Molecules at an Exhibition: Portraits of Intriguing Materials in Everyday Lifeby John Emsley
What ingredient in Coke can remove rust from chrome? What is the bitterest substance on earth? What is the worst smelling one? In this entertaining tour of chemistry, John Emsley answers these and many other questions as he illuminates the materials that make up our world. Dozens of lively articles explore such well-known molecules as water, oxygen, and glass;… See more details below
What ingredient in Coke can remove rust from chrome? What is the bitterest substance on earth? What is the worst smelling one? In this entertaining tour of chemistry, John Emsley answers these and many other questions as he illuminates the materials that make up our world. Dozens of lively articles explore such well-known molecules as water, oxygen, and glass; versatile plastics like polypropylene, polystyrene, and polyurethane; even "elements from hell" such as Sarin (a lethal nerve gas). With no formulas, equations, or molecular diagrams to baffle the non-expert, each piece blends history, science, and anecdote, with many intriguing facts added to the mix.
"The world of chemistry has never been made as entertaining," writes Nobel Prize-winning chemist Roald Hoffmann. Indeed, this book will fascinate everyone curious about the chemicals in the foods we eat, the clothes we wear, and the air we breathe.
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NEARLY AS NATURE INTENDED
An exhibition of some curious molecules in the
foods we eat
 AZTEC DREAMS  RHUBARB PIE  THE COCA-COLA CONUNDRUM
 RUST REMOVER  THE CURSE OF THE CURE-ALL
 THE WORST SMELL IN THE WORLD  CHINESE MEDICINE
 THE STATE OF THE HEART  THOSE UNSPEAKABLE MOLECULES
There are scores of myths surrounding the things we eat: chocolate is almost addictive; Coca-Cola is just a concoction of chemicals; garlic wards off heart disease and cancer; an aspirin a day keeps the doctor away. None of these statements is true, but they contain a germ of truth. In this gallery we can inspect the portraits of some of the natural and unnatural chemicals which a normal diet contains.
The pleasures of eating are sweet but fleeting, while the warnings about food seem bitter and never-ending. The warnings we should heed are those of professional dietitians, the front-line troops who are fighting the war against poor nutrition and unbalanced diets. While they help the people who are referred to them, the rest of us only hear their advice second-hand, and even then we do not heed it--which may explain why one person in five is now classed as obese (33% or more overweight) in the USA, and one in ten in Britain.
Behind the front-line dietitians is a regiment of armchair food commanders who offer their advice to anyone who listens. Often it is soundly based, telling us how to lose weight and still be properly nourished, but a lot is rather unhelpful, merely condemning some popular foods as `junk' without explaining why they are so (although this term is generally taken to mean that they contain too much sugar, salt, saturated fats and additives). Examples of junk food are chocolate, colas, hamburgers and french fries. Sadly the healthy alternatives, such as raw celery, mineral water and lentils, lack appeal for many, and especially for children.
Alongside claims about junk food come more dire warnings about the chemicals that are present in other foods, and especially if these have been added merely to make food look and taste more tempting, or if they are there as contaminants that come from pesticides and processing. Surprisingly, most food-related illness comes not from these, but from micro-organisms such as bacteria and fungi, and we are most at risk when we eat food that has not been properly stored or prepared. Ideally food should be free of all dangerous impurities, be they bacteria, fungi or chemicals.
Nature also has its chemicals, and some of these we are rather partial to, such as phenylethylamine and caffeine. Others we try to avoid, such as oxalic and phosphoric acids, and yet others we should take more of, such as salicylate and selenium. In this Gallery we will view a display of molecules that are there in the foods we eat, and all of them are perfectly natural, except one: phthalate (this comes courtesy of the plastics industry). The others are examples of molecules that make us feel better, those which can do us harm, and those which can make us smell.
Three of the molecules are to be found in chocolate. No food provokes the emotional responses of chocolate. To some is it the junk food and appears to be little more than a temptation of the devil. Why is it so irresistible? Most people love it, some cannot resist it, and a few unfortunate people have to avoid it. Some people stuff themselves with it until they are sick, while others claim that a mere lick of chocolate will trigger an allergic attack. The makers of chocolate confectionery advertise their products in a variety of ways. They emphasize its wholesomeness and nutritional value, and claim it is full of energy; they suggest you offer it as a gift to a loved one, or even eat it as a way of rewarding and pampering yourself. Whatever its benefits, it has its risks, and some people regard chocolate as junk because its sugar rots teeth, its fats damage the heart, its calories put on weight and its cocoa can trigger a migraine attack.
An analysis of chocolate buyers in the UK showed that most chocolate is bought by women, who account for around 40% of sales, while children buy 35% and men 25%. It tops the list of difficult-to-resist foods, and accounts for over half of all food cravings. Some women even claim to be chocoholics and say they find it impossible to resist, especially before their monthly period. Clearly for them, chocolate is more than just a tasty food or a treat. Chantal Coady, author of the book Chocolate, questions whether there really are such people as chocoholics. She writes: `Although chocolate contains many active chemicals, some of which mimic natural hormones, none of these is addictive.' She believes that women turn to chocolate for consolation when they need a little comfort, and that what they seek is the intense sweetness associated with chocolate confectionery, as well as its luxurious taste and texture in the mouth.
Chocolate is a fairly well-balanced food consisting of 8% protein, 60% carbohydrate, and 30% fat, although this last component is at the upper limit of what is desirable. A normal 100 g (4 ounce) bar provides 520 calories, but it also provides some essential minerals and vitamins:
Minerals Vitamins potassium 420 mg A 8 mcg chlorine 270 mg B1 0.1 mg phosphorus 240 mg B2 0.24 mg calcium 220 mg B3 1.6 mg sodium 120 mg E 0.5 mg magnesium 55 mg iron 1.6 mg copper 0.3 mg zinc 0.2 mg
We shall be viewing the dietary importance of minerals in an exhibition in the next Gallery. Looking at this list it is perhaps not surprising that chocolate bars make excellent emergency rations for soldiers and explorers, but there are a few things missing, like vitamins C and D, so it is far from being a complete food. It also has a few other things which are not nutrients, such as phenylethylamine, oxalic acid and caffeine. These have no nutritive value, but they do affect us, and two of them are abundant in other foods and drinks as well. The first three portraits in the exhibition concern these chemicals.
 Portrait 1
Aztec dreams--phenylethylamine (PEA)
The only thing in chocolate which comes anywhere near having a feel-good effect on our brain is phenylethylamine (PEA). The Mayas of Middle America, who flourished in Mexico from AD250 to 900, discovered the effects of this when they discovered chocolate, which they took as a drink and which they reserved for the ruling elite. By the time the Spaniards arrived at the end of the fifteenth century, the Aztecs were the dominant civilization and the economy was partly based on cocoa beans--levies from conquered tribes had to be paid in this currency. Aztec nobles also reserved chocolate for themselves, regarding it as an aphrodisiac, and yet forbidding women to drink it. When cocoa beans were taken back to Europe, chocolate's reputation as a love-stimulant sailed with them. This reputation grew: it was now drunk by both sexes, and in 1624, one author, Joan Roach, devoted a whole book to its condemnation, referring to it with puritanical disapproval as a `violent inflamer of the passions.' In the eighteenth century the great lover, Casanova, proclaimed chocolate to be his preferred drink.
Cocoa beans are harvested from the cacoa tree, which grows best in warm, moist climates and within 20 [degrees] latitude of the Equator. The world production of cocoa beans is two million tons a year, and they are grown in Brazil and Mexico for the North American market, and in West Africa for the European market.
