Effect of hydrogen and carbon dioxide on carboxylic acids patterns in mixed culture fermentation

► This study focuses on the effect of H2 and CO2 on the fermentation product spectrum. ► Headspace composition affects the hydrolysis degree rate but not the hydrolysis degree. ► H2 pressure up to 2 bars does not limit acetate and butyrate production. ► Up to 75% butyrate is selectively produced under a CO2 headspace. ► Propionate production can be ceased by H2 or CO2.