Relation between kinetic of alcoholic fermentation and must nitrogen content. Influence of nitrogen fertilization on <em>Vitis vinifera</em> cv Muscadelle with permanent grass cover. Nature and concentration levels of some amino acids of musts

An experiment concerning the introduction of nitrogen fertilization on permanent grass cover was undertaken in 1995. This was done in order to determine both the role and the influence of nitrogen on the fermentability of must. Located in the Sainte-Foy La Grande area, the plot studied has a deep silty soil. The cultivar Muscadelle was grafted on the 3309 C rootstock. Four tests with several N-fertilizations were compared : 1 (no-tillage and 0 kg N/ha/year), 2 (permanent grass cover without N-fertilization), 3 (permanent grass cover and 30 kg N/ha/year) and 4 (permanent grass cover and 60 kg N/ha/year). Nitrogen was added at springtime, on the total interrow area as weIl as on the narrow strip around the vines. The grass cover (with or without N-fertilization) had a great effect on yield : it decreased the latter by about 38 percent compared to the no-tillage trial. Moreover, the total acidity values were lower for the musts in trials 2,3 and 4 than in the control 1. Inversely, the initial sugar values were greater in the grass cover trials. The duration of alcoholic fermentation was very different with the no-tillage test and the others : 17 days were necessary for the first, and 42 to 50 for the other musts. This parameter was quite cIosely correlated to the content of nitrogen composites in the must and more particularly to amino acids. The global amino acid contents of must and wine were higher in test 1 than in the others. It is noteworthy that numerous kinds of amino acids were present but it seems likely that some of them were very important by virtue of their nature or their concentration levels. Among the principal amino acids, proline represented at least 20 percent of the global amino acids content for musts 2 and 4, and 14 to 19 percent for musts 3 and 1. Arginine was present only in the must of the grass-cover trials. Proline, arginine, alanine, asparagine/glutamine and aspartic acid represented about 50 to 60 percent of the global amino acids content of the musts. A statistical analysis of the four musts and wines showed a clear differentiation between the no-tillage trial (1) and the others. However, there seemed to be some composition similarities between musts 1 and 4, but the fermentation kinetic was different probably because of the lack of, or the lower concentration of, some of amino acids in case 4.