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30 { THE SOMM JOURNAL } FEBRUARY/MARCH 2017 { getting geeky } AFTER EXPLORING NUTRIENTS IN vineyards last issue, I wanted to extend my exploration to the role of nutrients during fermentation, this time with a focus on nitrogen. An interesting conversation with a budding-winemaker friend resulted in my exploration of nitrogen in the winemaking process. As I researched YAN (yeast assimi - lable nitrogen), the rabbit-hole deepened and widened. So, let's break it down. Yeast need nitro- gen to ferment; it is the most important nutrient during fermentation. Nitrogen, however, presents in different forms, and the yeast can use only a few of these. Nitrogen can be found in peptides and other larger proteins in grapes, but only YAN, basically free amino acids (organic) and ammonium (inorganic), along with a few other peptides, can be used by Saccharomyces cerevisiae. The amount of YAN will differ based on grape variety, condition of the fruit, vineyard soils and viti - cultural practices. It is monitoring YAN and supplementing the yeast during fermenta- tion that delivers different results. Don't give the fermenting juice enough nitrogen, and the fermentation might stall. Give the juice too much, and it will encourage other bacterial growth—quite the conundrum. The most frequently used nitrogen sup - plement is DAP (diammonium phosphate) although some organic supplements may be utilized—and further study is being con- ducted on the differences in end-product as a result of this choice. DAP is usually applied about a third of the way through fermenta - tion, when Saccharomyces cerevisiae really starts to take over as the dominant yeast strain. Another addition may be added about two-thirds of the way through when nitro - gen levels have been depleted by the active, hard-working yeast, but another addition is riskier and only applied if the fermentation threatens to get stuck. It seems there are two major effects of adjusting (boosting) nitrogen levels during fermentation. With more nitrogen to metabolize, the fermentation speed increases, and consequently the heat of fermentation increases, resulting in less aro - matic complexity. In addition, with extrane- ous nitrogen, spoilage organisms can mul- tiply, further compromising aromatics by presenting off-aromas, like Brettanomyces or hydrogen sulfide markers. Metabolism of amino acids results in higher alcohols, and the resulting glycerol and ethyl and acetate esters. A high level of ammonium leads to diminished thiol development, resulting in more muted aromatics. Measuring the YAN and applying just the correct amount of DAP if needed is the key to the desired wine profile. Mind you, I am by no means a wine - maker, so this is an over-simplification of a complex subject. For further study, see www.lallemandwine.com and aem.asm.org. CONTINUING WITH NUTRIENTS: NITROGEN IN FERMENTATION Source of Plant Life story and photo by Allyson Gorsuch The French oak fermentation vats at Bodegas Roda in Rioja, Spain.