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120 WINES&VINES January 2016 WINEMAKING PRACTICAL WINERY & VINEYARD varietal thiols). As a result, the nitrogen composition of the must can modulate the aroma profile of a wine. Organic nutrients have also been shown to positively in- fluence the formation of aroma compounds when added during alcoholic fermentation. Impact of different nitrogen sources on sensory profile Esters: The metabolism of amino acids (anabolism and catabolism) by yeast leads to the formation of higher alcohols, acetate esters and ethyl esters. The ester profile of Chardonnay (see "Ester Forma- tion With Organic Nutrition") was different when different forms of nitrogen were added. For example, comparing the synthesis of ester compounds with an addition of 50 mg/L of YAN in DAP form compared to 24 mg/L of YAN (Fermaid O) and 50 mg/L of YAN (DAP/Fermaid O) showed a significant increase for all aro- matic compounds that are ob- served with the organic nutrient, demonstrating the impact of or- ganic nitrogen compared to inor- ganic nitrogen on the formation of esters. Thiols: During fermentation it has been shown that excessive inor- ganic ammonium or addition at the beginning of alcoholic fermen- tation limits the release of varietal thiols by the yeast. 12 Catabolic repression by the inorganic nitro- gen in the form of ammonium prevents the synthesis of amino acid transporters in fermenting yeast. It also limits the entry of thiol precursors of the cysteinyl- ated form into the cell and, con- sequently, their intracellular conversion into volatile thiols (see Transport of Thiol Precursors in Yeast Cells"). In terms of thiol production ( s e e " T h i o l Fo r m a t i o n Wi t h DAP"), the thiols 3-MH and 3-MHA are lower when DAP is used. This observation, made in the laboratory and on a pilot scale, has led to implementation of a strategy for nitrogen nutri- tion during alcoholic fermenta- tion, with the addition of organic nutrient only, divided between the beginning of the alcoholic fermentation and at one-third of the alcoholic fermentation. Two different formulas of or- ganic nutrients have been tested on Colombard juice, which is very rich in thiol precursors. Looking at the results on the 3-MH release and its acetate of 3-MH, we con- firmed the positive impact of or- ganic nutrition on thiol conversion and expression (See "Thiol Forma- tion With Organic Nutrition"). Impact of nitrogen source on wine sensory profile In all trials, we were able to high- light the positive influence of or- ganic nutrient on the wine sensory profile, mainly when compared to inorganic nutrition such as DAP addition. In (See "Sensory Profile With Organic Nutrition"), the dif- ESTER FORMATION WITH ORGANIC NUTRITION Chardonnay from Yalumba Winery in Australia fermented with two different sources of nitrogen (DAP and Fermaid O). TRANSPORT OF THIOL PRECURSORS IN YEAST CELLS Transport of Cys-3MH amino acids into the yeast cell membrane via the Gap1p transport protein, and catabolic repression in the presence of excessive inor- ganic nitrogen in the must. 45 40 35 30 25 20 15 10 5 0 Percentage vs. DAP (50mg/L YAN 2-methyl- propyl acetate 2-methyl- butyl acetate 3-methyl- butyl acetate ethyl buatonate ethyl octanoate hexyl acetate phenyl- ethyl acetate Complex nutrition: DAP/Fermaid O @ 50mg/L YAN Organic nutrition: Fermaid O @ 24mg/L YAN THIOL FORMATION WITH DAP Thiol concentration in Sauvignon Blanc (Gers, France 2004) with DAP addition. 12 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0 No addition DAP 3MH 3MHA g/L