The Story of a Loaf of Bread

These facts suggest at once a method for estimating the shape and texture of the loaf which can be made from any given sample of wheat. An analysis showing the amount of soluble phosphate in the sample should give the desired information. But unfortunately such an analysis is not an easy one to make, and requires a considerable quantity of flour. In making these analyses it was noticed that when the flours were shaken with water to dissolve the phosphate, and the insoluble substance removed by filtering, the solutions obtained were always more or less turbid, and the degree of turbidity was found to be related to the amount of phosphate present and to the shape of loaf produced. On further investigation it was found that the turbidity was due to the fact that the concentration of acid and salts which make gluten coherent, also dissolve some of it, and gluten like other colloids gives a turbid solution. It was also found that the amount of gluten dissolved, and consequently the degree of turbidity, is related to the shape of the loaf which the flour will produce. Now it is quite easy to measure the degree of turbidity of a solution by pouring the solution into a glass vessel below which a small electric lamp is placed, and noting the depth of the liquid through which the filament of the lamp can just be seen. The turbidities were, however, so slight that it was found necessary to increase them by adding a little iodine solution which gives a brown milkiness with solutions of gluten, the degree of milkiness depending on the amount of gluten in the solution. In this way a method was devised which is rapid, easy, and can be carried out with so little wheat that the produce of one ear is amply sufficient. It can therefore be used by the plant breeder for picking out from the progeny of his crosses those individual plants which are likely to give shapely loaves. The method is as follows: An ear of wheat is rubbed out and ground to powder in a small mill. One gram of this powder, or of flour if that is to be tested, is weighed out and put into a small bottle. To it is added 20 c.c. of water. The bottle is then shaken for one hour. At the end of this time the contents are poured onto a filter. To 15 c.c. of the solution 1½ c.c. of a weak solution of iodine is added, and after standing for half an hour the turbidity test is applied. Working in this way it is possible to see through only 10 c.m. of the solution thus obtained from such a wheat as Red Fife, as compared with 25 c.m. in the case of Rivet. Other wheats yield solutions of intermediate opacity. This method is now being tested in connection with the Cambridge wheat breeding experiments.

CHAPTER V
THE MILLING OF WHEAT

In order that wheat may be made into bread it is necessary that it should be reduced to powder. In prehistoric times this was effected by grinding the grain between stones. Two stones were commonly used, the lower one being more or less hollowed on its upper surface so as to hold the grain while it was rubbed by the upper one. As man became more expert in providing for his wants, the lower stone was artificially hollowed, and the upper one shaped to fit it, until in process of time the two stones assumed the form of a primitive mortar and pestle.

The next step in the evolution of the mill was to make a hole or groove in the side of the lower stone through which the powdered wheat could pass as it was ground. This device avoided the trouble of emptying the primitive mill, and materially saved the labour of the grinder. Such mills are still in use in the less civilised countries in the East, and are of course worked by hand as in primitive times.

They gradually developed as civilization progressed into the stone mills which ground all the breadstuffs of the civilised world until about 40 years ago. The old fashioned stone mill was, and indeed still is, a weapon of the greatest precision. It consists of a pair of stones about four feet in diameter, the lower of which is fixed whilst the upper is made to revolve by mechanical power at a high speed. Each stone is made of a large number of pieces of a special kind of hard stone obtained from France. These pieces are cemented together, and the surfaces which come into contact are patiently chipped until they fit one another to a nicety all over. The surface of the lower stone is then grooved so as to lead the flour to escape from between the stones at definite places where it is received for further treatment. The grain to be ground is fed between the stones through a hole at the centre of the upper stone. It has been stated above that the surfaces of the two stones are in contact. As a matter of fact this is not strictly true. The upper stone is suspended so that the surfaces are separated by a small fraction of an inch, and it will be realised at once that this suspension is a matter of the greatest delicacy. To balance a stone weighing over half a ton so that, when revolving at a high rate of speed, it may be separated from its partner at no point over its entire surface of about 12 square feet by more than the thickness of the skin of a grain of wheat, and yet may nowhere come into actual contact, is an achievement of no mean order. Stone mills of this kind were usually driven by water power, or in flat neighbourhoods by wind power, though in some cases steam was used.

It was the common practice to subject the ground wheat from the stones to a process of sifting so as to remove the particles of husk from the flour. The sifting was effected by shaking the ground wheat in a series of sieves of finely woven silk, known as bolting cloth. In this way it was possible to obtain a flour which would make a white bread. The particles of husk removed by the sifting were sold to farmers for food for their animals, under the name of bran, sharps, pollards, or middlings, local names for products of varying degrees of fineness, which may be classed together under the general term wheat offals. The ideal of the miller was to set his stones so that they would grind the flour to a fine powder without breaking up the husk more than was absolutely necessary. When working satisfactorily a pair of stones were supposed to strip off the husk from the kernel. The kernel should then be finely pulverised. The husk should be flattened out between the stones, which should rub off from the inside as completely as possible all adhering particles of kernel. If this ideal were attained, the mill would yield a large proportion of fairly white flour, and a small proportion of husk or offals.

