Aspects of Distillation (8): Fermentation Speed!


Aspects of Distillation” is a series the iStill Blog hosts. It aims to cover as many aspects as possible. Aspects of – you guessed it! – the distillation process. Think alcohol formation, flavors, mashing, distillery design … and more. In fact, if you have a suggestion, please email us the aspect you want us to dive into. Via Today’s topic? Fermentation speed!

Fermentation speed

There are four things I want you to consider, when deciding on how long you want to ferment a certain brandy, rum, vodka or whiskey:

  1. It is during fermentation that the actual alcohol is formed;
  2. It is during fermentation that (the vast majority of) flavors are formed;
  3. Alcohol presence is a prerequisite for flavor formation;
  4. Alcohol formation and flavor formation do not take place at the same speed.

Most distillers are brought up with the idea that fermentation is important because it is the process where you make your alcohol. Faster fermentations translate into higher yield, and that is supposed to be a good thing. But that is only half of the story.

I want you to understand that fermentation is way more important, because it is also the process where most of your flavors are formed. Alcohol, in terms of yield, is important if you want to compete on economies of scale. Flavor composition is important if you want to compete on spirit quality. As a craft distiller I think you want to compete on quality. If that is the case alcohol yield only becomes the secondary goal of fermentation.

Flavors are formed where alcohol and organics meet. For flavor molecules to be created the presence of alcohol is required. Since fermentations start with 0% alcohol and – over the course of a few days – slowly grow towards (for example) 8%, flavor formation always lags behind. Alcohol formation comes first, flavor formation follows.

What this means for fermentation speed? Well, that if you look at fermentation as “alcohol production” only, you deal yourself a short hand.

As the drawing shows, it probably takes only three days to ferment from 0% to 7.8%. A longer fermentation – say five days – will only increase the alcohol percentage from 7.8 to 8.0%. The gain of 0.2% is minimal and not worth the extra time. You better start up a new ferment in order to make more alcohol quicker:


But, as stipulated above, as a craft distiller you are dealing yourself a short hand. Flavor formation only takes place when alcohol has formed. So a longer fermentation results in more flavor. As the drawing underneath shows: even though most alcohol production is done by day 3, it is at day 4 and 5 that most of the flavors are created:


With this in mind, we can now conclude that shorter fermentation cycles generate more alcohol faster, while severely limiting flavor formation. Longer ferments may be slightly less economical, but result in more flavorful outcomes.

What this means for your fermentation speed? Simply this: ferment short & fast for vodka and slow & long for taste-rich spirits.

Features & Benefits

The iStill Distilling University trains distillers extensively in the noble art of fermentation. Do you want to learn how to ferment and create the exact flavor profile for your drink? Purchase the iStill University Distilling Course via the link underneath.

Please know that the money you spend on the training program can be used as a discount, when ordering your iStill. Or if you want to purchase an iStill first, please be informed that the distilling training is included as a standard option.