Extending the Normalised Kumaraswamy Breakage Function for roller milling of wheat flour stocks to Second Break

The Normalised Kumaraswamy Breakage Function (NKBF), developed previously to describe First Break milling of wheat kernels, was applied here to describe the breakage of the stocks produced by First Break that then pass to Second Break milling. The breakage equation for roller milling assumes independent breakage of particles as they pass through the mill; that assumption was tested for Second Break milling and found to be appropriate. A single NKBF was not adequate to describe the full particle size distribution (PSD) created by Second Break milling, as the PSD is bimodal, and the NKBF is unimodal. Instead, the PSD was divided into the fraction of particles larger than 2000 μm, and those smaller than 2000 μm, and separate NKBF functions were applied to each fraction. In this way, it was demonstrated that the size distribution of particles larger than 2000 μm produced after Second Break milling depended strongly on the input particle size, but was essentially independent of the roll gap used for Second Break. By contrast, the PSD of particles smaller than 2000 μm depended strongly on the roll gap, but was essentially independent of the input particle size. This finding reflects the nature of Second Break milling, that it involves scraping endosperm material off bran particles, such that the endosperm material (which is produced as small particles) depends strongly on the size of the gap through which the bran particle passes, but the bran particle itself remains essentially intact, irrespective of roll gap. This scraping of endosperm particles, although strongly dependent on the roll gap, is independent of the size of the bran particles, because the bran particles pass through the roll gap oriented along their longest dimensions. Second Break milling of the entire output from First Break, as practised in double high systems, could be described adequately by a single NKBF. Extending the breakage equation via the NKBF to Second Break facilitates the application of simulation and optimisation to flour milling for both food and non-food processes.

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► A breakage equation model for Second Break milling of wheat stocks is presented.
► The particle size distribution (psd) of Fine particles depended strongly on roll gap.
► The psd of Coarse particles depended strongly on input particle size but not roll gap.
► The findings give insights into the physical mechanisms of Second Break milling