Agricultural residues

Food Standard and Quality Control

Lesson 18 : Food Toxicants - Contaminants

Agricultural residues

Pesticides: Pesticides used to control pests, fungi, insects, weeds etc. are deposited on crops and forage plants, and may build up in the soil as they are used over a number of years.

In the green revolution era phenomenal yield in grain production was possible by the development of high yielding varieties and concomitant use of agrochemicals to boost nutrient supply, minimize crop damage and post harvest losses. However, these chemicals are not always used appropriately, especially in less economically developed countries, where safety standards are not rigorously applied. Thus pesticides which are banned in economically developed countries may still be used to control the spread of insect-borne diseases such as malaria.
Organochlorine pesticides, such as dichloro-diphenyl trichloroethane (DDT), aldrin, dieldrin, lindane (gamma­hexachlorocyclohexane: 'Y-HCH; benzene hexachloride: BHC) or hexachlorobenzene (HCB) persist in the environment after they are used. DDT has a half-life of 2-15 years in most soils depending on the type of soil, but only 2 days in air as it is broken down by sunlight. Lindane has half-life of around 15 months in soil.
These residues accumulate in fatty tissues and increase in concentration as they pass up the food chain. Use of several pesticides of this group (e.g., DDT, aldrin, HCH) has been restricted or banned by many countries.

Most agrochemicals are toxic to humans, e.g. organophosphates can cause nerve damage, and some are potentially carcinogenic (atrazine, phenoxy acid herbicides, 2, 4, 5- T, lindane, methoxychlor and toxaphene). The levels of agrochemicals in food are well below those likely to cause harm and the main hazards are to the people working with them.
However, some agrochemicals are not readily excreted from the body and are able to accumulate in tissues, especially fatty tissues, and so low intakes could be cumulatively toxic.
The use of such persistent compounds is being phased out and foods are regularly monitored to ensure that maximum residue levels in foods are not exceeded.


Large amounts of nitrates are used as fertilizers throughout the world. Nitrates are very water soluble and excessive use of nitrates can pollute water supplies. Nitrate is regarded as relatively non-toxic but nitrite which can be derived from nitrate is regarded as toxic. Nitrate can be converted to nitrite by bacteria and this can happen during fermentation of food, e.g. pickling, as well as in the stomach when acidity is low.

Salt petre (potassium nitrate) has long been used to make salted meats such as bacon and ham. In this reaction, nitrate is reduced to nitrite and reacts with myoglobin to give a pink compound which gives ham and bacon their characteristic colour. In the body, a similar reaction occurs if the gastric acidity is low (e.g. in infants) permitting bacterial growth.

Nitrite is formed, absorbed and competes with oxygen for binding to haemoglobin with the subsequent formation of methaemoglobin. If excessive amounts of methaemoglobin are formed it impairs the oxygen carrying capacity and the infant appears blue (hence the term blue baby syndrome).

Nitrites can also react with secondary amines and amides to form nitrosamines and nitrosamides, which are potent carcinogens. The consumption of salted pickled fish in China is associated with a high risk of nasogastric cancer and studies have shown the salted fish to contain N-nitrosodirnethylamine which can cause cancer in animals. There is a need to develop a good predictor of level of nitrate in food and the nitrosamine formation. Furthermore, there is a need to investigate relationship between nitrate in food and drinking water and cancer risk.

Last modified: Thursday, 22 March 2012, 7:26 AM