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The aim of this study was to evaluate the combined effect of initial nitrogen content on the production of hydrogen sulphide and other volatile compounds during alcoholic fermentation. For that propose, three commercial wine strains of Saccharomyces cerevisiae were used to ferment synthetic grape juice media under different nitrogen concentrations. H(2)S was measured throughout fermentations and other aroma compounds were analyzed at the end of the experiments. The trigger levels at which an inverse relationship between the initial nitrogen present in media and total H(2)S production varied among the three strains tested. For UCD522 and PYCC4072, the highest H(2)S levels were produced in media with 267 mg N l(-1) of initial nitrogen, whereas the lowest levels were detected with nitrogen limitation/starvation conditions (66 mg N l(-1)). Moreover, 21 other aroma compounds belonging to different chemical classes were identified and quantified by solid phase micro-extraction (SPME) coupled to gas chromatography-mass spectrometry (GC-MS). The initial nitrogen concentration more than yeast strain had a decisive effect on the final aroma composition, suggesting that modulation of nutrients emerges as a useful tool for producing desired flavour and odour compounds.

Some non-Saccharomyces yeasts, including Metschnikowia pulcherrima, have been proposed as selected starters due to their contribution for the overall aroma and chemical profiles of wines. In this work, we aimed to evaluate the genetic and phenotypic diversity of Metschnikowia pulcherrima strains isolated from different locations of Douro Wine Region, and to explore their potential as co-adjuncts of S. cerevisiae in alcoholic fermentation. For that purpose, a set of 64 M. pulcherrima isolates were used. Polymerase chain reaction (PCR) fingerprinting with M13 primers demonstrated to be an efficient tool in intraspecific discrimination of M. pulcherrima strains. No significant associations were found between genotypic profiles and either geographical origin or winery. The isolates were screened for their stress resistance ability (ethanol, SO 2 , chitosan, copper, H 2 O 2 , and Grape Juice Medium), aroma-related activities (resistance to 5, 5 , 5 -trifluor-D, L-leucine and cerulenin and β-glycosidase, β-lyase and sulfite-reductase activities) as well as other relevant technological proprieties (protease activity and biogenic amines production). M. pulcherrima response to the different enological traits evaluated was greatly strain-dependent. The most discriminant features were the ability of the strains to grow in Grape-Juice Medium (GJM) and sulfite-reductase, and their β-lyase and protease activities. The enological potential of a selected M. pulcherrima strain in mixed-culture with S. cerevisiae was also assessed in natural grape-juice of a local variety, under two nitrogen regimes. M. pulcherrima proved to be promising for future industrial application as a co-starter, lowering ethanol, acetic acid and, reported here for the first time, lowering hydrogen sulfide levels in the wines.

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