“People produce the flour, not the machines”
09 September 201413 min reading
Known for his breakage equation for roller milling, Prof. Dr. Grant CAMPBELL emphasizes especially experience and knowledge while giving information on rolls and milling process. Campbell explains his view as following: “It is not the flour milling technology that produces the flour that quietly underpins the quality and affordability of much of the food industry; it is the people who design and operate the flour mills.”
Taking office in Huddersfield University as Professor of Chemical Engineering a while ago; Prof. Dr. Grant CAMPBELL is known for his studies on wheat features and grinding process in roller mills. Prof. Dr. CAMPBELL, who will establish the Centre for Cereal Process Engineering at Huddersfield University where he took office, states that grinding and screening are the two most important processes in the mills.
Giving information on the effects of rolls in the milling process, CAMPBELL expresses that there are many parameters related with the adjustments of the rolls in this process and these parameters have important effects on quality and yield. However; emphasizing that not all of the adjustments can be made systematically, Prof. Dr. CAMPBELL draws attention to the experience and knowledge. Campbell explains his view as following: “It is not the flour milling technology that produces the flour that quietly underpins the quality and affordability of much of the food industry; it is the people who design and operate the flour mills.” Grant CAMPBELL answers our questions on rolls and their roles on milling process.
Professor Campbell, firstly could you please introduce yourself? Could you give information on your studies related with the milling industry?
I have recently joined the University of Huddersfield as Professor of Chemical Engineering, having previously spent 19 years teaching chemical engineering at the University of Manchester and undertaking research in cereal process engineering. My research interests include wheat flour milling, aeration aspects of bread making, and cereal biorefineries. At Huddersfield I will be leading the introduction of a new undergraduate degree programme in chemical engineering, and establishing the Centre for Cereal Process Engineering to continue these research themes.
Regarding wheat milling, I have introduced and developed the breakage equation for roller milling as a basis for studying and interpreting the effects of wheat properties and roller mill operation on wheat breakage. First Break is the most critical stage of the flour milling process, as the particle size distribution produced at First Break determines the flows through the rest of the mill and hence the overall “mill balance” and flour yield and quality. My studies have therefore focussed on First Break and on the effects on breakage of roll gap and roll disposition, and of kernel hardness, size, shape and moisture content.
Mr. Campbell, could you please tell us what do rolls mean in milling and why are they important?
The milling of wheat into flour has been a key technology underpinning and influencing Western civilization, technology and culture for millennia. Over thousands of years this evolved from the use of saddlestones to rotary querns and eventually to millstones. Then, between 1880-1900, millstones were suddenly displaced by roller mills, which offered greater versatility and control and, crucially, were better suited to milling hard wheat which gave better bread. Modern flour milling is defined by the use of roller mills within the “gradual reduction process” in which wheat stocks are repeatedly milled and sifted to give a high yield of relatively pure white flour. Roller milling and sifting are thus the two central operations in flour milling; that fact that “milling” refers to the whole process as well as to just one of these two operations shows that it is the dominant and more important operation.
What is the effect of rolls on the milling process?
The wheat kernel is a baby plant; it contains the germ which, if placed in soil and given warmth and water, will germinate, sending a root down and a shoot up. Once the shoot pops out of the ground, it can begin to photosynthesise food for the growing plant. However, until it can photosynthesise its own food, the seed needs an on-board food supply to get the germination process going. Thus most of the wheat kernel is endosperm, the white part on the inside of the wheat kernel, which is food for the baby plant in the form of starch and protein, and potentially food for us as well – this is why wheat (and other cereals) are such an important part of the global food supply. The endosperm is also potentially food for insects and micro-organisms, so the whole kernel is surrounded by a protective layer of bran. The purpose of flour milling is to separate the floury endosperm from the bran.
