THE QUALITY OF WHEAT FROM THE MILLER’S POINT OF VIEW
To the miller the quality of wheat depends on three chief factors, the
percentage of dirt, weed seeds, and other impurities, the percentage of
water in the sample, and a complex and somewhat ill-defined character
commonly called strength.
With the methods of growing, cleaning and thrashing wheat practised in
Great Britain, practically clean samples are produced, and home grown
wheat is therefore on the whole fairly free from impurities. This is,
however, far from the case with foreign wheats, many of which arrive at
the English ports in an extremely dirty condition. They are purchased
by millers subject to a deduction from the price for impurities above
the standard percentage which is allowed. The purchase is usually made
before the cargo is unloaded. Official samples are taken during the
unloading in which the percentage of impurities is determined, and the
deduction, if any, estimated.
The percentage of water, the natural moisture as it is usually called,
varies greatly in the wheats of different countries. In home grown
wheats it is usually 16 per cent., but in very dry seasons it may be
much lower, and in wet seasons it may rise to 18 per cent. Foreign
wheats are usually considerably drier than home grown wheats. In
Russian wheats 12 per cent. is about the average, and that too is about
the figure for many of the wheats from Canada, the States, Argentina,
and parts of Australia. Indian wheats sometimes contain less than 10
per cent. This is also about the percentage in the wheats of the arid
lands on the Pacific coast and in Australia. These figures show that
home grown wheats often contain as much as 5 per cent. more water
than the foreign wheats imported from the more arid countries. The
more water a wheat contains the less flour it will yield in the mill.
Consequently the less its value to the miller. A difference of 5 per
cent. of natural moisture means a difference in price of from 1_s._
6_d._ to 2_s._ per quarter in favour of the drier foreign wheats. This
is one of the reasons why foreign wheats command a higher price than
those grown in this country.
Turning to the third factor which determines the quality of wheat from
the miller’s point of view, we may for the present define strength
as the capacity for making bread which suits the public taste of the
present day. We shall discuss this point more fully when we deal with
the baking of bread. At present the only generally accepted method of
determining the strength of a sample of wheat is to mill it and bake
it, usually into cottage loaves. The strength of the wheat is then
determined from their size, shape, texture, and general appearance. A
really strong flour makes a large, well risen loaf of uniformly porous
texture. Wheats lacking in strength are known as weak. A weak wheat
makes a small flat loaf. In order to give a numerical expression to
the varying degrees of strength met with in different wheats, the Home
Grown Wheat Committee of the National Association of British and Irish
Millers have adopted a scale as the result of many thousand milling
and baking tests. On their scale the strength of the best wheat
imported from Canada, graded as No. 1 Manitoban, or from the States
graded as No. 1 Hard Spring, is taken as 100, that of the well-known
grade of flour known as London Households as 80, and that of the
ordinary varieties of home grown wheat, such as Square Head’s Master,
Browick, Stand Up, etc., as 65. The strength of most foreign wheats
falls within these limits. Thus the strength of Ghirka wheat from
Russia is about 85, of Choice White Karachi from India 75, of Plate
River wheat from the Argentine 80, etc. The strongest of all wheats is
grown in certain districts in Hungary. It is marked above 100 on the
scale, but it is not used for bread making. The soft wheats from the
more arid regions in Australia and the States are usually weaker than
average home grown samples, and are marked at 60. Rivet or cone wheat,
a heavy cropping bearded variety much grown by small holders,--since
the sparrow, which would ruin small plots of any other variety, seems
to dislike Rivet, possibly on account of its beard,--is the weakest of
all wheats, and is only marked at 20, which means that bread baked
from Rivet flour alone would be practically unsaleable. Rivet wheat
finds a ready sale, however, for making certain kinds of biscuits.
[Illustration: Fig. 7. Loaves made from No. 1 Manitoba. Strength 100]
[Illustration: Fig. 8. Loaves made from average English wheat. Strength
65]
[Illustration: Fig. 9. Loaves made from Rivet wheat. Strength 20]
In order to make flour which will bake bread to suit the taste of the
general public of the present day, the miller finds it necessary to
include in the mixture or blend of wheats which he grinds a certain
proportion of strong wheats such as Canadian, American, or Russian.
