On Predicting Roller Milling Performance IV: Effect of Roll Disposition on the Particle Size Distribution from First Break Milling of Wheat

The breakage equation for roller milling allows prediction of the output particle size distribution from the feed particle size distribution in terms of a breakage function. Breakage functions were determined for First Break milling of hard and soft wheats under four roll dispositions: Sharp-to-Sharp (S-S), Sharp-to-Dull (S-D), Dull-to-Sharp (D-S) and Dull-to-Dull (D-D). The breakage functions were then used to predict the particle size distribution resulting from milling whole wheat samples at different roll gaps, for comparison with experimental results. First Break milling of both hard and soft wheat samples under a S-S disposition gave an even distribution of particle sizes over the range 250–2000 μm, which could be described by linear breakage functions. The effect of changing roll gap was to change the slope of the line i.e. to change the balance between small and large particles, with larger roll gaps giving a greater proportion of large panicles. Milling under S-D, D-S and D-D dispositions gave progressively more ‘U-shaped’ distributions; these were described by quadratic functions that indicated large quantities of small and large particles, but fewer in the middle size range. Again, increasing roll gap changed the balance towards more large particles, but had little effect on the quantity of particles in the middle size range. Predictions of output cumulative particle size distributions using the breakage equation gave excellent agreement with independently derived experimental results, with R2 in excess of 0·99 in all cases. The trend in wheat quality testing is to measure distributions of single kernel parameters. The breakage equation approach allows the link to be made between distributions of single kernel parameters and milling performance.