Biological pretreatment: a feasible environment friendly method for pretreatment, lignin recovery and ethanol production from paddy straw

Conversion of biomass entails four main steps: pretreatment, enzymatic hydrolysis, fermentation and ethanol recovery. Of these, pretreatment and enzymatic saccharification entail most cost. Pretreatment, which disintegrates Lignin-Carbohydrate Complex (LCC) and renders the carbohydrates vulnerable to enzyme attack, is the most critical step and determines the success and efficiency of processes downstream. Thermo-chemical methods involve harsh conditions and generate harmful byproducts which inhibit subsequent enzymatic saccharfication and fermentation. Alternatively, biological delignification, an environment friendly approach involving lignin degradation using microorganisms, can be applied to disrupt LCC. In this report, we pretreated paddy straw of aromatic rice (Pusa 2511) under solid state fermentation using hyperlignolytic, low cellulolytic fungi Trametes hirsuta, Myrothecium roridum at 30 ºC for seven days and extracted with mild alkali to remove partially degraded lignins. Upto 70% holocellulose enrichment in washed biologically pretreated solids was achieved and upto 25% of original lignin recovered from acidified extracts. Enzymatic hydrolysis of washed solids with Accellerase^®1500 (Genencor) yielded enhanced sugars with 55-100% efficiency.

Further, enzymatic hydrolysates were subjected to fermentation for ethanol production. Hydrolysis of Trametes pretreated paddy straw was carried out at ~10% wet solids loading, corresponding to 2% Cellulose (dry basis), with Accellerase^®1500 corresponding to (0.5 ml (~15 FPU)/ g glucan) yielded 1.3% sugar slurry. Hydrolysates, supplemented with minerals and 0.1% yeast extract, supported growth of Saccharomyces cerevisia LN well, complete sugar utilization and ethanol production and favorably compared with controls containing defined mineral medium and glucose. Thus, biological pretreatment is a feasible environment-friendly process for biomass conversion and ethanol production from paddy straw.