After cocoa pods are harvested, the beans are removed and left in the sun to ferment. This exposure turns them brown and converts some of their sugars first to alcohol and then to acetic acid, which we know best in the form of vinegar. The acetic acid kills the shoot and releases other flavour molecules. Phenylethylamine (PEA) forms during this fermentation stage. The beans are then roasted to remove most of the acetic acid, and milled, which causes the cocoa fat to become molten. The extent of the grinding process determines the different grades of chocolate.
Today when we speak of chocolate, we think of a piece of chocolate candy, but originally chocolate was a drink. The name is derived from the Aztec word xocalatl meaning bitter water, and it was served as a rather scummy liquid mixed with cinnamon and cornmeal. Later, vanilla and sugar were added to make it sweeter and more palatable for European tastes.
Despite what Casanova thought, chocolate is not an aphrodisiac, but there may be some truth in the idea that it affects the brain. Analysts have detected more than 300 chemicals in chocolate. Two of them are of stimulants: caffeine, which will be dealt with later in this gallery; and theobromine, which is chemically similar and was named after the cocoa tree, whose botanical name Theobroma cacoa means `food of the gods'. Theobromine is also present in tea.
The most likely chemical in chocolate that might explain its feel-good effect is PEA, of which there can be up to 700 mg in a 100 g bar (0.7%). Most chocolate contains much less than this, and a more typical amount would be 50-100 mg. In its pure state PEA is an oily liquid with a fishlike smell, and it can be made in the laboratory from ammonia. (PEA has the curious property of absorbing carbon dioxide from the air.) When people are injected with PEA, the level of glucose in their blood goes up and so does their blood pressure. These effects combine to produce a feeling of well-being and alertness. PEA may trigger the release of dopamine, which is the brain chemical that makes us feel happy, in which case PEA would be acting in the same way as amphetamines such as ecstasy. PEA and ecstasy molecules are roughly the same shape and size, and this has led to the suggestion that they might work in the same way, but scientific proof is lacking that they do.
Our own bodies produce tiny but detectable amounts of PEA naturally, and it is formed from an essential dietary amino acid called phenylalanine. The level of natural PEA varies and it increases when we are under stress. It is also higher than normal in schizophrenics and hyperactive children, but this is more likely to be a symptom of these conditions rather than their cause.
Not everyone can cope with a sudden influx of PEA, which is why some people are sensitive to chocolate, often suffering a violent headache if they eat too much. This happens because the excess PEA constricts the walls of blood vessels in the brain. The human body has little use for PEA and employs an enzyme, monoamine oxidase, to dispose of it. People whose bodies are intolerant of chocolate appear to have difficulty making enough of the enzyme to prevent the PEA building up to levels that triggers migraines.
That PEA is addictive seems unlikely, but there is another reason why some people deny themselves the enjoyment of chocolate. Its fat content, which is called cocoa butter, is primarily a saturated fat. In fact it is 60% saturated, the same as dairy cream, and should be viewed likewise. However, in Dr Herve Robert's book, Les vertus therapeutiques du chocolat, it is claimed that cocoa butter, unlike cream, does not lead to raised blood cholesterol levels.
The fat in chocolate is rather special in another way. Normal fats are a mixture of saturated and unsaturated fats which tend to soften and melt over a range of temperatures. This is not what we want to happen with a bar of chocolate. Chocolate has literally to melt in the mouth at a temperature of around 35 Celsius, just below the body temperature of 37 Celsius. This is why the best way to enjoy a bar of chocolate is to let a piece of it rest on the tongue until it melts and releases its rich flavour and aroma.
Cocoa butter itself can solidify in several different ways, and each melts at a different temperature. Only one form is right for solid chocolate, which explains why chocolate-making is still regarded as much as an art as a science, and why careful cooling of the molten chocolate is necessary to ensure that the correct form solidifies. If you keep chocolate too long it becomes covered with a greasy white bloom, which makes it look as if it has gone off. It hasn't: this is not a mould, but only another of the crystal forms of cocoa butter, and is perfectly edible.
When chocolate was regarded as a hot drink, the chemistry of its fat hardly mattered. Then in 1847, the Quaker confectioners, J.S. Fry & Sons, of Bristol, England, introduced a solid form that could be eaten as a sweet. They made it by pressing the molten chocolate in order to squeeze out the cocoa butter, and then added this to more molten chocolate. The result was plain chocolate with rather a strong flavour. Much more popular was milk chocolate, bars of which were first produced in 1876 by the Swiss chemist Henri Nestle. He added condensed milk, which made the product lighter in taste (and colour) and opened the market to children. Other Quaker families--the Cadburys, the Rowntrees and the Hersheys--entered the chocolate business and went on to establish equally large chocolate empires in the UK and the USA.
Since then chocolate has never looked back. Yet it is not without its hidden hazards, although these pose less of a threat than eating too much chocolate, especially if this leads to obesity.
 Portrait 2
Rhubarb pie--oxalic acid
Chocolate also harbours oxalic acid, a dangerous chemical that can kill--but rarely does. We take in oxalic acid every day from a variety of sources. It occurs in lots of foods in small amounts, and in a few foods in large amounts, cocoa having one of the largest with 500 mg per 100 g. Green leaf vegetables have the most, such as Swiss chard with 700 mg per 100 g, spinach with 600 mg, and rhubarb with 500 mg. Rhubarb, also known as pieplant in the USA, is popularly thought to have dangerously high levels because this food has killed people. Perhaps less well appreciated is that beetroot (300 mg) and peanuts (150 mg) also have a lot of oxalic acid.
The average person consumes about 150 mg of oxalic acid a day, and in countries where tea is popular the level is generally higher because a cup of tea provides 50 mg. A fatal dose of oxalic acid is around 1500 mg. Could we reach a deadly dose during the course of a normal day? And what effect do even the lower levels of oxalic acid have on us?
Rhubarb is less popular than it used to be, but it was once widely eaten stewed with sugar. It was famed for its laxative properties, and it works because it stimulates our gut to reject the natural toxin, oxalic acid. A bowl of stewed rhubarb could provide us with a sizeable fraction of the toxic dose. To poison yourself by eating bars of milk chocolate would be virtually impossible, no matter how chocoholic you felt, because these contain much less oxalic acid and you would be satiated with them before you reached even the laxative level.
Rhubarb became infamous in World War I when people ate its leaves as a vegetable, and some died through oxalic acid poisoning. The level of oxalic acid in rhubarb leaves is much higher than in the plant stalks, but you are not at risk from eating those.
Rhubarb has long been known as a medicament. In AD 70 the Greek physician and botanist Dioscorides recommended it for treating a variety of conditions. This was European rhubarb, and was used until the twelfth century, when a superior rhubarb appeared from the East. There was much speculation as to where it was grown. Most came from China and it continued to be imported on a large scale as powdered root for hundreds of years.
Rhubarb root has been used in traditional Chinese medicine for more than four thousand years. The Royal Society of Arts, Manufacturers and Commerce decided to promote the cultivation of the new rhubarb in the British Empire, and during the next 30 years it awarded several gold medals to those who grew the best varieties. In 1784 the Swedish apothecary Carl Wilhelm Scheele detected oxalic acid (which he knew as acid of sorrel) in rhubarb roots, and showed that the amounts in the plant's leaves were too large for these to be edible. The oxalic acid is thought to be the plant's protection against cattle. In 1860 the Victorian best-seller, Mrs Beeton's Book of Household Management, reported that rhubarb was in every kitchen garden, and she gave recipes for rhubarb pies, rhubarb jam and even rhubarb wine. The easiest way of cooking it was to stew the chopped stalks with sugar. When aluminium pans became popular, stewing rhubarb was discovered to have another bonus: it cleaned the pans beautifully. It did this because the oxalic acid dissolved off the top layer of metal, although the amount that was removed this way was so tiny it was no threat to health.