The quantity of strong wheat available is limited. Consequently strong
wheat commands a relatively high price. The average difference in price
of say No. 1 Manitoban and home grown wheat is about 5_s._ per quarter.
It is possible of course that the public taste in bread may change,
and damp close textured bread may become fashionable. In this case no
doubt the difference in price would disappear. Under present conditions
the necessity of including in his grinding mixture a considerable
proportion of strong foreign wheat is a distinct handicap against the
inland miller as compared with the port miller. The latter gets his
foreign wheat direct from the ship in which it is imported, whilst
the former has to pay railway carriage from the port to his mill. The
question naturally arises--is it not possible to grow strong wheats at
home and sell them to the inland miller?
This question has been definitely answered by the work of the Home
Grown Wheat Committee during the last 12 years. The committee
collected strong wheats from every country where they are produced,
and grew them in England. From the first crop they picked out single
plants of every type represented in the mixed produce, for strong
wheats as imported are usually grades and not pure varieties. From the
single plants they have established pure strains of which they have
grown enough to mill and bake. From most of the strong wheats they
were unable to find any strain which would produce strong wheat in
England. Thus the strong wheat of Hungary when grown in England was
no stronger than any of the ordinary typical home grown wheats. But
from the strong wheat of Canada was isolated the variety known as Red
Fife, which makes up a very large proportion of the higher grades of
American and Canadian wheats, and this variety when grown in England
was found to continue to produce wheat as strong as the best Canadian.
Year after year it has been grown here, and when milled and baked its
strength has been found to be 100 or thereabouts on the scale above
described. Finally it was found that a strain of Red Fife which had
been brought over from Canada 20 years ago, and grown continuously in
the western counties ever since, under the name of Cook’s Wonder, was
still producing wheat which when ground and baked possessed a strength
of about 100. Thus it was conclusively proved that in the case of
Red Fife at any rate the English climate was capable of producing
really strong wheat. The strength of Hungarian and Russian wheats
appear to be dependent on the climate of those countries. Red Fife,
however, produces strong wheat wherever it is grown. It is interesting
to note that this variety although first exploited in Canada and the
States is really of European origin. It was taken out to Canada by an
enterprising Scotchman called Fife in a mixed sample of Dantzig wheat.
He grew it for some time and distributed the seed. Pure strains have
from time to time been selected by the American and Canadian experiment
stations.
But the discovery that Red Fife would produce strong wheat in England
by no means solved the problem, for when the Home Grown Wheat Committee
distributed seed of their pure strain of that variety for extended
testing throughout the country, it was soon found to be only a poor
yielder except in a few districts. A yield of three quarters of strong
grain, even if it makes 40_s._ per quarter on the market, only gives to
the farmer a return of £6 per acre, as compared with a return of nearly
£8 from 4½ quarters of weak grain worth 35_s._ per quarter, which can
usually be obtained by growing Square Head’s Master, or some other
standard variety.
[Illustration:
Fig. 10. The left-hand loaf was made from average English
wheat. The loaf in the centre was made from Burgoyne’s Fife,
and is practically identical in size and shape with the
right-hand loaf which was made from imported No. 1 Manitoba]
It was at this point that Mendel’s discoveries came to the rescue.
