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Food The Chemistry of its Components
By T. P. Coultate The Royal Society of Chemistry
Copyright © 2002 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84755-090-3
CHAPTER 1
Introduction
For the chemists of the 18th and 19th centuries an understanding of the chemical nature of our food was a major objective. They realised that this knowledge was essential if dietary standards, and with them health and prosperity, were to improve. Inevitably it was the food components present in large amounts, the carbohydrates, fats, and proteins, that were first nutrients to be described in chemical terms. However, it was also widely recognised that much of the food, and drink, on sale to the general public was very likely to have been adulterated. The chemists of the day took the blame for some of this:
There is in this city [London] a certain fraternity of chemical operators who work underground in holes, caverns and dark retirements ... They can squeeze Bordeaux from the sloe and draw champagne from an apple.
The Tattler, 1710
but by the middle of the 19th century the chemists were deeply involved in exposing the malpractices of food suppliers. Chemistry was brought to bear on the detection of dangerous colourings in confectionery, additional water in milk, beer, wines and spirits, and other many other unexpected food ingredients. A major incentive for the development of chemical analysis was financial. The British government's activities were largely funded by excise duties on alcohol and tea and large numbers of chemists were employed to protect this revenue.
As physiologists and physicians began to relate their findings to the chemical knowledge of foodstuffs the need for reliable analytical techniques increased. 20th century laboratory techniques were essential for the study of vitamins and the other components that occur in similarly small amounts, the natural, and artificial colour and flavour compounds.
Until the use of gas chromatography (GC) became widespread in the 1960s the classical techniques of 'wet chemistry' were the rule but since that time increasingly sophisticated instrumental techniques have taken over. The latest methods are often now so sensitive that many food components can now detected and quantified at such low levels (parts per billion, 1 microgram per kilogram, are commonplace) that one can have serious doubts over whether the presence of a particular pesticide residue, environmental toxin or the like at the detection limits of the analysis really has any biological or health significance. It is perhaps some consolation for the older generation of food chemists that some of the methods of proximate analysis* that they, like the author, struggled with in the 1960s, are still in use today, albeit in automated apparatus rather than using extravagant examples of the glassblower's art.
By the time of World War II it appeared that most of the questions being asked of food chemists by nutritionists, agriculturalists, and others had been answered. This was certainly true as far as questions of the 'what is this substance and how much is there?' variety were concerned. However, as reflected in this book, over the past few decades new questions have been asked and many answers are still awaited. Apart from the question of undesirable components, both natural and man-made food chemists nowadays are required to explain the behaviour of food components. What happens when food is processed, stored, cooked, chewed, digested and absorbed? Much of the stimulus to this type of enquiry has come from the food-manufacturing industry. For example, the observation that the starch in a dessert product provides a certain amount of energy has been overtaken in importance by the need to know which type of starch will give just the right degree of thickening and what is the molecular basis for the differences between one starch and another. Furthermore that dessert product must have a long shelf-life and look as pretty as the one served in the expensive restaurant.
In recent years these apparently rather superficial aspects of food supply have begun to take on a wider significance. The nutritionists, physiologists and other scientists now recognise what consumers have always known – there is more to the business of feeding people than compiling a list of nutrients in the correct proportions. This is as true if one is engaged in famine relief as it is in a five-star restaurant. To satisfy a nutritional need a foodstuff must be acceptable, and to be acceptable it must first look and then taste 'right'.
In recent years we have become increasingly conscious of two other aspects of our food. As some of us have become more affluent, our food intake is no longer limited by our income and we have begun to suffer from the Western 'disease' of overnutrition. Our parents are appalled when our children follow sound dietetic advice and discard the calorie-laden fat from around their sliced ham and food scientists are called upon to devise butter substitutes with minimal fat content. Closely associated with this issue is the intense public interest in the 'chemicals' in our food. Sadly, the general public's appreciation of the terminology of chemistry leaves much to be desired. It is unlikely that many people would buy coleslaw from the delicatessen if the label actually listed the 'active ingredients':
ethanoic acid,
α-D-glucopyranosyl-(1,2)-β-D-fructofuranose,
p-hydroxybenzyl and indoylmethyl glucosinolates,
S-propenyl and other S-alkyl cysteine sulfoxides,
β-carotene (and other carotenoids),
phosphatidylcholine.
(The relationship of this list to the recipe for coleslaw will emerge later.)
