Yeast assimilable nitrogen

Knowing the YAN in the must allows winemakers to calculate the right amount of additive needed to get through fermentation, leaving only "nutrient desert" for any spoilage organisms that come afterwards.

[3] YAN is a measurement of the primary organic (free amino acids) and inorganic (ammonia and ammonium) sources of nitrogen that can be assimilated by S. cerevisiae.

There are several nitrogenous compounds found in must and wine including peptides, larger proteins, amides, biogenic amines, pyridines, purines and nucleic acids but these cannot be directly used by yeast for metabolism.

[1] The lack of protease enzymes, which break down larger peptides into smaller components, that can work outside the cell, limits the size of the molecules that yeast can use as a source for nitrogen.

However, well aerated starter cultures that contain must which hasn't had any diammonium phosphate added it to it will usually see some utilization of proline before the anaerobic conditions of fermentation kick in.

[4] In the cell, the inorganic ammonia and ammonium ions get "fixed" through a series of chemical reactions that ultimately yields the organic nitrogen source glutamate.

In the studies that put yeast cells through "ammonia starvation" the entire system shut down after 50 hours which gives strong evidence that a lack of ammonia/ammonium can create increase risk of having a stuck fermentation.

[2] Yeast can store amino acids in intracellular vacuoles and then later either use them directly, incorporating them into proteins, or break them down and use their carbon and nitrogen components separately.

However, there is not a direct correlation between YAN levels and hydrogen sulfide production since H2S can be produced by yeast even in the presence of abundant nitrogen but with instead other vital nutrients (such as the vitamin pantothenic acid) lacking.

Fruit that is damaged, moldy or botrytis infected will usually be more depleted of nitrogen (as well as other vitamin resources) when they come in from the vineyard than clean, intact grapes.

[10] However, not all winemakers will want to have a fermentation going at maximum rate (in terms of yeast biomass, temperature and speed) due to the impact that it can have on other sensory aspects of the wine such as aroma development and fruit retention.

Additionally, most bacteria used in MLF have the ability to produce extracellular protease enzymes that can also breakdown larger peptide chains into their base amino acid residues that can then be used for metabolism.

[1] Formol titration, invented by the Danish chemist S. P. L. Sørensen in 1907, utilizes formaldehyde in the presence of potassium or sodium hydroxide to measure amino acid concentration and ammonia with the aid of a pH meter.

[1] Winemakers have long known that some fermentations ran more predictable and "healthier" if pomace (the solid skins, seeds and remains left after pressing) from another wine was added to the batch.

[4] Urea was also used as an early nitrogen supplement but research linking it to the development of ethyl carbamate has led to its banning in many countries, including the United States since 1990.

Most of them are complex formulations that include nitrogen (from either amino acids or ammonium salts) along with vitamins, minerals and other growth factors and sold under brand names like Go-Ferm, Superfood, Fermaid K (the later two also containing some DAP).

The excess biomass can also create a scarcity of other yeast nutrients, such a vitamins and sterols, due to increase competition and may lead to the production of off-odors (such as hydrogen sulfide) and even stuck fermentations.

[1] Excessive levels of the amino acid arginine (greater than 400 mg/L), especially near the end of fermentation, can pose the risk increase the production of ethyl carbamate.

However, urea also reacts with ethanol if it is not completely metabolized which coupled with long term exposure (as well as high temperatures) can lead to the production of the ester ethyl carbamate.

[3] In the United States, the Alcohol and Tobacco Tax and Trade Bureau (TTB) limits the use of diammonium phosphate as a nitrogen additive to 968 mg/L (8 lbs/1000 gal) which provides 203 mg N/L of YAN.

Many winemakers split up the dosage of DAP with the first addition being made at the end of the lag phase when the yeast enter their period of exponential growth and alcoholic fermentation begins.

Yeast need a reliable source of nitrogen in forms that they can assimilate in order to successfully complete fermentation.
Most of a grape's YAN content is found in the skins and seeds which gets left behind as pomace after pressing.
Specialized transporter proteins (far left) within the plasma membrane of yeast cells bring amino acid residues and small peptides into the cell coupled with a hydrogen ion that later gets expelled by the cell.
Two buckets of red wine must with the top bucket showing the bluish color change after diammonium phosphate (an ammonia base) is added to the wine.
Saccharomyces cerevisiae can store amino acids in vacuoles until they are needed by the cell.
Malolactic fermentation inoculum packet and Optimalo, a nutrient supplement that contains amino acids.
YAN levels can be measured using the NOPA assay and a spectrophotometer.
Urea.
Fermaid-O is nitrogen supplement containing yeast hulls (a source of amino acids) and other vitamins but no diammonium phosphate.
Brettanomyces is one spoilage organism that can take advantage of excess nitrogen supplementation left in the wine.
When yeast inoculum (left) is first rehydrated, the ammonium salts in DAP would be too toxic so winemakers will often use a nutrient supplement (right) that contains primarily amino acids as the nitrogen source.