Yeast assimilable nitrogen
Yeast assimilable nitrogen or YAN is the combination of
However, the addition of excessive amounts of nitrogen can also create a hazard as other organisms besides beneficial wine yeast can utilize the nutrients. These include spoilage organisms such as
The amount of YAN that winemakers will see in their grape
Components
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
The amount of YAN that winemakers will see in their grape musts depends on a number of components including grape variety, rootstock, vineyard soils and viticultural practices (such as the use of fertilizers and canopy management) as well as the climate conditions of particular vintages. Infections by mold, such as
In the vineyard, nitrogen is taken up by the
Amino acids
Of the Free Amino Nitrogen (FAN) that make up YAN, the amino acids arginine, proline and glutamine are the most abundant followed by alanine, threonine, serine and aspartic acid in much smaller concentrations[1] though trace amounts of most known amino acids can be found in grape must.[2] Proline is usually the most concentrated and can represent up to 30% of the total amount of amino acids.[4] The exact amount FAN will vary and can range 22 to 1242 mg of nitrogen/liter of YAN being derived from free amino acids.[5]
While arginine, glutamine and other amino acids are rapidly consumed often very early in fermentation, proline is not consumed by yeast at all during the normal, anaerobic conditions of fermentations. This is because one of the enzymes required for its use is an
Yeast transport amino acids and small peptides (less than 5
Ammonia compounds
Throughout fermentation ammonium is the primary form of assimilable nitrogen available to yeast.[1] However, at crushing the juice may contain anywhere from 0 to 150 mg/L of ammonium salts, depending on the how much nitrogen the grapevine received in the vineyard.[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.[2] The ammonium ion also serves as an allosteric regulator for one of the enzymes used in glycolysis and may also have an effect on how the yeast cell transports glucose and fructose into the cell.[4] The proteins used in the main glucose transport system have been shown to have a half-life of 12 hours. 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.[3]
Glutathione (GSH: L-gamma-glutamyl-L-cysteinylglycine) is present in high concentrations up to 10 mM in yeast cells. It assumes a pivotal role in response to sulfur and nitrogen starvation.[6]
Ammonia is not used by bacteria such as Acetobacter and the lactic acid bacteria used in malolactic fermentation.[2]
Importance in winemaking
Assimilable nitrogen is an essential nutrient needed by
In the absence of nitrogen, yeast will begin to shut down and die off. Some strains will begin breaking down sulfur containing amino acids like cysteine and methionine releasing a sulfur atom that can combine with hydrogen to produce hydrogen sulfide (H
2S) which can impart rotten egg odors to the wine. 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. There are even some strains of S. cerevisiae that produce H2S as a response to having too much available nitrogen (particularly too much glutamic acid and alanine[3]). This is why a prophylactic approach of indiscriminately adding nitrogen supplementation to every fermentation may not have the desired results of preventing H2S.[2]
Nitrogen levels in the wine can have an influence on many sensory aspects of the resulting wine, including the synthesis of many aromatic compounds.
Estimates on how much is needed
The amount of YAN needed will depend on what the winemaker's goals are for fermentation, particularly whether or not wild fermentation is desired or if the wine will be fully fermented to dryness. The state of the grapes and the conditions of fermentation will influence the amount of nitrogen needed. Fruit that is damaged, moldy or
The suggested range given by enologists varies from 150 mg/L YAN
A study by the UC Davis Department of Viticulture and Enology found that recommendations on optimal nitrogen levels to complete a successful fermentation could be made based harvest brix level which have been adopted by many yeast and nutrient manufacturers.[11][12]
- 21°Bx = 200 mg N/L
- 23°Bx = 250 mg N/L
- 25°Bx = 300 mg N/L
- 27°Bx = 350 mg N/L
However, other studies have shown successful fermentation be conducted with YAN levels below these recommendations as well as sluggish/stuck fermentations occurring even when YAN levels are in line with recommendations.[1]
In malolactic fermentation
Like yeast, the lactic acid bacteria (LAB) used in malolactic fermentation (generally Oenococcus oeni) requires nitrogen. However, unlike S. cerevisiae LAB can not utilize ammonia and such additions like diammonium phosphate (DAP) offers no nutritional benefits. Winemakers who inadvertently use DAP as a nutrient additive for their MLF inoculation risk providing nutrients instead for spoilage organisms such as Brettanomyces.[2]
