Friend or foe?

Chemicals have been used to boost agriculture for decades, and have resulted in increased yields and better shelf-life. But at what cost asks Michael Wong

MOST people are aware of chemicals being added to food crops for a many reasons - whether it to increase yield, reduce disease, strengthen crops or control pests. And most of us believe that these additives far outweigh the possible negative human heath implications and environmental effects.

We’re also told that these additions are strictly controlled and are ‘safe’ to ingest but do we truly understand the accumulative effect, combined with contamination and exposure of additives present in other foods introduced through animal feed, food processing and packaging materials? What exactly are the potential long term effects to human health?

Unlike biological micro-organisms, chemical contamination is often unaffected by thermal processing. For the protection of public health, it is essential that contaminants are kept to an acceptable level. The monitoring of chemical contaminants in foodstuff can be complex and time consuming but a necessary task in order to ensure that the food we eat is within safety limits.

Chemical contaminants can be classified according to the source of contamination and by the mechanism by which they enter the crop. One of the main sources of additives is agrochemicals such as pesticides, fertilisers and veterinary drugs.

The underlying issue that affects the amount of contaminant in the crop relates to the farmer and his ability to judge the correct amount of contaminant required. A balance must be struck between the amount needed to increase productivity of the crop and also to ensure that the crop, after harvesting, will be below the legally allowed limit. Too little agrochemical and his crop may suffer disease or be overrun by pests, too much and he risks being fined and having his crop rejected as well as the detrimental effect caused to the surrounding environment.

One key agrochemical is nitrate. Adequate nitrogen (and potassium) fertility is critical for good crop yield. Nitrate is a good source of nitrogen and is an important element for amino acid production and subsequent protein in the plant. The process requires the enzyme nitrate reductase to reduce nitrate to nitrite and then to amides, which in turn is used for synthesis of amino acids. Since nitrate is highly water soluble, it makes it a good delivery mechanism for fertiliser.

The Council Regulation EEC 315/93 and subsequent Commission Regulation (EC) 466/2001 have set limits for the amount of contaminants in food. For example, fresh lettuce has a limit 2500 or 4500 ppm nitrite depending on the season harvested and whether it was grown under cover or in the open air. The maximum concentrations of contaminants allowed by legislation are often well below toxicological levels, because such levels can often be reasonably achieved by using good agricultural practices.

The enforcement of this directive has prompted farmers to look at their crop management plans to reduce nitrate content, this means that for every acre of land that nitrate is added, it has to be controlled. Nitrate crop management is more complex than it seems as it depends on many factors and there are also various sources of nitrate that have to be considered.

Firstly, soil. Bound organic nitrogen in the soil is converted to nitrate through mineralisation. Several factors influence this mineralisation process, generally, the warmer the climate, the better the conversion process, as well as adequate moisture levels and well aerated soils.

Secondly, organic fertilisers, i.e. manure and mineral fertiliser. Mineral fertiliser provides a continuous cycle of nutrients. Minerals extracted from the soil by the growing crop are reintroduced into the soil when the crop dies and goes through mineralisation. Obviously this circulation of nutrients only becomes balanced and stable if crops are not harvested or if the system is not affected by erosion or wash out.

Thirdly, rainfall. Nitrogen in the air and the formation of nitrogen oxides through climate changes and environmental pollution are washed from the atmosphere into the ground.

The take up of nitrate by the crop also depends on many factors and each species will have different capabilities to assimilate and store nitrate - as a general rule, those crops with many stalks show higher nitrate content. Light also influences nitrate content, the higher the intensity of light the lower the nitrate content. This explains the variances of nitrate concentrations during different seasons and the different limits imposed by the Commissions Regulation.

Ideally, the right amount of nitrate should be added to the crop to boost yields, with little excess, so most of the nitrate is taken up in the crop and is then removed by the harvested crop. Problems occur when there is insufficient crop to remove the nitrate, and so after harvesting any excess is flushed into streams. The link between agricultural activity and stream nitrate loading is clear. Nearly all agricultural surface water shows low levels of nitrate throughout the year with nitrate peaks at particular times of the year, typically May or June. Whilst this is generally not deemed a public health hazard, some countries issue alerts when concentrations reach a certain level and steps are taken to minimise exposure. Increasingly, people have been concerned over the levels of nitrate in surface water brought on by modern farming practices and contributing to the pollution of the nation’s drinking water.

