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Protein

the principal material of which animals are made. Proteins also play an important role in the life of plants and other living things; for example, all the enzymes that control chemical reactions in organisms are proteins. Thousands of kinds of protein are known, and more are constantly being discovered.

Proteins are composed of carbon, hydrogen, oxygen, and nitrogen; it is the presence of the last of these which distinguishes proteins from the other materials of life. There is often also a little sulphur and phosphorus. Proteins are highly complex substances with huge molecules containing thousands or millions of atoms. Each molecule is a long chain made up of smaller molecules known as amino acids. These are joined by a link called a ‘peptide bond’, which is fairly weak and easy to break.

Plants make proteins from substances in the soil and the air. Nitrogen is in relatively short supply; although there is plenty of this in the air, plants cannot absorb it directly. Leguminous plants such as beans have nodules on their roots, containing bacteria which take in atmospheric nitrogen and make it available to the plant. Other plants have to rely on nitrogen compounds in the soil, which come mainly from decaying plants and from the action of lightning which turns atmospheric nitrogen to nitrates, and also from fertilizers made from nitrate rocks.

Animals need proteins to provide the vital materials for the growth and repair of their bodies, but they cannot make them in the way plants do. They therefore have to eat plants or other animals. Luckily it is not necessary to eat the exact protein that is needed to repair a particular organ. Digestion breaks proteins into their component amino acids, which the body then reassembles as required.

A moderately active adult man needs about 45 g (1.6 oz) of protein a day and a woman 38 g (1.3 oz), assuming the protein to be of high biological value. Any protein above this amount, and any amino acids that cannot be used because protein is ‘incomplete’ (see below), are burnt as fuel, providing the same amount of energy as sugar: 4.5 calories (18.8 kJ) per gram or 127.5 calories (534 kJ) per ounce. However, when the diet consists almost entirely of protein, with very little carbohydrate, as in the well-known Atkins diet (see diet), the metabolism is pushed into an unbalanced state known as ketosis, in which energy from food is used inefficiently, which results in weight loss.

Complete and incomplete proteins

Of the twenty amino acids that play a significant role in the human body, eight or ten are an essential part of diet (the rest can be made by changing other amino acids). It is also necessary that there should be enough of each one, and that all should be eaten at the same meal, so that the body can have the full range to work on. This means that proteins which contain all the essential amino acids in the proportions in which they are needed are more valuable as foods than those which lack or do not have enough of a vital component. Foods of animal origin are, as might be expected, the most valuable. They are said to provide ‘complete’ proteins—though in fact this is an exaggeration, as is shown in the table.

Foods of vegetable origin provide ‘incomplete’ proteins, which are still useful but do not contain everything that is needed, or not enough of it. Cereals such as wheat, rice, and maize, which supply most of the protein in the diet of peoples all over the world, are notably low in the essential amino acid lysine. Legumes, in contrast, contain abundant lysine; and methionine, the amino acid they tend to lack, is plentiful in cereals. Thus cereals and legumes eaten together complement each other's protein, making it ‘complete’ and fully usable.

The availability of protein in legumes is further restricted by their indigestibility. This is especially true of dried beans. Soya beans, in particular, need to be treated in some way that breaks down their physical and chemical structure to make the proteins more available (see tofu; miso; tempe).

Protein foods can be assigned a ‘biological value’ showing how near they are to completeness. A more useful measurement is ‘net protein utilization’, which also takes into account how digestible they are (shown in the table published by the FAO in the 1970s).

Types of protein

From the cook's point of view there are two classes of protein, fibrous and globular. The difference lies in the way the long molecules are arranged. In a fibrous protein the chains lie lengthways like the strands of a rope. Examples are myosin, one of the chief proteins in muscles; and collagen, the main component of connective tissue such as cartilage. In a globular protein the chains are loosely bundled. Examples are ovalbumin, the main constituent of egg white; and casein in milk. All protein structures include a good deal of water, which makes up about three-quarters of the weight of muscle. A fibrous protein holds the water in a rigid network of strands. Globular proteins are dispersed in water, forming a thick liquid.

Proteins are also classed as ‘insoluble’ or ‘soluble’ in water. In fact, a ‘solution’ of protein is not much like a solution of (for example) salt in water, because the molecules are so enormous. It is more appropriate to think of it as a colloid system of small solid particles suspended in water. All enzymes are soluble proteins, and indeed can only work in a liquid environment. Globular proteins are always soluble, but fibrous proteins may be of either type.

Preparation and Digestion

Protein is digested by digestive enzymes (themselves proteins) which can break the peptide bonds that connect the amino acid links. This can happen only if the enzyme can physically reach a link. In some proteins few or no links are exposed, so the protein is indigestible. However, proteins can also be broken up by hydrolysis, the action of water, which proceeds more quickly at high temperatures and in acid conditions. Once the protein has begun to disintegrate, more links are revealed for the enzymes to work on. This is how cooking makes meat digestible. Hydrolysis also goes on in the strongly acid digestive juices of the stomach.

Some proteins are easily digestible in their natural state, but others need preparation if they are to provide any useful nutrition. Globular proteins are easy to digest. Raw egg is an excellent source of protein. Fibrous proteins are harder to deal with. Raw collagen is more or less indigestible. In muscle tissue it covers the strands of myosin, so that these cannot be digested either; therefore raw meat is a poor source of protein. This applies to the human digestive system; carnivorous animals are better equipped to extract nutrition from raw meat.

Cooking changes collagen into gelatin, which is still not very nutritious. But it is soluble, and is quickly removed to reveal the much more valuable myosin underneath.

The unravelling of protein in cooking processes is known as ‘denaturation’. Its effect varies with the type of protein and how far the process goes. When meat is cooked, hydrolysis frays and breaks the collagen. This begins at a temperature of about 60 °C (140 °F) and gets into full swing at about 70 °C (160 °F). Myosin behaves in a similar way at these temperatures; but if the meat reaches boiling point, myosin shrinks and coagulates into a tough, unyielding lump. That is why meat should be cooked gently. It is also important not to let it dry out, as hydrolysis can proceed only if water is present. Globular proteins partly unravel and become tangled, causing them to solidify, as when egg white sets. Again, overheating gives a tough, leathery texture.

Changes can also be made without cooking. Acids can cause hydrolysis without heating, as when tough meat is marinated in wine or lemon juice. Enzymes may also be used, as in the making of junket or cheese when milk is curdled by adding rennet (a digestive enzyme obtained from the stomachs of calves). Some fruits contain proteolytic (protein-breaking) enzymes, for example papayas, pineapples, and figs. Recipes from regions where tough meat is the norm often call for a marinade made with fruit or juice.

When egg white is beaten, the globular proteins are pulled out into strands which form a network of fibres to support the frothy result. Kneading bread dough pulls out the fibres of gluten, one of the proteins in wheat flour, in a similar way.

Gelatin is made by boiling down bones, skin, and meat trimmings, all containing collagen. The result is a protein that is still fibrous but has rather short strands and is soluble in water (in the limited sense already described). When hot water is poured onto gelatin the heat energy makes the strands vibrate, so that they loosen and the protein floats about freely. As the liquid cools, the strands move more slowly and eventually settle into a tangled mass, so that the liquid sets to a jelly.

Textured vegetable protein and mycoprotein are products made from proteins extracted from vegetables and fungi, and processed to resemble (though not very closely) some kind of animal food such as beef or chicken. From a tentative start in the last quarter of the 20th century, they have progressed to widespread use in the low-cost end of the catering industry and by vegetarians.

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