The picturesque windmills of the past have been superseded by huge modern mills. In the flour mill of to-day the wheat undergoes a number of operations in which separators, dust collectors, mellowing bins, conditioners, sifters, purifiers and other machines play their part
A GROUP OF REDUCTION ROLLS, which “reduce” purified stock to flour. The grain, having been cleaned and conditioned, is broken down on “break rolls” and sifted in special machines. It then passes down to the various reduction rolls, which have smooth or finely fluted surfaces.
FLOUR milling suggested until recent years quiet little country streams with pollard willows marking their banks, the gentle flow of water down the millrace, the rush and tumble of the water through the wooden trough that fed it on to the paddles of the waterwheel, which splashed and swished round to the grinding of the roughly made cogwheels, and the murmuring of the millstones in the loft above.
Then there were the windmills, now fast disappearing from the little hillocks dotting the flat countryside, their great sails or sweeps revolving under the gentle pressure of the breeze. The Domesday Book is said to have recorded some 7,500 watermills in England about the year 1086. Of the windmills few remain, some to be turned into week-end cottages, while others become more and more derelict. Yet much older methods of flour milling are still carried on in various parts of the world. The Australian aborigine still adopts the practice of the Stone Age, pulverizing the grain by handstones on larger stones. The native of the Nile valley has been seen to use the pestle and mortar of the ancient Egyptians. The dark-eyed Jewish maiden squats on her ground mat and with one hand turns the upper, or runner millstone, while she feeds in the corn with the other. In remote parts of India and China the native harnesses his buffalo or his mule, sometimes blindfolded, to the shaft of the millstone. Modifications of these systems are used in other parts of the world.
How different an impression is created by the modern flour milling machinery. From the time that the stream of golden grain pours in from the pneumatic elevator, sucking it up from river barges alongside, and is delivered into the first dark chute to the bins, it is not touched by human hands until, having passed through numerous boxed-in machines and little wooden spouts, it emerges at the sack mouth as snow-white flour. No murderous crushing between the upper and the nether millstones, those symbols of ruthless destruction, but by gentle persuasion and thoroughly efficient methods, that tender little grain of wheat is stripped of its hard outer husk, its nutritive contents are carefully extracted and even the tiny life germ is picked out without injury to add to the food value of certain patent breads.
Though the word flour is applied to the product of other cereals such as rice, bean and even potato crops, it is most generally used to describe the product of wheat for making bread, and is so dealt with here. The wheat grain, or “berry”, as it is technically called, is made up of several components, most of which take some part in the finished flour and its by-products.
GRAIN FROM OVERSEAS for the flour mills of Great Britain is discharged at the quayside by pneumatic elevators which may be fixed or may be run on rails along the quay. Vacuum pumps in the elevators suck the grain from the ship’s hold through pipes to a large tank known as a receiver. This has a dust extractor fitted. The grain is then stored in silos until it is required for milling.
First there is the more or less hard outer skin, or husk, which is made into bran. Just below this is a second skin made up of the aleurone (albuminoid) cells, which are a little higher in nutritive value than the bran. Inside this again is the endosperm, or flour bag, which is made up of cells of starch and gluten constituting about 90 per cent of the contents of the berry. From the endosperm the pure flour stock is chiefly made.
The whitest flour is made from the heart of the endosperm. The flour produced from the adjacent cells farther out is probably more nutritious, but not so white as people generally demand. The contents of the berry between this and the outer skin are made into a product known as “sharps”, which is the inseparable mixture of these adjacent finely reduced products. The average amount of pure manufactured flour now obtained from the wheat grain is about 70 per cent, as compared with only about 65 per cent obtained by the
older millstone methods. It was not until about the fifth century B.C. that mechanical means were discovered for driving millstones. The Greeks are credited with having produced the first of these; it was driven by a horizontal water-wheel. A century later the Romans made gearing for such wheels. For the next thousand years or so water power was the only mechanical aid apart from man and animal power.
Millstones Obsolete
About A.D. 600 the windmill was invented. It is claimed that the oldest windmill in England still in use is at Bourn, in Cambridgeshire. This mill is authoritatively recorded as having been built about 1636.
