The inhibitory effect of propionic acid P and biomass concentration X is studied in batch and continuous fermentations with cell recycle.
In batch fermentations, the specific growth rate decreases and cancels out at a critical propionic acid concentration Pc1; the formerly decreasing specific production rate becomes constant after Pc1 and cancels out when a second critical propionic acid concentration Pc2 is reached.
In continuous fermentation with cell recycle, a similar inhibition is observed with biomass. The specific rates decrease and become constant at a critical biomass concentration Xc. They cancel out at different high biomass concentrations.
In both cases, the specific production rate can be related to the specific growth rate by the Luedeking and Piret expression: ν=αμ+β, , where the constants α and β are determined by the fermentation parameters.