How Irish whiskey is made

I detail the production of:

• single pot still Irish whiskey
• single malt Irish whiskey
• Irish grain whiskey
• blended Irish whiskey

on their respective pages, but all styles of Irish whiskey share the same ageing and, ultimately, bottling processes.

Ageing

The Irish Whiskey Act specifies that Irish whiskey must be aged for at least three years in wooden casks in Ireland before it can be called Irish whiskey. Casks previously containing bourbon are most common, but casks seasoned with sherry and sometimes malaga, port or madeira are also used for this ageing process.

Unlike other nations' whiskies, Irish whiskey is permitted to be aged in wood other than oak. However, for numerous reasons, apart from a few experimental bottlings, oak is used for maturation. Two types of white oak - American white oak and European white oak- are used to make these casks. American white oak contributes more vanilla sweetness and usually comes in the form of 200-litre barrels previously used to age bourbon whiskey.

Bourbon production demands that the inside of these barrels are charred, but not all the American oak barrels used to make Irish whiskey are ex-bourbon barrels, and a small number of virgin oak (previously unused) barrels are also used, but even these are charred. The charring cooks the timber, allowing the sugars and vanillins in the oak to be released into the whiskey more quickly than from non-charred casks. Charring also does the following:

1. Burns off wood resins that could contribute to off notes.
2. Increases surface area to better allow sugars, vanillins and other wood constituents into the spirit.
3. Formation of heat degradation compounds that contribute flavour and aroma to the whiskey.
4. Formation of a charred layer plays an important role in the removal of immature character from the whiskey.

Different bourbon distillers specify their own degree of char when ordering casks from their cooperage. Some stipulate a light char, some medium and others a heavy char. American law dictates that bourbon must be aged in new charred white oak barrels, so U.S. distillers can only use the casks once. Consequently, the American whiskey industry provides a steady supply of ready-seasoned casks to Irish distillers. The Irish don't have a say in the char level, but most bourbon casks tend to be medium charred.

The European white oak casks tend to be merely toasted rather than charred and are mostly 500-litre butts which have been seasoned with sherry-style wine or occasionally port, madeira, malaga or masala. These European oak casks have a profoundly different influence on the whiskey aged in them compared to American oak.

European oak is a different species and has a different composition from American oak in terms of lignin, cellulose, tannins, and other volatile compounds. European oak imparts rich dried fruit cake, currant and date flavours, while American oak is associated with coconut and vanilla notes.

American oak casks are usually filled and emptied three times, and each time the cask is refilled, it contributes less to the whiskey. Consequently, after three fillings, some distillers recondition their casks for further use by scrapping the inside to reveal fresh oak and then re-charring.

Triple distillation means that Irish pot still distillates have a higher alcohol concentration than double-distilled Scottish malt whiskeys - typically 84-85% alc./vol. while the grain spirit is 94.4% alcohol by volume. Water is added to these distillates to reduce their strength. Different distilleries will have their own view on what is the optimum strength to ensure a balanced wood extraction whilst also keeping the number of casks to a minimum. (e.g. at Middleton Distillery, this is 63.5% for pot still whiskey and 70% for grain whiskey.)

Alcohol loss through evaporation, or 'Angel's Share', averages just 2% compared to some ten times that in the tropics. Ireland is not a hot country, so there is little difference in temperature and speed of maturation between the warehouse floor and roof. Consequently, there is little benefit from moving casks from the bottom to the top of the warehouse and vice versa, as is practiced by some whiskey distillers in Kentucky. Hence, in for Irish whiskey, once a cask is placed in a warehouse, it usually stays in the same position until ready to be emptied, and even then, a pump is often used so the cask doesn't move.

Occasionally, whiskeys are blended and then put back into cask for a final "finishing" period – typically 6 to 12 months, but this varies massively between whiskey brands.

Bottling

Once blended, the high-strength whiskey is reduced to bottling strength by blending with water, usually municipal water purified by reverse osmosis.

A small amount of caramel spirit is often added prior to bottling to standardise the colour of the bottled whiskey, which would otherwise vary due to different degrees of colour imparted by different casks. Even though a small amount, this colouring is frowned upon by some but has little effect on the taste and ensures all bottles of whiskey are a uniform shade of amber.

After blending, the whiskey is often chill-filtered to remove fine particles of barrel dust and char along with fatty acids, which might cause the whiskey to turn hazy when cold. The whiskey is cooled to 0°C or colder before being pumped through cellulose filters.

This stripping of fatty acids by chill-filtration is also frowned upon by some connoisseurs as it affects the mouthfeel of the whiskey. Hence, many higher-end bottlings forego both of these processes, a USP often publicised on labels with terms such as "natural colour and non-chill filtered. Such bottlings are often also at "natural cask strength" meaning their alcohol strength has not been diluted by the addition of water.