The human body can generally cope with low levels of nitrate, and there is only a slightly elevated health risk associated with high concentrations such as increased coronary circulation. Principally, nitrate is excreted by the kidneys and the concern of nitrate in the body is not the nitrate itself, but what it can be converted to.

The potential danger of high nitrate concentrations is when it is reduced to its more destructive form - nitrite. The use of low concentrations of nitrite in food has been commonplace for centuries - sodium nitrite not only acts as a preservative in meat but also imparts a pleasant taste and color to the meat. It is also speculated that dietary nitrite has an important role in augmenting salivary and gastric nitrite to concentrations that reduce the time required to kill ingested pathogens.

But the concern of elevated levels of nitrite present in the human body has been associated with methemoglobinemia - a disorder in which the body produces higher than normal levels of methemoglobin in the blood. Methemoglobin is a form of haemoglobin that binds to oxygen very tightly, and does not release it even in oxygen deprived tissues, thus oxygen is not supplied to the organs and this can lead to anaemia and tissue hypoxia. However, the human body is fairly robust, and is designed to carry oxygen. Clinical symptoms such as headaches, lethargy, cyanosis only present themselves with concentrations of methemoglobin greater than 20%.

Another concern over nitrites is that it may react with secondary amines or N-alkylamides to form N-nitroso compounds (NOCs) in acidic environments, such as in the stomach. NOCs belong to some of the strongest carcinogenic substances known, as well as being developmental toxicants. There is no existing evidence that direct nitrite ingestion itself is carcinogenic in humans.

The main source of nitrite to the human body is predominately through foods which have been cured or preserved, such as meat and poultry. However, this is not the only source, nitrite intake depends on dietary composition and it is estimated that 39% of dietary nitrite comes from cured meat, 34% from processed food and cereals, 16% from vegetables and 11% from other sources.

Excess nitrate also has a detrimental effect in the environment. Eutrophication refers to an increase of organic compounds within an ecosystem. This increase in chemical nutrients results in excessive plant growth and decay, favours certain weeds over others and is likely to cause severe damage to water quality. The enhanced growth of choking vegetation such as algal bloom disrupts the normal balance of the ecosystem. In fresh water or estuarine systems, high levels of nitrate (over 30 ppm) can inhibit growth, impair the immune system, cause stress and even death in some aquatic species.

Radical changes in farming practices and crop management plans are being implemented to reduce the nitrate content in crops and reduce excess being leached into the surrounding environment. For this to work effectively, an understanding of the relationship between agriculture and nitrate is necessary before any reasonable decision can be made. Nitrate levels already present in the soil, as well as take-up by the plant needs to be considered. Additional fertiliser is then calculated depending on desired yield. However, since the actual release of nitrate from organic matter cannot be predicted at time of planting, farmers add extra nitrate based on expected release. Even with the best judgment and management plans, bad weather conditions and unpredictable seasonal changes may result in a much lower crop yield, thereby resulting in an excess nitrate being washed away into streams and rivers. Good crop and soil management plans are slowly being introduced, for example sidedressing, where nitrogen fertiliser is applied after the crop emerges, as it is easier to determine the potential yield. Soil testing before and during crop growing should also be incorporated in crop management plans. Such plans may seem to be a burden initially, but ultimately reduce the cost of production for farmers as well reduce the quantities released into the environment.

There is no doubt that food additives play an important role in food, from seeding and growing to processing and packaging. Their use is justified from an economical, nutritional and safety point of view. But where is the line drawn in terms of justification for better produced food and the potential long term ill effects it may cause to human health and the environment? It seems fair to ask: at what point do food additives become a contaminant?

By Michael Wong. Michael is marketing manager at FOSS UK. He holds a masters degree in chemistry from Warrick University.