Organic acids from biomass by continuous fermentation

In continuous fermentation, more than 90% conversion of glucose and 86% conversion of xylose were achieved in a continuous stirred tank reactor (CSTR) for a 72-h retention time and a 30-g/L total sugar concentration. The fermentation in the CSTR was shown to be about four times faster than in the batch reactor. More than 92% conversion of glucose and 75% conversion of xylose were found at a 28-h retention time in an immobilized-cell reactor (ICR). Also, about 67% of the sugar can be converted into organic acids in this reactor, yielding more than 20 g/L of organic acids. High sugar concentrations were tested in the ICR, yielding a 54% conversion of glucose with a 10% sugar concentration. In fermenting concentrated solutions in the ICR, P. acidi-propionici showed a preference for glucose over xylose. In comparing the CSTR and ICR, the fermentation in the ICR is about 5 to 8 times faster than in the CSTR. Thus, the ICR system is the better system for organic-acid production by fermentation of sugars. Based on laboratory results, a 5-Gg/y plant for the production of organic acids from rice straw is proposed. The capital and operating costs of this plant are estimated.