Break Systems in Flour Mills

by Teresa Acklin
Share This:

      by David Sugden

   Start right, stay right is how any flour mill, beginning at the break system, needs to be set, given that wheat is properly tempered and clean.

   The break system is so called because its purpose is to break open the wheat kernel and continue to scrape endosperm from the wheat skin or bran, step-by-step, by sequential passages. An example of a classic break system is shown in Figure 1, although all sorts of variations are practiced.

   Cleaned and tempered wheat feeds the first break, or I Break, which consists of the first rollermill and its accompanying sifter. The top sieve tails (No. 20W) feed the II Break rollermill and sifter and so on, cascading down the system through a total of five steps, or passages.

   Bran exiting the V Break plansifter top sieve is destined, in this case, to a bran finisher to dust off any remaining endosperm before it is made ready for sale. This may be done by itself or in conjunction with other material combined from the later reduction system (not shown) as millfeed.

   The break system operates continuously. Each break passage produces flour. It also produces intermediate stocks to feed the remaining two parts of the flour mill, namely the purifier and reduction systems.

   Intermediate material, in the case of the I Break system tailing a No. 60W, is directed to the I and II (break) semolina purifiers, whereas the tails of No. 11N runs to the I and II middlings redresser. The remaining passages operate in a similar fashion, though with different qualities and different end points or destinations.

   Table 1 lists the metric aperture equivalent in microns (thousandths of a millimeter) of each mesh number used. For instance, No. 20W means a wire mesh of 20 threads per inch with a micron aperture of 1000. A No. 70W means 70 threads per inch with a 250 micron equivalent. However, No. 11N means an arbitrary No. 11 nylon mesh with apertures of 120 microns each.

   Grit gauze is sometimes used in place of wire mesh, though flours are virtually always sieved through nylon. Wire mesh tends to be more severe than either grit gauze or nylon.

   The intermediate destinations of the V Break passage (Figure 1) change to provide stock for the reduction system. This stock has higher mineral content, small particle size and more sticky material.

   The expression “2 tailings reduction/J/C10” refers to the reduction rollermill passage to be fed. The first part of that expression, “2 tailings,” is North American terminology; the “J” is U.K. terminology and the “C10” is European.

      TYPICAL ROLLERMILL DETAILS.

   Table 2 is an example of typical rollermill details, or configurations, for each break passage. It highlights the number of corrugations or teeth (flutes) per inch and per centimeter and the percent of spiral of corrugation.

   The latter means that the corrugations are not parallel, but are offset as in the rifling or swirling found in a gun barrel. The intention is to cut in to wheat or bran by a scissor rather than flat action. This is alternatively expressed as a ratio: 2% spiral = 1:50, 8% spiral = 1:12.5. The flatter spiral is reserved for the earlier passages, the more acute spiral for the later passages, meaning more severity.

   Well over 100 profiles or designs of corrugations exist. While the number of such profiles vary by plus or minus two or more per inch per passage, the designs show little commonality of approach.

   Some millers and engineers champion certain types, others different ones. Research is inconclusive. When it comes to wear, as all surely do over time, the differences become blurred. The classic material used for the construction of rolls is chilled iron for hardness and long life. Even longer life — double is claimed, with a price tag to go with it — can be gained from chrome-based metals.

   Some corrugations are designed with rounded peaks and valleys. Some have flat peaks and valleys. Others are sharp or a combination. Valley angles are more or less acute but seldom equal. The differential gear ratio — the speed of difference between the fast and slow rolls through which break passage material is ground — is almost always 2.5:1 worldwide. This has been found satisfactory by many years of experience and research, though minor variations exist. The fast and slow rolls run counter to each other in the form of a mangle. The slow roll holds the grain while the fast shears it in the nip or gap.

   Disposition is the relative position or configuration of the fast roll to the slow in terms of the profile of corrugations. The angle of a corrugation profile — normally more acute on the one side than the other from the vertical, such as 21° to 67° — means that a roll can be placed so that either its “sharp” face (21°) can be the cutting edge or, conversely, its dull edge (67°). Sharp is more severe.

   Since one of the main aims in milling is to make long extraction, low-mineral content flour, the break system needs to start off at the first break as gently as possible to reduce bran contamination. Hence, both the fast and slow rolls are disposed “dull to dull” in breaks one and two. The remainder are “sharp to sharp.”

    Variations on the disposition of the rollermills abound. Millers frequently install new rolls “dull to dull” and turn them either “dull to sharp,” “sharp to dull” or “sharp to sharp” as wear increases in order to maximize their life and reduce cost.

   Brushes facilitate the performance of corrugation by keeping the grooves clean, which is effective on the lower break rolls but not otherwise.

      MANY OPTIONS AVAILABLE.

   Roll speeds, that is the fast roll, can be anything up to 800 rpm, more typically 600 rpm. There are many options. Some mills have four break passages; others, especially the larger ones, have graded breaks, from break two downwards, from coarse to fine. Durum mills commonly have seven breaks and rye mills may have eight or more.

