Mashing: How Grain Starches Become Fermentable Sugars
At 90 °C, the third water in Crisp’s published mashing procedure has another batch ahead of it. Collected as weak worts, it becomes the first water of the next mash: one batch helping to start another.[4]
Mashing brings milled grain and hot water together so that enzymes can break starch into sugars. The sugary liquid is called wort. This is preparation for fermentation, not the making of alcohol, and in Scotch malt whisky production the enzymes keep working after the wort reaches the fermentation vessel, or washback.[1][7]
What mashing does
Inside the mash tun, several jobs happen together. Malt arrives as grist, the mixture produced by milling. Hot water and the grain’s enzymes turn its starch into material that can be fermented. Equipment maker Briggs of Burton identifies four tasks: mixing grist with water, breaking down proteins, gelatinising and liquefying starch, and converting starch into fermentable sugars.[1][3]
Separation draws the liquid away from the grain. Lautering draws wort away from the grain bed; sparging rinses the grain to recover the remaining extract. The spent grain is called draff. These are distinct tasks, even when one piece of equipment handles several of them, and conversion can continue after the wort has been drawn off.[3]
For Scotch whisky, the product specification requires cereals to be processed into a mash and converted using only endogenous enzyme systems: enzymes already present in the grain. Added enzymes are prohibited. Mashing, conversion, fermentation and distillation must all take place at the same distillery. These requirements apply to Scotch, rather than defining mashing worldwide.[1]
That choice of enzymes also shapes what follows. The specification explains that malt enzymes can produce a different fermentation substrate from added enzymes, affecting yeast selection and the flavour compounds produced. The comparison between malted barley and added enzymes reaches beyond starch conversion into fermentation and spirit character.[1]
Why the grind matters
The husk has a job to do: it forms the filter bed through which wort drains. Maltster Crisp describes a Scotch distilling specification of 10% flour, 70% grits and 20% husk. This is Crisp’s stated specification, rather than a universal requirement for every distillery.[4]
A finer grind is therefore not always a better grind. Exposing starch matters, but a mash tun also needs a grain bed that lets liquid through. A mash filter does not rely on that husk bed and can accept much finer grist. The right grind follows the method of separating liquid from grain, not simply the smallest particle size the mill can achieve.[4][9]
How enzymes make fermentable sugars
Before enzymes can readily attack starch, it must gelatinise. Making starch accessible and converting it into sugars are separate parts of mashing. A supplier’s table adapted from brewing literature gives barley malt a gelatinisation range of 64 to 67 °C; its figures for unmalted barley vary with granule size.[6]
The enzymes then tackle different parts of the starch structure. Briggs of Burton summarises their complementary roles.[3]
| Enzyme | Action during conversion |
|---|---|
| Alpha-amylase | Cuts starch chains at random.[3] |
| Beta-amylase | Releases maltose from chain ends.[3] |
| Alpha-glucosidase | Releases glucose.[3] |
| Limit dextrinase | Breaks the 1,6 branch points in amylopectin, a component of starch.[3] |
What emerges is a mixture, not a single sugar: glucose, maltose, maltotriose and dextrins of different lengths. A 2024 review by Abertay University and the Scotch Whisky Research Institute describes the sugars fermented by whisky yeast as primarily maltose.[2][6]
Temperature governs both access to starch and the enzymes’ ability to work. Briggs, citing research by Stenholm and Home, reports that limit dextrinase works best at about 62.5 °C and pH 5.5. Above 65 °C, its activity drops within about ten minutes. Simply making the water hotter is no shortcut to more complete conversion.[3]
Following the water through a mash
Each water addition has a purpose. In Crisp’s published procedure for a semi-lauter tun, the first hot water enters at roughly 68 to 70 °C, bringing the mash to 63.5 to 64 °C. This is the maltster’s typical regime, not a set of temperatures used by every Scotch distillery.[4]
| Stage in Crisp’s procedure | Published conditions | Purpose or destination |
|---|---|---|
| First water | About 4 litres per kilogram of grist | Forms the mash with the grist.[4] |
| Mashing in | 20 to 30 minutes | Introduces and mixes grist and water.[4] |
| Bed rest | Up to one hour | Some distilleries start running off sooner.[4] |
| Second water | About 2 litres per kilogram, at 75 °C | Sprayed through a sparge ring during run-off.[4] |
| Third water | 90 °C | Collected as weak worts for the next mash.[4] |
The third water goes forward to the next mash, not into the same fermentation as the main wort. Crisp describes keeping these weak worts above 70 °C and using them as the next mash’s first water. The sequence links batches together rather than sending every water addition to the washback.[4]
Drawing off wort also demands control. Crisp explains that an underback, also called a grant, breaks the direct connection between pump and tun, preventing the pump from sucking the grain bed down onto the plates. A falling level can warn of a compressing bed and a developing stuck mash.[4]
Why conversion continues in the washback
Leaving the mash tun does not stop conversion. Whisky wort is not boiled before fermentation, so starch-degrading enzymes remain active in the washback. Crisp notes that some Scottish distilleries run off sooner precisely because conversion will continue in the fermenter.[4][7]
