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.
[Illustration: Fig. 13. First break rolls seen from one end. The ribs
can just be seen where the two rolls touch]
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.
[Illustration:
Fig. 14. Break rolls. The large and small cog-wheels are the
simplest device used to give the two rolls different speeds.
The larger cog-wheel is driven by power and drives the smaller,
of course at a much higher rate of revolution]
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.
[Illustration:
Fig. 15. A pair of reduction rolls. They are smooth, and the
cog-wheels being nearly of the same size the speed of the two
rolls is nearly equal]
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.