Articles
INFLUENCE OF NITROGEN SUPPLY IN THE GRAPE MUST ON THE FERMENTATION CAPACITY AND THE QUALITY OF WINE
Increasing pollution of ground water with nitrate was one of the main reasons.
In the last ten vintages it could be demonstrated that in musts of grapes from vineyards with a normally sufficient N-fertilisation the amino acid content is often only half of the amount which was found in musts during the seventies.
Besides an increase in the occurrence of disagreeable sulfur (S) compounds, an untypical ageing of wine and problems in the course of fermentation were observed.
It is well known that nitrogenous deficiencies can lead to lagging and stuck fermentations, an increase of SO2-binding fermentation products, an increase of undesirable concentrations of volatile fermentation products like higher alcohols and H2S. Less information is available about the formation of other S-containing compounds under N-deficiencies like thiols, which also have an impact on wine quality.
Therefore the behaviour of several Saccharomyces cerevisiae strains which are used as commercial yeasts were tested for the production of disagreeable S-compounds in different musts.
It could be demonstrated that the quality of wine is influenced by the qualitative and quantitative composition of assimilable amino acids and by the yeast strain.
The commercial yeast strains varied in the production of volatile S-substances in dependence on the composition and concentration of the assimilable N-compounds in the must and in dependence on their requirement of assimilable N. Only in some cases the addition of diammonium phosphate which is allowed to add as N-source to grape musts before fermentation in a concentration of 0,3 g/l (under European law) can avoid the occurrence of S-off-flavours in wine.