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Focus on sulphur dioxide and sulphites

Sulphur dioxide and sulphites (E220 – E228) are chemical compounds, that are naturally occurring (sulphur dioxide is naturally present in wine or beer for example) and are most commonly used as preservatives in a wide range of foods. They are used to extend the shelf life of products as well as to kill bacteria, and are popular due to the fact that they will maintain the colour in products and can have a softening effect, which is particularly desired within the dried fruit market.

Unlike most allergens, protein or DNA cannot be extracted from sulphites – meaning that a highly sensitive analytical chemistry route is the only way to screen for them in food and food products.

Although there are historic techniques for determining sulphites, they have known limi­tations and accuracy issues with the limits set out in Annex II (as amended by Commission Dele­gated Regulation No. 78/2014).

Traditional analysis and its limitations

Sulphites are determined in foods by relatively straightforward pro­cedures. Most commonly, these involve a distillation process in which the sample is dispersed in an acidified solution to convert all sulphite forms to sulphurous acid (a solution of sulphur diox­ide) and the free sulphur dioxide is distilled over into a trapping solution to be quantified. In the Monier-Williams method, a standard approach used by many laboratories, the sulphur dioxide over whether the conventional titration with sodium hydroxide is measuring solely sulphuric acid or potentially other acid species co-distilled into the trapping solution.

This extra confidence is extremely important for the reliable measurement of low sulphite concentrations around the legal declaration limit.

The future of sulphites

With the UK food market moving to an increasingly health aware stance, as well as the growing popularity of the ‘food-to-go’ market, the demand for dried fruits and vegetables (and prod­ucts derived from these such as fruit bars) is increasing.

This means that manufacturers are looking for ways to prolong shelf life and therefore the profitability of their products – something that sulphites, when used properly, are extremely adept at doing for they maintain colour and texture whilst killing bacteria.

With new and sensitive techniques, manufacturers are able to effectively monitor sulphites within their products to ensure that they maximise their product potential whilst conforming to legislation.

The sensitivity of these new techniques also ensures that consumers have the confidence that they can safely devour products of their choice.

Dr Mike Jordan FRSC Head of Food Chemistry, PAS

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