Isolation and characterization of ethanol-tolerant yeasts from decaying oranges for the production of bioethanol

Abstract

Decaying orange fruits are readily available agricultural waste in Nigeria, yet they seem to be underutilized as potential growth medium for local yeasts strains, despite their rich carbohydrate content that can support yeast growth. The ability of different strains of yeasts to grow on orange biomass and its utilization as raw material for the production of bio-ethanol was investigated in this study. Five replicate samples of decaying oranges were collected at different time intervals from dump sites at five different local markets in Ile-Ife from which different species of yeasts were isolated. Standard biochemical tests were conducted to identify and confirm their identities. Fermentation of orange juice was done with the yeast strains singly and in combination at 30°C for seven days and parameters essential for ethanol production such as titratable acidity, pH, specific gravity, Brix and Ethanol tolerance and content were determined following Association of Official Analytical Chemist procedures. Data were analysed using ANOVA at p=0.05. A total of thirty yeast isolates were isolated and identified as Saccharomyces cerevisiae, Schizosaccharomyces japanicus, Candida valida, Candida fructus, Candida krusei, Kluyveromyces africanus and Rhodotorula gramis. Abundant production of bioethanol was derived from S. cerevisiae, C. fructus and C. valida (C. fructus and C. valida were used singly while C. fructus and Saccharomyces were combined in the ratio 1:1). The highest alcohol value (7.65%) was derived from the mixed strain (Candida fructus and Saccharomyces cerevisiae) and the lowest value of alcohol (6.98%) from a single culture of Candida fructus. Decaying oranges, its juice and its biomass could be used to produce bio-ethanol which is of a good quality. The method is effective and efficient with high yield eco-friendly ethanol. The fruit waste which provides readily available and cheap substrates for industrial use and at the same time solves the problem of environmental pollution that the decaying oranges cause if not disposed appropriately.