bread is a substance which is made in so many ways that it is quite useless to attempt to give average figures showing its composition. it will suffice for the present to assume a certain composition which is probably not far from the truth. this will serve for a basis on which to discuss certain generalities as to the food-value of bread. the causes which produce variation in composition will be discussed later, together with their effect on the food value as far as information is available. the following table shows approximately the composition of ordinary white bread as purchased by most of the population of this country. 109
per cent.
water 36
organic substances:
proteins 10
starch 42
sugar, etc. 10
fat 1
fibre ·3 63·3
ash:
phosphoric ·2
lime, etc. ·5 ·7
100·0
the above table shows that one of the most abundant constituents of ordinary bread is water. flour as commonly used for baking, although it may look and feel quite dry, is by no means free from water. it holds on the average about one-seventh of its own weight or 14 per cent. in addition to this rather over one-third of its weight of water or about 35 to 40 per cent. is commonly required to convert ordinary flour into dough. it follows from this that dough will contain when first it is mixed somewhere about one-half its weight of water or 50 per cent. about four per cent. of the weight of the dough is lost in the form of water by evaporation during the fermentation of the dough before it is scaled and moulded. usually 2 lb. 3 oz. of dough will make a two pound loaf, so that about three ounces of water are evaporated in the oven, this is about 110 one-tenth the weight of the dough or 10 per cent. together with the four per cent. loss by evaporation during the fermenting period, this makes a loss of water of about 14 per cent., which, when subtracted from the 50 per cent. originally present in the dough, leaves about 36 per cent. of water in the bread. as pointed out in the previous chapter this quantity is by no means constant even in the same loaf. it varies from hour to hour, falling rapidly if the loaf is kept in a dry place.
to turn now to the organic constituents. the most important of these from the point of view of quantity is starch, in fact this is the most abundant constituent of ordinary bread. nor is it in bread only that starch is abundant. it occurs to the extent of from 50 to 70 per cent. in all the cereals, grains, wheat, barley, oats, maize, and rice. potatoes too contain about 20 per cent. of starch, in fact it is present in most plants. starch is a white substance which does not dissolve in cold water, but when boiled in water swells up and makes, a paste, which becomes thick and semisolid on cooling. it is this property which makes starch valuable in the laundry. starch is composed of the chemical elements carbon, hydrogen, and oxygen. when heated in the air it will burn and give out heat, but it does not do so as readily as does fat or oil. it is this property of burning and giving out heat which makes starch 111 valuable as a foodstuff. when eaten in the form of bread, or other article of food, it is first transformed by the digestive juices of the mouth and intestine into sugar, which is then absorbed from the intestine into the blood, and thus distributed to the working parts of the body. here it is oxidized, not with the visible flame which is usually associated with burning, but gradually and slowly, and with the formation of heat. some of this heat is required to keep up the temperature of the body. the rest is available for providing the energy necessary to carry on the movements required to keep the body alive and in health. practically speaking therefore starch in the diet plays the same part as fuel in the steam engine. the food value of starch can in fact be measured in terms of the quantity of heat which a known weight of it can give out on burning. this is done by burning a small pellet of starch in a bomb of compressed oxygen immersed in a measured volume of water. by means of a delicate thermometer the rise of temperature of the water is measured, and it is thus found that one kilogram of starch on burning gives out enough heat to warm 4·1 kilograms of water through one degree. the quantity of heat which warms one kilogram of water through one degree is called one unit of heat or calorie, and the amount of heat given out by burning one kilogram of any substance is called its heat of combustion or fuel-value. thus 112 the heat of combustion or fuel-value of starch is 4·1 calories.
