Bran is a by‑product of wheat milling. The quantity of fine flour retained in bran has a direct impact on wheat flour extraction rate. In wheat milling, one of the key tasks is to promote bran‑flour separation so as to improve flour yield and quality. In actual production, if insufficient recognition is gained for the status and features of residual flour in bran and corresponding corrective measures are not adopted, the bran flour content may reach about 10%, far higher than the normal level of 4%‑6%. Under the premise of guaranteeing flour processing quality, how to minimize the flour content in bran has always been an urgent problem to be solved in the milling industry.
1 Status and Characteristics of Flour in Bran
Four main forms of flour mixed in bran are listed as follows:
Flour adhered to bran in connected state
Due to different degrees of scraping, spot‑shaped flour particles of varying size and thickness distribute on bran. Rubbing bran by hand brings an astringent feeling, with whitish bran colour and high bulk density. This type of flour accounts for a large proportion in bran with relatively hard texture.
Causes: insufficient scraping force or inadequate scraping depth during grinding. Endosperm particles are not stripped off from bran coats and closely bond with bran, whose tissue structure is still integrated with bran.
Free flour in bran
Free flour takes a relatively high proportion in bran, presenting fine granules with particle size close to or slightly smaller than bran flakes. Similar to bran fragments, it features higher bulk density, lower viscosity, good dispersibility and automatic grading performance. Excessive free flour will make bran turn pale and look unclean.
Causes: flour particles have already been scraped from bran but not separated through sifting and mixed with bran. Provided that proper sifting equipment is configured and sufficient sifting sections are reserved, this part of flour can be separated by sifting.
Attached flour on bran
This kind of flour is intermediate between connected and free state, accounting for a low proportion in bran. It features fine particles similar to raw flour and strong viscosity.
Causes: although endosperm particles are scraped from bran surface during grinding, the increase of material temperature and humidity in production makes partial particles re‑attach to the surface or inner layer of bran. Such flour has poor dispersibility and automatic grading capacity. After separated by sifting, it is easy to re‑adhere to bran surface, which cannot be separated merely by sifting. A bran beater is required to realize bran‑flour separation.
Loose flour
Loose flour occupies a small share in bran, different from connected flour. Although attached to bran surface without tight bonding, it comes from endosperm adjacent to bran coats, with crisp texture and uneven particle size.
Causes: insufficient scraping action during grinding. This fraction cannot be separated only by sifting and also needs bran beating treatment.
In summary, flour mixed in bran presents complex forms and diverse physical properties with great differences in proportion. To reduce bran flour content and facilitate bran‑flour separation, corresponding measures shall be taken according to different flour‑mixing states in bran.
2 Technical Measures for Reducing Flour Content in Bran
2.1 Rational tempering
Tempering is an essential measure to keep bran integrity and realize effective bran‑flour separation, which is mainly completed by damping and conditioning. It aims to make wheat absorb water and expand, build internal stress, change the physical properties inside wheat kernels, improve the tenacity of bran coats, reduce endosperm strength, weaken the bonding force between bran and endosperm, and benefit grinding and bran‑flour separation.
According to domestic production practice, to reduce bran flour content effectively and obtain clean bran, the moisture of conditioned wheat shall not be too high.
Moisture for ordinary flour: 13.5%‑14.0%
Moisture for standard flour: 14.0%‑14.5%
Moisture for special high‑grade flour:15.0%‑15.5%
For a short milling flow, the moisture before milling shall be controlled below 15%.
In winter, the tempering moisture before milling is preferably below 15%.
Excessively high or low moisture will decrease the binding force between bran and endosperm and make endosperm hard to be scraped completely from bran. Pre‑grinding spraying damping can strengthen bran tenacity, scrape bran thoroughly, lower bran flour content and create favourable processing conditions.
2.2 Optimize grinding performance
For flour tightly bonded with bran (connected state), bran‑flour separation cannot be achieved only by sifting and brushing. Scraping action must be strengthened to grind bran sufficiently, strip endosperm from bran to the maximum extent while meeting flour quality requirements. It is necessary to reasonably select roll surface parameters and carry out roll maintenance.
