Résumé
Bipolar membranes electrodialysis or cation-exchange resins are acidification processes for musts and wines, which allows to control extraction of cations. The impact of both processes on nitrogen molecules and mineral content has been studied in four grape musts (two red and two white) at different levels of acidification (pH decreases of O,2O, O,25 and 0.60 pH Units, respectively). The four tested musts were acidified on one hand by bipolar membrane electrodialysis, and on the other hand by contact with resins, which means that an exact quantity of resins was added to the must to obtain the pH decrease in the whole must under stirring. On one of the white musts, acidification with resins was also performed by percolation of a fraction of the must in a resin column, and the obtained acidified fraction was blended to the initial must in order to reach the three levels of acidification needed. Results show that potassium is the most extracted cation, and, on three of the four tested musts, the bipolar membrane electrodialysis process extracts most of the cations. Bipolar membrane electrodialysis allows more extraction of ammonium ions, but the resins remove more assimilable amino-acids. The impact of the acidification processes on the must nitrogen content depends on the color of the must: for red musts, bipolar membrane electrodialysis extracts more nitrogen than resins, and for whites musts the opposite situation is observed, whatever the acidification method used (contact or percolation). Resins extract more total nitrogen than bipolar membranes electrodialysis, especially for higher levels of acidification. Both processes, aimed to extract cations, reduce the mineral content of musts, as expected. The impact of both processes on the assimilable nitrogen contents of the musts is significant; resins extract more total nitrogen than bipolar membranes electrodialysis. The extraction of nitrogen is more important for high levels of acidification. Therefore, acidification of musts that exhibit a low content of assimilable nitrogen may require nitrogen supplementation to avoid slow or sluggish fermentations.