“Prior to milling, cleaning and conditioning of the grain is the most important aspect of milling. The number of break passages can vary according to the size of the mill. If, for example the mill is designed with five passages for maximum extraction and the number of roller mills in each passage is below the certain minimum, this will not assist in the milling operation to obtain the best extraction with the minimum production of bran powder and fine break flour. In each of the various break systems therefore, there is a certain amount of break roll surface for each sack of flour produced.”
Milling Technologist and Mechanical Specialist Consultant
Morning Star Milling Corporation-Manila
The break system is the beginning of the actual milling process. The object of the break system is:
1. On the head breaks, to shear open the wheat berry and scrape the endosperm away from the bran skin in as large sizes as possible and at the same time leaving the bran in as large flat flakes as possible.
2. On the lower breaks, to scrape off any adhering endosperm from the bran skins as cleanly as possible, with as little bran powder being generated as possible.
Throughout the breaking process, the skin should be kept as whole as possible and any excessive grinding, which produces bran powder, is to be avoided. Bran powder cannot be separated from flour.
Similarly any excess crushing of the released endosperm particles must be avoided, as the larger particle size, the easier it is to purifier and make separations necessary for correct quality flour.
Although it would be ideal to make nothing but large semolina particles, in practice however, some of the endosperm is reduced to medium sized particles (middlings) very fine particles (dunst) and some as flour which at this stage is called break flour. A break flour is of inferior quality and not suited for bread making as will be seen when we later study the mechanics of the milling process that to keep the production of break flour to a minimum, as well as bran powder, is an essential factor.
The extent that the endosperm must be scraped off the bran skin depends on the flour extraction required. Also the extent to which the endosperm can be scraped off the bran skins depends on many items such as the condition of the wheat used, the design of the fluting on the rolls and the roll adjustment itself. It depends on the number of roller mills used in the “surface” at each stage which is expressed as mm per 100 kgs per 24 hours.
Length of the break system
The number of breaks needed to release the stock with the minimum production of break flour depends on the percentage of flour extraction required overall i.e. 70%; 72%; 74%; 76%; or even higher according to the quality required. The normal number of breaks is four with an extraction of 72% to 74% as the target.
Apart from the extraction however, the type of wheat also has an influence on the number of passages, the increased addition of soft wheat in miller’s grists has resulted in the break passages increasing to five. This is due to the difficulty in cleaning the bran of endosperm in the soft wheat. Another factor that has influenced this change is the characteristics of wheat being produced currently where the vitreousness has drastically reduced due to demand by millers from producers, the hard wheats have more medium hard wheat mixed in consignments.
Also with the price of wheat rising more sharply over recent years as compared the selling price of wheat feed, the value of extraction has increased and so millers have also wished to raise the rate of extraction above 72to74%. This has also called for the number of passages being increased from to four to five.
Although reference is only made to break rollermills, it is a fact that bran finishers or brushers can supplement the work of the lower breaks.
The following is a general idea of the number of break passages according to the extraction required.
1. For 72-74% extraction 4 breaks
2. For 74-76% extraction 4 breaks and bran finishers
3. For 76-78% extraction 5 breaks and bran finishers.
The number of break passages can vary according to the size of the mill. If, for example the mill is designed with five passages for maximum extraction and the number of rollermills in each passage is below the certain minimum, this will not assist in the milling operation to obtain the best extraction with the minimum production of bran powder and fine break flour. In each of the various break systems therefore, there is a certain amount of break roll surface for each sack of flour produced. The original design was in inches per sack of flour, but in metric terms we now use mm per 100 kilograms per 24 hours of wheat ground.
First break 5.4 to 7.2 mm
Second break 6.6 to 8.4 mm
Third break 4.8 to 6.6 mm
Forth break 4.2 to 5.4 mm
It must be understood that these are only approximate indications, actual allocations will depend on whether the mill is to operate mainly on strong or weak wheat or an average blend. It is also influenced by the actual milling scheme in question. Roll speeds and feeds have drastically increased over the years. While four or five break systems are still the rule, surface allowance is as low as 2.1mm and are being successfully used, and are far more usual in practice.
Break roll operation
This breaking open of the wheat and releasing of the endosperm from the bran gradually is a mechanical process achieved by passing the wheat between rolls that are fluted; these flutes being cut in a spiral, and the two rolls running at different speeds to produce a cutting or shearing action. The amount of grinding is a question of degree to which the rolls are adjusted, and is measured by the amount of endosperm released from the bran in this operation. The flutes also have a sharp edge and a blunt edge, the arrangement of these is known as the disposition of the flutes.
