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Yeast and fermentation time in whisky - where the fruity or grainy character comes from

Fruity whisky, full of notes of pear, apple and tropical fruit, and grainy, nutty, dry whisky, can come from the same distillery, the same malt and the same water. One of the main causes of this difference is a stage hardly anyone thinks about when admiring the cask: fermentation. What yeast was used and how long the wort fermented in the vessel decide what aromas are formed, before the distillate even reaches the still. A short fermentation gives one character, a long one a completely different one, and not only yeast but also bacteria come into play here. Here is a guide to yeast and fermentation time: how they work, why short and long fermentation give different whisky and where the fruity esters come from.

What fermentation is in whisky

Fermentation is the stage in which the sweet wort, that is the extract of sugars from the malt, turns into an alcoholic liquid called the wash, ready for distillation. It happens in large fermentation vessels, called washbacks, to which yeast is added. The yeast eats the sugars from the wort and turns them into alcohol and carbon dioxide, and along the way produces a host of aromatic compounds. It is at this stage, long before the cask, that many of the aromas later recognised in the whisky are formed. Fermentation in whisky usually lasts from about forty to over a hundred hours, depending on the distillery. Understanding that it is not only the production of alcohol but also a forge of aromas is the key to grasping why yeast and fermentation time so greatly affect the flavour.

The role of yeast

Yeast is the engine of fermentation, but also a creator of aroma. Its main job is to turn the sugars into alcohol, but along the way it also produces compounds that shape the character of the future whisky, including fruity esters. Different yeast strains work somewhat differently and give a somewhat different profile, although in Scotch malt whisky the yeast traditionally used is aimed above all at the efficient production of alcohol. This shows that the choice of yeast is not indifferent to flavour, even if it plays a smaller role in whisky than fermentation time. Yeast is the first variable by which a distillery can influence the aroma of the wash. Together with fermentation time it forms the foundation of the distillate character, even before it touches the copper of the still or the wood of the cask.

Short fermentation

A short fermentation usually lasts about two to three days, roughly forty-eight to sixty hours. It gives a wash of relatively clean, grainy character, because there is less time for esters to develop, which keeps the fruity and floral aromas in check. As a result the distillate comes out more grainy, nutty and dry, with less emphasis on fruit. A short fermentation is the choice of distilleries that aim for a clean, malty-grainy style, where the grain plays first fiddle rather than fruity esteriness. It is a simpler and faster approach, giving a repeatable, legible profile. A short fermentation lays the foundation for a whisky of rather dry and grainy character, in which fruitiness is restrained. It is the first of the two poles between which a distillery chooses in shaping its style.

Long fermentation

A long fermentation often exceeds eighty, or even a hundred hours, that is four to five days and more. This extra time changes everything. It allows additional microorganisms to develop and more complex aromas to form, often fruity and floral. The longer the fermentation, the more esters responsible for fruity notes, and the distillate becomes more fruity and estery. A slower, longer fermentation at a lower peak temperature gives soft, fruity and slightly spicy esters. This is why distilleries seeking a fruity, complex character bet on long fermentation. It is a more time-consuming choice, but one giving a richer, more aromatic wash. A long fermentation is the second pole, leading whisky toward fruit, esters and complexity, far from the simple, grainy cleanness of a short fermentation.

A table: short versus long fermentation

Let us gather the two poles in one place:

The table shows the essence: the longer the wort ferments, the more esters and fruit, and the less simple, grainy cleanness.

The role of lactic acid bacteria

In a long fermentation not only yeast but also bacteria come into play, especially lactic acid bacteria. When the yeast has finished most of its work, on the third and fourth day of fermentation the wash begins to be colonised by bacteria, most often the good kind, of the Lactobacillus genus. The lactic acid they produce combines with the alcohol, forming esters responsible for ripe, tropical fruit notes. This is why only a long fermentation, giving the bacteria time, leads to such a fruity character. In a short fermentation the bacteria do not have time to develop, so this effect is missing. The cooperation of yeast and bacteria in a long fermentation is a subtle, often underrated mechanism of building fruit in whisky. It is another proof that the flavour of the spirit is born long before the cask, in the fermentation vessel.

Temperature and pace

The aroma is affected not only by the length of fermentation but also by its pace and temperature. A slower fermentation at a lower peak temperature favours the formation of soft, fruity and spicy esters. A faster, hotter fermentation gives a cleaner, more grainy distillate. This shows that a distillery steers the aroma not only through the choice of time but also through the control of conditions. The wooden fermentation vessels, traditional in many distilleries, also have their subtle share here, because their own microorganisms live in them. All these variables, yeast, time, temperature and bacteria, make up the character of the wash. It is proof of how complex and underrated a stage fermentation is, in which the seemingly simple process of turning sugar into alcohol hides a whole palette of decisions affecting flavour.

Why it matters despite the cask

One might think that since the cask gives most of the flavour of mature whisky, fermentation does not matter. That is a mistake. The character built in fermentation, fruity or grainy, is the foundation on which the cask only builds. The fruity esters of a long fermentation survive years in the cask and co-create the final profile. Distilleries deliberately choose the fermentation time to get a raw distillate of the desired character before it reaches the wood. We cover where the spirit aromas come from more in where whisky flavours come from, and the art of the cut in the cuts. Fermentation is one of the foundations, alongside the shape of the still and the cut, on which the character of the distillate stands. The cask enriches it, but does not create it from scratch.

How to sense it in the glass

The influence of fermentation can be sensed by comparing whisky styles. Clearly fruity whisky, full of notes of pear, apple, peach or tropical fruit, often comes from a long, ester-rich fermentation. More grainy, nutty, dry and simple whisky more often comes from a short fermentation. This is of course one of many factors, alongside the cask, the still and the cut, but a real one. Comparing a fruity whisky with a more grainy one, it is worth thinking that part of that difference began in the fermentation vessel, in the decision of how long the wort fermented. It is an invisible stage, yet one clearly heard in the flavour. We cover the whole road of production more in how whisky is made.

The essentials in brief

Let us gather it up. Fermentation is the stage in which yeast turns the sugars of the wort into alcohol, producing aromas along the way, including fruity esters. A short fermentation, about forty-eight to sixty hours, gives a clean, grainy, nutty and dry distillate, because esters do not have time to develop. A long fermentation, over eighty to a hundred hours, gives more esters and a fruity and floral character, and lactic acid bacteria also come into play, building tropical notes. The pace and temperature of fermentation also matter. The character built in the vessel is the foundation on which the cask only builds. Now you know where the fruity or grainy character of whisky comes from and why fermentation matters despite the cask.

Note every whisky in GustoNote - the style, the fruitiness and the notes you sense. Over time you will start to link the fruity or grainy character of a spirit to decisions made already at the fermentation stage, and understand more deeply where its flavour comes from.