Grapes contain a variety of nitrogenous compounds of which the most important
are the primary or alpha amino acids, ammonium ion and small peptides.
Proline, a dominant secondary amino acid in many grape varieties, cannot be
assimilated under anaerobic conditions (Ingledew et al. 1987).
These nitrogenous compounds, excluding proline, constitute what is commonly referred to as yeast
assimilable nitrogen (YAN).