PRETREATMENT OF LIGNOCELLULOSIC SUBSTRATES BY PLEUROTUS AND LENTINUS SPECIES FOR PRODUCTION OF BIOETHANOL USING SACCHAROMYCES CEREVISIAE

Abstract

Generation of bioethanol from food crops is a well-established industrial process. However, this poses serious challenge to global food security. Lignocellulosic substrates are possible alternatives for production of bioethanol, but there is need for their pretreatment, which may be expensive. Biological pretreatment is recognised as a low cost technique. However, there is paucity of information in literature for bioethanol generation from biologically pretreated lignocellulosic wastes. Therefore, this study was designed to convert lignocellulosic substrates waste for the production of bioethanol. Laboratory grown mushroom strains of Pleurotus ostreatus (PO) and Lentinus squarrosulus (LS) were screened for production of cellulase, xylanase and lignase using solid agar. Yeasts were isolated from palm wine and screened for ethanol production through gas evolution. The selected yeasts were genotypically and phenotypically characterised. Lignocellulosic substrates (groundnut shell, maize cob, maize stalk, sugarcane bagasse and rice straw) were degraded with PO and LS singly and in consortium (POLS) for 70 days, during which residual cellulose, hemicellulose, lignin and the reducing sugar content were determined at 7 days intervals using standard methods. The best substrate of the lot was equally pretreated with NaOH prior to degradation by the better mushroom and its sugar profile determined using HPLC. This was fermented with selected yeasts for bioethanol production. The effects of pH, temperature, sugar concentration, nitrogen sources, inoculum load and incubation period for optimum bioethanol production were determined. Data obtained were analysed using descriptive statistics. Pleurotus ostreatus and LS hydrolysed lignocellulose with hydrolytic zones (mm) of 35, 41 (cellulase); 35, 52 (xylanase); and 18, 31 (lignase), respectively. Sixty-four yeasts were obtained out of which two Saccharomyces cerevisiae (SA01 and SA02), had better carbon dioxide height (2 mm/hour). Highest maize stalk degradation of cellulose (5.60 %), hemicellulose (33.40 %), lignin (18.42 %) and highest reducing sugar (16.89 mg/g) were recorded in PO-degraded maize stalk, POLS-degraded maize stalk, POLS-degraded maize stalk and PO-degraded maize stalk at 42, 28, 7 and 21 days of degradation, respectively. The reducing sugar of alkaline PO-pretreated maize stalk was higher than that of PO-pretreated maize stalk. The sugar profile of the alkaline PO-pretreated maize stalk included (mg/100g) glucose (850.60), xylose (837.04), fructose (754.29), arabinose (502.76), ribose (2.066 x 10-4) and rhamnose (3.552 x 10-5). Higher ethanol (1.97 g/L) was recorded at pH 5.5 by both SA01 and SA02. At 30 °C, SA01 produced higher ethanol content (2.76 g/L) compared to SA02 (2.37 g/L). Supplementation with 2% glucose gave ethanol yield of 3.95 g/L by SA02. Corn steep liquor improved ethanol yield of SA01 (14.20 g/L) and SA02 (13.41 g/L) with 1% of 1.0 MacFarland standard inoculum load. The highest ethanol content (14.99 g/L) was produced by SA01 at pH 5.5, 30°C, 2% glucose supplementation, corn steep liquor and 1% of 1.0 MacFarland standard inoculum load after 72 hours of fermentation. Bioethanol was successfully obtained through fermentation of lignocellulosic substrates, with the maize stalk found to be the best substrate. Pleurotusostreatus and Saccharomyces cerevisiae could be employed in the conversion of lignocellulosic substrates into ethanol.