1902 Encyclopedia > Manufacture of Flour

Manufacture of Flour

MANUFACTURE OF FLOUR. Flour is the grain of wheat reduced to powder, and separated from the outer husk or coverings in which the seed is enveloped. The name is also applied to the grain of other cereals, and to the farinaceous seed of pulses similarly treated, and it is used generally to indicate any finely powdered dry sub-stance ; but when the term is employed without any quali-fication, it invariably means wheaten flour. As prepara-tions of flour form the staple food of all civilized communities of the West, the cultivation of wheat and the manufacture of flour are necessarily industries of the greatest magnitude and importance, rice being the only other grain which rivals (and which indeed possibly sur-passes) wheat in the number of human beings it feeds.

The cultivation of wheat was one of the earliest develop-ments of human civilization, and there are not wanting evidences that in making use of the grain the primitive races submitted it to a coarse pounding or grinding, thereby reducing it to a state resembling the meal of the present day. From remains found on the sites of the ancient lake dwellings of Switzerland it is obvious that the original form of corn-crushing or mealing apparatus con-sisted of a roundish stone—generally very hard sandstone _—about the size of a man's fist, with certain hollows or flattened surfaces on two opposite sides (figs. 1-3). The

Fig. 3.
FIGS. 1-3.—Primitive Corn-crushers (from Keller's Lake Dwellings).

rounded outline of the stone worked and fitted into a corre-sponding cavity in another stone in which the grain to be crushed or pounded was placed. By the deepening of the cavity in the under stone and the addition of a wooden handle to the upper stone-ball, would be formed the mortar and pestle; and in another direction, by fitting the upper stone for a motion of rotation within the cavity of the lower, the form of the quern would be produced, and the germ of the modern flour-mill elaborated. In early times, and indeed amid rude forms of society still, the preparation of meal and flour was a part of the domestic operations of preparing bread and otherwise cooking of food. At a period so remote as that of the patriarch Abraham it appears there was a distinction in the qualities of the flour or meal which could be produced, as Sarah was directed to " make ready quickly three measures of fine meal, knead it, and make cakes upon the hearth." There is much probability in the suggestion of Dr Livingstone that the grinding apparatus used by Sarah was similar to that still used in Central Africa, and figured in the frontispiece of his Zambesi and its Tributaries. In that work the apparatus is thus described :—

" The mill consists of a block of granite, syenite, or even mica-schist, 15 or 18 inches square, and 5 or 6 thick, with a piece of quartz or other hard rock, about the size of a half brick, one side of which has a convex surface, and fits into a concave hollow in the large and stationary stone. The workwoman kneeling grasps this upper millstone with both hands, and works it back-wards and forwards in the hollow of the lower millstone, in the same way that a baker works his dough when pressing it and push-ing it from him. The weight of the person is brought to bear on the movable stone, and while it is pressed and pushed forwards and backwards, one hand supplies every now and then a little grain, to be thus at first bruised, and then ground on the lower stone, which is placed on the slope, so that the meal when ground falls on to a skin or mat spread for the purpose. This is, perhaps, the most primitive form of mill, and anterior to that in Oriental count-ries, where two women grind at one mill, and may have been that used by Sarah of old when she entertained the angels."

That the two forms of grinding apparatus were familiar to the nations of antiquity is obvious from the allusions made to both in the Pentateuch. In the book of Numbers (xi. 8) we read that the Israelites gathered manna, " ground it in mills, or beat it in a mortar;" and again in Deuteronomy (xxiv. 6) there is an injunction that " no man shall take the nether or the upper millstone to pledge, for he taketh a man's life to pledge." Numerous other allusions to mills, mortars, and the grinding of corn are scattered throughout Scripture, from which it is made clear that the grinding of corn was, among the Hebrews, a domestic employment left entirely to women. Among the ancient Romans the mortar and pestle were alone used for pounding wheat, and the work was similarly done by women down to the year 173 B.C. At that date baking was established as a separate occupation, the craftsmen being called pistores, from pinsere, to pound, in allusion to their manner of preparing flour. At a subsequent date mills were introduced, of which the quern was the simplest and original form. It was called the mola manuaria, or mola trusatilis, and was worked chiefly by slaves, the labour being regarded as eminently degrading. Later the mola asinaria moved by animal power, and the mola aquaria or water mill, were employed as a substitute for hand-worked mills. Their mola aquaria approached in form and mechanism the rude small mills which existed in the more remote parts of Scotland and Ireland down till the early part of the present century. At the beginning of last century a pair of Roman millstones were found at Adel in Yorkshire, and described in the Philosophical Transactions. One stone, 20 inches in diameter, was con-vex in outline, while the other was concave, and retained traces of notching.

