(UMN.EDU) - Winemaking begins in the vineyard, and so does nitrogen. Nitrogen is one of the most common elements in the universe. On Earth, in its elemental form, it exists as a gas that forms 80% of our atmosphere. However, it is also a chemical constituent of many important components essential to life. Nitrogen makes up the building blocks of DNA, and it is also an important element in the composition of amino acids. When linked together, amino acids form the enzymes that drive all of life’s biochemical reactions. They are the building blocks to all proteins, hormones, and some plant metabolites that are responsible for wine flavor. Plants draw mineral nitrogen from the soil and convert it to amino acids and other compounds. Animals who consume plants in turn ingest the nitrogen that the plants have drawn from the soil. Even single-cell organisms, such as yeast, need nitrogen for survival.

Many of us are well aware of the effects of nitrogen on the growth of plants. Nitrogen is the most important nutrient involved in regulating vine growth, morphology, and tissue composition. Soils that are high in nitrogen cause an increase in vigor, which can lead to shaded canopies and high yields of unripe fruit in vineyards. However, it is also important to understand how the nitrogen that is in fruit at harvest can have an effect on fermentation.

What’s your YAN, man? When grapes or other fruits are harvested, they contain nitrogen in many different chemical forms. The most important nitrogen-containing compounds for fermentation are free amino acids (FAN), ammonium ions (NH3), and small peptides. These compounds can, for the most part, be consumed by yeast during fermentation and are collectively calledyeast assimilable nitrogen, or YAN.

The free amino acid content (FAN) of the grape juice can be measured by a variety of different methods, but the most commonly accepted way to measure it is the NOPA assay. I won’t detail the procedure here as there are plenty of resources available, but it is worth noting that a spectrophotometer is needed in order to interpret the results. For wineries looking to upgrade their lab, I’d highly recommend investing in this piece of equipment. 

The ammonia (NH3) content of juice (which is 83% nitrogen) is measured enzymatically, and the results are also determined by a spectrophotometer. The sum of the FAN and the NH3 collectively give us the amount of YAN in the juice.

Another method for measuring YAN is called the Formol titration method. While it is a simpler method, involving only a titration, it does involve using a Formaldehyde solution. In order to mitigate health and safety risks with this method, the titration must be performed under a fume hood – which is a much greater investment for a winery than the cost of a spectrophotometer. Newer methods of measuring YAN are also available, but require highly specialized lab equipment.

Nitrogen and fermentation. After sugar, nitrogen is the most important macronutrient for yeast. When juice is lacking in nitrogen, the yeast can exhibit sluggish fermentations, create off-odors, and eventually expire before consuming all the sugar resulting in stuck fermentations. Yet, while every winemaker I know carefully tracks the ºBrix (sugar) in their fruit, many winemakers don’t always measure the nitrogen content of the juice. Why? Well, many simply add a set amount of nitrogen (in the form of commercial yeast nutrients) as part of their regular fermentation protocol. Or, perhaps they don’t add a standard addition at the start of fermentation, but as soon as the wine starts smelling “stinky” (sulfide aromas like cooked cabbage or rotten eggs), they add nitrogen in the form of salts such as diammonium phosphate (DAP). When yeasts lack amino acids in their diet, they start to synthesize their own. Unfortunately, yeasts’ recipe for amino acids includes adding a bit of sulfur to create cysteine and methionene. When they then metabolize these amino acids, hydrogen sulfide is a byproduct.

Nonetheless, although a minimum amount nitrogen is important in preventing fermentation difficulties, it is possible to have too much of a good thing. When the nitrogen concentration in the grape must is too high (>450-500 mg/L YAN), it can stimulate the yeast to start overproducing undesirable aroma compounds such as ethyl acetate – an acetate ester with a nail polish aroma. Acetic acid production is also increased, as well as other aroma compounds that can be both beneficial and/or detrimental to a wine’s character. Even more disconcerting is the fact that wines made from high nitrogen juice contain greater amounts of the possibly carcinogenic compound ethyl carbamate. Bacteria can transform any excess amino acids following fermentation into biogenic amines like histamine and phenylethylamine – compounds which can cause headaches, nausea, or extreme reactions such as heart palpitations and shortness of breath in those who are sensitive. Thus, knowing the quantity of nitrogen at the start of fermentation can help prevent some of the undesirable consequences of adding more nitrogen than necessary (not to mention the added cost of using these nutrients!).

How much YAN do I need? The minimum amount of YAN needed for fermentation depends on a variety of factors such as the initial sugar concentration of the must, the fermentation temperature, and the strain of yeast used to ferment the wine. Nonetheless, it is generally accepted that juice with YAN less than 140-160 mg/L should be supplemented.