Heat-induced conformational changes in whey protein isolate and its relation to foaming properties
1994
Zhu, H. | Damodaran, S.
Heat-induced changes in the physicochemical properties of whey protein isolate (WPI) have been studied. WPI (5%) heated at 70 degrees C underwent rapid conformational changes within 1 min. The aperiodic structure content increased primarily at the cost of d-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 the protein surface increased. However, the surface hydrophobicity, as measured by the cis-parinaric acid binding method, decreased. In contrast, WPI (9%) heated at 90 degrees C did not exhibit significant changes in the secondary structure content. The surface hydrophobicity decreased, and only minimal changes in the hydrophobic character of the protein surface occurred. The specific viscosity and gel electrophoretic data indicated that the majority of proteins in WPI heated at 90 degrees C were polymerized via sulfhydryl-disulfide interchange reactions, whereas such polymerization was minimal in the case of WPI heated at 70 degrees C. Studies on the foaming properties showed that WPI heated for 1 min at 70 degrees C possessed better foamability and foam stability than the other heat-treated samples. The improvement in foaming properties was not only affected by conformational changes but, more importantly, 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 contributed to foamability, the polymeric species contributed to foam stability.
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