Yeast assimilable nitrogen

Carrau, F. M., Medina, K., Farina, L., Boido, E., Henschke, P. A., DeUacassa, E. (2008) Production of fermentation aroma compounds by Saccharomyces cerevisiae wine yeasts effects of yeast assimilable nitrogen on two model strains. FEMS Yeast Research DOI 10.11 ll/j.1567-1364.2008.00412.x. 

Ester formation is associated with yeast growth in the early phase of fermentation. Acetate esters are produced via the reaction between an alcohol and acetyl Co-A, which is catalysed by the enzyme alcohol acetyl transferases (ATFl and ATF2). Ethanol, branched-chain alcohols and 2-phenylethanol are the common moieties of acetate esters. Ethyl esters of medium-chain fatty adds are formed through the reaction between ethanol and respective fatty acyl Co-A, which is catalysed by the enzyme alcohol acyl transferases. Saccharomyces cerevisiae strains also produce esterases that hydrolyse esters, and thus the final concentration of esters in beers is the net balance between ester synthesis and hydrolysis. Strains of brewing yeasts produce predominantly ethyl esters of fatty acids, particularly ethyl octanoate, with relatively little formation of acetate esters. Ester production in beer is regulated by a number of factors such as yeast strain, temperature, hydrostatic pressure, wort composition, sugar type and concentration, type and amount of yeast-assimilable nitrogen, aeration, and unsaturated fatty acids (Hiralal, Olaniran, PiUay, 2014 Pires et al., 2014). 

The nitrogenous components of grapes and must that are metabolically available to yeast are present as ammonium salts (NH4 ) and amino acids primary, collectively known as yeast assimilable nitrogen (YAN). Therefore, a complete evaluation of the nutritional status of juice or must requires measurement of both fractions. The total nitrogen content of juice not only contains YAN but peptides and proteins as well. However, the latter two fractions are not thought to play a significant role in the nutritional needs of Saccharomyces during fermentation. 

Table 8.1. Yeast assimilable nitrogen (YAN) in 1523 grape musts obtained from Callifornia, Oregon, and Washington.

Must ripeness (°Brix) Yeast-assimilable nitrogen (mg N/L)... 

Butzke, C.E. 1998. Survey of yeast assimilable nitrogen status in musts from California, Oregon, and Washington. Am.J. Enol. Vitic. 49 220-224. 

Jiranek, V, Langridge, R, Henschke, P. A. (1996) Determination of sulphite reductase activity and its response to assimilable nitrogen status in a commercial Saccharomyces cerevisiae wine yeast. Journal of Applied Bacteriology, 81, 329-336. 

Vilanova, M., Ugliano, M., Varela, C., Siebert, T, Pretorius, 1. S., Henschke, P. A. (2007) Assimilable nitrogen utilisation and production of volatile and non-volatile compounds in chemically defined medium by Saccharomyces cerevisiae wine yeasts. Applied Microbiology and Biotechnology, 77, 145-157. 

Assimilable nitrogen sources readily utilized by all the tested strains were peptone, hydrolysed casein, yeast extract, glutamic acid. 

Let us now see how yeast acts in an industrial way in a brewery wort Here the conditions differ from those of the laboratory, in particular, aeration is insufifident. It results from this that the yeast does not absorb all the assimilable nitrogen contained in the wort, but only from 20 to 36 per cart ... 

Brewers yeast can assimilate some 50% of the amino nitrogen in wort [34] in general, bottom yeasts use the amino acids less completely than top yeasts [35]. The bulk of the yeast s nitrogen requirement is met by amino nitrogen (amino acids, peptides) rather than ammonium ions. 

In fermentation, the amino acids present in wort are preferentially used to supply nitrogen to the cell. Ammonium ions may or may not be used depending on the particular yeast strain [61]. The assimilated nitrogen is used... 

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