The Role of Yeast in Whiskey Fermentation

Despite being one of only three legal ingredients for Scotch Whisky, yeast is rarely discussed alongside barley and water.

The Process of Fermentation

Whisky fermentation begins when the distiller’s yeast is added to the wort, the sugary liquid recovered from the mashing stage, to begin the sugar-to-alcohol conversion process. Fermentation proceeds through a lag phase followed by a blossoming phase. The lag phase, which occurs after yeast is added to the worts but before fermentation begins, often lasts fewer than 12 hours.

The yeast enters the ‘bud’ stage of its asexual reproductive cycle once the lag phase is through.  Yeast produces alcohol and amino acids during reproduction, which are the precursors to taste compounds known as esters and congeners.  While the fermentation process may not be exciting to watch, it is essential to the final product’s flavor.


Scotch whiskey can only be made with water, barley, and yeast. Yeast plays a crucial role in the whiskey production process. Before the early 2000s, all distillers were required to use the same DCL M-strain yeast. However, the Scotch Whiskey Association (SWA) eventually relaxed this rule. Different yeast strains, including Mauri, MX, and even wild yeast strains, are now used to capture the distillery’s unique terroir. Yeast can be either liquid or dried when it arrives at the distillery. Dried forms can last for years, whereas the liquid form is only useful for a few weeks. Keep in mind that yeast is a live organism with a lifespan.

What Role Does Time Play in Fermentation?

Short fermentations (less than 60 hours), medium fermentations (60-75 hours), and long fermentations (greater than 75 hours) are the three most common categories.

A spirit with a malty flavor can be created by fermenting it for a short or medium amount of time. At Kilchoman, we let the wash ferment for more than 75 hours, which allows the amino acids to accumulate, imparting the wash with a floral, perfumed sweetness and tangy citrus overtones. After 85 hours of fermentation, our spirit has a unique floral flavor, with hints of peat smoke, maritime influence, and milder citrus notes.

Does Every Distillery Work With a Specific Kind of Yeast?

Yeast is available in three distinct forms: a liquid cream, a pressed semi-dry form, and a dried form. The most popular form of yeast is a liquid that is difficult to keep alive and only lasts a few weeks. Since dried yeast can be stored for years, it is common practice for distilleries to keep a supply on hand.

Because of its efficiency at fermenting maltose sugars into alcohol and desirable flavor profiles characteristic of Scotch Whisky, DCL M-Strain was once employed by all scotch whisky distilleries. Distillers still utilize this strain, but newer options including MX, Mauri, and Anchor/Bfp have entered the market. Only Mauri yeast, one of four strains used in a fermentation test, is used. We discovered that the Mauri yeast produced the most well-rounded flavor profile, with hints of citrus and tropical fruit.

Where Do Spirits Producers Get Their Yeast?

Buying yeast is the most convenient option for new distilleries. One of the largest manufacturers of commercial yeast is the Montreal-based Lallemand Biofuels & Distilled Spirits, which primarily produces baker’s yeast and sells over 90% of its output to bakeries, pizza makers, and similar establishments. However, Lallemand also offers yeast for brewing and distilling. There are already 10 kinds of “distilling yeast” produced and another strain tailored to malted grain fermentation will be added this summer.

Lallemand’s technical sales manager for distilled spirits, Annick Mercier, says the company has prioritized research on “flavor-derived” yeasts for distillers, especially the expanding number of craft distillers. These dried, vacuum-sealed sachets of yeast are bred for making particular alcohols and flavors. It may come as a surprise to those who have always believed that a spirit’s flavor is determined solely by the ingredients used to make it, such as grains, fruits, or molasses. Vodka, for instance, can be made with a more “neutral” yeast, while whiskey can be made with yeasts meant to produce a more (or less) fragrant spirit, or even, as Mercier puts it, a “complex whiskey with a Scottish style.”


Some manufacturers will try out a wide variety of yeast types before settling on the desired flavor profile. The flavors can be very diverse: Whiskey can pick up all sorts of interesting flavors during the fermentation process; Livermore has made a flavor wheel to show you the spectrum. He claims that yeast selection, in addition to variations in environmental factors like temperature, pH, oxygen, and nutrients, can influence all of these. Livermore also argues that fermentation can be deliberately sourened with competing microorganisms to create new flavors.

Wooden Wash Backs or Stainless Steel?

The materials of the Wash Backs are likewise debatable, much like the Yeast utilized in the fermentation process. European Larch, Douglas Fir, and Oregon Pine were the original materials used to construct washbacks. Because of the lack of knots in the long planks, these were all utilized. Stainless steel washbacks have recently become popular among distillers due to their low maintenance needs and ease of cleaning. Advocates of wooden washbacks, however, argue that the wood contributes to a somewhat sweeter wash, so the dispute continues.

Wooden washbacks are stunning to behold, yet they all react in somewhat different ways over time. Kilchoman is equipped with fourteen stainless steel washbacks, each with a capacity of 6,000 liters. Stainless steel is utilized because it guarantees uniformity in every batch, no matter the season or the number of times it has been used.