This affinity of oxalic acid for metals also explains another curious anomaly, and is the reason why nutritionists refer to it as an antinutrient. Oxalic acid interferes with the essential minerals iron, magnesium and especially calcium. Earlier this century spinach was advocated as a rich source of iron, and indeed it has higher levels of this metal than most vegetables. For example spinach has 4 mg of iron per 100 g, compared to peas which have 2 mg, brussels sprouts 1 mg, and cabbage only 0.5 mg. Despite having more iron, the oxalic acid in spinach renders 95% of this metal useless as a nutrient, and only 5% can be absorbed by our body. The cartoon character Popeye attributed his strength to spinach, but he was sadly misinformed. By all means eat spinach as a vegetable, enjoy it, but expect very little from it except a modest amount of vegetable protein and a little vitamin C.
Oxalic acid kills by lowering the calcium in our blood below a critical level. (The antidote is calcium gluconate.) Calcium is essential for keeping the blood at a constant level of acidity and viscosity, as well as for clotting and transporting phosphate around the body. But even in non-lethal doses the effect of oxalic acid on calcium is worrying, because it forms insoluble calcium oxalate, crystals of which can grow into painful stones in the bladder and kidneys. The development of these is more likely if our fluid intake is low. Doctors whose patients are prone to develop such stones put them on a low-oxalic acid diet, which excludes the foods we have been talking about. Although such foods can be avoided, we cannot exclude oxalic acid entirely from the body because there are other sources. For example, surplus vitamin C, which the body cannot store, may be turned into oxalic acid, and a side-effect of taking massive doses of this vitamin may also be kidney stones.
According to John Timbrell, a toxicologist at the London School of Pharmacy and author of Introduction to Toxicology, it is possible to get a fatal dose of oxalic acid in other ways. People who have accidentally or deliberately drunk ethylene glycol, which is used as antifreeze in cars, may die of oxalic acid poisoning because the body converts ethylene glycol to oxalic acid.
Plant cells are known to make use of oxalic acid, but it has no role in animal cells--or so it was once assumed. Recent research suggests otherwise. Despite the apparent toxicity of oxalic acid, the body will tolerate surprisingly high levels of it, and research scientists in Germany discovered that human tissue contains more oxalic acid than was previously suspected. Dr Steffen Albrecht and co-workers at Dresden University have challenged the view that oxalic acid is merely an unwanted end-product of metabolism. They have developed a sensitive method of analysis for oxalic acid, and can measure concentrations as low as millionths of a gram (mcgs) per litre of blood. Their work has revealed markedly different levels of oxalic acid in the blood: plasma, the fluid part of the blood, has 400 mcg per litre while serum, the clear solution which separates from blood after the plasma has coagulated, has 1200 mcg. Some blood cells have 250 000 mcg, which sounds a lot but when converted to milligrams is only 25 mg per 100 g, which is low compared to the levels in some foods. Albrecht says the high levels of oxalic acid point to its having an active role in human metabolism, although what that is remains unknown.
Oxalic acid is made commercially by treating either sugar with nitric acid or cellulose with sodium hydroxide. The acid is very soluble in water--a litre will dissolve 150 g--and it forms a corrosive solution. Industrially it is used in tanning leather, dyeing cloth, cleaning metals and for purifying oils and fats. The only guise in which it might be found in the home is in stain-removers for iron-based stains, such as rust and ink from fountain-pens.
 Portrait 3
The Coca-Cola conundrum--caffeine
Chocolate contains a little caffeine, but there are much richer sources, such as coffee, tea and cola. Most ingredients in colas have been criticized at one time or another, and yet the young people of the world continue to love them. But look at the label on a bottle or can and it appears that all you are drinking is a solution of chemicals in fizzy water. Colas contain little that can be described as natural. The main ingredients are sugar or an artificial sweetener, phosphoric acid, caffeine, and a blend of flavourings, which are supposed to be a secret. Once upon a time they were, and when Coca-Cola was first invented this was part of its attraction.
There is no denying that the secret formula of Coca-Cola has been highly successful: it has seduced the taste buds of billions of people around the world. Nor should we be surprised, because it is a refreshing, pleasantly flavoured drink, and a can of ice-cold coke can really quench your thirst on a hot summer's day. Not surprisingly, it has many imitators.
The story of Coca-Cola began on 28 June 1887 in [Atlanta, Georgia, when a pharmacist, Dr John Pemberton, then 56 years old, was granted the trademark, Coca-Cola, for a drink he had invented. The time was right for the new beverage because the city of Atlanta had just voted to ban alcohol, so perhaps it was not surprising that Coca-Cola sold well. Pemberton's new drink continued to sell even after prohibition was repealed in the city later that year.
Pemberton placed an advertisement in the Atlanta Journal describing his new drink as follows:
Delicious! Refreshing! Exhilarating! Invigorating! The new and
popular soda fountain drink contains the properties of the
wonderful coca plant and the famous cola nut.
And indeed the drink is still named after these ingredients--the coca plant, which is the source of cocaine, and the cola nut, which is rich in caffeine. I should hasten to add that neither of these plants provides ingredients for today's colas.
Pemberton had stumbled upon a recipe that was to become the world's best-selling soft drink. He kept the flavour ingredients a closely guarded secret, and the Coca-Cola company claim that only the top two executives in the company know what they are, and how they should be blended.
Most of the main ingredients of Coca-Cola have always been common knowledge: sugar, caramel, caffeine, phosphoric acid, lime juice, and vanilla essence. Together these make a passable concoction and the caramel, lime and vanilla are the dominant flavours. There never was any cocaine in Coca-Cola, although Pemberton, who was a regular user of this drug, may have experimented with it. Cocaine was certainly added to certain tonic wines of the day: Queen Victoria herself was reputed to be very partial to some of these. The cola extract was also dropped from the recipe early on, in preference to adding purified caffeine directly. The level of acidity of Coca-Cola, which was needed for its refreshing taste, was originally due to citric acid, which occurs in citrus fruits, but this ingredient was soon replaced with cheaper phosphoric acid.
Pemberton needed to make his new drink distinctive, and so he experimented with other flavours in smaller amounts. Finally he found a blend that he liked and gave it the code name 7X. So carefully did the Coca-Cola company protect the secret of 7X over the years that it was prepared to defy court orders rather than reveal it. In India manufacturers are obliged by law to say what is in a drink, but in 1977 the company decided to cease marketing Coca-Cola in India rather than reveal its secret.
Over the years there have been several attempts to guess what 7X consists of. Its natural essences are present in only tiny amounts, so it was almost impossible to discover what these were by chemical analysis, because each essence consisted of numerous flavour chemicals. In 1983, William Poundstone, author of the book Big Secrets, printed his list of what he thought was in 7X, which he said consisted of orange, lemon, nutmeg, cassia, coriander, neroli and lime. Cassia is also known as Chinese cinnamon, and neroli is extracted from the bitter-orange flower. Poundstone had made quite a shrewd guess, as we shall see.