Working on the Mendelian lines already explained, Biffen at Cambridge
crossed Red Fife with many of the best English varieties. From one of
the crosses he was able to isolate a new variety in which are combined
the strength of Red Fife and the vigour and cropping power of the
English parent. This variety, known as Burgoyne’s Fife, has been grown
and distributed by members of the Millers’ Association. In 1911 on the
average of 28 separate trials it yielded 38 bushels per acre, which
is well above the average of the best English varieties. It has been
repeatedly milled and baked, and its strength is between 90 and 100,
practically the same as that of Red Fife. It has been awarded many
prizes at agricultural shows for quality, and it commands on markets
where the local millers have found out its baking qualities about the
same price as the best foreign strong wheats, that is to say from
4_s._ to 5_s._ per quarter more than the average price of home grown
wheat. Taking a fair average yield of wheat as four quarters per acre,
Burgoyne’s Fife gives to the farmer an increased return over the
ordinary varieties of about 16_s._ per acre. The introduction of such a
variety makes the production of strong wheat in England a practicable
reality, and will be a boon both to the farmer and to the inland
miller. It is likely too that the possibility of obtaining a better
return per acre will induce farmers to grow more wheat. Anything that
tends to increase the production of home grown wheat and makes Great
Britain less dependent on foreign supplies is a national asset of the
greatest value.
It is of the greatest importance to the miller that he should be able
to determine the strength of the wheats he buys. Obviously the method
mentioned above, which entails milling enough of the sample to enable
him to bake a batch of bread, is far too lengthy to be of use in
assessing the value of a sample with a view to purchase. The common
practice is for the miller or corn merchant to buy on the reputation
of the various grades of wheat, which he confirms by inspection of
the sample. Strength is usually associated with certain external
characters which can readily be judged by the eye of the practised
wheat buyer. Strong wheats are usually red in colour, their skin is
thin and brittle, the grain is usually rather small, and has a very
characteristic horny almost translucent appearance. The grains are
extremely hard and brittle, and when broken the inside looks flinty.
On chewing a few grains the starch is removed and there remains in the
mouth a small pellet of gluten, which is tough and elastic like rubber,
but not sticky.
Weak wheats as a rule possess none of these characters. Their colour
may be either red or white, their skin is commonly thick and tough,
the grain is usually large and plump, and often has an opaque mealy
appearance. It is soft and breaks easily, and the inside is white, soft
and mealy. Very little gluten can be separated from it by chewing, and
that little is much less tough and elastic than the gluten of a strong
wheat.
These characters, however, are on the whole less reliable than the
reputation of the grade of wheat under consideration. To make a
reliable estimate of strength from inspection of a sample of wheat
requires a natural gift cultivated by continual practice. Even the
best commercial judges of wheat have been known to be deceived by
a sample of white wheat which subsequent milling and baking tests
showed to possess the highest strength. The mistake was no doubt due
to the great rarity of strength among white wheats. This rarity will
doubtless soon disappear now that a pure strain of White Fife has
been isolated and shown to possess strength quite equal to that of Red
Fife. Sometimes too the ordinary home grown varieties produce most
deceptive samples which show all the external characters of strong
wheats. Such samples, however, on milling and baking are invariably
found to possess the usual strength of home grown wheat, about 65 on
the scale. These considerations show the great need of a scientific
method of measuring strength, which can be carried out rapidly and on a
small sample of grain. This need is felt at the present time not only
by the miller and the merchant, but by the wheat breeder. For instance,
in picking out the plants possessing strong grain from cultures of the
second generation after making his crosses, the plant breeder up to the
present has had to rely on inspection by eye, and on the separation of
gluten by chewing, for a single plant obviously cannot yield enough
grain to mill and bake. This fact no doubt explains the differences of
opinion among plant breeders on the inheritance of strength, for it is
not every one who can acquire the power of judging wheat accurately by
his senses. Such a faculty is a personal gift, and is at best apt to
fail at times.
The search for a rapid and accurate method of measuring strength has
for many years attracted the attention of investigators. As might be
expected most of the investigations have centred round the gluten,
for as mentioned above the gluten of a strong wheat is much more tough
and elastic than that of a weak wheat. Gluten is a characteristic
constituent of all wheats, and it is the presence of gluten which
gives to wheat flour the power of making bread. The other cereals,
barley, oats, maize and rice are very similar to wheat in their general
chemical composition, but they do not contain gluten. Consequently they
cannot make bread.
