(626a) Pseudo-Continuous Fermentation Using a Novel Bioreactor to Facilitate the Study of a Co-Culture System for Ethanol Production

Keywords: bioreactor,
co-culture, fermentation, ethanol production, co-fermentation, bioethanol,
lignocellulose, mixed substrate, P.
stipitis, S. cerevisiae

The bioethanol industry has
developed rapidly in recent years to cope with the rising cost of fossil fuel.
The conversion of lignocellulosic materials into ethanol has been regarded as
one of the most promising renewable energy sources. This is not only because of
the moderate cost and high availability of lignocellulosic biomass, but also
because lignocellulosic ethanol process can greatly reduce CO₂
emission [1]. Glucose and xylose are two major components in the
lignocellulosic hydrolysates. While glucose fermentation is well established,
xylose fermentation remains an unsolved problem in the industrialized
lignocellulosic ethanol process [2]. In existing studies of ethanolic
fermentation, besides recombinant strategy, co-culture strategy has drawn much
research interest in the past few decades [3-7]. However, currently there is a
lack of systematic study on the properties of the co-culture system.

In this work, we focus on the
co-culture system consisting of Saccharomyces
cerevisiae and Pichia stipitis. One of the major difficulties of the co-culture
strategy for simultaneous glucose/xylose fermentation is the inability to
provide the optimal fermentation conditions for two different strains [8-10].
Most of studies on co-culture process reported that while the fermentation of
glucose in the mixed substrate proceeded efficiently with a glucose-fermenting
yeast strain, the fermentation of xylose with a xylose-fermenting strain often
exhibited low ethanol productivities due to the differences in the fermentative
regulation for ethanol production [11-15]. Another difficulty of co-culture
strategy is the diauxic growth of P.
stipitis due to its preferential utilization of glucose over xylose when
cultured on mixed substrates [7, 11,16].

In order to address these
difficulties and to improve the co-culture process efficiency, we have
developed a novel bioreactor which separates the two different yeast strains,
and enables different fermentative conditions for different strains. In
addition, we have built a cell retention module which enables the
pseudo-continuous fermentation mode, i.e. continuous fermentation with cell
retention. With pseudo-continuous operation, we can completely eliminate the
possible wash-out associated with continuous fermentation and can easily test
different operation conditions. In this study, we show that pseudo-continuous
operation using the developed bioreactor provides an effective tool to study
co-culture systems, which allow us to investigate the dynamic of co-culture
system for lignocellulosic ethanol production systemically.

References

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