| A guest post from Grant M. Campbell, Professor of Chemical Engineering, School of Applied Sciences, University of Huddersfield |
 |
| “In these two operations the story of milling… begins: the breaking up of cereal grain seeds, [and] the removal from the resulting meal of the unwanted portions… The story… is of how we have learned to do these tasks better and better, devising improved tools and new skills as time passed; enlisting the forces of nature to help us; enlarging our mechanical arts and our mental capacities as we struggled with the twin problems of increasing the quantity and improving the quality of our product; adopting new ways of life, forming new social organizations as a result of a growing dependence on this increasing food supply…There is no other single thread of development that can be followed so continuously throughout all [Western] history, and none which bears so constant a cause-and-effect relation to every phase of our progress in civilization.” Storck and Teague, Flour for Man’s Bread (1952, p5) “When you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.” William Thompson, Lord Kelvin (1889) This year I have been a chemical engineering academic for 30 years, about 15 of which were spent applying chemical engineering approaches to study wheat breakage. I recently took over teaching particle technology in my university’s chemical engineering programme, giving me a nostalgic opportunity to reconnect with my earlier research on wheat flour milling. A wheat field and windmill grace the cover of my module booklet, and my students are (I suspect) the only chemical engineering students in the world who are told that “the wheat kernel is the world’s most important particle”, in terms of influence on civilisation, international relations and science and technology. Much of what we know about particle technology we have learned from milling of wheat into flour. Modern flour milling starts with the initial breakage of the wheat kernels in what is called First Break, using counter-rotating fluted rollers to open up the wheat kernel. Roller milling breaks open the wheat kernel in such a way that the outer layer of bran tends to stay in large particles and the floury endosperm tends to break into small particles, so that flour and bran can be separated by size using sifting. Moisture content affects bran and endosperm breakage – optimally tempered wheat toughens the bran so that it stays as large particles, while softening the endosperm so that it breaks into smaller particles, facilitating the separation of bran from endosperm. Figure 1 illustrates the production of large bran particles (still with floury endosperm adhering) and smaller endosperm particles, and the factors that influence the distribution of particle sizes – the properties of the wheat kernels (size, shape, hardness and moisture) and the design and operation of the mill. Through repeated milling and sifting, high yields of relatively pure flour are obtained in a dry (and therefore cheap) process. |
 |
| Figure 1. Factors affecting wheat kernel breakage during First Break roller milling. |
| The distribution of particle sizes produced by this initial breakage of the wheat affects the flows throughout the rest of the mill, and hence the yield and quality of flour. First Break is therefore a critical control point in the mill. Ideally if the particle size distribution from First Break were kept constant, the flows to the rest of the mill would be constant and the mill would run smoothly. The problem is, the wheat entering the mill is constantly changing. There is a need, therefore, to understand how wheat properties and mill operation affect the particle size distribution coming out of First Break. |
| Modelling wheat breakage |
| What lights my fire in research is combining mathematical modelling with elegant experiments to reveal insights that could not be achieved any other way. I apologise for bringing an equation into this article (Steven Hawking in A Brief History of Time recalls how he was warned that for every equation he included, the book’s readership would be halved – hence he only included E=mc2!). It is not necessary to understand the following equation, but it is helpful to be aware that the basis of my wheat milling research was a breakage equation I developed that relates the size distribution of the output particles, described by a function 𝜌2(𝑥), to the size distribution of the wheat kernels, described by a function 𝜌1(𝐷), via a breakage function 𝜌(𝑥,𝐷): |
 |
| My work extended this equation to include effects of wheat kernel hardness, moisture and shape as well as roll gap and disposition. (The flutes on rolls are asymmetrical, with a sharp and a dull edge, and the rolls rotate at different speeds. This allows milling to be undertaken under different dispositions – a sharp edge of the fast roll can “meet” a sharp edge on the slow roll, to give Sharp-to-Sharp milling, and so on for Sharp-to-Dull, Dull-to-Sharp and Dull-to-Dull, giving different breakage patterns.) In a further extension, my students and I developed ways of predicting not just the size distribution of outlet particles from First Break, but also their composition (recalling that the point of roller milling is that large particles have more bran and small particles have more endosperm). At that point the maths and experimental work became very complicated! You can read the rest of Grant Campbell’s article here: https://new.millsarchive.org/plus2/uploads/libraryattachments/3025.pdf |
