GLUTEN—BREAD.
HAVING treated the cookery of the chief constituents of the roots
and stems of the plant, the fibre and the starch, I now come to food
obtained from the seeds and the leaves.
Taking the seeds first, as the more important, it becomes necessary to
describe the nitrogenous constituents which are more abundant in them
than in any other part of the plant, though they also contain starch
and cell material, or woody fibre, as already stated.
In the preceding chapter I described a method of separating starch from
flour by washing a piece of dough in water, and thereby removing the
starch granules, which fall to the bottom of the water. If this washing
is continued until no further milkiness of the water is produced, the
piece of dough will be much reduced in dimensions, and changed into a
grey, tough, elastic, and viscous or glutinous substance, which has
been compared to bird-lime, and has received the appropriate name of
_gluten_. When dried, it becomes a hard, horny, transparent mass. It is
insoluble in cold water, and partly soluble in hot water. It is soluble
in strong vinegar, and in weak solutions of potash or soda. If the
alkaline solution is neutralised by an acid, the gluten is precipitated.
If crude gluten, obtained as above, is subjected to the action
of hot alcohol, it is separated into two distinct substances, one
soluble and the other insoluble. As the solution cools, a further
separation takes place of a substance soluble in hot alcohol but
not in cold, and another soluble in either hot or cold alcohol.
The first, viz. that insoluble in either hot or cold alcohol, has
been named _gluten-fibrin_; that soluble in hot alcohol, but not
in cold, _gluten-casein_; and that soluble in either hot or cold
alcohol, _glutin_. I give these names and explain them, as my readers
may be otherwise puzzled by meeting them in books where they are
used without explanation, especially as there is another substance
presently to be described, to which the name of ‘vegetable casein’
has also been applied. The gluten-fibrin is supposed to correspond
with blood-fibrin, gluten-casein with animal-casein, and glutin with
albumen. Their composition is as follows, which I append for what it
is worth in connection with this theory, but mainly to show how small
is the difference between the chemical composition of the nitrogenous
constituents of animals and those of plants. I shall come to this
subject again:
+--------------------+---------------+---------------+--------+
| -- | Gluten-Fibrin | Gluten-Casein | Glutin |
+--------------------+---------------+---------------+--------+
| Carbon | 53·23 | 53·46 | 53·27 |
| Hydrogen | 7·01 | 7·13 | 7·17 |
| Nitrogen | 16·41 | 16·04 | 15·94 |
| Oxygen and sulphur | 23·35 | 23·37 | 23·62 |
+--------------------+---------------+---------------+--------+
+--------------------+---------------+---------------+--------+
| | Blood-Fibrin | | |
| -- | (Scherer) | Animal-Casein | Albumen|
+--------------------+---------------+---------------+--------+
| Carbon | 53·57 | 53·83 | 53·50 |
| Hydrogen | 6·90 | 7·15 | 7·00 |
| Nitrogen | 15·72 | 15·65 | 15·50 |
| Oxygen and sulphur | 22·81 | 23·37 | 24·00 |
+--------------------+---------------+---------------+--------+
Gluten is usually described as ‘partly soluble in hot water.’ My own
examination of this substance suggests that ‘partially soluble’ is a
better description than ‘partly soluble’ (Miller) or ‘very slightly
soluble’ (Lehmann). This difference is not merely a verbal quibble, but
very real and practical in reference to the _rationale_ of its cookery.
A partially soluble substance is one which is composed of soluble and
also of insoluble constituents, which, as already stated, is strictly
the case with gluten in reference to the solvent action of hot alcohol.
A very slightly soluble substance is one that dissolves completely, but
demands a very large quantity of the solvent. I find that the action of
hot water on gluten, as applied in cookery, is to effect what may be
described as a partial solution—that is, it effects a loosening of the
bonds of solidity without going so far as to render it completely fluid.
It appears to be a sort of hydration similar to that which is effected
by hot water on starch, but less decided.
To illustrate this, wash some flour in cold water so as to separate
the gluten in the manner already described; then boil some flour as in
making ordinary bill-stickers’ paste, and wash this in cold water. The
gluten will come out with difficulty from this, and, when separated,
will be softer and less tenacious than the cold-washed specimen. This
difference remains until some of the water it contains is driven out,
for which reason I regard it as hydrated, though I am not prepared to
say that the hydration is of a truly chemical character—a definite
chemical combination of gluten with water; it may be only a mechanical
combination—a loosening of solidity by a molecular intermingling of
water.
