Resistance of Lactobacillus casei in plastic-composite-support biofilm reactors during liquid membrane extraction and optimization of the lactic acid extraction system

Lactic acid fermentations were performed with plastic-composite-support (PCS) disks in solvent-saturated media with Lactobacillus casei subsp. rhamnosus (ATCC 11443). The PCS disks contained 50% (w/w) polypropylene, 35% (w/w) ground soybean hulls, 5% (w/w) yeast extract, 5% (w/w) soybean flour, and 5% (w/w) bovine albumin. Bioassays were performed by growing L. casei in solvent-saturated media after soaking the PCS disks. Eighteen different solvent and carrier combinations were evaluated. Overall, L. casei biofilm fermentation demonstrated the same lactic acid production in solvent-saturated medium as suspended cells in medium without solvents (control). To evaluate PCS solvent-detoxifying properties, two bioassays were developed. When solvent-saturated medium in consecutive equal volumes (10 mL then 10 mL) was exposed to PCS, both media demonstrated lactic acid fermentation equal to the control. However, when solvent-saturated medium with two consecutive unequal volumes (10 mL then 90 mL) was exposed to PCS, some degree of toxicity was observed. Furthermore, iso-octane, tributylphosphate (TBP), and Span 80 were optimized for recovery as 91%, 5%, and 4% (v/v), respectively, with a 1:1 ratio of 1.2 M Na₂CO₃ stripping solution. Also, recovery by emulsion liquid extraction in the hollow-fiber contactor was minimal due to low recovery at pH 5.0 and incompatibility of the solvent and hollow-fiber material. These results suggest that PCS biofilm reactors can benefit lactic acid fermentation by eliminating the toxic effect from solvent leakage into the fermentation medium from liquid-liquid extractive integrated fermentations.