“The amounts of fine products especially bran which are produced in the break system have to be minimized. Because the difficulties of separating flour from fine bran if produced in the early stages of break system the whole of the subsequent milling process will be affected in a bad way. So that the break system in flour milling is the most critical operation. This is where the miller has to use his skill and knowledge to minimize flour production as much as possible in the break system with that the amount and purity of white final flour that he can produce measure the skill of the miller.”
Prof. Farhan Alfin
The aim of wheat milling is to separate structural components of wheat kernel by break open the grain and recover the endosperm from other parts which are the bran skins and embryo and then reduce the endosperm to particles with a specified size distribution. Wheat has an oblong shape and embryo at one end and a beard at the other. Wheat kernels have not uniform size and the kernel has a crease which are the distinctive anatomical features that make the recovering pure endosperm a challenge. The bran skins of the crease are submerged within the kernel itself. All of these wheat kernel structure factors make separating the endosperm of wheat kernel not easy, which means that to do that, the grain has to be break opened through mechanical means (Bunn, 2000; Campbell, 2007).
Millers make their efforts to have three major objectives, which are the optimum extraction rate with the maximum quality of finished products and the capacity of the mill. Their efforts for these objectives are aimed to have the optimum economic balance of these objectives (Posner and Hibbs, 2005; Campbell, 2007). Miller to achieve his aim has to practice efficient milling techniques and to optimize the streams of every machine in the mill (Bunn, 2000). So intermediate streams are continuously monitored for quantity (distribution table), quality (ash curve), and granulation (break release and granulation curve) and this data is used for adjustment by the miller (Posner and Hibbs, 2005).
These techniques will be discussed in a series of articles. In this article break release technique will be discussed.
THE BREAK SYSTEM
Wheat milling process can be divided into three main system: break, sizing, and reduction. To obtain a high extraction rate of flour with low ash content, the miller’s first objective in the break system have to be to break open the wheat kernel and release as much as possible clean endosperm particles in large size of sizing and middlings, leaving the bran in large particles and minimum amount of flour (Bunn, 2000; Campbell, 2007; Serna-Saldivar, 2010).
The objective of the sizing and reduction systems is to reduce sizing and middlings into flour (Serna-Saldivar, 2010). The sizing system grades the endosperm, producing a narrower particle-size range, thus optimizing the performance of the reduction system (Miskelly and Suter, 2010).
There are generally 4–6 break roll passages (1BK, 2BK, 3BK, etc.) which have successively finer spiral flutes. Between each of the break operations, the stock is sieved in a plansifter separating flour from bran and larger endosperm particles (Miskelly and Suter, 2010; Serna-Saldivar, 2010). The break system products are bran, sizing, middling and flour. Sizing is the endosperm particles which have particle size in the range 210-500 μm. Middlings is the endosperm particles which passed from 210 μm and bigger than 150 μm. The amount of flour must be minimized and the particles are smaller than 150 μm sieve (Serna-Saldivar, 2010). Bran must be in flakes shape and free from adhering endosperm. The size of bran particles is over 500 μm. Bran finishers remove any remaining flour from the coarse bran after the break system (Miskelly and Suter, 2010).
1st and 2nd break rolls are to release the maximum amount of puro sizing and middlings in the correct quantities with regards to mill designed balance, whilst keeping the bran skins course, and with the minimum production of the bran fine particles. 3rd and 4th break rolls are to clean up the resulting break stocks and to recover any remaining endosperm still adhering to the bran while taking care not to create fine bran particles (Bunn, 2000).
Primary breaks (first, second and third) are the most heavily loaded in the mill and establish the load to purifiers, sizing rolls, and the reduction system. The secondary breaks (fourth and fifth) are not responsible for the intensive contribution to flour yield. However, recovery of fine middlings to the tail end of the reduction systems is critical for acceptable flour yields (Posner and Hibbs, 2005).
The amounts of fine products especially bran which are produced in the break system have to be minimized. Because the difficulties of separating flour from fine bran if produced in the early stages of break system the whole of the subsequent milling process will be affected in a bad way. So that the break system in flour milling is the most critical operation. This is where the miller has to use his skill and knowledge to minimize flour production as much as possible in the break system with that the amount and purity of white final flour that he can produce measure the skill of the miller (Bunn, 2000; Campbell, 2007).
Fig. 1 Schematic explanation of break release
Breakage of wheat kernels during first break roller milling depends on the characteristics of the wheat (including the distributions of kernel size, hardness and moisture content) and on the design and operation of the roller mill (including roll speeds and differential, roll disposition, fluting profile, number of flutes, fluting spiral, roll gap, the degree of roll wear and the feed rate). The effects of these factors are manifest in the particle size distribution exiting first break, the compositional distribution of those particles (as large particles tend to be richer in bran, while small particles are pure endosperm), the power required to mill the wheat and the rate of roll wear. (Campbell, 2007)
WHAT ARE BREAK RELEASES?
