Lactic acid production: extractive fermentation in aqueous two-phase systems
Abstract: There is an increased interest in the microbial production of lactic acid as a raw material for the synthesis of lactic acid polymers as biodegradable plastics. However, two factors limit the economical competitiveness of lactic acid production by microorganisms: lactic acid inhibition, which decreases both fermentation rate and the maximum lactic acid concentration, and the efficiency in the recovery of lactic acid. To minimise these effects, the production of lactic acid by extractive fermentation in aqueous two-phase systems (ATPS) was proposed: the bottom cell-rich phase is continuously recirculated while the product can continuously be removed from the top phase. A chromatographic method for the analysis of phase composition was developed and used to study the factors influencing lactic acid partitioning in ATPS, and to investigate the influence of bacterial cells, lactic acid and phosphate on ATPS. The presence of bacterial cells, lactic acid and phosphate were shown to influence both the shape of the binodial and the slope of the tie lines in the phase diagram. The stability of ATPS in continuous industrial processes is thus compromised by the changing conditions during fermentation. The chromatographic method described facilitates the control of ATPS-based processes. A new family of polymer conjugates was synthesised. Two different polymers carrying distinct attributes were combined to yield a new conjugate which retained the features of the original polymers. The new conjugates are based on polyethylene glycol (PEG) or ethylene oxide (EOPO), and polyethylenimine (PEI). PEG and EOPO furnishes the conjugate the capacity to form ATPS when mixed with polysaccharide polymers like dextran and starch. In the case of the EOPO-based conjugate, its recovery by temperature induced precipitation was achieved. PEI brings to the conjugate the capacity to be used as liquid anion exchanger to be used for the recovery of negatively charged molecules, from small organic acids like lactic acid to proteins. Lactic acid, and other organic acids as well as bovine serum albumin partitioned to the conjugate-rich top phase in EOPO·PEI-dextran ATPS. Furthermore, conditions were identified where the top phases of these systems showed temperature induced precipitation.
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