Blog 135

Distillation Part 3 - Maturation & Oxidation


Ben, is a director of Bar135 in Bristol.

All spirits when they come off the still are colourless. The ageing process for spirits originated by mistake when it was noticed that barrels of spirits tasted smoother and more complex after long sea voyages. The barrel is the most common vessel used for maturing spirits, usually 500-litre size. Smaller sizes can also be used, but these restrict the oxygen intake and change the character of the final spirit. A wooden barrel helps the chemical reactions, extraction of taste, extraction of bouquet and extraction of colour. A small amount of the alcohol and some of the lighter fusel oils will evaporate during this time.

Note that not all spirits are aged, and of those that are, not all are aged in wooden barrels. Nowadays some manufacturers are using artificial barrel tastes (chemicals can be bought to do this) from example Spain.


The Maturation Process

From the spirit receiver, the spirit passes, at about 70% abv, to the spirit store and into the spirit filling vat. There, the ABV is adjusted to the distillery’s normal cask fill strength by the addition of water, taken from the same pure source that the spirit was made from. From there, the casks are filled. The casks are usually made of oak (by law in some countries) and come in a number of sizes, each size having an important and different effect on the maturing spirits - generally, the larger the cask, the longer is the maturation time.

The casks may be newly made or more generally first-fill from sherry (a perfect size 110 gallons) which imparts a rich warm colour to the spirit, or a bourbon cask. Recently, some distilleries have been experimenting with other sources such as Madeira and port but those are generally used only to finish the spirit (for example whisky). Once filled, the barrels are rolled into the bonded warehouse where they are racked on staging up to five rows high and left to mature.

During storage in the bonded warehouse, various chemical reactions take place within, and in part influenced by, the cask, and the whisky itself changing gradually in nature. The oak wood imparts some chemical components to the whisky and these react to produce subtle changes in the spirit. The very atmosphere of the warehouse can exert an effect as well, for the casks are permeable and evaporation of around 1 to 2% of the contents per year (called the angels’ share) is allowed for by the Customs and Excise.



Some distilleries are now using a toasting process to aid the development of flavours.  They do this by gently heating specific American white oak barrels. The new American white oak barrels are made of smooth, clear, premium, slow growing Minnesota white oak. Built by hand and toasted to perfection, they are available in 5, 10, 15, 30 and 59 gallon sizes. The barrels are made out of bent planks of oak (called staves). Only the highest quality of oak is usually used in the manufacture of oak barrels, and the staves are always chosen by hand. The reason these are hand selected, is that the quality of the completed oak barrel lies heavily in these staves. The toasting level is achieved based upon the heating time (around 30 minutes). The flavours of the spirit partially depend on this. When heated longer than 30 minutes the toast level is heavier. When heated less than 30 minutes, it is lighter. There are multiple toasting levels, which can be adjusted according to what you need. These levels are heavy, medium, medium plus, and light. The toasting process happens when a caramelization in the wood occurs (during the heating of the staves). Some examples of the aromas this brings, are: a touch of nut, vanilla, fresh bread, or buttered bread. When you taste a wine or a spirit, you will often find these amazing aromas.


Wood finishes

More and more distilleries are producing whisky with various finishes, achieved by the last 6 months to 2 years of maturation being in ex-sherry, ex-port, ex-madeira, etc. casks. Once the casks are transferred to the bonded warehouse, by law they must remain there, under lock and key, for different maturation periods, for example a minimum of 3 years before it can be legally called Irish or Scotch whiskey. Until that point, it can only be called proof or British proof spirit. No duty has been paid on the spirit before it goes into bond. This occurs when the spirits are removed from bond at the end of its maturation period.

During its life in bond, usually much longer than the legal minimum - 8 years and upwards is typical - the spirit is gradually changing in character and composition until such time as it is required for bottling for example a single malt whisky is quite often at a variety of ages, for export in bulk form, or for use by the whisky or rum blending industry.


Bottling and cask strength distilled spirits

Distilled spirits are diluted to drinking levels by adding distilled water. This level is usually set at about 40% ABV but this figure differs according to local tastes or national liquor regulations. When the term ‘cask strength’ is used on a whiskey label, it does not mean that the product was never diluted, just that it was not diluted during the bottling process. This is usually a good sign, depending upon the age of the spirit which can vary considerably, distillery to distillery or age to age. Some cask strength spirits are as low as 55% ABV or under whilst others can be as high as 65% ABV or more. Once in the bottle, a spirit does not undergo any further changes.


Micro Distilleries

A new trend has developed in recent years brought about by the interest and desire by customers for craft produced distilled spirits. These small, independent distilleries are normally either farm based or are located in existing micro-breweries or small wineries, and are added on to the overall business operations of these small companies.


About our blog

From Aviation highs all the way down to a Dark and Stormy, the Bar 135 blog will guide you through the cocktail world and make sure you end up with the perfect drink in your hand.
Keep coming back for the latest food and drink fashions as well as expert advice on recreating your favourite drinks at home.






Home mixing


2015 Copyright Bar135 Cookies | Privacy