Thursday, May 10, 2012

Yeast Assimilable Nitrogen - Part I

As discussed in 'Importance of Nitrogen in Winemaking', there are several types of nitrogenous compounds to be concerned about during the winemaking process. Yeast assimilable nitrogen (YAN) is the combination of ammonia and free amino acids that are metabolically available to yeast during fermentation. Winemakers use the YAN value to determine if and how much nitrogen supplementation is required for a successful fermentation.

Nitrogen is considered the most limiting factor during fermentation. Lack of nitrogen in juice/must creates several different problems for yeast during fermentation: low yeast biomass (slow fermentation rate), inability to synthesize glucose transport proteins, and decreased ability to act under adverse conditions (high ethanol levels, high CO2 levels, extreme temperatures, etc.). Not only does this mean a high risk of incomplete fermentation, it also means high stress levels that will likely lead to the production of undesirable odors and flavors.

Besides avoiding the negative effects associated with nitrogen deficiency, there is a strong correlation seen between nitrogen levels and wine aroma/flavor intensity; wines fermented with sufficient YAN tend to have lower production of long-chain alcohols, aldehydes, and undesirable odors/flavors, while having higher production of low molecular weight esters and beneficial autolysis products.

The minimum amount of YAN required for a successful fermentation of normal table wine (i.e. juice/must with a starting brix of 21°) is 140 mg/L. Many wine scientists and winemakers (including myself) contend that this is far too low and highly likely to lead to an unsuccessful fermentation. I created the chart below as a guideline for brix and typical desired YAN value.

Yeast uptake different assimilable nitrogen sources preferentially during fermentation:
  1. Ammonia
  2. Glutamate and glutamine
  3. Alanine, serine, threonine, asparate, asparagine, urea, arginine
  4. Proline
  5. Glycine, lysine, pyrimides, thymine, thymidine
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.

Continue reading 'Yeast Assimilable Nitrogen - Part II'.

2 comments:

  1. I think that is very impotant to' bring the value of APA least 180mg /l for not having some problems of stress hints and marcaptan compounds tipically of reduction juice.

    ReplyDelete
  2. Thanks, I agree. I usually use the chart above to determine desired YAN based off brix. Does a pretty good estimation

    ReplyDelete