How to Whisky: How Fermentation Shapes Whisky’s Flavor

Distilleries have all kinds of “dials” they can tweak when crafting a spirit. They can vary the ingredients, or adjust the length of fermentation. What does that do to a whisky’s flavor? Discover it in this How to Whisky.

When distilleries set out to make whisky, they always follow a familiar sequence: malting, mashing, fermenting, distilling, maturing, and bottling. Fermentation is one of the most crucial steps, because it’s where sugars are converted into alcohol.

Although that might sound like a relatively simple biological process, the length of fermentation has a huge impact on a whisky’s final character. The difference between short and long fermentation shapes much of your dram’s personality. How does that work exactly?

After mashing, the sugar-rich wort is transferred to washbacks. Yeast is added here to convert the sugars into alcohol, heat, and carbon dioxide. In most distilleries this process takes roughly fifty to one hundred and twenty hours. Temperature is carefully controlled, because too much heat can kill the yeast or create off-flavors.

During fermentation, various esters, acids, and higher alcohols are formed. These compounds later define the aroma and complexity of the whisky.

A short fermentation usually lasts two to three days. It’s an efficient process that yields a wash with a relatively clean, cereal-driven profile. There’s less time for esters to develop, which keeps fruity and floral aromas in check.

Whiskies with a short fermentation often have:

A long fermentation typically lasts one hundred hours or more. In this extra time, bacteria and yeasts have the chance to form more esters and lactic acids.

Think of ripening, and even overripe, fruit. As a banana ripens, it becomes far more aromatic. When it’s on the verge of going off, it smells strongest thanks to esters.

During fermentation, those esters bring out notes of tropical fruit, floral tones, and a richer body.

Whiskies with a long fermentation often have:

Now that you know what fermentation does, it’s worth noting there’s no hard-and-fast rule for what counts as “long” or “short.”

Distilleries that ferment around 50 hours include Auchentoshan, Glenfiddich, and The Glenlivet. Producers that go a bit longer include The Balvenie, Springbank, and Benromach, typically running between 65 and 80 hours.

Some distilleries push it even further. With Kilchoman you can expect more than 100 hours of fermentation, and Glen Scotia will often let the mash ferment for over 120 hours.

What’s your favorite flavor profile? Do you lean towards those grainy notes, or do you prefer the funky, fruity style?