Single kernel effects on breakage during wheat milling

Abstract

Single kernel properties of wheat were investigated in order to improve predictive models of breakage during roller milling based on measured distributions of kernel properties. The breakage equation approach for describing First Break roller milling was also extended to include information about the composition of particles in the broken material. An image analysis-based approach to quantifying the bran content of flour stocks was adapted to allow a bran distribution function for First Break roller milling to be defined and quantified. The Perten Single Kernel Characterisation System (SKCS) measures the distributions of kernel hardness, mass, moisture content and diameter in a mixture of wheat kernels. For the first time, the particle size distribution (psd) resulting from breakage of kernels in the SKCS it self was quantified. Wheat varieties of different hardness, as measure by the SKCS, gave surprisingly consistent psd's on breakage in the SKCS. This indicates that the psd produced by the SKCS cannot be related directly to that produced on breakage during industrial roller milling. More positively, however, it signifies that the hardness index reported by the SKCS indicates relatively unambiguously the energy required to achieve a constant degree of breakage. This implies that the hardness index is inherently meaningful, and explains why it has been possible in previous work to relate the hardness index directly to breakage during First Break roller milling. Breakage equations have been constructed previously to predict the output particle size distribution from First Break roller milling based on distributions of kernel hardness, mass and moisture as measured by the SKCS. The current work added the fourth SKCS parameter, kernel mass, to breakage equations, and demonstrated that the ratio kernel mass:diameter was related to other kernel shape descriptors, thereby adding factors related to kernel shape to the breakage equation model. Further studies of kernel shape investigated the breakage of ancient emmer wheat lines, which have much more elongated kernels than modern varieties. These studies demonstrated that the hardness values reported when the SKCS is applied to these ancient wheats are not meaningful in terms of indicating their breakage patterns. The reason for this was considered to be the unusual shape of the kernels of these wheats. Evidence for this was presented by plotting breakage patterns versus shape descriptors rather than hardness, which appeared to show a smooth continuum between the ancient and modern wheat varieties. The particles produced on breakage of wheat kernels by roller milling vary in composition as well as size, such that large particles tend to have higher bran contents, and smaller particles higher endosperm contents. The breakage equation for First Break roller milling was extended to allow description of the composition of particles. A bran distribution function was defined, and the form of the function quantified by measuring bran contents in broken fractions using image analysis.