Wine Yeast Strains Under Ethanol-Induced Stress: Morphological and Physiological Responses

During alcoholic fermentation, ethanol accumulation significantly impacts yeast cells by disrupting membrane integrity, increasing permeability, and reducing cell viability. This study evaluated the effects of ethanol stress on the growth, membrane fluidity, and cell surface morphology of Saccharomyces cerevisiae and non-Saccharomyces yeast strains, specifically Torulaspora delbrueckii and Metschnikowia pulcherrima. These strains, commercialized by AEB SpA and preserved at the Unimore Microbial Culture Collection (UMCC), were tested for fermentative performance in grape must and grown under varying ethanol concentrations. Membrane fluidity was measured using Laurdan generalized polarization (GP), while Atomic Force Microscopy (AFM) assessed cell surface mor- phology. Results indicated that at 10% ethanol, membrane fluidity increased, particularly in strains able to tolerate up to 16% ethanol, which also demonstrated superior fermentative performance. Less tolerant strains, such as T. delbrueckii UMCC 5 and M. pulcherrima UMCC 15, showed smaller increases in fluidity. At 18% ethanol, these strains exhibited severely altered surface morphology and larger surface roughness values, suggesting increased instability under high ethanol stress, while more tolerant strains displayed better-preserved surface morphology and lower roughness values, reflecting enhanced adaptability. These findings offer insights into yeast responses to ethanol stress, supporting the development of more resilient strains for improved fermentation.