Nitrogen-containing compounds are naturally occurring in grapes and extremely important during their cultivation. They are also very important during winemaking procedures, particularly fermentation, clarification, and microbial stability. Nitrogen-containing compounds in juice/must include proteins, polypeptides, biogenic amines, amino acids, and ammonia.
The total nitrogen content of grapes is highly variable, ranging between 60 and 2,400 mg of nitrogen per liter. This variation is due to viticultural variables.
After veraison, protein synthesis in grapes occurs at a similar rate as sugar level increase. In juice/must, proteins usually represent less than 10% of total nitrogen content. In wine, levels are far higher and reach up to 40%. Only half of total wine protein is sourced from grape material, while the rest is derived primarily from yeast.
Read more about Wine Proteins & Stability.
Polypeptides are essentially protein fragments, long polymers of amino acids linked by peptide bonds. Depending on processing techniques that breakdown proteins, polypeptides can represent a very large portion of wine nitrogen content.
Polypeptides are important to wine because of their contribution to mouthfeel. The process of sur lie aging has a major effect on polypeptide level and enhancing mouthfeel.
Amines are a group of nitrogenous compounds that includes biogenic amines and amino acids. Biogenic amine levels are typically higher in red wines than whites, and largely indicated as the cause of wine headaches. Presence in wine can be attributed to grapes themselves and also as by-products of primary fermentation, but the majority is produced by bacteria such as Oenococcus, Lactobacillus, Pediococcus that convert amino acids into biogenic amines during a process called decarboxylation.
Since biogenic amines are present in very small concentrations (less than 0.3 mg/L), they can can be used as an indicator of spoilage. A strong correlation exists between biogenic amines and several undesirable attributes.
These are the building blocks of peptides and proteins. The levels of the twenty different amino acids naturally occurring in grapes vary widely depending on grape variety, viticultural practices, and processing techniques. Total amino acid level at harvest usually falls between 30-400 mg/L.
Amino acids are of utmost importance during fermentation due to their metabolic availability to yeast and their sensory effects in wine.
Read about Yeast Assimilable Nitrogen.
During grape maturation, ammonia levels decrease as protein and peptide levels increase. Typical ammonia levels at harvest range from 20-220 mg/L. Ammonia is also of utmost importance during fermentation due to its metabolic availability to yeast.