Stéphane Delpech

In alcoholic fermentation, under oenological conditions, the environmental parameters impacting fermentation kinetics and aroma production have been widely studied. The nitrogen content of grape must was found to be one of the most important parameters for both of these aspects of fermentation. Many studies have been performed on the effect of mineral nitrogen addition. However, it has increasingly been observed that the nature of the nitrogen added leads to different results. Our work focused on the effects of peptide addition on both fermentation kinetics and aroma production. Peptides are one of the less well understood sources of assimilable nitrogen, as their incorporation by yeast remains unclear. In this study, we compared the effect of the addition of a “classic” assimilable nitrogen source (ammonium + amino acids) with that of peptide addition in both white and red must fermentation by screening 18 Saccharomyces cerevisiae strains in total. Our data show that peptide addition enhances fermentation kinetics and leads to specific changes in the production of fermentative aromas. The impact of peptides on thiol synthesis is rather limited.

Since ten years, many studies conducted on beers showed an important impact of polyfunctionnal thiols of the aroma profiles. Among them, three thiols responsible for blackcurrant bud, passion fruit, citrus and rhubarb notes have been intensively studied: the 4‐methyl‐4‐mercaptopentan‐2‐one (4MMP), the 3‐mercaptohexan‐1‐ol (3MH) and its corresponding acetate. Their origin was very complex in beers since they probably came from odourless precursors present either in hops or in malts. Our work focused on the formal identification of thiol precursors in hops and on their quantification. By using pure synthetic standards and mass spectrometry characterization, we formally identified for the first time the occurrence of glutathionylated conjugates of 4MMP and 3MH and the cysteinylated conjugate of 4MMP in hops. First quantification results obtained on 10 hop varieties, showed that 3MH conjugates were more ubiquitous than 4MMP ones. Conjugates of 3MH occurred at very high level until 20 mg kg^‐1 in Cascade hop, which was considerably higher than concentrations found in grapes. Then, we compared the proportion of bound and free thiol fractions and we demonstrated that more than 99 % of 3MH occurred as precursors in hops. On the contrary, free 4MMP fraction represented the most important source of 4MMP in hops. Copyright © 2016 John Wiley & Sons, Ltd.