was considerably altered by heat treatment of milk. These results are discussed in terms of denaturation of whey proteins and subsequent interactions of proteins occurring during acidification and gel formation. (C) 1999 Elsevier Science Ltd. All rights reserved.