Occurrence of melatonin and indolic compounds derived from l-tryptophan yeast metabolism in fermented wort and commercial beers.

Melatonin and serotonin are bioactive compounds present in foods and beverages and related to neuroprotection and anti-angiogenesis, among other activities. They have been described in wines and the role of yeast in their formation is clear. Thus, this study evaluates the content of these bioactives and other related indolic compounds in beer. For this purpose, commercial beers were analyzed by a validated UHPLC-HRMS method and sample treatment optimized due to the low concentrations expected. Moreover, a wort was fermented with different commercial beer yeast (Abbaye, Diamond, SafAle, SafLager) in order to monitor the formation of these bioactives during the elaboration process. Results show that indolic compounds such as N-acetylserotonin and 3-indoleacetic acid are produced during the alcoholic fermentation of wort. Moreover, the occurrence of four indolic compounds (5-hydroxytryptophan, N-acetylserotonin, 3-indoleacetic acid, l-tryptophan ethyl ester) in commercial beers is reported for the first time.

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 production of aromatic alcohols in non-Saccharomyces wine yeast is modulated by nutrient availability.

Aromatic alcohols (tryptophol, phenylethanol, tyrosol) positively contribute to organoleptic characteristics of wines, and are also described as bioactive compounds and quorum sensing molecules. These alcohols are produced by yeast during alcoholic fermentation via the Erhlich pathway, although in non-Saccharomyces this production has been poorly studied. We studied how different wine yeast species modulate the synthesis patterns of aromatic alcohol production depending on glucose, nitrogen and aromatic amino acid availability. Nitrogen limitation strongly promoted the production of aromatic alcohols in all strains, whereas low glucose generally inhibited it. Increased aromatic amino acid concentrations stimulated the production of aromatic alcohols in all of the strains and conditions tested. Thus, there was a clear association between the nutrient conditions and production of aromatic alcohols in most of the wine yeast species analysed. Additionally, the synthesis pattern of these alcohols has been evaluated for the first time in Torulaspora delbrueckii, Metschnikowia pulcherrima and Starmellera bacillaris.