Effects of melatonin and tryptophol addition on fermentations carried out by Saccharomyces cerevisiae and non-Saccharomyces yeast species under different nitrogen conditions.

During wine fermentation, yeasts produce metabolites that are known growth regulators. The relationship between certain higher alcohols derived from aromatic amino acid metabolism and yeast signalling has previously been reported. In the present work, tryptophol (TrpOH) or melatonin (MEL), which are putative growth regulators, were added to alcoholic fermentations. Fermentations were performed with three different inocula, combining Saccharomyces cerevisiae and four non-Saccharomyces yeast species, under two nitrogen conditions. The combinations tested were: (i) only S. cerevisiae; (ii) the mixture of four non-Saccharomyces species; and (iii) the combination of all five species together. The results revealed that the TrpOH and MEL addition caused changes in fermentation kinetics, viability and species distribution during fermentation, but it was dependent on the nitrogen present in the media and the composition of the inocula. Low nitrogen condition seemed to favour the presence of non-Saccharomyces species until mid-fermentation, although at the end of fermentation the imposition of Saccharomyces was higher in this condition. The presence of high concentrations of TrpOH resulted in limited growth and a delay in fermentation, noticeably significant in fermentations performed with S. cerevisiae inocula. These effects were reversed by the presence of non-Saccharomyces yeast in the medium. Low TrpOH concentration allowed faster fermentation with mixed non-Saccharomyces and Saccharomyces inocula. Moreover, in the absence of S. cerevisiae, a low concentration of TrpOH increased the presence of Torulaspora delbrueckii during fermentation with high nitrogen availability but not under low nitrogen conditions, when the population of S. bacillaris was higher than that in the control. The effects of MEL were particularly evident at the beginning and end of the process, primarily favouring the growth of non-Saccharomyces strains, especially the first hours after inoculation.

The Influence of Sugar Cane Bagasse Type and Its Particle Size on Xylose Production and Xylose-to-Xylitol Bioconversion with the Yeast Debaryomyces hansenii.

In the present study, the effect of the type of sugar cane bagasse (non-depithed or depithed) and its particle size on the production of xylose and its subsequent fermentation to xylitol by Debaryomyces hansenii CBS767 was investigated using a full factorial experimental design. It was found that the particle size range and whether bagasse was depithed or not had a significant effect on the concentration and yield of xylose in the resulting hemicellulose hydrolysate. Depithed bagasse resulted in higher xylose concentrations compared to non-depithed bagasse. The corresponding detoxified hemicellulose hydrolysates were used as fermentation media for the production of xylitol. The hemicellulose hydrolysate prepared from depithed bagasse also yielded meaningfully higher xylitol fermentation rates compared to non-depithed bagasse. However, in the case of non-depithed bagasse, the hemicellulose hydrolysate prepared from larger particle size range resulted in higher xylitol fermentation rates, whereas the effect in the case of non-depithed bagasse was not pronounced. Therefore, depithing of bagasse is an advantageous pretreatment when it is to be employed in bioconversion processes.

Influence of inhibitory compounds and minor sugars on xylitol production by Debaryomyces hansenii.

To obtain in-depth information on the overall metabolic behavior of the new good xylitol producer Debaryomyces hansenii UFV-170, batch bioconversions were carried out using semisynthetic media with compositions simulating those of typical acidic hemicellulose hydrolysates of sugarcane bagasse. For this purpose, we used media containing glucose (4.3-6.5 g/L), xylose (60.1-92.1 g/L), or arabinose (5.9-9.2 g/L), or binary or ternary mixtures of them in either the presence or absence of typical inhibitors of acidic hydrolysates, such as furfural (1.0-5.0 g/L), hydroxymethylfurfural (0.01- 0.30 g/L), acetic acid (0.5-3.0 g/L), and vanillin (0.5-3.0 g/L). D. hansenii exhibited a good tolerance to high sugar concentrations as well as to the presence of inhibiting compounds in the fermentation media. It was able to produce xylitol only from xylose, arabitol from arabinose, and no glucitol from glucose. Arabinose metabolization was incomplete, while ethanol was mainly produced from glucose and, to a lesser less extent, from xylose and arabinose. The results suggest potential application of this strain in xyloseto- xylitol bioconversion from complex xylose media from lignocellulosic materials.

The combined effects of acetic acid, formic acid, and hydroquinone on Debaryomyces hansenii physiology.

The combined effects of inhibitors present in lignocellulosic hydrolysates was studied using a multivariate statistical approach. Acetic acid (0-6 g/L), formic acid (0-4.6 g/L), and hydroquinone (0-3 g/L) were tested as model inhibitors in synthetic media containing a mixture of glucose, xylose, and arabinose simulating concentrated hemicellulosic hydrolysates. Inhibitors were consumed sequentially (acetic acid, formic acid, and hydroquinone), alongside to the monosaccharides (glucose, xylose, and arabinose). Xylitol was always the main metabolic product. Additionally, glycerol, ethanol, and arabitol were also obtained. The inhibitory action of acetic acid on growth, on glucose consumption and on all product formation rates was found to be significant (p < or = 0.05), as well as formic acid inhibition on xylose consumption and biomass production. Hydroquinone negatively affected biomass productivity and yield, but it significantly increased xylose consumption and xylitol productivity. Hydroquinone interactions, either with acetic or formic acid or with both, are also statistically significant. Hydroquinone seems to partially lessen the acetic acid and amplify formic acid effects. The results clearly indicate that the interaction effects play an important role on the xylitol bioprocess.