Chemistry of sulphiting agents in food

The main reason for the reactivity of sulphites in food is the nucleophilicity of the sulphite ion. The factors which determine the activity of this nucleophile are summarized and critically evaluated for concentrated systems, e.g. dehydrated foods. The distinction between free and bound sulphite is explained, and reversible binding of the additive in beverages and dehydrated foods is discussed with reference to simple theory of chemical equilibrium. The inhibition of non-enzymic browning reactions accounts for a large proportion of sulphite which undergoes irreversible reaction in concentrated foods. The mechanisms of reactions between sulphite species and intermediates in the model Maillard reaction, glucose+glycine, are considered in depth together with supporting kinetic data. An interesting feature is the fact that sulphites seem to catalyse the reactions they are added to control. Implications of this to the level of use of sulphite are discussed. Reaction products from the inhibition of Maillard browning include 3,4-dideoxy-4-sulphohexosulose which is formed initially and polymeric substances which arise from the reaction of sulphites with melanoidins. A proportion of sulphite added to food becomes converted to sulphate. Mechanisms of autoxidation are critically appraised in view of the presence of considerable concentrations of antioxidants in foods. The autoxidation of sulphite involves reactive free radical intermediates which include effective oxidizing agents. Thus, a pro-oxidant effect by the additive is possible and demonstrable in model system experiments.