In making bread flour is mixed with water and yeast. The yeast feeds
on the small quantity of sugar contained in the flour, fermenting
it and forming from it alcohol and carbon dioxide gas. The gluten
being coherent and tough is blown into numberless small bubbles by
the gas, which is thus retained inside the bread. On baking, the high
temperature of the oven fixes these bubbles by drying and hardening
their walls, and the bread is thus endowed with its characteristic
porous structure. If a cereal meal devoid of gluten is mixed with water
and yeast, fermentation will take place with formation of gas, but the
gas will escape at once, and the product will be solid and not porous.
Evidently from the baking point of view gluten is of the greatest
importance. One of the most obvious methods that have been suggested
for estimating the strength of wheat depends on the estimation of the
percentage of gluten contained in the flour. The method has not turned
out very successfully, for strength seems to depend rather on the
quality than on the quantity of gluten in the wheat. Much attention has
been given to the study of the causes of the varying quality of the
gluten of different wheats. Gluten for instance has been shown to be
a mixture of two substances, gliadin and glutenin, and the suggestion
has been made that its varying properties are dependent on the varying
proportions of these two substances present in different samples. This
suggestion however failed to solve the problem.
After seven years of investigation the author has worked out the
following theory of the strength of wheat flours, which has finally
enabled him to devise a method which promises to be both accurate and
rapid, and to require so little flour that it can readily be used by
the wheat breeder to determine the strength of the grain in a single
ear. It has already been mentioned that a strong wheat is one that will
make a large loaf of good shape and texture. The strength of a wheat
may therefore be defined as the power of making a large loaf of good
shape and texture. Evidently strength is a complex of at least two
factors, size and shape, which are likely to be quite independent of
each other. Not infrequently, for instance, wheats are met with which
make large loaves of bad shape, or on the other hand, small loaves
of good shape. Probably therefore the size of the loaf depends on one
factor, the shape on another; and the failure of the many attempts
to devise a method of estimating strength have been caused by the
impossibility of measuring the product of two independent factors by
one measurement.
It seemed a feasible idea that the size of the loaf might depend on
the volume of gas formed when yeast was mixed with different flours.
On mixing different flours with water and yeast it was found that for
the first two or three hours they all gave off gas at about the same
rate. The reason of this is that all flours contain about the same
amount of sugar, approximately one per cent., so that at the beginning
of the bread fermentation all flours provide the yeast with about the
same amount of sugar for food. But this small amount of sugar is soon
exhausted, and for its subsequent growth the yeast is dependent on the
transformation of some of the starch of the flour into sugar. Wheat
like many other seeds contains a ferment or enzyme called diastase,
which has the power of changing starch into sugar, and the activity of
this ferment varies greatly in different wheats. The more active the
ferment in a flour the more rapid the formation of sugar. Consequently
the more rapidly the yeast will grow, and the greater will be the
volume of gas produced in the later stages of fermentation in the
dough. As a rule it is not practicable to get the dough moulded into
loaves and put into the oven before it has been fermenting for about
six or eight hours. If the flour possesses an active ferment it will
still be rapidly forming gas at the end of this time, and the loaf will
go into the oven distended with gas under pressure from the elasticity
of the gluten which forms the walls of the bubbles. The heat of the
oven will cause each gas bubble to expand, and a large loaf will be the
result. If the ferment of the flour is of low activity it will not be
able to keep the yeast supplied with all the sugar it needs, the volume
of gas formed in the later stages of the fermentation of the dough will
be small, the dough will go into the oven without any pressure of gas
inside it, little expansion will take place as the temperature rises,
and a small loaf will be produced.
From these facts it is quite easy to devise a method of estimating how
large a loaf any given flour will produce. The following method is that
used by the author. A small quantity of the flour, usually 20 grams,
is weighed out and put into a wide mouthed bottle. A flask of water
is warmed to 40° C., of this 100 c.c. is measured out, and into it 2½
grams of compressed yeast is intimately mixed, 20 c.c. of the mixture
being added to the 20 grams of flour in the bottle. The flour and
yeast-water are then mixed into a cream by stirring with a glass rod.
