Master Distiller Blog No.05 — The Kilning Curve
David Hsieh
Where beer malt and whisky malt part company — a distiller’s look at the drying step that decides what the enzymes leave behind
By David Hsieh — Master Distiller, Tankyu Distillery | Presenter, Business Whisky Guide Podcast
Adapted from Business Whisky Guide Podcast EP5 — ‘Peat and kilning, Part II’, recorded in the Scottish Highlands, March 2023.
Why does the distillery have to dry green malt straight away, and so stop germination? There are three reasons. First, if the malt is allowed to germinate further, the starch inside the grain is gradually consumed by the malt’s own metabolism — starch the distillery needs to ferment. Second, the enzymes awakened during malting will, if left running, continue working and consume the starch alongside the metabolic process. Third, the high humidity of the malting environment, given enough time, allows microbes to take hold; the longer the grain is left in that state, the greater the risk of contamination or spoilage.
The distillery’s task, then, is to press the pause button at the precise moment when the ratio of enzymes is optimal — and the most effective way to do that is to dry the malt.
The earliest drying method was to spread the green malt under the sun. As the moisture comes down, germination stops. The trouble with this approach in Scotland is meteorological: hours of sunshine are limited, cloud and rain frequent. Malt that does not dry quickly enough simply goes on germinating, and the starch is, in the end, exhausted.
Out of that constraint came the kiln — a furnace-like structure with the malt spread across a perforated floor above the heat source, with peat or coke burning below to drive a hot updraught. The bed was turned by hand every couple of hours until the moisture target was reached. The structure was later refined: the kiln was raised, and the chimney built up into a tall tapering tower. The temperature differential between the inside and the outside of the building helped draw the updraught, and the efficiency of drying improved markedly. The tapering chimney roof — the Pagoda Roof — went on to become the single most recognisable architectural signature of the Scotch whisky distillery. Even today, when many newly built distilleries no longer kiln their own malt on site, the Pagoda silhouette is often reproduced as a visual marker. Kavalan in Taiwan carries the same architectural cue on its building.
The kit now in commercial use is referred to as a Single-Floor Kiln — distinct from the traditional two-floor arrangement. The modern design concentrates drying within a single floor, sending hot air up through the bed of malt from below, with mechanical arms helping to turn the grain. The bed itself can be built up to 70 or 90 centimetres deep, with each square metre handling roughly 500 kilograms of malt at a time, and total drying time compressed to between 12 and 48 hours.
The two-phase kilning curve
Kilning runs in two distinct phases. The first is the Free Drying Phase, with the temperature set between 50 and 70°C. At the start, the malt is still wet, and the enzymes inside it are in an unstable state; raise the heat too quickly and the enzymes are destroyed. The phase therefore deliberately stays at the lower temperature, bringing the moisture down from roughly 40 per cent to around 12 per cent.
Once the malt is at 12 per cent, the outer husk is largely dry, but a great deal of water still sits trapped within the starch structure. That water is harder to drive out, requiring higher temperatures and more energy. By this point the malt is relatively dry and the enzymes more robust, less easily knocked out by heat; the temperature can therefore be raised to between 75 and 80°C. This is the Falling Rate Phase. The air-flow rate can drop, and the system can run on recirculated air rather than freshly heated; from there, the moisture is taken down from 12 per cent to a final 4 to 4.5 per cent.
There, in outline, is the rhythm of whisky malt kilning: low temperature at the start, gradual rise, with the aim of preserving the enzymes while bringing moisture down.
Why whisky malt avoids the third step
Beer malt sometimes goes through an additional fixed phase, raising the temperature further to between 85 and 100°C in order to drive the Maillard reaction. The high-heat treatment produces a deeper, char-and-chocolate character — the malt referred to as Chocolate Malt.
Whisky malt does not, ordinarily, take that step. The reason is straightforward: too much heat destroys the enzymes. What the distillery actually needs from the malt is the enzyme, not the flavour — and even what is formed at the kilning stage is not necessarily carried through intact by distillation.
There are, in recent years, distilleries that have begun to experiment with flavoured malts. Glenmorangie, in the Highlands, uses a proportion of more heavily kilned chocolate-style malt in part of its recipe, producing finished spirit with discernible coffee and chocolate notes. Holyrood Distillery, in Edinburgh, is a small operation that has built its reputation on experimentation and is continually trialling flavoured malts of various kinds. The approach is still a minority position within the trade, but it is gradually expanding.
Could you skip kilning altogether?
Could one bypass kilning entirely and run the whisky off green malt — malt that has germinated but never been dried? In principle, yes: enzyme activity is at its peak in green malt, and the mash could yield more sugar. The problem is logistical. Unkilned malt continues to germinate, cannot be stored for long, and tends to spoil on any but the shortest journey from maltings to distillery. Most Scotch distilleries sit in places not easily reached, and the transit time alone makes the malt’s condition impossible to control. The technique is therefore only really workable for distilleries that sit immediately next door to their maltings.
About the author
David Hsieh is master distiller at Tankyu Distillery. He has previously worked as a distiller at several whisky distilleries in Scotland. He holds an MSc in Brewing and Distilling from Heriot-Watt University, and is the host and producer of the Business Whisky Guide podcast, Taiwan's #1 whisky podcast.
About the distillery
Tankyu Distillery is one of Japan's few public-private (公設民営) craft distilleries, located in Higashikawa, Hokkaido. Founded in 2020, it produces single malt whisky and craft gin using pristine spring water from the Daisetsuzan mountain range — water so pure that Higashikawa remains the only municipality in Hokkaido without a municipal water supply. Learn more at tankyudistillery.jp.
Source
・ Business Whisky Guide podcast EP5 — ‘Peat and kilning, Part II’, recorded in the Scottish Highlands, March 2023.
Other distilleries, brands and producers mentioned in this article are referenced on the basis of publicly available information and the author’s personal observation, in the spirit of information-sharing rather than commercial comparison or evaluation. The references do not represent the position of Tankyu Distillery.