The issue of 'chemicals' in food is closely linked to the pursuit of 'naturalness' as a guarantee of 'healthiness'. The enormous diversity of the diets consumed by Homo sapiens as a colonist of this planet makes it impossible to define the ideal diet. Diet related disease, including starvation, is a major cause of death but it appears that while choice of diet can certainly influence the manner of our passing diet has no influence on it's inevitability. As chemists work together with nutritionists, doctors, epidemiologists and other scientists to understand what it is we are eating and what it does to us we will come to understand the essential compromises the human diet entails. After all, our success on this planet is to some extent at least owed to an extraordinary ability to adapt our eating habits to what is available in the immediate environment. Whether that environment is an Arctic waste, a tropical rain forest, or a hamburger-infested inner city, humans actually cope rather well.
This book sets out to introduce the chemistry of our diet. The early chapters (2 to 5), covering food's 'macro-components' are more overtly chemical in character because these are the substances whose chemical properties exert the major influence on the obvious physical characteristics of foodstuffs. If we are to understand the properties of food gels we are going to need a firm grasp of the chemical properties of polysaccharides. Similarly we will not understand the unique properties that cocoa butter gives to chocolate without getting involved in the crystallography of triglycerides.
In the next seven chapters we consider substances drawn together by the nature of their contribution to food, as colours, vitamins etc. rather than broad chemical classifications. Although this means that less attention can be devoted the chemical behaviour of these substances individually there is still much that the chemist contribute to our understanding of taste, appearance, nutritional value etc. Science is rarely as tidy as one might wish and it is inevitable that some food components have found their way into chapters where they do not really belong. For example, some of the flavonoids mentioned in Chapter 6 make no contribution to the colour of food. However, in terms of chemical structure they are closely related to the flavonoid anthocyanin pigments and Chapter 6 could be regarded as good a location for them as any. The final chapter is devoted to water. Apparently the simplest food component, and certainly the most abundant, it remains one of the most poorly understood. It could, as has often been suggested, be placed at the front of this book but it is feared that much of water's chemistry would appear so intimidating as to prevent many readers penetrating to the tastier chapters.
This book does not set out to be a textbook of nutrition, its author is in no way qualified to make it one. Nevertheless chemists cannot ignore nutritional issues and wherever possible the links between the subtleties of chemical nature of food components and nutritional and health issues have been pointed out. Similarly students of nutrition should find a greater insight into the chemical elements of their subject valuable. It is also to be hoped that health pundits who campaign for the reduction of this or that element in our diet will have a better appreciation of what exactly it is they are asking for, and what any knock-on effects might be.
Food chemists should never overlook the fact that the object of their study is not just another, albeit fascinating, aspect of applied science. It is all about what we eat, not just to provide nutrients for the benefit our bodies but also to give pleasure and satisfaction to the senses. Not even the driest old scientist compliments the cook on the vitamin content of the dish, it's the texture and flavour, and the company around the table, that wins every time.
A Brief Note on Concentrations
The concentrations of chemical components are expressed in a number of different styles in this book, depending on the context and the concentrations. Some readers may find the following helpful.
(a) However they are expressed, concentrations always imply the amount contained, rather than added. Thus '5 g of X per 100 g of foodstuff implies that 100 g of the foodstuff contains 95 g of substance(s) that are not X.
(b) The abbreviation 'p.p.m.' means 'parts per million' i.e. grams per million grams, or more realistically milligrams per kilogram. One 'p.p.b.', or part per billion, corresponds to one microgram per kilogram.
(c) Amounts contained in 100 g (or 100 cm3) are often referred to as simple percentages. Where necessary the terms 'w/w', 'v/v' or 'w/v' are added to indicate whether volumes or weights or both are involved. Thus '5% w/v' means that 100 cm of a liquid contains 5 g of a solid, either dissolved or in suspension. Note that the millilitre (ml) and litre (l) are no longer considered acceptable. Although the replacement for the ml, 'the centimetre cubed' ('cm3'), is widely recognised there is little sign that the cubic decimetre, or 'dm3' has taken over from the litre, except in the teaching laboratories that must always be seen to 'toe the party line'.
(d) Most often a strictly mathematical style is adopted, with 'per' expressed as the power of minus one. Since mathematically:
x-1 = 1/x
5 µg kg-1 becomes a convenient way of writing 5 micrograms per kilogram. This brief but mathematically rigorous style comes into its own when the rates of intake of substances such as toxins have to be related to the size of the animal consuming them, as in '5 milligrams per day per kilogram body weight', which abbreviates to:
5 mg day-1 kg-1
A quantity, say 10 mg, per cubic centimetre, cm-3, would be written: 10 mg cm-3.
FURTHER READING
Many readers, especially advanced students, will want to learn more about particular topics than can be accommodated in this volume. The 'Further Reading' sections at the end of each chapter will generally meet such needs and provide a gateway into the relevant primary literature, i.e. research papers. The review journal 'Trends in Food Science and Technology' is also an invaluable source of up-to-date information for students, as it was for the author when preparing this edition. Nowadays students have easy access to computerised databases, downloadable journals etc. that open up the scientific literature in ways that those of us brought up on the bound volumes of 'Chemical Abstracts' can only marvel at.