Steam was first used to drive flour mills in about 1784. All this time, and up till about sixty years ago, the millstone system was universally adopted for the grinding process. Grinding by rollers was first introduced in Hungary, to be soon followed by progressive millers in the United States and Canada. English millers were forced to follow suit, so that now roller milling is almost universal. Much finer products are obtained thereby. The millstones, generally from 4 feet to 5 feet in diameter, were made
in pairs and cut with grooves or furrows which radiated, not from the centre of the stone, but tangentially from the edges of what is called the “drift circle”. This falls within the circle of the hole in the top “runner” stone, called the “eye” or “swallow”, and through it the grain was fed.
The longest furrow, running the full length, was called the master furrow and the shorter ones the secondary furrows. The function of these was to distribute the grain over the grinding surface and to reduce it partly. At the same time the channels formed air passages which ventilated the surfaces and kept them cool. The top surface of the stones was “snecked” or “cracked” to give greater grinding power. The rotating action of the top runner over the surface of the lower, or “bed” stone, caused the grain gradually to flow outwards towards the periphery in a rather beautiful centrifugal action. This can be demonstrated by making a paper tracing of the furrows from a plan view, making a second, but reversed tracing and superimposing the reversed tracing over the centre of the other. The act of rotating will be seen to be almost kaleidoscopic in its effect. If the top tracing is turned in a clockwise direction there is a movement towards the outer edge, showing how the grain travels outwards. If the tracing is moved in the reverse direction the opposite movement takes place, towards the centre.
SELF-CONTAINED ROLLER PLANT designed to make flour for local consumption in wheat-growing areas. This unit carries out all the operations necessary for converting the grain into high-grade flour. It comprises wheat-cleaning plant, milling plant, break rolls, reduction rolls, sifter and dust collector. The plant has a capacity of 440 lb. an hour.
This action might also be likened to that of a pair of scissors, with its cross-cutting effect, so that the grain is not exactly pulverized as in the ancient pestle and mortar, but receives a slicing treatment as well as a certain amount of crushing. The grooves in millstones are more or less analogous to the grooving in the modern roller mills. A pair of old millstones of 4-feet diameter, running at 120 revolutions a minute, would grind about 300 lb. of meal an hour. The process, however, was severe on the stock ground, tearing up the bran skins and damaging the more delicate tissues and cells of the flour bag of the berry, with results not nearly so satisfactory as from modern machinery. The whole process nowadays is spread over a long series of finely-graded intermediate stages, so that, bit by bit, every particle is separated from its neighbours and diverted into its own particular path, finally to emerge in the finished state. Throughout, the greatest care is taken to clean, wash, condition and sift the grain of all impurities, leaving it pure and of the highest nutritive value. Even every scrap of inferior food value is made use of for “offals”, as food for animals, and for other products. The grain is brought into the mill from, say, river barges alongside, by a pneumatic elevating plant, as described in the chapter “How Grain Cargoes are Handled”.
The stream of grain is accompanied by all kinds of foreign matter such as straw, sack threads, bits of wood, iron, many kinds of seeds and other cereals which have at one time or another got mixed with it on its long journey from various countries. The grain first passes through an automatic weighing machine, which checks the gross weight of the consignment, then over an electromagnet which extracts all bits of iron, nails, bolts, nuts and so on, before they can do any damage to machines that follow.
The iron contents having been extracted, the grain passes into a receiving separator, which gives a preliminary cleaning to the wheat before it is delivered into the “dirty wheat” storage bins. The modern type of receiving separator consists of a slowly rotating cage-shaped reel.
THE ROTATING REEL of a receiving separator serves to give the wheat a preliminary cleaning. Curved baffle plates run parallel with the axis and are enclosed in a wire cage with wire meshes. The reel is rotated slowly in the manner of a squirrel’s cage. Grain passes through the reel, and impurities are separated as shown in the diagram below.
