What is a washback?

A washback is the large fermentation vessel in which a malt whisky distillery ferments the cooled wort with yeast into wash.

The wort comes out of the mash tun and consists of water with the dissolved sugars from malted barley. In the washback the distiller adds yeast, usually at 16 to 19 degrees. The yeast converts the sugars into alcohol and carbon dioxide, during which the temperature itself rises to 32 to 33 degrees; Scottish washbacks are not cooled. What is in it after fermentation is called wash. A washback holds thousands of litres and has a lid, because the foam rises high.

Washbacks are made of wood or stainless steel; with wood it is usually Oregon pine, the American Douglas fir, or larch.

The wood itself gives the wash no flavour. The difference lies in hygiene: wood is porous and full of seams and crevices where bacteria lodge. Cleaning is done with hot water under pressure, soap and about thirty minutes of steaming, and that kills most micro-organisms, but not all. Stainless steel is smooth, can be cleaned more reliably and can be sprayed with cold water from the outside; a wooden vessel cannot. Distilleries that keep their wooden washbacks do so for the bacterial life that survives the cleaning.

Fermentation in the washback takes two to three days and yields a wash of 8 to 10 percent alcohol.

How long exactly depends on the capacity of the distillery and on the character the distiller wants. A short fermentation of around fifty hours keeps the bacterial population low. Mashing does not stop at the mash tun: the wort is not boiled, so enzymes from the malt keep working in the washback and release extra sugar. After that the wash goes to the wash still, which draws low wines of 25 to 30 percent from it.

A longer fermentation gives lactic acid bacteria more time, and those bacteria only get going properly once the yeast has done its work.

Once the yeast has finished its work and dies off, nutrients are released that lactic acid bacteria can carry on with. They make lactic acid and acetic acid. From those acids esters such as ethyl lactate and ethyl acetate form, and these are the sweet, fruity and creamy notes; a lactone such as gamma-dodecalactone also belongs here, with a sweet and fatty character. Limited bacterial activity is part of the craft of malt distilling. Too many foreign bacteria cost yield and cause off-flavours, so it remains a narrow margin.

The washback sits between the mash tun and the stills, as the third step in a row of five.

The barley is first malted and dried. Mashing then follows in the mash tun, where hot water extracts the sugars from the ground malt. The wort is cooled and goes into the washback to ferment. The wash then goes to the wash still, and what comes out of that goes to the spirit still. There are separate question pages about mashing and about the two stills.

The law prescribes for Scotch whisky that fermentation takes place at the distillery itself and only by adding yeast.

The Scotch Whisky Regulations 2009 require the whisky to be distilled in Scotland from water and malted barley, the mash to be converted only by the grain's own enzymes, and fermentation at that distillery to use yeast only. The law says nothing about the length of fermentation, the size of the cask or the material. In the shop on the Brink in Bathmen there are 1,219 Scotch whiskies between 40 and 46 percent, from 249 brands, out of 2,824 Scotch whiskies and 3,945 whiskies in total.