The quern, and exceedingly rude water mills, were in use throughout Great Britain for many centuries, and con-tinued to be employed in outlying districts of Scotland and Ireland till very recent times. Strutt, in his Chronicle of England, says-—" At what time mills were first used in Britain cannot be determined ; hand-mills, which without doubt were the most ancient of any, we may conceive were known in the time of Ethelbert, king of Kent, who ruled that nation from the year 560 to the year 616 ; for in his laws a particular fine of twelve shillings is imposed upon any man who should corrupt the king's grinding maid i hence it is also evident that they were turned and tended by women ; but it is probable that before the end of the heptarchy water-mills were erected, because in ancient deeds and grants of lands we find mention made of mills, which are generally said to be situated near the water." Dr Johnson, in his Journey to the Western Islands of Scot-land, describes the working of the quern as seen by himself. " There are water mills," he says, " in Sky and Raasa, but where they are too far distant the housewives grind their oats with a quern or handmill, which consists of two stones about a foot and a half in diameter. The lower is a little convex, to which the concavity of the upper must be fitted. In the middle of the upper stone is a round hole, and on one side is a long handle. The grinder sheds the corn gradually into the hole with one hand, and works the handle round with the other. The corn slides down the convexity of the lower stone, and by the motion of the upper is ground in its passage." The accompanying woodcut (fig. 4) illustrates the leading forms of these primi-

FIG. 4.—Querns.

tive mills,—a representing a pair of rubbing stones, b a pot quern, and c an ordinary quern with hole in the centre of the upper stone into which the grain was fed, another hole towards one side for receiving the handle, and in the lower stone a spout through which the ground meal was delivered.

The nature of the water mills which were formerly common in Great Britain and Ireland, and which continued in use well into the present century, may be gathered from the following description of one visited by Sir Walter Scott during his voyage to the Shetland Islands, &c, in 1814 (Lockhart's Life). " In our return, pass the upper end of the little lake of Gleik-him-in, which is divided by a rude causeway from another small loch, communicating with it, however, by a sluice, for the purpose of driving a mill. But such a mill! The wheel is horizontal, with the cogs turned diagonally to the water ; the beam stands upright, and is inserted in a stone-quern of the old-fashioned con-struction. This simple machine is inclosed in a hovel about the size of a pig-stye, and there is the mill! There are about 500 such mills in Shetland, each incapable of grinding more than a sack at a time."

The ordinary flour mill of the present day is a structure of comparatively few essential parts; but in the arrange-ment and mounting of these the greatest amount of mechanical skill and experience has been exercised, and the accessories of the mill have been elaborated with much care, with the view of saving manual labour and perfecting the processes and results. Fully to appreciate the various processes of modern milling, it is necessary to bear in mind, not only that the wheat as delivered at the mill is dusty and mixed with sand and other refuse, but that it contains many light grains and seeds of foreign substances, which might be deleterious, and would certainly interfere with the appearance of the finished flour. Again the structure of the wheat grain itself must not be overlooked. A grain (caryopsis) of wheat is not a seed, but a fruit consisting of a pericarp or outer envelope tightly adherent to its contained single seed. The envelope consists of several layers of ligneous tissue, within which are the embryo and a peculiar fermentive nitrogenous principle termed cerealin, and finally a central mass of thin cells filled with a white powdery substance largely composed of starch granules. The object of ordinary milling is to grind as perfectly as possible, without breaking the minute granules, the central substance of the grain, and to separate it from the embryo and outer husks, the former constituting the flour, and the latter the bran of the miller. Whole wheaten flour, on the other hand, consists of the entire grain ground up to a uniform mass.

The machines and processes by which flour is prepared are very numerous, and are diverse in character; and it must further be said that, at the present moment, the whole industry is in a peculiarly unsettled and transition state. The system which has prevailed in the United Kingdom hitherto is what is known as ordinary or flat grinding with millstones ; but in the meantime a strong tendency is developing in favour of the use, either partially or entirely, of granulating or " high milling," and of some of the various systems of roller grinding which have been introduced. In Hungary and Austria the system of high milling prevails, in which the action of the millstones con-sists more in granulating than grinding; and in connexion with that system of milling, the use of rollers is a prominent feature. To a limited extent also the principle of the disintegrator has been brought into operation, in which the grain is broken by the violent impact of studs or projections revolving in opposite directions with enormous velocity. Thus we have these various systems :—(1) flat milling or grinding; (2) high milling or granulation ; (3) roller mill-ing or crushing; and (4) disintegrator milling or breaking.

