Results of study on natural toxins in food plants

The Centre for Food Safety (CFS) today (March 13) released results of a study on natural toxins in food plants.

The Centre's Consultant (Community Medicine) (Risk Assessment and Communication), Dr Ho Yuk-yin, said different types of natural toxins might be found in different food plants. If not handled properly, people consuming these plants could develop toxicological effects, including gastroenteritis, nausea and vomiting. In some severe cases, the central nervous system could also be affected.

But Dr Ho stressed that some measures, including post-harvest processing and thorough cooking, could destroy some of these natural toxins, or greatly reduce their toxicity.

"Common examples of natural toxins found in food plants include glycoalkaloids in potatoes and cyanogenic glycosides in bitter apricot seeds, bamboo shoots, cassavas and flaxseeds.

"Our current study focuses mainly on these two types of natural toxins and their levels in the food plants mentioned. It also assesses the effect of preparation and cooking in the reduction of the toxin levels," he said.

Five types of potato samples (new potato, russet potato, red-skinned potato, and two kinds of yellow-skinned potato) were taken by the CFS for analysis. Glycoalkaloids detected in these potatoes, mainly in the peel portion, ranged from 26-88 mg/kg. No detectable levels of glycoalkaloids were found in the flesh.

"Such levels of glycoalkaloid should not pose any adverse health effects. According to the Joint Food and Agriculture Organisation (FAO) / World Health Organisation (WHO) Expert Committee on Food Additives (JECFA), daily intake of potatoes, with normal glycoalkaloid levels (20-100 mg/kg), should not be of concern.

"However, results of sprouted potatoes are different. Glycoalkaloids, as high as 7,600 mg/kg, were found in the sprouts of red-skinned potato samples.

"Cooking cannot remove glycoalkaloids, so people should avoid eating potatoes that show signs of sprouting, greening or rotting," he said.

Meanwhile, the study showed that bitter apricot seed, bamboo shoot, cassava and flaxseed samples, in their raw state, contained releasable cyanide at levels ranging from 9.3 to 330 mg/kg.

Dr Ho said that to prevent toxicological effects, people could cut these food plants into small pieces and cook them thoroughly in boiling water. This method could reduce more than 90% of cyanogenic glycosides in food plants. For flaxseeds which are usually eaten raw, people should limit the intake to only small amounts.

Dr Ho said some other commonly consumed food plants like green beans, red kidney beans, and white kidney beans contained lectins, the toxicity of which could be reduced by thorough soaking and cooking. Colchicine was found in fresh "jinzhen", but drying would remove this toxic substance. Dried "jinzhen" was suitable for consumption. Cauliflower, broccoli and cabbage contained goitrogens. Soyabean contained trypsin inhibitors. Cooking could reduce the level of these toxins. Different types of wild mushroom contained different types of toxins, such as muscarine, most of which could not be destroyed by cooking. Therefore, people should avoid eating mushrooms picked in the wild.

According to the Department of Health, some 100 food poisoning cases due to natural toxins (ciguatoxin and tetrodotoxin) and histamine were reported in the past three years. Dr Ho reminded people not to eat large amounts of coral reef fish and not to handle or cook for themselves puffer fish and other fish species and fish products that contained tetrodotoxin.

Formation of histamine in fish is usually related to improper storage. It is commonly found in fish species such as tuna, mackerel, sardine and anchovy. To minimise the risk, fish products should be properly packaged and stored at four degrees Celsius or below.

Ends/Tuesday, March 13, 2007