Single Malt Scotch Whisky Production

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Words by Simon Difford

The barley must be partially germinated (sprouted) before it can release its starch reserves to be converted into fermentable sugars, but then dried by heat to arrest this germination before the grain uses the sugars to grow. This germination and drying process is called malting. The type of fuel, particularly smoke from peat if used during kilning, will greatly affect a whisky’s character.

The malting process induces physical and bio-chemical changes in the barley enabling distillers to utilise the starch within the grain to make alcohol.

Split a grain of barley and you'll see the white starch inside. Starch is present in barley as a food source to enable the embryo to grow a shoot and roots - to germinate. The starch is held in the endosperm, surrounded by protein and enclosed in cells, both barriers which need to be breached to allow the distiller to extract the starch. As the embryo starts to grow it breaks down the entrapping protein and the cell walls to access its starch reserve.


The harvested grain has a natural resistance to growth known as dormancy. This is nature's way of ensuring the grain does not start to germinate until the new growing season. Hence the dormant seed must be tricked into thinking it is spring, thus encouraging the barley to start to grow, known as germination.

The barley is coaxed into growing by a series of immersions in warm water at 16°C, a process known as steeping. Conversely, to store barley for prolonged periods it must be kept at below 15°C at a moisture level below 15% to maintain its dormancy.

Steeping raises the grains moisture content from storage levels of around 12% to 43-46% so initialising growth. Due to the grain's husk being relatively water proof, simply immersing the grain in water does not quickly raise moisture levels inside the grain, so steeping involves a series of 'wet stands' followed by 'dry stands' or 'air rests'. The grain is immersed in water for a short period to awaken the grain's embryo, then the water drained off to allow oxygen to reach the embryo. The now active embryo encourages the grain to absorb more water during the next soak or 'wet stand'. Different varieties of barley react to steeping in different ways so a pattern of wet and dry stands is repeated according to that particular barley variety. A typical steeping pattern might be: 7 hours wet, 10 hours dry, 9 hours wet, 7 hours dry, 7 hours wet and 2 hours dry.

In modern maltings air is blown through the wet grain during the wet standing periods and extraction fans remove carbon dioxide during the dry stands. The barley at the bottom of a steep develops slightly slower than that at the top. Blowing pressurised air through the barley during wet stands also serves to mix the barley thus ensuring even development.

Once the barley has absorbed enough water and oxygen the grain will start to germinate and the root (chit) will begin to emerge from the grain, initially visible as a white dot. The growing barley absorbs oxygen and emits carbon dioxide and heat. In order to provide the perfect conditions for the barley to continue to grow, the grain is moved to the next stage of the process, germination.

As the barley germinates a hormone called Gibberellic Acid activates synthesis of hydrolytic enzymes in the grain's aleurone layer which start to covert the grain's starch and protein reserve (the endosperm) into simpler starches and soluble sugars - the food source that allows initial growth of the germinating embryo and the shoot (acrosphere) and root (chit) to develop. The cell walls are broken down turning the previously hard shelled barley to easily crushable malt, allowing the starches to be released during mashing.


After steeping, the grain is spread out evenly and encouraged to germinate (starts to grow). During this period (four to five days) the maltster will control the humidity, temperature and air flow, which along with time, affect the final characteristics of malt. The grain must be turned at least twice a day to dissipate the heat produced by the germinating grain, allow carbon dioxide to be released and to prevent the fine rootlets becoming intertwined and creating an immovable thick matted slab.

Traditionally, barley is spread on the concrete floors of the malting house to a depth of 15 to 50cm (6 to 20 inches) to germinate with the depth varied to control temperature. This traditional method termed 'floor malting' is very labour intensive and relies on the back-breaking work of someone first spreading and then hand-turning the damp grain using flat wooden shovels called shiels as it lies on the malthouse floor to ensure even germination and prevent the rootlets of the germinating grain becoming entwined.

