Harmful substances

Substances in foods are termed harmful which can, at least potentially, have a harmful effect on human beings. Harmful substances are not necessarily foreign substances, i.e. ones that contaminate food through outside influence. Substances with toxic effects also occur naturally in some food plants.

As with foreign substances, it is primarily the amount ingested that decides whether a harmful substance indeed represents a danger to health.

Nitrate, nitrite, nitrosamines

Nitrates are nitrogen compounds that occur naturally in the earth. Plants require them as a source of nitrogen in order to form protein. Thus the plants themselves have a certain natural nitrate content, which varies with each plant. From this point of view, nitrates are not foreign substances to begin with.

They are nevertheless usually listed under foreign substances, due to the fact that a major portion of the nitrate enters the plants through fertilization with nitrogen - and thus again through outside influence. Particularly many vegetable plants thus have large concentrations of nitrate. Vegetables are divided into those with high, medium and low nitrate content (see Table).

Vegetables with high nitrate content
(i.e. 1000-4000 mg/kg fresh goods)
Vegetables with medium nitrate content
(i.e. 500-1000 mg/kg fresh goods)
Vegetables with low nitrate content
(i.e. less than 500 mg/kg fresh goods)
Iceberg lettuce
Curly endive, escarole
Lamb's lettuce, corn salad
Butterhead lettuce
Swiss chard
Small radishes
Chinese cabbage
Collard, kale
White cabbage
Red cabbage
Savoy cabbage
Chinese cabbage
Collard, kale
White cabbage
Red cabbage
Savoy cabbage

The nitrate content of plants depends on various factors. Some vegetables are by nature relatively rich in nitrate (e.g. different salads, small radishes, beetroot) and then accumulate even larger concentrations of nitrate through fertilization. In contrast, others such as tomatoes or cucumbers are poor in nitrate. In addition, solar radiation has an influence on the nitrate content. The highest contents are found during the spring and fall months with less light. Furthermore, the nitrate content of vegetables from greenhouse cultivation is usually higher that with outdoor cultivation.

Nitrate itself is not harmful for human beings. Under certain conditions, however (above all through the influence of intestinal bacteria or those already in the food), poisonous nitrite can be generated from nitrate.

Nitrite impedes the transportation of oxygen in the blood. Possible results of nitrite poisoning are nausea, stomach trouble and difficulty in breathing. One must be particularly careful to avoid nitrite poisoning in young children. The cause of such toxicity could be the consumption of spinach or carrots that have been over-fertilized with nitrogen, especially if these foods are reheated. Fortunately, serious symptoms of toxicity (cyanosis) practically never occur, but to be on the safe side, leftover baby foods consisting of vegetables with a high nitrate content should never be saved and reheated.

For their part, together with certain products of protein degradation, nitrites can be transformed further into so-called nitrosamines in the digestive tract. These are compounds which have been proven to be carcinogenic in animal experiments. To a large extent, the formation of nitrite and thus also of nitrosamines can be prevented if foods containing nitrate are at the same time rich in vitamin C.

Tips for reducing the content of harmful and foreign substances in fruits and vegetables

  • Always wash fruits and vegetables thoroughly in order to remove superficial residues and dirt.
  • The nitrate content of leafy vegetables can be greatly reduced if stalks, stems, large ribs and the outer leaves are removed.
  • Pour away the water in which nitrate-rich vegetables have been cooked.
  • Choose domestic products if possible; buy fruits and vegetables when they are naturally in season and whenever possible products from outdoor cultivation.

Oxalic acid

Oxalic acid and its salts, the oxalates, are very commonly found in plants. Those particularly rich in oxalic acid are spinach, mangold, beetroot and rhubarb.

Like oxalic acid itself, some of these salts have undesirable effects for human beings. Together with calcium, they form from the nutrients in the intestines insoluble salts that can then no longer be assimilated by the organism. In this way, part of the calcium that is important for the body is lost.

However, since oxalic acid-rich spinach or rhubarb is not normally eaten daily or in especially large amounts, really serious danger to the health of our bones is to be expected only in the extreme case. Nevertheless, eating oxalic acid-rich vegetables preferably together with milk or dairy products is widely recommended, in order to compensate somewhat for the calcium loss.

Higher concentrations of oxalic acid are undesirable also because they can lead to the formation of kidney stones in persons who are thus predisposed. These so-called oxalate stones account for a majority of kidney stones. Persons who have a tendency toward kidney stones should therefore eat spinach, mangold, rhubarb and beetroot only rarely.


Phasin is a nitrogen compound that is found in beans and is toxic. The consumption of only a few raw green beans can lead to inflammation of the bowels (i.e. enteritis, colitis).

Phasin is destroyed by cooking, however, so that cooked beans can be eaten without any danger. In contrast, green beans should never be eaten raw.


Solanine is a toxin that forms in the green parts of plants of the nightshade family. The best-known members of this family are potatoes and tomatoes.

Large amounts of solanine, which can lead to symptoms of poisoning, are found in green tomatoes and in the skins of green potatoes, or in seed potatoes. The symptoms are headache, nausea, vomiting, stomach and intestinal irritation, diarrhoea, and in severe cases respiratory disorders, cramps and paralysis.

Therefore, unripe, green tomatoes should not be eaten, not even as sweet-sour preserves. Potatoes should be stored in a dark place. Green spots and the eyes should be generously cut out. This will remove any danger from solanine.


Aflatoxins are poisonous metabolic products formed by moulds, above all the type Aspergillus flavus.

Aflatoxins may be contained (must not be, however!) mainly in nuts (pistachios, peanuts, hazelnuts, Brazil nuts, almonds) and various spices (pepper, chilli products, nutmeg, paprika). They form above all in a hot-humid climate - thus rarely under middle European climate conditions - and when the products are stored under warm, humid conditions.

Aflatoxins are not destroyed by heating. Moreover, they migrate into deeper parts of a food that may not appear mouldy. Therefore, if spots of mould are recognizable on nuts, these should be thrown away completely.

There are legally stipulated maximum values for aflatoxins in foods which may not be exceeded.


Ethylene is a gas that smells sweet at higher concentrations and is given off by many fruits, along with carbon dioxide. It is called the "ripening hormone" of the fruits and influences the plant metabolism in various ways. It accelerates the ripening process, but at the same time ageing and spoilage. The latter reduces the length of time that a fruit is suitable for storage.

A large amount of ethylene is formed mainly by pomaceous fruits such as apples and pears, but also by apricots, peaches and blueberries. High temperatures, low humidity and damage to the fruit promote the emission of ethylene.

Many other plants (e.g. cucumbers, tomatoes, broccoli) are sensitive to ethylene. When they are stored together with fruits that are strong emitters of ethylene their keeping quality is markedly reduced.

Ethylene is not a harmful substance, however, as it not absorbed by human beings and thus has no effects on health.