When Glenlivet is freshly distilled it is clear in colour and tastes cereal-rich and slightly smoky. But fifty years later of being stored in an oak barrel it develops a deep amber colour, the smoky aroma is gone and there are new caramel and chocolate characteristics. During maturation, whisky is largely influenced by the environment inside the barrel. Law has made it simpler by laying down that all casks have to be made of oak. Bourbon must be aged in previously unused casks whereas Scotch whisky should be held in barrels which were previously used to store sherry or bourbon.
Before storing the whisky, the barrels are charred. This creates a layer of charcoal on the inside which filters the spirit and removes the undesirable flavours. It would not be wrong to state that the environment outside also matters which includes temperature.
The various types of whisky casks are Gorda, Madeira Drum, Port Pipe, Butt, Puncheon,Barrique, Hogshead, ASB, Quarter Cask and Blood Tub. They differ in capacity. It is important to note that smaller the cask the more contact the whisky has with the wood.
Interestingly, in the olden days it was accidentally discovered that whisky matures in oak casks. Wooden barrels were merely used as vessels to store freshly produced distilled spirit. A lot of spirit was consumed before it matured into something like what we drink today. Nevertheless, a few casks were left laying for long. One can only imagine the delight of the Scotsmen who accidentally discovered the magical effects of wood on whisky. Soon drinkers developed a preference for matured whisky following which the process of oak cask maturation gained ground.