Sulphur dioxide: Regulations, testing and limitations

The term ‘sulphites’ is often used to refer collectively to substances with E-numbers 220 to 228 – for example, sodium metabisulphite (E224). Sulphite-containing substances have been used as functional ingredients in many food products, because they are efficient antimicrobials, enzyme inhibitors, bleaching agents, oxygen scavengers and reducing agents.

After sulphites are added to a food, eventually they are broken down to sulphur dioxide, which is the active component that preserves foodstuffs. Sulphur dioxide and sulphites can also occur naturally in some foods.

Although sulphites are chemical compounds and not proteins, as with most food allergens they are considered allergens in certain countries. This is due to adverse reactions that have been reported when sulphites or sulphur dioxide have been ingested, inhaled or injected into the body. The reactions may be immediate or delayed, and the recorded symptoms include dermatologic (skin), respiratory and gastrointestinal signs and symptoms.

Under the South African Foodstuffs, Cosmetics and Disinfectants Act (54 of 1972), Regulations Relating to the Labelling and Advertising of Foodstuffs, R146 of 2010, sulphur dioxide and related compounds are classified as ‘additives’ and must be declared on a label if the food product contains more than 10mg per kilogram of foodstuff as packed or ready to eat. However, it must be declared on the product label irrespective of the level present if it is transferred to the foodstuff through contact with the packaging material; or in certain cases, where fresh fruits or vegetables are treated with sulphur dioxide. The regulations relating to Preservatives and antioxidants, R. 965/1977, as amended, also provide specified levels of sulphur dioxide for particular foods.

Testing can be used as a measurement tool for sulphites, to determine whether a food product complies with regulations. The Optimised Monier-Williams method is one of the most well-known and widely used methods for quantifying sulphites in foodstuffs. It has a variety of advantages, including that accurate and reproducible results can be achieved continually once the analyst has familiarity and experience with the method.

Numerous modifications of this method exist for application to specific matrices, but there are discrepancies that can occur in test results. The results are dependent on the treatment given to a matrix when performing the method. Factors include distillation time, temperature, acid used and collection medium, among other variables. These contribute to deviations in test results, and there is evidence indicating that the Monier-Williams method is prone to overestimating sulphur dioxide in certain food matrices.

The class of methods that require prolonged distillation from a suspension or solution of the food in hot mineral acid dates back to the original publication of Monier-Williams. Although these methods are notably successful, there are a few foods that produce a false-positive result. These may include (but are not limited to) Allium and Brassica vegetables and isolated soya protein. Methods that rely on distillation cannot distinguish between sulphur dioxide derived from added mineral sulphite, and sulphur dioxide released as the result of hydrolysis of naturally occurring compounds. Fresh cabbage and onions typically give results above 10ppm, even in the absence of sulphite treatment. A greater range of results has been noted with the use of untreated garlic. Food products that contain substantial amounts of volatile fatty acids have also been reported to interfere with sulphur dioxide results.

There are many methods for determining sulphite levels in foodstuffs. They all require some means of isolating and/or capturing sulphur dioxide, and then quantifying the level found. Numerous attempts to differentiate between added and naturally-occurring sulphites have been unsuccessful. There are multiple food-matrix factors, such as processing, that may contribute to a deviation in results. Testing circumstances – for example, an operational error due to the manual nature of some of the analysis steps, with certain equipment set-ups – can also influence results.

All of these method limitations and matrix elements must be taken into consideration when submitting a food sample for sulphur dioxide testing.

FACTS can assist you with the correct solution for sulphur dioxide analysis of products, as this test falls within the scope of our facilitated testing services. If you require more information, please contact us.