As long as home grown wheats were used this ideal could be more or less attained because the husk of these wheats is tough and the kernel soft. Comparatively little grinding suffices to reduce the kernel to the requisite degree of fineness, and this the tough husk will stand without being itself unduly pulverised. Consequently the husk remains in fairly large pieces, and can be separated by sifting, with the result that a white flour can be produced. But home grown wheat ceased to provide for the wants of the nation more than half a century ago. Already in 1870 half the wheat ground into flour in the United Kingdom was imported from abroad, and this proportion has steadily increased, until at the present time only about one-fifth of the wheat required is grown at home. Many of the wheats which are imported are harder in the kernel, and thinner and more brittle in the husk, than the home grown varieties. Consequently they require more grinding to reduce the kernel to the requisite degree of fineness, and their thin brittle husk is not able to resist such treatment. It is itself ground to powder along with the kernel, and cannot be completely separated from the flour by sifting. Such wheats therefore, when ground between stones, yield flour which contains much finely divided husk, and this lowers its digestibility and gives it a dark colour.

In the decades before 1870 when the imports of foreign wheats first reached serious proportions, and all milling was done by stones, dark coloured flours were common, and people would no doubt have accepted them without protest, if no other flours had been available. But as it happened millers in Hungary, where hard kernelled, thin skinned wheats had long been commonly grown, devised the roller milling process, which produces fine white flour from such wheats, no matter how hard their kernels or how thin their skins. The idea of grinding wheat between rollers was at once taken up in America and found to give excellent results with the hard thin skinned wheats of the north-west. The fine white flours thus produced were sent to England, and at once ousted from the home markets the dark coloured flours produced from imported wheats in the English stone mills. The demand for the white well-risen bread produced from these roller milled imported flours showed at once that the public preferred such bread to the darker coloured heavier bread yielded by stone-ground flours, especially those made from the thin skinned foreign wheats.

This state of things was serious both for the millers and the farmers. The importation of flour instead of wheat must obviously ruin the milling industry, and since wheat offals form no inconsiderable item in the list of feeding stuffs available for stock keepers, a decline of the milling industry restricts the supply of food for his stock, and thus indirectly affects the farmer. At the same time the preference shown by the public for bread made from fine white imported flour, to some extent depreciated the value of home grown wheat.

It was by economic conditions of this kind that the millers were compelled in the early seventies to alter their methods. The large firms subscribed more capital and installed roller plant in their mills. These at once proved a success and the other firms have followed suit. At the present time considerably more than 90 per cent. of the flour used in this country is the product of roller mills. The keen competition which has arisen in the milling industry during the last 35 years has produced great improvements in roller plant, and the methods of separation now in use yield flours which in the opinion of the miller, and apparently too in the opinion of the general public, are far in advance of the flours which were produced in the days of stone milling.

Perhaps the first impression which a visitor to a modern roller mill would receive is the great extent to which mechanical contrivances have replaced hand labour. Once the wheat has been delivered at the mill it is not moved again by hand until it goes away as flour and offals. It is carried along by rapidly moving belts, elevated by endless chains carrying buckets, allowed to fall again by gravity, or perhaps in other cases transported by air currents. Another very striking development is the great care expended in cleaning the grain before it is ground. This cleaning is the first process to which the wheat is subjected. It is especially necessary in the case of some of the foreign wheats which arrive in this country in a very dirty condition. The impurities consist of earth, weed seeds, bits of husk and straw; iron nails, and other equally unlikely objects are by no means uncommon. Some of these are removed by screens, but besides screening the wheat is actually subjected to the process of washing with water. For this purpose it is elevated to an upper floor of the mill, and allowed to fall downwards through a tall vessel through which a stream of water is made to flow. As it passes through the water it is scrubbed by a series of mechanically driven brushes to remove the earthy matter which adheres to the grain. This is carried away by the stream of water.

After cleaning the grain next undergoes the process of conditioning. The object of this process is so to adjust the moisture of the grain that the husk may attain its maximum toughness compatible with a reasonable degree of brittleness of kernel, the idea being to powder the kernel with the minimum of grinding and without unduly powdering the husk. By attention to this process separation of flour and husk is made easier and more complete. The essential points in the process are to moisten the grain, either in the course of cleaning as above described, or if washing is not necessary, by direct addition of water. The moisture is given some time to be absorbed into the grain, which is then dried until the moisture content falls to what experience shows to be the most successful figure for the wheat in question.

Cleaning and conditioning having been attended to, the grain is now conveyed to the mill proper. This of course is done by a mechanical arrangement which feeds the grain at any desired rate into the hopper which supplies the first pair of rolls. These rolls consist of a pair of steel cylinders usually 10 inches in diameter and varying in length from 20 inches to 5 feet according to the capacity of the mill. The surfaces of the cylinders are fluted or ribbed, the distance from rib to rib being about one-tenth of an inch. The rollers are mounted so that the distance between their surfaces can be adjusted. They are set so that they will break grains passing between them to from one-half to one-quarter their original size. They are made to revolve so that the parts of the surfaces between which the grains are nipped are travelling in the same direction. One roll revolves usually at about 350 revolutions per minute, the other at rather less than half that rate (Fig. 14). It is obvious from the above description that a grain of wheat falling from the hopper on to the surface of the moving rollers will be crushed or nipped between them, and that since the rollers are moving at different rates, it will at the same time be more or less torn apart. By altering the distance between the rollers and their respective speeds of revolution the relative amounts of nipping and tearing can be adjusted to suit varying conditions.

