Sustainable Production of Bioethanol by Zymomonas mobilis and Saccharomyces cerevisiae using Rice Husk and Groundnut Shell as Substrates

Background of Study: Plant waste such as rice husk and groundnut shell are generated in large amounts, these waste presents a tremendous pollution to the environment. Worldwide, these wastes are often simply dumped into landfills and oceans or used as animal feeds. The recovery of food processing wastes as renewable energy sources represents a sustainable option for the substitution of fossil energy in order to minimize environmental damages and to meet energy demands of the growing population.

Aim: To produce bioethanol from rice husk and groundnut shell using local strains of Zymomonas mobilis and Saccharomyces cerevisiae.

Place and Duration of Study: Conducted at the Microbiology Laboratory of Abubakar Tafawa Balewa University Bauchi, Bauchi state, Nigeria, between April to June, 2021.

Methods: Groundnut shell and Rice husk were collected from local milling center. The wastes were powdered, sieved and used as carbon source. Proximate composition of the subsrate was done and the total carbohydrate was determined by difference. The sum of the percentage moisture, ash, crude lipid, crude protein and crude fibre was subtracted from 100. Zymomonas mobilis and Saccharomyces cerevisiae were isolated from rotten sweet oranges and locally fermented beverage (‘kunun-zaki’) respectively by growing them on Malt Yeast Peptone Glucose Agar (MYPGA) after which they were further screened for their ability to tolerate ethanol and they serve as organisms for fermentation. The enzyme α- amylase was used for hydrolysis. The fermented substrates were distilled at 78oC and the distillate was collected as bioethanol in a conical flask. UV-VIS spectrophotometer was used to determine the absorbance of each concentration (0, 0.2, 0.4, 0.6 and 0.8cm3) of reducing sugar content of the hydrolysates and the bioethanol produced by developing a standard curve at a wavelength of 491nm and 588nm respectively. The concentration of reducing sugar and bioethanol was determined using a reference line from the Standard curve.

Results: Proximate analysis done shows that rice husk have 70.09% carbohydrates while groundnut shell has 65.09% carbohydrates. Groundnut shell yielded the highest reducing sugar of 5.096%. Rice husk yielded the lowest quantity of reducing sugar with a total yield of 2.962%. Maximum concentration of bioethanol of 0.971% was produced from the combination of Saccharomyces cerevisiae and Zymomonas mobilis from groundnut shell. The lowest concentration of 0.121% of bioethanol was produced when Saccharomyces cerevisiae was used on rice husk hydrolysates. The synergistic relationship of Saccharomyces cerevisiae and Zymomonas mobilis yielded the maximum bioethanol when compared with the yield obtained when the organisms were used singly. Zymomonas mobilis produced highest bioethanol content when the organisms are used single.

Conclusion: This study demonstrates the potentiality of local strains of Saccharomyces cerevisiae and Zymomonas mobilis isolated from rotten sweet orange and locally fermented beverage (‘kunun-zaki’) to produce bioethanol by fermenting the rice husk and groundnut shell hydrolysates.