sugar has much the same food-value as starch, in fact starch is readily changed into sugar by the digestive juices of the alimentary canal or by the ferments formed in germinating seeds. from the point of view of food-value sugar may be regarded as digested starch. like starch, sugar is composed of the elements carbon, hydrogen, and oxygen. like starch too its value in nutrition is determined by the amount of heat it can give out on burning, and again its heat of combustion or fuel value 3·9 calories is almost the same as that of starch. it will be noted that the whole of the 10 per cent. quoted in the table as sugar, etc., is not sugar. some of it is a substance called dextrin which is formed from starch by the excessive heat which falls on the outside of the loaf in the oven. starch is readily converted by heat into dextrin, and this fact is applied in many technical processes. for instance much of the gum used in the arts is made by heating starch. the outside of the loaf in the oven gets hot enough for some of the starch to be converted into dextrin. dextrin is soluble in water like sugar and so appears with sugar in the analyses of bread. from the point of view of food-value this is of no consequence, as dextrin and sugar serve the same purpose in nutrition, and have almost the same value as each other and as starch. 113
bread always contains a little fat, not as a rule more that one or two per cent. but although the quantity is small it cannot be neglected from the dietetic point of view. fat is composed of the same elements as starch, dextrin, and sugar, but in different proportions. it contains far less oxygen than these substances. consequently it burns much more readily and gives out much more heat in the process. the heat of combustion or fuel value of fat is 9·3 calories or 2·3 times greater than that of starch. evidently therefore even a small percentage of fat must materially increase the fuel value of any article of food. but fat has an important bearing on the nutritive value of bread from quite another point of view. in the wheat grain the fat is concentrated in the germ, comparatively little being present in the inner portion of the grain. thus the percentage of fat in any kind of bread is on the whole a very fair indication of the amount of germ which has been left in the flour from which the loaf was made. it is often contended nowadays that the germ contains an unknown constituent which plays an important part in nutrition, quite apart from its fuel-value. on this supposition the presence of much fat in a sample of bread indicates the presence of much germ, and presumably therefore much of this mysterious constituent which is supposed to endow such bread with a special value in the nutrition particularly of young 114 children. this question will be discussed carefully in a later chapter.
white bread contains a very small percentage of what is called by analysts fibre. the quantity of this substance in a food is estimated by the analyst by weighing the residue which remains undigested when a known weight of the food is submitted to a series of chemical processes designed to imitate as closely as may be the action of the various digestive juices of the alimentary canal. theoretically, therefore, it is intended to represent the amount of indigestible matter present in the food in question. practically it does not achieve this result for some of it undoubtedly disappears during the passage of the food through the body. it is doubtful however if the portion which disappears has any definite nutritive value. that part of the fibre which escapes digestion and is voided in the excrement cannot possibly contribute to the nutrition of the body. nevertheless it exerts a certain effect on the well-being of the consumer, for the presence of a certain amount of indigestible material stimulates the lower part of the large intestine and thus conduces to regularity in the excretion of waste matters, a fact of considerable importance in many cases. the amount of fibre is an index of the amount of indigestible matter in a food. in white bread it is small. in brown breads which contain considerable quantities 115 of the husk of the wheat grain it may be present to the extent of two or three per cent. such breads therefore will contain much indigestible matter, but they will possess laxative properties which make them valuable in some cases.
we have left to the last the two constituents which at the present time possess perhaps the greatest interest and importance, the proteins and the ash. the proteins of bread consist of several substances, the differences between which, for the present purpose, may be neglected, and we may assume that for all practical purposes the proteins of bread consist of one substance only, namely gluten. the importance of gluten in conferring on wheat flour the property of making light spongy loaves has already been insisted upon. no doubt this property of gluten has a certain indirect bearing on the nutritive value of bread by increasing its palatability. but gluten being a protein has a direct and special part to play in nutrition, which is perhaps best illustrated by following one step further the comparison between the animal body and a steam engine. it has been pointed out that starch, sugar, and fat play the same part in the body as does the fuel in a steam engine. but an engine cannot continue running very long on fuel alone. its working parts require renewing as they wear away, and coal is no use for this purpose. metal parts must be renewed with metal. in much the same 116 way the working parts of the animal body wear away, and must be renewed with the stuff of which they are made. now the muscles, nerves, glands and other working parts of the body are made of protein, and they can only be renewed with protein. consequently protein must be supplied in the diet in amount sufficient to make good from day to day the wear and tear of the working parts of the body. it is for this reason that the protein of bread possesses special interest and importance.
