Master Distiller Blog No.11 — Inside the Mash Tun
David Hsieh
How the working parts of a mash tun turn grist and hot water into sweet wort
By David Hsieh — Master Distiller, Tankyu Distillery | Presenter, Business Whisky Guide Podcast
Adapted from Business Whisky Guide Podcast EP11 — ‘A Right Mess of Porridge (Part II)’.
This piece carries on from the last, turning to the mechanical workings of the mash tun itself.
The false bottom, and the bed that does the real work
At the foot of the mash tun lies a metal screen known as the false bottom, through which the wort is filtered. During mashing, though, it is not really the screen that does the filtering: it is the layer of husks that settles naturally on top of it, building up into a filter bed. That bed is where the wort is truly clarified.
Underletting: filling the gap first
Before mashing begins, the distiller performs a step called underletting — running a measured charge of hot water into the mash tun, just enough to cover the false-bottom screen. This water fills the void between the false bottom and the true base of the tun, so that the grist to come cannot wedge itself into the slots of the screen. Skip the step and the grist clogs the false bottom the moment it arrives, and the extraction rate and speed of the whole batch fall away sharply.
Through the mashing machine
Before grist and hot water enter the tun, they pass first through the mashing machine — shaped like a length of pipe laid on its side, with a screw running through it. Dry grist is fed in at the top, hot water at the bottom; the screw blades fold the two together and discharge them from the far end into the mash tun. The mashing machine usually holds the grist-to-water ratio at around 1:3 — too little water and the grist clumps; too much and the gravity of the wort suffers.
What the rakes are really for
Once grist and hot water have been delivered through the mashing machine, the distiller sets the rakes turning. They sweep slowly around the tun, but their purpose is not to stir the mash even — most of the mixing has already happened inside the mashing machine. What the rakes do is spread the grist evenly across the false bottom, building a husk filter bed of uniform thickness. If the bed is not laid level it heaps up near the inlet and thins out at the far end; and where the bed is uneven the wort runs off at different rates, so filtration falls away in patches.
Running off the wort
With the bed laid level and rested for some ten to thirty minutes, the distiller opens the valve at the base of the tun and lets the wort drain off under gravity. The level in the underback serves as the visual gauge for judging when the liquid in the mash tun has all been drawn.
Draff — and the rule every distiller learns once
Once all three worts are off, what remains in the tun is the solid residue: the draff. As a rule the mash tun has a draff port in its base, normally kept shut by a metal gate; to discharge, the gate is opened and the rakes restarted, and the arms push the draff towards the port, onto a conveyor and out to the draff lorry waiting outside, from where a farmer carts it off as feed for cattle and sheep. There is one iron rule here: before the rakes are started, you go outside and make sure the lorry is actually parked beneath the port. Miss that step and the entire batch of draff lands on the concrete floor — and the next two hours are spent shovelling tonnes of wet draff, spadeful by spadeful, back up into the truck bed. It is a mistake most distillers make exactly once, for the back will not straighten the following day, and the lesson goes in deep. Some mash tuns have no draff port at all, in which case the distiller really must pull on a pair of wellies, climb into the tun and dig the draff out by hand. At Edradour, the author once did precisely this as a matter of daily routine.
The mash tun is not, in itself, a complicated thing. Yet mashing is perhaps the stage a distiller watches more closely than any other in the making of whisky — and the one every distiller must take the most care to learn by hand.
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 EP11 — ‘A Right Mess of Porridge (Part II)’.
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.