Abstract
Volatile fatty acids (VFAs) can be readily produced from many anaerobic microbes and subsequently utilized as precursors to renewable biofuels and biochemicals. Megasphaera elsdenii represents a promising host for production of VFAs, butyric acid (BA) and hexanoic acid (HA). However, due to the toxicity of these acids, product removal via an extractive fermentation system is required to achieve high titers and productivities. Here, we examine multiple aspects of extractive separations to produce BA and HA from glucose and lignocellulosic hydrolysate with M. elsdenii. A mixture of oleyl alcohol and 10% (v/v) trioctylamine was selected as an extraction solvent due to its insignificant inhibitory effect on the bacteria. Batch extractive fermentations were conducted in the pH range of 5.0 to 6.5 to select the best cell growth rate and extraction efficiency combination. Subsequently, fed-batch pertractive fermentations were run over 230 h, demonstrating high BA and HA concentrations in the extracted fraction (57.2 g/L from ~190 g/L glucose) and productivity (0.26 g/L/h). To our knowledge, these are the highest combined acid titers and productivity values reported for M. elsdenii and bacterial mono-cultures from sugars. Lastly, the production of BA and HA (up to 17 g/L) from lignocellulosic sugars was demonstrated.
Highlights
The conversion of lignocellulosic sugars to bio-based products and biofuels presents an opportunity to replace non-sustainable, petroleum-based products while avoiding direct competition with food sources
These are the highest combined acid titers and productivity values reported for M. elsdenii and bacterial mono-cultures from sugars
Accumulation of butyric acid (BA) in the broth would favor chain elongation, limits exist where increasing increasing concentrations would become inhibitory to the organism. These results show that a concentrations would become inhibitory to the organism. These results show that a liquid–liquid liquid–liquid extraction process (Figure 3D) is not sufficient to provide high recoveries of extracted extraction process (Figure 3D) is not sufficient to provide high recoveries of extracted acids at an optimal acids at an optimal pH for bacterial growth
Summary
The conversion of lignocellulosic sugars to bio-based products and biofuels presents an opportunity to replace non-sustainable, petroleum-based products while avoiding direct competition with food sources. Often enabled by the use of strains that naturally accumulate products of interest, have garnered significant interest in the emerging bioeconomy. One such strategy, the “Carboxylate Platform”—namely the production of volatile fatty acids (VFAs, C2–C8 VFAs) from biomass sources via anaerobic fermentation either through mixed cultures or mono-culture—has emerged as an attractive approach to generate high yields of promising biofuel and biochemical precursors [1,2]. Most well studied VFA-producing microbes employ anaerobic chain elongation [3,4]. Both butyric acid (BA) and hexanoic acid (HA). For downstream catalytic transformations aimed at producing biofuels, mixtures are often desirable, creating an opportunity for heterofermentative production of mixed VFAs
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