Background:
In converting biomass to bioethanol, pretreatment is a key step intended to render cellulose more amenable and accessible to cellulase enzymes and thus, increase glucose yields. In this study, four cellulose samples with different degrees of polymerization and crystallinity indexes were subjected to aqueous NaOH and anhydrous liquid ammonia treatments. The effects of the treatments on cellulose crystalline structure were studied, in addition to the effects on the digestibility of the celluloses by a cellulase complex.
Results:
From X-ray diffractograms and NMR spectra, it was revealed that treatment with liquid-ammonia produced the cellulose III allomorph; however, crystallinity depended on treatment conditions. Treatment at low temperature (25degreesC) resulted in a less crystalline product, whereas treatment at elevated temperatures (130degreesC or 140degreesC) gave a more crystalline product. Treatment of cellulose I with aqueous NaOH (16.5 wt%) resulted in formation of cellulose II, but also produced a much less crystalline cellulose. The relative digestibilities of the different cellulose allomorphs were tested by exposing the treated and untreated cellulose samples to a commercial enzyme mixture (Genencor-Danisco; GC 220). The digestibility results showed that the starting cellulose I samples were the least digestible (except for corn stover cellulose, which had a high amorphous content). Treatment with NaOH produced the most digestible cellulose, followed by treatment with liquid-ammonia at low temperature. Factor analysis indicated that initial rates of digestion (up to 24 h) were most strongly correlated with amorphous content. Correlation of allomorph type with digestibility was weak, but was strongest with cellulose conversion at later times. The cellulose III samples produced at higher temperature had comparable crystallinities to the initial cellulose I samples, but achieved higher levels of cellulose conversion, at longer digestion times.
Conclusions:
Earlier studies have focused on determining which cellulose allomorph is the most digestible. In this study we have found that the chemical treatments to produce different allomorphs also changed the crystallinity of the cellulose, and this had a significant effect on the digestibility of the substrate. When determining the relative digestibilities of different cellulose allomorphs it is essential to also consider the relative crystallinities of the celluloses being tested.Source:
http://www.biotechnologyforbiofuels.com/rss/