Optimization of bio-ethanol production from cassava effluent using <i>Saccharomyces cerevisiae</i>

Abstract

The ethanol producing capability of Saccharomyces cerevisiae from cassava waste water was enhanced. Enhancement was obtained by varying pH, culture age and nutrient supplementation during fermentation until optimum conditions were attained. Optimum fermentation duration for ethanol production in acid and enzyme hydrolysates was 24 h. The start-up fermentation experiment process with S. cerevisiae resulted in 1.47 and 1.00% (v/w) ethanol in acid and enzyme hydrolysates, respectively. Appropriate adjustment in pH however, yielded 3.60 and 1.88% (v/w) ethanol in both acid and enzyme hydrolysates of cassava waste water. The pH value that resulted in optimum ethanol production by the test isolate was 5.5. Furthermore, culture age that resulted in highest ethanol yield was 3 days old culture. Consequently, when 3 days old cultures were employed in fermentation of acid and enzyme cassava waste water hydrolysates, ethanol generated increased to 3.61 and 2.91% (v/w), respectively. The addition of NPK fertilizer, NaNO₃ and K₂HPO₄ salts to each fermentor further enhanced the glucose utilization capacity of S. cerevisiae and concomitant ethanol generation such that ethanol produced increased to 14.5, 8.15 and 5.74 (%v/w) in acid hydrolysate containing NPK fertilizer, NaNO₃ and K₂HPO₄ salt supplements, respectively. The corresponding ethanol yield in enzyme hydrolysates were 8.30, 5.10 and 3.00 (%v/w) respectively. All the three nutrient supplements proved suitable for enhancing the fermentative capability of S. cerevisiae in a decreasing order NPK > K₂HPO < NaNO₃. The results reveal that optimum combinations of pH, nutrient concentrations and cultural status play a major role in getting maximum bioconversion of cassava waste water to ethanol.

Keywords: Saccharomyces cerevisae, cassava wastewater, hydrolysates, enhancement, nutrient supplement