Inhibition limits by undissociated acids in mixed culture fermentation and strategies to elevate process capacity

Using high substrate concentrations in mixed culture fermentation increases the products concentration achieved, making more cost-effective and sustainable its subsequent extraction. However, elevated levels of organic acids also result in inhibition by undissociated acids. This study investigates the concentration limits of undissociated acids that inhibit a lab-scale mixed culture carbohydrate fermenter operated under constant conditions (pH 5.4; HRT 20 h; 30 °C). Under these conditions, the conversion capacity was consistently inhibited at undissociated acids concentrations above 24.9 mM and substrate concentrations over 18.5 gCOD L_Feed⁻¹. This is attributed to the high maintenance costs incurred to maintain cell homeostasis at neutral intracellular pH in the presence of acids. This inhibition leads to lower yields and net biomass loss from the system, resulting in incomplete glucose conversion. Operating at a higher pH of 6 and utilizing granules proved to be effective strategies for expanding the conversion capacity.