Whichever type of mash tun is used the process begins in the same way: hot water and grist (ie. malted barley which has been milled) are combined in a separate tank called a mashing machine above the mash tun. The resulting 'mash' is conducted through a spout into the mash tun. From this stage the type of mash tun determines how the process continues.
A traditional mash tun means a large vessel fitted with a central column that operates a 'rake' mechanism, comprising curved steel arms possessing comb-like teeth. The arms rotate around the mash tun, the action resembling a swimmer doing front crawl, which stirs the mash and consequently aids drainage.
"Once the mash tun is three-quarters full we turn on the rake and give the rake three revolutions, which takes a few minutes. It's a very gentle stirring action, then we leave the mash sitting for half an hour before we start draining off the wort," says Gavin McLachlan, Springbank's distillery manager.
Turning a valve is essentially all that's required to start draining the wort (ie. sugary liquid) through perforated drainage plates at the base of the mash tun. Beneath the drainage plates is a network of pipes, arranged in a particular configuration that ensures an even rate of drainage from across the 'bed' (ie. the mash sitting in the mash tun), maximising efficiency.
Once the wort has drained, the process repeats with a second batch of hot water, added either from the spout of the mashing machine, or from a sparge ring positioned above the mash tun.
"There are various configurations of sparge rings in different distilleries, though typically this comprises two or three long stainless steel pipes with perforations through which water can be sprayed evenly across the mash tun. Some sparge rings are fixed, while at Deanston for example it rotates," says Ian MacMillan, master blender, Burn Stewart, which operates Bunnahabhain, Deanston and Tobermory.
Meanwhile, the rake is turned on and gently stirs in this second batch of water, creating more wort which is subsequently allowed to drain. These two batches of wort are stored together (ready for the next stage, which is fermentation).
A third batch of hot water is subsequently added to 'flush out' any remaining sugars. Referred to as 'sparge water,' this third water drains into a separate tank from the wort and is recycled as the first batch of water of the subsequent mashing cycle. Some distilleries also add a fourth water, which is recycled as the second water.
Using a traditional mash tun means that mashing can take around 10 hours, which compares to around four to six hours using a lauter.
"One reason for this is that once the mash tun has been filled hot water is added continuously, rather than in separate batches, and wort drains from the mash tun at the same rate as water is added, with the arms of the lauter stirring gently all the time," says Douglas Murray, Diageo's process technology manager.
Water is added using a sparge ring, with the arms of a lauter effectively a series of horizontal blades, from which smaller vertical blades protrude in a staggered format. Mash tuns which have arms that can only rotate are known as a semi-lauter, whereas a lauter means the arms can also be raised and lowered (while rotating). This enables the mashman to choose which part of the 'bed' is stirred to assist drainage. Having this additional degree of control does of course mean paying a higher price. Consequently, how much of an advantage a lauter may provide is a decision for each distiller, with numerous semi-lauters in service throughout Scotland.
"The mashman is constantly monitoring the process, and even with an automated display panel there are still a lot of decisions to make. If you spoil a mash it's an expensive mistake in terms of time and money, so the mashman has to know how to react if something happens," says Mickey Heads, distillery manager, Ardbeg.
Consequently, comprehensive training is required. "It can take several months to train a mashman how to operate a rake. We want them to understand why they're doing everything, not just how to do it," says Ian MacMillan.
The job also includes cleaning, which can mean pressing a button to operate an automated system, flushing the mash tun with hot water. Or it can be manual.
"Cleaning our mash tun takes a mashman around two and a half hours each time, three times a week, lifting the drainage plates, hosing underneath, and so on," says Robbie Hughes, distillery manager at Glengoyne, which employs three dedicated mashmen.