Since yeast uptake assimilable nitrogen rather quickly after the onset of fermentation, YAN needs to be measure prior to inoculation. YAN is the combination of the concentration of ammonia ions and the concentration of free amino acids (determined via two separate lab analyses). If the YAN level is found deficient for successful fermentation, the juice/must needs to be supplemented with additional nitrogen.

Additions

Yeast uptake different assimilable nitrogen sources preferentially during fermentation.

Ammonia

• Glutamate and glutamine
• Alanine, serine, threonine, asparate, asparagine, arginine
• Proline
• Glycine, lysine, pyrimides, thymine, thymidine

Diammonium phosphate (DAP) is the standard nitrogen supplement. Most proprietary blends of fermentation nutrients provide some amount of nitrogen, but usually not more than 4-5 ppm. DAP provides approximately one-fourth of its weight in assimilable nitrogen (to be extact, 100 ppm of DAP will provide 22.5 ppm of assimilable nitrogen). Using the chart above, determine the YAN deficiency in your juice/must and calculate the addition required.

Timing Additions

Timing is important when planning nitrogen supplementation.

• Adding high levels of YAN prior to fermentation can lead to rapid cell growth in yeast. This rapid cell growth can begin a cycle: yeast overpopulation and abnormally high rate of YAN depletion, followed by this overpopulation of yeast producing undesirable odors and flavors due to lack of YAN as fermentation continues.
• Adding YAN late in ferment is often useless. Yeast can uptake nitrogen throughout the growth phase, but alcohol inhibits uptake about halfway or two-thirds through fermentation. For example, 8-10° brix left when starting with 22° brix. Nitrogen supplementation past the halfway point will often have little if any effect on fermentation kinetics or undesirable odor/flavor production.

When possible, I like to complete both DAP and fermentation nutrient additions following on a basic schedule. The main goal is providing yeast with a steadier nitrogen supply throughout feremntation, while leaving some room for a later addition if undesirable odor/flavor appears.

• Smaller additions of YAN are best made at 1/3 brix depletion.
• Larger additions of YAN are best made in stages: approximately half at the beginning and the remainder around 1/3 brix depletion. 

Plenty of winemakers, including myself, have had success not following such schedules. It is not uncommon to add nitrogen to juice/must with just 5° brix remaining in hopes of reviving a troublesome ferment, or to add large amounts of YAN before transferring to ferment in barrels. Rules in winemaking are often broken.

Too Much YAN?

Too much of a good thing can pose problems, and it does with YAN. As mentioned above, yeast preferentially uptake ammonia and will synthesize virtually no other amino acids until no ammonia remains.more than one problem.

• Synthesis of amino acids provides a significant amount of wine's aroma and flavor precursors.
• After fermentation, remaining amino acids are left readily available to spoilage organisms that can lead to microbial instability.
• Yeast can metabolize some amino acids in urea, which can react with ethanol to create the carcinogen ethyl carbamate.
• Continue to read Yeast Assimilable Nitrogen  II, where I address some questions asked directly regarding this article.