Electro-fermentation and redox mediators enhance glucose conversion into butyric acid with mixed microbial cultures.

Electro-fermentation (EF) is an emerging and promising technology consisting in the use of a polarized electrode to control the spectrum of products deriving from anaerobic bioprocesses. Here, the effect of electrode polarization on the fermentation of glucose has been studied with two mixed microbial cultures, both in the absence and in the presence of exogenous redox mediators, to verify the viability of the proposed approach under a broader and previously unexplored range of operating conditions. In unmediated experiments, EF (with the cathode polarized at -700 mV vs. SHE, Standard Hydrogen Electrode) caused an increase in the yield of butyric acid production provided that glucose was consumed along with its own fermentation products (i.e. acetic acid and ethanol). The maximum obtained yield accounted for 0.60 mol mol-1. Mediated experiments were performed with Neutral Red or AQDS at a concentration of 500 μM both in the absence and in the presence of the electrode polarized at -700 mV or -300 mV vs. SHE, respectively. Mediators showed a high selectivity towards the generation of n-butyric acid isomer from the condensation of acetate and ethanol, hence suggesting that they provided microbial cells with the required reducing power otherwise deriving from glucose in unmediated experiments.