Bioethanol (Bio-Fuel) Production from Agro-Waste using Yeast Isolates from Nigeria
A Thesis Submitted to the School of Postgraduate Studies, University of Lagos
Bio-fuels are fuels derived from biological materials or their by-products such as agricultural waste. A study was carried out to assess the production of bio-fuel (bioethanol) from some hydrolyzed agricultural wastes using yeast species isolated from Cola acuminata, Ipomoea batatas, Manihot esculenta, Pennisetum glaucum, Sorghum bicolor, Solanum tuberosum, Zea mays and palmwine (from Elaeis guineensis). Eight yeast species including Candida krusei, Candida tropicalis strain A, Candida tropicalis strain B, Kluyveromyces marxianus, Pichia caribbica, Saccharomyces cerevisiae strain A, Saccharomyces cerevisiae strain B and Schizosaccharomyces pombe were used. The agro-wastes and starchy substrates were pretreated through milling, saccharification with mineral acid (H2SO4/HCl) and Anacardium occidentale (cashew) Nut Shell Extract (CNSE). Substrates were distilled after 72 h of fermentation. The quantity of bioethanol produced varied with substrates and organisms used. The analysis of the CNSE revealed the presence of hydrolytic enzymes such as endoglucanase, exoglucanase and xylanase in varying concentrations. Biochemical analysis and DNA sequencing revealed that some of the fungal species are probably new strains. Using 100 g hydrolyzed substrates, the highest amount (33.34 ± 2.81 g/L) of bioethanol was obtained from sweet potato tuber hydrolyzed with 50 ml of 50% HCl and fermented with S. pombe. Plantain peel hydrolyzed with CNSE and fermented with S. pombe produced 28.12±1.61 g/L of bioethanol. The quantity of reducing sugar in the agro-waste hydrolyzed with CNSE was maximum of 491 mg/g in sugarcane chaff and minimum of 46 mg/g in rice husk. Sugar fermentation test of the yeasts showed that they can ferment sugars such as glucose, lactose, maltose, fructose and xylose. Lactose and xylose were fermented atypically by S. cerevisiae strain B. All strains tested were resistant to 30 µg/l of chloramphenicol and they were able to grow at 37°C except Candida krusei. In the urease hydrolysis test, S. pombe, P. caribbica and S. cerevisiae strain B were positive. Growth study of the yeast strains on eight broth media showed that potato dextrose broth, malt peptone broth and millet dextrose broth are the best media for growth and reproduction. This is probably the first report of isolation and characterization of fermenting yeast from maize, kolanut and sweet potato in Nigeria. Gas chromatographic (GC) assay of the CNSE hydrolysates showed the presence of simple sugars in varying concentrations with cassava peel possessing the highest concentration of glucose (38.19 mg/L) while plantain peel yielded the lowest concentratiom of 5.44 mg/L. The GC analysis of some of the agro-waste distillates showed considerable concentrations of bioethanol. Highest concentration of 2.41 x 104 mg/L was obtained from sweet potato peel fermented with S. cerevisiae strain A whereas the lowest concentration of 2.75x103 was obtained from plantain peel fermented with S. pombe. The cashew nutshell extract was able to biodegrade the agro-wastes due to the synergy of the cellulases that are present in it. This study showed that agro-waste which are diverse and commonly pose significant disposal problems can be used for the production of bio-fuel and other organic compounds. Fossil fuel causes environmental pollution and replacing it with bio-fuel like bioethanol from agro-waste will be more environmental friendly and will also reduce pressure on crop plants.