The passage of the grain through such a pair of rollers is known technically as a break. Its object is to break or tear open the grain with the least possible amount of friction between the grain and the grinding surfaces. Since the rollers are cylindrical it is obvious that the grain will only be nipped at one point of their surfaces, and even here the friction is reduced as much as possible by making both the grinding surfaces move in the same direction. As already explained it can be diminished, if the condition of the wheat allows, by diminishing the difference in speed between the two rolls. The result of the first break is to tear open the grains. At the same time a small amount of the kernel will be finely powdered. The rest of the kernel and husk will still remain in comparatively large pieces. The tearing open of the grain sets free the dirt which was lodged in the crack or furrow which extends from end to end of the grain. This dirt cannot be removed by any method of cleaning. It only escapes when the grain is torn open in the break. It is generally finely divided dirt and cannot be separated from the flour formed in this process. Consequently the first break flour is often more or less dirty, and the miller tries to adjust his first break rolls so that they will form as little flour as possible. The first break rolls not only powder a little of the kernel, but they also reduce to a more or less fine state of division a little of the husk.

The result of the passage of the grain through the first break rolls is to produce from it a mixture of a large quantity of comparatively coarse particles of kernel to many of which husk is still adherent, a small quantity of finely divided flour which is more or less discoloured with dirt, and a small quantity of finely divided husk. This mixture, which is technically known as stock, is at once subjected to what is called separation, with the object of separating the flour from the other constituents before it undergoes any further grinding. It is one of the guiding principles of modern milling that the flour produced at each operation should be separated at once so as to reduce to a minimum the grinding which it has to undergo. Separation is brought about by the combination of two methods. The stock is shaken in contact with a screen made of bolting silk so finely woven that it contains from 50 to 150 meshes to the inch, according to the fineness of the flour which it is desired to separate. The shaking is effected in several different ways. Sometimes the silk is stretched on a frame so as to make a kind of flat sieve. This is shaken mechanically whilst the stock is allowed to trickle over its surface, so that the finely divided particles of flour may fall through the meshes and be collected separately from the larger particles which remain on the top. These larger particles are partly heavy bits of broken kernel and partly light bits of torn husk. In order to separate them advantage is taken of the fact that a current of wind can be so adjusted that it will blow away the light and fluffy husk particles without disturbing the heavy bits of kernel. By means of a mechanically driven fan a current of air is blown over the surface of the sieve, in the direction opposite to that in which the stock is travelling. As the stock rolls over and over in its passage from the upper to the lower end of the inclined sieve the fluffy particles of husk are picked up by the air current and carried back to the top of the sieve where they fall, as the current slackens, into a receptacle placed to receive them. Thus by the combination of sifting and air carriage the stock is separated into a small quantity of finished flour, a small quantity of finished husk or offal, and a large quantity of large particles of kernel with husk still adhering to some of them. These large particles, which are called semolina, of course require further grinding. Different methods of sifting are often used in place of the one above described, especially for completing the purification of the flour. Sometimes the silk is stretched round a more or less circular frame so as to form a long cylinder covered with silk. The stock is delivered into the higher end of this cylinder which is made to revolve. This causes the stock to work its way through the cylinder, and during its progress the finely ground flour finds its way through the meshes, and is separated as before from the coarser particles. Such a revolving sieve is known as a reel. In a somewhat similar arrangement known as a centrifugal a series of beaters is made to revolve rapidly inside a stationary cylindrical sieve. The stock is admitted at one end and is thrown by the revolving beaters against the silk cover. The finer particles are driven through the meshes of the silk, the coarser particles find their way out of the cylinder at the other end. Sometimes for separating very coarse particles wire sieves of 30 meshes, or thereabouts, to the inch are used. Whatever the method the object is to separate at once the finished flour and offal from the large particles of kernel which require further grinding.

These large particles, semolina, are next passed between one or more pairs of smooth rolls known as reduction rolls (Fig. 15). These are set rather nearer together than the break rolls, and the difference in speed between each roll and its partner is quite small. The object of reduction is to reduce the size of the large particles of semolina and to produce thereby finely divided flour. The stock from the first pair or pairs of reduction rolls contains much finely ground flour mixed with coarser particles of kernel with or without adherent husk. It is at once submitted to the separation and purification processes as above described. This yields a large quantity of finished flour which is very white and free from husk. It represents commercially the highest grade of flour separated in the mill and is described technically as patents. A small amount of finished offal is also separated at this stage.

