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76 W i n e s & V i n e s D e C e M b e r 2 0 1 4 I n order for proper fermentation to occur, yeast must have adequate nutrients available. One essential nutrient re- quired for yeast health during the fermen- tation process is nitrogen: It contributes to the development of essential yeast molecules, which allow for healthy yeast growth and metabolism. The yeast assimi- lable (or available) nitrogen (YAN) content can be measured at harvest (in grape juice or must) and indicates the level of nitrogen (N) available at the start of fermentation. The YAN value for a given lot of grape must or juice directs winemakers to determine what nutrient additions need to be made to ensure a complete fermentation and minimize the potential for hydrogen sulfide (H 2 S) production. YAN is composed of inorganic (ammo- nium ion) and organic (amino acid) nitrogen components. Amino acids are brought into the yeast cell through transport across the cell membrane. The presence of alcohol and ammonium ions (i.e., diammonium phosphate or DAP) inhibits amino acids from being brought into the cell. This is why winemakers are advised not to add DAP at inoculation or at the beginning of fermentation, as yeast can actively absorb organic nitrogen in the juice (aqueous) environment. Once alcohol concentrations begin to increase as a result of primary fermentation progression, transport of amino acids from the wine into the yeast cell will be inhib- ited. Therefore, the primary source of nitro- gen will then come from inorganic sources such as DAP. High YAN levels can be a fermentation nightmare A high-YAN concentration of more than 300mg of nitrogen per liter of must can create a challenging situation for the winemaker. Due to the excessive amount of available nutrients, yeast can grow and reproduce quickly, which often leads to very rapid fermentations. The speed of fermentation, of course, can affect the aromatics and quality of the wine, and fast fermentations often lead to more simple aroma and flavor profiles. This may not be an issue with some fermenta- tions, but for many white wine or fruit (other than grapes)-based fermentations, aromatic retention should be a priority of the winemaker. Higher concentrations of the inorganic component of YAN can lead to a high initial biomass of yeast. This is a problem because the rapid increase in yeast populations can lead to starvation by the majority of the yeast by mid- to late fermentation. Yeast starvation leads to yeast stress, which can increase the incidence of stuck or sluggish fermentations. Another stress response by yeast is the production and release of hydrogen sulfide. This can obviously cause hydrogen sulfide issues in wine by the time fermentation is complete. When the initial YAN is very high, the yeast population may not utilize all of the nitrogen contents by the end of fermenta- tion, and nitrogen could remain suspended in the finished wine. This excess "food" could be available for other microorgan- isms such as acetic acid bacteria or Brettanomyces, creating potential spoilage problems if the wine is not properly stabi- lized. Such spoilage obviously is detrimen- tal to wine quality. High initial YAN values may also lead to increased concentrations of ethyl carba- mate, according to a recent report from Cor- nell University. Ethyl carbamate is naturally produced by fermentation, but it is a mild carcinogenic compound. For this reason, many countries have established maximum legal levels for ethyl carbamate concentra- tions in wine. How to manage high YANs The first step toward managing YAN levels is to know what that level is. Winemakers should analyze for YAN during their normal pre-harvest assessment of their grapes one to two weeks before those grapes are har- vested. When the fruit arrives at the winery, the grapes can be tested again, although it is not required to repeat the pre-harvest test. Whether using the pre-harvest num- bers or those from delivered grapes, am- moniacal and amino acid-derived nitrogen WineEast Wine East HIGHLIGHTS • it is critical that winemakers test grapes for yeast assimilable nitrogen (YAn) before fermentation begins. • Careful observation of the fermentation and modification of nutrient additions can reduce H 2 s development. • excess nitrogen not utilized by yeast during fermentation may be food for microorganisms and lead to spoilage. • DAP should not be added with yeast hydration, as it is toxic to the yeast and can inhibit primary fermentation. High-YAN Fermentations Problems and solutions for managing nitrogen to achieve better fermentation results By Denise Gardner Winemaking Adding hydration nutrients during yeast in- oculation is one way to achieve desired YAN. DENISE GARDNER