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| ISBN-13: | 9781782625896 |
|---|---|
| Publisher: | Royal Society of Chemistry |
| Publication date: | 11/09/2015 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 221 |
| File size: | 1 MB |
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A Healthy, Wealthy, Sustainable World
By John Emsley
The Royal Society of Chemistry
Copyright © 2010 John EmsleyAll rights reserved.
ISBN: 978-1-84755-862-6
CHAPTER 1
Food and Chemistry
[A word in bold means there is more information in the Glossary.]
The way to produce more food on less land and to do it sustainably requires chemistry. Here we look at four topics relating to food where chemistry is vital: fertilisers, pesticides, preservatives and additives. In addition, three topics are discussed where an understanding of chemistry explains how some components of food, other than the nutrients, can also affect us: natural toxins, neutraceuticals and food fraud.
During the time it takes you to read this sentence, the world's population will have increased by 20. By the time you have read this chapter, enough babies will have been born to populate a small town. In fact, the world's population will continue to grow for most of this century and may even reach 9 billion. They will all have to be fed.
According to the UN, the world population in 1950 was 2.5 billion living off 1.3 billion hectares of arable land. In 2000 there were more than twice as many people but, thanks to more productive agriculture, there was only a ten per cent increase in the area of farmed land. By 2030 there will be three times as many people and possibly no increase in the area of land for food production, although by then there may be twice as much land devoted to biofuel crops (which is the topic of Chapter 4). If no one is to go hungry we must farm scientifically, and that must include genetically modified crops. Even those will need fertilisers and pesticides, and the food from them will need preservatives so that it doesn't go bad before we can eat it.
Many have been told to suspect all things 'chemical' because this means it is unnatural. This use of the word 'chemical' betrays a scientific ignorance. I hope that by the time you have read this chapter you will appreciate that what chemists are mostly doing is reinforcing what Nature already does.
Maybe one day there will be GM (genetically modified) crops which make their own nitrogen fertiliser; produce more of their edible components; provide the range of nutrients which humans need; and resist all forms of pest and microbe attacks. However, until that happy day we will have to rely on chemistry to do many of these things, and that chemistry must be sustainable. We need fertilisers whose manufacture requires much less energy; agents to boost plant growth to maximise output; pesticides to protect crops; and preservatives to ensure the edible parts remain safe to eat after storage.
We eat and drink to provide our body with the energy we need for warmth and movement, and the chemicals it needs to repair and renew damaged and worn out tissue. The nutrients it needs are carbohydrates, proteins, fats, vitamins, minerals and, of course, water. Most of what we eat comes from plants either directly or indirectly via animals, and plants also need nutrients. The first topic in this chapter is all about providing these in the form of fertilisers, and the most important one is nitrogen.
1.1 FERTILISERS
Without the ammonium nitrate produced by the chemical industry the world would starve.
Local famines, even national famines, have threatened humans with starvation throughout history. Could we one day face a global famine? Worldwide hunger was envisaged and warned against in the past by Thomas Malthus (1766–1834). Malthus predicted that there must eventually be a catastrophe for the human race because we reproduce 'geometrically', in other words we were multiplying, whereas food production could only increase 'arithmetically', in other words by the simple addition of more land devoted to farming.There must come a time when there were just too many mouths to feed and no more land to farm and there would be mass starvation. This didn't happen, however, because in the 1800s the plains of North America began to produce food in abundance much of which was exported.
Worries about overpopulation surfaced again in the 1960s as numbers continued to increase. In 1968, Paul Ehrlich's book The Population Explosion predicted a future crisis with hundreds of millions of people dying of starvation. Another group to warn of an impending food crisis was an influential NGO (non-governmental organisation) called The Club of Rome. In 1972 their report, entitled The Limits to Growth, forecast that that there would be severe food shortages by the year 2000. That crisis was averted by the application of even better agrochemicals and better strains of the major food crops. Today the majority of people in the world get enough to eat, but many get barely enough to survive and stand little chance of making much of their lives. Eventually all will need to be fed by sustainable agriculture which will have to rely on a sustainable agrochemical industry.
A plant needs nutrients to grow and the elements it needs most are carbon, nitrogen, potassium and phosphorus. The first of these is not a problem because plants take their carbon from the carbon dioxide in the air, and there is enough of that as we know. The other three have to come from the soil and the one which most limits plant growth is nitrogen. Why is this? The reason is that all living things need amino acids which have nitrogen as part of their molecular make-up. These acids link together to form proteins and enzymes, and the result is that the average person contains 2 kg of nitrogen.
