Sulphur dioxide: Part 2

Most foods to which sulphur dioxide is added contain significant amounts of protein and it is conceivable that reaction of the additive with disulphide groups may account for a considerable part of the sulphite which is lost irreversibly. The reaction takes place by SN2 displacement at the disulphide bond as shown in figure 1 with formation of a thiosulphonic acid (Bunte salt) and a thiol. In biscuit manufacture, sulphite ion is added to weaken the dough through cleavage of disulphide groups thereby modifying its rheological properties; analysis of the organic sulphite derived product has shown that it is all in the form of S‐sulphoprotein. The secondary and tertiary structure of many enzymes are due to disulphide links and it is reasonable to suggest that cleavage of these bonds is one way in which enzymes are inactivated by sulphite. Damage to structural proteins in microbial cells could also form part of the basis for its antimicrobial effect.