Recent developments in the understanding of the chemistry of sulphur(IV) oxospecies in dehydrated vegetables

Abstract

The understanding of the mechanism of action of sulphur(IV) oxospecies as an additive to dehydrated foods is complicated by the low water activity environment and the possible interconversions between sulphur(IV) oxospecies. An analytical procedure, involving combustion analysis, Monier-Williams analysis and the determination of sulphate ion, shows significant differences in ionic composition of the additive in dehydrated carrot and cabbage, blanched at pH 5 and pH 9–10 respectively, before dehydration. A kinetic model for the sulphite-inhibited Maillard reaction is discussed in relation to dehydrated vegetables and it is possible that the importance of hydroxysulphonate adducts in the inhibition of browning is greater in the dehydration of vegetables than in model systems composed of relatively dilute solutions of reactants. The possible involvement of sulphur-oxo free radicals in sulphited food systems has received little attention in the past. Solutions of sulphur(IV) oxospecies undergoing autoxidation are able rapidly to convert β-carotene to products containing 10 to 15 atoms of oxygen per β-carotene molecule when experiments are carried out in homogeneous solution. Significant differences exist between such homogeneous reactions and the heterogeneous systems formed when hydrocarbons are dispersed in foods. The conditions under which such systems are susceptible to ‘sulphite-mediated oxidation’ are discussed in order to reconcile the apparent anti-oxidant effect of the additive in many food systems with the pro-oxidant effect observed in model systems.