Changes in nitrogen compounds in must and wine during fermentation and biological aging by flor yeasts

Urea, ammonium, and free amino acid contents were quantified in a must from Vitis vinifera cv. Pedro Ximenez grapes and in fermented wine and after a short aging of this wine by Saccharomyces cerevisiae race capensis yeast un der variable oxygen availability conditions. The previous compounds were al so determined in a wine in which the nitrogen source was depleted by the sa me race of flor yeast (old wine) and also following the addition of ammoniu m ion, L-glutamic acid, and L-proline. Under specific conditions such as lo w oxygen level and the absence of some nutrients, the yeasts release some a mino acids including L-threonine, L-tryptophan, L-cysteine, and L-methionin e to the medium. These amino acids must originate primarily in a de novo sy nthesis from ethanol that regenerates NAD(P)(+). On the basis of these resu lts, the yeasts may be able to use amino acids not only as nitrogen sources but also as redox agents to balance the oxidation-reduction potential unde r conditions of restricted oxygen, when electron transport along the respir atory chain may be hindered or limited.