Food Safety Focus (13th Issue, August 2007) – Food Safety Platform

Natural Toxins in Food Plants

Reported by Dr. Anna S.P. TANG, Research Officer,
Risk Assessment Section, Centre for Food Safety

Introduction

Natural toxins are present in a wide variety of plants, some of which are commonly consumed as food. These toxic substances when ingested can be potentially harmful to human health. From time to time, food poisoning cases due to plant toxins have been reported.

What are Natural Plant Toxins and Why are they Present?

Natural toxins may be present inherently in plants. They are usually metabolites produced by plants to defend themselves against various threats such as bacteria, fungi, insects and predators, which may be species specific and give the plant its particular characteristics, e.g. colours and flavours. Common examples of natural toxins in food plants include lectins in beans such as green beans, red kidney beans and white kidney beans; cyanogenic glycosides in bitter apricot seed, bamboo shoots, cassava, and flaxseeds; glycoalkaloids in potatoes; 4'-methoxypyridoxine in ginkgo seeds; colchicine in fresh lily flowers; and muscarine in some wild mushrooms.

Poisoning Caused by Natural Plant Toxins

These types of poisoning mainly occur in the following three scenarios:

1. Consumption of plants not intended for human consumption:

   Some wild plants, such as wild mushrooms and giant elephant ears, contain potent toxins that are not easily destroyed by cooking. These wild plants may be mistaken as edible plants. Severe symptoms may occur even when they are eaten in small amounts.

2. Consumption of food plants without proper cooking or processing:

   Green beans often cause poisoning if they are not thoroughly cooked before consumption. Cyanogenic plants such as bitter apricot seeds can cause food poisoning when eaten raw and in sufficient amount, but are safe for consumption when thoroughly cooked in boiling water, such as in Chinese soups. For plants such as cassava and bamboo shoots, toxic cyanide can be removed more effectively by soaking in water or by cutting into small pieces before cooking.

3. Consumption of plants where the toxins cannot be destroyed by cooking or processing:

   Some edible plants can also cause food poisoning when eaten in excess, where the toxins present cannot be effectively reduced through normal processing. Cases of poisoning have been reported following consumption of as few as ten gingko seeds because not all the toxins present can be readily
   destroyed by cooking. High levels of plant toxins may be present in some food plants such as potatoes when they are greened or sprouted. Since the toxins are heat stable, food poisoning can occur even when the potatoes are thoroughly cooked. Different natural toxins may cause different symptoms ranging from mild gastrointestinal symptoms to severe central nervous system symptoms. In general, whether poisoning will occur depends on the amount of the plant ingested, the level of toxins present and susceptibility of the individual. The level of toxins present in a plant can vary widely according to the species, growth conditions and geographical factors.

Advice to the Public

The public is advised to take measures for reducing risk prior to ingestion of food plants containing natural toxins (see Table 1), and not to pick and consume wild plants. Particular attention should be given to children, the elderly and individuals with poor health conditions. The public should follow the conventional ways of food processing that are known to be safe, and maintain a balanced diet.

Table 1: Measures for reducing risk associated with the consumption of food plants containing natural toxins

Illustration: Green bean

Illustration: Red kidney bean

Illustration: White kidney bean

Illustration: Soya bean

Illustration: Bamboo shoot

Illustration: Cassava

Illustration: Bitter apricot seed

Illustration: Flaxseed

Illustration: Greened potato

Illustration: Ginkgo seed

Illustration: Fresh lily flower (fresh Jin Zhen)