a Through the use of high partial pressures of CO2 (pCO(2)) to facilitate temporary pH reductions in two-phase partitioning bioreactors (TPPBs), improved pH dependent partitioning of butyric acid was observed which achieved in situ product recovery (ISPR), alleviating end-product inhibition (EPI) during the production of butyric acid by Clostridium tyrobutyricum (ATCC 25755). Through high pressure pCO(2) studies, media buffering effects were shown to be substantially overcome at 60 bar pCO(2), resulting in effective extraction of the organic acid by the absorptive polymer Pebax (R) 2533, yielding a distribution coefficient (D) of 2.4 +/- 0.1 after 1 h of contact at this pressure. Importantly, it was also found that C. tyrobutyricum cultures were able to withstand 60 bar pCO(2) for 1 h with no decrease in growth ability when returned to atmospheric pressure in batch reactors after several extraction cycles. A fed-batch reactor with cyclic high pCO(2) polymer extraction recovered 92 g of butyric acid to produce a total of 213 g compared to 121 g generated in a control reactor. This recovery reduced EPI in the TPPB, resulting in both higher productivity (0.65 vs. 0.33 g L-1 h(-1)) and yield (0.54 vs. 0.40). Fortuitously, it was also found that repeated high pCO(2)-facilitated polymer extractions of butyric acid during batch growth of C. tyrobutyricum lessened the need for pH control, and reduced base requirements by approximately 50%. Thus, high pCO(2)-mediated absorptive polymer extraction presents a novel method for improving process performance in butyric acid fermentation, and this technique could be applied to the bioproduction of other organic acids as well. (C) 2014 Wiley Periodicals, Inc.