HEAT-INDUCED CONFORMATIONAL-CHANGES IN WHEY-PROTEIN ISOLATE AND ITS RELATION TO FOAMING PROPERTIES

Heat-induced changes in the physicochemical properties of whey protein isolate (WPI) have been studied. WPI (5%) heated at 70 degrees C unde rwent rapid conformational changes within 1 min. The aperiodic structu re content increased primarily at the cost of beta(-)sheet structure. The hydrophobic character, as measured by changes in the pH-solubility profile and the solubility profile at pH 4.6 in NaCl solutions, of th e protein surface increased. However, the surface hydrophobicity, as m easured by the cis-parinaric acid binding method, decreased. In contra st, WPI (9%) heated at 90 degrees C did not exhibit significant change s in the secondary structure content. The surface hydrophobicity decre ased, and only minimal changes in the hydrophobic character of the pro tein surface occurred. The specific viscosity and gel electrophoretic data indicated that the majority of proteins in WPI heated at 90 degre es C were polymerized via sulfhydryl-disulfide interchange reactions, whereas such polymerization was minimal in the case of WPI heated at 7 0 degrees C. Studies on the foaming properties showed that WPI heated for 1 min at 70 degrees C possessed better foamability and foam stabil ity than the other heat-treated samples. The improvement in foaming pr operties was not only affected by conformational changes but, more imp ortantly, by the ratio of monomer to polymeric protein species present in WPI. Maximum foam stability was observed when the ratio of monomer to polymer was 40:60, whereas maximum foamability occurred at a 60:40 ratio. The results suggested that while monomeric proteins contribute d to foamability, the polymeric species contributed to foam stability.