Historically, each distillery had its own on-site floor maltings but the distillery's output was restricted by the capacity of its maltings. Floor malting takes some seven days to complete and almost all distilleries now outsource malting to third-party maltsters who use the modern mechanised malting methods described below.

Germination must be checked around about the time the shoot has grown two-thirds of the way round within the husk and the root is about as long as the grain. If germination is not stopped in time, the grain will be 'over-modified', meaning that the shoot has used too much of the soluble sugars whilst growing, leaving little for the yeast to feed on during fermentation. If germination is stopped too soon, the grain will be under-modified, meaning that not enough soluble sugars will have been produced.

Two traditional tests allow malsters to check the development of the grain so access when to stop germination. One is to check the length of the shoot (acrosphere) which can be seen growing just under the grain's husk. Another, often preferred by malsters, is the 'rub' test: The grains white inner part (the endosperm) is exposed by splitting the grain open with a finger nail and the endosperm rubbed between forefinger and thumb, or chalk-like down a wall. If no mark is left by the barley then the starch is still encased within the cell walls and protein so requires more germination. If a white powder is left then germination has reached an optimum stage having only used around 10% of the available starch, leaving 90% for the distiller.

To arrest germination, the 'green malt' is dried by heating, so is transferred from the germinating vessels to the kiln for the next stage in the malting process, kilning. (Modern combined germination and kilning vessels do not require the grain to be moved.)


Heat applied during kilning arrests germination and reduces the water content in the green malt from around 40-46% to 3.5-6.5%. Complex chemical reactions also take place during this heating process. Amino acids and sugars are degraded, or react together to produce roasted, biscuity flavours crucial to the taste of whisky. The resulting low moisture content also prevents mould growth, so allowing the malted barley to be stored for prolonged periods. During kilning it is important not to overheat and kill the gain's natural enzymes, which are important in the mashing stage.

Kilning takes 24 to 48 hours. The damp barley is spread across the kiln floor, which has thousands of small holes large enough to let heat (and sometimes smoke) from the heater or fire below rise up through the floor and then barley.

Traditionally, a distillery's kiln is topped by chimney with a distinctive pagoda-shaped roof. Such pagoda roofs still crown most Scottish malt whisky distilleries, although most are now purely decorative. Originally designed by Charles Cree Doig and dubbed the Doig Ventilator, these once vented the kilns. In the few remaining distilleries still malting their own grain, the pagoda sits like a hat atop the outlet from the kiln and serves to create a low pressure area, thus drawing the smoke from the fire through the grain and then out from under the pagoda.

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Pagoda roof (top left shot) at Royal Lochnagar

There are four main phases to the modern kilning process:

1. Forced drying: Lasts 3 to 4 hours - moisture is driven from the interior of the grain.

2. Pre-break: Lasts 12 hours - large volume of air at around 60°C is blown upwards through the bed to drive off moisture from the surface of the grain.

3. Break: This is the point when most of the moisture has been driven off the surface of the green malt. Post-break: The temperature is increased and the flow of air through the bed decreased.

4. Curing: Lasts 2-3 three hours. The temperature is increased to 70 to 90°C to colour the malt. During this toasting phase the fan speed is reduced and the re-circulation of air increased. The degree of toasting will affect the flavour of the malted barley with lower temperatures retaining hay-like cereal notes while higher temperatures produce chocolatey flavours.

In Scotland, particularly on the islands where there are few trees, kilning traditionally used heat from a peat fire as it was a freely available local fuel. Many of the whiskies produced on the islands (most notably Islay but also Arran, Jura, Lewis, Mull, Orkney and Skye) continue to use malt kilned with peat, although peat is now usually supplemented with more modern fuels such as oil, gas or anthracite (coal is no longer used).

Peat forms in poorly drained wetland conditions from a build-up of partially decomposed vegetation. The constantly wet conditions starve microbes of the oxygen needed to fully decompose the vegetation. Waterlogged dead grass, sphagnum moss, heather and other plant matter accumulate over thousands of years and are compressed and partially carbonised (not quite coal) to form peat bogs.

