Understanding Whisky Production Unit 6: Maturation & Blending
Chapter 14: Oak Maturation Chemistry: Wood Polymers, Toasting, and Extraction
Quercus alba versus Quercus robur, the thermal breakdown of cellulose, hemicellulose, and lignin, charring, and additive versus subtractive reactions.
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Fresh new-make spirit running off a still is crystal-clear, intensely alcoholic, and dominated by raw cereal notes. The colour, sweetness, vanilla, and complexity associated with mature whisky come entirely from the wood of the oak cask.
Oak is a complex composite of three biological polymers: crystalline cellulose (which gives the staves their physical strength), hemicellulose (which breaks down under barrel toasting into simple sugars and caramelised furfurals), and lignin (which thermal degradation cleaves into vanillin, syringaldehyde, and spicy eugenol). Maturation is a three-way interaction: additive extraction of wood flavours, subtractive absorption of unwanted compounds by charred barrel interiors, and slow interactive oxidation over decades.
Changes during Maturation
We firstly need to understand the changes in flavour and colour that occur when we mature spirit in wood.
There are three types of reactions, or changes, which take place:
Additive reactions
Subtractive reactions
Sensory interactions
We will now look at each of these in more detail.
Additive Reactions
As the name suggests, additive reactions are processes in which something is added to the spirit. Compounds are extracted from the wood into the spirit. There are two categories of additive compounds: those that give flavour and those that give colour.
Maturation causes quite a significant flavour change. You might assume that a large number of flavour compounds are extracted from the wood. However, this is not the case. Relative to the number of flavour compounds coming from the raw materials, or those created by yeast during fermentation, the number of compounds that come from the wood is relatively small. However, these compounds tend to be very flavour active, i.e. they significantly influence the flavour. Vanillin is an example of an additive flavour compound extracted from the cask. Vanillin, as the name suggests, smells of vanilla. It increases perceived sweetness during maturation.
We generally understand the chemistry of the additive flavour compounds well. In contrast, we know very little about the nature of the colour enhancing compounds coming from the wood. What we do know is that the additive reactions (both flavour and colour) depend on the type of cask used, and the storage conditions. We will cover this in more detail later here.
Subtractive Reactions
As you may have guessed, subtractive reactions occur where flavour compounds are either removed or reduced. There are three types of subtractive reactions:
Evaporation. Casks are not gas-tight. Some of the most volatile compounds simply evaporate through the wood over time. It is not only flavouring compounds that are lost in this way; water and alcohol also evaporate. This loss by evaporation is often referred to as the “angels’ share” (which we have already discussed in previous lessons).
Flavour compounds can also be removed through interaction with the wood’s surface. This primarily happens in a charred cask (we will cover cask heat treatment later). The char on the internal surface of the cask selectively removes sulphur compounds, leaving other flavour compounds broadly unchanged. This reduces the harshness of the original spirit, as sulphur compounds tend to have undesirable flavours such as rotten vegetable, rubbery or meaty aromas.
Flavour compounds can react with other components in the spirit over time. They are reduced or removed by being transformed into other compounds. Oxygen enters the cask from the atmosphere. Thus oxidation (reactions involving oxygen) may play a role in these types of subtractive reactions. In wines, oxidation reactions are known to be important in developing mature flavour. However, we understand less about the role of oxidation in spirits production, as well as the precise changes that occur.
These additive and subtractive reactions ARE maturation. It is important to understand these reactions, as we will keep coming back to them throughout this lesson.
Sensory Interactions
Additive and subtractive reactions physically change the spirit’s composition. The final change that can occur during maturation is one where we perceive a change in flavour, even though the levels of the compounds giving this flavour have not altered. We refer to this as a sensory interaction: a change that we can detect with our senses.
An example of this might be a reduction in cereal-related aromas in whisky over the course of maturation. This may not be due to a reduction in the level of compounds giving cereal aromas. It may instead be because their perception is masked by other flavours that have developed in the spirit.
Factors that Influence Maturation
We will now look at the factors that influence the flavour/colour changes occurring during maturation.
Several factors influence flavour and colour development. These include:
- Factors relating to the cask itself and its construction
- How the spirit is treated before filling into the cask
- Storage conditions
We will examine each of these in turn under the following headings:
- Wood type and origin
- Heat treatment
- Cask size
- Choice of cask: New, previously used or finishing casks
- Fill strength
- Type of warehousing
- Storage conditions
- Length of maturation
- Cask management and regeneration
Wood Type and Origin
When building a cask, we must first consider which type of wood to use. Some wood types are more suitable than others are. The wood needs to be pliable, so that it can be physically bent into shape. It also needs to be available in reasonably long lengths that are free from defects, such as knots. It is also important that the wood is not too porous, so that the cask does not leak.
As the above figure shows, oak ticks all of these boxes and is thus by far the most popular wood used for cask manufacture. In addition, oak also contains many desirable additive flavour compounds.
The main flavour compounds naturally present in oak, ready for extraction by the spirit, are oak lactones and tannins.
- Oak lactones have coconut aromas.
- Tannins give bitterness and astringency.
The compound levels vary between oaks and depend on both the species of oak and the conditions in which it has been grown (where in the world).