The importance of this in the cookery of grain-food is very great, as
anybody who aspires to the honour of becoming a martyr to science may
prove by simply making a meal on raw wheat, masticating the grains
until reduced to small pills of gluten, and then swallowing them. Mild
indigestion or acute spasms will follow, according to the quantity
taken and the digestive energies of the experimenter. Raw flour will
act similarly, but less decidedly.
Bread-making is the most important, as well as a typical example, of
the cookery of grain-food. The grinding of the grain is the first
process of such cookery; it vastly increases the area exposed to the
subsequent actions.
The next stage is that of surrounding each grain of the flour with
a thin film of water. This is done in making the dough by careful
admixture of a modicum of water and kneading, in order to squeeze
the water well between all the particles. The effect of insufficient
enveloping in water is sometimes seen in a loaf containing a white
powdery kernel of unmixed flour.
If nothing more than this were done, and such simple dough were baked,
the starch granules would be duly broken up and hydrated, the gluten
also hydrated, but, at the same time, the particles of flour would be
so cemented together as to form a mass so hard and tough when baked,
that no ordinary human teeth could crush it. Among all our modern
triumphs of applied science, none can be named that is more refined and
elegant than the old device by which this difficulty is overcome in the
everyday business of making bread. Who invented it, and when, I do not
know. Its discovery was certainly very far anterior to any knowledge
of the chemical principles involved in its application, and probably
accidental.
The problem has a very difficult aspect. Here are millions of
particles, each of which has to be moistened on its surface, but each,
when thus moistened, becomes remarkably adhesive, and therefore sticks
fast to all its surrounding neighbours. We require, without altogether
suppressing this adhesiveness, to interpose a barrier that shall sunder
these millions of particles from each other so delicately as neither to
separate them completely nor allow them to completely adhere.
It is evident that, if the operation that supplies each particle with
its film of moisture can simultaneously supply it with a partial
atmosphere of gaseous matter, the difficult and delicate problem will
be effectively solved. It is thus solved in making bread.
As already explained, the seed which is broken up into flour contains
diastase as well as starch, and this diastase, when aided by moisture
and moderate warmth, converts the starch into dextrin and sugar. This
action commences when the dough is made; this alone would only increase
the adhesiveness of the mass, if it went no further, but the sugar
thus produced may, by the aid of a suitable ferment, be converted into
alcohol. As the composition of alcohol corresponds to that of sugar,
minus carbonic acid, the evolution of carbonic acid gas is an essential
part of this conversion.
With these facts before us, their practical application in bread-making
is easily understood. To the water with which the flour is to be
moistened some yeast is added, and the yeast-cells, which are very
much smaller than the grains of flour, are diffused throughout the
water. The flour is moistened with this liquid, which only demands a
temperature of about 70° Fahr. to act with considerable energy on every
granule of flour that it touches. Instead, then, of the passive, lumpy,
tenacious dough produced by moistening the flour with mere water, a
lively ‘sponge,’ as the baker calls it, is produced, which ‘rises’
or grows in bulk by the evolution and interposition of millions of
invisibly small bubbles of gas. This sponge is mixed with more flour
and water, and kneaded and kneaded again to effect a complete and equal
diffusion of the gas bubbles, and finally, the porous mass of dough is
placed in an oven previously raised to a temperature of about 450°.
The baker’s old-fashioned method of testing the temperature of his oven
is instructive. He throws flour on the floor. If it blackens without
taking fire, the heat is considered sufficient. It might be supposed
that this is too high a temperature, as the object is to cook the
flour, not to burn it. But we must remember that the flour which has
been prepared for baking is mixed with water, and the evaporation of
this water will materially lower the temperature of the dough itself.
Besides this, we must bear in mind that another object is to be
attained. A hard shell or crust has to be formed, which will so encase
and support the lump of dough as to prevent it from subsiding when the
further evolution of carbonic acid gas shall cease, which will be the
case some time before the cooking of the mass is completed. It will
happen when the temperature reaches the point at which the yeast-cells
can no longer germinate, which temperature is considerably below the
boiling point of water.
