Master Distiller Blog No.07 — The Golden Ratio of the Grist
David Hsieh
From roller mills to the 2:7:1 grist split — how the milling step quietly sets the rest of the whisky in motion
By David Hsieh — Master Distiller, Tankyu Distillery | Presenter, Business Whisky Guide Podcast
Adapted from Business Whisky Guide Podcast EP7 — ‘No sponsor from the small mill this time’.
Once the malt has arrived at the distillery from the maltings, the first thing that happens to it is milling.
The reason for milling before mashing is the same reason a barista grinds a coffee bean before brewing it: by breaking the kernel open, the surface area in contact with the mashing water increases, and the sugars and flavour compounds inside the malt can be drawn out efficiently. Run an un-milled, intact kernel into a mash tun and the husk keeps the hot water away from the enzymes; conversion is dramatically less efficient.
Before any malt reaches the mill, the foreign matter has to come out. There is always some. Small stones get carried in from the field at harvest, metal fragments shed from shipping containers find their way into a load, and occasionally a stray fitting falls into the kit during malting. When a delivery arrives at the distillery, the receiving team checks the previous cleaning records of the container for food-safety reasons, and the top of the mill itself carries a vibrating screen or powerful magnets to take out stones and metal physically before the malt drops in. If this step is slipshod, the consequences of a stone or a metal shard going into the mill run from half a day’s downtime at one end to a written-off mill at the other — and in the worst case, a spark from the metal meeting the malt-dust cloud in the air. So the screening is taken seriously.
Milling whisky malt is not the same thing as polishing sake rice
There is a worthwhile contrast here with sake brewing. Whisky milling pulverises the malt while preserving the husk for use later as a filter bed; rice polishing for sake, by contrast, grinds away the outer layer of the rice — the protein- and lipid-rich part that produces off-flavours — to leave the cleaner starch at the centre. The distinction follows from what comes next. Whisky is distilled afterwards, and many of the unwanted components are removed in the still; what counts is mashing efficiency and grain character. Sake is fermented but not distilled, so the cleanness of the raw rice has to be created at the polishing step itself.
The four-roller mill
The vast majority of Scottish malt distilleries use the roller mill, and the most common configuration is the four-roller mill: two pairs of rollers stacked, with the malt fed in from above. It passes through the first pair, which open the husk; then through the second pair, which crush the kernel. Within each pair, one roller turns at high speed while the other is fixed or turns slowly; the difference in speed between the two creates the shearing action that separates the husk from the kernel. The roller surfaces are usually grooved — tyre-tread fashion — to increase grip and so the efficiency of the grind.
Why two pairs rather than one? The analogy is cracking a sunflower seed: the first move opens the shell, the second extracts the kernel. The first pair handles a clean dehusking; the second pair crushes the dehusked kernel into a meal. Husk and kernel meal come out mingled together at the bottom — and that mixture is the grist.
The grist contains three components of different particle size: the coarse husk, the mid-sized cracked-kernel pieces called grits, and the very fine flour.
Why the husks have to stay
If almost all the available starch is in the kernel meal and flour, and the husks contain little of it, why not sieve the husks out and only mash the grits and flour? Because the husks have a critical second job in the mash tun. Once the grist is mixed with hot water at mashing, the husks — being heavier — settle out onto a metal mesh at the base of the mash tun, the false bottom. There they form a filter bed several centimetres thick. When the wort runs off through the false bottom at the end of mashing, it is that extra layer of husks that traps the fine particles and keeps the wort clear. The husks therefore have to be preserved in usable form. To stop them shattering too finely during milling, some distilleries even spray a mist of water onto the malt just before it enters the mill — wet husks are tougher and break up less.
The 2:7:1 golden ratio
The composition of the grist is one of the distillery’s core observation points. Mill too coarsely and the kernels do not open up properly; the contact surface in mashing is too small, sugar extraction falls, and the spirit yield drops. Mill too finely and the flour fraction climbs too high — and when the flour hits the hot water it balls up, the inside of the ball stays dry, mashing efficiency drops anyway, and the surplus flour finds its way through the husk filter bed and blocks the gaps in the false-bottom mesh, choking the run-off.
The golden ratio that a typical Scottish malt distillery aims for is 2:7:1 — 20 per cent husk, 70 per cent grits, 10 per cent flour. This keeps enough husk to maintain a usable filter bed, keeps the kernel/flour fraction high enough to extract sugars properly, and holds flour down enough to avoid balling and choking.
To check whether the milled grist is meeting that ratio in practice, the distillery uses an instrument that looks rather like a tiered wooden box, with stacked sieves of progressively finer mesh inside. A small sample of grist is placed on the top, the lid is shut and the whole thing is shaken for about a minute; the residues from each layer are then weighed, and the actual husk/grits/flour proportions emerge. Not every batch is checked this way, but if the mill has just come back from maintenance or has just been recalibrated, the test is run again to confirm the setup.
The hammer mill and where it belongs
Some distilleries use a different machine: the hammer mill. A high-speed rotor carrying swinging metal hammers smashes the malt against a screen, with the screen’s mesh size controlling the output particle size. The grist that comes out of a hammer mill is very fine and very uniform — husks and kernels alike beaten down to something approaching flour. That grist cannot form a husk filter bed in a traditional mash tun, and so a mash filter — a plate-and-frame press of the kind used in brewing — is needed to filter the wort instead.
Hammer mills are mostly seen in Scotch grain distilleries, in American bourbon distilleries, and in the handful of malt distilleries that have built around a mash filter — among them Tankyu Distillery in Hokkaidō. Building around a mash filter adds capital cost and a more demanding cleaning regime, but the finer grist together with the pressure extraction of the mash filter produces a wort with higher sugar concentration and fuller malt-derived flavour. The character of the resulting whisky is notably different from the malt-distillery norm.
Storage, freshness, and what is decided here
Milled grist is held in a grist bin until mashing begins. Once milled, grist is exposed to air and oxidises readily, so it does not keep for long; the principle is much the same as the advice to grind coffee just before brewing. Some smaller distilleries do not buy a mill of their own and purchase pre-milled grist from the maltings, but the long distances and storage time involved reduce freshness, and the choice is, like much else, a balance between cost and quality.
From the moment the malt enters the mill to the moment it leaves the grist bin, the milling step has already, quietly, decided a good part of the whisky to come.
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. The company was established in 2020, and the distillery opened in August 2025. 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/en.
Source
・ Business Whisky Guide podcast EP7 — ‘No sponsor from the small mill this time’.
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.