GROWTH, DEATH, AND OXYGEN-UPTAKE KINETICS OF PICHIA-STIPITIS ON XYLOSE

Pichia stipitis NRRL Y-7124 has potential application in the fermentation of xylose-rich waste streams produced by wood hydrolysis. Kinetic models of cell growth, death, and oxygen uptake were investigated in batch and oxygen-limited continuous cultures fed a rich synthetic medium. Variables included rates of dilution (D) and oxygen transfer (K(i)a) and concentrations of xylose (X), ethanol (E), and dissolved oxygen (C(ox)). Sustained cell growth required the presence of oxygen. Given excess xylose, specific growth rate (mu) was a Monod function of C(ox). Specific oxygen uptake rate was proportional to mu by a yield coefficient relating biomass production to oxygen consumption; but oxygen uptake for maintenance was negligible. Thus steady-state C(ox) depended only on D, while steady-state biomass concentration was controlled by both D and K(i)a. Given excess oxygen, cells grew subject to Monod limitation by xylose, which became inhibitory above 40 g/L. Ethanol inhibition was consistent with Luong's model, and 64.3 g/L was the maximum ethanol concentration allowing growth. Actively growing cells died at a rate that was 20% of mu. The dying portion increased with E and X.