In spite of this high outside temperature, that of the inner part of
the loaf is kept down to a little above 212° by the evaporation of
the water contained in the bread. The escape of this vapour and the
expansion of the carbonic acid bubbles by heat combine to increase the
porosity of the loaf.
The outside being heated considerably above the temperature of the
inner part, this variation produces the differences between the
crust and the crumb. The action of the high temperature in directly
converting some of the starch into dextrin will be understood from what
I have already stated, and also the partial conversion of this dextrin
into caramel, which was described in Chapter VII.
Thus we have in the crust an excess of dextrin as compared with
the crumb, and the addition of a variable quantity of caramel. In
lightly-baked bread, with a crust of uniform pale yellowish colour, the
conversion of the dextrin into caramel has barely commenced, and the
gummy character of the dextrin coating is well displayed. Some such
bread, especially the long staves of life common in France, appear as
though they had been varnished, and their crust is partially soluble in
water.
This explains the apparent paradox that hard crust, or dry toast, is
more easily digested than the soft crumb of bread; the cookery of the
crumb not having been carried beyond the mere hydration of the gluten
and the starch, and such degree of dextrin formation as was due to
the action of the diastase of the grain during the preliminary period
of ‘rising.’ In the crust some of the work of insalivation is already
done by the baker. The digestibility of toast is doubtless aided by its
brittleness, causing it to be more broken up and mixed with the saliva.
Everybody has, of course, heard of ‘unfermented bread,’ and many have
tasted it. Several methods have been devised, some patented, for
effecting an evolution of gas in the dough without having recourse to
the fermentation above described. One of these is that of adding a
little hydrochloric acid to the water used in moistening the flour, and
mixing bicarbonate of soda in powder with the flour (to every 4 lbs.
of flour ½ oz. bicarbonate and 4½ fluid drachms of hydrochloric acid of
1·16 specific gravity). These combine and form sodium chloride, common
salt, with evolution of carbonic acid. The salt thus formed takes the
place of that usually added in ordinary bread-making, and the carbonic
acid gas evolved acts like that given off in fermentation; but the
rapidity of the action of the acid and carbonate presents a difficulty.
The bread must be quickly made, as the action is soon completed. It
does not go on steadily increasing and stopping just at the right
moment, as in the case of fermentation.
Other methods similar in principle have been adopted, such as adding
ammonia carbonate with the soda carbonate. The ammonia salt is volatile
itself, besides evolving carbonic acid by its union with the acid.
In spite of the great amount of ingenuity expended upon the manufacture
of such unfermented bread, and the efforts to bring it into use, but
little progress has been made. The general verdict appears to be that
the unfermented bread is not so ‘sweet,’ that it lacks some element of
flavour, is ‘chippy’ or tasteless as compared with good old-fashioned
wheaten bread, free from alum or other adulteration. My theory of this
difference is that it is due to the absence of those changes which
take place while the sponge or dough is rising, when, if I am right,
the diastase of the grain is operating, as in germination, to produce
a certain quantity of dextrin and sugar, and possibly acting also on
the gluten. Deficiency of dextrin is, I think, the chief cause of the
chippy character of aerated bread. It must be remembered that, in
ordinary bread-making, the fermentation is protracted over several
hours, during which the temperature most favourable to germination is
steadily maintained.
The practical importance of the fermentation is strikingly shown by the
fact that, in the course of sponge rising, dough rising, and baking,
a loaf becomes about four times as large as the original mixture of
flour, water, &c., of which it was made; or, otherwise stated, an
ordinary loaf is made up of one part of solid bread to more than three
parts of air bubbles or pores. French rolls and some other kinds of
fancy bread are still more gaseous.
So far I have only named the flour, water, salt, and yeast. These,
with a little sugar or milk, added according to taste and custom, are
the ingredients of home-made bread, but ‘bakers’ bread’ is commonly,
though not necessarily, somewhat more complex. There is the material
technically known as ‘fruit,’ and another which bears the equivocal
name of ‘stuff,’ or ‘rocky.’ The _fruit_ are potatoes. The quantity
of these prescribed in Knight’s ‘Guide to Trade’ is one peck to the
sack of flour. This proportion is so small (about 3 per cent. by
weight) that, if not exceeded, it cannot be regarded as a fraudulent
adulteration, for the additional cost involved in the boiling,
skinning, and general preparing of the small addition exceeds the
saving in the price of raw material. The fruit, therefore, is not added
merely because it is cheaper than flour, as many people suppose.
