Deactivation extents of cellobiohydrolase, endoglucanase, and a total cellulase mixture (Spezyme CP) were studied independently as functions of incubating time and mixing intensity. It was found that the decrease in total cellulase activity was more strongly related to deactivation of cellobiohydrolase 1 (CBH1) than endoglucanase. The mass-averaged shear in orbiting flasks at 50, 150, and 250 rpm was quantified by computational fluid dynamics and was two-orders smaller than shear in typical stirred tanks. Endoglucanase activity did not change significantly with mixing speed, but CBH1 and total cellulase activities were 10-25% higher at 250 rpm compared to the lower speeds after a 24-h incubation. Total deactivation due to mechanical mixing (similar to 20%) may be too low to account for all the rate reduction during cellulose hydrolysis. Thermal deactivation was independent of enzyme concentration while deactivation due to mechanical stress decreased when cellulase loading increased over 0.15 filter paper unit/ml. (C) 2011 Elsevier Ltd. All rights reserved.