The bottle is then placed in a vessel of water which is kept by a small
flame at 35° C. The bottle is connected to an apparatus for measuring
gas, and the volume of gas given off every hour is recorded. As already
mentioned all flours give off about the same volume of gas during the
first three hours. After this length of time the volume of gas given
off per hour varies greatly with different flours. Thus a flour which
will bake a large loaf gives off under the conditions above described
about 20 c.c. of gas during the sixth hour of fermentation, whilst a
flour which bakes a small tight loaf gives off during the sixth hour of
fermentation only about 5 c.c. of gas.
Having devised a feasible method of estimating how large a loaf any
given flour will make, the problem of the shape and texture still
remains. Previous investigators had exhausted almost every possible
chemical property of gluten in their search for a method of estimating
strength. The author therefore determined to study its physical
properties. Now gluten is what is known as a colloid substance, like
albumen the chief constituent of white of egg, casein the substance
which separates when milk is curdled, or clay which is a well known
constituent of heavy soils. Such colloid substances can scarcely be
said to possess definite physical properties of their own, for their
properties vary so largely with their surroundings. The white of a
fresh egg is a thick glairy liquid. On heating it becomes a white
opaque solid, and the addition of certain acids produces a similar
change in its properties. Casein exists in fresh milk in solution. The
addition of a few drops of acid causes it to separate as finely divided
curd. If, however, the milk is warmed before the acid is added the
casein separates as a sticky coherent mass. Every farmer knows that
lime improves the texture of soils containing much clay, because the
lime causes the clay to lose its sticky cohesive nature.
Such instances show that the properties of colloid substances are
profoundly modified by the presence of chemical substances. Wheat,
like almost all plant substances, is slightly acid, and the degree of
acidity varies in different samples. Accordingly the effect of acids on
the physical properties of gluten was investigated, and it was found
that by placing bits of gluten in pure water and in acid of varying
concentration it could be made to assume any consistency from a state
of division so fine that the separate particles could not be seen,
except by noticing that their presence made the water milky, to a tough
coherent mass almost like indiarubber (Fig. 11). It was found, however,
that the concentration of acid in the wheat grain was never great
enough to make the gluten really coherent.
[Illustration: Fig. 11.
Gluten in pure water; soft, but tough and elastic
Gluten in very weak hydrochloric acid (3 parts in 100,000 of water); it
floats about in powder, having entirely lost cohesion, and makes the
water milky
Gluten in hydrochloric acid (3 parts in 1000 of water); very hard and
tough]
But wheat contains also varying proportions of such salts as chlorides,
sulphates and phosphates, which are soluble in water, and the action
of such salts on gluten was next tried. It was at once found that
these salts in the same concentration as they exist in the wheat grain
were capable of making gluten coherent, but that the kind of coherence
produced was peculiar to each salt. Phosphates produce a tough and
elastic gluten such as is found in the strongest wheats. Chlorides and
sulphates on the other hand make gluten hard and brittle, like the
gluten of a very weak wheat (Fig. 12).
The next step was to make chemical analyses to find out the amount of
soluble salts in different wheats. Strong wheats of the Fife class
were found to contain not less than 1 part of soluble phosphate in
1000 parts of wheat, whilst Rivet wheat, the weakest wheat that comes
on the market, contained only half that amount. Rivet, however, was
found to be comparatively rich in soluble chlorides and sulphates,
which are present in very small amounts in strong wheats of the Fife
class. Ordinary English wheats resemble Rivet, but they contain rather
more phosphate and rather less chlorides and sulphates. After making a
great many analyses it was found that the amount of soluble phosphate
in a wheat was a very good index of the shape and texture of the loaf
which it would make. The toughness and elasticity of the gluten no
doubt depend on the concentration of the soluble phosphate in the wheat
grain, the more the soluble phosphate the tougher and more elastic
the gluten, and a tough and elastic gluten holds the loaf in shape as
it expands in the oven, and prevents the small bubbles of gas running
together into large holes and spoiling the texture.
[Illustration: Fig. 12.
Gluten in water containing both acid and phosphate; very tough and
elastic
Gluten in water containing both acid and sulphates. It shows varying
degrees of coherence, but is brittle or “short”]
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.