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 called yeast 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.