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.

Malting is essentially an exercise in biological deception. You take dormant barley, soak it in water until it decides spring has arrived, allow it to begin growing just long enough to manufacture an arsenal of starch-splitting enzymes, and then abruptly dry it in a kiln before the young shoot can eat the very sugar you were after.

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

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• 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 - Technology

The ideal conditions involve maintaining the barley at 43-46% moisture, and between 16-21˚C. The germination step in the malting process typically takes four to six days and happens within a germination vessel. These vessels can be either circular, a drum, or a box.

The figure below is known as the Saladin box.

A germination vessel requires a couple of key components in order to provide the ideal conditions for the barley. We will look at a circular germination vessel as an example, to help us explain these critical components:

Like the flat-bottomed steep, the grain is loaded on to a false perforated bottom. This creates a plenum chamber, where the humidified (moist) air is pushed through the grain bed to control the temperature and remove the carbon dioxide. The other key components for a germination vessel are:

• A barley inlet and outlet
• A false bottom (perforated deck), creating a plenum chamber between the vessel wall and barley
• Carbon dioxide extraction exhaust
• A rotating turner arm with helical screws and water sprays

Like the giracleur in the steep, the turner arm with the rotating helical screws helps to load an even grain bed and unload the vessel. It also turns the bed during the germination step in the malting process. Loosening the bed is essential to avoid the rootlets tangling and forming a mat. Water nozzles positioned in front of the helical screws can spray water at specific times, to maintain the ideal moisture of 43-46%.

• Adjustable louvres

Louvres are horizontal metal panels, positioned at various angles to control the mixing of fresh air from outside the vessel, and hot, humid air from within the vessel. We commonly mix the air in order to increase the air temperature before it passes through the fan and is forced through the grain bed.

• Fresh air inlet
• Fan
• Water atomisation units

Water passes through nozzles to form a fine mist. When the air passes through the mist, the air’s humidity (moisture level) increases which avoids the grain drying out which would lead to incomplete germination/modification and “hard ends”.

There are other types of germination vessel available. For example, a rotating drum can be used to gently turn the green malt, preventing the matting of the rootlets while providing airflow and temperature regulation.

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.

What we now call floor maltings were the first large scale malting houses. The barley would be received into the top of these houses and dropped into deep cisterns for steeping. The grain would then be deposited in a pile onto the stone floor for germination. Once the grain had accumulated enough heat, the pile will be levelled using shovels. Thin layers of 8-15 cm were required for sufficient temperature control. The grains were manually turned with wooden shovels to ensure no excessive heat or carbon dioxide build-up was present. The room temperature had limited control using the windows in the walls. Kilning occurred with limited air flow due to lack of electricity to drive fans at many malting/distilling sites. Air flow through the drying green malt bed was enhanced by the shape of the building hence the distinctive outline of malt whisky distilleries in Scotland with their “pagoda” roofs. There are still a few floor maltings in use today such as the one at Highland Park Distillery in Orkney but by far most of the malt produced today is made in large scale commercial malting plants.

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:

Making malt for distilling: Link

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

John Fegan · Editor

John has run Uisge Beatha since 2015 and Whiskipedia.com since 2020. He owns whisky casks and writes from direct experience of their storage, insurance and bottling decisions.

• johnfegan.org