The coarse particles of kernel with adherent husk from which the flour and offal have been separated are now passed through a second pair of break rolls more finely fluted than before, known as the second break. These are set closer together than the first break rolls. Their object is to rub off more kernel from the husk. The stock from them is again separated, the flour and finished offal being removed as before. The coarser particles are again reduced by smooth reduction rolls, and a second large quantity of flour separated. This is commercially high grade flour and is usually mixed with the patents already separated. The coarse particles left after this separation are usually subjected to a third and a fourth break, each of which is succeeded by one or two reductions. Separation of the stock and purification of the flour take place after each rolling, so that as soon as any flour or husk is finely ground it may be at once separated without further grinding. The last pair of fluted rolls, the fourth break, are set so closely together that they practically touch both sides of the pieces of husk which pass through them. They are intended to scrape the last particles of kernel from the husk. This is very severe treatment, and usually results in the production of much finely powdered husk which goes through the sifting silk and cannot be separated from the flour. The flour from the fourth break is therefore usually discoloured by the presence of much finely divided husk. For this reason it ranks as of low commercial grade. The later reductions too yield flours containing more or less husk, which darkens their colour. They are usually mixed together and sold as seconds.

The fate of the germ in the process of roller milling is a point of considerable interest, both on account of the ingenious way in which it is removed, and because of the mysterious nutritive properties which it is commonly assumed to possess. The germ of a grain of wheat forms only about 1½ per cent. by weight of the grain. It differs in composition from the rest of the grain, being far richer in protein, fat, and phosphorus. Its special feeding value can, however, scarcely be explained in terms of these ingredients, for its total amount is so small that its presence or absence in the flour can make only a very slight difference in the percentages of these substances. But this point will be discussed fully in a subsequent chapter. Here it is the presence of the fat which is chiefly of interest. According to the millers the fat of the germ is prone to become rancid, and to impart to the flour, on keeping, a peculiar taste and odour which affects its commercial value. They have therefore devised with great ingenuity a simple method of removing it. This method depends on the fact that the presence in the germ of so much fat prevents it from being ground to powder in its passage between the rolls. Instead of being ground it is pressed out into little flat discs which are far too large to pass with the flour through the sifting silks or wires, and far too heavy to be blown away by the air currents which remove the offals. The amount which is thus separated is usually about 1 per cent. of the grain so that one third of the total quantity of germ present in the grain is not removed as such. Considerable difficulties arise in attempting to trace this fraction, and at present it is impossible to state with certainty what becomes of it. The germ which is separated is sold by the ordinary miller to certain firms which manufacture what are known as germ flours. It is subjected to a process of cooking which is said to prevent it from going rancid, after which it is ground with wheat, the product being patent germ flour.

CHAPTER VI
BAKING

In discussing the method of transforming flour into bread it will be convenient to begin by describing in detail one general method. The modifications used for obtaining bread of different kinds, and for dealing with flours of different qualities will be shortly discussed later when they can be more readily understood.

Bread may be defined as the product of cooking or baking a mixture of flour, water, and salt, which is made porous by the addition of yeast. It is understood to contain no other substances than these—flour, salt, water and yeast.

In the ordinary process the first step is to weigh out the flour which it is proposed to bake. This is then transferred to a vessel which in a commercial bakery is usually a large wooden trough, in a private house an earthenware bowl. The necessary amount of yeast is next weighed out and mixed with water. Nowadays compressed or German yeast is almost always used at the rate of 1 to 2 lbs. per sack or 280 lbs. of flour. For smaller quantities of flour relatively more yeast is needed, for instance 2 ozs. per stone. Formerly brewers’ yeast or barm was used, but its use has practically ceased because it is difficult to obtain of standard strength. Some people who profess to be connoisseurs of bread still prefer it because as they say it gives a better flavour to the bread. The water with which the yeast is mixed is warmed so as to make the yeast more active. The flour is then heaped up at one end of the vessel in which the mixing is to take place, and salt at the rate of 2 to 5 lbs. per sack is thoroughly stirred into it. A hollow is then made in the heap of flour into which the mixture of yeast and water is poured. More warm water is added so that enough water in all may be present to convert all, or nearly all, the flour into dough of the required consistency. When dealing with a flour with which he is familiar the baker knows by experience how much water he requires per sack. In the case of an unaccustomed brand of flour he determines the amount by a preliminary trial with a small quantity (Figs. 16 and 17). Flour from the heap is then stirred into the water until the whole of the flour is converted into a stiff paste or dough as it is called. By this method a little dry flour will always separate the dough from the sides of the vessel and this will prevent the dough from sticking to the vessel and the hands. The dough is then thoroughly worked or kneaded so as to ensure the intimate mixture of the ingredients. The vessel is then covered to keep the dough warm. In private houses this is ensured by placing the vessel near the fire. In bakeries the room in which the mixing is conducted is usually kept at a suitable temperature. The yeast cells which are thoroughly incorporated in the dough, find themselves in possession of all they require to enable them to grow. The presence of water keeps them moist, and dissolves from the flour for their use sugar and salts: the dough is kept warm as above explained. Under these conditions active fermentation takes place with the formation of alcohol and carbon dioxide gas. The alcohol is of no particular consequence in bread making, the small amount formed is probably expelled from the bread during its stay in the oven. The carbon dioxide, however, plays a most important part. Being a gas it occupies a large volume, and its formation throughout the mass of the dough causes the dough to increase greatly in volume. The dough is said by the housewife to rise, by the professional baker to prove.

