Yeast assimilable nitrogen, or YAN, as it is more commonly
referred to, comprises ammonia and alpha amino acids and is,
apart from sugar, by far the most important nutritional requirement
in wine fermentations. To determine the YAN value of a juice, the
FAN or the free alpha-amino groups of the primary amino acids is
measured. This value is then added to the ammonia concentration
value. Proline, a very dominant amino acid in grape juices, is
excluded from YAN measurements because yeast cannot utilise it
under the anaerobic conditions of fermentation.