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 stipitis, it is important to use bioreactors that (a) allow a high concentration 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 κ-carrageenan was amended through the subsequent treatment of the bioparticles with a hardening agent (Al³⁺). The variables examined were the cellular mass/gel mass ratio, the concentration of the hardening solution, 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 conventional fixed-bed reactor. By using these bioreactors, various effects are found: a lower retention of carbon dioxide in the reactor, a closer 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 using bioparticles untreated with the hardening agent.