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 121 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, 122 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 123 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.
description of flour
or offal protein
per cent. phosphoric acid
per cent.
flours:
patents 10·0 0·18
straight grade, about 70 per cent. 10·6 0·21
households 10·9 0·26
standard flour, about 80 per cent. 11·0 0·35
wholemeal 11·3 0·73
offals:
germ 24·0 2·22
sharps 14·5 1·66
bran 13·5 2·5
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 12? per cent. in the hard foreign wheats as compared with 10 per cent. in home grown wheats. thus the presence of a larger 124 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 125 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 126 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.
kind of flour from
which bread
was made percentage of
the grain
contained in
the flour amount of protein digested
per 100 parts eaten
american
experiments cambridge
experiments
patents 36 — 89
straight grade 70 89 —
standard 80 81 86
brown 88 — 80
brown 92 — 77
wholemeal 100 76 —
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 127 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.
when all the facts are taken into account it appears that the contention of the food reformers, that the various breads which contain those constituents of the grain which lie near the husk are capable of supplying more protein for the needs of the body than white breads, cannot be upheld. from statistics collected by the board of trade some few years ago as to the dietary of the working classes it appears that the diet of workers both in urban and in rural districts contains about 97 grams of total protein per head per day. this is rather under than over the commonly accepted standard of 100 grams of protein which is supposed to be required daily by 128 a healthy man at moderate work. consequently a change in his diet which increased the amount of protein might be expected to be a good change. but the suggested change of brown bread for white, though it appears to increase the total protein, turns out on careful examination to fail in its object, for it does not increase the amount of protein which can be digested.
from the same statistics it appears that the diet of a working man includes on the average about 1? lb. of bread per day. this amount of bread contains about 60 grams of protein, or two-thirds of the total protein of the diet. now it was pointed out in the last chapter that the protein of wheat was very rich in glutaminic acid, a constituent of which animals require comparatively small amounts. it is also correspondingly poor in certain constituents which are necessary to animals. apparently therefore it would be better to increase the diet in such cases by adding some constituent not made from wheat than by changing the kind of bread. from the protein point of view, however we look at it, there appears to be no real reason for substituting one or other of the various kinds of brown bread for the white bread which seems to meet the taste of the present day public.
but important as protein is it is not everything in a diet. as we have already pointed out the food 129 must not only repair the tissues, it must also supply fuel. it has been shown also that the fuel-value of a food can be ascertained by burning a known weight and measuring the number of units of heat or calories produced. many samples of bread have been examined in this way in the laboratories of the american department of agriculture, and it appears from the figures given in their bulletins that the average fuel value of white bread is about 1·250 calories per pound, of wholemeal bread only 1·150 calories per pound. these figures are quite in accord with those which were obtained in cambridge in 1911, in connection with the digestion experiments already described, which were also extended so as to include a determination of the proportion of the energy of the bread which the diet supplied to the body. the energy or fuel-value of the diet was determined by measuring the amount of heat given out by burning a known weight of each of the kinds of bread used in the experiment. the energy which was not utilised by the body was then determined by measuring how much heat was given out by burning the excreta corresponding to each kind of bread. the following table gives side by side the average results obtained in several such experiments in america and in cambridge.
the agreement between the two sets of figures is again on this point quite satisfactory. it is evident 130 that a greater proportion of the total energy of white bread can be utilised by the body than is the case with any of the breads made from flours of lower commercial grades which contain more husk. in fact it appears that the more of the outer parts of the grain are left in the flour the smaller is the proportion of the total energy of the bread which can be utilised. combining this conclusion with the fact that brown breads contain on the average less total energy than white breads, there can be no doubt that white bread is considerably better than any form of brown bread as a source of energy for the body.
kind of flour from
which bread
was made percentage of
the grain
contained in
the flour amount of protein digested
per 100 parts eaten
american
experiments cambridge
experiments
patents 36 96 96
straight grade 70 92 —
standard 80 87 95
brown 88 — 90
brown 92 — 89
wholemeal 100 82 —
there is one more important substance in respect of which great superiority is claimed for brown breads, namely phosphoric acid. from the table on page 122 there can be no doubt that flours containing more of the outer parts of the grain are very much richer in phosphoric acid than white flours, and the disparity is so great that after allowing for the larger 131 proportion of water in brown breads they must contain far more of this substance than do white breads. in the cambridge digestibility experiments quoted above the proportion of the phosphoric acid digested from the different breads was determined. it was found that for every 100 parts of phosphoric acid in white bread only 52 parts were digested, and that in the case of the brown breads this proportion fell to 41 parts out of 100. again, as in the case of protein and energy, the phosphoric acid in white bread is more readily available to the body than that of brown bread, but in this case the difference in digestibility is not nearly enough to counterbalance the much larger proportion of phosphoric acid in the brown bread. there is no doubt that the body gets more phosphoric acid from brown bread than from the same quantity of white bread. but before coming to any practical conclusion it is necessary to know two things, how much phosphoric acid does a healthy man require per day, and does his ordinary diet supply enough?
