Specific Phenotypic Traits of Starmerella bacillaris Related to Nitrogen Source Consumption and Central Carbon Metabolite Production during Wine Fermentation

Abstract

Over the last few years, the potential of non-<i>Saccharomyces</i> yeasts to improve the sensory quality of wine has been well recognized. In particular, the use of <i>Starmerella bacillaris</i> in mixed fermentations with <i>Saccharomyces cerevisiae</i> was reported as an appropriate way to enhance glycerol formation and reduce ethanol production. However, during sequential fermentation, many factors, such as the inoculation timing, strain combination, and physical and biochemical interactions, can affect yeast growth, the fermentation process, and/or metabolite synthesis. Among them, the availability of yeast-assimilable nitrogen (YAN), due to its role in the control of growth and fermentation, has been identified as a key parameter. Consequently, a comprehensive understanding of the metabolic specificities and the nitrogen requirements would be valuable to better exploit the potential of <i>Starm. bacillaris</i> during wine fermentation. In this study, marked differences in the consumption of the total and individual nitrogen sources were registered between the two species, while the two <i>Starm. bacillaris</i> strains generally behaved uniformly. <i>Starm. bacillaris</i> strains are differentiated by their preferential uptake of ammonium compared with amino acids that are poorly assimilated or even produced (alanine). Otherwise, the non-<i>Saccharomyces</i> yeast exhibits low activity through the acetaldehyde pathway, which triggers an important redistribution of fluxes through the central carbon metabolic network. In particular, the formation of metabolites deriving from the two glycolytic intermediates glyceraldehyde-3-phosphate and pyruvate is substantially increased during fermentations by <i>Starm. bacillaris</i> This knowledge will be useful to better control the fermentation process in mixed fermentation with <i>Starm. bacillaris</i> and <i>S. cerevisiae</i><b>IMPORTANCE</b> Mixed fermentations using a controlled inoculation of <i>Starmerella bacillaris</i> and <i>Saccharomyces cerevisiae</i> starter cultures represent a feasible way to modulate wine composition that takes advantage of both the phenotypic specificities of the non-<i>Saccharomyces</i> strain and the ability of <i>S. cerevisiae</i> to complete wine fermentation. However, according to the composition of grape juices, the consumption by <i>Starm. bacillaris</i> of nutrients, in particular of nitrogen sources, during the first stages of the process may result in depletions that further limit the growth of <i>S. cerevisiae</i> and lead to stuck or sluggish fermentations. Consequently, understanding the preferences of non-<i>Saccharomyces</i> yeasts for the nitrogen sources available in grape must together with their phenotypic specificities is essential for an efficient implementation of sequential wine fermentations with <i>Starm. bacillaris</i> and <i>S. cerevisiae</i> species. The results of our study demonstrate a clear preference for ammonium compared to amino acids for the non-<i>Saccharomyces</i> species. This finding underlines the importance of nitrogen sources, which modulate the functional characteristics of inoculated yeast strains to better control the fermentation process and product quality.