Natural Toxins in Fruits and Vegetables
Most fruits and vegetables produce small amounts of natural toxins which are of no concern when the food is consumed as part of a varied diet and eaten in moderation. The reason for the presence of these plant-produced natural toxins is not always known. In some foods, a toxin is present as a naturally occurring pesticide to ward off insect attacks. A toxin may also be formed to protect the plant from spoilage when damaged by weather, handling, UV light or microbes. Moldy or damaged plants, for example, may contain more natural toxicants than uninfected or undamaged plants.
Familiar foods can sometimes be unsafe as a result of eating the wrong plant part, improper preparation, inadequate cooking or abnormal eating patterns. While many Canadians know not to eat rhubarb leaves or potato sprouts, newcomers to Canada may not be aware of the potential danger of some of our traditional foods. Likewise, adventurous eaters who try unfamiliar ethnic foods, or plants collected from the wild, first need to learn how to prepare and cook such foods safely. Unusual eating patterns (such as fad diets that put undue emphasis on a single food) may also cause harmful effects if natural toxicants in foods are ingested above safe levels.
Many kinds of raw or dried beans and lentils contain a harmful toxin called lectins (Phytohaemagglutinin) that may cause an upset stomach. The highest concentrations are in red and white kidney beans and soybeans. Lectins are destroyed by soaking the beans for at least five hours and then boiling them briskly in fresh water for at least 10 minutes. Do not cook beans at a low temperature, as it may not destroy the toxin. Improperly cooked beans can be more toxic than raw ones. Canned beans can be used without further cooking.
Raw lupini beans contain a harmful toxin called alkaloids that may produce symptoms of dizziness, dry mouth and vomiting when ingested. Alkaloids can be dissolved in water. A minimum of 7 days soaking is required for lupini beans and the water should be changed daily. After soaking the beans may be cooked until soft.
Rhubarb leaves contain a harmful toxin called oxalic acid and anthraquinones that can cause severe poisoning and kidney damage which may be fatal. Therefore, do not eat the leaves; the stalks are the edible part of the plant.
Cassava is also known as yucca, tapioca (a processed product of cassava), gaplek or manioc. Cassava contains a naturally occurring toxin called cyanogenic glycosides in raw or unprocessed cassava that may cause breathing difficulties, staggering, or paralysis and may be fatal. The amount of toxin depends on the variety and growing conditions. "Bitter cassava" contains toxic levels of the compounds than the small-rooted non-bitter variety, which is sometimes called sweet cassava. Much of the toxin in the non-bitter varieties may be removed by peeling and cooking the tuber in boiling water. Bitter cassava needs special processing techniques (e.g., soaking, fermenting, cooking) to reduce toxicity.
Bamboo shoots, a traditional ingredient of Asian cuisine, are sourced from the underground stems of the bamboo plant. Bamboo shoots contain a type of naturally occurring toxin (cyanogenic glycosides) that can lead to exposure to hydrogen cyanide and its related toxicity. Fresh bamboo shoots should be sliced in half lengthwise; the outer leaves peeled away and any fibrous tissue at the base trimmed off. The remaining fresh shoots should then be thinly sliced into strips and boiled in lightly salted water for eight to ten minutes.
Fruit pits, Seeds and Twigs:
Peach, apple, cherry, plum, apricot seeds, shoots or twigs contains a glycoside, amygdalin, which on hydrolysis yields hydrogen cyanide. Therefore, eat only the fleshy part and skin of fruit, raw or cooked. Health Canada produced an advisory about the safety of bitter apricot kernels in 2009 due to an associated illness.
Comfrey leaves contain toxic alkaloids whose long term concern may cause cancer. Therefore, do not eat them.
Bracken Ferns and Fiddleheads:
Young bracken ferns are sometimes mistaken for fiddleheads which are young ostrich ferns. Bracken ferns contain thiaminase, a substance which destroys thiamine (Vitamin B1) so can be toxic and should not be eaten. Raw or undercooked fiddleheads may cause also cause illness, as reported in an outbreak investigation, but health officials are not sure what causes the illness. For instructions on handling fiddleheads, refer to Health Canada's Food Safety Tips for Fiddleheads.
Potatoes contain the natural toxin, solanine which is a glycoalkaloid. Solanine levels are usually low but higher levels are present in the sprouts, eyes, peel and where there is green coloration. Areas that are green likely have higher concentrations of solanine than non-green areas. This is because exposure to light encourages both glycoalkaloid and chlorophyll production. The chlorophyll produces the green discolouration which indicates the possible presence of solanine.