Modern analytical techniques will lay bare the intimate details of any secret mixture, and so perhaps it was not unduly worrying for Coca-Cola when Mark Prendergast finally published the recipe for 7X in his book For God, Country and Coca-Cola in 1993. He says he came across it in the tattered remains of one of Pemberton's laboratory notebooks in the company archives. The mysterious 7X was a blend of the oils of lemon (120 parts), orange (80), nutmeg (40), cinnamon (40), neroli (40) and coriander (20). Pemberton mixed these with alcohol and then left it to stand for 24 hours to give him his secret extract. Today there is no alcohol, but it may be that the use of that ingredient during prohibition explains Pemberton's original need for secrecy.
It is still claimed by the Coca-Cola company that it is the sequence in which the ingredients of 7X are blended that is the key to producing the 'real thing', and it may still be that only two executives of the company know this. There are people who say they can identify the different colas that are available, but a discriminating palate for this type of drink is never likely to be considered the mark of a connoisseur. Colas are simply refreshing drinks that do no harm and keep many people employed in making, distributing, selling and advertising them. When you buy a can of cola, it hardly matters that packaging, promotion and profits account for 95% of what you spend. You can be under no illusion that you are buying something essential, and a glass of water is even better at quenching your thirst and costs virtually nothing. What you are really buying is a solution of caffeine, and this can have an effect on you.
The amount of caffeine in a can of cola is 40 mg, the same as in a cup of tea, and about half that in a cup of freshly ground coffee. The same volume of instant coffee provides 60 mg, and over the last 50 years this has become the most popular way to take caffeine. Instant coffee was first produced by the Swiss company Nestle in 1938, and sold as Nescafe (the Brazilian Institute for Coffee had shown that coffee could be reduced to a soluble powder in 1930). Instant coffee really came into its own in World War II when it was widely used by US troops, and thereafter it became part of everyday living.
Young people may get their daily dose of caffeine from colas, but most adults get it from coffee and tea. While flavour is the most important part of these drinks, it is the caffeine that explains their enduring popularity. Tea is mainly drunk in the countries in which it is grown, such as India, Sri Lanka, and especially China, but a few countries are large importers, such as Great Britain and Australia. Coffee, on the other hand, is mainly grown as a crop for export in countries like Brazil, Colombia, Indonesia and Kenya. International trade in coffee beans exceeds $7 billion a year, making them one of the top four traded commodities (along with coal, grain and oil).
Worldwide consumption of caffeine is now estimated to be over 120 000 tons per year, which works out at about 60 mg per person per day. Scandinavians have the largest caffeine intake, generally from coffee, with over 400 mg per day; the British consume around 300 mg per day, much of it as tea; and the Americans, long regarded as big coffee and cola drinkers, get a surprisingly low 200 mg per day.
A fatal dose of caffeine taken by mouth would be about 5000 mg, the amount in 80 cups of coffee or 120 cups of tea. When you take in caffeine your body mobilizes its defences to dispose of the invading toxin, which is how it sees this non-nutrient. It rids itself of the offending molecule by plucking off carbon atoms, although at first this has little effect, because it leads to new molecules, such as theophylline and paraxanthine, which are just as potent. However, the process continues and finally the product is xanthine, which the body can eliminate in the urine or put to other uses. All this explains why the effects of caffeine in the body persist for around five hours. Curiously, cigarette smoking stimulates the liver to generate more caffeine-destroying enzymes and for smokers the effects last about three hours.
More than 60 plant species produce caffeine, and it is believed that this chemical probably protects them against attack by insects. The coffee bush is indigenous to Ethiopia and was cultivated there over a thousand years ago. It reached Europe around AD 1600, probably via Turkey where it got its name, kehveh. Tea has a much longer tradition, and was being drunk in China in 2500BC, but it too did not reach Europe until the seventeenth century. The cola, or kola, plant is an evergreen tree of tropical Africa which produces glossy nuts with a high caffeine content. The way to release their caffeine was to chew them.
Caffeine is not only a pick-me-up: it also has medicinal benefits, and is used in painkillers, asthma treatments, and diet aids. These rely on its effect of stimulating the metabolism and relaxing the bronchial nerves. Caffeine has long been used to increase physical endurance. In Tibet, not only do Tibetans drink a lot of tea themselves, but they give their horses and mules large vessels of the drink. Distances were once measured in the number of cups of tea deemed necessary for a journey: three cups of tea would give you enough `fuel' for around 8 kilometres.
Chemically, caffeine is a white powder which was first isolated in 1820 by the German chemist Friedlieb Ferdinand Runge, but it was not until 1897 that its molecular structure was deduced. It can be made in the laboratory, but the commercial market is supplied by the caffeine which is produced as a by-product of decaffeinating coffee. Removing caffeine without affecting the taste of coffee is relatively simple, and involves extracting it with liquefied carbon dioxide.
There are many popular myths about caffeine. It is accused of causing sleepless nights, indigestion and bad breath, and as if that were not enough it has also been blamed for raising cholesterol levels in those who drink a lot of it, and so putting them at risk from heart disease. There was even a suggestion in the 1970s that it might cause liver cancer, but that scare turned out to be completely unfounded. Nor does it cause insomnia, indigestion or heart disease, and this was the conclusion of 175 scientists from around the world who attended the International Caffeine Workshop which was held in Greece in 1993. As more and more data have been collected and analysed, the many scares about caffeine have been shown to be little more than the artefacts of poorly designed epidemiological studies into people's eating habits.
Caffeine affects us in many ways. It is metabolised by the liver, which takes about 12 hours to remove 90% of any caffeine we have consumed. The first few times we have caffeine it raises our heart rate and blood pressure quite dramatically, but as we become regular drinkers of colas, coffee and tea, our body stops reacting in this way. Because of these physiological effects it was not surprising that caffeine was thought to be a factor in some common diseases. A report in 1973 suggested that the risk of thrombosis was doubled if a person consumed 400 mg a day, equivalent to drinking five cups of fresh coffee. However, a study in 1990 on 45 000 men failed to find any connection between thrombosis and coffee drinking. A supposed link with heart disease was also shown to be wrong by a large survey in Scotland, where both men and women have a particularly high incidence of this condition. The researchers there questioned more than 10 000 middle-aged men and women and could find no link between caffeine intake and heart disease.
Caffeine acts as a stimulant and its drinks are advertised to emphasize this, so we are told that coffee wakes us up and colas refresh, while a cup of tea revives. It works by boosting the brain's own stimulant dopamine, and this responds up to around four cups of coffee, after which extra cups have no more effect on the level of this in the brain. Caffeine in excess is popularly believed to keep us awake at night, but it probably does not have this effect on most people, unless they drink too much. There are those who metabolise caffeine only slowly, and they may suffer this way. Despite earlier reports that we cannot become addicted to caffeine, caffeine withdrawal symptoms now seem to be accepted, and they are, in order of occurrence: headache, depression, fatigue, irritableness, nausea, vomiting.
In addition to its caffeine, tea may have hidden benefits in the form of three other chemicals it contains. These are salicylate, epicatechin gallate and epigallocatechin gallate. We shall look at the portrait of salicylate a little further along in this gallery. The other two molecules are part of a group known as flavinoids, and are thought to protect the body against free radicals. These are highly dangerous natural chemicals which have a rogue electron, and it is this which enables them to attack key components of the living cell such as DNA, thereby possibly causing cancer. It is their relentless attack on the body which is thought to be the underlying cause of ageing.
Perhaps tea-drinking can help in the fight against free radicals. A Dutch research team carried out a 15-year study on men aged 50 and over, and in 1996 reported their findings which showed that tea-drinkers had a much reduced incidence of stroke. They attributed this to the flavinoids capability of destroying free radicals. Other research has shown that the tea flavinoids also protect against tumours, at least in animals.
