Nitrogen addition in the vineyard affects aminoacids conentration

 

 

 

 

The total concentration of amino acids, as opposed to that of ammonia 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 complex nitrogen (Ingledew et al. 1986, Munoz and Ingledew 1990).

 

 

The total amount of amino acids is important for the metabolism of the plant, but the assimilable (the total 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 did not receive any nitrogen fertilization (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 must at harvest is estimated to be between 51-92% of nitrogen assimilable by yeasts (Bell 1994, Spayd et al. 1994, Conradie 2001).

However, the effect of adding nitrogen to the vineyard on the concentration of amino acids as a percentage of nitrogen assimilable by yeasts at harvest is variable. Conradie (2001) showed that the addition of nitrogen had very little effect on the concentration of assimilable amino acids as a percentage of nitrogen assimilable by yeasts. In contrast, Bell (1994) found that the concentration of assimilable amino acids as a percentage of nitrogen assimilable by yeasts generally increased by adding nitrogen to two of the three fields applied, but this increase was not significant. In a different study, however, the concentration of assimilable amino acids, as a percentage of nitrogen assimilable by yeasts, in musts from a vineyard to which a quantity of nitrogen was added was reduced by 10% compared to musts from a vineyard that did not have nitrogen fertilization 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 ammonia 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 ammonia 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 for the biosynthesis of amino acids in yeasts in general (Large 1986).

The production of urea, which is a product of arginine metabolism, in some yeast strains is associated with the formation of carbamic acid esters in wine (Henschke and Ough 1991, Monteiro and Bisson 1991b, Ough et al. 1991). Arginine can also be incorporated directly into proteins, while much of the arginine is shifted to the yeast chymotope as a 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. 1971).