The instructions concerning its use given in the work above named
clearly indicate that the potato flour is used to assist fermentation.
These instructions prescribe that the peck of potatoes shall be boiled
in their skins, mashed in the ‘seasoning tub,’ then mixed with two or
three quarts of water, the same quantity of patent yeast, and three or
four pounds of flour. The mixture is left to stand for six or twelve
hours, when it will have become what is called a _ferment_. After
straining through a sieve, to separate the skins of the fruit, it is
mixed with the sack of flour, water, &c.
It is evident from this that it would not pay to add such a quantity in
such a manner as a mere adulterant. The baker uses it for improving the
bread, from his point of view.
The _stuff_ or _rocky_ consists, according to Tomlinson, of one part of
alum to three parts of common salt. The same authority tells us that
the bakers buy this at 2_d._ per packet, containing 1 lb. in each, and
that they believe it to be ground alum. They buy it thus for immediate
use, being subject to a heavy fine if they keep alum on the premises.
The quantity of the mixture ordinarily used is 8 oz. to each sack of
flour weighing 280 lbs., so that the proportion of alum is but 2 oz. to
280 lbs. As one sack of flour is (with water) made into eighty loaves
weighing 4 lbs. each, the quantity of alum in 1 lb. of bread amounts to
1/160th of an oz.
The _rationale_ of the action of this small quantity of alum is still
a chemical puzzle. That it has an appreciable effect in improving
the _appearance_ of the bread is unquestionable, and it may actually
improve the quality of bread made from inferior flour.
One of the baker’s technical tests of quality is the manner in which
the loaves of a batch separate from each other. That they should break
evenly and present a somewhat silky rather than a lumpy fracture, is a
matter of trade estimation. When the fracture is rough and lumpy, one
loaf pulling away some of the just belongings of its neighbour, the
feelings of the orthodox baker are much wounded. The alum is said to
prevent this impropriety, while an excess of salt aggravates it.
It appears to be a fact that this small quantity of alum whitens the
bread. In this, as in so many other cases of adulteration, there
are two guilty parties—the buyer who demands impossible or unnatural
appearances, and the manufacturer or vendor who supplies the foolish
demand. The judging of bread by its whiteness is a mistake which has
led to much mischief, against which the recent agitation for ‘whole
meal’ is, I think, an extreme reaction.
If the husk, which is demanded by the whole-meal agitators, were as
digestible as the inner flour, they would unquestionably be right, but
it is easy to show that it is not, and that in some cases the passage
of the undigested particles may produce mischievous irritation in the
intestinal canal. My own opinion on this subject (it still remains in
the region of opinion rather than of science) is that a middle course
is the right one, viz. that bread should be made of moderately-dressed
or ‘seconds’ flour rather than over-dressed ‘firsts’ or undressed
‘thirds’—_i.e._ unsifted whole-meal flour.
Such seconds flour does not fairly produce white bread, and consumers
are unwise in demanding whiteness. In my household we make our own
bread, but occasionally, when the demand exceeds ordinary supply, a
loaf or two is bought from the baker. I find that, with corresponding
or identical flour, the baker’s bread is whiter than the home-made, and
proportionally inferior. I may describe it as colourless in flavour,
it lacks the characteristic of wheaten sweetness. There are, however,
exceptions to this, as certain bakers are now doing a great business in
supplying what they call ‘home-made’ or ‘farmhouse’ bread. It is darker
in colour than ordinary bread, but is sold nevertheless at a higher
price, and I find that it has the flavour of the bread made in my own
kitchen. When their customers become more intelligent, all the bakers
will doubtless cease to incur the expense of buying packets of ‘stuff’
or ‘rocky,’ or any other bleaching abomination.
Liebig asserts that in certain cases the use of lime-water improves the
quality of bread. Tomlinson says that ‘in the time of bad harvests,
when the wheat is damaged, the flour may be considerably improved,
without any injurious result whatever, by the addition of from 20 to 40
grains of carbonate of magnesia to every pound of flour.’ It is also
stated that chalk has been used for the same purpose. These would all
act in nearly the same manner by neutralising any acid, such as acetic,
that might already exist or be generated in the course of fermentation.