The effect of rolls is that they break open the wheat kernel in such a way that the bran tends to stay as large particles and the floury endosperm breaks into smaller particles, so that bran and endosperm can be separated by sifting. By using repeated milling and sifting, a high yield of relatively pure white flour is obtained using a dry, and therefore cheap, process.
The wheat kernel features a crease, in which the bran layers fold in to the centre of the kernel. The rice kernel does not have a crease, such that for rice, bran can be separated from endosperm simply by polishing the bran layers off the outside. For wheat, however, complete separation of bran from endosperm requires breaking open the wheat kernel in order to remove the crease bran. Thus the technologies for wheat and rice milling are very different, which has influenced the technological and social evolution of Western versus Eastern civilisation over the last several thousand years. However, in recent years rice debranning technology has been applied as the first stage in wheat milling, to remove some of the bran prior to conventional milling, which has given advantages in terms of improved bread and pasta quality.
What are the types of rolls and according to which area of usages they vary?
The flour milling process can be divided into two broad areas, the Break system and the Reduction system. The Break system typically uses 4-5 sets of fluted rolls with progressively finer fluting to open up the wheat kernel and scrape the endosperm from the bran. Flour is endosperm material smaller than about 200 µm; some flour is produced at each Break stage. The Reduction system takes the larger endosperm material and uses around 10-12 pairs of smooth rolls to reduce the size of these particles. Break rolls operate with a gap between the rolls, the size of which affects the degree of breakage and the effectiveness of scraping of endosperm from bran.
Break rolls also operate under different speeds, with the fast roll typically rotating around 2.7 times as fast as the slow roll. Reduction rolls operate without a gap and with a lower differential of around 1.3:1. Reduction rolls operate under pressure, which causes a degree of damage to the starch granules and hence their susceptibility to enzyme attack and their ability to absorb water. The final flour is made up of the flours produced at each Break and Reduction stage; the overall quality of the final flour depends on the quality and amounts of the flours produced at each stage.
First Break, where the wheat first enters the process, is the critical control point in the flour milling process, as the particle size distribution produced at this point determines the flows through the rest of the mill, and hence the overall “mill balance” and the yield and quality of the final flour. A wide range of particle sizes can be produced at this stage, ranging from smaller than 200 µm to larger than 3000 µm; the bran tends to stay as the larger particles, with adhering endosperm which is subsequently scraped off, while the smaller particles tend to be mostly endosperm with a small amount of bran dust. The particle size distribution depends on the design and operation of the mill, including the roll fluting and the gap between the rolls, and depends on the wheat properties, in particular hardness.
The challenge of flour milling is to try to keep the output particle size distribution from First Break relatively constant in the fact of a constantly changing feedstock, in order to keep the performance of the rest of the mill relatively constant.
How do the details of the roll fluting and operation affect milling?
First Break rolls typically have 10.5 flutes per inch, with each flute having a sharp face and a dull face (i.e. referring to the angle of the face of the flute). Pairs of rolls can be operated such that the flutes on the faster roll “meet” the flutes on the slower roll with their sharp face first, called a “Sharp-to-Sharp” disposition, or with their dull face first, called “Dull-to-Dull”. Sharp-to-Sharp milling gives a narrower particle size distribution, while Dull-to-Dull milling produces large proportions of very large and very small particles, with fewer in the mid-size range. This wider particle size distribution is easier to separate by sifting; hence millers tend to use Dull-to-Dull milling at First Break. It is also possible to operate the mills under Dull-to-Sharp or Sharp-to-Dull dispositions, which give in-between particle size distributions. As the rolls wear down during operation and the flutes become more dull, millers may change the disposition progressively from D-D, then D-S, then S-D and final S-S, to try to maintain a consistent particle size distribution for longer before sending the rolls for regrinding of the flutes.
What changes should be done in settings of such parameters depending on the type of the milled raw material? How do the different settings affect the milling quality?
