Yeasts have a strict need for nitrogenous compounds during fermentation of grape juice. Most grape must is deficient in nitrogen and so supplements are normally added to increase the level of nitrogen compounds in the must. By having a sufficient level of nitrogen, the yeast can ferment the must to dryness with no deleterious side effects, for example sulfide formation. This article investigates the commercial products that are used to provide the extra nitrogen needed and the source materials of these nitrogen compounds.
Nitrogen needs of yeast in fermentation
The main metabolite in grape juice that yeasts ferment is sugar. This is very plentiful in grape juice – in the 20 % range (~200g/L). The main nutrient required so that yeast can perform these fermentations trouble free is nitrogen. The level generally accepted that is needed is in the order of >200 parts per million, that is 0.02%. As can be seen, the difference between these 2 parameters is huge with the sugar concentration approximately 1000 times the amount of available nitrogen.
Yeast nutrient needs have been discussed previously in this journal (GGWM Sept 2011 G Howell). As previously stated, the main nutrient that needs to be supplemented is nitrogen. And as seen above it is obvious that nitrogen is the metabolite that is in much lower quantity in the original must.
Other nutrients are also required in trace amounts but in much lower quantities than the nitrogen compounds. These trace compounds won’t be discussed in this article – the main focus is to discuss how nitrogen can be added to provide the ingredients for a good fermentation.
Commercial nitrogen-containing yeast nutrients
The simplest source of nitrogen for yeast fermentation is diammonium phosphate (DAP). It provides ammonium as a nitrogen source. This has been and is still widely used throughout Australia as a simple and cheap source of ammonium, however, anecdotally it appears that winemakers are experimenting with other nitrogenous materials and using less DAP. This may be because DAP is a chemical product and one that is produced in fertiliser operations. The source materials for producing DAP are phosphate rock and sulphuric acid.
Organic nutrients contain materials from living matter, in this case from yeast by-products. The nitrogenous compounds provided from these yeast materials are predominantly amino acids. Little or no ammonium is available.
There are some products on the market that are yeast-based derivatives that contain yeast metabolites and or yeast hulls but do not contain any DAP. Again anecdotally, these products are becoming more popular and this is certainly the case for those winemakers who are interested in using natural materials instead of industrial chemical materials such as DAP.
Complex yeast nutrients
Although this expression is widely used, there does not appear to be a formal definition in the literature for it. A loose definition that can be used for a “complex yeast nutrient” is a product that contains both DAP and yeast by-products. The DAP provides ammonium and the yeast by-products provide the amino acids. Yeast hulls may also be used – not as a source of nitrogen compounds but to assist the fermentation in other ways.
Rehydration conditioning products
There is at least one product on the market used to “protect” yeast during the rehydration step. From the literature available on this product, it appears that it is not a nitrogenous fermentation nutrient as it does not provide nitrogen to the yeast during fermentation.
Yeast Assimilable Nitrogen
Yeast has an essential need for nitrogen during fermentation. That is, it must have access to some form of nitrogen that can be assimilated during the fermentation process. This nitrogen can come from two main sources: ammonium ions and amino acids. The ammonium ion is an inorganic compound, the amino acids are organic. Both forms and definitions are discussed below.
As previously discussed (Howell Sept 2011) the Yeast Assimilable Nitrogen (YAN) content of a juice is now easily measured by using a spectrophotometer and Ammonia and Amino Acid Nitrogen kits. The YAN result so obtained can then be used to determine the level of nitrogen supplement required to achieve a trouble free ferment.
The forms of nitrogen that yeast can assimilate
The main inorganic nitrogen compound needed by yeast is ammonium. This is formed when ammonia is dissolved in water and forms the inorganic ammonium ion:
NH3 + H20 => NH4+ + OH–
It is these soluble ammonium ions that are utilised by yeast.
The ammonium supplement that is used in Australia and New Zealand is diammonium phosphate (correctly known as diammonium hydrogen phosphate) and known usually as DAP. When this is added to aqueous solutions such as grape juice the following occurs
(NH4)2HPO4 => 2NH4+ + HPO42—
That is, two ions of ammonium and one ion of hydrogen phosphate are released in the solution when the DAP dissolves and the ammonium ions are metabolised by yeast during the fermentation process. This is an inorganic molecule (Inorganic in the chemical lexicon refers to any compound that is not organic – and an organic compound is one that contains carbon).
Amino acids are organic compounds; that is, they are compounds of carbon. The use of the word ‘organic’ in chemistry should not be confused with the other popular use of that word – ‘organic farming’. (Wikipedia defines ‘organic farming’ as the technique of farming that ‘uses fertilizers and pesticides but excludes or strictly limits the use of manufactured or synthetic fertilizers and pesticides’).
Amino acids contain nitrogen that is available to yeast (the amino acid proline being an exception). The general formula for an amino acid is RCOOHNH2, where R is a carbon side chain of varying formula.
For example one of the simplest amino acids is glycine (aminoacetic acid). Its carbon backbone is 2 carbons long; so in this case R = CH3
As can be seen, glycine contains the -NH2 amino and the –COOH carboxylic acid functional groups. By definition this is an alpha or primary amino acid, a very important group of compounds in biochemistry.
It is this amino group in amino acids that can also as a source of nitrogen by yeast.
The main source of amino acids used in yeast nutrient supplements are from yeast itself. Typically the yeast Saccharomyces cerevisiae is autolysed and the resultant extract contains metabolites that include a high level of the amino acids that can be used by the yeast in the fermentation process.
The main nutrient that needs to be supplemented during fermentation is nitrogen. It comes in 2 main chemical forms that can be assimilated by yeast: ammonium ions and amino acids. There are various commercial products that are used to supply this nitrogen, and they can be classified as either simple inorganic, simple organic or complex yeast nutrients. Rehydration conditioning products are not relevant in this classification as they do not provide nitrogen to the yeast during the fermentation process.
Article from the Australian and New Zealand Grapegrower and Winemaker, Issue 588, 2013
Page Number(s): 57-58
Author: Greg Howell
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