Production

Into the melting pot

The mash tun is a vital vessel,accommodating the conversion of starch within the grist into sugars,using asequence of three,or even four waters at a progressively higher temperatures. But rather than the mashingprocess,the aim of this article is to explore the design of a mash tun,and the practicalities that enable mashingto take place – Ian Wisniewski reports.
By Ian Wisniewski
Mash tuns were traditionally fashioned from cast iron, with the move to stainless steel dating from around 20 years ago,though Springbank and Royal Lochnagar are two examples of distilleries still using cast iron. One factor is that cast iron retains heat longer than stainless steel, but this is a difference that can be compensated for.“Stainless steel is always insulated, basically with a 70mm thick layer of insulation, which is normally mineral wool clad with a light stainless steel sheet,”says Richard Forsyth of Forsyth’s,who manufacture distilling equipment.While stainless steel mash tuns typically feature a matching lid, some are colour co-ordinated with a copper lid, as at Bunnahabhain, Glenfiddich and Auchentoshan. Copper’s excellent malleability, and aesthetics, are two advantages.A rare option is no lid at all, practised by distilleries such as Royal Lochnagar and Springbank.“We don’t have a lid on the mashtun. Conversion takes place regardless of the heat above the bed of the mash,”says Springbank’s Stuart Robertson.Mashing begins by combining the first water with grist in a mashing machine (also known as a mixer), and being conducted through a spout into the mash tun.The ‘strike temperature’ (ie.temperature at which water strikes the grist) must be accurate to prevent damaging enzymes within the grist (as they play a vital role in converting starch into sugar). Reaching the exact strike temperature is usually a case of combining hot and cold water.Some distilleries allow the first water to drain away before adding the second. Others begin adding the next water while there is still a residual amount of the first water remaining in the grist, as a lack of residual water could see sticky sugars compact and create additional challenges.With the first water conducted through an external spout, two or even three subsequent waters are usually sprayed on, using a sparge ring, effectively a series of spray nozzles suspended from the lid. In the absence of a lid, subsequent waters follow the same route as the first water.“At Springbank we put each subsequent water after the first water through the mash spout, this tends to appear quite violent as it gouges a hole in the mash bed unlike the spray from a sparge ring positioned above the mash tun creating even distribution of the second and third waters,”says Stuart Robertson.Drainage plates at the base of the mash tun feature various dimensions and configurations.“The principle of drainage is the same, with stainless steel slots around 0.7 mm wide being usual, this allows the worts to go through but holds back the grains.The number of slots is relative to the overall size of the mash tun,”says Richard Forsyth.Diageo’s Douglas Murray adds, “The greater the area of slits the quicker the drainage, but the bigger the slits and the quicker you drain the more risk there is of solids going through, as the weight of the bed is pushing the liquid through.” Needless to say mash tuns vary in size, and one of the largest examples in Speyside is at Glenfarclas, with a diameter of 10 metres.“There are consecutive rows of very small slits,2 mm wide and 3 cm long, all across the plates.These slits are in the opposite direction from the stirrer, so maybe less sediment collects, which is what we want,”says Ian McWilliam of Glenfarclas Distillery.The interior design of a mash tun features either a rake, the more historic option, or a lauter, a standard choice of the industry for the past 20 years.A lauter comprises arms that resemble a series of vertical blades, from which smaller blades (known as knives) protrude horizontally in a staggered format to create drainage channels within the bed (ie. grist and water).Knives culminate in small ‘feet’covered in nylon, to prevent damaging the drainage plates.Gently rotating arms maximise drainage through the bed, with variable speeds available, in a clockwise and anti-clockwise manner. Moreover, a lauter also provides the option of raising or lowering the arms, and so choosing the height of the bed which is agitated,whereas the arms of a semi-lauter are fixed.“Many in the industry feel a semi-lauter is enough.The process is all about getting sugars from the grain, and they feel they can do it just as well with a semi-lauter.There’s not a huge difference in price between a lauter and a semilauter, it’s only about 5 per cent,” says Richard Forsyth.Mash tuns fitted with a rake are still in active service at distilleries such as Springbank and Royal Lochnagar. This effectively means rotating arms possessing comblike teeth, that resemble a swimmer doing front crawl. When using a rake each batch of water is typically ‘stirred in’ a few times (ie.the rake completes a few rotations of the mash tun) to keep the bed in suspension, with a complete rotation taking between two and two and a half minutes at Springbank distillery.“Using a rake takes about twice as long as a lauter for the same tonnage, as each individual water has to be drained separately, whereas with a lauter spraying on water and draining is a continuous process,”says Douglas Murray.Once mashing has been completed, and the grist becomes draff (ie. spent grist), the lauter is rotated anti-clockwise to assist in removing the draff.“Once you’ve collected the wort you reverse the gearing, knives have feet that run on a swivel which act as a plough to push all the spent grain to the discharge outlet,”says Richard Forsyth.At Glenfarclas, for example, the base of the mash tun features four port holes, 30 cms wide, which are opened after draining the third water.“The draff falls about two metres from the mash tun and meets a screw forcing it away from the mash tun, then compressed air blasts it out into a draff hopper,an external vessel where trucks can take the draff away.We contract the disposal of draff to one company and give them disposal times.We have 11 hours between mashes, and as the storage vessel can hold two mashes, there is some flexibility on time,”says Ian McWilliam.Meanwhile, at Springbank the bottom of the rake features a scraper that moves wet draff round as it’s being emptied,and rather than the dry draff being blown out into a draff tank it’s transferred to a drainer, which resembles another mash tun.“We pump draff as a slurry from the mash tun to the drainer, with the liquid drained off producing sparge for the second water.Once the liquid is drained off and the draff is quite dry, which takes 30-45 minutes,we open the draff ports and it takes approximately 30 minutes to manually shovel the draff from the drainer down through the draff ports into a trailer.The same farmer collects and feeds it to his cattle, taking draff from someone else during our silent or malting season,”says Stuart Robertson.With a mash tun catering for various requirements, design is of course a major factor.“It’s quite an intricate piece of kit, with six people working 6-8 weeks to produce a mash tun.Internally the mash tun has a centre shaft with arms and knives so there’s quite a lot of design and engineering work.Mash tuns with a larger diameter need a ‘valley’ bottom,pressed with up to 15-20 valleys to collect the wort, each valley being a ‘dip’ of around 20-30 mm in the surface. A CIP system refers to Cleaning In Place,an automatic cleaning system,using nozzles that can project a hot water flush cleaning the mash tun,”says Richard Forsyth.Meanwhile, regular checks ensure that mash tuns remain on top form.“A stainless steel mash tun will last forever. It’s important to check the gear box on an annual basis, as you have gearing which will wear.We can bring worn parts back to the workshop and make replacements. Nylon scrapers on the feet at the end of the knives need to be replaced every two years, but it’s not a big job,”adds Richard Forsyth.