Many of you have probably tried--and thoroughly enjoyed--whisky labeled as "Port Cask" or "Sherry Cask". But what are these?
Both port and sherry are fortified wines, the former from Portugal and the latter from Spain. However, they are made quite differently. "Fortified" simply means these wines have been made stronger with extra added alcohol. Most well-known, respected European fortified wines have strict guidelines specifying this must be from grapes growing in the area, although cheaper-grape brandy from further away may be used sometimes.
Why fortify wines? Originally, this was done so that wines travelling a long distance by sea would still be drinkable when they arrived. The Portuguese following Vasco de Gama to India in particular had a need for this. So, like many fine things, necessity was the mother of invention.
How does this change the taste of your whisky? We first need to understand the flavours in these two unique, often misunderstood wines. Most port is made from red grapes, with fermentation stopped prematurely by adding extra brandy. This kills the yeast, resulting in wine that retains much of the grapes' original sweetness. Ruby port, darker in colour, is generally either kept in an oak barrel (cask) for a short time or bottled immediately. Tawny port, however, spends a long time in barrel, which lightens the colour and lessens the primary fruit character, accentuating the caramel, marmalade and toasty type flavor spectrum. These are the subtle leftover characteristics of the wine in the oak barrels sold to whisky producers after they are no longer of use to the port producers.
Sherry is made from white grapes, usually Palomino for dry wines and Pedro Ximenez for sweet wines, sometimes blended together to produce a wine with the desired amount of sweetness. Sherry is deliberately oxidized during the ageing process, when wines are placed in barrels with a "two fist" gap at the top. A natural yeast called "flor" grows on the top and both it and the slightly-porous oak barrel allow a controlled amount of air to react with the wine. The result can be a brown-coloured, nutty-tasing wine with savoury, yeast flavours--which usually blend with the toasty vanilla from the oak barrels and are transferred to the whisky.
Enjoy your whisky, as well as some port or sherry!