Understanding Whisky Production Unit 2: Raw Materials
Chapter 3: The Malting Process: Steeping, Germination, and Kilning
How steeping, germination, and kiln drying activate enzymes, break down cell walls, and impart peat smoke to barley grains ready for the mill.
On this page
Malting is essentially an exercise in cruel biological deception. You get the dormant barley slightly damp, convince it that spring has arrived, allow it to begin growing just long enough to manufacture an arsenal of starch-splitting enzymes, and then roast it alive in a kiln the moment it shows genuine enthusiasm. Preferably over peat smoke so it dies with character.
If the drying kiln burns peat beneath its perforated floor, the rising smoke coats the damp husks in aromatic phenols, cresols, and xylenols. The plant wanted to become a leafy stalk; instead, it becomes friable, enzyme-rich malt carrying a permanent smoky fingerprint.
The Malting Process
A malting process typically consists of three stages. First, you have steeping, followed by germination and finally, kilning.
The Steeping Process
Steeping increases the kernel moisture from storage moisture levels, to an ideal moisture level that will encourage the grain to grow.
Are you familiar with sprouting grains? With sprouting grains, you allow the grains to grow just enough to be eaten raw. Malting barley is similar, but the grain is dried to ensure safer storage and easier transport.
For maltsers to sprout the barley, they first submerge the grain in water. This process is known as steeping. Steeping increases grain moisture from its storage levels of typically 12% to around 43-46%. Increasing the moisture tricks the grain to start growing. When the moisture of the kernel is greater than 30%, the kernel will germinate (begin to grow).
Analytical documents, such as a certificate of analysis, report results for barley or malt ‘as-is’ or on a dry weight basis. ‘As-is’ represents the actual weight of the grain; no corrections are made for moisture levels. Therefore, when you are looking at this figure, remember that we must consider the actual percentage of moisture.
The first sign of germination is the chit (the first rootlet) appearing at the embryo (proximal end). The chit is also called the coleorhiza, or root sheath. It extends and branches to form the seminal (temporary) roots. The figure below shows that the rootlet forms within the coleorhiza, and the coleorhiza acts as a protective sheath (cover) for the rootlet. The acrospire, which develops into the shoot, is protected by the coleoptile. As the embryo grows, this triggers the production of plant hormones such as gibberellins. Gibberellins then trigger the production of other enzymes such as α-amylase.
Steeping Technology
A steeping vessel uses water and air to control the barley’s growth rate. This section will explore vessel design and examine the various operational requirements.
The steeping vessel can be a conical or flat-bottomed design. The figure below gives an example of a cylindro-conical steep vessel.
The vessel design promotes the ideal conditions for the barley to germinate and grow. The growing grains will produce carbon dioxide and heat in a biological reaction known as respiration.
The figure above shows a flat-bottomed steep vessel. We will use this example to further explain the process. The essential components of the vessel will be:
- An inlet and outlet for the barley
Ideally, the barley is fed into the vessel by gravity (saving electricity costs).
- A rotating giracleur, which spreads the grain evenly over the floor surface
- A water inlet and outlet
- A false bottom (perforated deck), creating a plenum chamber between the vessel base and the barley
The barley rests on this false bottom, allowing water and air to be pumped evenly through the grain bed via the plenum chamber.
- Compressed air and aeration tubes
- Carbon dioxide extraction duct
It is essential to remove carbon dioxide from the grain bed as the grain starts to respire and produce it . Otherwise, the grain will suffocate and die.
The steeping process involves one or more water stands, which submerge the barley underwater. During water stands, air bubbles up through the grain bed to lighten the bed and supply further oxygen to the grain. Care must be taken not to drown the barley with prolonged water stands, especially if the barley is water sensitive.
The barley seed is not actually sensitive to the water, but rather the lack of oxygen during submerged state.
Steeping also includes dry stands (sometimes referred to as air rests). During air rests, the water drains off, and air is sucked down through the bed to remove any carbon dioxide being produced by the grain. This again supplies oxygen to the growing seeds. We optimise the wet and dry stands to deliver the ideal conditions for the barley to achieve a final moisture level of 43-46% at the end of steeping. A typical steeping schedule could be 6 hours wet, 12 hours dry, 8 hours wet, 14 hours dry, 2 hours wet, then drain and transfer to the germination vessel (6w,12d, 8w,14d, 2w).
