Reduction systems in flour mills
November 01, 1998
by Teresa Acklin
This is the final article in a series by industry consultant David Sugden explaining the gradual reduction system in a flour mill. The first two articles, on break systems and purification, appeared in the September and October issues of World Grain.
It is important to differentiate between the three separate, yet combined, continuous functions of breaks, purifiers and reductions. The first “breaks” open the wheat kernel and step-by-step scrapes endosperm from the bran. The second lifts bran particles away from coarser middlings or semolina to ensure a longer extraction of flour.
The aim of a reduction system, widely used across the globe, is to finally and deliberately mill the center particles of the wheat grain into flour. There are many variations of reduction systems, but the logic is essentially the same: to grind the released, graded and purified endosperm from the two earlier systems into flour.
Rollermills are used to reduce particle size and are fitted throughout the mill mostly with smooth rolls. Some exceptions include the 1 sizings passage, which is equipped with finely corrugated rolls, and the last reduction, 8 middlings, which is occasionally furnished with even finer corrugations or flutes. However, the majority of rolls are smooth, with a matted finish as opposed to a “glass” finish.
Each pair of rolls is cambered, or tapered, slightly. The degree is largely a matter of trial and error the harder or closer a rollermill is set, the greater the camber or taper required for an even grind along the length of the roll. Hard grinding creates heat and so the roll expands. The differential ratio between the fast and slow roll typically is 1.25:1. This is to ensure that feedstock is drawn through the hip of the roll rather than queueing up to pass through. Higher differentials up to 1.8:1 are sometimes found on the higher reduction passages, such as 1 sizings, in order to create extra starch damage by greater shearing.
Extra frictional heat is generated in such circumstances. Three types of roll cleaners are used: felt pads, brushes or scrapers. Each needs maintaining for top-line efficiency. The latter type requires careful bedding or running in.
The enormous and unique benefit of the rollermill on the reduction system is that it is power efficient and can flatten germ and bran as well as reducing both coarse and fine semolina to flour.
The flattening effect on bran particles and germ means that each plansifter that follows can separate material more easily and accurately than any other known grinding method. It is for this reason that the rollermill has been and still is the basis for more than 120 years of efficient milling of white flour at long extraction.
CLASSIC 12-STEP FLOW.
A classic example of a 12-step flow that gradually reduces material fed from the break system and purifier is shown in the accompanying illustration. The 1 sizings, 1 middlings and 2 middlings along the top row produce the whitest and lowest ash flours. The first two gain their feed from the I and II break purifiers. The second row 2 quality, 3 middlings and 4 middlings draws its feed not only from the top row but also from the lower end of earlier purifiers.
The third and fourth rows draw feed material similarly and are higher in ash content. In particular, the 1 tailings and 2 tailings passages can produce flattened wheat germ to be sold separately.
Each of the 12 passages produce flour, although of differing characteristics. Millfeed exits the system from the 2 tailings, 7 middlings and 8 middlings plansifters.
A daily inspection of the germ and millfeed destinations from these passages will reveal any faults.
The system operates like a cascade. The plansifter cover aperatures, shown in microns (1,000th mm), are alterable depending on the quality of the wheat used and its hardness and softness. Softer wheats require more open covers, with the exception of germ.
The proportion of total flour milled by the reduction system will be from 80% at best, ranging down to 70%. The balance comes from the breaks. Moreover, the top six reductions collectively produce around three-quarters of the total.
The telltale sign of worn corrugations in the break system is an increase in the proportion of break flour and a decrease in reduction flour. This also generally means a shorter extraction of higher ash flour.
The first three passages show a flake disruptor between each rollermill and sifter. The flake disruptor is normally only used on head-end, low ash passages. The remaining passages have detachers to break up endosperm flakes of flour created by the rollermill. At the same time, these machines tend to not reduce bran or germ material in particle size, which allows the plansifter to seive out more flour.
The flake disruptor is a disc that rotates up to 3,000 rpm, often with a remotely set variable speed controller. The detacher has a rotating shaft with beaters that run at up to 700 rpm.
Top patent flours, around 0.4% content dry basis, are made by selecting and “dividing” the flours made from the 1 sizings, 1 middlings and 2 middlings passages. Others can be added during the day and are usually also the best baking quality flours from any plant.
By contrast, the 2 tailings, 7 middlings and 8 middlings passages result in low-grade flours with high mineral content, dark color and high protein. Any high amylase activity in the parent grain also finds its way here, as well as to the lower break flours. These characteristics can be demonstrated in the mill laboratory.
