Bioconversion of d-xylose to xylitol by Debaryomyces hansenii UFV-170: Product formation versus growth

The relationship between d-xylose-to-xylitol conversion and cell growth was investigated in the yeast Debaryomyces hansenii UFV-170. A first phase of rapid growth (average specific growth rate, μ, of 0.24 h−1) dissociated from xylitol accumulation was followed by a second phase during which growth was associated with low xylitol formation (ΔP = 1.27 g l−1) and a third phase characterized by the largest d-xylose consumption (Xylc = 21.4 g l−1) and xylitol production (ΔP = 14.4 g l−1) but exhibiting very slow growth. Xylitol accumulation in the medium was then associated with a decelerated growth rather than being completely dissociated from it. During the bioconversions without a nitrogen source performed at very high inoculum (Xo = 24.0 g l−1), non-growth conditions were able to address most of consumed d-xylose (86%) to xylitol formation, thus ensuring the best xylitol production performance (QP = 4.09 g l−1 h−1, qP = 0.17 g g−1 h−1 and YP/S = 0.78 g g−1). Xylitol formation was simultaneously influenced by the physiological state of the culture and the concentration of biomass. After four operations of cell recycling in the absence of the nitrogen source, long-term viability of the system but unstable xylitol formation was observed.