Studies on Bioethanol Production with Thermo Tolerant Yeast Isolates and their Co-Cultures using African Wild Cocoyam as Feedstock

Abstract

In this work different ways of optimally producing bioethanol at various pH with thermotolerant yeasts and their cocultures using a non-human edible starchy food as feedstock was examined. African wild cocoyam, Xanthosoma roseum, sourced from abandoned farmlands in Obukpa, Nsukka, Nigeria was used as the substrate, while strains of Kluyveromyces marxianus and Pichia stipitis were used to ferment them. First the tubers were gelatinized by boiling under pressure above 100oC before hydrolysis with concentrated H2SO4. The hydrolysates were then fermented at 35oC with the thermotolerant yeasts for five days at different pH. Results obtained showed that gelatinized sample of the substrate gave optimum glucose yield when hydrolysed with 1M H2SO4 for 60 minutes. Kluyveromyces marxianus produced more ethanol than Pichia stipitis at all the four fermentation pH values tested. However, optimum ethanol production was obtained when the two yeast strains were used as coculture at pH 4.5. The peak time for ethanol production was 96 hours for the individual yeast cultures while that of their coculture was 72 hours. The results of the study indicated that wild cocoyam is an excellent feedstock for bioethanol production with many advantages including being non-edible, thereby eliminating concerns for food security, and containing high amount of carbohydrate. The study also revealed that fermenting sugar hydrolysates with a coculture of microorganisms during bioethanol production is a more efficient process than using individual cultures.