The Germination Process
The second step in the malting process is germination. Germination creates the ideal conditions in which the barley kernels can grow. Temperature-controlled, humidified air keeps the grain at ideal temperatures and additional water sprays maintain the bed’s moisture level.
Germination aims to achieve complete and even modification. Good modification is critical to ensure problem-free processing in downstream operations. Adequate modification is achieved by maintaining the moistures that resulted during steeping (43-46%), and by providing ideal growing conditions for the barley. Ideal growing conditions are created by removing heat and carbon dioxide. We also need an oxygen supply, and we need to turn the grain bed in order to get even malting. This loosens the bed thus allowing easier movement of oxygen and carbon dioxide and reduces the risk of rootlets “matting” together.
During germination, many important changes happen in the barley kernel. Enzymes start to break down the cell walls and their contents, providing the embryo with nutrients. The grain is tricked into thinking it is in the ground, and that it is time to grow into a new plant.
Let us discuss these changes in more detail. The figure below shows the major components surrounding the starch granules within a single endosperm cell:
In barley, the cell walls and protein matrix all surround the starch granules. This makes the starch inaccessible for conversion to fermentable sugars. The barley needs specific enzymes to break down the components surrounding the starch granules. The start of germination, and the growth of the embryo, both trigger the production of gibberellins (a plant hormone). This then stimulates the aleurone layer to produce the relevant enzymes through activation (those already present in the barley, e.g. β-amylase), or synthesis (will be produced by the aleurone layer, e.g. α-amylase).
Enzymes such as β-glucanases and pentosanases break down the cell walls which then exposes the protein matrix. Enzymes can then break down the protein; these are called proteases. This action exposes the starch granules. During modification (the germination step in malting), enzymes convert some of the starch into sugars to provide nutrients for the embryo. The enzymes responsible for this are α- and β-amylase. Gibberellic acid can sometimes be used to promote modification but due to regulations in some countries, e.g. in Scotland for Whisky production, external agents such as growth hormones and enzyme sources are not permitted.
Towards the end of germination/modification, if you rub the starchy endosperm between your fingers, it will ‘rub out’ and there will be no ‘hard ends’. It will feel pasty, but there will be no formation of little balls between your fingers. This pasty consistency indicates that the starchy endosperm is well modified, and that no cell walls remain. At this point, the barley is ready to transfer to the kiln; it then becomes known as green malt.
Some distillers prefer working with green malt as the enzyme levels are higher compared than those within kilned malt. Some distillers think the green malt contributes different flavours to the new make spirit compared to dried malt. Green malt will not last long in storage, as it is biologically active; it is thus best to use it immediately. Furthermore, at high moisture, it still produces carbon dioxide and heat, making it prone to microbial growth. Microbial growth can introduce mycotoxins (any toxin produced by a fungus) into the malt, which can be dangerous for human consumption.
Another dangerous compound associated with malting is ethyl carbamate, which is a carcinogen. During germination of barley, a precursor of the ethyl carbamate (epi-heterodendrin) forms. Therefore, to reduce the risk of ethyl carbamate forming during the distillation processes, barley breeders select parents’ plants producing low levels of epi-heterodendrin. Ethyl carbamate formation is not usually a problem these days, but it requires careful monitoring, due diligence sampling, and analysis to confirm this.
The key changes occurring during germination are: Enzyme production During germination, the key enzymes, α- and β-amylase are synthesised and activated respectively. These two enzymes are responsible for breaking down the starch to fermentable sugar. Cell wall degradation Cell walls prevent the enzymes from accessing starch. During germination, the barley enzymes β-glucanase and pentosanase break down the endosperm cell walls. Protein breakdown Like the cell walls, the protein matrix isolates the starch granules. Proteases break down the matrix during germination, exposing the starch granules.
Germination Systems: From Stone Floors to Modern Vessels
The biological objective of germination is identical regardless of equipment: maintain the grain bed between 16°C and 21°C at 43% to 46% moisture, supply abundant oxygen, flush away suffocating carbon dioxide, and periodically turn the sprouting kernels so their delicate rootlets do not tangle into an impenetrable fibrous mat. Over two centuries of distilling history, the engineering solutions to this challenge have progressed through four distinct eras.
