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Yeast aroma compounds – The impact of DAP on the sensorially important yeast-derived flavor compounds is summarized in Figure 2 together with the varietal compounds. Of the yeastderived flavor compounds that were affected by DAP supplementation, 12 compounds, whose OAV were greater than 1.0, are potentially important to the sensory profile of wine.
The fruity ethyl esters, represented by nine compounds and especially ethyl butanoate and ethyl hexanoate, were highest with moderate DAP supplementation (350 mg/L YAN). Among the volatile fatty acids, hexanoic acid showed a significant contribution to wine aroma with large nitrogen addition. More detailed discussion of DAP modulation of yeast aroma compounds can be found in AWRI publication #137511 and in the section below on Chardonnay wine.
Overall, these results suggest that wines made with moderate DAP supplementation of juice already having moderate YAN content exhibit the largest potential aromatic impact, whereas large DAP addition reduces potential aromatic impact.
Fermentation nitrogen – effects on yeast flavor-active compounds in Chardonnay
Compared with floral varieties in which terpenes are major contributors to wine aroma, the aroma profile of non-floral varieties, such as Chardonnay, depends largely on grape C13-norisoprenoids and yeast-flavor compounds.2 Chardonnay is a good example in which to study yeast contribution to wine aroma.
A Chardonnay juice (22° Brix), having low YAN (160 mg N/L), was chosen to investigate the effect of inorganic N (ammonium nitrogen) supplementation on the major aroma metabolites produced by yeast. Inorganic N was added to produce a wide range of initial YAN values ranging from 160 (low N), 320 (moderate N), to 480 (high N) mg N/L; the first value typically being just sufficient for risk-free fermentation whereas the latter typically exceeding yeast’s total demand for nitrogen and leading to residual wine YAN.3
Figure 3: Effect of ammonium nitrogen supplementation on the formation of yeast aroma active metabolites. Clarified Chardonnay must (160 mg/L YAN) was supplemented with ammonium nitrogen to produce 320 and 480 mg/L YAN, and fermented with (Maurivin) AWRI 796. (Adapted from AWRI publication #1277.)
Fermentations were carried out with AWRI 796 yeast at 18° C. Fermentation duration was 13, 8, and 5 days for low, moderate, and high N juices, respectively.8
Supplementation of juices with ammonium nitrogen affected all yeastderived aroma metabolites measured, including acetates, ethyl esters, higher alcohols (fusel alcohols), and volatile fatty acids.4,10 The results are summarized in Figure 3.
Ethyl acetate, the major ester produced by yeast, increased in concentration from 40 mg/L to a maximum of 100 mg/L with the largest nitrogen addition, whereas total esters (excluding ethyl acetate) reached a maximum value at moderate nitrogen. Higher alcohols, on the other hand were inversely related to YAN, such that their aroma impact strongly diminished. Volatile fatty acids showed a variable pattern.
Overall, nitrogen supplementation of Chardonnay strongly increases wine ester content and decreases the alcohol content, which results in wines acquiring a more fruity, floral aroma profile, as described in the following sensory section.
Since yeast strains vary widely in ester-producing capacity and their responses to nitrogen supplementation, 3,7,15,17 the results obtained in this
Chardonnay trial should be considered a guide. Furthermore, the timing of DAP supplementation affects production of yeast aroma compounds, including H2S, with early additions tending to have greatest impact.13,14 Late DAP additions have reduced affect on aroma compounds and increase the risk of residual nitrogen in finished wine.
Fermentation nitrogen – effect on sensory profile of inorganic vs organic Chardonnay
There has been little study on the effect of nitrogen supplementation on the sensory profile of wine, despite the fact that DAP is widely used in winemaking. The Chardonnay wines, referred to in the previous section, were profiled by quantitative sensory descriptive analysis.8
In addition to investigating the effect of inorganic nitrogen (ammonium nitrogen) on aroma profile, organic nitrogen was also studied. Composition of organic nitrogen supplement was based on amino acid analysis of the Chardonnay juice. This nitrogen supplement therefore more closely models the nitrogen composition of juices that have naturally moderate to high (480 mg/L) YAN content. Of course, other changes in grape must composition as the result of vineyard nitrogen application will not be modelled.3
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