Break roll release percentage
As already remarked, the main object of the break system is to break open the wheat kernels and release the endosperm. Experience has proved that this is best done gradually so as to obtain optimum efficiency of the separation, and also to keep the different qualities of the released materials suitably distinct for further treatment. For this reason the amount of material released at each stage is limited and is normally adjusted to a predetermined percentage of the feed to the break rolls in question. This is checked by ascertaining the percentage amount passed through a test sieve having mesh size similar to those of the actual sifter scalping or overtailing covers following the break stage.
Percentage release figures differ considerably for different mills, depending on the length and design of the system, and in particular on the number of break passages and the allocation of roll surface for each stage. At some stage, releases have to be adjusted to suit the changes in the wheat blend or the wheat characteristics, usually being increased for soft wheats and correspondingly reduced on the succeeding breaks. Release figures may also be adjusted to suit the type of flour being produced. An example in this case is that higher early releases may be required when a higher percentage of patent flour is required or for higher flour extraction. This was the case during war times when millers produced high extraction flours as high as 80 percent, but this was the extreme and abnormal.
For this reason it is not feasible to suggest hard and fast release percentage figures, but the following will serve as a guideline of possible release ranges for a reasonably well surfaced mill having four or five break passaged, when milling normal blends of bakers flour:
First break 35/45% thru 20 wire
Second break 50/55% thru 20 wire
Third break 30/35% thru 26 wire
Fourth and fifth break normally set to clean out the bran with as little bran powder production as possible.
In modern mills the stocks fed to the second and lower breaks are separated into coarse, medium and fine segments which are respectively fed to coarse, medium and fine fluted rolls for each stage. Where this is the case the percentage release for these stages will normally be lower on the coarse rolls than those indicated above, and considerably higher for the fine passages.
Grinding consists either of shearing open the wheat grain kernels and scraping off the endosperm from the bran skins, or in reducing the endosperm to consistency of the granulation of the flour required to be produced.
Prior to the reduction passages, the different granulations of semolina’s are treated on purifiers where the miller attempts to remove as much of the smaller particles of bran as possible.
Sifter surface allocation is important and the miller has to ensure that the sieves are properly replaced after maintenance and when repairs are carried out. Sifter separation inspections on a shift basis has to be done by every shift to be able to detect any anomalies that can occur during the normal operation of the mill. For this to be maintained throughout the run of a mill, flour stream inspection must also be done on a per shift basis or when the QA. department informs millers of out of spec production. Engineering staff have to ensure that planned lubrication and belt drive inspection if carried out.
These machines do what their name imply, in that the stock to the reduction and recovery passages are, cleaned of any light branny particles and the clean wholesome endosperm, then directed to the head reduction roller mills while the composite particles of endosperm and branny stock is directed to the lower reduction or recovery rollermills. Purifiers are to be set in such a manner to ensure that the stocks are thoroughly separated and that there is not contamination in the separations. The miller must exercise his skill and knowledge when adjusting purifiers. Sieve choice and cleaning brush movement is important in order to achieve the desired separations and produce high quality flour or semolina.
In this section of the mill, the miller must have knowledge of the degree of roll grind for each stage of reduction passages, to ensure that flour produced is free from bran contamination and that the extraction value required is achieved, one must not sacrifice quality for extraction.Balancing the feed to like rolls is extremely important for this to be attained.
Regulating the feed to the nip of the rolls across the entire roller milling process will help extend the life of the fluted rolls and avoid inconsistent flour quality due to rollermills engaging and disengaging too often, as this in turn results in sifter rich, then bare dressing because of the feed being on and off. Smooth roll scrapers have also to be inspected regularly and adjustments or replacement made as required.
Impact detachers and bran finishers play an important part in achieving flour quality and recovery. These pieces of equipment must be inspected on regular intervals to ensure proper functioning during normal operation.
Flour collection and transfer to storage is equally important and equipment and flour streams must be part of a millers’ checklist per shift.
I must state once again, that in order for your mill to perform to desired standard, the miller and management have to be committed in ensuring that everything is done with diligence. Any mill anywhere around the world will perform only as good as the miller responsible for its operation.
We also suggest you to read our previous article titled "The future of Pest Management".