Very high gravity ethanol fermentation from sweet sorghum stem juice using a stirred tank bioreactor coupled with a column bioreactor

A stirred tank bioreactor (STR) coupled with two column bioreactors (CRs) was used for ethanol production from sweet sorghum stem juice by Saccharomyces cerevisiae SSJ01KKU in a very high gravity fermentation. The effects of the medium circulation rate between the STR and CRs (2.6 and 5.2 mL/min, corresponding to 25 and 50 % of the S. cerevisiae specific growth rate), the starting time of medium circulation (0 and 4 h) and cell inoculation were investigated. The results showed that a medium circulation rate of 5.2 mL/min, starting the medium circulation at the beginning of fermentation (0 h) with cell inoculation into the STR only were appropriate conditions for ethanol production. This yielded an average ethanol concentration (PE) of 120.96 g/L and ethanol productivity (QP) of 2.52 g/L.h. When a repeated-batch (RB) ethanol fermentation in the STR coupled with CR was carried out using a drain and fill technique at different volumes (75 and 90 %, referenced as RB1 and RB2, respectively), it was found that at least eight successive cycles could be operated under both RB1 and RB2. The average PE and QP for RB1 and RB2 were not significantly different. However, the average total ethanol production rate in RB2 (3.25 g/h) over the eight cycles was significantly higher than that of RB1 (2.60 g/h).