Difficulties with testing for sulphite in some foods

Sulphites occur naturally in some foods but are added artificially to wines, nuts and dry fruits, as well as to some seafoods and meats, to help preserve the colour and appearance of food and reduce spoilage. Sulphite can cause severe allergic reactions in people with sulphite allergies. Those with weakened respiratory systems, such as asthmatics, are especially at risk. Research from the US Centre for Disease Control suggests that approximately 5% of asthmatics are at risk for severe reaction to sulphites.

There are five well-known methods for the determination of residual sulphite in foods, however because each method has its own characteristic specificity, results can differ considerably. It is well known that added sulphite is present in foods both in free and combined forms. Some methods appear to be only suitable for the determination of free sulphite, or only for free and weakly-combined sulphites.

The optimised Monier-Williams method is widely used in the determination of sulphite content, expressed as sulfur dioxide, in foodstuffs in which the content of sulphite is at least 10 mg/kg. The method is applicable in the presence of other volatile sulfur compounds, but is not applicable to cabbage, dried garlic, dried onions, ginger, leeks and soy proteins. False positive results have also been reported in food products with high levels of volatile fatty acids such as butter flavourings.

Earlier this year, the Hong Kong Government published on their website a briefing session on analysis of sulphur dioxide in food. This presentation sets out the International Standard methods for the determination of sulphites, describes some of these methods in detail, plus provides information about validated standards and proficiency tests for the determination of sulphites in foods. As with many chemical detection methods, it is important that the laboratory performing the test validates and if necessary, optimises the test procedure for the sample type being tested.