Flat or Ordinary Milling.—In the ordinary or flat mill-stone milling of the United Kingdom there are three main points to consider—(1) the cleaning and preparation of the wheat; (2) the grinding; and (3) the bolting or dressing of the ground products. The ordinary cleaning or screening apparatus through which the wheat, as received, passes, consists of a kind of cylindrical sieve of wire cloth, mounted in a sloping position, and having internal partitions so as to resemble an Archimedean screw. When the apparatus is set in motion, the grain, fed in at its upper end, tumbles from one division into another, thereby being freed from small refuse and sand, and as it issues at the lower extremity is subjected to a fan blast. For cleaning grain there are other kinds of apparatus, in which the prin-ciple of aspiration, or drawing currents of air through the grain, is now extensively employed, the most frequently used being Child's aspirator. A further cleaning is sometimes given by Child's decorticator, an implement which can be adjusted at will, for simply rubbing and scouring the grain, or for removing the thin bran and germ previous to the operation of grinding. The " Victor " brush machine is a recently introduced and highly approved apparatus for polishing and finishing wheat, its peculiar feature being that the " opposed brushes are constructed and worked in such a manner that they come in contact with every kernel of wheat in every conceivable position, and with as much force as the miller chooses to use—thus polishing it on the ends better than any other machine can do, and this not only on one pair of brushes, but on several." The pre-pared grain is next conveyed to the grinding apparatus, and here it may be said that, in moving the grain or flour horizontally, Archimedean screws working within an in-closed casing are employed, while in lifting from one floor to another, small boxes mounted on an endless band worked over pulleys and similarly encased are used. The grinding machinery consists first of a bin containing the grain to be ground, from which it passes by a spout to the hopper, whence it is delivered by a feeding adjustment to the stones. These constitute the distinctive feature of the entire mill, and upon their condition and delicate adjust-ment the whole success of the milling operation turns. They consist of two flat cylindrical masses inclosed within a wooden or sheet metal case, the lower or " bed-stone" being permanently fixed, while the upper or " runner" is accurately pivoted and balanced over it. The average size of millstones is about 4 feet 2 inches in diameter by 12 inches in thickness; and they are made of a hard but cellular silicious stone called buhr-stone, the best qualities of which are obtained from La-Ferte-sous-Jouarre, department of Seine-et-Marne, France. Millstones are generally built up of segments, bound together around the circumference by an iron hoop, and backed with plaster of Paris. The bed-stone is dressed to a perfectly flat plane surface, and a series of grooves or shallow depressions are cut in it, generally in the manner shown in fig. 5, which represents

FIG. 5.—Upper Millstone, lower surface.

the grinding surface of an upper or runner stone. The grooves on both are made to correspond exactly, so that when the one is rotated over the other the sharp edges of the grooves, meeting each other, operate like a rough pair of scissors, and thus the effect of the stones on grain sub-mitted to their action is at once that of cutting, squeezing, and crushing. The dressing and grooving of millstones is generally done by hand-picking, but sometimes black amorphous diamonds (carbonado) are used, and emery wheel dressers have likewise been suggested. The upper stone or runner is set in motion by a spindle on which it is mounted, which passes up through the centre of the bedstone, and there are screws and other appliances for adjust-ing and balancing the stone. Further, provision is made within the stone case for passing through air to prevent too high a heat being developed in the grinding operation; and sweepers for conveying the flour to the meal spout are also provided. The ground meal delivered by the spout is carried forward in a conveyer or creeper box, by means of an Archimedean screw, to the elevators, by which it is lifted to an upper floor to the bolting or flour-dressing machine. The form in which this apparatus was formerly employed consisted of a cylinder mounted on an inclined plane, and covered externally with wire cloth of different degrees of fineness, the finest being at the upper part of the cylinder where the meal is admitted. Within the cylinder, which was stationary, a circular brush revolved, by which the meal was pressed against the wire cloth, and at the same time carried gradually towards the lower extremity, sifting out as it proceeded the mill products into different grades of fineness, and finally delivering the coarse bran at the extremity of the cylinder. For the operation of bolting or dressing, hexagonal or octagonal cylinders, about 3 feet in diameter and from 20 to 25 feet long, are now commonly employed. These are mounted horizontally on a spindle for revolving, and externally they are covered with silk of different degrees of fineness, whence they are called "silks" or silk dressers. Piadiating arms or other devices for carry-ing the meal gradually forward as the apparatus revolves are fixed within the cylinders, and there is also an arrangement of beaters which gives the segments of cloth a sharp tap, and thereby facilitates the sifting action of the apparatus. Like all other mill machines, the modifica-tions of the silk dresser are numerous. Mill products are differently assorted and classified in various localities and different mills—some distinguishing many qualities of flour and bran, and others making only three or four divisions. The following (from Professor Church's Food) may be taken as a fair average representation of the product of 100 B) of good white wheat:—

