sodium metabisulphite is currently listed as an authorised food additive with maximum permitted residues in crustacean products of 150ppm. Sodium metabisulphite is regarded as one cause of asthma attacks and can trigger allergic reactions in vulnerable individuals. In response to this, EU Directive (2003/89/EC), which came into force on 25 November 2005, will make allergen labelling a requirement for all metabisulphite treated foods. These regulations will require that foods treated with metabisulphite are clearly displayed as such on the final packaging.

Following recent court cases relating to high levels of residue present on prawns and new requirements for allergen labelling, there is now significant interest within the industry to source an alternative treatment. Further motivation has come from the highly corrosive nature of sodium metabisulphite, leading to high maintenance costs of both vessels and processing machinery and hazardous conditions during operation.

A number of treatments appeared to perform competitively with sodium metabisulphite for inhibiting melanotic blackening. Selected alternatives provide comparable shelf life to high concentrations of sodium metabisulphite. However, prawns treated with 5% metabisulphite lose some visual qualities through bleaching and attain high sulphite residues, often above the permitted 150ppm. Neither of these qualities are evident with alternative treatments. Furthermore, prawns with high sulphite residues, through treatment with high concentration sodium metabisulphite, were found to have poor flavour.