Plants take in nitrogen as either ammonia or nitrate, and these are in the soil and come from the debris of dead plants, animal excreta, and soil fauna and flora. A little nitrate even arrives with rainwater, which dissolves the nitrogen oxides produced in thunderstorms and from vehicle exhausts. There is plenty of nitrogen on Earth because it constitutes around 80% of the atmosphere, totalling an incredible 4 trillion tonnes, but this is mainly inaccessible to plants. However, some plants can draw on atmospheric nitrogen, such as legumes like peas and beans, thanks to the rhizobia bacteria which are present in the root nodules of these plants. The rhizobia take carbohydrate from the plant in return for soluble nitrogen salts which they produce. Most plants rely on the microbes in the soil breaking down plant and animal residues for their ammonium and nitrate, however, and this source of nitrogen is limited and can become exhausted. The key to boosting crop yields is to add ammonium nitrate from another source.
Early attempts by chemists to react nitrogen and hydrogen failed to produce any ammonia (NH3), no matter how strongly they were heated together. What the reaction needed was a catalyst and that's what the German chemist Fritz Haber discovered, and chemical engineer Carl Bosch turned it into a commercial process which is carried out at around 450 °C and under high pressure. On 3rd July 1909, the first successful Haber–Bosch chemical plant started up at Oppau, Germany. Today, there are Haber–Bosch plants around the world producing 150 million tonnes of ammonia a year, the vast majority of which goes into making fertilisers. Two billion people worldwide are now reliant on this source of agrochemical nitrogen for their food supply. Industry produces almost as much 'fertiliser' nitrogen as the biomass fixes naturally.
Nitrate is made from ammonia by reacting the gas with oxygen and water. There is also a process for making nitrate from the oxygen and nitrogen in the atmosphere, known as the Birkeland–Eyde process, which was patented by two Norwegians in the early 1900s and operated successfully for many years. Using a powerful electric discharge to mimic the effect of lightning, nitrogen and oxygen react together to form nitric oxide (NO) which then reacts with more oxygen and water to form nitrate, as nitric acid (HNO3).
Combine ammonia and nitric acid and you have ammonium nitrate. Spread or spray a dilute solution of this on the land when a growing crop needs it and you can double, triple and even quadruple the yield. Opponents of the agrochemical industry call such fertiliser 'artificial' (i.e. meaning unnatural), but of course it matters not to the roots of a plant whether the ammonia or nitrate comes from rotting matter or from the chemical industry.
At the present time the production of ammonium nitrate is not sustainable because the energy to make it comes from fossil fuels. Sometime in the future we will either have to genetically modify plants so that, like legumes, they can support rhizomes on their roots, or we must devise a way of manufacturing ammonium nitrate using only renewable energy. It may be possible one day to do it on a local scale with the Birkeland–Eyde process using energy derived from a waterwheel or windmill. If there were such a unit in every farming community, the environmental impact would be minimal and crop yields would remain large and sustainable.
In the 1980s there were scare stories based on epidemiology about nitrate from fertilisers getting into the water supply and causing stomach cancer and the scary sounding blue-baby syndrome, although the last case of this in the UK was 60 years ago. Happily 11 other studies showed no link, and 7 even showed a negative correlation, in other words more nitrate meant fewer cases of cancer. Indeed, supposedly 'healthy' foods like lettuce, spinach, beetroot, celery and potatoes have naturally high levels of nitrate, and eating lots of those has never been linked to cancer. In 1985 it was discovered that the human body even generates its own nitrate at around 70mg per day, similar to that coming from food and water. Cells release nitrate in response to infections and even to strenuous physical exercise such as running and cycling. Nitrate protects the human body against disease pathogens.
No benefit comes totally without cost and so it is with nitrogen fertilisers. Overuse leaves this in the soil, there to be acted upon by microbes which release some of the nitrogen as nitrous oxide (N2O), a powerful greenhouse gas. Indeed the world is using much too much ammonium nitrate to fertilise crops – some estimates suggest that we could make do with less than half of that which we now use if there were better management strategies, such as tailoring application to crop needs. This has been undertaken in the Netherlands where a 40% reduction of nitrogen fertilisers has had little effect on crop yields.
And what of the other macro nutrients that crops need? These are phosphate and potassium. Are they sustainable?
Human sewage and animal manure represent an important source of phosphate. Both can be processed to recover phosphate especially that from large intensive livestock farms or chicken sheds which can be turned into struvite (ammonium magnesium phosphate). This can be used as a slow-release fertiliser. Sewage is less easy to process because it is greatly diluted with water. Nevertheless, there is a growing need to find a valid use for the phosphate that has to be removed from it. Phosphate can be precipitated as an insoluble form, either as iron phosphate or aluminium phosphate, but these are unsuitable as fertiliser because the phosphate is chemically too tightly bound for plant roots to extract it.
Biological phosphorus reclamation was first suggested in 1955 when it was observed that aerated sewage sludge absorbed phosphate. The first biological process was known as the Phostrip process, and the sludge that settled from such treatment was suitable for use as an agricultural fertiliser. The bacteria with an appetite for phosphate are Acinetobacter, Aeromonas and Pseudomonas, There are now several commercial systems of biological phosphorus removal in use. As yet, the recycling of reclaimed phosphate for other uses is in its infancy, but research has shown that it is feasible.
