'The Mystery of Sherry Oak Maturation' For reasons not even science can wholly explain, whisky has always matured best in oak casks that have contained sherry. Due to increasing expense and scarcity, other distillers no longer insist on sherry casks. The Macallan directors do. Journeying annually to the bodegas of Jerez, Spain, they buy fresh oak casks into which they pour mature , carefully chosen sherries, then keep them for two further years in Spain before having them shipped over to be filled with whisky. The results are shown partly in The Macallan's rich golden colour, partly in the 'nose' and wholly we may venture?,... in the tumbler.