Identificador: TDX:432
Autores: Beltran Casellas, Gemma
Resumen:
Wines produced at low temperatures (10-15ºC) are known to develop certain characteristics of taste and aroma, not only related to primary aroma retention. However, low temperature fermentations have also some disadvantages that comprise an increase of the duration of the process and a higher risk of stuck and sluggish fermentation.In order to improve the fermentation performance and the quality of wine, we established the following objectives: - The study of wine yeast metabolism at low temperature fermentation (13ºC), and its influence in aspects as the fermentation kinetic, the yeast growth, the yeast lipid metabolism, the production of aromatic compounds, and the global yeast gene expression.- The study of nitrogen metabolism of yeast in alcoholic fermentation, as well as the study of nitrogen supplementations at different points of the fermentation.Our results showed that low temperatures increased the length of fermentation, the yeast viability along the process, but also modified the lipid composition of yeast cells, increasing the membrane fluidity, and improved the aromatic composition of the wine, increasing the flavour-active compounds and decreasing the unpleasant ones such as acetic acid and fusel alcohols. To identify the molecular mechanism that causes these changes in aroma profiles and to verify that 13°C-fermentation does not hinder other cellular properties, we compared the expression programs during wine fermentation at 13ºC and 25°C (using Microarrays technology), and tentatively correlated the differential genes expression with changes in intracellular lipid content, and in the production of flavour-active metabolitesThis genome-wide analysis carried out for the first time with a commercial yeast strain under true industrial conditions revealed many major differential genes expression both during the course of the wine fermentation and between two fermentation temperatures. With respect to industrial output, wine fermentation conducted at 13°C presents the advantage to induce an early cold stress response that apparently does not penalize the wine fermentation process, further than the longest fermentation length. In the study of the nitrogen metabolism of yeast along the fermentation we observed that in wine fermentations the cells evolve from a nitrogen-repressed situation at the beginning of the process to a nitrogen-derepressed situation as the nitrogen is consumed. These nitrogen-repressed/derepressed conditions determined the different patterns of ammonium and amino acid consumption. Arginine and alanine were hardly used under the repressed conditions, while the uptake of branched-chain and aromatic amino acids increased. The repression of GAP1 and MEP2 genes in the cells, low arginase activity or inhibition of arginine uptake could be considered as a good Nitrogen Catabolite Repression markers. Winemakers systematically supplement grape musts with diammonium phosphate to prevent nitrogen-related fermentation problems. The timing of the nitrogen additions influenced the biomass yield, the fermentation performance, the patterns of ammonium and amino acid consumption, and the production of secondary metabolites. These nitrogen additions induced a nitrogen-repressed situation in the cells, and this situation determined which nitrogen sources were selected. Nitrogen assimilation also depends on fermentation temperature. Fermentation temperature is an important factor determining utilization of nitrogen sources during fermentation of grape juice, and influences the quantity and the quality of nitrogen requirement. Ammonium and glutamine, the preferred source for biomass production, are less consumed at low temperature. Likewise amino acids that are only taken up under derepressed conditions (arginine, alanine, asparagine, etc.) are more consumed at low temperature.The information provided by this thesis represents a starting point for deciphering the regulatory circuits during wine fermentation, overall at low temperature, and should help us to understand the properties of wine yeasts. Our results open up a lot of interesting perspectives that will further our knowledge of wine yeast metabolism during wine fermentations.
Repositori URV