 Portrait 4
Rust remover--phosphoric acid
The ingredient of colas which looks rather odd, and rather menacing, is phosphoric acid. Generally we are more familiar with this acid as the active agent in rust-removers, and with its salts, which are called phosphates, and are used in detergents. In the 1970s and 80s, phosphates became a dirty word, and were blamed for the pollution of rivers and lakes, with detergents being fingered as the most important source. We will look a little closer at this issue in Gallery 6, where there is a portrait of phosphates.
People need phosphate in their diet as an essential nutrient to make DNA, build their bones and form their membranes. It also is needed for the chemical adenosine triphosphate (ATP), which plays a central role in helping get the energy we need from food. Phosphate-containing molecules also act as messengers, and govern calcium transport. In addition to these major roles phosphate has many minor uses in the body. It might seem that such a key element could be in danger of being in short supply in our diet, but this rarely happens because the body recycles it very effectively and in any case we have an enormous store of phosphate in our skeleton. The phosphoric acid in colas can be regarded as making a useful but minor contribution.
Sometimes the phosphoric acid in colas has found other uses. Motorists, motorcyclists and truck drivers in the 1950s and 60s used cola to clean the chrome bumpers (fenders), grilles and headlights which lavishly adorned their vehicles in the fashion of those times. The phosphoric acid reacted chemically with the chrome to form a hard surface layer of chromium phosphate which protected it. It also dissolved any rust that formed and protected any of the underlying steel which had become exposed. Industrially phosphoric acid is still used for this purpose, and all anti-rust paints rely on it.
There is nothing sinister about phosphoric acid or its salts. To say that colas contain an industrial cleaner, as one book has claimed, is strictly true, but this is no reason to not drink them. Any phosphate in our food becomes phosphoric acid in the acidic conditions of the stomach. Every living cell needs phosphoric acid to function and it matters not where it comes from.
The phosphoric acid in colas presents no threat to health; indeed, we could regard it as an essential mineral. Plants begin the process of supplying the food chain with phosphate by extracting it from the soil, and they store phosphate in their seeds as the chemical phytic acid. They use this store when they germinate so they can put down roots without needing to take in any phosphate from the environment. Although seeds are highly nutritious on account of the protein, carbohydrate, fats and minerals they contain, they provide us with little in the way of phosphate because we cannot digest the phytic acid store since we lack the enzymes to release its phosphoric acids. So the phytic acid passes straight through us--not that we need it, because every plant and animal cell that we eat contains more than enough phosphate.
We get most of the phosphate in our diet from natural sources such as fish, meat, eggs and dairy products, and a little from unnatural sources such as colas, processed cheese, cheese spreads, sausages and cooked meats, to which it is added to improve texture and regulate acidity.
 Portrait 5
The curse of the cure-all--dipropenyl disulfide
When is something simply a flavouring and when is it a medicine? Garlic is admired by many for having both these properties, and the chemical which is responsible is a simple molecule called dipropenyl disulfide. But can it really be a healing drug? And if it is, should it not be subject to the same kinds of tests that all pharmaceutical drugs are exposed to, so that we know it works and that it is perfectly safe?
Of course you don't need to bother with such tests if the material you are testing is basically a food flavouring ingredient that has long been part of the human diet. Time has tested it for you, although sometimes Time can be proved wrong--witness the once popular herb, comfrey, long used in salads and to make comfrey tea. The sale of comfrey is now banned in Europe because of the harmful chemicals it contains. So perhaps chemists are not being too finicky when they suggest that everything that purports to be a healing drug should be tested in the same way as pharmaceuticals. In other words we should put dipropenyl sulfide through a programme of tests on animals such as mice and rats, dogs and cats, and finally monkeys and humans. Of course it would fail early on, because it has undesirable side-effects, the worst of which is to give the patient an advanced case of halitosis. No creature deserves to have this obnoxious material forced on them, except human volunteers. Nevertheless, it is perfectly natural, and it is the most popular of the so-called alternative medicines on sale today. It is sold as garlic oil and is purchased by millions of people all over the world and taken in the form of capsules. In Germany it is the best-selling over-the-counter drug. We can get used to garlic and eventually come to like it.
Garlic-growing is big business, as well as being an essential part of the domestic garden in many countries. The USA produces around 65 000 tons a year, worth $180 million, and this is grown mainly in California, especially around the small town of Gilroy (pop. 33 500), at whose annual garlic festival it is possible to consume garlic ice cream, garlic cheesecake and garlic scones. In Europe garlic tends to be used in cooking, especially for casseroles and soups, or in salads, and all over the world garlic bread has become a popular way of enjoying it.
Garlic used in cookery loses much of its sting while adding piquancy to soups and savoury dishes. Uncooked garlic in salads can be enjoyable to the eater but not to those they come in contact with afterwards. Yet some people prefer to eat it raw for health reasons, believing it is effective in warding off cancer and cardiac disease. Those who eat it regularly may find their bad breath protects them against illness because it keeps others at a distance. Many are even willing to take it daily in large amounts as though it really was a medicinal drug--but it isn't.
The active ingredient in garlic, dipropenyl disulfide, has two sulfur atoms at its centre, and it is these which produce the odour that its users have to endure, along with their family and close friends. Any chemical we consume which has a lot of sulfur, such as garlic, onions and certain forms of protein, poses a slight social problem for us, if not for our body. One way to get rid of some of the sulfur is to turn it into the obnoxiously smelling molecule methyl mercaptan, which we can breathe out. This is the main cause of halitosis, and we shall look more closely at its portrait next.
A clove of garlic has almost no smell until it is cut or crushed, but when this happens an enzyme called allinase works on an amino acid called allin, and converts it to allicin which is the main constituent in garlic extract. This is the precursor of dipropenyl disulfide--basically the same molecule but with an oxygen atom bonded to one of the two sulfurs. Allicin easily loses its oxygen atom and reduces to the more volatile molecule dipropenyl disulfide. This is the compound which gives garlic its odour.
Raw garlic will give you plenty of this disulfide, but cooking gets rid of it because it is volatile enough to evaporate during cooking. This is the reason you can safely eat a soup or stew that has lots of garlic in the recipe, and still enjoy a friendly tete-a-tete with someone. Some claim that eating parsley or lettuce with raw garlic actually neutralizes its odour, which may well be so, but the evidence is not compelling, and in the end some of it will still be exhaled on the breath.
Epidemiological studies in China and Italy reported that garlic eaters had fewer gastric cancers, and a survey of 40 000 US women appeared to show a link between garlic consumption and lower rates of colon cancer. However, when Elisabeth Dorant and colleagues at the University of Limburg, Maastricht, fed laboratory animals fresh garlic or garlic extracts, they did not observe fewer cases of cancer, although they found tumours were slightly slower in growing in those rats with the disease.
Garlic is known to lower blood cholesterol levels by 10%, if you eat a clove a day, and so it might help prevent cardiovascular disease. However, the evidence that it does so is again less than convincing. In 1994 Christopher Silagy of Flinders University, Adelaide, and Andrew Neil of Oxford University reviewed several tests of the effect of garlic on blood pressure. They concluded that it only helped those with slightly elevated blood pressure, and they could not recommend it as routine clinical therapy.
Such scientific evidence will not impress those who are still convinced that garlic harbours something rather remarkable, and they can point out that garlic has been used in medicine for hundreds of years. Garlic's supporters can even validate their cure-all claims with a bit of chemistry by pointing out that allicin and dipropenyl disulfide are antioxidants, and as such are highly regarded because they can mop up peroxides in the body, thereby preventing the formation of free radicals.
