The Effect of UHT and VAT Thermal Processing Systems on Whey Protein Denaturation and Gel Strength of yoghurt

Milk was thermally processed by Ultra-High-Temperature (UHT, 132 to 154 C for 0 to 12 sec) and Vat (VAT, 66 C to 88 C for 0 to 30 min) systems. Representative milks were fermented into gels by a traditional yoghurt fermentation procedure. Selected functional properties, i.e. whey protein denaturation, gel strength, and their interrelationship were studied. Denaturation behavior of the five whey proteins indicates different heat effects produced by the UHT and VAT heat treatments. The degree of whey protein denaturation increased at a faster rate in the UHT than in the VAT heat treatments. The UHT heat treatment resulted in yoghurt gels of lower strength than those of VAT heat treatments. A critical point was found in the optimal gel strength in the UHT (149 C, 3.3 sec) heat treatment. An approximate correlation between the degree of whey protein denaturation and the gel strength was observed in the VAT heat treatment, whereas no such correlation was found in the UHT heat treatment. It appeared, therefore, that whey protein denaturation in the UHT and VAT heat treatments plays a markedly different role in the mechanism of yoghurt gelation. Moreover, the VAT but not the UHT heat treatment may provide an index of the gel strength.