The break release percentage is the amount of ground material obtained, consisting mostly of sizing, middlings, flour, and fine bran, reported as a percentage of the original material being tested through a certain sieve aperture. A schematic of a break release test is presented in Figure I.
The following equation should be used to calculate break releases:
in which W1 = weight of sample, and W2 = weight of overs on the sieve after sifting. The break release values are actually the percentages of the stock passing through the sieve.
WHY ARE BREAK RELEASES NECESSARY?
Before contacting the break releases necessary, mill balance term should be explained. Mill balance is defined in “Dictionary of Milling Terms” as “the proper distribution of stock to the various parts of the milling system, as determined by the flow sheet, to ensure proper loading of equipment and high milling efficiencies” (Gwirtz, 2002).
In order to achieve optimum efficiencies of all machines within a mill, all sections of the mill should be fully utilized and the miller ensures the correct amount of stock from the break rolls to the subsequent roller mills/sifters/purifiers is entering as it was designed to receive to keep it in balance. The amount of stock is as important as the type of stock - quality and particle size. If these quantities are changed for any reason then some areas of the mill will be underloaded and other areas will be overloaded, thereby causing an imbalance in the milling system. This imbalance will result in poor milling efficiencies, further resulting in poor extraction, poor flour quality, higher milling costs and lower mill capacity (Bunn, 2000; Posner and Hibbs, 2005).
An imbalance in the mill flow to plansifter causes bare or over dressing that results bad flour quality. When a sifter is receiving too much product then there will be a higher percentage of fines over tailing which will automatically overload the passages further down the system. If a sifter is not receiving enough product then the percentage of over tails will be low, which will result in more lower quality fines passing through the sieve also causing all following machines to have less quantity of product than they were designed to have therefore causing bare dressing to be accentuated right down the flow (Bunn, 2000).
Break releases are used to (Bunn, 2000; Posner and Hibbs, 2005).
1) Optimize mill performance by keeping the balance of the mill at fluctuation resulting from conditioning or changes in the mix which led to changes in quantity of sizing, middlings and flour of streams of the early stages of the milling process. Optimizing mill performance led to obtaining high quality and consistent flour at maximum mill capacity and flour extraction.
2) To evaluate roller mill performance over a period of time. In the other worlds, to monitor the wearing factor of roll flutes. As roll fluting wears, the release lessens, therefore the miller must exert higher pressures on the rolls to try to achieve the desired amount of fine product from the break system. This will require a higher input of power which in most modern mills can be readily monitored by Amp meters on each roll drive motor.
HOW CAN BREAK RELEASES BE MONITORED?
Break releases measurement method should be controlled and uniform to avoid variation in test results unless data is of no value. The head miller has to train the procedure and ensure the same procedure and equipment is used each and every time, without exception (Bunn, 2000; Fowler, 2012).
Taking the sample for sieve analysis can be a challenging act that demands some experience. Representative samples are taken under the grinding rolls in such a way that a truly representative sample of the flow can be repeatedly obtained. A sample should be taken from under the rolls from each side of the ground material stream and if necessary, from the middle, the sample must be collected in a consistent manner (Bunn, 2000; Fowler, 2012). Some millers use specially designed containers equipped with a cutting lid to take a fixed volume of material under the rolls. Containers of different volumes are used for different materials. The weight of the total sample is recorded and used for the break release test (Posner and Hibbs, 2005; Fowler, 2012).
The collected sample can be sifted directly as is or weighing out a small portion of it. The first way which use the entire sample prevents segregation but requires collecting comparable sample sizes and performing calculations to determine the percentage through the screen.
Weighing out a small portion of a sample taken under a roll stand prevents calculations but might cause vibration and unintentional segregation in the sample (Bunn, 2000; Fowler, 2012). Finer particles will settle to the bottom of the sampling container. Weighing off or sub-sampling of the original sample most likely will result in a break release calculation that is lower than the actual break release (Fowler, 2012). Accordingly, the whole sample of about 100-200 g should be sifted. Sample size can be controlled by container volume and/or the amount of time the sample is taken. Weighing of the sample must be done in a manner that will not result in size separation. Accurate weighing of the sample is important, as well as accurate weighing of the separate components after test sifting (Posner and Hibbs, 2005).
Proper test sifter procedures are important in determining break or grinding releases from roller mills. The test sifter used must be large enough to process the entire sample collected (Fowler, 2012).