① Appropriate roll surface characteristics
With the development of grinding theory, new concepts such as speed differential, linear speed and grinding active area have been put forward, which provide theoretical guidance for practical production. The selection of roll surface parameters is closely related to raw material condition and finished product requirement, directly influencing grinding efficiency. Roll wear, tooth angle and action angle impose significant effects on grinding results.
② Process scheme
The middle‑break milling system is widely adopted for grade flour and high‑precision flour, with reduction mill as the main flour‑producing section (accounting for 60% of total extraction rate). The front break rolls (Ⅰ‑ⅢB) mainly produce high‑quality grits and middlings with more intermediate products and less direct flour so as to keep bran intact. The rear break rolls (Ⅳ‑ⅤB) are designed to scrape residual endosperm off bran.
Therefore, dull‑to‑dull disposition with 4%‑8% corrugation is preferred for front break rolls to produce more grits and fewer middlings. Sharp‑to‑sharp disposition with a larger action angle (60‑75°) and bigger tooth angle (100‑110°) is suitable for rear break rolls, which works well in decreasing bran flour content and improving product quality.
③ Roll maintenance
After a certain running period, roll corrugations will wear and become blunt, decreasing the scraping efficiency of materials. The larger the contact area during grinding, the higher the scraping efficiency, and vice versa. If rolls appear concave‑waist or convex‑belly shape, partial roll sections will be in idle state, resulting in unstable scraping effect. Worn rolls cause fluctuating roll gap and periodic variation. Excessive roll gap leads to poor grinding effect while too narrow roll gap brings intense force on materials. It is required to replace rolls for each system regularly as planned.
2.3 Give full play to plansifter sifting function
Free‑state flour accounts for the largest proportion in bran, which is separated by plansifter. Sifting of plansifter shall achieve thorough sifting and good classification. When handled by plansifter, large bran flakes at the bottom can pass through sieve compartments to realize maximum separation. Sifting performance in production mainly depends on sifting route and material bed thickness.
① Proper sifting time
Sifting efficiency is positively correlated with the length of sifting route. Longer sifting route thins the material bed on sieve surface and improves sifting efficiency. However, overlong sifting time will lead to over‑sifting and reduce the quality of under‑sieve materials.
② Proper bed thickness
Thicker material bed reduces sifting efficiency. Partial light materials are pressed at the bottom and fine particles fail to contact sieve mesh. Material humidity and temperature also affect bed thickness. A thicker bed increases the bonding probability between coarse and fine particles and weakens separation effect. Too thin bed causes material jumping during sifting, reduces the contact frequency between materials and sieve cloth and lowers sifting efficiency.
Besides, sufficient grading sieve compartments are indispensable. Insufficient compartments will cause poor material classification, bring impurities into downstream processes, deteriorate ground material quality and raise bran flour content. In normal processes, materials on plansifter shall be pre‑classified (pre‑sifted) first so as to reduce bran flour content. Sieve surface condition cannot be ignored either. Poor sieve cleaning, damaged sieve cloth or excessive mending will impair sifting effect and increase bran flour content.
2.4 Make good use of bran beaters and bran brushes
For flour particles attached to bran surface, scraping and beating treatment can be adopted.
① Optimized technical parameters for beaters and brushes
The linear speed of beaters and brushes determines the effect of bran beating and brushing. Too low linear speed leads to insufficient brushing & beating and higher bran flour content, thus reducing flour yield. In practical production, linear speed can be adjusted according to feeding and discharging conditions. The clearance between beaters/brushes and sieve surface has a prominent influence on brushing effect. Excessive clearance brings mild beating and poor brushing effect with high residual flour in bran; too small clearance causes excessive beating force. For bran brush installation, uniform clearance shall be guaranteed between brushes and sieve surface both circumferentially and axially. For the last pass of bran brushing, bristles shall just lightly touch the sieve cloth.
② Reasonable layout of bran beaters & brushes
When bran beaters and brushes are arranged in the front section, materials are in flaky form with high endosperm flour content and relatively high temperature (e.g. ⅡB), obtaining better treatment effect. For granular materials with low flour content and low temperature, the treatment effect is relatively poor. Such equipment is more suitable for large bran flakes while less effective for small bran pieces. It only plays an auxiliary role in grinding and sifting, and achieves better performance in the front section than in the rear section.