The process of kneading causes the particles of gluten to absorb water and to adhere to one another, so that the dough may be regarded as being composed of innumerable bubbles each surrounded by a thin film of gluten, in or between which lie the starch grains and other constituents of the flour. Each yeast cell as above explained forms a centre for the formation of carbon dioxide gas, which cannot escape at once into the air, and must therefore form a little bubble of gas inside the particular film of gluten which happens to surround it. The expansion of the dough is due to the formation inside it of thousands of these small bubbles. It is to the formation of these bubbles too that the porous honey-combed structure of wheaten bread is due. Also since the formation of the bubbles is due to the retention of the carbon dioxide by the gluten films, such a porous structure is impossible in bread made from the flour of grains which do not contain gluten.

The rising of the dough is usually allowed to proceed for several hours. The baker finds by experience how long a fermentation is required to give the best results with the flours he commonly uses. When the proper time has elapsed, the dough is removed from the trough or pan in which it was mixed to a board or table, previously dusted with dry flour to prevent the dough adhering to the board or to the hands. It is then divided into portions of the proper weight to make loaves of the desired size. This process is known technically as scaling. Usually 2 lbs. 3 ozs. of dough is allowed for baking a 2 lb. loaf. Each piece of dough is now moulded into the proper shape if it is desired to bake what is known as a cottage loaf, or placed in a baking tin if the baker is satisfied with a tinned loaf. In either case the dough is once more kept for some time at a sufficiently warm temperature for the yeast to grow so that the dough may once more be filled with bubbles of carbon dioxide gas. As soon as this second rising or proving has proceeded far enough the loaves are transferred to the oven. Here the intense heat causes the bubbles of gas inside the dough to expand so that a sudden increase in the size of the loaf takes place. At the same time the outside of the loaf is hardened and converted into crust.

After remaining in the oven for the requisite time the bread is withdrawn and allowed to cool as quickly as possible, after which it is ready for use or sale.

The method of baking which has been described above is known as the off-hand or straight dough method. It possesses the merit of rapidity and simplicity, but it is said by experts that it does not yield the best quality of bread from certain flours. Perhaps the commonest variation is that known as the sponge and dough method, which is carried out as follows. As before, the requisite amount of flour is weighed out into the mixing trough, and a depression made in it for the reception of the water and yeast. These are mixed together in the proper proportions, enough being taken to make a thick cream with about one quarter of the flour. This mixture is now poured into the depression in the flour, and enough of the surrounding flour stirred into it to make a thick cream or sponge as it is called. At the same time a small quantity of salt is added to the mixture. The sponge is allowed to ferment for some hours, being kept warm as in the former method. As soon as the time allowed for the fermentation of the sponge has elapsed, more water is added, so that the whole or nearly the whole of the flour can be worked up into dough. This dough is immediately scaled and moulded into loaves, which after being allowed to prove or rise for some time are baked as before. This method is used for flours which do not yield good bread when they are submitted to long fermentation. In such cases the mellow flours, which will only stand a very short fermentation, are first weighed out into the mixing trough, and a depression made in the mass of flour into which a quantity of strong flour which can be fermented safely for a long time is added. It is this last addition which is mixed up into the sponge to undergo the long preliminary fermentation. The rest of the flour is mixed in after this first fermentation is over, so that it is only subjected to the comparatively slight fermentation which goes on in the final process of proving.

Many other modifications are commonly practised locally, their object being for the most part to yield bread which suits the local taste. It will suffice to mention one which has a special interest. In this method the essentially interesting point is the preparation of what is known as a ferment. For this purpose a quantity of potatoes is taken, about a stone to the sack of flour. After peeling and cleaning they are boiled and mashed up with water into a cream. To this a small quantity of yeast is added and the mixture kept warm until fermentation ceases, as shown by the cessation of the production of gas. During this fermentation the yeast increases enormously, so that a very small quantity of yeast suffices to make enough ferment for a sack of flour. The flour is now measured out into the trough, and the ferment and some additional water and salt added so that the whole can be worked up into dough. Scaling, moulding, and baking are then conducted as before. This method was in general use years ago when yeast was dear. It has fallen somewhat into disuse in these days of cheap compressed yeast, in fact the use of potatoes nowadays would make the process expensive.

In private houses and in the smaller local bakeries the whole of the processes described above are carried out by hand. During the last few decades many very large companies have been formed to take up the production of bread on the large scale. This has caused almost a revolution of the methods of manipulating flour and dough, and in many cases nowadays almost every process in the bakery is carried out by machinery. In many of the larger bakeries doughing and kneading are carried out by machines, and this applies also to the processes of scaling and moulding. A similar change has taken place too in the construction of ovens. Years ago an oven consisted of a cavity in a large block of masonry. Wood was burned in the cavity until the walls attained a sufficiently high temperature. The remains of the fuel were then raked out and the bread put in and baked by radiation from the hot walls.

Nowadays it is not customary to burn fuel in the oven itself, nor is the fuel always wood or even coal. The fuel is burned in a furnace underneath the oven, and coal or gas is generally used. Sometimes however the source of heat is electricity. In all cases it is still recognised that the heat should be radiated from massive solid walls maintained at a high temperature. In the latest type of oven the heat is conducted through the walls by closed iron tubes containing water, which of course at the high temperatures employed becomes superheated steam. It is recognised that the ovens commonly provided in modern private houses, whether heated by the fire of the kitchen range, or by gas, are not capable of baking bread of the best quality, because their walls do not radiate heat to the same degree as the massive walls of a proper bake oven.

