The transcriptomic response of a wine strain of Lachancea thermotolerans to oxygen deprivation

Differences in the management of intracellular redox state between wine yeast species dictate their fermentation performances and metabolite production

The maintenance of the balance between oxidised and reduced redox cofactors is essential for the functioning of many cellular processes in all living organisms. While the electron transport chain plays a key role in maintaining this balance under respiratory conditions, its inactivity in the absence of oxygen poses a challenge that yeasts such as Saccharomyces cerevisiae overcome through the production of various metabolic end-products during alcoholic fermentation. In this study, we investigated the diversity occurring between wine yeast species in their management of redox balance and its consequences on the fermentation performances and the formation of metabolites. To this aim, we quantified the changes in NAD(H) and NADP(H) concentrations and redox status throughout the fermentation of 6 wine yeast species. While the availability of NADP and NADPH remained balanced and stable throughout the process for all the strains, important differences between species were observed in the dynamics of NAD and NADH intracellular pools. A comparative analysis of these data with the fermentation capacity and metabolic profiles of the strains revealed that Saccharomyces cerevisiae, Torulaspora delbrueckii and Lachancea thermotolerans strains were able to reoxidise NADH to NAD throughout the fermentation, mainly by the formation of glycerol. These species exhibited good fermentation capacities. Conversely, Starmerella bacillaris and Metschnikowia pulcherrima species were unable to regenerate NAD as early as one third of sugars were consumed, explaining at least in part their poor growth and fermentation performances. The Kluyveromyces marxianus strain exhibited a specific behaviour, by maintaining similar levels of NAD and NADH throughout the process. This balance between oxidised and reduced redox cofactors ensured the consumption of a large part of sugars by this species, despite a low fermentation rate. In addition, the dynamics of redox cofactors affected the production of by-products by the various strains either directly or indirectly, through the formation of precursors. Major examples are the increased formation of glycerol by S. bacillaris and M. pulcherrima strains, as a way of trying to reoxidise NADH, and the greater capacity to produce acetate and derived metabolites of yeasts capable of maintaining their redox balance. Overall, this study provided new insight into the contribution of the management of redox status to the orientation of yeast metabolism during fermentation. This information should be taken into account when developing strategies for more efficient and effective fermentation.

Multiparametric Approach to Interactions between Saccharomyces cerevisiae and Lachancea thermotolerans during Fermentation

The aim of a significant part of current wine technology research is to better understand and monitor mixed culture fermentations and optimize the microbiological processes and characteristics of the final wine. In this context, the yeast couple formed by Lachancea thermotolerans and Saccharomyces cerevisiae is of particular interest. The diverse results observed in the literature have shown that wine characteristics are dependent on both interactions between yeasts and environmental and fermentation parameters. Here, we took a multiparametric approach to study the impact of fermentation parameters on three different but related aspects of wine fermentation: population dynamics, fermentation, and volatile compound production. An experimental design was used to assess the effects of four independent factors (temperature, oxygenation, nitrogen content, inoculum ratio) on variables representing these three aspects. Temperature and, to a lesser extent, oxygenation and the inoculum ratio, were shown to constitute key factors in optimizing the presence of Lachancea thermotolerans during fermentation. The inoculum ratio also appeared to greatly impact lactic acid production, while the quantity of nitrogen seemed to be involved more in the management of aroma compound production. These results showed that a global approach to mixed fermentations is not only pertinent, but also constitutes an important tool for controlling them.

An Integrative View of the Role of Lachancea thermotolerans in Wine Technology

The interest in Lachancea thermotolerans, a yeast species with unusual characteristics, has notably increased in all ecological, evolutionary, and industrial aspects. One of the key characteristics of L. thermotolerans is the production of high quantities of lactic acid compared to other yeast species. Its evolution has mainly been driven by the influence of the environment and domestication, allowing several metabolic traits to arise. The molecular regulation of the fermentative process in L. thermotolerans shows interesting routes that play a complementary or protective role against fermentative stresses. One route that is activated under this condition is involved in the production of lactic acid, presenting a complete system for its production, showing the involvement of several enzymes and transporters. In winemaking, the use of L. thermotolerans is nowadays mostly focused in early–medium-maturity grape varieties, in which over-ripening can produce wines lacking acidity and with high concentrations of ethanol. Recent studies have reported new positive influences on quality apart from lactic acid acidification, such as improvements in color, glutathione production, aroma, malic acid, polysaccharides, or specific enzymatic activities that constitute interesting new criteria for selecting better strains. This positive influence on winemaking has increased the availability of commercial strains during recent years, allowing comparisons among some of those products. Initially, the management of L. thermotolerans was thought to be combined with Saccaharomyces cerevisiae to properly end alcoholic fermentation, but new studies are innovating and reporting combinations with other key enological microorganisms such as Schizosaccharomyces pombe, Oenocous oeni, Lactiplantibacillus plantarum, or other non-Saccharomyces.