* The main amino acids are an important source of assimilable nitrogen for , yeasts but , differ in their effectiveness (Cooper 1982, Jiranek et al. 1995a,b). Some of the amino acids are precursors to volatile compounds formed during fermentation, for example, higher alcohols (Henschke and Jiranek 1993, Rapp and Versini 1996).
· The total concentration of , amino acids as opposed to that of , ammonium ions is determined in the , vineyard as it is not common practice to add amino acids to fermented musts although some commercial , products such as yeast shells provide a small amount of compound nitrogen (Ingledew et al. 1986, Munoz and Ingledew 1990).
The concentration for each amino acid varies significantly. * The content and concentration of amino acids may vary depending on the variety , the underlying , the location and prevailing conditions and degree of maturation (Nassar and Kliewer 1966, Kliewer 1970a, Huang and Ough 1989, Sponholz 1991, Gockowiak and Henschke 1992, Dellas 1993, Spayd and Andersen-Bagge 1996, Treeby et al. 1998, Hernandez-Orte et al. 1999, Stines et al. 2000).
· Cultivation practices such as the amount and timing of application of , nitrogen, , support system, soil management techniques and even botrytis infestation are determinants of the synthesis and concentration of amino acids in the rail (Ough and Bell 1980, Sponholz 1991, Dellas 1993, Spayd et al. 1994, Miele et al. 2000, Conradie 2001, Rodriguez-Lovelle and Gaudillere 2002).
· The total amount of amino acids is important for the metabolism of the , plant but the assimilable (the total of amino acids without the secondary amino acids , proline and hydroxyproline) part of the total amount of amino acids is what is of oenological interest.
The addition of nitrogen to the vineyard increases the total and assimilable concentration of amino acids in the must compared to vineyards that received no nitrogen fertilisation at all (Bell 1994, Spayd et al. 1994, Conradie 2001).
· The percentage of nitrogen assimilated by yeasts corresponding to amino acids increases as maturation progresses ,, but this trend is unaffected by the addition of nitrogen to the vineyard (Bell 1994). The assimilable amino acid content of the must at harvest is estimated to be between 51-92% nitrogen digestible by yeasts (Bell 1994, Spayd et al. 1994, Conradie 2001).
However, the effect of the addition of nitrogen in the , vineyard on the concentration of amino acids as a percentage of the nitrogen assimilable to the yeasts at the time of harvesting has a variability. Conradie (2001) showed that nitrogen addition had very little effect on the concentration of assimilable amino acids as a percentage of the nitrogen assimilable by yeasts. · In contrast to , Bell (1994) found that the concentration of assimilable amino acids as a percentage of the nitrogen assimilable by yeasts generally increased with the addition of nitrogen in two of the three fields applied , but this increase was not significant. In a different survey, however, the concentration of assimilable amino acids as a percentage of the nitrogen assimilable by yeasts in musts from a vineyard to which a quantity of nitrogen was added decreased by 10% compared to must from a vineyard in which nitrogen fertilization was not applied (Spayd et al. 1994).
* L-Arginine and L-Proline , generally account for the largest percentage of the concentration of total amino acids present in the rail (Kliewer 1970a, Ough and Bell 1980, Bell 1994, Stines et al. 2000). · Arginine is an important source of nitrogen for , yeasts but its assimilation is regulated by the presence of other , more desirable sources of , nitrogen such as ammonium ions (Middelhoven 1964, Grenson and Hou 1972, Monteiro and Bisson 1992a, Henschke and Jiranek 1993, Jiranek et al. 1995a, Salmon et al. 1998). · Thus , the addition of diammonium phosphate , prevents the use of arginine until the metabolism of ammonium ions is completed. * Arginine acts as a source of nitrogen for yeasts because its metabolism under the influence of the enzyme arginase forms L-Ornithine and urea , which under the right conditions eventually form glutamic acid and ammonium , respectively. * These compounds are precursors to the biosynthesis of generally , amino acids in yeasts (Large 1986).
· The production of , urea, which is a product of the metabolism of , arginine in some yeast strains, is related to the formation of carbamate esters in wine (Henschke and Ough 1991, Monteiro and Bisson 1991b, Ough et al. 1991). * Arginine can also be directly incorporated into , proteins while much of the arginine is shifted to yeast juice as nitrogen reserve (Watson 1976).
* L-Proline is used to a limited extent by yeasts under the usual anaerobic environment of alcoholic fermentation due to the oxygen needs of proline oxidase , which catalyzes the first step of its metabolism (Duteurtre et al. 2009). 1971).
Source: Bell S-J and P.A. Henschke , 2005. Implications of nitrogen nutrition for grapes , fermentation and wine. Australian Journal of Grape and Wine Research 11 , 242-295.
