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.
per cent.
Water 36
Organic substances:
Proteins 10
Starch 42
Sugar, etc. 10
Fat 1
Fibre ·3 63·3
Ash:
Phosphoric acid ·2
Lime, etc. ·5 ·7
------
100·0
The above table shows that one of the most abundant constituents of
ordinary bread is water. Flour as commonly used for baking, although it
may look and feel quite dry, is by no means free from water. It holds
on the average about one-seventh of its own weight or 14 per cent. In
addition to this rather over one-third of its weight of water or about
35 to 40 per cent. is commonly required to convert ordinary flour into
dough. It follows from this that dough will contain when first it is
mixed somewhere about one-half its weight of water or 50 per cent.
About four per cent. of the weight of the dough is lost in the form
of water by evaporation during the fermentation of the dough before
it is scaled and moulded. Usually 2 lb. 3 oz. of dough will make a
two pound loaf, so that about three ounces of water are evaporated in
the oven, This is about 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.