Yeast Assimilable Nitrogen Requirements Of Innoculated And Spontaneous Fermentations Of Riesling (V. Vinifera L.)

Abstract

Nitrogen plays a major role in the metabolic processes of fermentative microorganisms, affecting fermentation kinetics and formation of flavor-active compounds. Though a Yeast Assimilable Nitrogen (YAN) concentration of 140 mg N/L is generally recognized as the minimum required to avoid stuck or sluggish fermentations, most research on YAN requirements has focused on warm climate cultivars and conditions, and has been extrapolated for use in other regions. This work aimed to define optimal YAN concentrations for cool-climate fermentations, specifically those of Riesling. Riesling grape juice with YAN concentrations adjusted to 130-300 mg N/L with diammonium phosphate (DAP) additions was fermented with three yeast strains commonly used in the Finger Lakes, NY, wine region. Fermentation kinetics suggested that 130 mg N/L concentration was enough to complete fermentation for the three yeast strains studied, and DAP additions improved fermentation kinetics only for yeast strain EC1118. Analysis of select volatile compounds via GC-MS showed that ester concentrations increased with nitrogen addition, but the unsupplemented control wine was preferred by panelists in a sensory study. In a separate experiment, spontaneous fermentations of Riesling were monitored in two Finger Lakes wineries to assess their YAN requirements and microbiome composition. NonSaccharomyces yeast species were isolated through late stages of fermentation, and most of the S. cerevisiae isolates identified were found to be similar to commercial strains. Fermentation kinetics and microflora were markedly different in the two wineries studied, but in all spontaneous fermentations the YAN consumption range was lower than that in the inoculated fermentations described above. This work suggests that nitrogen requirements for cool climate Riesling fermentations are moderate (140 mg N/L or lower), and that DAP supplementation should be applied with caution to avoid excess residual nitrogen and possible negative effects on sensory properties. The initial assessment of spontaneous fermentations indicated that nitrogen requirements are likely lower than inoculated fermentations, precluding the need for nitrogen additions in the winery.