AbstractIn order to improve the biotechnological production of xylitol, the metabolism of Debaryomyces hansenii NRRL Y‐7426 in corncob hemicellulose hydrolyzate has been investigated under different conditions, where either maintenance or growth requirements predominated. For this purpose, the experimental results of two sets of batch bioconversions carried out alternatively varying the starting xylose concentration in the hydrolyzate (65.6 ≤ S0 ≤ 154.7 g L−1) or the initial biomass level (3.0 ≤ X0 ≤ 54.6 gDM L−1) were used to fit a metabolic model consisting of carbon material and ATP balances based on five main activities, namely fermentative assimilation of pentoses, semi‐aerobic pentose‐to‐pentitol bioconversion, biomass growth on pentoses, catabolic oxidation of pentoses, and acetic acid and NADH regeneration by the electron transport system. Such an approach allowed separately evaluating the main bioenergetic constants of this microbial system, that is, the specific rates of ATP and xylose consumption due to maintenance (mATP = 21.0 mmolATP C−molDM−1h−1; mXyl = 6.5 C‐mmolXyl C−molDM−1h−1) and the true yields of biomass on ATP (YATPmax = 0.83 C‐molDM molATP−1) and on xylose (YXylmax = 0.93 C‐molDM C‐molXyl−1). The results of this study highlighted that the system, at very high S0 and X0 values, dramatically increased its energy requirements for cell maintenance, owing to the occurrence of stressing conditions. In particular, for S0 > 130 g L−1, these activities required an ATP consumption of about 2.1 molATP L−1, that is, a value about seven‐ to eightfold that observed at low substrate concentration. Such a condition led to an increase in the fraction of ATP addressed to cell maintenance from 47% to 81%. On the other hand, the very high percentage of ATP addressed to maintenance (>96%) at very high cell concentration (X0 ≥ 25 gDM L−1) was likely due to the insufficient substrate to sustain the growth. Biotechnol. Bioeng. 2009;102: 1062–1073. © 2008 Wiley Periodicals, Inc.