It is commonly agreed that bread, in the usual acceptation of the term, should contain nothing but flour, yeast, salt, and water; or if other things are present they should consist only of the products formed by the interaction of these four substances in the process of baking. Millers and bakers have, however, found by experience that the addition of certain substances to the flour or to the dough may sometimes enable them substantially to improve the market value of the bread produced by certain flours. The possible good or bad effect of such additions on the public health will be discussed in a later chapter. It may be of interest here to mention some of the substances which are commonly used as flour or bread improvers by millers and bakers, and to discuss the methods by which they effect their so called improvements.

In a former chapter we have discussed the quality of wheat from the miller’s point of view, and during the discussion certain views were enunciated on the subject of strength. It was pointed out that a strong flour was one which would make a large well-shaped loaf, and that the size of the loaf was dependent on the flour being able to provide sugar for the yeast to feed upon right up to the moment when the loaf goes into the oven. This can only occur when the flour contains an active ferment which keeps changing the starch into sugar. That this view is generally accepted in practice is shown by the fact that, when using flours deficient in such ferment, bakers commonly add to the flour, yeast, salt, and water, a quantity of malt extract, the characteristic constituent of which is the sugar producing ferment of the malt. This use of malt extract is now extending to the millers, several of whom have installed in their mills plant for spraying into their flour a strong solution of malt extract. It seems to be agreed by millers and bakers generally that such an addition to a flour which makes small loaves distinctly increases the size of the loaf. There can be no doubt that this effect is produced by the ferment of the malt extract keeping up the supply of sugar, and thus enabling the yeast to maintain the pressure of gas in the dough right up to the moment when it goes into the oven.

The view that the shape of the loaf is due to the effect of salts, and particularly of phosphates, on the coherence of the gluten has also been put to practical use by the millers and the bakers. Preparations of phosphates under various fancy names are now on the market, and are bought by bakers for adding to the flour to strengthen the gluten and produce more shapely loaves. A few millers too are beginning to spray solutions of phosphates into their flours with the same object in view, and such additions are said to make material improvements in the shape of the loaf produced by certain weak flours.

These two substances, malt extract and phosphates, are added to the flour with the definite object of improving the strength and thus making larger and more shapely loaves. But there is a second class of substances which are commonly added to flours, not in the mill but in the process of bread making, with the object of replacing yeast. Yeast is used in baking in order that it may form gas inside the dough and thus produce a light spongy loaf. Exactly the same gas can be readily and cheaply produced by the interaction of a carbonate with an acid. These substances will not react to produce acid as long as they remain dry, but as soon as they are brought into close contact with each other by the presence of water, reaction begins and carbon dioxide gas is formed. These facts are taken advantage of in the manufacture of baking powders and self-rising flours. Baking powders commonly consist of ordinary bicarbonate of soda mixed with an acid or an acid salt, such as tartaric acid, cream of tartar, acid phosphate of lime, or acid phosphate of potash. One of these latter acid substances is mixed in proper proportions with the bicarbonate of soda, and the mixture ground up with powdered starch which serves to dilute the chemicals and to keep them dry. A small quantity of the baking powder is mixed with the flour before the water is added to make the dough. The presence of the water causes the acid and the carbonate to give off gas which, as in the case of the gas formed by the growth of yeast, fills the dough with bubbles which expand in the oven and produce light spongy bread. When using baking powders in place of yeast it must not be forgotten that gas formation in most cases begins immediately the water is added, and lasts for a very short time. Consequently the dough must be moulded and baked at once or the gas will escape. This is not the case, however, with those powders which are made with cream of tartar, for this substance does not react with the carbonate to any great extent until the dough gets warm in the oven. For some purposes it is customary to use carbonate of ammonia, technically known as volatile, in place of baking powder. This substance is used alone without any addition of acid, because it decomposes when heated and forms gas inside the dough. Sometimes too one or other of the baking powders above described are added to the flour by the miller, the product being sold as self-rising flour. Such flour will of course lose its property of self-rising if allowed to get damp. Occasionally objectionable substances are used in making baking powders of self-rising flours. Some baking powders for instance contain alum which is not a desirable addition to any article of human food. Baking powders and self-rising flours are far more frequently used by house-wives for making pastry or for other kinds of domestic cookery than for breadmaking.

Bread is made on the large scale without the intervention of yeast by the aeration process, which is carried out as follows. A small quantity of malt is allowed to soak in a large quantity of water, and the solution thus obtained is kept warm so that it may ferment. This charges the solution with gas and at the same time produces other substances which are supposed to give the bread a good flavour. Such a solution too retains gas much better than pure water. This solution is then mixed with a proper proportion of flour inside a closed vessel, carbon dioxide gas made by the action of acid on a carbonate being pumped into the vessel whilst the mixing is in progress. The mixing is of course effected by mechanical means. As soon as the dough is sufficiently mixed, it is allowed to escape by opening a large tap at the bottom of the mixing vessel. This it does quite readily being forced out by the pressure of gas inside. As it comes out portions of suitable size to make a loaf are cut off. These are at once moulded into loaves and put into the oven. The gas which they contain expands, and light well risen bread is produced. This process is especially suited for wholemeal and other flours containing much offal, which apparently do not give the best results when submitted to the ordinary yeast fermentation.

