Fermentation: How the Wash Is Made

Fermentation turns sugary wort from the mash tun into wash, the alcoholic liquid that will be distilled to make whisky. Yeast produces alcohol alongside other volatile compounds, so the washback is already helping to shape the character of the future spirit, not simply supplying its alcohol.[1]

What happens during whisky fermentation?

Wort is the sugary liquid produced during mashing; a washback is the vessel in which it ferments. Once yeast is added, the sugary liquid becomes wash, described in the Scotch Whisky Technical File as similar in aroma and taste to unhopped beer.[1]

That comparison is a useful starting point, but it does not tell the whole story. Research on Scotch whisky fermentation describes an unboiled, nonsterile process in which malt enzymes remain active and bacteria can participate alongside yeast. The washback therefore holds more than a straightforward mixture of sugar and yeast.[2][5]

Three overlapping activities matter: yeast produces alcohol and other volatile compounds; surviving malt enzymes continue breaking down material left from mashing; and bacterial activity can influence the developing mixture. Their relative importance changes over the course of fermentation, which helps explain why fermentation time matters to more than alcohol yield.[1][2][5]

Yeast needs more than sugar

The major carbohydrates available during whisky fermentation include maltose and maltotriose. These are among the sugars yeast encounters in the wort, rather than a single, uniform sugar supply.[4]

The Scotch Whisky Technical File connects the use of the barley's own enzymes with a complex fermentation substrate, meaning the material available for fermentation. It says this complexity requires specialised yeast strains. The Scotch Whisky Association's cereals technical note also explains that some cereal protein is necessary for yeast growth during distillery fermentation.[1][9]

This makes mashing and fermentation closely connected stages. Mashing determines the material reaching the washback, while active enzymes can continue working after yeast has been added. The boundary between making fermentable sugars and fermenting them is therefore less tidy than a simple production diagram might suggest.[2]

Yeast's contribution is not limited to ethyl alcohol, also called ethanol. The Technical File identifies other volatile compounds produced during fermentation and specifically lists fermentation congeners among the factors shaping the new spirit. Here, congeners means compounds accompanying the ethanol that contribute to the developing spirit's character.[1]

Temperature, time and wash strength

There is no single fermentation clock that describes every Scotch distillery. The Technical File gives grain Scotch fermentation as taking two to three days, whereas Springbank's own process account describes fermentation lasting up to 110 hours. These are different production accounts, not competing definitions of the correct duration.[1][6]

Temperature also belongs to the process rather than being a detached specification. The Technical File describes starting fermentation at low temperatures in Scotland's cool climate and allowing it to warm naturally to a maximum of about 33 °C. It identifies that fermentation pattern as a factor affecting the final distillate's character.[1]

About 8% ABV is a useful reference for Scotch malt wash, rather than a guaranteed strength for every fermentation.[3]

The practical reading rule is to keep each number attached to its context. A named producer's fermentation duration, a laboratory wash strength and a modelling baseline answer different questions; combining them would create a supposed standard that none of the sources actually establishes.

Why an unboiled wort matters

In the Scotch fermentations described by Daute and colleagues, wort is not boiled before fermentation. Malt enzymes therefore remain active, including alpha-amylase, beta-amylase, limit dextrinase and alpha-glucosidase, and continue breaking down residual starch during fermentation.[2]

That continuing activity is one reason wash should not be understood simply as beer awaiting a different destination. The comparison with unhopped beer describes a resemblance in aroma and taste, while the unboiled wort explains an important feature of the production process.[1][2]

Bacteria also arrive through more than one route. Van Beek and Priest describe bacteria from malt surviving mashing and entering fermentation, alongside organisms that colonise distillery plant. Their study followed the changing lactic acid bacterial community during a commercial malt whisky fermentation.[5]

Nonsterile does not mean uncontrolled. Trade guidance identifies poor cleaning as the most common cause of failed fermentation and notes that lactobacilli compete with yeast for wort sugars. A bacterial contribution to character and unwanted competition during fermentation are not the same thing.[8]

What longer fermentation can change

Long fermentation is often discussed as though the extra hours simply produce extra alcohol. The more useful distinction is between yeast's alcohol production and the later development of compounds that can influence spirit character.[1][5]

In the commercial malt fermentation studied by van Beek and Priest, the final phase began at about 70 hours. The researchers also noted that spirits from long fermentations, described as greater than 55 hours, tend to have higher ester levels. Those figures belong to that research context, not a universal threshold separating short and long fermentations.[5]

The same study explains a connection that continues into the next production stage: lactic acid formed during fermentation reacts with ethanol during distillation to produce ethyl lactate. Fermentation therefore supplies compounds whose significance is not confined to the washback itself.[5]

Springbank provides a documented example of an extended schedule: its own account says yeast converts sugars over a fermentation period of up to 110 hours. The wording “up to” matters, and the duration alone is not a tasting note or evidence that a longer fermentation necessarily makes a better whisky.[6]

