Courtesy of The Drifting Winemaker
By Mike Horton
Nitrogen-containing compounds are naturally occurring in grapes and extremely important during their cultivation. In turn, they are also very important during winemaking procedures (particularly during fermentation, clarification, and microbial stability). The total nitrogen content of grapes is highly variable, ranging between 60 and 2,400 mg (N)/L and due to viticultural factors (grape varietal and rootstock, fertilization, grape maturity at harvest, soil, climate, and disease). Nitrogen-containing compounds in juice/must include amino acids, amines, ammonia, peptides, polypeptides, and proteins.Proteins – Protein synthesis in grapes occurs rapidly after veraison (onset of ripening), 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%. This is a bit deceptive because only half of the total wine protein is sourced from grapes. The rest are derived from fermentation; small amounts of protein are released by yeast during fermentation, and large amounts are released during autolysis (yeast breakdown) post-fermentation. Proteins derived from yeast autolysis (extended lees contact will be discussed in a later post) contribute to a wine’s mouthfeel. Soluble proteins are of major concern during clarification procedures because their precipitation in bottled wines leads to ‘protein haze’ or deposits.
Polypeptides – Essentially protein fragments, polypeptides are long polymers of amino acids linked by peptide bonds (chains of peptides). These can represent a very large portion of wine nitrogen content depending largely on processing techniques that breakdown proteins. Polypeptides are important to wine because of their contribution to mouthfeel; the process of sur lie aging (to be discussed in a later post) has a major effect on polypeptide level and enhancing mouthfeel.
Biogenic Amines – Amines are a group of nitrogenous compounds that includes biogenic amines and amino acids. Biogenic amines present in wine result largely from bacterial activity, both from malolactic fermentation or non-desirable organisms (decarboxylation of amino acids by Oenococcus, Lactobacillus, Pediococcus, etc.). They can also be sourced from the grapes themselves, and as by-products of primary fermentation and wine maturation. Usually only present in very small concentrations (less than 0.3 mg/L), biogenic amines can be used as an indicator of spoilage (strong correlation between high levels of biogenic amines and volatile acidity, as well other negative compounds such as butyric acid, acetic acid, ethylacetate, etc.). Biogenic amines levels are typically higher in red wines than whites, and are largely indicated as the cause of wine headaches.
Amino Acids – Amino acids serve as the building blocks for peptides and proteins. The levels of the twenty different amino acids naturally occurring in grapes vary widely depending on grape varietal, viticultural practices, and processing techniques, though 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.Ammonia – 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.