Abstract
Nitrogen sources in the must are important for yeast metabolism, growth, and performance, and wine volatile compounds profile. Yeast assimilable nitrogen (YAN) deficiencies in grape must are one of the main causes of stuck and sluggish fermentation. The nitrogen requirement of Saccharomyces cerevisiae metabolism has been described in detail. However, the YAN preferences of non-Saccharomyces yeasts remain unknown despite their increasingly widespread use in winemaking. Furthermore, the impact of nitrogen consumption by non-Saccharomyces yeasts on YAN availability, alcoholic performance and volatile compounds production by S. cerevisiae in sequential fermentation has been little studied. With a view to improving the use of non-Saccharomyces yeasts in winemaking, we studied the use of amino acids and ammonium by three strains of non-Saccharomyces yeasts (Starmerella bacillaris, Metschnikowia pulcherrima, and Pichia membranifaciens) in grape juice. We first determined which nitrogen sources were preferentially used by these yeasts in pure cultures at 28 and 20°C (because few data are available). We then carried out sequential fermentations at 20°C with S. cerevisiae, to assess the impact of the non-Saccharomyces yeasts on the availability of assimilable nitrogen for S. cerevisiae. Finally, 22 volatile compounds were quantified in sequential fermentation and their levels compared with those in pure cultures of S. cerevisiae. We report here, for the first time, that non-Saccharomyces yeasts have specific amino-acid consumption profiles. Histidine, methionine, threonine, and tyrosine were not consumed by S. bacillaris, aspartic acid was assimilated very slowly by M. pulcherrima, and glutamine was not assimilated by P. membranifaciens. By contrast, cysteine appeared to be a preferred nitrogen source for all non-Saccharomyces yeasts. In sequential fermentation, these specific profiles of amino-acid consumption by non-Saccharomyces yeasts may account for some of the interactions observed here, such as poorer performances of S. cerevisiae and volatile profile changes.
Highlights
The main sources of yeast assimilable nitrogen (YAN) in grape must are ammonium and amino acids
At 20◦C, the total NS yeast population varied from 1.9 × 107 cells/mL for M. pulcherrima BB810 (Supplementary Figure 1C) to 2.5 × 107 cells/mL for P. membranifaciens BB3 (Supplementary Figure 1D), whereas that for S. bacillaris BBMV5FA17 (Supplementary Figure 1B) reached ∼108 cells/mL like S. cerevisiae Selectys R La Marquise” (SLM) (Supplementary Figure 1A)
At the start of fermentation, NS yeasts consume some of the nutrients present, including Yeast assimilable nitrogen (YAN), which is known to be a source of volatile compounds precursors in S. cerevisiae (Hazelwood et al, 2008)
Summary
The main sources of yeast assimilable nitrogen (YAN) in grape must are ammonium and amino acids. It is generally agreed that at least 120–140 mg N/L YAN is required for satisfactory fermentation kinetics and final product quality (Bely et al, 1990; Jiranek et al, 1995). Ammonium could be added which will influence biomass formation, fermentation kinetics, viability, and the production of aroma compounds (Hernández-Orte et al, 2005, 2006; Hazelwood et al, 2008). Preferred nitrogen sources are assimilated more rapidly and are generally used earlier in the fermentation than poor nitrogen sources. These differences can be attributed principally to differences in the efficiency of the corresponding transport systems
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