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

Vasileios Englezos,Sylvie Dequin,Luca Cocolin,Kalliopi Rantsiou,Anne Ortiz-Julien,Audrey Bloem,Carole Camarasa

doi:10.1128/aem.00797-18

Abstract

Over the last few years, the potential of non-Saccharomyces yeasts to improve the sensory quality of wine has been well recognized. In particular, the use of Starmerella bacillaris in mixed fermentations with Saccharomyces cerevisiae 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 Starm. bacillaris 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 Starm. bacillaris strains generally behaved uniformly. Starm. bacillaris strains are differentiated by their preferential uptake of ammonium compared with amino acids that are poorly assimilated or even produced (alanine). Otherwise, the non-Saccharomyces 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 Starm. bacillaris This knowledge will be useful to better control the fermentation process in mixed fermentation with Starm. bacillaris and S. cerevisiaeIMPORTANCE Mixed fermentations using a controlled inoculation of Starmerella bacillaris and Saccharomyces cerevisiae starter cultures represent a feasible way to modulate wine composition that takes advantage of both the phenotypic specificities of the non-Saccharomyces strain and the ability of S. cerevisiae to complete wine fermentation. However, according to the composition of grape juices, the consumption by Starm. bacillaris of nutrients, in particular of nitrogen sources, during the first stages of the process may result in depletions that further limit the growth of S. cerevisiae and lead to stuck or sluggish fermentations. Consequently, understanding the preferences of non-Saccharomyces 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 Starm. bacillaris and S. cerevisiae species. The results of our study demonstrate a clear preference for ammonium compared to amino acids for the non-Saccharomyces 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.

Highlights

IMPORTANCE Mixed fermentations using a controlled inoculation of Starmerella bacillaris and Saccharomyces cerevisiae starter cultures represent a feasible way to modulate wine composition that takes advantage of both the phenotypic specificities of the non-Saccharomyces strain and the ability of S. cerevisiae to complete wine fermentation
Cerevisiae strains were grown in duplicate in SM200 synthetic medium with a high sugar concentration (229 g/liter) and 202 mg/liter of yeast-assimilable nitrogen (YAN), which consisted of a mixture of 19 amino acids and ammonium ions
The growth and the kinetics of metabolite formation from central carbon metabolism (CCM) were monitored according to the fermentation and profiles of the produced volatile compounds determined at the end of culturing

Summary

Introduction

IMPORTANCE Mixed fermentations using a controlled inoculation of Starmerella bacillaris and Saccharomyces cerevisiae starter cultures represent a feasible way to modulate wine composition that takes advantage of both the phenotypic specificities of the non-Saccharomyces strain and the ability of S. cerevisiae to complete wine fermentation. Along with the addition of an S. cerevisiae strain, the use of mixed starter cultures with selected non-Saccharomyces and S. cerevisiae yeasts by simulating spontaneous fermentation can result in a greater complexity of wine and produce unusual aromas and flavors in ways not that cannot be attained with a pure starter culture of S. cerevisiae [6] The production of these complex aromas and flavors is mainly due to the ability of the nonconventional species to produce target metabolites or hydrolyze aromatic precursors [7]. Despite these positive aspects, in recent years, concern regarding the use of sequential mixed-culture fermentations has been noted, because the initial growth of non-Saccharomyces yeasts may compete with S. cerevisiae for nutrients, limiting their subsequent growth and increasing the risk of sluggish or stuck fermentation [8]. In the presence of amino acids and ammonium, wine yeasts sequentially take up nitrogen sources, and the order of assimilation is controlled by molecular mechanisms [15]

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