(43a) Supercritical CO2 Based Pretreatment of Guayule Biomass for High Sugar Yield and Low Inhibition Hydrolysate

Guayule (Parthenium argentatum Gray) is a commercial crop for hypoallergenic latex production. Latex (rubber) and resins, however, make up only about 20% of guayule biomass, rest of which, already powderized, is potentially a good lignocellulosic source for biorefinery. Lignocellulosic biorefinery has three common steps: pretreatment, enzymatic hydrolysis and fermentation. In the pretreatment step, the biomass is chemically and/or physically treated to help advance the enzymatic hydrolysis efficiency. In our study supercritical CO₂ based pretreatment was found very effective to achieve higher monomeric sugar yield in enzymatic hydrolysis. This pretreatment involved: raising system temperature, adding water to the bagasse and load them into the reactor, pressurizing using CO₂, holding the system for a period of time, and the quick release of pressure. The supercritical pretreatment preserved essentially all carbohydrate without losing, e.g., hemicellulose sugars as in the widely used dilute acid pretreatment. It had both reaction effect and explosion effect. Because of higher temperature and mildly acidic condition (carbonic acid) in this pretreatment, some fermentation inhibitors such as furfural, hydroxymethyl furfural, and acetic acid could be produced. In this study we optimized the pretreatment conditions for minimal degradation/inhibition products and maximal sugar yield in subsequent enzymatic hydrolysis. It was found that at harsher condition reaction effect dominated without apparent explosion effect. Higher temperature (>180⁰C) and pressure (>12.2 MPa) and longer time (>30 min) gave higher sugar yield: as high as 80% of glucose, xylose and total reducing sugar were obtained in the enzymatic hydrolysate. But these conditions also caused more degradation: per g total sugar released, 4-6 mg HMF, 16-19 mg furfural, and 160-200 mg acetic acid were found in hydrolysate. With shorter pretreatment time, the explosion effect was observed and degradation was even less: per g total sugar released, 1-2 mg HMF, 5-6 mg furfural and 65-75 mg acetic acid were formed. Results show that HMF and furfural production by this pretreatment method is very low and the acetic acid produced is below the minimum inhibitory concentration for yeast. In conclusion the supercritical CO₂ based pretreatment is very effective for preparing guayule biomass for enzymatic hydrolysis, to give high fermentable sugar yield and very low degradation to inhibitory compounds and it does not involve use of hazardous solvent.