Curved baffle plates run parallel with the axis and are enclosed in an outer wire cage with wide meshes. Certain baffles retard the grain from passing too quickly through the reel. While the reel rotates slowly all large impurities, being too big to pass through the wire mesh, are carried over, discharged clear of the grain and carried away to the dust collector. The grain passes through the feed hopper which automatically spreads it over the full length of the reel and through the wire-meshed cover in such a way that there is always a mass of grain between the cover and the baffles at the point where the grain enters the reel. This has the effect of preventing the undesirable foreign matter from getting through the mesh as some of it, say, the sticks and straws, might do if meeting it on end. These are thus turned over and discharged outside. Air is drawn through the stream of grain and lifts out light impurities, which are then deposited in a settling chamber from which they are removed.
THE RECEIVING SEPARATOR removes larger impurities from grain in the first stages of treatment. Grain from the hopper A falls on to an adjustable plate B, which directs it on to the rotating reel C (see illustration above). Impurities which are too large to pass through the wire mesh are carried over and fall on to a smaller reel D, which extracts any grain carried over by the impurities. Grain from the reel C falls on the seal gate E and out of the machine at F. Air is drawn through the stream of grain and carries light impurities through G to the settling chamber H. The air passes to the fan inlet J and is discharged from the fan back to the machine.
From here further extraction of foreign matter and dust is effected by a “Cyclo-Pneumatic” separator. The wheat enters the machine through the feed tube and falls on to a revolving cast-iron disk which distributes it in a circular spray evenly inside a hood. As the wheat falls from the disk the air is drawn through in the opposite direction to that of the travel of the wheat, lifting the dust and the screenings, which are deposited in an expansion chamber, where the same operation is repeated. The wheat then passes on by elevators or conveyers to the “dirty wheat” bins. These are large enough to take in perhaps one or two months’ supply of several varieties from different sources, as they can be filled only when the bulk consignments come along and are discharged irrespective of the daily demand from the working mill.
On being drawn off from these bins, the wheat may be. passed through a combined measurer and magnetic separator. This measures off the quantities required of the different kinds necessary for blending various mixtures to meet the market demands. The magnetic separator extracts any remaining small bits of iron which had escaped the receiving separator. The grain is again weighed automatically and passes on to some form of milling separator, a machine which is sometimes used at the receiving stage to perform a similar function to the receiving separator.
In the milling separator there are three reciprocating sieves having different sized perforations or meshes. The first rejects large rubble, straws and the like; the second rejects peas and large grains, and the third deals with sand and small seeds. A powerful aspiration, or suction air draught, acts upon the feed and the delivery, separating the lighter particles and depositing them in an expansion chamber above the sieves, from which they are continually removed.
ACCURATE FLUTING of a roll used in the “breaking” processes, as it appears when seen through a magnifying glass. The rolls are made of chilled cast iron. The number of grooves, or flutes, in a roll varies from about ten to an inch of circumference on rolls of the first “break” to about twenty-five on rolls used in the fourth and last breaking operation. The flutes are cut not parallel with the axis, but in a slight spiral.
This function of air suction is carried out in almost every stage throughout the whole process, either by a large independent fan installation to which all the machines concerned are connected by air ducts, or else by individual fans on the machines themselves. Sometimes a combination of both systems is used. The whole system of separation of the particles may be said to be effected in accordance with either of two general principles — by size, such as when the particles are passed through sieve plates with holes or meshes of varying size according to the material treated; or by weight, such as by water treatment when the heavier matter sinks to the bottom, or by air current which lifts the lighter particles from the heavier.
A variety of the former method is the indented cylinder, or indented disk plate machine. This is for separating a grain which is either larger or smaller than the wheat being treated. In either instance the basic principle is virtually the same. The plate, or disk, is covered with hundreds or perhaps thousands of small pockets or indents. These are made to retain the grain or seeds to be lifted out. Supposing there is a quantity of oats in the mass of wheat. The pockets will retain the wheat grains; the oats, which are longer, will not enter the pockets but will pass over them. As the cylinder or the disks rotate slowly the retained grain is carried round and deposited in the delivery hopper, while the oats drop off on their way round into a different part of the machine and are conveyed away by a small worm conveyer at the bottom.
