Dehydration stress responses of yeasts Torulaspora delbrueckii, Metschnikowia pulcherrima and Lachancea thermotolerans: Effects of glutathione and trehalose biosynthesis.

In food industry and winemaking, the use of active dehydrated yeast (ADY) Saccharomyces cerevisiae is a frequent practice because of the long-term stability and high efficiency of ADY. Nowadays, there is an increasing interest for new yeasts strains, such as Torulaspora delbrueckii (Td), Metschnikowia pulcherrima (Mp) and Lachancea thermotolerans (Lt). However, the yeasts transformation processes into the solidified form generate several stresses that reduce the cell viability. In this case, understanding the phenomena of yeast cell resistance before, during and after dehydration is of great importance. In this study we analyzed two compounds associated with resistance to stress and produced by cells, glutathione (total, oxidized and reduced) and trehalose, at different stages of the process. The impact of growing and dehydration conditions on cell viability was analyzed by flow cytometry and two-photon laser scanning microscopy. The results showed that cells naturally enriched in glutathione or trehalose acquired resistance to dehydration, preventing the oxidation of glutathione in a growth/dehydration condition dependent manner. This is the first time that simultaneous metabolic and dehydration responses were observed in three non-Saccharomyces strains. These findings represent an opportunity to better understand the yeast's dehydration resistance phenomena and thus to promote the efficient industrial production of new dried yeasts.

Oxidative stress resistance during dehydration of three non-Saccharomyces wine yeast strains.

The development of new fermented foods and beverages requires more and more the use of new dehydrated yeasts species. In this context, the non-Saccharomyces (NS) yeasts Torulaspora delbrueckii, Metschnikowia pulcherrima and Lachancea thermotolerans are developed especially in winemaking as co-culture in the fermentation of the must or for the must bioprotection. However, during formulation-dehydration the yeast cells are exposed to several stresses that reduce cellular activity. Among these, the oxidative stress induced by the stabilization processes has been described as one of the main causes of cell death. In this study, we analyzed the effects of growth medium associated with two dehydration kinetics on the accumulation of reactive oxygen species (ROS) and lipid peroxidation levels. The cultivability of tested yeast strains was dependent on growth and dehydration conditions. The L. thermotolerans strain was the most sensible to dehydration when growing in nutrient-poor media, which was characterized by high levels of ROS, lipid peroxidation and reduced cultivability. In contrast, this yeast was able to restore its cultivability when growing in nutrient-rich medium before dehydration. The other NS yeast strains acquired resistance to oxidative stress similarly but in a growth-dehydration condition less dependent manner. These results showed that modulation of growing medium composition is a simple way to improve resistance to oxidative attack imposed by dehydration in NS yeasts. This was the first time that multiple quantitative and qualitative indicators of oxidative stress response in these three NS yeast strains were explored.