When gluten is kept in a moist state, it slowly loses its soft,
elastic, and insoluble condition; if kept in water for a few days, it
gradually runs down into a turbid, slimy solution, which does not form
dough when mixed with starch. The gluten of imperfectly-ripened wheat,
or of flour or wheat that has been badly kept in the midst of humid
surroundings, appears to have fallen partially into this condition, the
gluten being an actively hygroscopic substance.
Liebig’s experiments show that flour in which the gluten has undergone
this partial change may have its original qualities restored by mixing
100 parts of flour with 26 or 27 parts of saturated lime-water and a
sufficiency of ordinary water to work it into dough. I suspect that
the action of the alum is of a similar kind, though this does not
satisfactorily account for the bleaching.
The action of sulphate of copper, which has been used in Belgium and
other places for improving the appearance and sponginess of loaves, is
still more mysterious than that of alum. Kuhlmann found that a single
grain in a 4-lb. loaf produced a marked alteration in the appearance
of the bread. Fortunately this adulteration, if perpetrated to a
mischievous extent, may be easily detected by acidulating the crumb,
and then moistening with a solution of ferrocyanide of potassium.
The brown colour thus produced betrays the presence of copper. The
detection of alum in small quantities is extremely difficult.
I should add that the ancient method of effecting the fermentation of
bread, which I understand is still employed to some extent in France,
differs somewhat from the ordinary modern English practice.
When flour made into dough is kept for some time moderately warm, it
undergoes spontaneous fermentation, formerly described as ‘panary
fermentation,’ and supposed to be of a different nature from the
fermentation which produces yeast.
Dough in this condition is called _leaven_, and when kneaded with fresh
flour and water its fermentation is communicated to the whole lump;
hence the ancient metaphors. In practice the leaven was obtained by
setting aside some of the dough of a previous batch, and adding this
to the next when its fermentation had reached its maximum activity.
One reason why the modern method has superseded this appears to be
that the leaven is liable to proceed onward beyond the first stage of
fermentation, or that producing alcohol, and run into the acetous, or
vinegar-forming fermentation, producing sour bread. Another reason may
be that the potato mixture above described, which is but another kind
of leaven, is more effectual and convenient.
Dr. Dauglish’s method (patented in 1856, 1857, and 1858) is based on
the fact that water under pressure absorbs and holds in solution a
large quantity of carbonic acid gas, which escapes when the pressure
is diminished, as in uncorking soda-water, &c. Dr. Dauglish places the
flour in a strong, air-tight iron vessel, then forces water saturated
with carbonic acid under high pressure into this; kneading-knives mix
the dough by their rotation. When the mixture is completed a trap at
the lower part of the globular iron vessel is opened. The pressure of
the confined carbonic acid above forces the dough through this in a
cylindrical jet or flat ribbon as required, and this squirted cylinder
or ribbon is fashioned by suitable cutters, &c., into loaves. The
compressed gas expands, and the loaves are smartly baked before the
expansive energy of the gas is exhausted. It is justly claimed for this
process that it is far more cleanly than the ordinary method of making
bread, as with suitable machinery such ‘aerated bread’ can be made
without handling.
The difference between new and stale bread is familiar enough, but
the nature of the difference is by no means so commonly understood.
It is generally supposed to be a simple result of mere drying. That
this is not a true explanation may be easily proved by repeating the
experiments of Boussingault, who placed a very stale loaf (six days
old) in an oven for an hour, during which time it was, of course,
being further dried; but, nevertheless, it came out as a new loaf. He
found that during the six days, while becoming stale, it only lost 1
per cent. of its weight by drying, and that during the one hour in the
oven it lost 3½ per cent. in becoming new, and apparently more moist.
By using an air-tight case instead of an ordinary oven, he repeated
the experiment several times in succession on the same piece of bread,
making it alternately stale and new each time.
For this experiment the oven should be but moderately heated—260°
to 300° Fahr. is sufficient. I am fond of hot rolls for breakfast,
and frequently have them _à la Boussingault_, by treating stale
bread-crusts in this manner. My wife tells me that when the crusts
have been long neglected, and are thin, the Boussingault hot rolls are
improved by dipping the crust in water before putting it into the oven.