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Peat bog on Islay

Parts of the Scottish Highlands and islands, particularly Islay, have the vast tracks of peatland and the peat differs depending on the vegetation in a particular place. When burnt and used to kiln barley, the smoke emitted by peat cut from bogs in Islay imparts very different flavours to the barley than from peat cut on the Scottish mainland.

The seaweed, tar and iodine so typical of Islay malts are partly due to decomposed marine vegetation and sea salt in the island's peat. The peat on the Orkney Islands is composed almost entirely of heather so imparts Orcadian malts with a heathery, gentian-like smokiness. Highland peat varies again according to location but contains more tree lignin so tends to impart a more wood fire-like smokiness.

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Islay peat

Obviously, the longer the malt is exposed to peating, the peatier the whisky will be. Peat smoke is more easily absorbed by the green malt when the surface is still wet so peat is burned at the start of kilning, prior to the point known as the 'break'.

It is chemicals called phenols (isomeric cresols, guaiacol and xylenols) in the peat smoke which flavour the malted barley, and so too the whisky. Typical flavours imparted by peat smoke phenols include: vegetal, medicinal, carbolic, tar, creosote, clove and seaweed.

The amount of peatiness, or 'peat reek' of malted barley is measured and expressed in parts per million (ppm) of phenols, the flavour element imparted by the peat smoke. Each distillery specifies a specific peat level to be supplied by its maltster, traditionally distilleries on Scotland's west coast use more peat reek than those in Speyside and the east coast.

Peating levels

Level of phenols expressed as parts per million (ppm)

Very heavily peated: 50ppm-170ppm (Bruichladdich Octomore uses 167ppm)
Heavily peated: 30-50ppm (Ardbeg uses 50ppm)
Moderately high: 20-30ppm (Lagavulin uses 30ppm)
Medium peated: 15-20ppm (Talisker & Highland Park use 20ppm)
Moderately low: 5-15ppm
Lightly peated: 1-5ppm (typical Speyside malt uses 2ppm)
Unpeated: None (even unpeated malt can have phenolics of 0.6 to 0.8 ppm)

While it is easy to accurately measure how phenolic malt is, it is more challenging to accurately peat malt to a specified level. Peating levels vary by 5 to 10% so it is easier for maltsters to consistently make heavily peated malt, and for the maltsers or distillers requiring medium peat levels to blend heavily peated malt with unpeated malt to dependably achieve their desired medium peat level.

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Peat cut by hand on Islay

The use of dried peat as a fuel in Scotland dates back to medieval times when the Little Ice Age left few trees in Scottish Highlands. Although now mainly reserved for the whisky industry, some Scottish households still exercise their right to cut peat to burn as a domestic fuel. The surface turf above the peat is cut away with a peat spade. A tool called a 'Fal' or 'peat iron' is then used to cut and extract the peat itself in long bricks. A long-handled three pronged folk is used to haul peat bricks up from the wet trench and onto the bank where they are spread out to dry. After being left to dry for a few weeks, in good weather the peats will be dry enough to be stood on end without collapsing. This step is called 'lifting' and the peat bricks are lined up to further dry for four to six weeks, resting on each other in a line called a 'rùdhan'. The hand-cutting of peat is a dying art and most commercially cut peat is now dug out by machines which cut in a chain saw-like action to produce sausage-like peat rather than traditional bricks.

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Prior to the building of railways which supplied coke and other fuels to Highland distilleries, peat was pretty much the only locally available fuel. The warm smoke from the peat not only dries the grain but gives protection against bacterial spoilage as well as adding a distinctive flavour to the whisky.

Today, sulphur is sometimes added to the fuel used for kilning instead of peat due to cost and the environmental concerns of using peat.

Old & New Malting Methods

After malting, the grain superficially resembles the original barley, but is physically and bio-chemically much changed. Over time the equipment used by maltsters has become more technically advanced, but the natural changes in the grain induced by malting remain the same. The malt whisky industry is wonderfully traditional with outdated equipment and techniques enduring due to their perceived contributions to flavour and the marketing story. Consequently, the oldest malting technology, floor malting, may be very inefficient and labour intensive but not only are floor maltings still used, they are being recommissioned.

