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116 WINES&VINES January 2016 WINEMAKING PRACTICAL WINERY & VINEYARD N itrogen is a key factor that has a significant impact on wine fermentation. It is the most important yeast nutrient, influencing both fermentation kinetics and wine quality. In this article, we look at the different forms of nitrogen used by yeast and their impact on wine. Nitrogen is present in grape must in different forms: inorganic (ammonium) and organic (amino acids, peptides and proteins). 1 Nitrogen that can be used by yeast during alcoholic fermentation is called "assimilable nitrogen." The needs for assimilable nitrogen are different according to the wine yeast used. Yeast cells also contain nitrogen in the form of proteins, peptides— particularly tripeptides—and amino acids. YAN (yeast assimi- lable nitrogen) includes free α-amino acids (AA), ammonium and some peptides. 9 Inactivated yeast-based nutrients can also be used by wine yeast to efficiently carry out alcoholic fermentation. YAN = free α-amino acids + NH 4 + + some small peptides Proline is the only free α-amino acid (AA) not assimilated by yeasts during alcoholic fermenta- tion, even if it is one of the AAs most abundant in grape must. FAN = Free α-Amino Nitrogen, which is equivalent to the free α-amino acids The main free AAs (in quan- tity) in grapes include proline, arginine, glutamate A study by L. Crépin showed that amino acids could be classi- fied into three groups based on when they were taken up by the yeast during alcoholic fermenta- tion. Lysine was the first amino acid consumed by the yeast, fol- lowed by asparagine, threonine, glutamate, leucine, histidine, me- thionine, isoleucine, serine, glu- tamine and phenylalanine. The last amino acids used by the yeast included valine, arginine, ala- nine, tryptophan and tyrosine. 5 The initial concentrations of these compounds did not alter the order in which they were con- sumed, except for arginine and ammonium. Nitrogen is essential for yeast growth and yeast metabolism. In winemaking, YAN plays a key role at two different levels: • It represents an important nutritional factor for yeasts during alcoholic fermenta- tion due to its function in protein synthesis and sugar transport. • It is essential for the biosyn- thesis of wine quality mark- ers like higher alcohols, thiols and esters by wine yeast. YAN concentrations in natural grape musts range from about 60 mg/L to 500 mg/L, depending on grape variety, vintage and micro- climate. In nitrogen-deficient con- d i t i o n s , y e a s t g r o w t h a n d fermentation speed are limited. A low initial YAN concentration has been shown to cause slow and sluggish fermentations, which is why nitrogen addition to the must has become a necessary step in wine production. YAN measurement is a good way for a winery to assess the gen- eral state of grapes in terms of ni- trogen quantity, but not necessarily nitrogen quality, which is another key parameter from the point of view of fermentation and the sen- sory quality of the final wine. Different form of YAN In order to properly manage alco- holic fermentation, the best fer- mentation practices recommend both the addition of nitrogen in inorganic (ammonium) and/or an organic form. Organic nitrogen has been shown to be the most efficient and complete nutrient to c o m p l e t e f e r m e n t a t i o n a n d achieve the full aromatic potential of the grape. 10 In must, one-third of the nitro- gen is found in ammonium form and two-thirds in amino acids, but musts are often deficient in YAN (less than 150 mg/L). 8 Nitrogen from external sources can be added to the must to assist with fermentation. They come in two forms: Inorganic nitrogen: ammonium salts such as diammonium phos- phate (DAP), which is added dur- ing alcoholic fermentation. Organic nitrogen: in the form of proteins, peptides, tripeptides and free amino acids that are part of the cells from yeast (see "Nitro- gen-Containing Compounds in Yeast Cells"). When yeast cells are inactivated through various pro- duction processes (inactivated yeast and yeast autolysate), the Nitrogen Plays Many Roles During Fermentation Uncovering the relationship between nitrogen and aroma development By Anne Ortiz-Julien, Ann Dumont, Edouard Lordat and Gordon Specht Nitrogen-containing compounds (amino acids, peptides and proteins) are among the contents of a yeast cell. NITROGEN-CONTAINING COMPOUNDS IN YEAST CELLS Amino acid Proteins Proteins Proteins Tripeptides Amino acid Tripeptides Amino acid Peptides