Paper I of this series showed that the relationship between the inlet and outlet particle-size distributions in a roller milling operation can be found by integrating, over the range of input particles, the breakage of each individual particle as a function of its physical characteristics. This is possible because particles break independently during roller milling. The pattern of breakage for individual particles is called the breakage function. This paper considers the form of the breakage function and determines it experimentally for roller milling of wheat.The breakage function is shown to depend critically on the ratio of roll gap to input particle size (the milling ratio). For a given ratio, the breakage function for wheat grains is linear with respect to output particle size, over a wide range. This is quite different from the particle-size distribution produced by, e.g., hammer milling. It perhaps explains why roller milling 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; distributions 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.