Statistically designed cellulase mixture for saccharification of pretreated Sorghum durra stalk

Abstract

The potential of least explored Sorghum durra as a feedstock for enzymatic saccharification in bioethanol production was evaluated as only one report is available. The pretreatment specific, optimization of saccharification of Sorghum durra stalk (Sds) was carried out. The concentrations of crude recombinant endoglucanase (BaGH5-UV2), cellobiohydrolase (CtCBH5A) and β-glucosidase (CtGH1) in cocktail were optimized for saccharification of acid or base pretreated Sds by using mixture design approach. Optimized percentage ratio of BaGH5-UV2, CtCBH5A and CtGH1 with total enzyme loading 2000 U/g of pretreated Sds for acid pretreated Sds was 43.1:10.0:46.9 and for base pretreated Sds was 80:10:10 at 48 h of saccharification. The base pretreated Sds showed 1.8-fold (165.6 mg/g of pretreated Sds) higher total reducing sugar (TRS) yield and 1.2-fold (81.7 mg/g of pretreated Sds) higher glucose yield than acid pretreated Sds. This low sugar yield from acid pretreated Sds was due to the presence of higher acid insoluble lignin content, 11.9 % (w/w) than the base pretreated biomass (2.2 %, w/w). In acid pretreated Sds, ANOVA data showed 3.7-fold higher F value (111.58) for BaGH5-UV2 and CtGH1 interaction than the base pretreated Sds. This indicated that the synergism between BaGH5-UV2 and CtGH1 for acid pretreated Sds was significantly higher. This was due to lower intact hemicellulose content (10.6 %, w/w) in acid pretreated Sds than base pretreated Sds (21.8 %, w/w). The optimized enzyme ratios for saccharification of acid pretreated Sds gave maximum TRS and glucose yield at 72 h, 113 and 85 mg/g of pretreated Sds, respectively, unlike base pretreated Sds at 96 h, 211 and 174 mg/g of pretreated Sds, respectively. These results showed that the ratio of cellulase enzymes in cocktail and their synergism depends on the type of pretreatment used for biomass. Thus, this in-house cocktail design specific to pretreatment can improve the biomass saccharification at large scale.