== TABLE ==

In the Crown Mills, Glasgow, belonging to John Ure & Son, the classification and general average mill products, dealing with 50 qrs., are as follows :—

== TABLE ==

An additional proportion of fine flour is obtained by dressing and remilling tailings and middlings, and the purification and regrinding of these products have now become of much consequence in connexion with the changed systems of milling rapidly coming into use. A great variety of middlings purifying machines have been introduced and eagerly pushed within the last few years, showing that this branch of economic milling is now receiving great attention.

The Hungarian System or High Milling.-—The object of the low or flat milling process, as practised in Great Britain, is to produce at one grinding operation as large a propor-tion of good finished flour as possible. In high milling, on the other hand, the stones are kept so far apart that grain is merely bruised in the first operation, and by a series of such grindings or bruisings, alternated with elaborate sifting, the bran and all the outer envelopes with the cerealin are detached, and a nucleus of very pure semolina only left. In this way a large proportion of very inferior branny flour is obtained in these early millings, and the proportion of exceedingly fine strong flour for which Austro-fiungarian millers are famous is comparatively small. It is only to the hard brittle wheats that the Hungarian system of mill-ing is applicable, and the method is only practicable under circumstances where there is a demand for the two extreme qualities of mill product which result from the system. Within the last few years the Hungarian millers have very largely adopted the roller mills, either to supplant entirely or to supplement the stone grinding.

Roller Mills.—In this form of mill a pair of horizontal rollers rotate face to face, and the grain or other material is submitted to their action by passing between them. The nature of that action varies according to the modification of roller surfaces, the closeness of the rollers to each other, and the equal or differential rate at which they revolve. Rollers of metal, either steel or chilled iron, having a toothed surface, revolving at different rates of speed and at definite distances apart, have a cutting action on the grain submitted to them. Such rollers are employed in the Buchholz system for reducing hulled wheat to the condition of semolina, and a similar arrangement is employed in the machine of Ganz & Co. of Buda-Pesth. Fig. 6 shows a section of the face of a pair of such rollers, where B revolv-ing slowly serves as a holder for the grain, which is cut by the sharp edges of A, revolving at a speed three times greater than B. Smooth surface rollers are mounted to press hard against each other, and are made in some cases of polished chilled metal; and in other instances cylinders of porcelain are employed. Their principal function is for the reduction of purified middlings and semolina, and when no differentia] motion is given the action is simply that of squeezing, but when the opposing rollers revolve at different rates of speed a grinding effect is further superadded. A roller mill which has met great and sudden public accept-ance is that invented and patented by F. Wegmann, a miller of Naples. In Wegmann's mill, fig. 7, porcelain rollers are employed, there being two pairs fed from opposite ends of one hopper in the machine figured. This machine possesses a self-acting pressure and a differential motion ; but its most valuable feature consists in the employment of porcelain for rollers, by which a surface at once exceedingly hard and slightly rough or porous is secured. It is claimed for these rollers that,

FIG. 7.—Wegmann's Roller-Mill.