Whether garlic really is effective in warding off cancer and heart disease is doubtful, but the plant is not without its uses. Indeed, it is essential at Halloween, when ghosts walk, witches dance, demons pounce and vampires feast. This is the time to get out the garlic, which is guaranteed to be 100% effective in warding off evil spirits. What probably deters them is the smell of methyl mercaptan on their victim's breath, and this molecule is the subject of our next portrait.
 Portrait 6
The worst smell in the world--methyl mercaptan
There are official standards for acceptable levels of noxious smells, and methyl mercaptan heads the list. This molecule makes the news wherever it is emitted in large amounts, and sometimes it does so because it is used industrially, for example for making the insecticide Dimethoate. When it was accidentally released from a factory in Waltham Abbey, England, the local residents were so sickened by its smell that some rushed to hospital assuming they were being poisoned by a deadly pollutant. Others rang the local gas company. This is not as surprising as it seems, because compounds similar to methyl mercaptan are used to odourize natural gas, so that leaks are easy to detect.
Methyl mercaptan is also produced naturally from bacteria in the environment, and the shoreline near Edinburgh, Scotland, often exudes it, much to the distress of the residents of the select suburb which overlooks that beautiful stretch of water.
The methyl mercaptan we breathe out after eating garlic or taking a garlic capsule is produced in the body as we digest allicin. Bacteria are also responsible for the methyl mercaptan we generate in our own mouths and may breathe out continuously as bad breath. This is formed from our own body protein as it breaks down under bacterial attack. We can easily detect methyl mercaptan when someone speaks to us--humans can detect it at levels of parts per billion in air--but, curiously, we cannot smell the gas we produce ourselves. In Japan, you can test your own breath with an Oral Checker of which thousands have already been sold. Katunori Nakamura has patented his halitosis detector, which is about the size of a powder compact and works on the principle that a metal oxide, like tin oxide, changes its electrical resistance when it absorbs a gas like methyl mercaptan onto its surface.
Bad breath is caused by several molecules, such as hydrogen sulfide and dimethyl sulfide, but the main culprit is methyl mercaptan. Hydrogen sulfide, the traditional stink of the chemistry laboratory, is much less smelly, and the same is true of dimethyl sulfide, which is part of the aroma of fresh coffee. Graham Embery of the University of Wales at Cardiff researches the sulfur-containing molecules found in the mouth which arise as a result of the activity of bacteria. These break down the protein residues and release methyl mercaptan from the amino acids cysteine and methionine. If the smell of methyl mercaptan is very strong, it indicates gum disease. Methionine is essential for all living things, and animal protein contains up to 4% of this amino acid; therefore bacteria are capable of releasing enough methyl mercaptan to make the victim's breath smell vile.
Embery and Gunnar Rolla of the University of Norway in Oslo are authors of the book Clinical and Biological Aspects of Dentifrices, which devotes a whole chapter to halitosis. Embery's advice to those who suspect that they exhale methyl mercaptan is to use a toothpaste that contains anti-plaque agents, such as zinc or tin salts. These metals interfere with the enzymes in the bacteria which produce the methyl mercaptan. Traditionally, mouthwashes are supposed to cure halitosis, but they do little more than clean the mouth and disguise the offending smell. The best known one, Listerine, consists of water and alcohol, with benzoic acid and natural flavours, such as thymol and menthol. A good rinse with a mouthwash will remove about half the oral bacteria. A more popular way to clean the mouth is to increase saliva flow by chewing gum.
Our feet can also harbour microbes that give off methyl mercaptan, especially if we provide them with a perfect environment of unwashed socks and unventilated shoes. Staphylococci and aerobic coryneform bacteria are to blame, and these flourish in the increasingly alkaline conditions which are a feature of such socks and shoes. If you have smelly feet, then the chemical answer is to insert into your shoes special charcoal-filled insoles, which have layers of carbon that absorb the methyl mercaptan. Since the amount of methyl mercaptan is tiny, the insoles will go on working for weeks.
Methyl mercaptan is the simplest member of a series of compounds in which there can be chains of up to 20 carbon atoms attached to a sulfur atom. Methyl mercaptan has one carbon atom. Mercaptans with three and four carbons are those we encounter when we smell a leak of gas. A mercaptan with 18 carbons attached to a chain is used as a wax in silver polishes.
A major drawback in manufacturing and transporting methyl mercaptan for industry is its low boiling point of 6 Celsius. Luckily, it can easily be turned into the chemically similar dimethyl disulfide (DMDS), a yellow liquid which boils at 110 Celsius. This consists of two methyl mercaptans joined through their sulfur atoms. It is only slightly less odorous, but is much safer to transport, and most is made at Lacq in south-west France, where natural gas wells bring up large amounts of hydrogen sulfide. This is reacted with methanol to form methyl mercaptan and then converted to DMDS.
Methyl mercaptan is used industrially to make pesticides, and especially for weedkillers for cereal crops like wheat, maize and rice. Its chief use in industry is to regenerate the catalysts used in the refining of petrol. Methyl mercaptan is also used to make methionine, an amino acid which may be deficient in the diet. Some animal feeds are now fortified with methionine, thereby increasing the amount of this in the animals' meat and milk.
 Portrait 7
Merthyl mercaptan and dimethyl sulfide may be the worst smells we come across in normal life, but there are even worse variants of these molecules; their selenium versions. Selenium is chemically very similar to sulfur, but when it replaces sulfur in a volatile molecule the smell intensifies dramatically. Research chemists who work with selenium compounds have to be very careful to avoid contact with them. Any that gets on to the skin, or even on to clothing, is liable to be expelled as a methyl compound by any micro-organism that is around. If you accidentally ingest some, then your breath will smell appalling. If you take too much selenium then you could even poison yourself.
Despite this unpleasantness, selenium is essential to many species, including humans. We need it only in microgram quantities, but even so every cell of our body contains over a million atoms of selenium. At such low levels it poses no threat to our social life.
It is difficult to measure how much selenium we take in, how much we excrete, and how much we really need. The daily intake varies between 6 and 200 mcg according to the type of food we eat. The average Westerner takes in about 60 mcg per day, which is more than enough to prevent the symptoms of selenium deficiency--a mere 10 mcg may be all we need, provided we get it regularly. Some days our body may lose more selenium than it absorbs, but because the average adult holds about 15 000 mcg (or 15 mg) this does not pose a threat. A single dose of 5000 mcg (5 mg) would be dangerous, and 50 000 mcg (50 mg) would be lethal for many humans. We store most of our selenium in our skeleton, but the parts of the body with the highest levels of selenium are hair, kidneys and testicles.
Most people get most of their selenium from wheat products such as breakfast cereals and bread. The foods richest in selenium are:
* seafoods, such as tuna, cod and salmon
* offal, such as liver and kidney
* nuts, such as Brazil nuts, cashews and peanuts
* wheat germ, bran and Brewer's yeast.
All these have 30 mcg or more of selenium per 100 g of product, although in the case of wheat and meat products the level depends upon the soil of the farm from which they came. The only people who might just be at risk of selenium deficiency are pregnant and breast-feeding women, and children--and only if they carefully avoided all the types of foods listed above. Essential though selenium is, we can have too much, and the recommended maximum daily intake is 450 mcg. Above this we risk selenium poisoning, the most obvious symptom of which is bad breath and body odour. The smell is caused by volatile methyl selenium molecules which our body produces as it rids itself of the selenium it does not need.