Before closing this chapter it may be of interest to add a short account of the sale of bread. Bread is at the present time nominally sold by weight under acts of Parliament passed about 80 years ago. That is to say, a seller of bread must provide in his shop scales and weights which will enable him to weigh the loaves he sells. No doubt he would be prepared to do so if requested by a customer, in which case he would probably make up any deficiency in weight which might be found by adding as a makeweight a slice from another loaf. For this purpose it is commonly accepted that the ordinary loaf should weigh two pounds. But in practice this does not occur, for practically the whole of the bread which is sold in this country is sold from the baker’s cart, which delivers bread at the houses of customers, and not over the counter. Customers obviously cannot be expected to wait at their doors whilst the man in the cart weighs each loaf he is delivering to them. In actual practice therefore the bread acts, as they are called, are really a dead letter, and bread is sold by the loaf and not by weight, though it must be remembered that the loaf has the reputed weight of two pounds. There are no doubt slight variations from this weight, but for all practical purposes competition nowadays is quite as effective a check on the bona fides of the bread seller as enforced sale by weight would be likely to be. If a baker got the reputation of selling loaves appreciably under weight his custom would very soon be transferred to one of his more scrupulous competitors. Altogether it may be concluded that the present unregulated method of sale does not work to the serious disadvantage of the consumers. A little consideration will show that the sale of bread could only be put on a more scientific basis by the exercise of an enormous amount of trouble, and the employment of a very numerous and expensive staff. No doubt the ideally perfect way of regulating the sale of either bread or any other feeding stuff would be to enact that it should be sold by weight, and that the seller should be compelled to state the percentage composition, so that the buyer could calculate the price he was asked to pay per unit of actual foodstuff. Now bread normally contains 36 per cent. of water, but this amount varies greatly. A two pound loaf kept in a dry place may easily lose water by evaporation at the rate of more than an ounce a day. The baker usually weighs out 2 lbs. 3 ozs. of dough to make each two pound loaf, and this amount yields a loaf which weighs in most cases fully two pounds soon after it comes out of the oven. But if the weather is hot and dry such a loaf may very well weigh less than two pounds by the time it is delivered to the consumer. In other words the baker cannot have the weight of the loaves he sells under complete control. Furthermore the loss in weight when a loaf gets dry by evaporation is due entirely to loss of water, and does not decrease the amount of actual foodstuff in the loaf. To sell bread in loaves guaranteed to contain a definite weight of actual foodstuff might be justified scientifically, but practically it would entail so great an expense for the salaries of the inspectors and analysts required to enforce such a regulation that the idea is quite out of the question. Practically, therefore, the situation is that it would be unfair to enforce sale by weight pure and simple for the weight of a loaf varies according to circumstances which are outside the baker’s control, and further because the weight of the loaf is no guarantee of the weight of foodstuff present in it. Nor is it possible to enforce sale by guarantee of the weight of foodstuff in the loaf, for to do so would be too troublesome and expensive. Finally the keenness of competition in the baking trade may be relied on to keep an efficient check on the interests of the consumer. Quite recently an important public authority has published the results of weighing several thousand loaves of bread purchased within its area of administration. The results show that over half the two pound loaves purchased were under weight to the extent of five per cent. on the average. Legislation is understood to be suggested as the result of this report, in which case it is to be hoped that account will be taken of the fact that the food value of a loaf depends not only on its weight but also on the percentage of foodstuffs and water which it contains.

CHAPTER VII
THE COMPOSITION OF BREAD

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.

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 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 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 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.

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 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 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 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 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 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 rôle of protein in the animal economy apply to adults in which protein is required for wear 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 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.

CHAPTER VIII
CONCERNING DIFFERENT KINDS OF BREAD

The table given in the last chapter states the average composition of ordinary white bread baked in the form of cottage loaves, and the remarks on the various constituents of bread in the preceding pages have for the most part referred to the same material, though many of them may be taken to refer to bread in general. It will now be of interest to inquire as to the variation in composition which is found among the different kinds of bread commonly used in this country. This enquiry will be most readily conducted by first considering the possible causes which may affect the composition of bread.

The variation in the composition of bread is a subject which is taken up from time to time by the public press, and debated therein with a great display of interest and some intelligent knowledge. In most of the press discussions in the past interest has been focussed almost entirely on the effect of different kinds of milling. The attitude commonly assumed by the food reform section of the contributors may be stated shortly as follows: In the days of stone milling a less perfect separation of flour and bran was effected, and the flour contained more of the materials situated in the grain near the husk than do the white flours produced by modern methods of roller milling. Again the modern roller mills separate the germ from the flour, which the stone mills fail to do, at any rate so completely. Thus the stone ground flours contain about 80 per cent. of the grain, whilst the whole of the flour obtained from the modern roller mill seldom amounts to much more than about 72 per cent. The extra eight per cent. of flour produced in the stone mills contains all or nearly all the germ and much of the material rich in protein which lies immediately under the husk. Hence the stone ground flour is richer in protein, and in certain constituents of the germ, than white roller mill flour, and hence again stone ground flour has a higher nutritive value. Roller mill flour has nothing to commend it beyond its whiteness. It has been suggested that millers should adopt the standard custom of producing 80 per cent. of flour from all the wheat passing through their mills and thus retain those constituents of the grain which possess specially great nutritive value.

