Fermentation

1. The Process for Malt Whiskey

Milling

After the starch has been converted by enzymes into sugar, it needs to be extracted from the corn. Malt mills grind down the malt into a coarse substance called grist. This grist is not as fine as flour and still contains all the elements of the shell.

Irish Malt and Grist
Irish Malt and Grist

Mashing

The grist is mixed with hot water to wash out the sugar. Three times grist and water are mixed in mash tuns. Each time the temperature is increased until 95 °C. The last time the extracted sugar is least. And this last water is used for the next batch in the mash tun. The resulting sweet water is called wort and goes on to the fermentation. The remaining mash is used as animal feed.

A cast iron Mashtun of the Midelton distillery
A cast iron Mashtun of the Midelton distillery

2. The Process for Grain Whiskey

Milling and Mashing

Grain that is not malted still contains starch. For the alcoholic fermentation to work, it needs to be transformed without the help of natural enzymes into sugar. Thus the grain is milled and then pressure cooked. Two things happen together. The starch solutes out of the husks and the long molecules of the starch are cut into shorter sugars. The resulting sugar solution is called wort, the same as with Malt whiskey.

A big stainless steel tun with an open manhole
The stainless steel mash tun of the Cooley distillery

3. Fermentation

The Chemical Process

The wort is cooled down to about 20 °C. Then yeast can be added. The solution is left in wash backs for 48-96 hours. In this time the yeast works and produces alcohol from sugar. The yeast cultures also create a lot of CO2 and excess heat. If wash backs are placed in a cold environment the fermentation process is slower and the whiskey is said to taste better. In big distilleries CO2 is collected and sold in pressurized steel bottles for industrial purposes.

A look into the washback through the manhole
The washbacks at the Cooley distillery
The wash with bubbles on top
The wash in a washstill

The Wash Backs Today

Wash backs were used to be made from pine wood, because the containing resin made it highly resistant to bacteria.

Wash backs at the Teeling distillery
Wash backs at the Teeling distillery

Today the wash backs are made most often of stainless steel and are sometimes equipped with cooling systems to precisely control the speed of the fermentation.

During the fermentation you see bubbles of CO2 rising. Watch out if you look into one of these manholes. The alcoholic fumes rising are very sharp. The floors of the Washroom are always made of grids and the rooms are well ventilated to prevent the CO2 from building up and suffocating the workers.

A stainless steel tun in a hall
Wash backs at the Cooley distillery