Yeasts are essential in biotechnological processes like alcoholic beverage production, where they face various stresses, including ethanol-induced stress. Ethanol's solubility in water and lipids allows it to penetrate cell membranes, increasing fluidity, compromising integrity, and enhancing permeability. This can disrupt mitochondrial function, reduce ATP levels, and promote oxidative stress, decreasing cellular vitality.
This study aimed to investigate the effects of ethanol stress on the growth, membrane fluidity, and cell surface morphology of yeast strains from Saccharomyces cerevisiae and non-Saccharomyces species, specifically Torulaspora delbrueckii and Metschnikowia pulcherrima. These strains, commercialized as wine starters by AEB SpA, are preserved at the Unimore Microbial Culture Collection (UMCC).
The strains' fermentative fitness was assessed in grape must, and their growth under ethanol stress was evaluated using selective media with varying ethanol concentrations. Membrane fluidity was measured using a Laurdan generalized polarization, and cell surface morphology was observed through Atomic Force Microscopy (AFM).
The results showed that all S. cerevisiae strains exhibited high ethanol tolerance, sustaining growth up to 14% (v/v) ethanol, with the most tolerant strains growing even at 16% (v/v). Non-Saccharomyces strains showed compromised growth above 10% (v/v) ethanol, though T. delbrueckii was less inhibited at 10% ethanol than M. pulcherrima. Strains with higher ethanol tolerance and better fermentative aptitude demonstrated increased membrane fluidity at 10% (v/v) ethanol, while less tolerant strains showed lower fluidity. Additionally, non-Saccharomyces strains had higher Root Mean Square (RMS) values at 18% (v/v) ethanol, indicating greater instability under high ethanol stress, whereas more tolerant strains had the lowest RMS values, reflecting superior adaptability.
This study provides valuable insights into yeast strains' distinct responses to ethanol stress, highlighting the importance of membrane fluidity and cell surface alterations in developing more resilient strains for enhanced fermentation processes. This research was supported by AEB SpA.