Effect of sulphur dioxide on the activity of trypsin

Effect of wine inhibitors on free pineapple stem bromelain activity in a model wine system

The influence of potential inhibitors, naturally present in wine, on the activity of stem bromelain was investigated in order to evaluate the applicability of this enzyme for protein stabilization in white wine. Bromelain proteolytic activity was tested against a synthetic substrate (Bz-Phe-Val-Arg-pNA) in a model wine system after adding ethanol, sulfur dioxide (SO(2)), skin, seed, and gallic and ellagic tannins at the average range of their concentration in wine. All the inhibitors of stem bromelain activity tested turned out to be reversible. Ethanol was a competitive inhibitor with a rather limited effect. Gallic and ellagic tannins have no inhibitory effect on stem bromelain activity, while both seed and skin tannins were uncompetitive inhibitors. The strongest inhibition effect was revealed for sulfur dioxide, which was a mixed-type inhibitor for the enzyme activity. This study provides useful information relative to a future biotechnological application of stem bromelain in winemaking.

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.

Inhibition of browning by sulfites

The present state of understanding of the mechanisms by which sulfites inhibit browning reactions in food is reviewed. The difficulties of specifying the composition of sulfur(IV) oxospecies in sulfited foods arise from the existence of labile equilibria between SO2, HSO3-, SO3(2-) and S2O5(2-), whose position depends on concentration, ionic strength and the presence of non-electrolytes. A proportion of the additive is also found in a reversibly bound form. The main reason why sulfites are able to inhibit a wide range of browning reactions is the nucleophilic reactivity of sulfite ion. The mechanism of reactions between sulfite species and intermediates in the model Maillard browning reaction, glucose + glycine, are considered in depth and are supported by kinetic data. A most interesting feature is the fact that sulfites seem to catalyse the reactions they are added to control. Reaction products include 3,4-dideoxy-4-sulfohexosulose which is formed initially and polymeric substances arise from the reaction of sulfite species with melanoidins. It is found that melanoidins from glucose + glycine react with sulfite to such an extent that one sulfur atom is incorporated for every two glucose molecules used to make up the polymer. The mechanisms of inhibition of ascorbic acid browning, enzymic browning and lipid browning are reviewed briefly. The known toxicological consequences of the formation of reaction products when sulfites are used for the control of Maillard browning give little cause for concern. Little is known of the implications of the formation of reaction products during the inhibition of other forms of browning. Consideration of the requirements for alternatives to sulfites is given.