RAMAN-SPECTROSCOPIC STUDY OF THERMALLY-INDUCED GELATION OF WHEY PROTEINS

Alpha-lactalbumin and beta-lactoglobulin solutions [15 % (w/v) in D2O, pD 6.8, 20 mM NaCl] were heated at 50, 70, or 90-degrees-C for 30, 60, or 90 min. Only the samples heated at 90-degrees-C formed transparent gels. The amide I' and amide III' bands of the Raman spectra implicated an increase of beta-sheet structure with a simultaneous decrease of helical structure in heated alpha-lactalbumin, while an increase of beta-sheet with a simultaneous decrease of turn structure was suggested in heated beta-lactoglobulin. Changes in other regions of the Raman spectra could be interpreted as changes in disulfide conformation as well as in microenvironment around several amino acid residues, i.e., Trp, Tyr, and His. These changes were observed after heating at 70 and 90-degrees-C but were more intense in the gelled (90-degrees-C) than in the ungelled samples and in beta-lactoglobulin than in alpha-lactalbumin. These observations are in accord with the fact that beta-lactoglobulin forms gels much more easily than alpha-lactalbumin.