It would probably be extremely difficult to produce 80 per cent. of flour from many kinds of wheat, but for the present this point may be ignored, whilst we discuss the variation in the actual chemical composition of the flour produced as at present and on the 80 per cent. basis. In comparing the chemical composition of different kinds of flour it is obvious that the flours compared must have been made from the same lot of wheat, for as will be seen later different wheats vary greatly in the proportions of protein and other important constituents which they contain. Unfortunately the number of analyses of different flours made from the same lots of wheat is small. Perhaps the best series is that published by Dr Hamill in a recent report of the Local Government Board. Dr Hamill gives the analyses of five different grades of flour made at seven mills, each mill using the same blend of wheats for all the different kinds of flour. Calculating all these analyses to a basis of 10 per cent. of protein in the grade of flour known as patents, the figures on the opposite page were obtained, which may be taken to represent with considerable accuracy the average composition of the various kinds of flours and offals when made from the same wheat.

Accepting these figures as showing the relative proportions of protein and phosphoric acid in different flours as affected by milling only, other sources of variation having been eliminated by the use of the same blend of wheat, it appears that the flours of commercially higher grade undoubtedly do contain somewhat less protein and phosphoric acid than lower grade or wholemeal flours. Taking the extreme cases of patents and wholemeal flours, the latter contains one-ninth more protein and four times more phosphoric acid than the former, provided both are derived from the same wheat.

In actual practice, however, it generally happens that the higher grade flours are made from a blend of wheats containing a considerable proportion of hard foreign wheats which are rich in nitrogen, whilst wholemeal and standard flours are usually made from home grown wheats which are relatively poor in nitrogen. From a number of analyses of foreign and home grown wheats it appears that the relative proportions of protein is about 12½ per cent. in the hard foreign wheats as compared with 10 per cent. in home grown wheats. Thus the presence of a larger proportion of protein in the hard wheats used in the blend of wheat for making the higher grade flours must tend to reduce the difference in protein content between say patents and wholemeal flours as met with in ordinary practice. Furthermore much of the bread consumed by that part of the population to whom a few grams per day of protein is of real importance is, or should be, made, for reasons of economy, from households flour, and the disparity between this grade of flour and wholemeal flour is much less than is the case with patents. It appears, therefore, on examining the facts, that there is no appreciable difference in the protein content of the ordinary white flours consumed by the poorer classes of the people and wholemeal flour or standard flour.

In the above paragraphs account has been taken only of the total amount of protein in the various kinds of bread and flour. It is obvious, however, that the total amount present is not the real index of food-value. Only that portion of any article of diet which is digested in the alimentary canal can be absorbed into the blood and carried thereby to the tissues where it is required to make good wear and tear. The real food-value must therefore depend not on the total amount of foodstuff present but on the amount which is digestible. The proportion of protein which can be digested in the different kinds of bread has been the subject of careful experiments in America, and lately in Cambridge. The method of experimenting is arduous and unpleasant. Several people must exist for a number of days on a diet consisting chiefly of the kind of bread under investigation, supplemented only by small quantities of food which are wholly digestible, such as milk, sugar and butter. During the experimental period the diet is weighed and its protein content estimated by analysis. The excreta are also collected and their protein content estimated by analysis, so that the amount of protein which escapes digestion can be ascertained. The experiment is then repeated with the same individuals and the same conditions in every way except that another kind of bread is substituted for the one used before. From the total amount of protein consumed in each kind of bread the total amount of protein voided in the excreta is subtracted, and the difference gives the amount which has been digested and presumably utilised in the body. From these figures it is easy to calculate the number of parts of protein digested for every 100 parts of protein eaten in each kind of bread. This description will have made evident the unpleasant nature of such experimental work. Its laboriousness will be understood from the fact that a series of experiments of this kind carried out at Cambridge last winter necessitated four people existing for a month on the meagre diet above mentioned, and entailed over 1000 chemical analyses.

The following table shows the amounts of protein digested per 100 parts of protein consumed in bread made from various kinds of flour, as based on the average of a number of experiments made in America, and in the experiments at Cambridge above referred to.

The American and the Cambridge figures agree very well with each other, and this gives confidence in the reliability of the results. It appears to be quite certain therefore that the protein in bread made from the higher grade flours is very considerably more digestible than that contained in bread made from flours containing greater amounts of husk. The percentages following the names of the various grades of flour in the first column of the table indicate approximately the proportion of the whole grain which went into the flour to which the figure is attached. Looking down these figures it appears that the digestibility of the protein decreases as more and more of the grain is included in the flour. It follows, therefore, that whilst by leaving more and more of the grain in the flour we increase the percentage of protein in the flour, and consequently in the bread, at the same time we decrease the digestibility of the protein. Apparently, too, this decrease in digestibility is proportionally greater than the increase in protein content, and it follows therefore that breads made from low grade flours containing much husk will supply less protein which is available for the use of the body, although they may actually contain slightly more total protein than the flours of higher grade.