This is not necessary in experimenting with a whole loaf or a thick
piece of stale bread.
The crumb of bread, whether new or stale, contains about 45 per cent.
of water. Miller says ‘the difference in properties between the two
depends simply upon difference in molecular arrangement.’
This ‘molecular arrangement’ is the customary modern method of
explaining a multitude of similar physical and chemical problems, or,
as I would rather say, of evading explanation under the cover of a
vague conventional phrase.
I have made some simple experiments which supply a visible explanation
of the facts without invoking the aid of any invisible atoms or
molecules, or any imaginary arrangements or rearrangements of these
imaginary entities.
I find that, as bread becomes stale, its porosity _appears_ to
increase, and that when renewed by reheating, it returns to its
original _apparently_ smaller degree of porosity. That this change can
be only apparent is evident from the facts that the total quantity of
solid material in the loaf remains the same, and its total dimensions
are retained more or less completely by the rigidity of the crust.
I say ‘more or less,’ because this depends upon the thickness
and hardness of the crust, and also upon the completeness of its
surrounding. Lightly-baked loaves shrink a little in dimensions in
becoming stale, and partly regain the loss on reheating, but this
difference only exaggerates the apparent paradox of varying porosity,
as the diminished bulk of a given quantity of material displays
increased porosity, and the increase of total dimensions accompanies
the diminished porosity.
I have obtained a reconciliation of this paradox by careful examination
of the structure of the crumb. This shows that the larger or decidedly
visible pores are cells having walls of somewhat silky appearance. The
silky lustre and structure is, I have no doubt, due to a varnish of
dextrin, the gummy nature of which I have already described. On looking
a little more closely at this inner surface of the big blow-holes
with the aid of a hand-lens of moderate power, I find that it is not
a continuous varnish of gum, but a net-work or agglomeration of gummy
fibres and particles, barely touching each other.
My theory of the change that takes place as the bread becomes stale is,
that these fibres and particles gradually approach each other either
by shrinkage or adhesive attraction, and thus consolidate and harden
the walls of each of the millions of easily visible pores, these walls
forming the solid material of which the loaf is made up. In doing so
they naturally increase the dimensions of the visible pores, while the
microscopic interstices or spaces between the minute fibres of the cell
walls are diminished by the approximation or adhesion of the fibres to
each other.
This adhesion is probably aided by an oozing out or efflorescence
of the vapour held by the fibres, and its condensation on their
surfaces. This point, be it understood, is merely hypothetical, as
the efflorescence is not visible. All the other phenomena I have just
described are visible either with the naked eye or by the aid of a lens.
When the stale bread is again heated, a general expansion occurs by
the conversion of liquid water into aqueous vapour, every grain of
water thus converted expanding to 1,700 times its former bulk. As
this happens throughout, _i.e._ upon the surface of every one of the
countless fibres or particles, there must be a general elbowing in
the crowd, breaking up the recent adhesion between these fibres and
thrusting them all apart in the directions of least resistance; _i.e._
towards the open spaces of the larger and visible pores, producing that
_apparent_ diminution of porosity that I have observed as the easily
visible characteristic of the change.
This explanation may be further demonstrated by cutting a loaf through
the middle from top to bottom, and exposing the cut surfaces. In this
case the bread becomes unequally stale, more so near the cut surface
than within. The unequal pull due to the greater approximation and
adhesion of the fibres and small particles causes a rupture of the
exposed surface of the crumb, which becomes cracked or fissured without
any perceptible alteration of the size of the visible pores. If the two
broken faces be now accurately placed together, the halves thus closely
joined, firmly tied, and placed for an hour in the oven, it will be
seen on separating them that the chasms are considerably closed, though
not quite healed. Careful examination of the structure of the inside,
by breaking out a portion of the crumb, will reveal that loosening
which I have described.
‘Popped corn’ is a peculiar example of starch cookery. Here a certain
degree of porosity is given to an originally close-compacted structure
of starch by the simple operation of explosive violence due to the
sudden conversion into vapour of the water naturally associated with
the starch. The operation is too rapid for the production of much
dextrin.