alpha-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.