ALCOHOLIC FERMENTATION OF XYLOSE BY IMMOBILIZED PICHIA-STIPITIS IN A FIXED-BED PULSED BIOREACTOR

To enhance the productivity of the fermentation of xylose by Pichia st ipitis, it is important to use bioreactors that (a) allow a high conce ntration of yeast (this can be achieved by using immobilized yeasts), (b) reduce the diffusional limitations resulting from dead volume (due mainly to the supplied and produced gas), and (c) diminish the effect of inhibition by ethanol. The process of immobilizing P. stipitis in K-carrageenan was amended through the subsequent treatment of the biop articles with a hardening agent (Al3+). The variables examined were th e cellular mass/gel mass ratio, the concentration of the hardening sol ution, and the contact time between the bioparticles and the hardening agent. We determined the optimum conditions of immobilization, which resulted in higher productivities. We also determined the improvements that can be achieved when a pulsating system is connected to a conven tional fixed-bed reactor. By using these bioreactors, various effects are found: a lower retention of carbon dioxide in the reactor, a close r approach to the plug flow model, and a better oxygen transfer along the bioreactors. As a result, the productivity was increased up to 4.4 g l-(1) h(-1) an increase of 23% compared to the fixed-bed bioreactor without pulsation, and of more than 150% compared to batch process us ing bioparticles untreated with the hardening agent.