11-24: Anhydrous ammonia treatment of corn stover and its effects on enzymatic digestibility

In the conversion of biomass to bioethanol, pretreatment of biomass is a key step intended to render cellulose more amenable and accessible to cellulase enzymes and thus, increase glucose yields.  In this study, extractives-free (EFCS) and deacetylated corn stover (DCS) were subjected to liquid ammonia treatments at 25 and 130°C, respectively.  The effects of anhydrous liquid-ammonia treatment on the crystalline structure of cellulose were studied by X-ray diffraction. From the X-ray diffractograms it was revealed that treatment with liquid-ammonia resulted in transformation of the cellulose I crystal structure to the cellulose III_I allomorph. The relative digestibilities of the EFCS and DCS were tested by exposing the untreated and liquid-ammonia treated corn stover samples to a commercial enzyme mixture (Genencor-Danisco; GC 220 at an enzyme loading of 20 mg enzyme /g glucan content).  The digestibility results showed that for both EFCS and DCS, treatment with liquid-ammonia at 130°C produced the most digestible cellulose, followed by treatment at 25°C. A maximum cellulose conversion of about 90% and 95% was obtained for the liquid-ammonia treated (130°C) DCS and EFCS, respectively after 96 h. A three-fold enhancement in the cellulose conversion was achieved for the liquid-ammonia treated DCS whereas for the liquid-ammonia treated EFCS a more than four-fold enhancement in the cellulose conversion was achieved when compared to the untreated corn stover samples. These results show that liquid-ammonia treatment can significantly overcome the recalcitrance of biomass and increase the digestibility of the substrate.