Production of ethanol from cassava starch hydrolysate by immobilised cells of 'Zymomonas mobilis'

Abstract

Cells of Zymononas mobilis (ATCC 10988) were immobilised in calcium alginate beads and used to convert enzymatically hydrolysed cassava starch to ethanol. Optimum operating conditions were investigated in batch experiments. The ogtimum pH range and temperature were found to be 3-0 to 8.0 and 30 C, respectively. The maximum rates of glucose consumption and ethanol formation were obtained with an initial glucose concentration of 100 g/1. There was no fermentation inhibition below an initial ethanol concentration of 60 g/1. Ethanol productivity was the same using pure cassava hydrolysate or a medium composed of cassava hydrolysate, yeast extract and mineral nutrients. The immobilised Zymomonas mobilis cells were studied in a packed-bed reactor system operating under optimised parameters from the batch experiments. Volumetric ethanol productivities of 8.91 g/l.h and 22.5 g/l.h were obtained at 100% and 75% of glucose utilization, respectively; these productivities correspond to 1.5 times that of a free cell reactor when glucose utilisation was 100% and 3 times that of a free cell reactor when glucose utilisation was 75%. The maximum specific ethanol formation rate and the maximum specific glucose uptake rate were found to be 1.4 g/g.h and 2.8 g/g.h, respectively. The immobilised-cell reactor was operated continuously at a constant dilution rate of 0.2 h -1 for 20 days resulting in only a 20% loss of the original fermentative activity corresponding to an estimated half- life of 63 days. Based on experimental data, a mathematical analysis has been made and rate equations proposed.