Siew Kim Lee

Changes in the rheology and microstructure of processed cheese during cooking

Abstract The ‘creaming reaction’ of a processed cheese can be traced by the change in its viscosity profile. The viscosity profile can be measured by shearing the processed cheese at a low but constant shear rate using a rheometer. We showed that this change in the viscosity profile during cooking occurred in processed cheese made from a typical formulation as well as from a fat-free model system. This indicated that the ‘creaming reaction’ is primarily a protein-based interaction, which takes places with or without the presence of fat. The observed viscosity profile can be explained in terms of changes in the protein structure of the molten processed cheese during processing. A model is proposed.

Effect of heat treatments and homogenisation pressure on the acid gelation properties of recombined whole milk

The homogenisation pressure and the order of heating and homogenisation were varied in the preparation of recombined whole milks. The fat globule sizes decreased from ∼2 to 0.2μm as the homogenisation pressure increased from 50 to 850bar for samples heated before (HEHO) or after (HOHE) homogenisation. For acid gels prepared from the milks, small increases in elastic modulus of the set gels were observed with decreasing fat globule size. The yield strain of the acid gels increased linearly with decreasing fat globule size, and HOHE gels had higher yield strains than HEHO gels. The yield stress of the set gels increased with decreasing fat globule size, and the yield stress for HOHE gels were higher than the HEHO gels. Confocal micrographs of the gels revealed an almost continuous protein network structure without pores for gels from milks treated at low homogenisation pressures, and the large fat globules did not actively interact with the strands in the gel network. In contrast, a porous protein network structure with distinct strands was observed for the samples treated at high homogenisation pressures, and the small fat globules were intimately involved in the strands in the network structure. The HOHE gels had a more porous protein network with thicker strands than the HEHO gels. The size of the fat globules and their incorporation in the protein network during acidification is proposed to affect the acid gel structure and properties.