Comparison of ethanol production by Zymomonas mobilis from sugar beet substrates

Abstract

The potential of four sugar beet substrates from the sugar industry [syrup (S), crystallizer effluent 1 (CE1), crystallizer effluent 2 (CE2) and molasses (M)] were compared for ethanol production using an osmotolerant mutant strain of the bacterium Zymomonas mobilis. Sucrose of the substrates was enzymatically hydrolysed to avoid levan formation during fermentation. Nutrient supplementation experiments have shown that reproducible growth and ethanol production could be obtained on the four substrates supplemented only with magnesium sulphate (CE2 and M) or additionally with ammonium sulphate (S and CE1). Thus, addition of costly yeast extract could be avoided. All 20% (w/v) substrates showed nearly complete sugar conversion (>94.9%), good growth (0.16 h(-1)) and ethanol production (>40 g 1(-1)). However, sorbitol formation reduced the ethanol yield (73-79% of the theoretical value) significantly. Batch kinetic parameters and studies of instantaneous parameters showed that enhanced osmolality of substrates (S<CE1<CE2<M) inhibited biomass production more strongly than ethanol production. In conclusion, all four sugar beet substrates could be utilized for ethanol production using this mutant strain of Z. mobilis with appropriate supplementation.