The 12 passages categorize the steps into two distinct groups: coarse and fine. The coarse reductions are 1 sizings, 2 quality, 1 tailings and 2 tailings. The remaining eight passages are fine reductions. The logic of the flow reveals that the four coarse passages all take their feed from purifiers and the overtail covers of most reduction sifters.
The segregation between coarse and fine particles of feedstock allow each passage to grind with as narrow a range of micron dimensions as possible. This enables each rollermill to reduce material to flour more efficiently.
A rollermill will not work properly with a wide range of particle size because the large ones will be ground but the small ones will not. This creates a cushion and less capability.
STARCH DAMAGE, ROLL SURFACE.
The harder the grind of the reduction system, the higher the starch damage. Increased starch damage is beneficial for bread flours in some areas, particularly those with no regulation as to dry matter content of bread.
Even then there are practical limits not of a mill, but of the effect of an excessive water uptake when making bread. Roughly speaking, undamaged starch holds between one-third and one-half its own weight of water. Damaged starch holds its own weight and protein holds twice its own weight.
If the damaged starch figure is too high in relation to the protein content of flour, loaf volume will collapse. In other words, there may not be enough protein to carry the extra water demanded by a particular flour due to unduly high starch damage.
This phenomenon is well known in England, where flour proteins are low, damaged starch high and no law to stipulate the minimum quantity of dry matter in fermented goods (bread).
In a 12-step reduction process, there is considerable rerolling or remilling, resulting in less flour seived from the plansifter of, say, 1 sizings, 1 middlings or 2 middlings than from the overtails of each passing to the 2 quality passage. Those will be rerolled for a second time at 2 quality and so on via 1 tailings to 2 tailings, not forgetting the finer non-flour material being remilled at 3 middlings to 8 middlings.
Consequently, a cumulative build-up happens. This can be determined in the laboratory by measuring not just all machine flour samples but also the tailed material of each sifter passage. In this way, it is possible to characterize a starch damage profile determine its effects.
The reduction system in a medium to large modern flour mill will be somewhere around 6.5 millimeters per 100 kilograms per day. The North America equivalent is 0.155 inches per hundredweight per day. Many mills are more, some are less particularly those with double high rollermills.
Sifter surface will approximate 0.03 square meters per 100 kg wheat per day, equivalent to 0.195 square feet per cwt per day.
The advantages of roll cooling include more efficient sifter dressing of stocks, less humidity inside sifters, a less frequent need to strip and reassemble sifter boxes, less condensation in mill spouting, slightly less proneness to infestation and longer runs without stopping for maintenance.
Milling loss between wheat moisture content and flour is reduced by about 0.5% to 1%. There are, however, two disadvantages: greater capital cost and equipment maintenance.
Keep in mind the need to control and monitor the water temperature feeding into the roll. On the one hand, no such control leads to a roll gradually heating up, increasing the roll and water temperature. This expands the roll, the grind becomes closer and so the roll heats further, as does the water.
The solution is to control the water temperature going in to 25° to 28° C (77° to 83° F). If the feed water is much warmer, there is less point in water cooling. If it is much cooler, the roll surface will become relatively cold. This is sufficient on breaks but hard grinding reductions will tend to ring. This phenomenon is the build-up of ground material about ½-inch to 1-inch wide in the form of a ring, which prevents the roll from grinding. Scrapers or brushes will counteract this up to a point.
A number of options are available. Some reduction passages number up to 15, but this is rare. In conjunction with debranning, the number of steps is about six or seven. Double high rollermills placed throughout the reduction system will end up with a similar number, with a commensurate drop in sifter surface. On one hand, this can save money in terms of machinery, power and space but it also can compromise performance and flexibility.
Another option is to replace some of the passages with flake disruptors, pin mills and even hammermills. This option is valid only on higher reductions because of the need to avoid mineral matter contamination when grinding, especially when milling soft wheat.
With many permutations in adjustments and alterations in the flow, the classic 12-step reduction system has inherent flexibility and the possibility to bleed off specialty flours. Conversely, there are a number of mills successfully operating with restricted mini, compact or short flows that fill a need.
David Sugden, independent consultant to the grain industries, may be reached at The Coach House, Killigrews, Margaretting, Ingatestone, Essex CM4 0EZ, U.K. Tel: 44-1245-352048. Fax: 44-1245-251162.