Zheng Yuguo

Citric acid production from the mash of dried sweet potato with its dregs by Aspergillus niger in an external-loop airlift bioreactor

Abstract An external-loop airlift bioreactor (11.5 l), with a ratio of height to diameter of the riser of 2.9, was used for the production of citric acid from the mash of dried sweet potato with its dregs by Aspergillus niger . The effects of air flow rate, liquid volume and sparger hole diameter on citric acid production were investigated. A comparison of citric acid fermentation was made between the external-loop airlift bioreactor and the mechanically stirred tank bioreactor. After a total fermentation time of 65 h, an average of 10.6 mg/ml citric acid concentration was obtained in the external-loop airlift bioreactor under an air flow rate of 1.3 vvm, liquid volume of 8.5 l as compared to 9.6 mg/ml citric acid concentration in the 10 l mechanically stirred tank bioreactor under optimised operation conditions: agitation rate 200 rpm, air flow rate 1.0 vvm and liquid volume 6.5 l. It was demonstrated conclusively that the external-loop airlift bioreactor could be used to produce citric acid economically using A. niger from the mash of dried sweet potato with its dregs.

α-Amylase production by Bacillus subtilis with dregs in an external-loop airlift bioreactor

An external-loop airlift bioreactor, with a low ratio 2.9 of height-to-diameter of the riser and a ratio 6.6 of riser-to-downcomer diameter, was used to produce alpha-amylase from fermentation with dregs by Bacillus subtilis. The effects of gas flow rate and liquid volume on alpha-amylase production were investigated. After a 36-h fermentation time, an average of 432.3U/ml alpha-amylase activity was obtained under the conditions of liquid volume 8.5l and gas flow rate 1.2vvm for the first 12h of fermentation, 1.4vvm from 12 to 27h, and 1.2vvm from 27h to the end. The activity was higher than that obtained in shaking flasks (409.0U/ml) and in a mechanically stirred tank bioreactor (397.2U/ml) under optimized operating conditions. The fermentation cycle of the airlift bioreactor was shorter than the 48h required for the shaking flasks and close to the 36h of the mechanically stirred tank bioreactor. It was demonstrated that the external-loop airlift bioreactor could substitute for the traditional mechanically stirred tank bioreactor to produce alpha-amylase from fermentation by Bacillus subtilis with dregs.

Production of Extracellular Protease from Crude Substrates with Dregs in an External‐Loop Airlift Bioreactor with Lower Ratio of Height to Diameter

Bacillus subtilis AS1.398 was cultivated in a 11.5‐L total volume external‐loop airlift bioreactor with a low height‐to‐diameter ratio of 2.9 and a riser‐to‐downcomer diameter ratio of 6.6 for the production of protease from crude substrates with dregs. The influence of aeration rate, liquid volume, and sparger hole diameter on protease production was investigated. An average of 8197 u/mL protease activity was obtained after a total fermentation time of 32 h in the external‐loop airlift bioreactor with a liquid volume of 8.5 L, air flow rate of 1.5 vvm, and sparger hole diameter of 1.5 mm. The addition of one stainless steel sieve plate in the riser of the airlift bioreactor increased productivity of protease. After 32 h of fermentation, an average of 8718 u/mL protease activity was achieved in the external‐loop airlift bioreactor with one sieve plate and an air flow rate of 1.2 vvm, liquid volume of 8.5 L, and gas sparger hole diameter of 1.5 mm. This was 9.0% higher than the typical averages of about 8000 u/mL protease activity in the mechanically stirred tank bioreactors of the enzyme factory using the same microorganism. It is possible to make a scale‐up of the external‐loop airlift bioreactor and feasible to operate it for production of protease from crude substrate with dregs.