pin mills offer an alternative means of grinding These consist of two, contra-rotating discs each of which is fitted with hardened steel pins. Nibs are subject to a pulverising action, which produces a mobile paste of particle size between 90 and 120 µm This requires further reduction and the paste passes through a set of three refining rollers, Roll pressures are hydraulically controlled and the rollers cooled by circulating water.
The most recent development in cocoa nib grinding is the vertical ball mill. Grinding involves frictional and impact forces generated by balls of hardened steel moving freely within a defined space. 6,3,9 vertical ball mills consist of a steel column containing a motor driven spindle fitted with a series of horizontal agitator arms which carry the grinding balls. Cocoa beans, direct from winnowing, can-not be handled, and the feed consists of a mobile paste produced by a single stage disk or pin mill, The paste is pumped up through the base of the mill and ground by the steel balls, set in motion by the spindle and agitator arms. The agitator arms also serve to mix the paste and ensure even grinding.