protein like starch, sugar, and fat contains the elements carbon, hydrogen, and oxygen, but it differs from them in containing also a large proportion of the element nitrogen, which may be regarded as its characteristic constituent. when digested in the stomach and intestine it is split into a large number of simpler substances known by chemists under the name of amino-acids. every animal requires these amino-acids in certain proportions. from the mixture resulting from the digestion of the proteins in its diet the amino-acids are absorbed and utilised by the body in the proportions required. if the proteins of the diet do not supply the amino-acids in these proportions, it is obvious that an excessive amount of protein must be provided in order that the diet may supply enough of that particular amino-acid which is present in deficient amount, and much of those amino-acids which are abundantly present must 117 go to waste. this is undesirable for two reasons. waste amino-acids are excreted through the kidneys, and excessive waste throws excessive work on these organs, which may lead to defective excretion, and thus cause one or other of the numerous forms of ill health which are associated with this condition. again, excessive consumption of protein greatly adds to the cost of the diet, for protein costs nearly as many shillings per pound as starch or sugar costs pence.
these considerations show clearly the wisdom of limiting the amount of protein in the diet to the smallest amount which will provide for wear and tear of the working parts. the obvious way to do this is to take a mixed diet so arranged that the various articles of which the diet consists contain proteins which are so to speak complementary. the meaning of this is perhaps best illustrated by a concrete example. the protein of wheat, gluten, is a peculiar one. on digestion it splits like other proteins into amino-acids, but these are not present from the dietetic point of view in well balanced proportions. one particular amino-acid, called glutaminic acid, preponderates, and unfortunately this particular acid does not happen to be one which the animal organism requires in considerable quantity. other amino-acids which the animal organism does require in large amounts are deficient in the mixture of 118 amino-acids yielded by the digestion of the protein of wheat. it follows, therefore, that to obtain enough of these latter acids a man feeding only on wheat products would have to eat a quantity of bread which would supply a great excess of the more abundant glutaminic acid, which would go to waste with the evil results already outlined. from this point of view it appears that bread should not form more than a certain proportion of the diet, and that the rest of the diet should consist of foods which contain proteins yielding on digestion little glutaminic acid and much of the other amino-acids in which the protein of wheat is deficient. unfortunately information as to the exact amount of the different amino-acids yielded by the digestion of the proteins even of many of the common articles of food is not available. but many workers are investigating these matters, and the next great advance in the science of dietetics will probably come along these lines. by almost universal custom certain articles of food are commonly eaten in association: bread and cheese, eggs and bacon, are instances. such customs are usually found to be based on some underlying principle. the principle in this case may well be that of complementary proteins.
the remarks which have been made above on the subject of the r?le of protein in the animal economy apply to adults in which protein is required for wear 119 and tear only and not for increase in weight. they will obviously apply with greatly increased force to the case of growing children, who require protein not only for wear and tear, but for the building up of their muscles and other working parts as they grow and develope. consequently the diet of children should contain more protein in proportion to their size than that of adults. for this reason it is not desirable that bread should form an excessive proportion of their diet. the bread they eat should be supplemented with some other food richer in protein.
the ash of bread although so small in amount cannot be ignored, in fact it is regarded as of very great importance by modern students of dietetics. the particular constituent of the ash to which most importance is attached is phosphoric acid. this substance is a necessary constituent of the bones and of the brain and nerves of all animals. it exists too in smaller proportions in other organs. like other working parts of the body the bones and the nervous system are subject to wear and tear, which must be replaced if the body is to remain in normal health. a certain daily supply of phosphoric acid is required for this purpose, and proportionally to their size more for children than for adults. considerable difference of opinion as to the exact amount required is expressed by those who have investigated this question, nor is it even agreed whether all forms of 120 phosphoric acid are of the same value. there is however a general recognition of the importance of this constituent of the diet, and the subject is under investigation in many quarters.
