an important category of substances in food, include all sugars, starch, cellulose, hemicelluloses, pectin, and the various sorts of gum. They make up most of the solid matter in plants. Animals, however, whose solid matter consists mostly of proteins, contain only a little carbohydrate (apart from any undigested food they may have on board).
Plants store energy in the form of carbohydrates. When people and animals eat the plants they release this energy and can use it for their own purposes. Carbohydrates are the main source of energy for most peoples of the world. In an average western diet, for example, 55 to 65% of the energy comes from carbohydrates, the rest from fats and proteins. In a typical Japanese diet the figure for carbohydrates is about 80%.
Carbohydrates are not a requirement for survival as are proteins, essential fatty acids, vitamins, or minerals. The Inuit (better known as Eskimos) live largely on meat and fish and get their food energy from protein and fats. However, in most parts of the world this would be a wasteful way to live, since foods rich in protein and fat are relatively expensive, and their vital nutrients are lost if they are simply ‘burnt as fuel’. A diet with plenty of carbohydrates and a smaller, but sufficient, quantity of protein and fat allows the body to ‘burn’ the carbohydrates and exploit the vital nutrients in the other substances to its best advantage. Moreover, a diet with virtually no carbohydrates throws the body into an abnormal metabolic condition known as ketosis, characterized by foul-smelling breath. The popular Atkins slimming diet, which virtually excludes carbohydrates, has this effect (see diet).
In chemical terms, carbohydrates consist of carbon, hydrogen, and oxygen. Most of them have the general formula C6H10O5. (The term carbohydrate means ‘carbon with water’ and water consists of hydrogen and oxygen.) Plants make them by taking in carbon dioxide from the atmosphere and water from the soil; energy from sunlight falling on the leaves powers a series of chemical reactions which convert these substances to carbohydrates, which the plants retain and use, and to oxygen. This process is known as photosynthesis. The oxygen replenishes the supply in the atmosphere which is steadily used up by animals breathing and by the burning of fuels.
The molecules of carbohydrates may be relatively simple, or may consist of thousands of atoms. But they are all based on molecules of simple or ‘single’ sugars, which can be joined together in long, often branched, chains to make the more complex carbohydrates. These simple or ‘single’ sugars, called monosaccharides, are described in the entry for sugar, where it is also explained that all digestion of carbohydrates is a process of breaking down large molecules to monosaccharides and changing them to dextrose. Slightly more complex carbohydrates are the disaccharides or ‘double’ sugars which include sucrose (ordinary white sugar), maltose (malt sugar), and lactose (milk sugar). Oligosaccharides (‘few’ sugars) include raffinose (three units) and stachyose (five units), both found in legumes.
Carbohydrates with a large number of sugar units are called polysaccharides (‘many’ sugars). They include starch, cellulose, hermicelluloses, pectin, and gums, all of which occur naturally in plants. In starch all the units are of the sugar dextrose, but this is not true of all carbohydrates. Inulin, a substance found in Jerusalem artichokes and asparagus, is made of fructose units.
The links between the units in these substances cannot all be undone by the human digestive system, so undigested portions of molecules pass through the gut. Here there are bacteria which can break the links. The chemical reactions by which they do this evolve gas, which is why peas and beans cause wind.
Carbohydrates present in animals include some dextrose (glucose) in the blood. The amount is small, only about 5 g (0.2 oz) in an adult man, but it is important because of its role as a reserve of short-term energy. When the level falls low it is topped up from a 250 g (11 oz) store of glycogen, another carbohydrate, in the liver, the conversion being regulated by a hormone, insulin. Faulty control results in the disease diabetes.
There are a few other animal carbohydrates, notably chitin, the substance which constitutes the hard outer casing of insects and crustaceans.
The main mechanism by which carbohydrates are broken into their constituent sugars is hydrolysis, in which the links between the monosaccharide molecules are undone by the insertion of a molecule of water. This is assisted by heat; by acid conditions, as in the stomachs of most animals; and by digestive enzymes.
The exact way in which carbohydrate bonds are made differs from substance to substance. Particular enzymes undo particular types of bond. The human digestive system has enzymes which can hydrolyse starch, but only to a certain degree. One type of starch molecule has a branched, treelike structure. An enzyme begins at the tip of a branch and dismantles the bonds all down it until it reaches a fork, where there is a differently shaped bond which it cannot break. The result is that the branched molecule is reduced to a form like a lopped tree. The indigestible remainder is called a limit dextrin.
There are other kinds of dextrin. Starch hydrolysed by heating it in acid breaks more indiscriminately to form a type of dextrin found, for example, in corn syrup. It is slightly sweet, and sticky enough to be used as a gum (‘British gum’). Starch heated dry forms pyrodextrins, some of which are compounds with a brown colour and a distinctive flavour (see browning).
The human digestive system has no enzyme which can break down cellulose. But cows and other herbivorous animals have bacteria in special auxiliary stomachs ‘upstream’ of their true stomach which can and do break the bonds. Thus cows can gain nourishment from grass, which is practically non-nutritious to humans. However, indigestible carbohydrates do play a role in the diet. Dieticians refer to them as ‘crude fibre’ or ‘roughage’. They act as a bulky medium to help food travel through the intestines (see cellulose).
Digestion of carbohydrates is given a head start by cooking food, which, since the process involves heat and water, performs some preliminary hydrolysis. This, and the swelling and disruption of the cellulose and hemicelluloses and dispersion of the pectin which together form the cell walls of plants, cause vegetables and fruits to soften when cooked. The cellulose is not actually dissolved or much dispersed in normal cooking conditions, though hemicelluloses are dispersed to some extent even by the fairly weak acids and very weak alkalis used in cooking.
One further class of carbohydrates occasionally encountered is the dextrans: slimy, gummy, or rubbery substances, complex in composition. These are formed by bacteria, including those active in pickles. Usually they are no more than a nuisance, something to be skimmed off; but some are now grown deliberately and used as gums by the food industry (see also nata).
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
