NATURAL TOXIN SUBSTANCES IN EVERYDAY FOOD

Natural toxins are present in a wide variety of plants. Some of these plants are commonly consumed as food. These toxic substances when ingested in significant amount or when they are not processed appropriately can be potentially harmful to human health causing food poisoning. This study conducted by the Centre for Food Safety of the Food and Environmental Hygiene Department aimed to review natural toxins in food plants commonly consumed in Hong Kong and the measures that can be employed to prevent poisoning from consumption of these food plants. Laboratory study of two natural toxins, glycoalkaloids and cyanogenic glycosides, was carried out to determine the levels of these toxins in food plants commonly consumed in Hong Kong. The effects of preparation and cooking on the reduction of the toxin levels were also studied. Results showed that glycoalkaloid contents varied among the different types of the fresh potatoes tested which ranged from 26-88 mg/kg (average 56 mg/kg). This was within the normal range of glycoalkaloid contents in potatoes of 20 - 100 mg/kg, which JECFA considered that consumptions on a daily basis were not of concern. The highest concentrations of glycoalkaloids were found in potato sprouts. Cyanide was detected in bitter apricot seed, bamboo shoot, cassava, and flaxseed samples in their raw state at levels of 9.3 mg/kg to 330 mg/kg. Cyanide contents were found to be higher in bitter cassava than sweet cassava. Cyanide concentration was found to be highest at the tip portion of bamboo shoot, followed by the middle portion, then the base portion. Cutting cyanogenic food plants into small pieces and cooking them in boiling water reduced cyanide contents of the food commodities by over 90%. Dry heat could not reduce cyanide contents effectively and only reduced around 10% of the cyanide contents in flaxseeds following oven-heating for 15 minutes. Consumers should avoid buying or eating potatoes that show signs of sprouting, greening, physical damage or rotting since glycoalkaloids are not decomposed by cooking. Cutting the cyanogenic plants into smaller pieces and cooking thoroughly in boiling water help release toxic hydrogen cyanide before consumption. When the cooking method chosen is heating under dry-heat or at low moisture contents, limit the intake of the cyanogenic plants to only small amounts. KEY WORDS: glycoalkaloids, cyanogenic