In order of technological advance, the four main malting methods employed today are:
1. Floor maltings
2. Saladin boxes
3. Rotary drums
4. Germinate and kilning vessels.

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The inside of a rotary drum at Port Ellen

Floor Maltings

Advocates of floor malting believe this traditional method allows the grain to retain more flavour from peat smoke in the kiln. Modern mechanised malting methods, using warm air, dry the grain's husk. In contrast, old-fashioned and slower floor-maltings leave the grain's surface moist. It is the surface moisture that the phenols (the flavour elements in the peat smoke) stick to during 'peating'. An analogy sometimes used compares a man in a smoky bar wearing a dry sweater with his neighbour, whose sweater is damp. When they leave the bar, the man with the damp sweater will have a stronger smell of smoke than the man with the dry sweater.

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Floor malting at Highland Park

Traditional floor maltings add to a distillery's character, with their long rooms, grain-laden floors and maltmen still using chariots (a wheelbarrow-like tool) for spreading barley. All very romantic but floor maltings are inherently inconsistent and very labour intensive to operate compared to modern, mechanised malting methods, hence only seven single malt Scotch whisky distilleries still operate floor maltings. And most of these buy malt from third-party maltsters as their own floor maltings are only able to supply a small proportion of their malt requirements.

Scottish Distilleries with operational floor maltings:
1. Balvenie, Speyside
2. BenRiach, Speyside (reinstated in November 2012)
3. Bowmore, Islay
4. Highland Park, Orkney
5. Kilchoman, Islay
6. Laphroaig, Islay
7. Springbank, Campbeltown (produces 100% of malt used)

Saladin Boxes

Invented by a Frenchman, Colonel Charles Saladin in the late 1800s, Saladin maltings were the early replacement for floor maltings. There are two common designs: Saladin Boxes and Circular Saladin. Saladin Boxes are horizontal boxes fitted with turners that move through the bed several times a day, raising the barley so preventing the rootlets becoming entangled. As the name suggests, Circular Saladin are circular vessels fitted with turners attached to an arm that rotates around the vessel. Both designs also allow air to be blown through the barley for cooling. The germinated 'green malt' is then transferred from the Saladin Box to the kiln.

Saladin boxes can process beds of barley between 60 cm and 80 cm deep in batches of 200 tons at a time and were made obsolete when drum maltings were introduced.

Rotary Drums/ Boby Drums

The wet grain is loaded into large drums, which intermediately revolve to prevent the rootlets becoming matted. Cool humid air is blown through the drum to control the heat generated by the grain as it germinates. Once the barley has reached the optimum degree of germination, the green malt is transferred from the germinating vessels to the kiln.

Rotary drums offer a gentle method of producing good quality malt, typically for small production runs. Such drums hold between 9 and 50 tons. The seven huge, Boby drums which operate at the Port Ellen Maltings on Islay are the largest malting drums in the northern hemisphere and hold 50 tonnes of barley at its original weight (65 tonnes of barley at 45% moisture content). [See photo of drum at top of page.]

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Peat kiln at Port Ellen

Germination and Kilning Vessels

Also known as GKVs, these are the most modern malting vessels and are operated at Diageo's Roseile maltings. As the name indicates these allow the barley to be germinated and kilned in the same vessel. On completion of germination the humidified air is stopped and replaced with heated air from kiln burners.

The UK Malting Industry

Around 1.5 million tonnes of malt is produced in the UK each year, mostly by seven large and seven smaller maltsters. Some 80% of this output is for use by Britain's domestic distillers (48%), brewers (40%) and food industry (3%). The remaining 9% is exported to places such as Japan for whisky production. The malting industry purchases nearly two million tonnes of barley annually, approximately one-third of the UK crop.

Continue reading: 3. Milling the barley...

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