Batch- and continuous propionic acid production from glycerol using free and immobilized cells of Propionibacterium acidipropionici

Propionic acid production from glycerol was studied using Propionibacterium acidipropionici DSM 4900 cells immobilized on polyethylenimine-treated Poraver (PEI-Poraver) and Luffa (PEI-Luffa), respectively. Using PEI-Luffa, the average productivity, yield and concentration of propionic acid from 40gL ^-1 glycerol were 0.29gL ^-1h ^-1, 0.74mol _PAmol _Gly ^-1 and 20gL ^-1, respectively, after four consecutive recycle-batches. PEI-Poraver supported attachment of 31 times higher amounts of cells than PEI-Luffa and produced 20, 28 and 35gL ^-1 propionic acid from 40, 65 and 85gL ^-1 glycerol, respectively (0.61molPAmolGly-1). The corresponding production rates were 0.86, 0.43 and 0.35gL ^-1h ^-1, which are the highest reported from glycerol via batch or fed-batch fermentations for equivalent propionic acid concentrations. Using a continuous mode of operation at a dilution rate of 0.1h ^-1, cell washout was observed in the bioreactor with free cells; however, propionic acid productivity, yield and concentration were 1.40gL ^-1h ^-1, 0.86molPAmolGly-1, and 15gL ^-1, respectively, using immobilized cells in the PEI-Poraver bioreactor. The choice of the immobilization matrix can thus significantly influence the fermentation efficiency and profile. The bioreactor using cells immobilized on PEI-Poraver allowed the fermentation of higher glycerol concentrations and provided stable and higher fermentation rates than that using free cells or the cells immobilized on PEI-Luffa.