Immobilisation of yeasts for continuous fermentation

Continuous ethanol fermentation from cheap sugar cane molasses was studied. Two strains of Saccharomyces cerevisiae were used, one of them flocculent, in order to evaluate the performance of fluidized tubular reactors, with and without support for yeast immobilisation (polyurethane foam and expanded clay argilate granules were tested). In this heterogeneous continuous system, with three phases, flow conditions are difficult to control, mostly due to CO2 production. Different long term continuous experiments were performed using three different reactors. Operating parameters such as feedstock medium, temperature and pH were not changed and ethanol productivity was followed up as the dilution rate was increased. Different steady states were reached and the productivity increased with the dilution rate. Using a 1L volume reactor, 25 g Lh of ethanol were achieved at 0.8 h dilution rate, although in a non stable operation mode. The performance of the two strains is comparable, but the flocculent one raises problems due to fast growth and support particles aggregation.