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

Summary Commercial protein-stabilized emulsions are often stored for many months. Reversed phase high performance liquid chromatography (HPLC) analysis of protein displaced from β-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 interface did. The changes in the retention time of the displaced protein were accompanied by an increase in the mass of the protein. When 2-mercaptoethanol was added prior to emulsification, the rate of modification was significantly slower than in its absence. Tryptic digests of the displaced, modified protein showed that the changes were specific. Analysis of volatile components present in the emulsions showed that emulsification induced the autoxidation of the polyunsaturated fatty acyl chains of the soya oil resulting in the time-dependent formation of low molecular weight compounds. 2-Mercaptoethanol inhibits the process probably by reacting with hydroperoxides and terminating the chain reaction. Aldehydes, particularly enals, are known to react with nucleophilic amino acid residues such as lysine via addition and/or condensation reactions, leading to the observed increase in the mass of the protein.