Traditional Floor Maltings
For centuries, all malt was made by hand on open stone or concrete floors. After steeping in deep cisterns, the swollen barley was tipped onto the malting floor and spread out in beds roughly 8 to 15 cm deep.
Because growing barley generates significant metabolic heat and carbon dioxide through respiration, the grain bed had to be turned continually by hand using wooden shovels known as shiels. Maltmen walked the vast, dim floors day and night, tossing the grain into the air to cool it and untangle the sprouting rootlets. Decades of heavy, repetitive one-sided shovelling famously afflicted workers with a permanent postural distortion known as “monkey shoulder”, where one arm hung lower than the other.
Floor malting is intensely romantic, highly atmospheric, and commercially ruinous on a modern industrial scale. Floor-malted grain requires vast floor areas, remains entirely at the mercy of ambient weather, and typically yields 5 to 8 fewer litres of pure alcohol per tonne compared to computer-controlled modern facilities. Consequently, the vast majority of Scottish malt floors were mothballed or converted into barrel stores during the 20th century.
Today, only a handful of traditional floor maltings remain operational in Scotland, serving a blend of craft heritage and tourist appeal:
- Springbank: The only Scottish distillery that carries out 100% of its malting on its own traditional floors, remaining completely self-reliant from grain to bottle.
- Bowmore: Produces roughly 30% to 50% of its barley requirements on its Islay malt floors.
- Laphroaig: Malts approximately 15% to 20% of its grain on-site, kilning it over its own pungent peat fires.
- Highland Park: Prepares around 20% of its malt on its Orcadian floors, smoking it with heather-rich peat cut from nearby Hobbister Moor.
- Balvenie: Malts around 10% to 15% of its annual needs on its Speyside floor.
- Kilchoman: Features an on-site farm floor malting that provides approximately 20% of its barley.
- Benriach: Revived its historic 1898 malt floor in 2012 for seasonal production runs.
- Ardnamurchan: Commissioned an operational floor malting on the remote Ardnamurchan peninsula.
The Saladin Box: The Dawn of Mechanised Germination
In the late 19th century, French engineer Colonel Charles Saladin set out to liberate maltmen from the backbreaking toil of the shovel. His invention, the Saladin box, introduced the first successful forced-air mechanised malting system.
A Saladin box is an elongated rectangular trough, typically constructed of concrete or brick, between 60 and 80 cm deep. The grain rests on a perforated steel false bottom. Instead of relying on open windows and drafty attics, a large mechanical fan forces humidified, temperature-controlled air upward through the plenum chamber and into the grain bed, precisely regulating temperature and sweeping away carbon dioxide.
To turn the grain, a motorised carriage travels along rails mounted on the trough walls. Suspended from the carriage are rotating vertical helical screws (augers) that burrow through the grain bed. As the carriage crawls back and forth, the screws gently lift grain from the bottom of the bed to the surface, turning up to 200 tonnes of barley without tangling the rootlets or requiring human intervention.
In Scotland, Glen Mhor was the pioneer, installing the country’s first Saladin box in 1950. Tamdhu followed shortly afterward. While Saladin boxes revolutionized the industry in the mid-20th century, most were eventually replaced by rotating drums. Today, Tamdhu is the sole remaining distillery in Scotland that still operates working Saladin boxes, utilizing two massive boxes to supply its on-site kilns.
Drum Maltings: Cylindrical Tumbling
The precursor to the Saladin box was invented in 1873 by Nicholas Galland, who developed drum maltings. Rather than moving turners through a stationary grain trough, drum malting places the steeped barley inside a colossal horizontal steel cylinder.
As the germination drum rotates very slowly (often just a few revolutions every hour), the entire grain bed gently cascades and tumbles against the curved walls. This rotation prevents rootlets from matting together without bruising the grain. Conditioned, humidified air is blown through a central perforated pipe running along the drum’s axis and drawn outward through the tumbling grain.
