Natural immobilisation of microorganisms for continuous ethanol production

Using a growth medium based on cane blackstrap molasses, we compared ethanol production by two strains of Saccharomyces cerevisiae that were immobilized in polyurethane foam cubes in a fluidised-bed fermenter. One strain (NCYC 1119) was adhesive and extremely flocculent, whilst the other strain was not adhesive and only weakly flocculent. The strong flocs of NCYC 1119 caused blockage of the bed, so that stable operation could not be achieved beyond 15 days. Nevertheless, it was able to produce 40 g L−1 ethanol at a rate up to 16 g L−1 h−1 (D = 0.4 h−1), although this production period was limited to 192 h. In contrast, the non-adhesive strain was only capable of producing 28 g L−1 ethanol at a rate of 11 g L−1 h−1 at the same dilution rate, even though production continued for 576 h. Despite the conversion of sugars to ethanol not being complete during these trials (up to 47 g L−1 was expected), it was clearly demonstrated that the productivity of the adhesive strain was higher than that of the non-adhesive one. However, further work is required to develop this process into a robust, industrial system.