acting on middlings, they produce, with less expenditure of power, more and better flour than can be obtained by either stones or metal rollers. For the grinding of fine middlings English experience is, however, in favour of chilled iron rollers as the best. The roller mills have not yet been long enough in operation in Great Britain to enable any safe conclusion to be drawn as to their adapta-bility to British milling ; but though the system originated only within the last few years, it has in the meantime practically superseded all other methods in the Austro-Hungarian districts, where milling is carried to greater perfection than in any other part of the world. To show the progress made in the introduction of roller mills in Britain, it may be noted that, while in January 1877 there was only one Wegmann's machine in operation, there were by the close of the year 350 in various mills through-out the country.
The Disintegrator.—Under this name a form of machine was invented and introduced a few years ago by the late Thomas Carr for, among other purposes, the manufacture of flour. Carr's disintegrator consists of a pair of circular discs of metal set face to face, and studded with circles of projecting bars so arranged that the circles of bars on the one disc alternate with those of the other. The discs are mounted on the same centre, and so closely set to each other that the projecting bars of the one disc come quite close to the plane surface of the other; and they are inclosed within an external casing into the centre of which a spout passes by which the grain is delivered into the machine. The discs are caused to rotate in opposite directions with great rapidity, making about 400 revolutions per minute, and as the outer circle of bars is 6 feet 10 inches from the centre, they move at a rate of 140 feet a second, or about 100 miles per hour. The grain in passing through the machine is struck by the oppositely revolving studs with enormous force,—a force which increases as the shattered grains approach the outer circles of studs, and it is almost instan-taneously reduced to a powder which falls under the discs and is carried away by a spiral creeper. It is stated that one of these machines 7 feet in diameter is capable of doing the work of as many as 27 pairs of millstones, and they have been fully tested in the flour mills of Gibson and Walker, Bonnington, near Edinburgh, where two of them have been in operation for a period of seven years. Notwithstanding this success, however, the disintegrator has not met with general acceptance as a flour mill, having been introduced into only a very limited number of establishments. The following is a statement of the average results Messrs Gibson and Walker have obtained by the disintegrator from old Scotch wheat and from a mixture of Baltic and Ghirka wheats respectively:—

== TABLE ==

The semolina is reground under stones, and yields a fine strong flour; but it may be and sometimes is reduced by passing it again through the disintegrator.
Qualities of Flour.—There appears to be at present some conflict between public demand, as indicated by the in-creasing attention paid to the production of a fine strong white flour, and the current of scientific opinion expressed, though with some hesitation and doubt, in favour of whole meal or flour in which the richly nitrogenous outer portions of the wheat are retained. The fact that the outer portions of the wheat are 'richest in nitrogenous principle, and that also in a peculiarly active form, is indisputable ; but it has not been satisfactorily determined whether the nutritive value of that portion of the grain is exactly measured by its chemical composition. The condi-tion of the nitrogenous substance, the amount and irritat-ing nature of the ligneous tissue which accompanies it, and its peculiar influence on the other constituents of the wheat grain may and probably do affect its value. It is certain also that white flour is deliberately preferred by the labouring population, whose instinct is probably right, and it is also preferred by and for many purposes indispensable to the baker and cook. Good flour should present the appearance of a pure uniform white powder, only faintly tinged with yellow, free from all grit and lumps; and when pressed in the hands it ought to show a certain amount of adhesiveness. It should be free from all smell of damp or mouldiness, and it should have no acidity of taste. For the purposes of the baker its strength is measured by the amount and quality of its gluten or nitrogenous constituents, which may be roughly estimated by making up a little dough and observing its tenacity when drawn out; but a more accurate means of measuring its elasticity is provided by the aleurometer of M. Boland, a Parisian baker. The following analytical statement of the constituents of flour, and other calculations concerned therewith, are extracted from Professor Church's Handbook of Food, prepared for the Science and Art

== TABLE ==

" One pound of good wheaten flour, when digested and oxidized in the body, might liberate force equal to 2283 tons raised one foot high. The greatest amount of external work which it could enable a man to perform is 477 tons raised one foot high. For one
art of flesh-formers in fine wheaten flour there are 7J parts of eat-givers reckoned as starch. One pound of wheaten flour can-not produce more than about If oz. of the dry nitrogenous sub-stance of muscle or flesh."

Statistics of the Flour Trade.—The disproportion between the grain producing and consuming capacity of the United Kingdom is very great and yearly increasing, and the deficiency in our own crops is made good from almost all quarters of the globe. In the discussion of this principal portion of the British food supply, the whole grain crops of the kingdom and the entire imports of all kinds of grain would require to be taken into consideration, and this large question cannot be here entered upon. The extent of our import trade in wheat and flour is indicated by the follow-ing table, relating to the five years 1872-6; and the detailed statement for 1876 gives the principal sources whence these supplies are drawn :—

== TABLE ==

The subjoined extract from the London Com Exchange Report for 4th March 1878 will suffice to show the market classification of flour, and give an approximately accurate idea of the relative value of the various descriptions :

== TABLE ==

(J. PA.)

The above article was written by James Paton, F.L.S.; Superintendent of Museums and Art Galleries of Corporation of Glasgow from 1876; Assistant in Museum of Science and Art, Edinburgh, 1861-76; President of Museums Association of the United Kingdom, 1896; editor and part-author of Scottish National Memorials, fol. 1890.

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