from the board of trade statistics already quoted it appears that, on the assumption that the average worker eats white bread only, his average diet contains 2·4 grams of phosphoric acid per day, which would be raised to 3·2 grams if the white bread were replaced by bread made from 80 per cent. flour containing ·35 per cent. of phosphoric acid. information 132 as to the amount of phosphoric acid required per day by a healthy man is somewhat scanty, and indicates that the amount is very variable, but averages about 2? grams per day. if this is so, the ordinary diet with white bread provides on the average enough phosphoric acid. exceptional individuals may, however, be benefited by the substitution of brown bread for white, but it would probably be better even in such cases, for the reasons stated when discussing the protein question, to raise the phosphorus content of their diet by the addition of some substance rich in phosphorus but not made from wheat.
finally comes the question of the variation in the composition of bread due to the presence or absence of the germ. the first point in this connection is to decide whether germ is present in appreciable proportions in any flour except wholemeal. the germ is a soft moist substance which flattens much more readily than it grinds. consequently it is removed from flour by almost any kind of separation, even when very coarse sieves are employed. if this contention is correct no flour except wholemeal should contain any appreciable quantity of germ, and it is certainly very difficult to demonstrate the presence of actual germ particles even in 80 per cent. flour. indirect evidence of the presence of germ may, however, be obtained as already explained by estimating 133 by chemical analysis the proportion of fat present in various flours. the figures for such estimations are given by dr hamill in the report of the local government board already referred to. they show that the percentages of fat in different grades of flours made from the same blends of wheat are on the average of seven experiments as follows: patents flour ·96: household flours 1·25: 80 per cent. or standard flour 1·42. these figures show that the coarser flours containing more of the whole grain do contain more germ than the flours of commercially higher grade, in spite of the fact that it is difficult to demonstrate its presence under the microscope.
remembering, however, that the whole of the germ only amounts to about 1? per cent. of the grain, it is clear that the presence or absence of more or less germ cannot appreciably affect the food-value as measured by protein content or energy-value. it is still open to contention that the germ may contain some unknown constituent possessing a peculiar effect on nutrition. such a state of things can well be imagined in the light of certain experimental results which have been obtained during the last few years.
it has been shown for instance by dr hopkins in cambridge, and his results have been confirmed at the carnegie institute in america, that young rats fail to thrive on a diet composed of suitable amounts of purified protein, fat, starch, and ash, but that they 134 thrive and grow normally on such a diet if there is added a trace of milk or other fresh animal or vegetable substance far too small to influence either the protein content or the energy-value. another case in point is the discovery that the disease known as beri beri, which is caused by a diet consisting almost exclusively of rice from which the husk has been removed, can be cured almost at once by the administration of very small doses of a constituent existing in minute quantities in rice husk. the suggestion is that high grade flours, like polished rice, may fail to provide some substance which is necessary for healthy growth, a substance which is removed in the germ or husk when such flours are purified, and which is present in flours which have not been submitted to excessive purification.
the answer is that no class in great britain lives on bread exclusively. bread appears from the government statistics already quoted to form only about half the diet of the workers of the country. their diet includes also some milk, meat, and vegetables, and such substances, according to dr hopkins’ experiments, certainly contain the substance, whatever it may be, that is missing from the artificial diet on which his young rats failed to thrive.
one last point. it will have been noticed in the figures given above that the variations in protein content, digestibility, and energy-value, between 135 different kinds of bread are usually not very large. there is, however, one constituent of all breads whose proportions vary far more widely, namely water. during last summer the author purchased many samples of bread in and around cambridge, and determined the percentage of water in each sample. the samples were all one day old so that they are comparable with one another. the results on the whole are a little low, probably because the work was done during a spell of rather dry weather, when the loaves would lose water rapidly.
the average figures are summarised below:
percentage
of water
cottage loaves made of white flour 31·7
tinned loaves made of white flour 32·7
small fancy loaves made of white flour 33·7
tinned loaves made of “standard” flour 35·9
tinned loaves made of brown or germ flour 40·0
the figures speak for themselves. there must obviously be more actual food in a cottage loaf of white flour containing under 32 per cent. of water than in any kind of standard or brown loaf in which the percentage of water is 36 to 40. it is quite extraordinary that no one who has organised any of the numerous bread campaigns in the press appears to have laid hold of the enormous variation in the water content of different kinds of bread, and its obvious bearing on their food-value.