Drum maltings offered precise control over airflow and temperature while occupying a fraction of the land area required by floor maltings. During the distilling boom of the 1960s and 1970s, Scottish Malt Distillers invested heavily in centralized drum plants:
- Burghead Maltings: Opened in 1967 with 24 drums and expanded in 1971 to 48 drums, becoming one of the largest drum malting complexes in Europe with an annual capacity of 85,000 tonnes.
- Glen Ord Maltings: Built with 18 large germination drums, producing 35,000 tonnes of malt annually for northern distilleries.
- Port Ellen Maltings: Established on Islay in 1973 with seven massive drums following the closure of the distillery’s malt floors, built to supply all of Islay’s distilleries (including Ardbeg, Caol Ila, and Lagavulin) with precisely calibrated peated malt.
Industrial Maltings and Combined Vessels (SGKV)
Today, more than 95% of all malt used in Scotch whisky production is produced off-site by large commercial maltsters or centralized distilling complexes. Modern UK and Scottish malthouses operate on an immense industrial scale, utilizing computerized circular vessels:
- Circular Germination Vessels: Large circular tanks equipped with rotating bridge turners that sweep through the bed, turning grain with vertical helical screws while conditioning air is pushed from beneath.
- Combined GKV and SGKV Vessels: In modern Germination and Kilning Vessels (GKV) or Steeping, Germination, and Kilning Vessels (SGKV), the entire process takes place inside a single automated chamber. The barley is steeped, aerated, germinated, and kilned without ever being transferred between vessels, dramatically reducing handling losses, energy consumption, and labour costs.
Specialist commercial maltsters like Crisp, Bairds, Simpsons, and Boortmalt, alongside dedicated distilling facilities like Diageo’s Roseisle, operate around the clock to supply distilleries with malt tailored to exacting specifications of diastatic power, nitrogen content, and peat smoke phenols.
The Kilning Process
The last step in the malting process is kilning. Kilning reduces the moisture within the green malt, to stabilise it for long-term storage. Malt production for distilling maximises the enzyme potential of the malt, hence the higher moistures ex-kiln, e.g. five percent or more.
Kilning Technology
The kilning step happens in a kilning vessel (sometimes referred to as a kiln).
The figure below highlights its key components:
- A green malt (barley ex-germination vessel) inlet and a kilned malt outlet
- A false bottom (perforated deck), creating a plenum chamber between the vessel base and green malt
- Exhaust
- A rotating turner arm with helical screws
An alternative design keeps the turners in a fixed position and rotates the whole floor and grain bed past the turners.
Not all malting plants turn the bed during kilning. It is good practice to avoid disturbing the bed once the bed is even after loading.
- Adjustable louvres
Fresh air heats up when passing through the heating unit. This process reduces the air’s moisture, allowing it to pick up moisture as it moves through the grain bed. Air recirculation is possible. It can be recirculated within the same kiln. Alternatively, we can use warm air from a linked kiln, in different stages during the drying process. This saves energy. It is common to have two kilns linked via a system that uses the heat from the one kiln to heat the air for the other kiln.
- Fresh air inlet
- Fan
Huge fans push warm, dry air through the grain bed to reduce the moisture. They are the biggest users of electricity in most plants.
- Heating unit
Historically, heating units were directly fired. In other words, their heat source was in direct contact with the green malt. Depending on the fuel used e.g. if natural gas is directly fired then dangerous chemicals can form during this process, such as nitrosodimethylamine (NDMA). Some kilns are still direct fired, especially those that burn peat and special measures are employed such as burning rock sulphur to prevent NDMA formation and produce “safe” malt. However, for the most part indirect heating took its place, in order to produce a safe product. In indirect heating, steam flowing through radiators heats the air, before it forces through the grain bed. Another design for indirect kilns uses the hot flue gas to heat up fresh incoming air in a heat exchanger. This heated, fresh air is used to dry the malt.
The kiln uses heated air to evaporate the moisture from the grain, ultimately reducing the moisture from 43-46% to 4-5% for malted barley distilling, and 5-6.5% for grain distillation. The initial stages of kilning are at low temperatures. High temperatures combined with high moistures will de-activate the enzymes produced during steeping and germination.
