Grist to themill

Grist to themill

Ian Wisniewski gets to grips with an often\roverlooked yet vitally important stage of the\rproduction process – milling

Production | 11 Oct 2006 | Issue 59 | By Ian Wisniewski

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With commercial maltsters preparing the vast majority of the industry’s malt requirement, milling is usually the first process undertaken at a distillery.Although this may seem an entirely practical stage, any deviation from the usual regime can have significant consequences.“The mill is a very important piece of kit,” says Highland Park’s Russell Anderson. “I go to the mill every day, guaranteed, and check grist samples, I like to physically see how the husk and grits are looking. If we don’t have the correct grind extraction we will suffer, i.e the mash will stick in the mashtun, which can also have a knock-on effect on our fermentations.” The first specification to consider is the moisture content of the malt when it’s delivered to the distillery. A moisture level exceeding the usual spec can alter, or even prevent milling.“Too high a moisture content could mean the malt sticking to the rollers, which would cause all sorts of problems,” says Alan Winchester of Chivas Bros.Mills are typically the work of Robert Boby or Porteus, examples of which can been seen at numerous distilleries, though there are a few other manufacturers.“Glenfarclas has a Buhler mill, manufactured in Uzwil, Switzerland, which we describe as the Bentley of mills, installed in 1974,” says Ian McWilliam of Glenfarclas.The malt first passes through a dresser, effectively a revolving drum fitted with wire mesh, which catches any extraneous items such as pieces of straw and small stones (apart from damaging the rollers of the milling machine, stones can cause other, far more serious damage, of which more later).Magnets are also fitted in order to catch any possible metal items.Another optional extra is to have a separate destoner after the dresser, effectively a tray or sieve set at an angle, propelling the malt forward to the next stage, while any stones pass to the rear of the unit and are caught in a mesh.Milling inevitably creates plenty of dust, and as this is combustible a stone or piece of metal such as a screw or nail striking the rollers could create a spark that would cause the dust to combust.Consequently, strict preventative measures are in place.“There’s always going to be dust when handling barley and malt. We minimise this with dust extraction units and try to eliminate any ignition source such as a stone hitting the rollers, a small part of a screw could break off for example, you need safeguards in place. We have vents so that if there was an explosion it would take the easiest way out, and minimise damage. Adust explosion can be very serious and explode like a ball of fire,” says Russell Anderson.A mill can be fitted with various arrangements and numbers of rollers. “It’s usual to have a four roll mill, comprising two sets of two rollers, some distilleries use a six roll mill, comprising three sets of two rollers, the key is that you achieve the same result,” says Diageo’s Douglas Murray.Further variations include “The mills at Allt a’Bhainne and Braeval, which have five rollers, grouped as a set of three rollers, and another set of two,” says Alan Winchester.The first set of rollers squeezes the grains to pop the husk, with subsequent rollers grinding further.“A pair of rollers on the top provide the initial grind, rollers have longitudinal lines to assist pulling barley through the rollers, then it goes through three screens which spread the malt out more evenly for maximum efficiency.We mill at a relatively slow rate so it doesn’t stress the mill, this helps the longevity of the mill, and helps us check the consistency which is important to get the maximum extraction,” says Ian McWilliam.Milling yields three separate ‘grades’ of malt: husk, grits (or ‘middles’) which are medium-ground, and the much finer ground flour (or ‘fines’), collectively called grist. A typical specification is 20 per cent husk, 70 per cent grits and 10 per cent flour, although this will vary among distilleries.A traditional way of checking the spec is a sample box. This comprises three sections each fitted with a different sized mesh, so that each grade ends up in its alloted tray.“Sample boxes are low-tech but very effective, though labs back this up with analysis,” says Alan Winchester.The spec provides malt with an optimum total surface area, enabling the maximum amount of sugar to be extracted during the subsequent mashing process (when adding hot water converts the starches into sugars).The proportions of each grade also reflect practical considerations. Husks help water drain through the grist, and give bouyancy to the bed, though too much husk would prevent the grist from mashing properly, which would compromise the degree of extraction. Douglas Murray raises another consideration, “With some barley varieties the husk has the ability to retain more water than others, so this can change the way the water drains through in the mash tun.” Meanwhile, excessive levels of flour can result in an equivalent of porridge within the mash tun.“Too high a level of flour influences the draining process during mashing, water can become clogged up whereas we want consistent drainage.“A lower percentage of flour means the water drains more quickly affecting the sugar extraction, so consistency is important,” says Ian McWilliam.Douglas Murray adds, “Flour is kept to a minimum, otherwise this can choke the bed during mashing and it’s less porous.“We want the clarity of the wort to be consistent, which is mainly governed by the amount of fine powder in the grist, so it’s vital to maintain consistency.” Maintaining the milling spec may also entail adjusting the rollers, when changing barley varieties, as some varieties have a relatively small grain size compared to others.But the barley variety is only one factor.“The operator checks each load, you can make an adjustment when changing barley varieties, though even with the same variety you can get a different corn size due to the climate each year, a dry summer for example can mean a smaller corn size,” says Douglas Murray.Russell Anderson adds, “There are variations in the crop year on year, but also batch on batch, as steeping conditions can have a fundamental impact on the malt. So we constantly monitor whether the rollers need to be adjusted, it’s a daily check.” While milling demands precision, the mills themselves aren’t particularly demanding, and only require minimal maintenance.“Boby and Porteus mills are slightly different in appearance but do the same job. The mills are reconditioned but the casing is hardly ever changed, so many are 50-70 years old, we replace rollers and mechanically moving parts as required,” says Douglas Murray.Ian McWilliam adds, “Malted barley does wear away the steel mesh, not so much the roller, we’ve had no problems with the rollers or replacements in 32 years.”
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