Heriot-Watt research found continuing starch-degrading activity during an unboiled, all-malt wash fermentation, but none in the boiled comparison wort. The study’s abstract identifies boiling as destroying malt enzyme activity. The experiment shows why wort leaving the tun and completed conversion are different milestones.[8]
A separate experiment by Abertay and the Scotch Whisky Research Institute compared pretreatments of distillery wort. Boiling or autoclaving reduced alcohol yield by at least 6.2 litres of pure alcohol per tonne under the study’s conditions. That measured reduction belongs to the experiment, rather than representing a fixed loss for every distillery.[7]
Active enzymes can release more fermentable material, though they do not make every remaining dextrin fermentable. In the next stage, how fermentation makes the wash, yeast uses sugars while enzymes continue their work.[6][7]
Mash tuns, lauter tuns and mash filters
Different equipment handles the same essential tasks in different ways. Briggs describes traditional stirred mash tuns, semi-lauter tuns with fixed knives, and full lauter tuns with automated raking and run-off. Its approximate cycles are 6 to 8 hours for traditional tuns, 5 to 6 hours for semi lauter tuns and 3 to 4 hours for full lauter tuns.[3]
These are the equipment maker’s comparisons, not fixed cycle times for every installation. Briggs also describes full lauter equipment as capable of producing clear wort without a time or extract penalty. Mash filters take another route: a 2018 trade explanation describes conversion in a tank, followed by liquid separation in the filter.[3][9]
Clearer does not necessarily mean better. Briggs says the requirement for clear or cloudy wort is specific to the distillery, while Crisp states that wort clarity can influence final spirit character. Clarity is a production choice, not a universal score for successful mashing.[3][4]
How grain whisky mashing differs
Scotch grain whisky takes a different route with its unmalted cereals, mainly wheat and maize. They are mixed with hot water and heated to liquefy their starch. The mixture is then cooled to about 65 °C before malted barley is added. The product specification notes that green malt, meaning unkilned malt, is sometimes used.[1]
Most grain whisky distillers also keep the solids in for fermentation, according to the specification. The mixture is cooled to about 15 to 23 °C. The contrast with the malt whisky process described above is in fermenting with the solids rather than separating wort from grain, not in whether the wort is clear or cloudy.[1]
Measuring the result and using the draff
The measure that matters is fermentable extract, not just the volume of liquid recovered. Crisp describes malt assessment through fermentability and Predicted Spirit Yield, expressed in litres of pure alcohol per tonne. The Scotch Whisky Research Institute identifies its method for measuring fermentability and predicting spirit yield as the industry standard.[4][10]
The grain left behind has another use. A Scottish Government report gives draff a typical dry matter content of 23%, within a range of 20% to 30%. Its main destination is livestock feed, with some going to anaerobic digestion or combined heat and power plants. Draff is a by-product with uses beyond the mash tun.[11]
Where to go next
- Enzymes in Whisky: Malted Barley vs Added Enzymes: compare the sources of conversion power.
- Fermentation: How the Wash Is Made: follow the wort into the next production stage.
- Washbacks: Wooden vs Stainless Steel: explore the vessels that receive the wort.
Sources
[1] UK Government. 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. and Walker, G. The potential for Scotch Malt Whisky flavour diversification by yeast. FEMS Yeast Research, volume 24, foae017, 2024.
https://pmc.ncbi.nlm.nih.gov/articles/PMC11095643/
[3] Briggs of Burton plc. Davies, S., Philips, M. and Hancock, J. Evolution of a Mash Tun. Presentation, undated; file name indicates 2014. Equipment maker’s account.
https://www.briggsplc.com/wp-content/uploads/technical-papers/davies-mashing-wdsc2014_complete.pdf
[4] Crisp Malt. Scotch Distilling Mash Procedure. Undated. Maltster’s account.
https://cdn.shopify.com/s/files/1/0002/9006/2342/files/Crisp_Malt_Scotch_Distilling_Mash_Protocol.pdf
[6] Murphy and Son. Johnson, A. It Will All Come Out In The Wash: Enzymes and Distillers. 9 April 2021. Trade supplier’s account.
https://murphyandson.co.uk/it-will-all-come-out-in-the-wash-enzymes-and-distillers
[7] 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/
[8] Vriesekoop, F., Rathband, A., MacKinley, J. and Bryce, J. H. The evolution of dextrins during the mashing and fermentation of all-malt whisky production. Journal of the Institute of Brewing, volume 116, issue 3, pages 230 to 238, 2010. Heriot-Watt research portal abstract.
https://researchportal.hw.ac.uk/en/publications/the-evolution-of-dextrins-during-the-mashing-and-fermentation-of-/
[9] ScotchWhisky.com. The Whisky Professor. What's a mash filter and how does it work? 8 January 2018.
https://scotchwhisky.com/magazine/ask-the-professor/17361/what-s-a-mash-filter-and-how-does-it-work/
[10] Scotch Whisky Research Institute. Timeline: 1981. Undated.
https://www.swri.co.uk/timeline/1981-2/
[11] Scottish Government. Distillery by-products, livestock feed and bio-energy use: report. 1 July 2019.
https://gov.scot/publications/distillery-products-livestock-feed-bio-energy-use-scotland/pages/4
This desk-researched article uses the supplied verified fact sheet, distinguishing research findings from the product specification and equipment maker, maltster and trade accounts.
Last checked: November 2026