Extractive biobutanol fermentation using immobilized cells with organic acid esters as novel co-substrates in large extractant volume

Organic acid co-substrates increase butanol production; however, their toxicity to cells limits their utilization during fermentation. In the ester form, the substrate is storable in the extractant as fermentation feedstock. This study investigated butanol fermentation by immobilized cells in a large extractant volume, using glucose and esters as co-substrates. By employing ethyl butyrate at a substantial co-substrate to glucose ratio of 1.4, an impressive total butanol of 50.1 g/L was achieved. In contrast, when utilizing acids, this ratio yielded only 10.9 g/L. Both batch and fed-batch fermentations incorporating ester feeding exhibited comparable total butanol concentrations, affirming the gradual release of esters into the medium while ensuring their concentration remained below the toxicity threshold. A high butanol yield of 0.64 was obtained using ethyl butyrate as co-substrate, reducing the sugar required to produce butanol. This result could be applied for the utilization of butyrate ester to obtain high butanol yield.