Whisky in Port Casks
1. The Origins
The Port wine is the Portuguese version of the Sherry wine. It is a grape juice that has been fermented and then fortified with distilled grape spirit (brandy). The Port wine or sometimes referred to as just Port has been bound to the region Douro Valley. But like the trademark "Scotch" Port wine is now a trademark of Portugal. So every Port wine is from Portugal.
2. Production and post treatment
Port wine is produced from fermented grape juice. But there are a few differences between different styles of Port wine. The most distinctive difference between the wines is the dryness and the sweetness. The dryness and sweetness is defined by the level of residual sugar. A very low residual sugar is called a dry wine and a wine with a lot of residual sugar is called sweet.
The level of residual sugar is defined in a few ways. First it is defined by the grape. A fully ripe grape has more sugar than an unripe grape. Also some grape varieties contain more sugar than others. The second way of defining the residual sugar is through the fermentation. The sugar in the grape juice is turned into alcohol by the yeast until all sugar is used up or the yeast dies of alcohol poisoning. To artificially get a higher residual sugar the fermentation process is stopped by adding grape spirit (Brandy). This leads to a higher ABV and the yeast dies of alcohol poisoning and doesn't turn the rest of the sugar into alcohol.
3. Effect on the Taste
The maturation in a Port wine casks depends on the dryness/sweetness of the port. To find out the effect of the cask on your whisky you have to find out what kind of Port wine was matured in the cask. The majority of the Port wine are rather sweet so the general assumption that a Port wine finish/maturation sweetens the whisky and adds fruity flavours is not a bad one.
4. Side Note
Is Port wine a 100% Portuguese product? No! The port wine just has to be made inside of Portugal. This does not mean that all the ingredients have to come from Portugal, too. For example the vast amount of grape spirit doesn't come from Portugal but from developing countries mostly in South America.
5. Effect on Colour
As with the taste, the colour depends on the Port wine. A very dry Port wine will have no effect on the colour of the whisky and a sweet one will darken the whisky a lot. This is a way of telling, if a Port wine cask contained a sweet or a dry wine. But you have to be careful. First of all you need to verify, that your whisky is not artificially coloured. Second of all you need to know how long the whisky was stored in the Port wine cask. If the whisky was matured in the Port cask for longer than 5 years and is not coloured, than it was almost certainly a dry Port wine.
5. Whisky Examples
This Port Wine finished Scotch is a top example of a sweet Port cask. It has a dark gold and slightly red colour even though it was not artificially coloured. The taste comes with dark fruits and sweetness.
Another great example of sweet fruity Port Wood finishes
The colour of this no age, non-chill filtered, not artificially coloured whisky is remarkably dark. This comes from the Port wine casks used for the finishing.