Pretreatment of guayule biomass using supercritical carbon dioxide-based method

Guayule, a desert shrub harvested for commercial production of hypoallergenic latex and resins constitutes <20% of the biomass. Converting the remaining bagasse to biorefinery feedstock for value-added products is an optimal economic option. A supercritical CO2-based process had been developed previously for resin extraction. In this study, the feasibility of including a supercritical CO2-based bagasse pretreatment method was evaluated. The pretreatment involved: adding water to the bagasse, raising system temperature, pressurizing using supercritical CO2, holding the system for a period of time, and exploding the bagasse. The pretreated biomass was subjected to enzyme hydrolysis. The yields of released sugars were used as pretreatment effectiveness indicators. Supercritical method outperformed other methods and gave much higher overall sugar yields for guayule (as high as 77% for glucose and 86% for total reducing sugars through both pretreatment and hydrolysis, as compared to 50% for glucose and 52% for total sugars with the dilute-acid pretreatment and 36% for glucose and 52% for total sugars with the delignification pretreatment). The enzymatic hydrolyzates were tested on the cellulase-producing fungus Trichoderma reesei Rut C-30. No inhibitory/toxic effects were apparent in terms of cell growth, sugar consumption, and cellulase and xylanase production. The supercritical CO2-based method was found to be very promising for pretreatment of waste biomass as the feedstock for subsequent enzymatic hydrolysis and fermentation to produce value-added bioproducts.