The strong, bold flavours of preserved and pickled food products tickle many people's taste buds, but beyond the taste, what kinds of chemical are we putting into our body?


Sulphur dioxide is a commonly used preservative in preserved and pickled food, but oddly enough, it is not applied directly onto food. In fact, sulphur dioxide is released from a food additive — sulfite — which is used to prevent or slow down the growth of micro-organisms, such as mould or bacteria, in food. It can also inhibit, retard or arrest the process of fermentation, acidification or other food deterioration processes so as to extend the expiry date. Some sulfites turn into sulphur dioxide, which can be measured to give an estimate of the amount of additives in food.

Sulphur dioxide is usually added to dried produce and fruits, pickled vegetables, processed food such as sausages, hamburger patties and even wine and beer. According to the Preservatives in Food Regulation, it is an offence to add sulphur dioxide to fresh or chilled meat, and the amount cannot exceed 10mg/L. Moreover, according to the Food and Drugs (Composition and Labelling) Regulations, the preservative or antioxidant’s functional class and its name or number as per the International Numbering System for Food Additives (INS) should be stated in the ingredient list of most prepackaged foods. Sometimes, the prefix "E" or "e" may be added. For instance, if sulphur dioxide is added to the food as a preservative, it could appear in the ingredient list as Preservative (sulphur dioxide), Preservative (220), Preservative (E220) or Preservative (e220).

Dr Kelvin Leung, Associate Professor of the Department of Chemistry said, “Sulphur dioxide has low toxicity. Since it can be metabolised by the human body and discharged by way of urine, the impact of residual sulphur dioxide in food on health is likely to be insignificant. Yet, intake of sulphur dioxide by allergic individuals may result in headache, vomiting and diarrhoea, and may even trigger asthma for allergic children.”

Since sulphur dioxide is water-soluble, Dr Leung suggests immersing dried food in water for 30 minutes or more, and rinsing it again before cooking. Furthermore, sulphur dioxide evaporates, so cooking food for three minutes has the desirable effect of removing most of the residual sulphur dioxide in food.

Dr Leung also suggests consumers not to choose food with long shelf life or sharp colours. With growing awareness of food safety, one could also seek out quality and reliable testing organisations with a good reputation to conduct food safety testing. There are a number of certified food testing organisations in Hong Kong such as The Chemical Testing Services (CTS) established under the Faculty of Science, Hong Kong Baptist University which has been granted the status of an accredited laboratory under The Hong Kong Laboratory Accreditation Scheme (HOKLAS). Apart from testing food for heavy metals, pesticide residues, antibiotics and microorganisms, CTS is accredited by HOKLAS to conduct food tests, such as chemical analysis of sulphur dioxide concentration, to ensure it meets food safety and quality standards.

The Chemical Testing Services (CTS), Hong Kong Baptist University

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