The addition of nitrogen to vine plants increases the concentration of nitrogen in the rail. Many components of the rail that contribute to the taste and aroma of wine are affected by the addition of nitrogen directly or indirectly. Thus, changes in the concentration and composition of nitrogen compounds in must can positively or negatively affect the composition of the wine produced. Many of these effects are modified through yeast metabolism during fermentation.
In order to understand how the type of nitrogen compounds and their concentration in the rail affect the growth and metabolism of yeasts, and the conversion of flavor and aroma precursors derived from the grape, it is necessary to know which nitrogenous components of the rail are important. Despite the fact that must is relatively rich in nutrients necessary for yeasts, nitrogen content was found to show strong variability, not so much in concentration but in the type of nitrogenous compounds present. Only some of the main nitrogenous compounds of the rail are important for yeasts.
Despite the fact that must is relatively rich in nutrients necessary for yeasts, nitrogen content was found to show strong variability, not so much in concentration but in the type of nitrogenous compounds present. Only some of the main nitrogenous compounds of the rail are important for yeasts.
The basic yeast used in fermentations, Saccharomyces cerevisiae, preferentially uses simple sources of nitrogen such as ammonia ions and nitrogenous compounds with free α amino group, which occur in the form of primary amino acids
Secondary amino acids such as proline and hydrooxyproline are not metabolized to a remarkable extent under normal winemaking conditions.
Low molecular weight peptides may also be used.
In contrast, grape proteins cannot be used as a source of nitrogen as Saccharomyces cerevisiae lacks significant extracellular proteolytic activity.
Thus, the fraction of nitrogen that can be used is often referred to as assimilable nitrogen (Υeast Αssimable Νitrogen).
The remaining components of total nitrogen, including proline and hydrooxyproline, high molecular weight peptides and proteins are referred to as Yeast Non Assimilable Nitrogen. Depending on the analytical method used, nitrates are included in the non-digestible nitrogen, as Saccharomyces cerevisiae lacks the ability to digest nitrates.
The determination by analytical methods of nitrogen assimilable by yeasts is now a crucial measurement for fermentation management, as the determination of nitrogen by plant tissue analysis does not ensure reliable prediction of nitrogen assimilable by yeasts at harvest.
The content of assimilable nitrogen from the yeasts in the rail is the key factor that will determine the composition and concentration of the nitrogen components of the must assimilated by the yeasts.
Losses of assimilable nitrogen due to harvesting and transport conditions should be taken into account.
Factors such as the choice of processing methods (effect on assimilable nitrogen extraction), the management of the existing yeast population and the addition of complementary assimilable nitrogen to the winery have their role in determining the final composition and concentration of assimilable nitrogen in the must.
Πηγή : 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.
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