Selection of Starter from Decaying Oranges and Pineapple for Bioethanol Production

Abstract

Screening of yeasts for bioethanol production is an important aspect of industrial microbiology and biotechnology. Optimal yeast nutrition also requires availability of nutrients allowing for growth and ethanol production and ability to withstand the increasing concentration of the medium during fermentation. This study seeks to use appropriate methods to isolate, characterize and identify yeasts isolates with essential attributes for bioethanol production. Microbiological and Physicochemical assessment of the isolated strains was carried out on yeast maintenance media for the determination of ethanol, temperature and salt tolerance ability, growth at different pH and temperature, chloramphenicol and nalidixic acid test, determination of killer toxins production capacity of yeast, nitrate and carbon assimilation and sugar fermentation tests were carried out to select the best starter for production.
 Fifteen yeasts were isolated belonging to the genera Saccharomyces, Candida, Rhodotorula, Kluvyreomyces, Trichosporon, Pichia. S. cerevisae and K. marxianus showed efficient physico-chemical attributes. Proximate analysis of fruits juice showed a moisture content between 81.83 – 86.37%, crude protein 1.33 – 2.00%, Ash 0.73 – 0.83%, carbohydrate 11.87 – 15.67% and Ether 0.1 – 0.5%. The total sugar ranged from 1.83 – 13.17, equal mixture of oranges and pineapple juice showed a sugar content of 15 which reduced during fermentation while the alcohol content increased. Organisms were able to tolerate a percentage of 4-30% ethanol content, tolerating 15% sodium chloride which is an index of osmotolerance. At higher concentration, growth reduced, organisms produced catalase, reduced nitrate and showed a variation in utilization of different sugars.
 At the end of the screening, S. cerevisiae and K. marxianus showed the best attribute essential for bioethanol production and were chosen as starters. The ability of the selected yeast isolates to produce Killer toxins against E. coli showed a negative result. Antibiotic sensitivity test was carried out on the selected yeast isolates using chloramphenicol and nalidixic and the two organisms were resistant to the antibiotics tested.
 The ability of the organisms to grow in changing environmental conditions and ethanol tolerance are attributes essential for production while Saccharomyces cerevisiae showed the highest attribute followed by Kluveromyces marxinus. This study concluded that Saccharomyces cerevisiae can be employed as starter in the industry for the production of bioethanol and in the conversion of agricultural waste to wealth.