Les acides gras libres du vin; observations sur leur origine

L'addition d'écorces de levure ou de lies fraîches dans un milieu alcoolique entraîne une augmentation de la teneur en acides gras libres suivie d'une diminution progressive; les lies fraîches ne cèdent pratiquement pas d'acide palmitoléique, inhibiteur de l'activité malolactique. La teneur en acides gras libres augmente au cours de la macération des raisins rouges, mais elle est peu affectée par la macération préfermentaire des raisins blancs. Par ailleurs, les teneurs des vins en acides gras libres varient en fonction du cépage. +++ This work deals with the free latty acids (C12 to C18:3) content of different mediums; these substances seem to have an activity on the alcoholic and the malotactic fermentations. ln a first step we have studied the influence of the addition to the medium of yeast ghosts and fresh yeast cells (lees). We have also determined free fatty acids in red wines (Merlot, Cabernet Sauvignon and Malbec) in function of the time of maceration (0 to 10 days) and in white wines (Semillon and Sauvignon blanc) in function of skin contact time (0 to 6 hours) and SO2 which have been added, or not, to the medium belore skin contact. The free fatty acids were extracted from 20 ml of wine by three times 10 ml of dichloromethane ; the organic phase was concentrated under vacuum to 1 ml. The extract was analysed by gas chromatography with a capillary CP WAX 57 column from Chrompack (10 m x 0,22 mm); the chromotograph was equiped with a spitless system injector. The addition of yeast ghosts and fresh lees into an alcoholic medium causes a temporary increase of the free fatty acids followed by a constant decrease. However, the fresh lees do not give palmitoleic acid which is an inhibitor of the malolactic activity. The time of maceration plays a positive part on the free fatty acids content of the wine; this is more important for the red wines than for the white ones. The free fatty acids content of the wines changes according to the grape variety.