Supercritical CO2 Hydrolysis and Explosion as Pretreatment of Guayule Bagasse for Fermentation Feedstock

Guayule is a desert shrub grown in the dry regions of Arizona and California for commercial production of hypoallergenic latex. Its resins also have unique applications. These products are non-allergenic to most people unlike the products made from the rubber from b. Latex and resins, however, make up only about 20% of biomass. Converting the waste bagasse to biorefinery feedstock for value-added products makes good economic sense. A supercritical CO2-based process had been developed for extracting resins and rubber from guayule. In this study, we demonstrated the feasibility of using a supercritical explosion-hydrolysis procedure, as part of the integrated process, to prepare/pretreat the bagasse for subsequent enzymatic and microbial conversion. Our approach used supercritical CO2 and moisture for mild hydrolysis and subsequent explosion, created by sudden pressure release, to loosen and expose the lignocellulosic structure for enzymatic hydrolysis. 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 the indicators for pretreatment effectiveness. Comparison studies showed that this method was more effective than other techniques such as acid hydrolysis and delignification. The enzymatic hydrolysates were also tested on Trichoderma reesei Rut C-30. No inhibitory/toxic effects were apparent in terms of cell growth, sugar consumption, and cellulase and xylanase production. Optimizing the operating parameters like temperature, pressure, moisture and duration of hydrolysis can increase the sugar yield. A factorial design was performed to find the optimum conditions of temperature, pressure, moisture and duration of hydrolysis for maximum sugar yields from the supercritical CO2 pretreatment of the guayule. The yields of sugars released from the enzyme hydrolysis were used as the response variable in the central composite design used. The optimum glucose yield was 55% and pentose yield was 60% from enzyme hydrolysis. XRD analysis of the bagasse samples pretreated under different supercritical conditions confirmed the relationship between the crystallinity index and the ease of hydrolysis. SEM pictures showed the effect of explosion and hydrolysis. SEM and XRD data correlations provided additional insight into the pretreatment process.