Does sugar inhibition explain mixing effects in enzymatic hydrolysis of lignocellulose?

BACKGROUND: Enzymatic hydrolysis of lignocellulose is associated with mixing issues, which are likely to affect process performance. The aim of this study was to investigate how viscosity and sugar inhibition influence the mixing-dependence in hydrolysis of steam-pretreated spruce. RESULTS: The effect of agitation on low-viscosity, low-solid hydrolysis (5% water insoluble solid (WIS)) was marginal, as the conversion after 72 h decreased by 9% when decreasing the agitation rate from 600 to 100 rpm. However, when the viscosity at 5% WIS was increased by Xanthan addition, the effect of agitation was greater, and conversion decreased by 21% when decreasing agitation from 600 to 100 rpm. For high-viscosity, high-solid hydrolysis (16% WIS), the conversion decreased by 54% when decreasing the agitation from 600 to 100 rpm. However, when the product concentration was kept low by simultaneous saccharification and fermentation (SSF), the effect of agitation was weaker, and conversion decreased by only 14%. CONCLUSION: The results of this study strongly suggest that poor mixing in viscous lignocellulose hydrolysis causes local product accumulation, leading to increased inhibition and decreased hydrolysis rates. Decreasing the glucose and cellobiose concentration removes the mixing-dependence, highlighting SSF as an attractive option for large-scale hydrolysis and fermentation of steam-pretreated softwood