Islay whisky making

Distilling on Islay Fermentation

Fermentation is the conversion of carbohydrates into alcohol, carbon dioxide and other compounds by a variety of microorganisms, including yeasts and bacteria. It is the process by which our sugary wort is converted into an alcoholic liquid called wash.

Different varieties of yeast will have very different production characteristics, and most Scottish distillers now use one of a small number selectively bred over the years to meet their needs. These distillers yeasts survive the sugar concentrations of the wort, operate in the correct temperature range, produce desirable flavour compounds and work at an appropriate rate.

The wort and yeast are combined into a large vessel called a washback, and left to work. After an initial phase of seeming dormancy, during which the yeast acclimatises to its new surroundings, it grows rapidly, consuming nutrients in the wort and producing alcohol and other compounds. Once the nutrients needed for further growth are exhausted, overall growth slows and stops. After a period of stability, the yeast begins to die off and without available nutrients to grow more, yeast activity declines.

This process commonly takes about 48 hours in a distillery environment, but rather than immediately moving the wort on to the next stage of production, it is left to continue to develop. Both bacterial action and autolysis of the yeast – the process by which yeast enzymes convert the dead yeast cells into other compounds – create further flavour.

  • FACT: Caol Ila uses both wooden and steel washbacks
< Back – Mashing

As the yeast grows rapidly in the early stages of fermentation, heat is produced. This raises the initial temperature of the wort from about 20°C to about 35°C. In the early stages of fermentation, carbon dioxide is also produced, which can cause the wash to foam. If unchecked, this can cause washbacks to overflow, and in distilleries where this is a common issue, ‘switchers’ are installed – mechanical blades that spin above the surface of the liquid in the washback, cutting the top off the foam and preventing it from overflowing.

The length of time the wort is left to ferment will be different between distilleries, and can even vary within the same distillery. Generally, fermentation times fall between 48 and 120 hours, with most around the 60-hour mark.

Traditionally, washbacks are made from wood but more recently many distilleries have replaced them with steel models. Steel vessels are easier to clean, but wood is a much better insulator, regulating fermentation temperature without outside heating or cooling. Wooden vessels can also harbour bacteria which can both help and hinder the fermentation process, although modern steam cleaning between batches has reduced this aspect.

Once fermentation has concluded, the alcoholic wash is pumped to the stills for the next step of the production process.

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