Yeast nutrition during fermentation

Karien O'Kennedy - 17 May 2018

Important yeast nutrition facts:

Yeast nutrition is added to fermentation primarily to ensure the completion of fermentation and to limit the formation of sulphur-like off odours. It is a fairly common practice seeing that grape juice often contain insufficient nitrogen.

There are five types of yeast nutrition available on the market: basic yeast nutrition (mainly inorganic nitrogen – DAP), complex yeast nutrition (inactivated yeasts, inorganic nitrogen), rehydration nutrition (inactivated yeasts), organic yeast nutrients (inactivated yeasts) and yeast cell walls. Technically, yeast cell walls have no nutritional value but rather serve as detoxifying agents. The type and dosage of yeast nutrients needed depends on the following factors:

  • The initial YAN of the juice
  • The nutrient demand of the specific yeast used
  • Fermentation temperature
  • Oxygen availability during fermentation
  • The potential alcohol concentration
  • The grape variety and wine style desired
  • The amount of SO2 added at crushing

It has been proven scientifically and experienced in practice that the addition of nitrogen to fermenting must suppresses H2S formation.

It has also been proven scientifically and experienced in practice that the addition of too much nitrogen to the juice before fermentation can lead to extensive biomass formation, the initiation of H2S formation by yeasts and their subsequent release into the must. The opposite effect of what was aimed for is thus obtained.

H2S formation early in fermentation is very manageable and easily removed via CO2 during active fermentation. H2S formation towards the end of fermentation poses a bigger problem, as fermentation is not so active anymore and CO2 therefore cannot remove it effectively. It therefore can stay in the wine and result in the formation of mercaptans and other, more complex, sulphur-like off odours.

The natural YAN in juice comprises mainly amino acids (organic nitrogen) and, to a lesser extent, ammonium (inorganic nitrogen).

The higher the YAN (natural or adjusted), the higher the yeast biomass and the faster the fermentation speed of yeasts.

Yeasts prefer to take up ammonium ions if they are in excess, as in the case of supplementation with DAP. This suppresses amino acid uptake.

Certain amino acids are aroma precursors, e.g. leucine, iso-leucine, valine, tyrosine and phenyl-alanine. Adding excess DAP to the juice phase will supress the uptake of these amino acids and, as a result, aroma formation will be influenced negatively. Pure DAP should ideally be added only 48 to 72 hours after the start of fermentation.

DAP added at the correct times and in the correct amounts during fermentation has a very positive effect on wine aroma (enhances ester formation, lowers sulphur-like off odours).

The use of too much DAP on low YAN juice can lead to the formation of negative aromas such as ethyl acetate.

It has been demonstrated that DAP addition 72 hours after the onset of fermentation leads to lower final alcohol, total SO2 and VA concentrations in comparison to adding it right at the start of ferment. Some yeasts, however, will require an earlier addition, so do not only add it after exactly 72 hours as a rule. Some yeasts can be finished with fermentation after 72 hours, especially on Pinotage.

Some yeasts have very high nutrient demands and, irrespective of the grape juice nutrient status or fermentative conditions, will always need complex nutrition for optimal performance.

Low YAN juice must always be supplemented with some form of complex nutrition and not just with DAP and a yeast with a low nutrient demand must be used for fermentation.

Rehydration nutrients can strengthen the yeast cell membrane by being a source of additional sterols and long-chain fatty acids (cell membrane building blocks). They improve yeast viability and vitality.

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