Risk in Brief - Naturally Occurring Toxins in Vegetables and Fruits

Naturally Occurring Toxins in Vegetables and Fruits

Introduction

  1. Fresh vegetables and fruits contain nutrients, vitamins and minerals essential for growth and health and are important components of a healthy diet. However, some vegetables and fruits may contain natural toxins which could be potentially harmful to our health. Every now and then, there have been reports of food poisoning cases suspected to have been caused by consumption of raw vegetables and fruits containing natural toxins.
  2. Natural toxins are poisonous substances present naturally in fruits and vegetables. They are produced by plants to defend themselves against fungi, insects and predators, and offer a protective mechanism for the plant.

What fruits and vegetables involved?

Fruits and vegetables commonly found with natural toxins include -

  1. Beans 豆類 (such as green beans 四季豆 (photo), red kidney beans 紅腰豆 (photos) and white kidney beans 白腰豆) - These varieties of beans contain naturally a toxin known as phytohaemagglutinin. Food poisoning caused by this toxin in raw and inadequately-cooked beans has a short onset time (1-3 hours) with symptoms of nausea, vomiting and diarrhoea. However, this toxic substance can be destroyed by subjecting the bean to thorough soaking and then cooking thoroughly at boiling temperature. Tinned beans which have been subjected to thorough heat-treatment are safe to eat without further cooking.
  2. Cassavas 木薯 (photo) - They refer to the edible root of cassava plants. Cassavas contain cyanogenic glycoside. The bitter type of cassavas has higher levels of toxins than the sweet type. When raw or inadequately-cooked cassavas are ingested, the toxin will be transformed into a chemical called hydrogen cyanide, which may result in food poisoning. Symptoms of cyanide poisoning occur within a few minutes and may include constriction of the throat, nausea, vomiting, headache, etc., and death has been reported in severe cases.
  3. Bamboo shoots 竹筍 (photo) - The toxicological profile of bamboo shoots is similar to that of cassavas. The natural toxin, cyanogenic glycoside, in fresh bamboo shoots may lead to food poisoning when consumed. To render them safe for consumption, fresh bamboo shoots should be sliced into smaller pieces and cooked thoroughly. Symptoms of poisoning are same as those by cassavas.
  4. Fruit seeds and stones* 果核和種子 - These refer to seeds or stones of apples, apricots, pears, plums, prunes, cherries, peaches, etc. The flesh of these fruits is not toxic, but the seeds (such as bitter apricot seeds 北杏 (photo)) and stone contain cyanogenic glycoside. When the consumer chews the fresh seeds or stone, the cyanogenic glycoside in it can be transformed into hydrogen cyanide, which is poisonous to the consumer. Young children are more susceptible and swallowing only a few seeds/stone may cause cyanide poisoning. Symptoms of poisoning are same as those by cassavas and bamboo shoots.
    (*A stone refers to the large hard seed in the middle of a fruit.)
  5. Fresh Jin Zhen 鮮金針 (photo) - It is the floral part of a plant harvested before it was in blossom. Fresh Jin Zhen has been used in dishes as fresh vegetables. A natural toxin known as colchicine is concentrated in the root of the plant and it is also found in the floral part of fresh Jin Zhen. Poisoning may be resulted from consumption of fresh Jin Zhen which has not been soaked well in water and cooked thoroughly. Symptoms of poisoning may include gastrointestinal discomfort, such as abdominal pain, vomiting and diarrhoea. However, colchicine in Jin Zhen is destroyed during cooking and processing and therefore commercially processed Jin Zhen and Jin Zhen in the dried form are non-toxic (photo).
  6. Potatoes 馬鈴薯 - Potatoes contain natural toxins called glycoalkaloids. The levels of these toxins are usually low and do not pose adverse effects in humans. However, potatoes that show signs of greening, sprouting, physically damaged or rotting may contain high level of glycoalkaloids and the majority of the toxins are present in the green area, in the peel, or just below the peel of the potatoes (photo). High levels of glycoalkaloids have a bitter taste and symptoms of poisoning may include a burning sensation in the mouth or severe stomach ache nausea and vomiting. Cooking and frying cannot destroy glycoalkaloids.
  7. Ginkgo seeds 銀杏/白果 - Ginkgo seeds contain toxins which are mainly neurotoxic. Vomiting, irritability, and tonic or clonic convulsions are the classical symptoms in ginkgo seeds poisoning and usually begin 1 to 12 hours after ingestion. Children are especially susceptible to this type of food poisoning. In severe cases where large amounts have been taken or in susceptible individuals, loss of consciousness and deaths may occur. It has been reported that ingestion of 10-50 of the cooked seeds at one time can cause acute poisoning in humans. The unripe and uncooked seeds are more toxic and the toxicity is reduced after cooking.

How to Reduce the Risk?

The risk of poisoning by natural toxins in fruits and vegetables can be avoided or significantly reduced by taking the following measures -

Selection of Foods

  1. Buy foods from reputable sources and do not patronise illegal hawkers.
  2. Do not buy green potatoes or potatoes which are sprouting.
  3. Do not eat vegetables and fruits raw or undercooked if they are usually consumed cooked.

Preparation and Consumption

  1. Cook beans such as green beans, red kidney beans and white kidney beans, cassavas, bamboo shoots thoroughly at boiling temperature after thorough soaking in clean water.
  2. Do not use raw or inadequately-cooked green beans or other bean species in the preparation of salad dishes. Always bear in mind a few raw beans can cause food poisoning symptoms.
  3. When eating fresh fruits, avoid eating seeds of fruits, such as apples, apricots, pears, etc., whereas the flesh of these fruits is nutritious and safe to eat.
  4. Cook bitter apricot seeds thoroughly and eat them in strict moderation.
  5. If fresh Jin Zhen is chosen in food recipe, soak it well in water before cooking and cook it thoroughly.
  6. Store potatoes in a dark, cool and dry place and avoid eating potatoes that show signs of greening, sprouting or rotting.
  7. Limit the intake of the ginkgo seeds to not more than a few seeds per day and avoid eating uncooked seeds, especially for children.
  8. Take a balanced and varied diet containing plenty of fruits and vegetables as they are nutritious and safe to eat after observing the above risk avoidance or reduction measures.

Risk Assessment Section
December 2005