What is a worm tub in whisky?

A worm tub lets the vapour from the still run through a copper spiral tube that lies in a tub of cold water, so that the vapour cools and becomes liquid.

The tube itself is called the worm. It can be up to about a hundred metres long and is full of joints, because such a length of copper is assembled from separate pieces. The tub around it is made of wood or cast iron and at many distilleries stands outside against the still house.

The cooling water in the tub takes over the heat of the vapour. At the end of the worm the new distillate runs out. Two things determine how that goes: the temperature of the water in the tub and the copper surface on the inside of the worm.

A worm tub has much less copper surface than a shell and tube condenser, so the vapour touches less copper along the way.

A shell and tube condenser contains a bundle of dozens of thin copper tubes in a jacket with cooling water around it. The copper surface is thereby large, and condensing is fast and under pressure. Trade journals describe the distillate from it as lighter in character.

A worm tub contains only one long tube. Copper binds sulphur compounds out of the vapour, and with less copper surface more of those compounds remain in the distillate. The heavier, meatier and sulphury character of worm tub distillate is attributed in the trade to that lesser copper contact. That is an attribution from the trade literature, not a property measured on a bottle. What you smell and taste in the glass, we will tell you at the counter.

The Scotch Whisky Regulations 2009 regulate the entire production process, but say nothing about the shape of the still or the way of condensing.

Article 3 of that regulation requires that the mash is made at the distillery itself, that fermentation is done with yeast only, and that distillation takes place below 94.8 percent alcohol. Single malt Scotch whisky must come from pot stills. Maturation takes place only in Scotland, in oak casks of at most 700 litres, for at least three years. The bottle must contain at least 40 percent alcohol, and nothing may be added except water and plain caramel.

There is nothing about a worm tub or a condenser. A distillery is therefore free in that, and that choice goes with the distillate it wants to make. The shape of the still and the neck also falls outside the rules.

Warmer cooling water in summer cools the vapour more slowly, and that changes the character of the distillate coming out of the worm tub.

The temperature difference between the vapour and the cooling water becomes smaller in a warm summer. Less then condenses on the copper and distilling becomes more laborious. A distiller who lowers the water flow lets the tub heat up further: the vapour stays longer in the worm, touches more copper and yields a slightly lighter distillate.

Distilleries absorb a warm summer by distilling more slowly and less often, and accept a lower output in doing so. In the hot summer of 2018 Glenfarclas stood still for ten weeks. Talisker and Caol Ila installed cooling circuits with seawater for more than a million pounds, so that they need less fresh water.

About twenty Scottish distilleries still condense with a worm tub, and at Diageo about a third of the malt distilleries have one.

Trade magazines mention Benrinnes, Talisker, Mortlach, Craigellachie, Glen Elgin, Oban, Dalwhinnie, Speyburn, Glenkinchie, Royal Lochnagar and Edradour, among others. Dalwhinnie actually went back from condensers to worm tubs in 1995, to bring back the complexity of the distillate. New distilleries such as Ballindalloch and Ardnahoe chose a worm tub when they were built.

Whoever keeps such a tub does not do so out of nostalgia alone. A worm tub needs upkeep, is harder to keep clean than a condenser and is sensitive to the weather. Distilleries accept this because the spirit that comes out suits their house style.

In the shop on the Brink in Bathmen there are 144 bottles of whisky from these distilleries, from 40 to 60.4 percent alcohol, all from Scotland. That is part of the 3,900 bottles of whisky on the shelf.