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Glycerol, on the other hand, varies widely in response to DAP supplementation (Figure 1). The pattern of glycerol production is dependent on the yeast strain and must composition, such that glycerol production varies to the greatest extent at low and high YAN concentrations.4,8 Since glycerol production also responds to various stress conditions, such as osmotic stress imposed by over-ripe grapes or to grape solids, nitrogen supplementation cannot be used reliably to modulate wine glycerol content without knowledge of yeast characteristics and must composition.
Organic acids – Fermentation typically leads to a variable but small change in both volatile and non-volatile acids, depending on yeast strain and fermentation conditions. Because each acid serves different metabolic roles, the physiological regulation of each acid, including that by nitrogen, is different (Figure 1).6
Acetic acid, the major volatile acid in wine, is produced by yeast in response to various stresses, most notably osmotic stress, which results from high sugar concentration. Most yeast strains produce the lowest volatile acidity at moderate YAN values (around 200 to 250 mg N/L), which increases towards lower and greater nitrogen concentrations.4,13
Malic acid consumption is typically limited to less than 5% during fermentation; however, its consumption increases in response to higher YAN values, depending on the strain. Succinic acid, the major non-volatile acid produced by yeast, which can range 0.5 to 2 g/L, tends to increase as juice YAN increases.4,6 Abnormal amounts of succinic acid can result from high levels of γ-amino butyric acid, which forms in grape must under certain conditions.
These changes in the acid profile, which result from nitrogen supplementation, can contribute to a decrease in titratable acidity (TA) and pH. DAP addition of 0.5 g/L (100 mg/L YAN) can reduce TA by up to 1 g/L and pH by 0.1 unit, depending on yeast strain, and must composition and buffer capacity. Significant DAP addition can therefore affect perceived wine acidity. Further research is required to confirm whether beneficial changes to wine palate can be achieved within the constraints of legally permitted nitrogen addition in each country.
SO2 Most yeasts have been selected to produce little acetaldehyde and SO2 during fermentation. In a low YAN medium, some strains produce more acetaldehyde and SO2 in response to DAP addition.8,16 This may increase the risk of MLF failure.
Fermentation nitrogen – effects on grape and yeast flavor-active compounds in Albariño
Albariño is an aromatic white grape variety which, like other floral varieties such as Riesling and Traminer, requires fermentation to reveal its varietal potential. 11 Its distinctive citrus, fruit character, which is, in part, derived from glycosidic precursors, results from hydrolytic release during fermentation.
Hydrolytic release of glycosidically bound flavor compounds involves a combination of acidic and enzymatic catalyzed reactions. This variety, which dominates the northwest region of Spain and northern Portugal, is gaining rapid consumer acceptance in the New World, including Australia and the U.S.
In order to understand how DAP addition can modulate flavor contributions from the grape and the yeast, a Galician Albariño juice (20.5° Brix) with moderate YAN (250 mg N/L) was chosen for study. The control juice was supplemented with 0.5 and 1.0 g/L DAP to produce juices with 350 and 450 mg/L initial YAN concentrations, respectively. Fermentation was carried out with Vitilevure M05 at 18°C.11
Grape aroma compounds – Compared to control wines, moderate DAP addition (0.5 g/L DAP) produced wines with the highest concentration of varietal compounds, including free monoterpenes and norisoprenoids as well as most yeast-derived fermentation products (Figure 2). Only the statistically significant odor-active compounds are shown.
The important grape-derived aroma compounds analyzed were the monoterpenes, which included limonene (lemon note), linalool (floral, citrus) and α-terpineol (lilac), and the C13- norisoprenoids α-ionone (tobacco) and β-damascenone (apple, rose). At the highest YAN studied (450 mg/L), only β-ionone (violet) was present at its highest concentration.
In order to assess the likely sensory impact of these compounds on wine aroma as the result of DAP addition, an analysis of odor activities of the various volatile compounds was made. Of the six compounds with odor activity values (OAV) that exceeded 1.0, meaning that their concentration in wine exceeded their odor threshold concentration, five were significantly affected by DAP supplementation. This result suggests that DAP could be used to moderately modulate varietal character, at least in a floral variety. The impact of DAP on grape-derived varietal compounds backed up with sensory analysis requires closer attention.
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