Innovative strategies for operation of mist trickling reactors for enhanced hairy root proliferation and secondary metabolite productivity

Nutrient feeding modes, root tip inoculation density, and oxygen tension in a mist trickling reactor (MTR) were investigated in depth. On-line monitoring of conductivity, pH, and DO were performed to facilitate the investigation. A linear relationship between biomass concentration and sucrose consumption rate as well as conductivity decrease were established in shake flasks. The biomass concentration in a MTR was estimated with these relationships. For a 3 L MTR, the optimum root tip inoculum density was 80–120 roots. Intermittent feeding of nutrient medium in the later half of cultivation promoted the productivity of l-DOPA, especially with a misting cycle of 1 h on and 1 h off. Due to the shortage of a nitrogen source during the later stage of cultivation, fed-batch supplementation of B5 medium was initiated after continuous nutrient feeding. The results showed that fairly high l-DOPA productivity was attained (0.644 g/L). The distribution of PO2 in the radial and axial directions was measured by employing two and three micro-oxygen electrodes. PO2 was relatively uniform under the normal packing density of hairy roots. An adequate aeration rate was also achieved.