Alcoholic fermentation (Source: http://alcoholicfermentation.net/) |
Proteins are used by the yeast as (i) enzymes for the glycolytic pathway (indicated above), (ii) permeases in the cell membrane responsible for the transportation of compounds into the cells, (iii) cellular constituents (Kennedy and Reid, Yeast nutrient management in winemaking, The Australian and New Zealand Grapegrower and Winemaker, 537, October 2008). These proteins are synthesized by the yeast and nitrogen (N) is a key component in that process. According to Kennedy and Reid, "Efficient protein synthesis is needed for efficient sugar transport and overall yeast metabolism."
According to Schwarcz and Schoeninger (Stable Isotope Analysis in Human Nutrition, Yearbook of Physical Anthropology 34, pp. 293-321), almost 100% of exchangeable nitrogen is found in the atmosphere or dissolved in the world's oceans and is transferred from these environments into the biological system through the processes illustrated in the figure below. Grape vine plants receive their nitrogen through this terrestrial nitrogen cycle.
Source: http://tolweb.org/notes/?note_id=3920 |
Nitrogen is required throughout the fermentation process with larger amounts being utilized during the exponential growth phase of the yesats and small amounts during the stationary phase. In some cases the grape must does not provide adequate amounts of assimilable nitrogen and is supplemented by added nitrogen in the form of diammonium phosphate (DAP). Juice levels of < 25 mg/L ammonia or < 150 mg N/L (measured as YAN) is considered nitrogen-deficient (UCDavis). Insufficient nitrogen can result in sluggish/stuck fermentations or sulfide formation (sulfur-like off-odors, mercaptans, and sulfur-containing acetic esters; the less assimilable nitrogen in the must, the more hydrogen sulfide will be produced). Supplements are best added incrementally and proportional to yeast growth (UCDavis).
Excessive nitrogen in the must can lead to elevated levels of ethyl carbamate (a supposed carcinogen) or urea excretion. The levels of nitrogen required for a successful AF is dependent on (Kennedy and Reid):
- Initial must YAN
- Yeast strain
- Fermentation temperature
- Initial grape sugar
- Other factors.
Amino Acid | Characteristics |
Proline | Not metabolized appreciably by yeasts under winemaking conditions One of the predominant amino acids along with Arginine and Glutamine Main amino acid from low-fertilization vineyards |
Arginine | One of the predominant amino acids Breakdown results in formation of urea and ammonia (During wine storage, urea can react with ethanol to form ethyl carbamate, a carcinogen). Located mostly in grape skin so processing practices could influence content in juice Main amino acid from low-fertilization vineyards |
Glutamine | One of the predominant amino acids Favored by yeasts because it can be broken down to glutamate and ammonia |
The less-important amino acids taken up by the yeast cells are alanine, serine, and theronine.
©Wine -- Mise en abyme
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