Chemical changes in beta-Lactoglobulin structure during ageing of protein-stabilized emulsions

Commercial protein-stabilized emulsions are often stored for many months. R eversed phase high performance liquid chromatography (HPLC) analysis of pro tein displaced from beta-lactoglobulin-stabilized oil-in-water emulsions by the competitive adsorption of Tween 20, showed that whereas the retention time of protein displaced from a tetradecane-water interface did not change greatly upon ageing, that of the protein displaced from a soya oil-water i nterface did. The changes in the retention time of the displaced protein we re accompanied by an increase in the mass of the protein. When 2-mercaptoet hanol was added prior to emulsification, the rate of modification was signi ficantly slower than in its absence. Tryptic digests of the displaced, modi fied protein showed that the changes were specific. Analysis of volatile co mponents present in the emulsions showed that emulsification induced the au toxidation of the polyunsaturated fatty acyl chains of the soya oil resulti ng in the time-dependent formation of low molecular weight compounds. a-Mer captoethanol inhibits the process probably by reacting with hydroperoxides and terminating the chain reaction. Aldehydes, particularly enals, are know n to react with nucleophilic amino acid residues such as lysine via additio n and/or condensation reactions, leading to the observed increase in the ma ss of the protein.