Microbial elongation of short chain carboxylates : investigation with environmental inoculum and complex biomass substrate

A mixed culture of microorganisms is able to metabolize complex fermentable substrate like beet pulp (BP) into short chain fatty acids (SCFA). Some microorganisms, like Clostridium kluyveri, are able to elongate the carbon chain of SCFA to produce medium chain fatty acids (MCFA). They require a reducing equivalent source like ethanol or H2. The aim of this work is to identify the key parameters that enable the elongation of SCFA towards caprylate by a non-axenic culture under anaerobic conditions. The influence of several parameters was tested in pressurized bioreactors (PR) or in bio-electro-reactor (BER). Three substrates were used: BP, brewer spent grain (BSG) and BSG + trub. Three different pH were tested: 5,5; 6,5 and 7. Two gas phase compositions were used: H2 and N2 at 1,3 bar. Two different electron supply methods were tested: as electric current in the BER or as gas (H2) diffusing from the headspace in the liquid phase of the PR. The PR were running in three different operating modes: batch, fed-batch and semi continuous mode. The best conversion ratio (0,05 ± 0,01 gCOD_caprylate/gtotal_COD_in_the_mixed_liquor_(ML)) was obtained in the batch mode PR reactors at pH 6,5 with BP as substrate and H2 as supply gas. The fed batch reactors had the best conversion ratio (0,09 ± 0,01 gCOD_caproate/gtotal_COD_in_the_ML) under the same conditions and the semi continuous PR reactors at pH 7 with BP and H2 had the best conversion ratio (0,20 ± 0,05 gCOD_butyrate/gtotal_COD_in_the_ML) towards butyrate. Globally, BP showed better conversion ratios towards carboxylates than BSG or BSG + trub. No caprylate was observed in PR reactors at pH 5,5. BER did not give better results than pH 6,5 PR using H2 as reducing equivalent source. When replacing H2 by N2 in the gas phase, little elongation was observed and methane production occurred. Based on those experiments, the key parameters enlightened towards the production of caprylate were a batch mode PR at pH > 6,5, H2 as reducing equivalent source and feeding BP as substrate rather than BSG.