The influence of shear rate and temperature on the tempering of different mixtures of cocoa butter, sugar and lecithin has been studied using a concentric cylinder viscometer as a shearing device and using viscosity measurement to monitor crystallization. Shear rates ranging from 1 to 50 s(-1) were tested at four different isothermal temperatures (13 degrees C, 17 degrees C, 20 degrees C and 23 degrees C). Three different material compositions were investigated: plain cocoa butter, a cocoa butter/sugar mixture (44 wt% sugar) and a cocoa butter/sugar/lecithin mixture (44 wt% sugar and 0.2% lecithin). In each case a gradual remelt was performed in the viscometer to obtain an indication of the sample onset melting point. The results for cocoa butter show that at lower temperatures induction times were much shorter, unaffected by shear and generally lead to a lower melting point sample. At 23 degrees C, induction times were shear dependent, with higher shear rates producing higher melting samples, suggestive of higher melting polymorphs. The addition of sugar caused a universal and substantial decrease in induction times, and although tempering at 23 degrees C was still shear dependent the onset melting point was lower than for the sugar only sample. The addition of lecithin caused a slight delay in the onset of crystallization. The results tend to suggest that sugar crystals provide sites for heterogeneous nucleation, which is slightly weakened by lecithin which coats the sugar surfaces. (c) 2005 Elsevier Ltd. All rights reserved.