Issue link: https://read.dmtmag.com/i/705606
w w w. v w m m e d i a . c o m J u l y - A u g 2 016 | V I N E YA R D & W I N E RY M A N A G E M E N T 4 9 reduction in Chardonnay: The min- eral character this produces is often attributed to terroir and, indirectly, it could be a site character (because s o m e s i t e s r e g u l a r l y p r o d u c e must with low YAN, leading to the development of reductive charac- ters). But these good VSCs are the exceptions. The majority of H 2 S produced during winemaking is a side prod- uct of attempts by the yeast to syn- thesize the sulfur-containing amino acids methionine and cysteine. They're found in low concentra- tions in grape juice and the yeast tries to synthesize them via the sulfate reduction sequence (SRS). The yeast takes sulphate from the grape juice and transports it into the cell before reducing it to sulfite and then again to sulfide. This sulfide is combined with a nitrogen-contain- ing precursor to form cysteine or methionine. If there isn't enough YAN present, sulphide builds up Typical YAN concentrations natu- rally occurring in wine would be 50 to 450 mg/L, with the most com- mon range being 100 to 200 mg/L. The recommendations are that, as a minimum, whites need 150 mg/L and reds 100 mg/L, with a more comfortable level being around 250 mg/L. The maximum demand from the yeast is around 400 mg/L. As a rough guideline, 150 mg/L is the tipping point. Below this, it's likely you'll see some flavor impact from reduced nutrient availability and the risk of stuck fermentation increases. WIDELY USED NUTRIENTS There are two different sorts of nutrients widely used in wine- making. The first, and least expen- sive, is diammonium phosphate (DAP), which is an inorganic nitro- gen source. The second is a set of organic nitrogen supplements, most of which are based on yeast and is released from the yeast cells as H 2 S. This is the beginning of reduction problems. OTHER MICROBES Other microbes present in the must can also deplete nutrients. For example, during a pre-fermen- tation cold soak, other microbes can deplete nutrients before the yeasts get going and, as sampling for YAN usually takes place at the juice stage, this change wouldn't be picked up. There can also be prob- lems the opposite direction: If you don't know the nutritional needs of the yeast strain you're using, you can supplement too much and leave nutrients in the wine to feed other microbes. Adding too much nitrogen can also result in flavor defects. Excess nitrogen leads to production of acetate esters, espe- cially ethyl acetate, which smells of nail polish.