Yeast and bacteria are living organisms that, like us, require a balanced diet to thrive. A diet solely based on sugar isn’t sufficient. Without proper nutrition, yeast can struggle, leading to stuck or sluggish fermentation and the production of hydrogen sulphides (H2S). The aromas associated with H2S range from struck matches and cabbage to sauerkraut and rotten eggs. While a small amount of reduction can be tolerated in some wine styles, excessive levels are undesirable, especially in sparkling base wines where reductive secondary fermentation can exacerbate the problem.
Winemakers have long supplemented yeast’s diet with nitrogen. Measuring Yeast Available Nitrogen (YAN) in grape juice at pressing allows fine-tuning of any needed additions or the decision that none are required. Testing for YAN is relatively inexpensive, especially considering the issues arising from sticky fermentation, but it requires specialist lab equipment and skilled operators. Sending samples to an external lab adds time and admin during the busy harvest period, tempting winemakers to add nutrients as a precaution. Most commercial nutrient additions are diammonium phosphate (DAP) or mineral nitrogen (NH4+). DAP can be seen as the “Big Mac” of yeast food, whereas naturally occurring nitrogen in grapes is about two-thirds organic nitrogen (amino acids). Once fermentation starts, yeast are not picky about their food’s origin, but the type of nitrogen may affect the resulting flavour profile. Adding amino acids (organic) can increase thiols, while DAP (mineral), in excess, can suppress them. For base wines, you may wish to avoid such an increase, while for aromatic whites, it may be highly desirable.
The amount of YAN required depends on the expected alcohol level and the yeast strain used. Common “champagne” strains like Fermentis SafŒno SPK 05, IOC 18-2007, and Lalvin EC-1118 have lower requirements than those for still wine ferments. Sparkling base wines are typically expected to ferment to 11% maximum, so their YAN requirements are lower than a still wine at 12% or more. For example, a juice with a potential alcohol of 12% requires around 200mg/L of YAN.
Insufficient YAN can lead to higher volatile acidity (VA) levels, which are legally limited in finished wine. This is another reason to check YAN at the juice stage. While classic varieties rarely have YANs below 200mg/L, aromatic whites and other varieties may be more susceptible to soil and microclimate variations.
Excess nitrogen can cause the very problems it aims to prevent. High YAN musts without other crucial nutrients can still lead to H2S production. Vitamin B5 (pantothenate) is essential for maximizing nitrogen effectiveness. Excess nitrogen at fermentation’s start can cause excessive yeast reproduction, reduction, or rapid die-off, especially in lipid-depleted musts. For musts with initially high YAN, no addition should be made at the beginning; instead, a timely YAN addition at one-third fermentation or the stationary phase ensures enough food for the yeast.
