Continuous ethanol production using yeast immobilized on sugar-cane stalks

Sugar-cane stalks, 2.0 cm long, were used as a support for yeast immobilization envisaging ethanol production. The assays were conducted in 38.5 L fermenters containing a bed of stalks with 50% porosity. The operational stability of the immobilized yeast, the efficiency and stability of the process, as well as the best dilution rate were evaluated. Molasses from demerara sugar production was used in the medium formulation. It was diluted to obtain 111.75 +/- 1.51 g/L without any further treatment. Sulfuric acid was used to adjust the pH value to around 4.2. Every two days Kamoran HJ (10 ppm) or with a mixture containing penicillin (10 ppm) and tetracycline (10 ppm), was added to the medium. Ethanol yield and efficiency were 29.64 g/L.h and 86.40%, respectively, and the total reducing sugars (TRS) conversion was 74.61% at a dilution rate of 0.83 h(-1). The yeast-stalk system was shown to be stable for over a 60 day period at extremely variable dilution rates ranging from 0.05 h(-1) to 3.00 h(-1). The concentration of immobilized cell reached around 10(9) cells/gram of dry sugar-cane stalk when the fermenter was operating at the highest dilution rate (3.00 h(-1)). Keywords: alcoholic fermentation, continuous fermentation, yeast, sugar-cane stalks, yeast immobilization, ethanol.