Despite the smell, we would die without selenium. In 1975 it was proved essential for humans when Yogesh Awasthi discovered it was part of a human enzyme called glutathione peroxidase. In 1991 Dietrich Behne in Berlin found selenium in a second enzyme, deiodinase, which promotes hormone production in the thyroid gland. If the amount of selenium in our body is too low, then we appear to be at risk from several conditions, such as anaemia, high blood pressure, infertility, cancer, arthritis, premature ageing, muscular dystrophy and multiple sclerosis. As yet there is no proof that a lack of selenium in the diet leads to these conditions, and it is more likely that this element is having a second-order effect, in other words it controls other components which actually do the harm.
Selenium is known to protect us against the effects of other toxic metals such as mercury, cadmium, arsenic and lead: for example, the damage that cadmium can do to our reproductive organs and to a foetus is thought to be prevented by selenium. Tuna fish, which accumulate higher-than-expected levels of mercury, are also thought to be protected by selenium, and analysis shows that for every mercury atom in a tuna there is also an atom of selenium. This 1:1 ratio appears to be true for other marine mammals such as seals, and for men who work in mercury mines.
Most normal diets contain more than enough selenium, so there is little need for people to take selenium supplements, although these are on sale in health foods shops and pharmacies. As a dietary supplement, selenium is taken in the form of sodium selenite, which is a white crystalline material that is soluble in water. The daily dose is 50 mcg. Selenium was first popularized as a dietary aid by Alan Lewis, whose book Selenium: the Essential Trace Element You Might Not be Getting Enough Of was published in 1982. Lewis reported that selenium could be used to treat rheumatism, arthritis, heart disease and cancer, and that it would even delay old age. While most of these claims appear fanciful, and are based mainly on anecdotal evidence, two at least are well founded: trials in China have shown that selenium does prevent certain types of heart condition, and that the body needs a certain level if it is to ward off cancer effectively.
The Chinese have long had a special interest in selenium because large areas of that country have selenium-deficient soil, and this affects the health of the local population. Children in the Keshan region were prone to a heart condition known as Keshan disease, which is caused by a lack of selenium. This disease results in a swelling of the heart and kills half of those afflicted. A large-scale trial in south China in 1974 involved 20 000 children, half of whom were given selenium-containing tablets and half given a placebo. Of those on the placebo, 106 developed Keshan disease and 55 died, while of those on the selenium supplement only 17 got Keshan and one died.
Another test in China also found that selenium was beneficial in reducing cancer cases. Among the Chinese people living in the north central province of Linxian there is a high incidence of stomach cancer. The people there agreed to take part in a five-year project and 30 000 middle-aged people were given different combinations of dietary supplements such as vitamins A, [B.sub.2], C and E, zinc and selenium. The study showed a remarkable drop in cancer cases in the group taking vitamin E plus selenium.
Selenium was discovered in 1817 by Jons Jacob Berzelius at Stockholm, Sweden. He named it after selene, the Greek word for the Moon, to match the name of the related element tellurium, which was based on the Latin tellus meaning Earth. He found it when he investigated a red-brown sediment which collected at the bottom of the chambers in which sulfuric acid was made. The element selenium is available either as a silvery metal or as a red powder. The main producing countries are Canada, the USA, Bolivia and Russia, and most comes from copper smelters and refiners. Copper sulfide ores have copper selenide as an impurity. The most important source of selenium is the slime which settles at the bottom of tanks when impure cop per is refined electrolytically, and this sediment may contain up to 5% selenium. This source accounts for about 90% of selenium production. Each year about 150 tons of selenium is recycled from industrial waste and reclaimed from old photocopying machines.
The metallic form of selenium has the curious property of generating an electric current when light falls on its surface, and it is used in photoelectric cells, light meters, solar cells and photocopiers. These electronic uses account for about a third of all selenium production, and require high grade selenium of 99.99% purity. The second largest user is the glass industry, where selenium goes into special glass such as the bronze architectural glass which screens out the Sun's rays. The third main use is to make sodium selenite for animal feeds and food supplements. Selenium is also used in metal alloys, such as the lead plates used in storage batteries; in rectifiers to convert electric current from a.c. to d.c.; and in anti-dandruff shampoos.
Selenium is rarer than silver, and one day mineral sources of the element will be exhausted. Then we may have to harvest it by growing crops like milk-vetch on high-selenium soils. This could yield as much as 7 kg per hectare (3 kg per acre). The current world demand for selenium, of about 1500 tons per year, would require about 200 000 hectares to be farmed this way. But as reserves of selenium in ore deposits amount to over 100 000 tons, it will be quite some time before this type of farming will be needed.
The effects of high-selenium soils have been known for a long time. Animals grazing on such pasture may suffer from the so-called `blind staggers'. Marco Polo (1254-1324) wrote that the animals of Turkestan behaved this way. The plant responsible for the staggers was probably vetch, which can concentrate up to 1.4% of its weight as selenium. The cowboys of the Wild West knew that this plant could affect their herds, and called it locoweed, from the Spanish word loco meaning insane. In 1934 the biochemist Orville Beath proved that the staggers were caused by excess selenium in the diet. When the vetch had an offensive smell, it was a sure indication that it had absorbed a high level of selenium.
 Portrait 8
The state of the heart--salicylates
In 1763 the Reverend Edmund Stone, an English parson living in the Cotswolds, made an infusion of the bark of the white willow and gave the drink to people in his village who had fever. Today we can only guess at what his parishioners were suffering from, but one suspects most of them probably had a mild virus infection, such as `flu. In any event, they clearly had high temperatures and the treatment was successful in bringing these down. We now know that it would have been effective because Stone had given the villagers a solution that in the human body would produce salicylic acid, which is good at reducing high body temperatures.
In the century which followed, this simple but effective treatment continued even though it had unpleasant side effects. Salicylic acid is a strong irritant, causing bleeding and ulcers in the mouth and stomach. It was not until two chemists working for the German chemical company Bayer made the derivative, acetylsalicylic acid, that the treatment became relatively safe. That was in 1893, and the chemists were Felix Hoffmann and Heinrich Dreser. Their product was named aspirin, and for over a century it has brought relief to millions of people around the world. Aspirin works by blocking an enzyme that makes prostaglandins, the chemicals which signal that the body has been injured or invaded by a micro-organism. Protaglandins are generated in excess, and the result is inflammation, pain and fever.
Today in the USA around 20 billion aspirin tablets are taken each year, even though it is still a risky remedy and can cause stomach inflammation in some people. The best known form of aspirin is Alka Seltzer, whose tablets also contains citric acid and sodium bicarbonate. The bicarbonate reacts with the aspirin to form its sodium salt, thereby making it soluble in water and supposedly quicker acting, and reacts with the citric acid to generate bubbles of carbon dioxide. The citric acid also masks the taste of the aspirin.
Although aspirin has been used for a long time it is not without its more serious risks, and for some young children aspirin has proved fatal, when they have been given it to treat a viral infection like `flu or chicken pox. They developed what is known as Reye's syndrome, and although this is extremely rare, it is best never to give aspirin to a child under the age of 12.
Despite its disadvantages, aspirin is much more than just a painkiller, and is prescribed by doctors to patients who have suffered a heart attack because it inhibits the formation of those chemicals which cause blood platelets to aggregate together, which is what starts a blood clot. Aspirin is normally sold as 300 mg tablets, and these can safely be taken at a rate of two every four hours to a maximum dose of a dozen tablets (4 g) per day. A single dose of 10 g (30 tablets) can kill an adult because it makes their blood too acidic. The body tries to cope by rapid breathing to dispel [CO.sub.2] and thereby reduce acidity, and by boosting the action of the kidneys, which leads to dehydration. If the acidity cannot be corrected by natural means, tissue damage occurs and eventually death.
