Ethanol production from sucrose-based raw materials using immobilized cells of Zymomonas mobilis

The fermentation of sucrose-based raw materials has been studied using immobilized cells of Z. mobilis in a laboratory-scale bioreactor. This follows an earlier evaluation under similar conditions using glucose media. With 150 g litre −1 sucrose medium, steady-state ethanol levels of 65–68 g litre −1 (8–8·5% v/v) were sustained for dilution rates D = 0·05–0·10 h −1 . At higher dilution rates the production of the by-product levan (and possibly sorbitol) caused a yield reduction when compared to fermentation of glucose medium. Sugar cane juice, molasses and synthetic mill liquor (a mixture of cane juice and molasses) required no nutrient supplementation and were all suitable substrates for ethanol production by immobilized cells of Z. mobilis . From comparative studies with molasses (155 g litre −1 total sugars) it was concluded that immobilized Z. mobilis are likely to be less effective than immobilized yeast cells for ethanol production from molasses. The reasons for this lie in the by-products produced by Z. mobilis on sucrose media and possible salt inhibition effects.