Toxicity in Foods

Toxicity in Foods

NATURALLY-OCCURRING TOXICANTS IN FOODS

Many plants and animals that man uses for food contain as natural constituents chemical substances known to have toxic properties. By experimentation man has learned to avoid dangerous exposure to the natural chemical components of his foods. Although acute poisoning is usually avoided, the public health significance associated with naturally-occurring toxicants is usually in the realm of chronic toxicity. A number of possible cause and effect relationships of this kind exist, but usually they only point to the presence of a toxicant. This is true because the intake is too small to cause severe effects, and because chronic effects are difficult to identify. Recognition of factors in the environment that may affect public health is basic to the eventual control of those factors. Some naturally-occurring toxicants have already been identified and are listed under the following categories.

Antienzymes or Inhibitors

Seed proteins such as cereal grains, legumes, and oil seeds are important sources of dietary protein in many areas of the world but some of them contain enzyme inhibitors. Perhaps the best known and most extensively studied of these toxic factors in seeds used as food are the trypsin inhibitors. Trypsin is an enzyme involved in protein digestion and trypsin inhibitors can result in a decreased availability of protein. However, under conditions of controlled processing the antitryptic factor in the seeds can be partially or completely altered and the nutritional value improved. Trypsin inhibitors, which for the most part are completely destroyed by heat, have been reported in wheat flour, soybeans, lima beans, mung beans, peanuts, oats, buckwheat, barley, sweet potatoes, garden peas, corn and white potatoes. Chymotrypsin is another enzyme involved in protein digestion and a chymotrypsin inhibitor has been found in potatoes.

Carcinogens

In the past three decades a large number and variety of synthetic chemical carcinogens have been discovered, and the increasing use of chemicals in the modern world has caused much concern about their hazard to man. Until recently little attention has been given to the possibility that various forms of life might produce agents which give rise to tumors in other living systems. Today we are aware of a small but increasing number of such agents. Some of these agents could find their way into human food, and indeed this has occurred in some instances. Aflotoxin, a type of mycotoxin, is produced by the organism, Aspergillus flavus. The fungus usually occurs on peanuts, corn, and wheat that have been improperly stored and where the temperature and humidity favor its growth. The carcinogenicity of aflatoxins and their sporadic occurrence in food have given rise to considerable concern. Some toxin appears in milk from cows fed toxic meals, and trace amounts of aflatoxins have been reported in some peanut butters. Commercial peanut butter producers carefully monitor their peanuts and have instruments that can detect aflotoxin at one-half part per billion. Peanut oils are free of the toxin because the alkaline treatment used in processing destroys the toxins. Toxic corn has also been reported, and in the United States there have been cases of livestock toxicity from moldy feed.

Cholinesterase Inhibitors

The cholinesterases represent a group of enzymes that are of great significance in both the physiological and economic sense. Their principal characteristic appears to be control of the conduction of nerve impulses, a function that makes them of unique importance to the lives of both higher and lower animals. The inhibition of cholinesterases is one of man's more powerful chemical weapons against insect enemies. Two of the general classes of synthetic organic insecticides, the organophosphates and the carbamates, owe their effectiveness to this mechanism. Study of the chemistry and toxicology of natural cholinesterase inhibitors such as glycoalkaloids, is important, for they represent a real hazard under certain conditions. Several instances of poisoning have been attributed to human consumption of "green" potatoes high in solanine. The toxic properties of green potatoes is not due to the green color, however, for the greening is due to chlorophyll which often accompanies the synthesis of the toxic sub- stance solanine, a glycoalkaloid. Glycoalkaloid synthesis increases when the tuber is exposed to stress conditions such as light, cutting, or bruising. Potato varieties differ widely in their glycoalkaloid content and some varieties have had to be taken off the market because their glycoalkaloid content exceeded the level considered safe for human consumption; a glycoalkaloid level above 20 milligrams per 100 grams of fresh weight is considered unsafe. The fruit of eggplant and the root and leaves of tomatoes also contain cholinesterase inhibitors.

References

~Dack, G. M. Food Poisoning, 3rd ed. The University of Chicago Press: Chicago, IL, 1956.
~Committee on Food Protection, Food and Nutrition Board, National Research Council, National Academy of Sciences, Toxicants Occurring Naturally in Foods. Washington, DC, 1973.
~Liener, Irvin E., Ed. Toxic Constituents of Plant Foodstuffs, 2nd ed., Academic Press: New York, NY, 1980.
~Committee on Nitrite and Alternative Curing Agents in Food, National Academy of Sciences, The Health Effects of Nitrates,
Nitrite, and N-Nitroso Compounds. Washington, DC, 1981.