Continuous ethanol production byZymomonas mobilis andSaccharomyces cerevisiae in biofilm reactors

Abstract

Continuous ethanol fermentations were performed in duplicate for 60 days withZymomonas mobilis ATCC 331821 orSaccharomyces cerevisiae ATCC 24859 in packed-bed reactors with polypropylene or plastic composite-supports. The plastic composite-supports used contained polypropylene (75%) with ground soybean-hulls (20%) and zein (5%) forZ. mobilis, or with ground soybean-hulls (20%) and soybean flour (5%) forS. cerevisiae. Maximum ethanol productivities of 536 gL⁻¹ h⁻¹ (39% yield) and 499 gL⁻¹ h⁻¹ (37% yield) were obtained withZ. mobilis on polypropylene and plastic composite-supports of soybean hull-zein, respectively. ForZ. mobilis, and optimal yield of 50% was observed at a 1.92h⁻¹ dilution rate for soybean hull-zein plastic composite-supports with a productivity of 96gL⁻¹h⁻¹, whereas with polypropylene-supports the yield was 32% and the productivity was 60gL⁻¹h⁻¹. With aS. cerevisiae fermentation, the ethanol production was less, with a maximum productivity of 76gL⁻¹h⁻¹ on the plastic composite-support at a 2.88h⁻¹ dilution rate with a 45% yield. Polypropylene-support bioreactors were discontinued due to reactor plugging by the cell mass accumulation. Support shape (3-mm chips) was responsible for bioreactor plugging due to extensive biofilm development on the plastic composite-supports. With suspensionculture continuous fermentations in continuously-stirred benchtop fermentors, maximum productivities of 5gL⁻¹h⁻¹ were obtained with a yield of 24 and 26% withS. cerevisiae andZ. mobilis, respectively. Cell washout in suspensionculture continuous fermentations was observed at a 1.0h⁻¹ dilution rate. Therefore, for continuous ethanol fermentations, biofilm reactors out-performed suspension-culture reactors, with 15 to 100-fold higher productivities (gL⁻¹h⁻¹) and with higher percentage yields forS. cerevisiae andZ. mobilis, respectively. Further research is needed with these novel supports to evaluate different support shapes and medium compositions that will permit medium flow, stimulate biofilm formation, reduce fermentation costs, and produce maximum yields and productivities.