Potassium can also be sustainable. In the 1800s the demand for potassium was met from forest clearances in the US. Trees were burned and their ash stirred with water to extract the water-soluble potassium salts. The solution so obtained was heated to dryness in large pots to yield potassium carbonate, hence its common name pot-ash. Most potassium for fertiliser is currently obtained by mining the minerals sylvite (potassium chloride) and carnallite (potassium and magnesium chlorides) of which there are deposits in excess of 10 billion tonnes, which means that sustainability is never likely to be an issue. Even so, potassium for fertilisers could be derived in part from the ash left after burning biomass.
1.2 PESTICIDES
Modern pesticides are our best weapons in the fight to protect the food we grow.
Let's start with a simple fact: half the fruit and vegetables grown by farmers would be unfit for market if pesticides were not used. The yields of 'organic' crops are typically 30% less than normal, and in the case of 'organic' potatoes is 40% less, which is part of the reason they are so expensive. Erich-Christian Oerke of the University of Bonn in Germany has demonstrated that without the products of the agrochemical industry, crop yields would fall to about half and crops like cotton would fall by as much as 80%. Protection of our food supply is needed at all stages, starting with the soil itself which is a hostile environment of pests and microbes. Protection is needed for seeds, for growing plants and for the harvested crop. All are under attack.
There are those who would ban pesticides because they think the traces of pesticides in their food represent a risk to their health. These people are prepared to pay premium prices for 'organic' food in the mistaken belief that this is produced without the need for chemical pesticides – it isn't, as we shall see. Pesticide residues are blamed for causing a variety of conditions ranging from lethargy to liver cancer, including several illnesses whose cause is as yet unknown. When it comes to modern pesticides these claims of damaging human health don't stand up to close inspection.
Those working for agrochemical companies make many new molecules which have potential as crop protection agents, but very few ever reach the farmer. Indeed, it has been calculated that only one compound in 100 000 will end up in the market place, and then only after years of testing to make sure it works and is safe. Consequently it costs around £150 million to launch a new pesticide. The hurdles it must pass include effectiveness; testing in various formulations and application routes; and biological testing against a variety of insects, weeds or diseases on various crops under a variety of conditions. It is also necessary to know how the pesticide actually works and what its breakdown products are in the soil and in plants. Only when all this data has been amassed, and approved by numerous agencies, will a licence be issued. Only large companies have the resources to invest in this area.
Pesticides must not harm those who come into contact with them, must only destroy the target pests, and must be short-lived and break down into harmless substances. Pesticides in the past often failed in one of more of these aspects. What chemists also do is produce better targeted pesticides, so less is needed. Ideally, a modern pesticide will not bio-accumulate as did some of the early ones, like DDT. In assessing whether a pesticide poses a risk, the guideline is the No-Observable-Adverse-Effect (NOAE) level. When this has been determined, then the amount that could possibly be consumed is set at 100 times less. Crop protection now requires much less insecticide than was once used. For example, the traditional herbicide 2,4-D has to be applied at a rate of 1 kg per hectare, whereas the best modern herbicides require only around 0.01 kg (10 g) per hectare. Some, such as glyphosate, will affect only plants because it blocks an enzyme which only plants have. The enzyme is used by plants to make plant protein, whereas animals make protein in a different way.
Plants produce their own pesticides to protect themselves against microbes and insects – and even animals. In fact most crop protection chemicals are produced naturally by plants. The world-famous molecular biologist Dr Bruce Ames, who devised the test for assessing the effects of chemicals on living things, pointed out that we ingest 10 000 times as many natural crop protection products than we do those made by the chemical industry. Some are remarkably effective, such as the products produced in the leaves of the neem tree of India. The leaves of this tree can even resist attack by a plague of locusts; their secret lies in the compound azadirachtin which actually deters an insect from further feeding once it has take a bite. In 2007 Steve Ley of Cambridge University Chemistry Department showed how azadirachtin could be produced chemically.
An example of a modern pesticide is indoxacarb, which was discovered in 1972 by Dutch researchers Rudolph Mulder and Kobus Wellinga, and kills only the target pest, in this case caterpillars. These little beasts are some of the most voracious crop feeders which attack cotton, soyabean, cabbage, peppers, tomatoes and alfalfa, which is a legume grown throughout the world mainly as cattle fodder. In 2000 DuPont launched indoxacarb. This chemical interferes with the caterpillar's ability to chew. It does not affect other creatures, such as beneficial insects, because the victim has to ingest the pesticide for it to kill. Indoxacarb itself is not the molecule which kills; the toxin which kills it is formed from indoxacarb by the caterpillar's own metabolic processes.
(Continues...)
Excerpted from A Healthy, Wealthy, Sustainable World by John Emsley. Copyright © 2010 John Emsley. Excerpted by permission of The Royal Society of Chemistry.
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