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 75 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, 76 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. 77
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 78 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 79 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 80 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 81 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.
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 82 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 83 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.
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 84 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 85 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 86 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 87 flour and offal from the large particles of kernel which require further grinding.
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 88 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 89 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 1? 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 90 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.