In the disk separator shown in the diagrammatic drawing below the grain enters the head end of the machine through a feed hopper furnished with slides to control the amount of grain entering. The grain is discharged into an open space at the head of the machine, and the disks revolve in the mass of grain at a speed of from 40 to 60 revolutions a minute. The grain lifted by the pockets is collected in troughs arranged between each disk and either passes out of the machine through the bottom hopper or is diverted to the worm conveyer and returned to the head of the machine for further treatment. The blades in the centre of each disk form a screw conveyer which forces the rejected grain from disk to disk until it is finally “tailed” away at the end of the machine.
GROUP OF SIFTERS in a large modern flour mill. After the grain has passed through the break rolls it comprises such products, of varying sizes, as bran, flour, middlings and semolina. The sifters grade and separate these products for different subsequent treatment.
Now that most of the coarser separation has been effected from the dirty wheat, it passes on to the next stage of washing and conditioning. A useful and effective machine carries out the three operations indicated by the names washing, stoning and whizzing. The stock is discharged into the water tank of this machine, which also contains two worm conveyers, one larger for the wheat and a smaller one at the bottom for collecting the stones. The stones fall to the bottom of the trough. The upper worm propels the wheat towards the whizzer, at the same time churning up the water and preventing the wheat from falling to the bottom of the trough.
Most of the dirt still adhering to the wheat is washed off, and the wheat is passed forward into the whizzer, where surface moisture is removed by powerful air currents produced by “lifters” in combination with the perforated metal cover. The stones are conveyed back to the head of the machine to a collecting box, from which they can be removed as desired. The wheat is carried up to the top of the machine by a number of inclined blades on the rotating element of the whizzer, which at the same time throws off all surplus water by centrifugal action, assisted by the strong air current. In some instances steaming processes are adopted for treating hard wheats.
THE MACHINERY OF A MODERN FLOUR MILL is completely encased. The sifters are extracting the coarser stock before its purification.
At this stage the wheat goes through a conditioning process which improves the milling and baking qualities of all wheats and flours. Various methods of applying the necessary heat for drying and various cooling systems have been and are used. Some wheats require more moisture applied to the outer husk; others need less, according to whether it is desired to soften or harden it. The heat may be applied by steam radiators, by heated air or by a combination of both methods.
The general practice has been to raise the temperature of the wheat to a heat of not more than about 125° Fahrenheit, but recent experiments are going higher The time during which such heat is applied and the duration of the cooling depend on the nature of the stock treated. The machine is of great size and height, running through two or three floors. The wheat is fed in at the top and arranged to pass downwards slowly over a number of sections through, or over which, the warm air current flows and permeates the grain.
Clearing House for Dust
After this operation, lasting about an hour, the wheat is conveyed into mellowing' bins, where it may rest quietly for about thirty hours to mellow for the milling operations.
From the bins the wheat is drawn off as required for milling. Having passed again through a measurer and a weighing machine, it goes into a scourer, which is one of the most important machines in the wheat cleaning process. This machine contains a large fixed cylinder which is lined with a specially prepared mixture of emery. Inside the cylinder a shaft rotates at high speed, carrying a
number of beaters which throw the wheat out centrifugally against the emery lining, thereby scouring its outer skin. A powerful air current is again passed through to remove the scourings and light screenings.
THE SIEVES OF A SIFTER of the type illustrated here are arranged in nests and can easily be withdrawn for inspection while the machine is running. The sieves have a rotary motion in a horizontal plane. The machine is supported by sets of flexible canes suspended from the ceiling. The flour is fed through canvas sleeves at the top and the sifted products are delivered out of sleeves at the bottom of the machine.
The dust collector for the cleaning department may be placed in- any convenient position, and has the function of collecting all the dust drawn off various machines, bringing it to a sort of central clearing house for dust. The collector contains numerous sections, the number depending on the size of the installation, as well as filtering sleeves of specially woven fabric for filtering out the finest particles. The machine works in conjunction with an exhaust fan, which first draws the air into an expansion chamber, where in a quieter atmosphere the heavier dust settles. The air containing the lighter dust is then drawn off through the sleeves, which collect the remainder.