The five most important factors that affect the particle size distribution produced from First Break milling are the roll disposition, the roll gap, the size of the wheat kernel, its moisture content and, most importantly, its hardness. As noted above, most millers operate under a Dull-to-Dull disposition at First Break, to give a wide particle size distribution that is easily separated by sifting. Breakage depends on the ratio between the roll gap (G) and the wheat kernel size (D), called the milling ratio (G/D); increasing the milling ratio (by increasing roll gap or by decreasing the input kernel size) results in larger particles. Roughly speaking, increasing the First break roll gap from 0.5 to 0.6 mm will increase the average output particle size by about 8-10%. Millers tend to condition wheat to around 16% moisture prior to milling, which toughens the bran and softens the endosperm, such that the bran stays more intact in larger particles and the endosperm shatters more readily into smaller particles, facilitating the separation of bran from endosperm. But the most important property of wheat in relation to milling is hardness. Soft wheat breaks easily to produce a lot of small endosperm particles, with the bran staying relatively intact as large particles. In hard wheat the bran and endosperm tend to break together to give a much narrower particle size distribution with fewer large particles and fewer small particles, and more in the mid-size range.
The widest particle size distribution is therefore produced when soft wheat is milled under a Dull-to-Dull disposition. The narrowest particle size distribution is produced from hard wheat milled under a Sharp-to-Sharp disposition. Hence millers tend to employ Dull-to-Dull initially, to give a wider particle size distribution that is easier to sift. It might also appear, from this logic, that soft wheat are easier to mill for the same reason. However, hard wheat produces particles from which endosperm is more easily scraped from bran during the subsequent milling stages, such that it is easier to produce cleaner flour, with less bran contamination, from hard wheat.
How do the features of the rolls and the parameters related with these rolls affect the consistency in the production?
The flour milling process is very complex; as noted above, First Break is critical in determining the initial particle size and hence the flows through the rest of the mill, but the rest of the mill features numerous stages arranged in complex configurations such that there is not an easy relationship between the settings at any one point and the overall flour yield and quality. The major control parameters available to the miller are the rolls gaps between Break rolls and the pressure applied to Reduction rolls. Millers aim to run as much as possible at a steady state using a consistent feedstock, such that small fluctuations in wheat properties can be addressed by constant small adjustments in roll gaps, based on experience and guided by a sensible understanding of how roll gap affects breakage (larger gaps give less breakage) and how that will affect downstream operations. In general millers have a set of operating parameters that they know will work tolerably well for wheat within a given range of hardnesses, based on long experience, and adapted each year in response to the new harvest.
However, the complexity of the flour milling process means that it is unlikely these operations are optimal; mills are no doubt overdesigned, which gives them a degree of robustness and flexibility to respond to changes in the wheat and to produce a consistent product, but at a cost. A greater understanding of milling operations throughout the process would allow a cheaper and more sophisticated operation that responds more effectively to changes in wheat properties and that delivers more targeted and consistent flour quality. Flour milling is a very long-standing industry and somewhat conservative; there is scope to understand flour milling more deeply and, on the basis of that deeper understanding, to design and operate flour mills more effectively.
Finally, what would you like to add about this subject?
Ultimately, it is not the flour milling technology that produces the flour that quietly underpins the quality and affordability of much of the food industry; it is the people who design and operate the flour mills. The benefit of my research into flour milling, I hope, is that it can stimulate those people to think about their process in different ways and with greater insight, as well as a greater appreciation of the fascination of this process and its importance, and hence a greater sense of personal pride and commitment. I have studied flour milling all these years simply because I find it a fascinating process, and my hope is not just to generate new understanding, but to inspire flour millers to deepen their own fascination and insight, and hence to operate this important industry with greater effectiveness. Wheat is the world’s most important cereal, in terms of its influence on world history and its ongoing importance for global trade, food security and sustainability. Wheat is also playing an important role in developing non-food uses of cereals. In both cases, however, the value of wheat is only realised by breaking it open, by milling it. The effectiveness with which we mill wheat is key to exploiting and maximising the value of this most important seed.
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