Macromolecular interactions of food proteins studied by Raman spectroscopy - Interactions of beta-lactoglobulin, alpha-lactalbumin, and lysozyme in solution, gels, and precipitates

Coagulation and gelation of food proteins are a manifestation of intermolecular interactions occurring at high solute concentrations. In order to study these interactions, analytical methodologies are required which can be applied to study the structural characteristics of proteins at these high concentrations, in solutions as well as in insoluble or gelled phases. Visible laser Raman spectroscopy is a useful tool for this purpose. Changes reflecting the microenvironment and interactions of aromatic and aliphatic residues, disulfide bonds, hydrogen bonds and relative proportions of secondary structural types can be monitored, by detailed investigation of various bands in the Raman spectrum arising from vibrational motions of the different amino acid residue side chains as well as the polypeptide backbone. This chapter illustrates a few applications of Raman spectroscopy to study interactions of food proteins in solutions, gels and coagula. Changes in protein structure during thiol- or heat-induced conversion of solutions of hen egg white lysozyme to opaque gels, and of beta-lactoglobulin and alpha-lactalbumin into transparent gels have been monitored. Evidence of protein-protein interactions in unheated as well as heated (90 degrees C, 30 minutes) binary mixtures of the two whey proteins and lysozyme is also presented.