Long-Term <i>n</i>‑Caproic Acid Production
from Yeast-Fermentation Beer in an Anaerobic Bioreactor with Continuous
Product Extraction
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Abstract
Multifunctional reactor microbiomes
can elongate short-chain carboxylic
acids (SCCAs) to medium-chain carboxylic acids (MCCAs), such as <i>n</i>-caproic acid. However, it is unclear whether this microbiome
biotechnology platform is stable enough during long operating periods
to consistently produce MCCAs. During a period of 550 days, we improved
the operating conditions of an anaerobic bioreactor for the conversion
of complex yeast-fermentation beer from the corn kernel-to-ethanol
industry into primarily <i>n</i>-caproic acid. We incorporated
and improved in-line, membrane liquid–liquid extraction to
prevent inhibition due to undissociated MCCAs at a pH of 5.5 and circumvented
the addition of methanogenic inhibitors. The microbiome accomplished
several functions, including hydrolysis and acidogenesis of complex
organic compounds and sugars into SCCAs, subsequent chain elongation
with undistilled ethanol in beer, and hydrogenotrophic methanogenesis.
The methane yield was 2.40 ± 0.52% based on COD and was limited
by the availability of carbon dioxide. We achieved an average <i>n-</i>caproate production rate of 3.38 ± 0.42 g L<sup>–1</sup> d<sup>–1</sup> (7.52 ± 0.94 g COD L<sup>–1</sup> d<sup>–1</sup>) with an <i>n</i>-caproate yield
of 70.3 ± 8.81% and an <i>n</i>-caproate/ethanol ratio
of 1.19 ± 0.15 based on COD for a period of ∼55 days.
The maximum production rate was achieved by increasing the organic
loading rates in tandem with elevating the capacity of the extraction
system and a change in the complex feedstock batch