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 studiedwith twomixed 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−700mV vs. SHE, Standard Hydrogen Electrode) caused an increase in the yield of butyric acid production
provided that glucosewas consumed alongwith 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.