YAN & The Home Winemaker A lot of people think that making wine is just crushing the grapes (called must) into the fermenter and adding yeast. The yeast will digest (eat) the grape sugars and produce alcohol, carbon dioxide gas (CO2) and heat. This will generally work as long as the crushed grapes have the correct balance of nitrogen, usually called YAN (yeast assimilable nitrogen). Every load of grapes comes from the vineyard with a varying level of YAN naturally. However, if the levels are too low, the yeast will die without converting all the sugar to alcohol. This is called a “stuck fermentation”. A stuck wine that is bottled with residual sugar can cause the wine to spoil and often will taste bad. Home winemakers usually solve this problem of unknown YAN by adding specified amounts of yeast rehydration nutrients and yeast "vitamins" to the must.
Yeast Nutrients & Vitamins Yeast rehydration nutrient is a natural product that has a balance of nutrients and vitamins and is usually introduced into the grape must at the time the yeast is added. It is an effective way for helping the yeast survive the fermentation process and helps insure that all the sugars are converted to alcohol thus creating a “dry” wine.
Another natural product that is often added in combination with rehydration nutrient, and in the middle of fermentation, are yeast vitamins. They are a complete yeast nutrient that is a complex blend of several compounds such as ammonia salts (DAP), amino acids (organic nitrogen), sterols, unsaturated fatty acids and other key nutrients (thiamin, niacin, calcium, inactive yeast, etc.) These vitamins help ensure yeast survival and help the yeast maintain an alcohol resistance (excess alcohol will kill the yeast). They are typically added both at the start of fermentation and again when the fermentation is 1/3 complete.
YAN & The Commercial Winery Wineries (vs. home winemaking) want to know the YAN, which is composed of organic Primary Amino Nitrogen (PAN) and inorganic Ammonia (NH3) and Ammonium (NH4) that can be used by the wine yeast (saccharomyces cervisiae). Knowing YAN saves money and will help prevent stuck fermentations. If the YAN is too high, other bad organisms can utilize the nutrients to create wine spoilage. Knowing the YAN in the grape must allows for the precise calculation of the nutrients that need to be added to the must to make the best wine. The amount will vary depending on the sugar (Brix) and will range from <200mg to 400mg of nitrogen per liter. The amount of nutrients is usually measured by the enologist in the lab by using a spectrophotometer.
The Importance of a Successful Fermentation There are many more factors involved in the fermentation process. The first is getting healthy grapes, which occurs in the vineyard and depends on the amount of water, the rootstock, the pruning, fruit maturity, crop load, type of growing season, the fungal degradation, fertilizer, organic sprays, leaf thinning, etc. The second is the actual fermentation itself which includes the type of grape crush (cluster, whole berry or crush), the size/type of fermentation vessel, the heat generated and gases produced, the pH and acidity, the amount of alcohol, etc. Also a key factor is proper sanitation; always sterilize your equipment before and after use.
So, you just don’t pitch the yeast into the grape must and expect good wine, because it probably isn’t going to happen.