Glycoalkaloids produce a bitter taste that causes a burning sensation in the mouth and can cause stomach upset, vomiting and headaches; they may also cause death. Glycoalkaloids are not destroyed by cooking, so it is important to remove any green or damaged parts, and eyes and sprouts, before cooking. Solanine is water soluble so some will be removed during boiling of potatoes. Do not eat cooked potatoes that taste bitter and do not use potato water from potatoes that were green or had eyes or sprouts. Remember to store potatoes in a dark place to avoid light-induced glycoalkaloids, and store in a cool and dry place to slow the production of eyes/sprouts.
Kumara, a member of the sweet potato family, can produce toxins in response to injury, insect attack and other stress. The most common toxin, ipomeamarone, can make the kumara taste bitter. There have been reports of cattle deaths after they have eaten moldy kumara. The toxin levels are usually highest near the area of damage. It is recommended that any damaged parts on kumara are removed before cooking. Do not eat it if it tastes bitter after cooking.
Parsnips commonly contain a group of natural toxins known as furocoumarins. These are probably produced as a way of protecting the plant when it has been stressed. The concentration of the toxin is usually highest in the peel or surface layer of the plant or around any damaged areas. One of the furocoumarin toxins can cause stomach ache and may also cause a painful skin reaction when contact with the parsnip plant is combined with UV rays from sunlight. It is important to peel the parsnip before cooking and remove any damaged parts. The levels of toxin are reduced when the parsnip is cooked by baking, microwaving or boiling. Discard any cooking water.
Zucchini may occasionally contain a group of natural toxins known as cucurbitacins. These toxins give zucchini a bitter taste. Bitterness in wild zucchinis has been known for a long time but is rarely found in commercially grown zucchinis. Eating bitter zucchinis have caused people to experience vomiting, stomach cramps, diarrhea and collapse. Do not eat zucchini that have a strong unpleasant smell or taste bitter.
Health Canada’s Food Guide:
It is important that you include a wide variety of fresh fruit and vegetables in your diet. Eating fresh fruits and vegetables reduces the risk of serious health problems such as heart disease, cancer, diabetes and obesity.
Health Canada recommends that adults eat at least 7 to 8 servings of fruits and vegetables each day. It is suggested to choose at least one dark green and one orange vegetable each day to obtain enough of the vitamin folate and vitamin A. Dark green vegetables include arugula, broccoli, chard, dandelion greens, gai lan, kale/collards, mustard greens, and salad greens including romaine lettuce, spinach or mesclun mix. Orange vegetables include carrots, sweet potatoes, squash, pumpkin or yams. Apricots, cantaloupes, mangoes, nectarines, papaya and peaches also contain vitamin A and can replace an orange vegetable.
A pulse is an edible seed that grows in a pod. Pulses are a great source of protein for vegetarians, but they are also a very healthy choice for meat-eaters. Pulses include the whole range of beans, peans and lentils. Pulses count as one of the variety of at least 7 portions of fruit and vegetables we should be aiming to eat each day (3 heaped tablespoons of pulses = one portion). But pulses can only make up a maximum of one portion a day. This is because we need to eat a variety of fruit and vegetables to obtain all the nutrients we need.
Canadian Food Inspection Agency. (2005). Natural Toxins in Fresh Fruit and Vegetables. Retrieved fromhttp://www.inspection.gc.ca/english/fssa/concen/specif/fruvegtoxe.shtml
Food Safety Network. (2005). Fact sheet on Cassava. Retrieved from http://www.foodsafetynetwork.ca/articles/533/cassava_factsheet.pdf Now at: http://www.uoguelph.ca/foodsafetynetwork/sites/uoguelph.ca.foodsafetynetwork/files/cassava_factsheet.pdf
Grace, M.R. (1977). "Cassava Processing", FAO Plant Production and Protection Series No. 3. Food and Agriculture Organization. Rome, Italy. Retreived from http://www.fao.org/docrep/X5032E/x5032E00.htm.
Health Canada. (2008). Cyanide. Retrieved from http://www.hc-sc.gc.ca/ewh-semt/pubs/water-eau/cyanide-cyanure/index_e.html
Health Canada. (2008).Health Canada’s Food Guide. Retrieved from http://www.hc-sc.gc.ca/fn-an/food-guide-aliment/choose-choix/fruit/tips-trucs-eng.php
New Zealand Food Safety Authority. (n.d.).Natural Toxins in Food. Retrieved fromhttp://www.nzfsa.govt.nz/consumers/chemicals-toxins-additives/natural-toxins/index.htm
U.S. Food and Drug Administration. (1992). Phytohaemagglutinin. Retrieved fromhttp://www.cfsan.fda.gov/~mow/chap43.html For updated link, click here.
Date modified: 2012-07-09