Nonsulphite dipping

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Enzymic discoloration of fresh prepared produce is one of the major causes of quality loss and spoilage during post-harvest handling, processing and storage (Sapers, 1993; Laurila et al., 1998). PPO (EC is the enzyme primarily responsible for the discoloration of fresh prepared potatoes, apples, carrots, parsnips, swede, pears, mushrooms, bananas, peaches, grapes and lettuce, and this discoloration is often the shelf-life limiting quality attribute for these items (Duncan, 1999). PPO activity also results in detrimental changes to the texture and flavour of fresh prepared produce and losses of nutritional quality (Whitaker, 1996).

Given the deleterious effects of PPO activity upon the sensory and nutritional quality of fresh prepared produce, it is not surprising that considerable research has been devoted to inhibit the activity of this enzyme (Duncan, 1999). Sulphites have long been used as food additives to inhibit enzymic and non-enzymic discolorations, to control the growth of microorganisms and to act as bleaching agents and antioxidants (Sapers, 1993; Laurila et al., 1998). The most frequently used sulphiting agents for fresh prepared produce are sodium and potassium bisulphites and metabisulphites. Sulphites act as PPO inhibitors and antimicrobial agents and are most effective in acidic conditions (e.g. pH 3-5). For low-acid (e.g. pH 5-8) fresh prepared produce items such as mushrooms, bananas, potatoes and lettuce, sulphites have the tendency to accelerate bacterial decay by adversely affecting cell wall or membrane integrity which may stimulate the growth of certain spoilage bacteria (Duncan, 1999). Also, there are several negative attributes associated with sulphite use which has led to decreased consumer acceptance. In particular, sulphites can induce severe allergic reactions or even anaphylactic shock in a proportion of the asthmatic population (Sapers, 1993). Consequently, the adverse health effects of sulphite consumption have resulted in stricter regulatory restrictions and consumer labelling requirements (Anon., 1991).

The increased regulatory restrictions on the use of sulphites have created an urgent need for safe, practical and functional alternatives which are economically viable (Ahvenainen, 1996). Proprietary chemical non-sulphite formulations (containing, for example, mixtures of ascorbic acid or erythorbic acid or their sodium salt in combination with citric acid, malic acid, tartaric acid, succinic acid, calcium chloride, sodium chloride, 4-hexylresorcinol, sodium acid pyrophosphate and/or cysteine hydrochloride) are commercially available but further research is required to optimise appropriate formulations and dipping protocols for fresh prepared produce items. New opportunities exist for the use of approved starch and pectin-based edible coatings and safe biological agents such as enzymes and PPO inhibitors produced by lactic acid bacteria (Ahvenainen, 1996; Laurila et al., 1998).

It should be appreciated that different produce cultivars show large differences in their tendency to discolour after tissue wounding upon preparation. Such differences can be exploited by selecting raw material cultivars that have a low tendency to discolour after preparation so that treatments to inhibit enzymic discoloration can be minimised (Sapers, 1993). In addition, research has demonstrated that combining chemical non-sulphite dipping treatments with optimal MAP yields extended shelf-life and quality benefits greater than those achieved with either dipping or MAP alone (Duncan, 1999). Such combination treatments are likely to be the focus of future research aimed at minimising enzymic discolorations and maximising the maintenance of fresh prepared produce quality.

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