The wheat passes from the scourer to the clean wheat bins. Thus great care and ingenuity have been exercised to clean the grain thoroughly before it is passed on to the mills, which do the grinding. But there are still one or two refining processes to follow.
A useful machine for the final cleaning and polishing of wheat is the brush machine. One type of brush machine consists of a horizontal cylinder and casing containing a spiral revolving brush. A fan and settling chambers are provided also for dealing with the dust, “beeswing” (flakes of the outer bran skin) and light wheat. Screenings and dust are separated from the wheat and collected and delivered by subsidiary self-contained dust collectors and this process is independent of the general dust collection of the earlier stages. The revolving brushes drive through the wheat, giving it a thorough polishing, and, being set at an angle, they gradually propel the wheat towards the delivery end.
After another weighing,, the wheat passes to the first of the grinding processes in what are called the “break” rolls. This part of the system consists of four distinct operations in breaking down the wheat, which has now been delivered in almost perfect condition. Each machine contains two pairs of main rolls made of chilled cast iron, which provides a hard surface to stand the continual grinding under pressure. These rolls are provided with accurately cut grooves, or “flutes”, of varying sizes according to the stage. They vary from nine or ten grooves to every inch of the circumference of the roll in the first break, to about twenty-five or twenty-six an inch in the fourth break. The grooves are not parallel with the axis of the roll, or they would, so to speak, get into gear with each other and jam, but are cut in a slight spiral.
The break rolls are required not to crush the berry, but to give a slicing action to remove the outer skins gently and at the same time to break up the centre of the berry and release the contents. The wheat is fed on to the main rolls by small feed rollers cut with circumferential grooves, which distribute the grain uniformly along the whole length in a thin stream and provide an even feed of uniform thickness.
FLOW CHART showing machines in which wheat is treated in a modern flour mill.
Note: the Dust Collector (left) may be in any suitable position in the mill.
After the grain has passed through the respective break rolls, it comprises various sized products, such as bran, flour, middlings and semolina. These different substances are brought from the rolls on to a sifting machine which grades them for the various successive machines.
No process of sifting has ever bettered the age-old method of working a sieve by hand with a horizontal rotary movement. A modern machine of great capacity is based on this principle. It is capable of dealing either with a large volume of any one grain or with several varieties at one time in separate sieves. It comprises large nests of numerous separate sieves. The whole machine is suspended by long canes from above and is driven by a central shaft which rotates in ball bearings. The ground wheat is fed through canvas sleeves above each set of sieves, and delivered out of similar flexible sleeves at the bottom.
The sieve has stretched across it a silk screen, over which the ground stock is given a rotary motion and made to travel along a rectangular tray, up the one side of a central slot and back by the other until the siftings have been discharged at the return end. All this time a brush which is carried on an arm working in the slot is made to sweep over the surface of the screen in an ingenious but simple manner, thereby preventing it from getting choked up. The machine runs at a speed of 190 revolutions a minute.
A purification before grinding on the smooth “reduction” rolls is carried out on the coarse material extracted by the sifter. One form of this purifier contains silk screened sieves, of various mesh according to the nature of the stock treated. The sieves are given a gentle reciprocating motion. Above them are a number of channel trays over which the brushes travel. The finer particles are drawn off in a gentle upward current of air, and those which are too heavy fall on the trays and are swept to a side channel and conveyed away. Finally the stock is put through a centrifugal dressing machine which discharges the flour perfectly clean and free from specks or grey particles.
The finest possible flour has now been produced from the wheat, which has gone through its many processes of dry cleaning, washing, scouring, brushing, roll grinding, sifting, purifying and repurifying, ready to be delivered finally into the sack. The empty sack or bag is fixed to the bottom of the automatic packing machine by an instantaneous clamp. The flour is run into the sack in a measured quantity and is automatically weighed, the weight of each sack and the number filled being also automatically recorded, all at a rate of from forty-five to sixty sacks an hour.
WHEAT BRUSH used for giving wheat a final cleaning and polishing before it passes to the break rolls. A brush machine of this type consists of a horizontal cylinder containing brushes fitted to a rotor. Propellers fitted to the rotor distribute the wheat round the casing to assist the action of the brushes.
