Can Xylose Be Fermented to Biofuel Butanol in Continuous Long-Term Reactors: If Not, What Options Are There?

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This study applied concentrated xylose (60–250 g/L) medium to produce butanol (acetone butanol ethanol, or ABE). A control batch fermentation of 61 g/L initial glucose using Clostridium beijerinckii P260 resulted in a productivity and yield of 0.33 g/L·h and 0.43 g/g, respectively. Use of 60 g/L xylose in a batch system resulted in productivity and yield of 0.26 g/L·h, and 0.40 g/g, respectively. In these two experiments, the culture fermented 89.3% glucose and 83.6% of xylose, respectively. When ABE recovery was coupled with fermentation for continuous solvent removal, the culture fermented all the added xylose (60 g/L). This system resulted in a productivity and yield of 0.66 g/L·h and 0.44 g/g, respectively. When the sugar concentration was further increased above 100 g/L, only a small fraction of the sugar was fermented in batch cultures without product removal. However, with simultaneous product removal, all the xylose (150 g/L) was fermented provided the culture was fed with nutrients intermittently. In this system, 66.32 g/L ABE was produced from 150 g/L xylose with a productivity of 0.44 g/L·h and yield of 0.44 g/g. Using the integrated culture system allowed sugar consumption to be increased by 300% (150 g/L). The continuous system using xylose as a feed did not sustain and after 36 days (864 h) of fermentation, it produced only 2–3 g/L ABE. Rather, the culture became acidogenic and produced 4–5 g/L acids (acetic and butyric). This study suggested that xylose be fermented in batch reactors coupled with simultaneous product recovery rather than in continuous reactors.