BACKGROUND Isobutyrate is a platform chemical that is currently produced from a non‐renewable fossil‐based feedstock. This study aimed at developing a renewable isobutyrate production process by using methanol chain elongation, a novel bioprocess that uses organic waste as primary feedstocks and an undefined reactor microbiome as the catalyst. RESULTS A continuous anaerobic bioreactor experiment was first performed using a synthetic medium containing acetate, butyrate and methanol, all of which are common derivatives from organic residues. Continuous isobutyrate (2.0 g L−1 day−1) and caproate formation (0.2 g L−1 day−1) from methanol, acetate and butyrate were demonstrated. A batch test to synthesise isobutyrate from a real organic waste, i.e. acidified supermarket waste (ASW), was performed. Up to 6.2 g L−1 isobutyrate was formed which accounted for 63% of all identified products. CONCLUSION In this study, a proof‐of‐principle for isobutyrate production from organic waste via methanol chain elongation was demonstrated. The continuous accumulation or supply of butyrate, the suppression of methanogenic activity and methanol addition were shown to be of use to provide conditions for isobutyrate formation. © 2016 Society of Chemical Industry

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chain elongation is a microbial process in which an electron donor, such as ethanol, is used to elongate short chain carboxylic acids, such as acetic acid, to medium chain carboxylic acids. this metabolism has been extensively investigated, but the spread and differentiation of chain elongators in the environment remains unexplored. Here, chain elongating communities were enriched from several inocula (3 anaerobic digesters, 2 animal faeces and 1 caproic acid producing environment) using ethanol and acetic acid as substrates at pH 7 and 5.5. This approach showed that (i) the inoculum's origin determines the pH where native chain elongators can grow; (ii) pH affects caproic acid production, with average caproic acid concentrations of 6.4 ± 1.6 g·L −1 at pH 7, versus 2.3 ± 1.8 g·L −1 at pH 5.5; however (iii) pH does not affect growth rates significantly; (iv) all communities contained a close relative of the known chain elongator Clostridium kluyveri; and (v) low pH selects for communities more enriched in this Clostridium kluyveri-relative (57.6 ± 23.2% at pH 7, 96.9 ± 1.2% at pH 5.5). These observations show that ethanol-consuming chain elongators can be found in several natural and engineered environments, but are not the same everywhere, emphasising the need for careful inoculum selection during process development.Carboxylic acids with carbon chains of six to twelve carbon atoms, also named medium chain carboxylic acids (MCCA), are a group of chemicals with a wide range of potential applications; as antimicrobial agents in feed and fodder these chemicals could replace conventional antibiotics, while they can also be used as a corrosion inhibitor 1 . Furthermore, chemical derivatization of MCCA creates additional product opportunities, e.g. esterification of MCCA can lead to a range of fragrance and flavour products for use in cosmetics and food 2 . On top of that, conversion of the carboxylic acids to their relative alcohols and alkanes, can supply solvents and fuels 3 , while Kolbe electrolysis of MCCA allows the production of long chain alkanes from MCCA, for instance decane, usable as jet-fuel 4 .Caproic acid, the shortest of the MCCA (six-carbon chain), is currently sourced from plant oils and animal fats 5 with a production capacity of 25,000 tonnes per year and a market value of approx. €2000/tonne 6 . It has generated great research interest because of the aforementioned application potential, in combination with the possibility of sustainable sourcing of caproic acid through bioproduction from waste streams, such as organic wastes and syngas 7-9 . This bioproduction occurs through the so-called reversed-β oxidation pathway, by which an electron donor, such as ethanol, is converted to acetyl-coA. This acetyl-CoA is then used in a cyclical process that converts acetic acid to butyric acid, which is in turn converted to caproic acid. This stepwise two-carbon increase of the carboxylic acid acyl-chain is why this metabolism is often named chain elongation. Clostridium kluyveri was the first isolated bacter...

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