While some winemakers will inoculate their LAB with nutrients that include nitrogen, most of the nutrients needed for MLF come from the breakdown (or
Measurements and tests
The nitrogen by
Ammonia and ammonium can be measurement using an ion-selective electrode probe and a pH meter.[1]
Nitrogen supplementation
Winemakers have long known that some fermentations ran more predictable and "healthier" if
As
There are many types of nitrogen supplements available for winemakers to use. 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).[2] Amino acids can be added directly to the must though as of 2010 only glycine is permitted to be added to must in the United States.[4]
Yeast hulls (or Yeast ghosts) are the remnants of yeast cell walls left over from the commercial production of yeast strains to be used for inoculation. In addition to providing a source of assimilable nitrogen from amino acids, they also provide
Risk in adding too much
Nitrogen supplements, particularly DAP, stimulates yeast reproduction and can greatly increase the biomass. This could have the consequence of speeding up the fermentation rate faster than what a winemaker may desire and will also increase the fermentation temperature due to the heat being generated by the yeast. 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. This is because arginine gets broken down into urea which can be reabsorbed and utilized by yeast or metabolized into ammonia. 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.[1]
However, the greatest risk of over supplementing a must is that excess nitrogen and other nutrients will be left behind after fermentation is complete. This can create microbial instability as spoilage organisms can use these excess nutrients.[3]
Wine laws and regulations
In the United States, the
Influence of timing
As most nutrient supplements feed all living microorganism in the must (whether desirable or not), winemakers will often wait to add the nutrients until they are ready to inoculate the must with their desired S. cerevisiae strain. Producers who are using wild ferments may also wait until after sulfur dioxide additions have killed off unwanted microbes or feed early because they would like the potential complexity that other microbes could add to the wine. When added, the nitrogen is usually in the form of amino acids, combined with vitamins and minerals to help kick start the fermentation.[2]
Soon after inoculation, yeast begin to rapidly consume the available assimilable nitrogen with up to 46% of YAN being fully consumed by the onset of full fermentation.
References
- ^ ISBN 0834217015
- ^ ISBN 0387333495
- ^ ISBN 9780123736468
- ^ ISBN 978-1-4419-5190-8
- ^ a b Sara E. Spayd and Joy Andersen-Bagge "Free Amino Acid Composition of Grape Juice From 12 Vitis vinifera Cultivars in Washington" Am. J. Enol. Vitic 1996 vol. 47 no. 4 389-402
- ^
Penninckx, MJ (2002). "An overview on glutathione in Saccharomyces versus non-conventional yeasts". FEMS Yeast Research. 2 (3): 295–305. PMID 12702279.
- ^ Lallemand "Yeast Nutrition and Protection for Reliable Alcoholic Fermentations" The State of The Art. Accessed: March 31st, 2013
- ^ S.M. Weeks and P.A. Henschke "Yeast assimilable nitrogen" The Australian Wine Research Institute. Accessed: March 31st, 2013
- ^ Maurizio Ugliano, Paul A. Henschke, Markus J. Herderich, Isak S. Pretorius "Nitrogen management is critical for wine flavour and style" The Australian Wine Research Institute. VOL 22 NO 6 NOVEMBER/DECEMBER 2007
- ^ Bruce W. Zoecklein "I. Nitrogen Compounds" Vintner's Corner, Virginia Tech University Enology Notes, Vol. 13, No. 4 July - August, 1998
- ^ Linda F. Bisson and Christian E. Butzke "Diagnosis and Rectification of Stuck and Sluggish Fermentations" Am. J. Enol. Vitic 2000 vol. 51 no. 2 168-177
- ^ Chris Gerling "FAQs about YAN" Veraison to Harvest #6, Cornell University Cooperative Extension. October 2010
- ^ UC Davis Cooperative Extension "NOPA Procedure" Butzke & Dukes (1998) Accessed: March 31st, 2013
- ^ Barry H. Gump, Bruce W. Zoecklein, Kenneth C. Fugelsang and Robert S. Whiton "Comparison of Analytical Methods for Prediction of Prefermentation Nutritional Status of Grape Juice" Am. J. Enol. Vitic 2002 vol. 53 no. 4 325-329
- ^ Virginia Tech University "Estimate of FAN by Formol Titration" Adapted from Zoecklein et al., 1999 and Gump, Zoecklein and Fugelsang, 2002. Accessed: March 31st, 2013
- ISBN 0-671-68702-6
- ISBN 0-19-860990-6
- ^ CHRISTIAN E. BUTZKE & LINDA F. BISSON "Ethyl Carbamate Preventative Action Manual"' UC Davis Cooperative Extension. Accessed: March 31st, 2013
- ISBN 0824792904