The Food Safety Centre reminds people to ensure food is properly cooked, as natural toxins can be found in some plants. It said post-harvest processing and thorough cooking can destroy some natural toxins, or greatly reduce their toxicity.

The centre today released the results of a study on natural toxins in food plants. The centre's consultant Dr Ho Yuk-yin said different types of natural toxins can be found in different plants. If not handled properly, people consuming these can develop toxicological effects, including gastroenteritis, nausea and vomiting. In some severe cases, the central nervous system can be affected.

"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," he said.

Potato tests

Five types of potato samples - new, russet, red-skinned, and two kinds of yellow-skinned potatoes - were analysed. Glycoalkaloids detected in them, mainly in the peel, ranged from 26 to 88mg-per-kg. No detectable levels of glycoalkaloids were found in the flesh.

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

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

Since cooking cannot remove glycoalkaloids, people should avoid eating potatoes that show signs of sprouting, greening or rotting.

The study also 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 330mg-per-kg.

Food handling

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

According to the Department of Health, 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 species and products that contain 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.