On predicting roller milling performance Part II. The breakage function

Paper I of this series showed that the relationship between the inlet and o utlet particle-size distributions in a roller milling operation can be foun d by integrating, over the range of input particles, the breakage of each i ndividual particle as a function of its physical characteristics. This is p ossible because particles break independently during roller milling. The pa ttern of breakage for individual particles is called the breakage function. This paper considers the form of the breakage function and determines it e xperimentally for roller milling of wheat. The breakage function is shown to depend critically on the ratio of roll ga p to input particle size (the milling ratio). For a given ratio, the breaka ge function for wheat grains is linear with respect to output particle size , over a wide range. This is quite different from the particle-size distrib ution produced by, e.g., hammer milling. It perhaps explains why roller mil ling is so suited to milling of wheat to produce flour: the broad and even distribution of particle sizes produced allows effective separation of bran and efficient recovery of white flour. Breakage functions depend on wheat variety and physical characteristics and on the design and operation of the roller mill. Single-kernel testing is becoming widespread in wheat quality testing; dist ributions of individual kernel parameters, such as size, mass, hardness and moisture content, are measured. The breakage function approach potentially provides a link between single-kernel testing and milling performance. (C) 2001 Elsevier Science B.V. All rights reserved.