Modern cooks have shown a great interest in exotic vinegars made with fermented fruit such as plums or berries. Such vinegars are expensive, however, because of the need to ferment the fruit first to make a wine before then turning it into a vinegar, which then in turn requires a lengthy ageing process of up to 6 months. It is stored in barrels in cold cellars hewn out of rock, where the temperature means that the wine reaches only 4-5% oxidation. The addition of essence made from the fruit enhances the flavour.