The Importance of Nitrogen in Wine Fermentation

After hearing Dr. Sara Spayd speak about "Nitrogen Nutrition in Fermentation", it recalled to mind the UC Davis lectures on nitrogen, so I went back to my lecture notes as well as the internet to find references regarding nitrogen in ferementation.
It is important to know the initial nitrogen status of a juice or must, because it affects the growth of yeast and the fermentation of sugars, and contributes to wine flavour. When talking about the nitrogen content in grape must or juice, you might run into the following acronyms:

FAN = Free Amino Nitrogen -- all amino acids except proline
PAN = Primary Amino Nitrogen - same as FAN
YAN = Yeast Available Nitrogen = [FAN or PAN + Ammonia]

The amino acid proline can be one of the major nitrogen containing compound in the must, but because proline is a secondary amine and not a primary amine, the nitrogen is not available to yeast under aerobic conditions.

Both the total available nitrogen and the balance of amino acids and ammonia can significantly affect the production of different groups of fermentation-derived volatile compounds.

When adding diammonium phosphate (DAP) to the must, it is important to add the appropriate amount. The amount of DAP to add is dependent on the Brix level of the must or juice. Too little nitrogen may lead to a stuck fermentation, while too much addition of DAP will lead to the over production of hydrogen sulfide (H₂S) as well as ethyl acetate, leading to unwanted organoleptic characteristics in the finished wine.
Additional reading:
1. AWRi Report: Nitrogen management is critical for wine flavour and style
2. FAQs About YAN Chris Gerling, Enology Extension Associate, Cornell University - NYSAES.