Tolerance of Yeast to Formic Acid during Ethanol Fermentation

This study was carried out to investigate the tolerance of yeasts isolated from some Nigerian traditional fermented foods to formic acid during laboratory-scale fermentation of ethanol. A total of 27 yeast strains were isolated from burukutu, ogi, kunu and palm wine. The yeasts were screened for formic acid tolerance using spot plate technique on two culture media. One strain was selected based on its ability to tolerate up to 15 mM concentration of formic acid on Yeast Extract Peptone Dextrose Agar and was further identified as Candida tropicalis strain IFM 63517. C. tropicalis was used for fermentation of ethanol with varying concentrations of formic acid, ethanol and residual glucose concentrations which were monitored at intervals. The total viable cell count was determined using plate count technique. The highest ethanol yield of 8.36% (v/v) with a residual glucose concentration of 0.33 g/L was obtained from 0 mM formic acid (control fermentation vessel) with a total viable cell count of 8.7x109 cfu/ml, while the lowest ethanol yield of 8.00% (v/v) with a residual glucose concentration of 0.14g/L was obtained from 15 mM concentration of formic acid with a total viable cell count of 6.1x109 cfu/ml. The yeast strain used in this work exhibited a high ethanol yield despite the presence of an inhibitory compound (formic acid) when comparing the ethanol yield at its tolerance threshold (15 mM of formic acid) to the control fermentation vessel without formic acid.