Contribución de las levaduras al aroma de los vinos espumosos durante la segunda fermentación y la lisis celular

Abstract

Abstract • Introduction The sparkling wines belong to the category of special wines and are obtained through the second fermentation of a base wine with adequate oenological characteristics. The methodologies used for this purpose are diverse, the best known being the traditional or Champenoise method, which consists of a second fermentation in a closed bottle, after adding yeast and sugar, followed by a minimum period of contact with the yeast lees, where its autolysis takes place. Both stages, fermentation and ageing, constitute what is called ‘prise de mousse’. In recent decades, the consumption of sparkling wines has increased notably, which has motivated producers and the scientific community to make significant efforts to introduce improvements in production processes to innovate, maintain quality and demand for these wines or attract emerging consumers [1]. Thus, the proposals presented in the scientific literature cover three fundamental areas: quality, technology / biotechnology and health [2], which allows to achieve the main goals of the oenological industry regarding the identity, better production and quality, health and food safety. Sensory quality is a determining factor for the selection of a food product by consumers, highlighting the visual and aromatic quality as the first characteristics that they value in the acceptance of a certain wine. Several factors influence the sensory properties of sparkling wines (foam, colour, aroma and taste), among which stand out: viticultural aspects and edaphoclimatic conditions, grape variety, health and maturation status, methodology used in the elaboration of the base wine and the 'prise de mousse' phase [2-4]. However, the characteristic aroma of sparkling wines is greatly influenced by the yeast involved in the two phases of the foam making process (fermentation and aging on lees). On the one hand, during fermentation, yeasts are alive and subjected to harsh conditions imposed by the high ethanol content and the overpressure of carbon dioxide, which are related to their metabolism and therefore to the production of aroma compounds. In a second phase (aging on lees), when the yeast is dead, cell lysis occurs and during it, some intracellular metabolites, proteins and enzymes are released and contribute to the aroma of these wines [5, 6]. Current research and innovation trends in sparkling winemaking focus on choosing unconventional yeast strains and grape varieties traditionally used for making still wines. Studies carried out in this regard have allowed the selection of yeast strains for their tolerance to stress conditions, for the production of extracellular enzymes (pectinases, amylases, lipases, proteases and glucosidases) and also for their capacity for lysis and flocculation [2, 4, 5, 7, 8]. Nevertheless, there is little research on the contribution of yeasts to the composition of the aroma during the in -bottle second fermentation, since existing studies do not usually differentiate between this stage and the ageing on lees [9, 10]. Generally, the studies carried out address the effect of some factors involved in the production process, but the interaction effect between them is hardly considered. Furthermore, there are very few studies on the application of new biotechnologies, such as emerging yeast immobilization systems. • Content of the thesis In this Doctoral Thesis, the volatile metabolites with odorant activity have been studied during the fermentation and ageing stages of sparkling wines obtained with two strains of yeast Saccharomyces cerevisiae, P29 and G1. The first yeast is commonly used in the production of sparkling wines from the Penedès wine region, while the second is a veil-forming yeast isolated from the surface of the wine, typical in the production of fine-type biological ageing wines and used for the first time to make sparkling wines. This yeast was isolated from the Montilla-Moriles wine-growing area and has been proposed to carry out the second fermentation of base wines by the traditional method due to its high tolerance to ethanol, and its flocculation properties; which make it an ideal candidate to shorten the time of riddling stage. The research carried out in this Thesis on the yeasts’ volatilome completes the study carried out in recent years by members of the Vitenol group of the University of Córdoba on the proteomic and metabolomic profile of the yeasts used for the production of sparkling wines [10, 11]. The experimental design applied in each stage of this thesis allowed to study the influence of key factors in the production process, related to yeast stress and, consequently, with the aromatic quality of sparkling wines. Thus, during the bottle fermentation stage, the endogenous CO2 pressure was studied and, together with the ageing stage, the combined effect of the fermentation temperature, the ageing time and the yeast inoculation format. Furthermore, the potential of fast and non-destructive techniques for the analysis of the aroma fraction has been also evaluated, such as the electronic nose based on quartz microbalances. Finally, the statistical studies carried out on organoleptic and analytical data matrices have made it possible to clarify the existing associations between the composition of the volatile fraction and the sensory quality of sparkling wines. • Conclusions The study carried out in this doctoral thesis makes it possible to establish the effects of temperature, yeast strain and ageing time on the analytical and sensory quality of sparkling wines during the two phases of the ‘prise de mousse’. During the in -bottle second fermentation, a lesser influence of the CO2 overpressure is observed on the viability of the typical yeast cells for the production of sparkling wines, whose formation kinetics is faster compared to the veil yeast strain; which is justified for being a strain that is not adapted to this type of winemaking. Although the viability of the flor-yeast is lower compared to the conventional one, it appears to be a good candidate for wine clarification in the riddling phase, due to its ability to form thick flocs with rapid sedimentation and less adherence to the walls of the bottle. This observation is also reinforced with the results of the volatile fraction, obtaining significant differences in the contents of the chemical families of the aroma that depend on the yeast strain and the pressure of the CO2 gas generated. The CO2 overpressure causes a decrease in the chemical and fruity odorants series in the wines obtained by the tested yeasts. During the ageing period, a greater effect of the time factor is observed on the volatile fraction, compared to the yeast strain. Through sensory analysis, the wines made with flor yeast obtained higher scores, which is related to the greater limpidity of the wines obtained and, probably, to the interactions of odorant compounds that are only detectable by tasting. During the 'prize de mousse' stage, which involved the analysis of the remaining factors (temperature, format and ageing time), a slowdown in fermentation kinetics is observed when using immobilized yeasts at low temperatures, mainly caused by the stress suffered during the immobilization process. The study carried out allowed the establishment of key volatile compounds as markers of the influence of each factor studied. The good results obtained in the classification of wines through the use of the electronic nose make evident its usefulness as a tool for quality control and fraud detection in sparkling wines. In general terms, the influence of the studied factors on volatilome and its relationship with sensory attributes will allow better quality control of the sparkling wine making process, product diversification and greater innovation in this sector of the agri-food industry. • References 1. OIV: OIV Focus: The global sparkling wine market. In. http://www.oiv.int/en/; 2020. 2. Di Gianvito P, Arfelli G, Suzzi G, Tofalo R: New trends in sparkling wine production: Yeast Rational Selection. In: Alcoholic Beverages. Edited by Grumezescu AM, Holban AM: Woodhead Publishing; 2019: 347-386. 3. Jones JE, Kerslake FL, Close DC, Dambergs RG: Viticulture for sparkling wine production: A review. American Journal of Enology and Viticulture 2014, 65(4):407-416. 4. Kemp B, Alexandre H, Robillard B, Marchal R: Effect of production phase on bottle-fermented sparkling wine quality. Journal of Agricultural and Food Chemistry 2015, 63(1):19-38. 5. 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