Oxalic acid pretreatment, fungal enzymatic saccharification and fermentation of maize residues to ethanol
Dilute oxalic acid pretreatment was investigated as a feasible method for conversion of maize residues into sugars and subsequent production of bioethanol. Oxalic acid concentration of 200 mM for samples treated at 160°C for 10 min was found to be the best pretreatment conditions. Cellulases, mainly cellobiohydrolases from Trichoderma reesei, recorded activities of up to 116 U/ml during saccharification of the cellulosic substrates. The optimum saccharification conditions using 0.2% (v/v) enzyme concentrate were the incubation temperature of 45°C for 48 h. Up to 46% (w/w) glucose and 28% (w/w) xylose were obtained from the pretreated maize residues (stalk, leaf, husk and cobs). Furthermore, a native Saccharomyces cerevisiae (strain KB) was able to convert 78% (w/w) of glucose (and other fermentable sugars) to ethanol after 60 h of incubation at 32°C, under stationary culture conditions. The challenges encountered in chemical and biological conversions included incomplete hydrolysis and fermentation due to substrate recalcitrance and the inability of the yeast cells to utilize 5-carbon sugars such as xylose. This study therefore provided baseline research data and information that could be used for more elaborate and scaled-up studies for possible industrial-scale conversion of the readily available crop residues into bioenergy.
Key words: Bioethanol, cellulases, fermentation, hydrolysates, maize residues, oxalic acid, pretreatment, saccharification.