Break releases were originally monitored by using a hand-held sieve to mimic the coarse separations of the sieve or centrifugal. More recently, a mechanical sieve, which has a pre-set timing device, is commonly used. Accordingly, the miller selects for break release adjustment sieves with apertures similar to those used at the top of the corresponding break sifters sections in the mill and sift each sample the same amount of time (Posner and Hibbs, 2005; Fowler, 2012). Appropriate sifting time can be determining by sifting in 10-second time increments and determining when less than a 1% change in through stock based on the original sample weight has been identified, with the sample screen remaining clear and no fines observed in the over tail (Gwirtz, 2018).
Generally, the stock over the screen is weighed and subtracted from the original sample weight to calculate the weight of the through stock that can be expressed as a percent of the original sample weight (Gwirtz, 2002).
In new developments millers take different approaches to automatically determining the break release on-line. The test sifters are modified for continuous on-line separation of ground material to overs and throughs, and the weights of those are used to determine the break release and to activate roll gap adjustment. Another approach is to determine the weight of one of the sifter outlet materials continuously with a flow-through scale and feedback information to the roll gap control (Posner and Hibbs, 2005).
An innovative solution for monitoring the granulation distribution is with Particle Size Measurement (PSM) apparatus, which can be either installed fixed in a plant or as a portable. Particle Size Measurement (PSM) instrument provides an alternative to the classic laboratory sifter (Ziolko, 2018).
WHEN SHOULD BREAK RELEASES BE CARRIED OUT?
Break releases should be checked whenever a change has occurred in the mill - roll change or sieve change.
Small changes such as incorrect wheat moisture, inconsistent conditioning times, softness or hardness, unwell adjusted feed rolls, poorly adjusted roll gap, ineffective roll brushes, incorrect sieves, damaged or worn sieve cleaners, poorly adjusted air settings on purifiers, incorrect sieves can individually or collectively cause a good mill to produce poor milling results (Bunn, 2000).
The break releases are determined on a routine basis and at least once each shift. Because of changes in the composition of the different streams that occur depending on the wheat mix that is processed the break releases of each of the break passages should be adjusted by the head miller after mill mix changes (Bunn, 2000; Posner and Hibbs, 2005).
When a mill is about to carry out an Ash Curve, then it is imperative that the break releases are checked and set as per a pre-determined protocol.
WHAT IS THE OPTIMUM BREAK RELEASES VALUES?
Wheat type, wheat moisture, length of milling flow, condition of the grinding rolls and flour quality desired are among of the factors that affect the optimum break release schedule for any milling operation (Fowler, 2012). The ash of produced flour is a good indication of variance from optimum break releases. The break releases adjustments of first three breaks (primary break) have an effect on cumulative ash of resulting flours and intermediate stock distribution in mill (Sakhare and Inamdar, 2014).
When determining the optimal break release schedule for the mill, proper management for testing and setting the break releases is critical to maintain the correct mill balance and to meet the ultimate objective, maximizing extraction of quality product (Fowler, 2012, Fowler, 2014).
For example, in an operating mill, the rolls are adjusted to the following possible break release ranges of mill having four or five break passaged (Posner and Hibbs, 2005; Sebastian, 2018)
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Campbell G. M., 2007, Roller Milling of Wheat, in: Handbook of Powder Technology, Volume 12, Elsevier Ltd., Amsterdam
Fowler, M., 2012, Optimizing Break Release, Milling, 20(4):30.
Fowler, M., 2014, Adjusting the break system, World Grain 32(4):114.
Gwirtz, J., 2002, Break Release Measurement, A Management Issue?, AOM Bulletin April, 7777- 7781.
Gwirtz, J., 2018, Measuring break release, World Grain, 36(9):72-76
Miskelly D. and Suter D. A. I., 2010, Processing wheat to optimise product Quality, in: C. W. Wrigley and I. L. Batey, Cereal grains Assessing and managing quality, Woodhead Publishing Limited.
Posner E. S. and Hibbs A. N., 2005, Wheat Flour Milling, American Association of Cereal Chemists, Inc, St. Paul Minnesota. U.S.A. 489p.
Sakhare, S. D. and Inamdar, A. A., 2014, The cumulative ash curve: a best tool to evaluate complete mill performance, Journal of Food Science and Technology, 51(4): 795–799.
Sebastian, R., 2018, The importance of break system in flour milling process, Miller Magazine 12(105):80.
Serna-Saldivar S. O., 2010, Cereal Grains Properties, Processing, and Nutritional Attributes, CRC Press. 747p.
Ziolko, T., 2018, Measuring particle sizes and optimizing processes, Miller Magazine, 12(105):76.