   Debranning can reduce the number of breaks from five to three for regular flour mills, and from seven to four for durum mills. Two-high or eight-roller mills reduce the roll and sifter surfaces and pneumatics but not usually the number of steps, although these are combined. Some pre-break systems use impactors, sifters and aspirators to increase germ extraction and decrease filth counts. Further, a smooth “cracking” roll is occasionally found.

   So far as roll surface is concerned, in the classical flow depicted in Figure 1, it is normal to find a third of the complete flour mill allocated to break surface and two-thirds to reductions. Many conventional plants today allocate around 3.3 millimeters per 100 kilograms of wheat per 24 hours to the break system, or 0.079 inches per hundredweight of flour per 24 hours. This practice is subject to debate and varies widely, however, and many systems are shorter still. Soft wheats, such as low protein European and U.S. soft white, demand consistently sharp corrugathe larger ones, have graded breaks, from break two downwards, from coarse to fine. Durum mills commonly have seven breaks and rye mills may have eight or more.

   Debranning can reduce the number of breaks from five to three for regular flour mills, and from seven to four for durum mills. Two-high or eight-roller mills reduce the roll and sifter surfaces and pneumatics but not usually the number of steps, although these are combined. Some pre-break systems use impactors, sifters and aspirators to increase germ extraction and decrease filth counts. Further, a smooth “cracking” roll is occasionally found.

   So far as roll surface is concerned, in the classical flow depicted in Figure 1, it is normal to find a third of the complete flour mill allocated to break surface and two-thirds to reductions. Many conventional plants today allocate around 3.3 millimeters per 100 kilograms of wheat per 24 hours to the break system, or 0.079 inches per hundredweight of flour per 24 hours. This practice is subject to debate and varies widely, however, and many systems are shorter still.

   Soft wheats, such as low protein European and U.S. soft white, demand consistently sharp corrugations to enable high performance (extraction).

   In a conventional mill, sifter surface allocation for the total will split 50-50 between the break and reduction systems. Looking at the breaks, this will be about 0.03 square meters per 100 kg of wheat per 24 hours or 0.195 square feet per cwt of flour per 24 hours. Again, practice varies but not as widely as rollermill surface because a rollermill can be loaded in capacity while a sifter has definite limits.

      CAREFUL SETTING NEEDED.

   Break rolls need careful setting and should be checked at least daily or on every mill or grist change. When adjusting a rollermill, an even grind along its length is obvious. What is not so clear is the effect of cumulative releases.

   Release means the percentage of material sieved through a known sieve aperture, i.e. “released,” with the tails proceeding to the next break passage and so on.

   Table 3 shows the cumulative release resulting from increasing breaks one and two by only 5% each, which moves the release to 83.62% from 80.59%.

   Although no flour mill is 100% efficient, the ideal percentage release would equate to the extraction obtained. For long extraction white flours within specification, the aim should be to release the same percentage as the actual extraction.

   Suppose the actual extraction is 76%. The release, in a perfect world, would be the same.

   It is possible with well-maintained equipment to obtain a 76% extraction from a release of 80.59% or even more, depending on wheat millability, tempering, moisture and so on. However, just increasing the release will not necessarily gain more extraction because the theoretical amount of endosperm is about 82.5%. More release in fact may reduce extraction because the mineral content of flour may have drifted too far out of specification.

   New-crop wheat usually brings new questions to be resolved, such as break releases and sifter sieve clothing. This is a matter of observation of not only the breaks, but the whole plant, flour specification, extraction and tempering. It is a complex and time-consuming issue, requiring several weeks of analysis in difficult cases. In other words, the mill will probably need retuning.

   Accordingly, high mineral content wheat may result in fluffy endosperm and overlarge bran particles, and may require the miller to open out flour covers, increase the release, open the top sifter covers or lower the extraction rate. Easily friable wheats will tend to lead to high mineral content flours when the solution may be to fine up flour covers or increase air on the purifiers.

   David Sugden, independent consultant to the grain industries, may be reached at The Coach House, Killi-grews, Margaretting, Ingatestone, Essex CM4 0EZ, U.K. Tel: 44-1245-352048. Fax:

Table 1
Sifter sieve equipment
mesh sizes
MeshAperture
number microns
20W 1,000
22W 910
60W 300
64W 280
70W 250
11N 120
12N 110

Table 2
Typical rollermill details
Break Number of Number of Spiral Differential Disposition Brushes Corrugation purpose corrugations corrugations % gear ratiotype
per inch per centimeter
I 10 3.9 2% 2.5:1 Dull to dull No See text
II 14 5.5 2% 2.5:1 Dull to dull No See text
III 20 7.9 4% 2.5:1 Sharp to sharp Yes See text
IV 24 9.5 6% 2.5:1 Sharp to sharp Yes See text
V 28 11 8% 2.5:1 Sharp to sharp Yes See text

Table 3
Cumulative effect of increasing I & II Break releases by 5%
Break passage % release 1 % cumulative release 1 % release 2 % cumulative release II I303035 35
II45 31.550 32.5
III30 11.5530 9.75
IV20 5.3920 4.55
V10 2.1610 1.82
Total 80.59 83.62
Note: Cover numbers used for release measurements: 20W for I & II Breaks, 28W remainder.

Partners