P34: Long-term conversion of n-butyrate to n-butanol with Clostridium saccharoperbutylacetonicum using a two-stage continuous culture and in-situ product removal

We are operating anaerobic bioreactors with undefined mixed cultures to convert waste lignocellulosic biomass into useful products. A traditional product of anaerobic bioreactors (i.e., anaerobic digesters) is methane. Recently, we modified bioreactor conditions to more efficiently produce a range of carboxylates, including n-butyrate rather than methane. The objective of the present study was to upgrade n-butyrate into the biofuel n-butanol via fermentation, using pure cultures of solventogenic Clostridia.

Here, we describe a two-stage continuous fermentation system with Clostridium saccharoperbutylacetonicum strain N1-4 designed for conversion of n-butyrate into n-butanol. In the first stage, glucose was added as a source of energy and reducing equivalents, while in the second stage undissociated n-butyrate was maintained at a concentration of 0.3 g/L with a pH-auxostat to induce a high solventogenic activity (i.e., production of n-butanol). We integrated a gas-stripping and condensation system in the second stage as a strategy for in-situ product removal. The two-stage fermentation successfully minimized metabolic oscillations that are common to n-butanol fermentation carried out by Clostridia. In addition, culture degeneration was avoided by periodical heat shocking and reinoculating stage 1. The average n-butyrate consumption was 0.251 g/(L*h), the conversion efficiency for n-butyrate to n-butanol was 93%, the overall n-butanol production was 0.39 g/(L*h), the molar ratio of butyrate:glucose consumed was 0.358, the molar yield Y_{n-butanol/n-butyrate} was 2.01, the molar yield Y_{n-butanol/glucose} was 0.718, and the yield Y_{n-butanol/carbon} was 0.386. This is the first report of a continuous fermentation system specifically designed and optimized for the conversion of n-butyrate into n-butanol.