Historically, before the advent of electrically driven fans, kiln airflow relied on natural convective updraft. Hot air rising from the kiln furnace below was drawn through the grain bed and exhausted through the roof vents.
Drive far enough through whisky country, and once you grow immune to the breathtaking rolling hills and the scent of malted dreams, you will start to notice something odd: all the distilleries appear to be wearing hats. Not normal hats, either, but tall, pointy affairs you might expect on a Japanese temple if said temple also smelled faintly of peat smoke and optimism and accidentally settled in the Highlands. These are Scotland’s pagoda roofs (more accurately cupolas), which somehow manage to look both exotic and entirely local, like a kilt with ideas above its station.
They exist because of a man called Charles Chree Doig of Elgin, who thought that if barley had to dry, it might as well do so with a little grace. Before Doig was summoned to Dailuaine in 1889 with instructions to ‘make it work better’, distillery roofs were about as exciting as a tax return: smoke went up, rain came down, and sometimes they met halfway in a brief but meaningful exchange that usually ended with someone swearing. When the wind blew, the malt dried too fast; when it didn’t, it sulked.
Doig reshaped the chimney. The pagoda’s upturned eaves harnessed the wind’s chaos to steady the draught, drawing smoke evenly upwards: a brilliant exercise in controlled ventilation. It worked so well that within a few years, half of Speyside sprouted matching pagodas until the Highlands resembled a slightly confused Zen garden with an excellent drinks policy. Today, modern industrial maltings rely on high-volume centrifugal fans and indirect heat exchangers, but the pagoda endures: whisky’s signature silhouette, dedicated not to the gods, but to better airflow.
Peated Malt
In distilling, a product known as peated malt imparts unique flavours to the final product.
Peat is decayed plant material, or organic matter, which forms over thousands of years.
The key ingredient for producing peated malt is the smoke produced by burning peat. The peat burns in a separate unit (firebox) under controlled conditions to produce as much smoke and as little flames as possible. The burning peat’s smoke carries the flavour and aroma compounds, known collectively as phenols. However, there are also many other similar compounds in the smoke.
It is crucial that the peat has a moisture content of 50-60%. Controlling the peat moisture level is essential as smoke is required when we burn the peat, not flames.
The high levels of organic material contribute to the unique characteristics of a peat smell. These include phenol, cresol, eugenol and guaiacol.
Malting Other Cereals
Barley is the most common, commercially malted, cereal in the world. However, other malted cereals are available for distilling.
Wheat malts
The malted wheat used in distilling is usually pale and lightly kilned. The malted wheat used in food is typically darker, and gives a sweet, luscious flavour to the product.
Like barley, the wheat’s nitrogen influences the selection of the grain for malting. Typical wheat nitrogen is less than 1.6%. Soft wheat is good for malt dedicated to alcohol production. Since there is no husk, the grain will compact more than barley would within the malting process. Therefore, the layers of the grain bed are not as deep as they are in barley malting. Wheat absorbs water more readily and is normally steeped to a moisture of 42%. Wheat is germinated at a cool temperature, typically lower than 16°C. Like barley, the wheat is in the germination vessel for up to five days before kilning. As in barley malt production, gibberellic acid can be used to promote modification if it is allowed by local legislation. In some parts of the world like Scotland, it is not allowed.
Wheat malts are often rich in free amino nitrogen (FAN); this is good for fermentation.
Rye malts
Rye malts have high levels of α- and β-amylase enzymes, which make them suitable for distilling. Since rye is also naked, e.g., it has no husk, we need to follow the same precautions as we did with wheat. During steeping, rye rapidly takes up water. Unless we use gibberellic acid to promote modification, it has a seven-day germination process.
Sorghum malt
These malts differ significantly in their quality. Germination can be anything from four to six days. Both α- and β-amylase enzyme levels increase during germination.