The internet can provide a wealth of information but it must be used with very great caution. It has always been the case that one should not necessarily believe something just because it was printed in a book, but such cynicism is an absolute essential when looking at web sites. For example, an internet search using 'choline' and 'vitamin' as its keywords will locate the official statement that choline is not a vitamin (see Chapter 8) together wit innumerable commercial sites that claim the opposite in order to enhance sales of choline as a 'supplement'. The temptation to include addresses of relevant and authoritive websites was resisted since, unlike the printed scientific literature, they tend to be rather ephemeral and unlikely to remain available throughout the lifetime of a textbook.
CHAPTER 2
Sugars
Sugars, such as sucrose and glucose, together with polysaccharides, such as starch and cellulose, are the principal components of the class of substances we call carbohydrates. In this chapter we will be concerned with the sugars and some of their derivatives; the polysaccharides, essentially a special class of their derivatives, will be considered in Chapter 3. Although chemists never seem to have the slightest difficulty in deciding whether or not a particular substance should be classified as a carbohydrate, a concise, formal definition has proved elusive. The empirical formulae of most of the carbohydrates we encounter in foodstuffs approximate to (CH2O) n, hence the name. More usefully, it is simpler to regard them as aliphatic polyhydroxy compounds which carry a carbonyl group (and, of course, derivatives of such compounds). The special place of sugars in our everyday diet will be apparent from the data presented in Table 2.1.
Sugars are automatically associated in most peoples' minds with sweetness and it is this property that normally reveals to us their presence in a food. In fact there are a great many foods, especially elaborately processed products, where the sugar content may be less obvious. Fortunately it is now common to find information about the total sugar content (as in: 'Total carbohydrates ... of which sugars ...') on the labels of packaged food products but this tells us little about which sugars are present. In fact only a few different sugars are common in foods, as shown in Figure 2.1.
MONOSACCHARIDES
The monosaccharides constitute the simplest group of carbohydrates, and, as we shall see, most of them can be referred to as sugars. The monosaccharides have a backbone of between three and eight carbon atoms, but only those with five or six carbon atoms are common. The names of monosaccharides that have a carbonyl group have the suffix '-ose', and in the absence of any other identification the number of carbon atoms is indicated by terms such as triose, tetrose, and pentose. The chain of carbon atoms is always straight, never branched (2.1):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.1)
All but one of the carbon atoms carry a hydroxyl group, the exception forming the carbonyl group. It is the presence of carbonyl group that confers reducing properties on monosaccharides and many other sugars and the carbonyl is often referred to as the 'reducing group' in considerations of sugar structure. The prefixes 'aldo-' and 'keto-' are used to show whether the carbonyl carbon is on the first or a subsequent carbon atom, i.e. the sugar is an aldehyde or a ketone. Thus we refer to sugars as, for example, aldohexoses or ketopentoses. To complicate matters further, the two triose monosaccharides are almost never named in this way but are referred to as glyceraldehyde (2,3-dihydroxypropanal) (2.2) and dihydroxyacetone (1,3-dihydroxypropan-2-one) (2.3).
[FORMULA NOT REPRODUCIBLE IN ASCII] (2.2)
[FORMULA NOT REPRODUCIBLE IN ASCII] (2.3)
Of greatest concern to us will be the aldo[(2.4) and (2.6)] and keto[(2.5) and (2.7)] pentoses [(2.4) and (2.5)] and hexoses [(2.6) and (2.7)], shown here with the conventional numbering of the carbon atoms:
[FORMULA NOT REPRODUCIBLE IN ASCII] (2.4)
[FORMULA NOT REPRODUCIBLE IN ASCII] (2.5)
[FORMULA NOT REPRODUCIBLE IN ASCII] (2.6)
[FORMULA NOT REPRODUCIBLE IN ASCII] (2.7)
The carbon atom of each CHOH unit carries four different groups and is therefore asymmetrically substituted. This asymmetry is the source of the optical isomerism characteristic of carbohydrates. Almost all naturally occurring monosaccharides belong to the so-called D-series. That is to say their highest numbered asymmetric carbon atom (e.g. carbon 4 in a pentose or carbon five in a hexose), the one furthest from the carbonyl group, has the same configuration as D-glyceraldehyde (2.8) rather than its isomer L-glyceraldehyde (2.9).
(Continues...)
Excerpted from Food The Chemistry of its Components by T. P. Coultate. Copyright © 2002 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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