More than half the people in developed societies die of heart disease. Rather than wait until their heart is showing signs of weakness, when they would be prescribed aspirin by their doctor, many now believe they can escape this fate by the simple expediency of taking a junior aspirin tablet every morning as a preventative. Such a tablet contains a quarter of the normal dose, in other words 75 mg of acetyl salicylic acid. What they may not realize is that they are also getting salicylate from other sources, notably their diet.
Some who fear for the health of their heart have been persuaded that they can fend off the grim reaper by eating the right type of fat. They avoid all animal fats and hydrogenated vegetable oils, and go for those vegetable oils which are mainly mono-unsaturated. They may also have read that those who drink red wine are also less prone to heart disease. All this advice for a healthy life appears to be sound, and those who advocate it can point to the people of the Mediterranean region where heart disease is much less common than elsewhere. Clearly the diet of that region must hold the key, they say, and the focus tends to fall on olive oil and red wine. A chemical explanation is usually offered in terms of mono-unsaturated fits, the main component of olive oil, and polyphenol antioxidants, which are particularly abundant in the skins of black grapes and which are extracted into red wine.
It may well be that the Mediterranean-style diet has another factor: salicylate. This is also present in many vegetables, herbs and fruits. Gazpacho, the soup made from tomatoes, onions and tarragon, and served cold, may contain a healthy dose of salicylate, while ratatouille, the vegetable dish of aubergine, courgettes, red peppers and tomatoes, could be brimming with the stuff. Other foods from warm climes are relatively rich in salicylate, such as pineapples, melons and mangoes, while curry powder has over 200 mg pre 100 g.
It is possible to plan a diet that will garner salicylate in gentle stages throughout the day. For example, if you like fruit at breakfast go for raspberries: a bowl of them will provide 4 mg of salicylate. If you want a salad at lunch then choose chicory leaves and add a couple of gherkins: both have lots of salicylate. A liberal sprinkling of tomato ketchup on your hamburger and fries is also a good idea, and if you need a snack during the day then nibble a handful of currents or raisins.
The easiest way by far to boost your salicylate intake is to drink tea. A cup, made with one tea bag, will provide 3 mg, and if you drink the average 5 cups a day you will be getting a life-enhancing 15 mg. Coffee drinkers, on the other hand, would need to take in 20 mugs of their brew to get this amount. Other foods to boost your daily dose of salicylate are almonds, peanuts, coconut, honey, Worcester sauce, licorice, peppermint, broccoli, cucumbers, olives and sweetcorn. And only eat potatoes with their skins on: peel them and all the salicylate is gone. The same is true of pears. If you are going to a party you can enjoy salicylate in fruit juices, wines and beer.
Of course you may be one of the unlucky few per cent who react badly to salicylate, and are advised to avoid aspirin because it can cause stomach bleeding and ulcers. In which case you are probably likely to get indigestion from a diet rich in salicylate, so the best advice is to avoid foods like these. If you are hypersensitive to salicylate then you may even be put on a salicylate-free diet, but you need not feel deprived because there are lots of zero-salicylate foods to choose from: meat, fish, milk, cheese, eggs, wheat, oats, rice, cabbage, brussels sprouts, celery, leeks, lettuce, peas and bananas have none at all. And if you fancy a drink then stick to spirits, but be careful to choose the right mixer. Gin and tonic is fine, and so is rum and coke, but avoid Bloody Marys (vodka and tomato juice).
 Portrait 9
Those unspeakable molecules--phthalates
Finally in this gallery we come to a portrait of a molecule that is present in everything we eat: phthalate. There have been several scares about phthalates over the years: a recent one in the UK concerned their presence in formula feeds for babies. Mothers were alarmed to be told that phthalates were contaminating their baby's feed, and that these molecules were being described, somewhat mischievously, as `gender-bending' chemicals. The panic that resulted echoed an earlier phthalate scare of the 1970s when they were said to leach from plastic wrapping into food, and were then accused of causing cancer. Despite these worrying assertions, there is no need for alarm, because phthalates cause neither cancer nor infertility in humans, as we will discover. Phthalates are derivatives of phthalic acid, which consists of a benzene ring with two acid groups attached. These groups may be next to each other, when the molecule is called simply phthalate, or on opposite sides of the ring, when it is called terephthalate. (There is a third form in which the groups are one atom apart, but these have little commercial significance.) Phthalates were first made in the 1850s and called naphthalates, from naphtha, the ancient Greek name for natural petroleum, but this was soon shortened to phthalate.
Phthalates are entirely manufactured and worryingly widespread; even in remote regions of the planet analysts have recorded 0.5 ppm of phthalates in rainwater, so even the peoples of the high Himalayas and the remote Pacific islands get a daily dose. The alarm over baby foods came from a report by the UK's Ministry of Agriculture, Fisheries and Food, which released surveys entitled Phthalates in Paper & Board Packaging (1995) and Total Diet Survey (1996) which reported them to be present in almost all food analysed, not just in baby milk. Levels in milk and milk products were reported to be around 1 ppm, and for a time it looked as though this might be coming from the PVC tubing used in milking machines, but investigation showed that this source accounted for only a tenth of what was present.
Both kinds of phthalate are produced industrially. Terephthalate is used to make polyester for bottles and fibres; it is permanently fixed as an integral part of the polymer and poses no threat. We will be inspecting its portrait in Gallery 5. The other kind of phthalate goes into plastics like PVC to make them pliable. PVC is a tough, rigid solid used for window frames and drainpipes, but when phthalate is added to it the plastic becomes flexible because this allows the polymer chains to move over one another. In this way we get PVC that can be used as garden hoses, wallpapers, shower curtains, clothes, blood bags and water beds. However, it is electric cable and vinyl flooring which uses most of the phthalate. This phthalate is not fixed, and is simply blended in to act as a molecular lubricant. If one of these phthalate molecules finds itself near the surface of the PVC it is free to escape--to be rubbed off or to evaporate into the air.
Because of earlier fears about their safety, plasticizer phthalates are now among the most investigated of all chemicals. The leading plasticizer is DEHP, short for di(ethylhexyl) phthalate, but according to David Cadogan, of the European Union's Council for Plasticizers and Intermediates in Brussels, this poses little risk: `As far as humans are concerned it causes neither cancer nor reproductive effects. Nor are phthalates accumulating in the environment because they are biodegradable, and levels are falling. In Rhine sediment, for example, there has been a reduction of 85% since the 1970s. Phthalates are very insoluble in water--about a millionth of a gram per litre--so leakage from plastics in old landfill sites is tiny.'
In 1990 the EU Commission said that DEHP should not be classified as a carcinogen, because no carcinogenic or oestrogenic activity was found with fish, hamsters, guinea-pigs, dogs or monkeys. However, rats did show increased risks of liver tumours and smaller testes, but these animals, unlike humans, are known to be particularly prone to respond this way because they have been specially bred to be sensitive to cancer-forming chemicals. Humans are not at risk. The Danish Institute of Toxicology concluded that an intake of 500 mg a day was without effect. Our average daily intake is around 0.35 mg, which over a lifetime would amount to less than 10 g (a dessert spoonful). For babies, the tolerable daily intake is 0.05 mg per kilogram of body weight, but no formula feed would provide anything like this amount of DEHP. In any case the 0.05 guideline has a large inbuilt factor and is based on the tests on rats. The danger from phthalates is negligible, even to babies. If all the phthalates in a year's supply of milk were to be consumed at one feeding, it would still not be enough to make a baby sick, let alone anything more serious.
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