Malt Quality
We determine the quality of malt by studying its certificate of analysis (CoA). Let us look at an example of a CoA:
Scroll sideways to see all columns.
| Malt quality aspect | Test parameter | Unit | Typical range |
|---|---|---|---|
| Process yield | Moisture | % | 4.5-5.5 |
| Process yield | Soluble extract | % as-is | 77.0-79.0 |
| Process yield | Fermentability | % | > 87.5 |
| Process yield | Fermentable extract | % | 67.0-69.0 |
| Process yield | Predicted spirit yield | L/t | 405-418 |
| Protein modification | Total nitrogen of barley | % dry mass | < 1.65 |
| Protein modification | Soluble nitrogen | % dry mass | 0.45-0.55 |
| Protein modification | Soluble nitrogen ratio (SNR) | % | 36-42 |
| Protein modification | Free amino nitrogen (FAN) | mg/L | 130-160 |
| Cell wall breakdown | Friability | % | 88-95 |
| Cell wall breakdown | Whole corns | % | < 2.0 |
| Cell wall breakdown | Homogeneity | % | > 98 |
| Cell wall breakdown | Residual wort β-glucan | mg/L | < 130 |
| Enzyme potential | Diastatic power (DP) | ˚IoB | > 75 |
| Flavour | Total phenols (peated character) | mg/L | 2-5 (lightly peated) |
| Flavour | Total phenols (peated character) | mg/L | 12-16 (peated) |
| Flavour | Total phenols (peated character) | mg/L | 35-55 (heavily peated) |
| Food safety | Glycosidic nitrile | g/t | < 0.2 (non-producer) |
| Food safety | Glycosidic nitrile | g/t | < 1.5 (low-producer) |
| Food safety | NDMA | μg/L | < 1.0 |
When buying malt for distilling, you want your CoA to meet the basic specifications for distilling malt. Let us discuss some of the common analyses below.
Moisture
Distillers prefer higher moistures than brewers do. Distillers want to preserve more enzymes during kilning. They achieve this by producing malt with higher final moistures. Lower kilning temperatures result in lower denaturation of starch degrading enzymes, promoting higher alcohol levels at the end of fermentation. Less than 5% is a safe percentage for storage, as well as for the preservation of enzymes.
Good predicted spirit yield (PSY)
PSY is a calculated number from laboratory analyses. The first requirement is the amount of soluble material obtainable from the grain (soluble extract). The second requirement pertains to how much of this soluble material is fermentable by yeast (fermentability).
Total soluble nitrogen (TSN) and free amino nitrogen (FAN)
There is a correlation between enzyme levels and nitrogen levels. Rule of thumb is that higher nitrogen barley will have higher levels of enzymes and lower starch. Farming inputs and the weather determine the nitrogen level. Nitrogen and soluble nitrogen are both essential for maintaining enzyme levels. The analytical analysis in the laboratory identifies the total soluble nitrogen (TSN), as free amino nitrogen (FAN) is critical for proper fermentation.
Diastatic Power (DP)
The enzymes responsible for breaking down starch are critical to producing fermentable sugars. The levels of α- and β-amylase are therefore vital when selecting malt for distilling. The diastatic power (DP) indicates both the α- and β-amylase activity. Typical values on the CoA will be > 75 ˚IoB.
Friability and homogeneity
In order to optimise the ease of processing malt in the distillery during milling and mashing, you need to consider the impact of friability and homogeneity in the process.
What is friability?
Friability is an indirect indicator of malt modification. It measures the amount of force required to crush the grain.
Remember that modification is part of germination; the cell walls are broken down to expose the starch.
The method for determining friability is a friabilimeter. The analysis uses a specified amount of malt, placing it in a perforated drum where it is subjected to pressure from a roller. The pressing of the malt against the perforated drum separates the soft, chalky endosperm from the part of the kernel that is not fully modified during the malting process.
The homogeneity test uses the same equipment as the friability test, but this time the focus is on the material left in the drum that could not press through the perforated drum. The material in the drum is weighed and classified as either partially unmodified grains (PUGs), or wholly unmodified grain (WUGs). The higher the fraction of PUGs and WUGs, the more cell wall and protein matrix that remains. This leads to less soluble extract, as the starch is surrounded by protein and β-glucan cell walls. The cell wall material can also cause future problems such as slow run-off during mashing.
Key Takeaways
Careful control of moisture during steeping and airflow during germination maximises enzyme potential without burning through starch reserves. Kilning then halts growth, locks in diastatic power, and determines whether the finished spirit will be clean and malty or profoundly peated.
About the author
