4.1.2.a Management of available nitrogen

What is nitrogen's role?

Nitrogen enables yeast cell multiplication, protein synthesis and correct functioning of yeast metabolism during alcoholic fermentation.

What are the consequences of a deficiency?

The most common risk is the development of a reduction flavour. Yeasts normally use sulphur to form sulphur-containing amino acids such as cysteine or methionine. To do this they need nitrogenous bases. If there is a deficiency, they will produce, among other things, H2S, a typical reductive molecule reminiscent of rotten eggs.
Another risk is a sluggish fermentation or stuck fermentation. In such cases, it is necessary to enrich the must with nitrogen and to create a fermentation starter vat.
Finally, a nitrogen deficiency during alcoholic fermentation can also lead to a deficiency in amino acids for the lactic bacteria during malolactic fermentation, and consequently to problems with fermentation.

What are the consequences of an excess?

While it may seem counterintuitive, an excess of nitrogen is also detrimental to a qualitative alcoholic fermentation.
An excessive amount of ammoniacal nitrogen can lead to very rapid and excessive growth of the yeast population (during the first third of the alcoholic fermentation), which will ultimately create a nitrogen deficiency for the remainder of the fermentation. This leads back to the previous problems (reduction, etc.). In such situations, an addition of amino acids at around 1050 must density is possible to meet the fermentation environment's requirements.

Excess nitrogen also has an impact on the development of volatile thiols. In excess, nitrogen binds to the yeast thiol precursor transporters, which prevents their break down by the yeast and thus any volatile thiol aromatic expression. This is known as NCR (Nitrogen Catabolic Repression) complex.

The other potential problem of nitrogen excess is the creation of a residual substrate that is ideal for contaminating microorganisms such as Brettanomyces bruxellensis.

How is the nitrogen requirement determined?

The standard proportion regarding ammoniacal nitrogen/amino acids (organic nitrogen) is 1/3 and 2/3. A must is considered deficient when its assimilable nitrogen content is less than 140 mg/L.
In real terms, 0.8 mg of assimilable nitrogen is needed to ferment 1 g of sugar.

For example: to ferment 220 g/L of sugar requires:
220 x 0.8= 176 mg/L of assimilable nitrogen (minimum).

To simplify, most people assume that 200 mg/L of assimilable nitrogen is required, which is sufficient for most situations.

How and when should nitrogen be added?

Two main forms are used:

• Ammoniacal nitrogen (diammonium phosphate)
• Yeast autolysates or deactivated yeast typically rich in amino acids

Thiamine is often added. This vitamin is necessary for yeast metabolic function. It is already present in the skins but is often added in an ammoniacal nitrogen solution. It reduces the stress on the yeast and therefore the production of volatile acidity and compounds that strongly combine with SO2 such as pyruvic acid or acetaldehyde.
The best way to add nitrogen is in fractional doses during pumping over which homogenizes the addition.

At the beginning of fermentation, ammoniacal nitrogen and thiamine can be added to enable good cell multiplication. The addition is made after the first point drop. This usually represents 2/3 of the added nitrogen.

Subsequently, more ammoniacal nitrogen or preferably organic nitrogen (amino acids) can be added corresponding to the remaining third of the total quantity added. The ideal moment for this second application is after the first third of the AF and up to about 1050 density.

Note that it is also possible to use solely organic nitrogen for both additions.