Comparison of heat and pressure treatments of skim milk, fortified with whey protein concentrate, for set yogurt preparation: effects on milk proteins and gel structure

Heat (85 degrees C for 20 min) and pressure (600 MPa for 15 min) treatments were applied to skim milli fortified by addition of whey protein concentra te. Both treatments caused > 90 % denaturation of beta-lactoglobulin. Durin g heat treatment this denaturation took place in the presence of intact cas ein micelles; during pressure treatment it occurred while the micelles were in a highly dissociated state. As a result micelle structure and the distr ibution of beta-lactoglobulin were different in the two milks. Electron mic roscopy and immunolabelling techniques were used to examine the milks after processing and during their transition to yogurt gels. The disruption of m icelles by high pressure caused a significant change in the appearance of t he milk which was quantified by measurement of the colour values L*, a* and b*. Heat treatment also affected these characteristics. Casein micelles ar e dynamic structures, influenced by changes to their environment. This was clearly demonstrated by the transition from the clusters of small irregular ly shaped micelle fragments present in cold pressure-treated milk to round, sepal ate and compact micelles formed on farming the milk to 43 degrees C. The effect of this transition mas observed as significant changes in the c olour indicators. During yogurt gel formation, further changes in micelle s tructure, occurring in both pressure and heat-treated samples, resulted in a convergence of colour values. However, the microstructure of the gels and their rheological properties were very different. Pressure-treated mill; y ogurt had a much higher storage modulus but yielded more readily to large d eformation than the heated milk yogurt. These changes in micelle structure during processing and yogurt preparation are discussed in terms of a recent ly published micelle model.