Development of biotechnological systems based on the application of oenological bio activators to enhance the cellular vitality and fermentative capacity of yeasts in the presence of stress factors

The present doctoral thesis investigated the physiological status of wine yeasts and the resulting impact on alcoholic fermentation performance and final wine quality under conditions of environmental stress, with particular emphasis on oxidative stress and low pH. These stress factors are among the most critical challenges in modern enology, as they can significantly affect yeast viability, metabolic activity, fermentation kinetics, and the chemical and sensory characteristics of wine. In parallel, increasing consumer demand for wines with reduced chemical inputs has stimulated the development of alternative microbiological and technological strategies aimed at enhancing yeast resilience and protecting must and wine quality through more sustainable approaches.The first part of the thesis focuses on oxidative stress occurring during the pre-fermentative and early fermentative stages. Oxygen exposure can promote undesirable oxidative reactions, resulting in browning, aroma degradation, and the proliferation of spoilage microorganisms. To address these challenges, different microbiological and biochemical strategies were evaluated. The application of a glutathione-rich inactivated yeast product was investigated both alone and in combination with the non-Saccharomyces yeast Metschnikowia pulcherrima. This approach demonstrated a significant reduction in oxygen uptake and browning phenomena during the pre-fermentative phase, while also improving the aromatic profile of the resulting wines, particularly with respect to fruity and floral volatile compounds. These findings highlighted the protective role of glutathione-rich products and selected non-conventional yeasts in mitigating oxidative damage and preserving wine quality, supporting their potential as alternatives to conventional antioxidant practices.Building on these results, the second part of the thesis explored the innovative use of a Saccharomyces cerevisiae strain applied both as a bio-protective agent during the pre-fermentation phase and as the main fermentative starter culture. This dual-function strategy was evaluated for its ability to limit oxidative phenomena, improve microbial control, and ensure efficient alcoholic fermentation. Early inoculation of S. cerevisiae promoted rapid oxygen consumption, reduced oxidation-related compounds, and enhanced the production of desirable volatile compounds associated with positive sensory attributes. Furthermore, this approach ensured stable fermentation kinetics and contributed to improved overall sensory quality, demonstrating its effectiveness as a simplified and sustainable microbiological strategy capable of simultaneously protecting must integrity and supporting fermentation performance.The third part of the thesis investigated yeast adaptation to low pH stress, a condition frequently encountered in grape musts intended for specific wine styles, such as sparkling and aromatic white wines. Particular attention was given to the early stages of alcoholic fermentation, which represent a critical window for yeast adaptation. The physiological and molecular responses of S. cerevisiae were evaluated under standard and low pH conditions, with and without the addition of yeast-derived bio-activators. The results demonstrated that low pH significantly affects yeast viability and prolongs the lag phase, potentially compromising fermentation efficiency. However, the addition of bio-activators improved cell viability, accelerated fermentation onset, and enhanced the expression of stress-related genes involved in antioxidant defense and cellular protection. These findings provided important insights into the mechanisms underlying yeast adaptation to acidic environments and highlighted the potential of bio-activators to improve fermentation robustness under stressful conditions.Finally, the thesis addresses the microbiological and oenological implications of white winemaking with prolonged skin maceration, commonly referred to as orange wine production. This peculiar winemaking approach is associated with increased extraction of phenolic compounds and enhanced oxidative and antioxidant dynamics, which can influence both yeast metabolism and wine composition. The study evaluated the antioxidant capacity and color characteristics of orange wines produced under controlled conditions, providing preliminary insights into the oxidative balance and stability of these products. Additional metabolomic and chemical investigations are currently ongoing and will further contribute to understanding the complex interactions between yeast physiology, oxidative processes, and wine composition in this unique winemaking context.Overall, this thesis provided a comprehensive evaluation of yeast physiological responses and fermentation performance under oxidative and acidic stress conditions, integrating microbiological, chemical, molecular, and sensory analyses. The results contributed to a deeper understanding of yeast adaptation mechanisms and demonstrated the effectiveness of targeted microbiological and nutritional strategies to enhance fermentation reliability, reduce oxidative damage, and improve final wine quality. These findings supported the development of more sustainable and biologically driven winemaking practices, with reduced reliance on chemical additives and improved control over fermentation processes and product quality.