Heat-induced changes in sulphydryl and disulphide levels of β-lactoglobulin A and the formation of polymers

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Abstract
Summary: The effects of heat and pH on sulphydryl (–SH) and disulphide (–SS–) groups of β-lactoglobulin (β-lg) A have been studied by heating at different temperatures and pH values in air and at pH 6·9 in the absence of air. At pH 6·9 under aerobic conditions a decrease of –SH groups and an increase of –SS– groups was observed with increasing time and temperature. Only small changes were found under anaerobic conditions. At pH from 3·0 to 9·8 the –SH level decreased while the –SS– level increased up to pH 6·9 and then dropped rapidly. In addition to –SH/–SS– interchange there were reactions to other sulphur-containing compounds as seen from the losses in the total amount of –SH plus –SS– sulphur. The results of gelchromatographic investigations suggest that –SH-initiated –SS– exchange-reactions play an important role in the formation of high molecular weight polymers of β-lg A during heat treatment, and that the formation of intermediates depends on the presence of air.