How American sour mash fits in

Sour mash describes an acidified mash, not a promise that the finished whiskey tastes sour. The Oxford Companion to Spirits & Cocktails explains that the thin portion of spent stillage, called backset, is added to the mash cooker with water and fresh grain meal.[10]

Its entry gives a typical backset addition of 10 to 25% of fresh mash by volume. The acids lower mash pH and suppress spoilage bacteria; the entry also describes a traditional second step in which lactic bacteria sour the yeast mash.[10]

This is a useful contrast with the late bacterial activity discussed in malt whisky research. Both involve acidity and microorganisms, but backset addition is a particular production practice, not another name for leaving a Scotch fermentation longer. The Scotch Technical File similarly describes sour mash in Tennessee and bourbon whiskey as acidification associated with local water conditions.[1][5][10]

Why wash strength matters beyond fermentation

Wash strength affects the production system beyond the washback. A 2026 Heriot-Watt modelling study examined increasing Scotch malt wash strength from an approximately 8% ABV baseline to higher strengths, rather than reporting that those stronger washes were standard distillery practice.[3]

Increasing the modelled strength from 8 to 14% ABV progressively reduced energy use and carbon footprint by 15.8 to 16.1% per litre of pure alcohol. Modelled pot ale volumes fell by 55.0% and spent lees by 30.4%; these are the study's calculated outcomes, not measurements from an industry-wide conversion to stronger wash.[3]

The model found that distilling energy dominated total energy demand, while fermenter cooling energy was negligible. Its lesson is about the relationship between fermentation and the wider process: changing the wash can change downstream requirements. It does not establish a flavour advantage for stronger wash.[3]

Where to go next

• Mashing: How Grain Starches Become Fermentable Sugars: follow the production of the wort that feeds fermentation.
• Enzymes in Whisky: Malted Barley vs Added Enzymes: examine the enzymes behind sugar production.
• Washbacks: Wooden vs Stainless Steel: explore the vessels in which fermentation happens.
• Lactic Acid Bacteria and Whisky Fermentation: look more closely at the microorganisms working alongside yeast.

Sources

[1] GOV.UK, Department for Environment, Food & Rural Affairs. Technical File for Scotch Whisky, June 2019.
https://assets.publishing.service.gov.uk/media/5fd36667e90e07662ed92c85/Scotch_Whisky_Technical_File_-_June_2019.pdf

[2] Daute, M., Jack, F., Harrison, B. and Walker, G. Experimental Whisky Fermentations: Influence of Wort Pretreatments. Foods, volume 10, issue 11, article 2755, 2021.
https://pmc.ncbi.nlm.nih.gov/articles/PMC8624260/

[3] Zhuang, S., White, J. and Boulton, C. Quantifying the impact of increasing wash strength for Scotch malt whisky production: a modelling study. Journal of the Institute of Brewing, volume 132, issue 2, pages 78 to 91, 2026.
https://jib.cibd.org.uk/index.php/jib/article/view/97

[4] Kido, Y., Maeno, S., Tanno, H., Kichise, Y., Shiwa, Y. and Endo, A. Niche-specific adaptation of Lactobacillus helveticus strains isolated from malt whisky and dairy fermentations. Microbial Genomics, volume 7, issue 4, article 000560, 2021.
https://pmc.ncbi.nlm.nih.gov/articles/PMC8208680/

[5] van Beek, S. and Priest, F. G. Evolution of the lactic acid bacterial community during malt whisky fermentation: a polyphasic study. Applied and Environmental Microbiology, volume 68, issue 1, pages 297 to 305, 2002.
https://pmc.ncbi.nlm.nih.gov/articles/PMC126549

[6] Springbank Distillers. Our Process, undated producer's account, version recorded in the supplied fact sheet in November 2026.
https://www.springbank.scot/about/process/

[8] ScotchWhisky.com, The Whisky Professor. Are wooden washbacks better than steel?, 16 May 2016. Trade explanation.
https://scotchwhisky.com/magazine/ask-the-professor/9275/are-wooden-washbacks-better-than-steel

[9] Scotch Whisky Association. Scotch Whisky Cereals Technical Note, 6th edition, August 2021.
https://www.scotch-whisky.org.uk/media/1900/cereals-technical-note-6th-edition-240821.pdf

[10] Smith, K. Sour mash, in Wondrich, D. and Rothbaum, N. (editors), The Oxford Companion to Spirits & Cocktails, Oxford University Press, online entry reproduced by Spirits & Distilling, version recorded in the supplied fact sheet in November 2026.
https://www.spiritsanddistilling.com/dictionary/id/acref-9780199311132-e-669

This desk-researched article uses the supplied verified fact sheet, distinguishing technical descriptions, research findings, modelling results and the producer's own account.

Last checked: November 2026