- The most common types of oak used to construct casks are American oak ( Quercus alba ) and European oak ( Quercus robur ).
- American oak tends to contain higher levels of oak lactones and lower levels of tannins than European oak, but there is some variation.
- More recently, other oak species have increased in popularity. One example is Quercus mongolica , Japanese oak. This contains a different balance of oak lactones from either American or European oaks. It has been used in the maturation of some Japanese whiskies.
Heat Treatment
Once built, the internal surface of the cask is heat-treated. This heat treatment is an essential step in generating flavour. Heating breaks down some of the structural components of the oak, increasing the additive flavour compounds available for extraction. Without heat treatment, the only flavour compounds present would be those naturally occurring in the wood (oak lactones and tannins), as described in the previous section.
What does heat treatment do to the wood? It breaks down lignin. Lignin is the main structural component of oak. It is found in the cell walls and gives the wood its rigidity. As the figure below shows, it is a complex, highly branched molecule:
Heating the lignin breaks it down into smaller fragments. Some of these fragments are flavour active. The main flavour compounds created from the heat treatment of oak are vanillin (vanilla), eugenol (spicy, cloves) and 4-ethyl guaiacol (smoky, spicy). The levels generated depend on the heat treatment’s duration and intensity. Casks can be either toasted or charred (as defined below).
Toasting - This is where we heat the internal surface of the cask, without igniting the wood. Charring - This is where we allow the inside of the cask to ignite for a period, forming a layer of char.
Both types of heat treatment form additive flavour compounds. In the toasted cask, these compounds will be near the surface. In the charred cask, the char layer will contain little or no additive flavour compounds, as the heat treatment has been so severe that it has driven off any flavour created in the early stages of heating. However, the layer of wood below the char is the equivalent of a toasted layer. Here, the heat has been less severe, but high enough to break the lignin down to give flavour. Regardless of the type of heat treatment used, it must be carefully controlled in order to achieve consistent maturation from one cask to the next.
An internal char layer on a cask helps to remove sulphur compounds - this is one of the main subtractive reactions described previously. Sulphur compounds are not reduced in this way if the cask has only been toasted.
Cask Size and Options
Maturation is faster in smaller casks, as they have a high surface area relative to the volume of spirit. Since maturation is slower in bigger casks, the legislation for some categories of spirit specifies a maximum cask size. This ensures the desired matured quality.
Cask choice has the biggest impact on flavour and colour development. Flavour changes depend on:
- Cask type
- Cask age
- Whether the cask has previously been used
- Whether it is a more specialised finishing cask
New cask
New casks are the most active for creating flavour and colour reactions. The Bourbon industry specifies maturation in new, charred oak barrels. These contribute a high level of additive flavours, while the char layer efficiently removes sulphur compounds.
Previously used cask
The other option is to use a cask that has previously held another product. Since the Bourbon industry does not reuse its casks, many ex-Bourbon barrels are available. These are very popular with other spirits producers. Ex-sherry butts are also popular. These are larger capacity toasted casks, which have previously been used to store sherry. Although these are the two main types, spirits producers have experimented with a wide range of other casks.
Used casks are not as active as new casks, as some of the additive flavour compounds have already been extracted from the wood. The amount left depends on how long the cask was used. The longer the previous maturation, the lower the remaining flavour compounds. The flavour compounds left in the wood also depend on the nature of the previous cask contents. A low strength wine, for example, will have extracted different components from the wood compared to a high strength spirit. It is not just the additive reactions that are lower in a used cask. The char is also less able to remove sulphur compounds. However, subtractive reactions that involve evaporation or oxidation, should progress at the same rate regardless of the cask’s previous history. We must also note that the flavours (or colour) from the previous cask’s contents will also potentially carry over into your maturing spirit.
Finishing cask
Spirit does not need to remain in the same cask for the whole storage period. Finishing is the term used when a spirit is moved from one cask into another, typically for a shorter period at the end of maturation. Transferring the spirit in this way is labour-intensive and costly. It is thus only done if the distiller thinks that it will significantly influence the flavour. Finishing is carried out in casks that have been selected to contribute specific flavours that would not be given by the standard cask types.
Key Operating Insight
Welcome back. So, what were your thoughts? What type of cask did you decide to use? There is no right or wrong answer to this question. It all depends on what you want to achieve. However, a suggestion for each spirit is as follows: Cereal-based pot still spirit - You have decided that you want to add flavour from the cask, but not overwhelm the original spirit character. You also want to remove the sulphury notes, as you consider this to be undesirable to your target consumer. A fresh ex-Bourbon barrel would be a good choice to achieve what you are looking for. Rum - This is a light spirit, so you probably only want to impart a low level (hint) of additive flavour, so as not to overwhelm spirit character. An older cask, which has been used previously for several cycles of maturation, might be a good option here. Alternatively, you could decide to use a newer cask, but only mature for a short period.
Fill Strength
Fill strength is the term used to describe the alcohol strength of the spirit when it is filled into the cask. In the unit covering distillation, we learned that spirit from a double pot still distillation typically has an alcohol strength of around 70% abv, while a continuous (column) still can produce spirit that is more than 90% abv.
Spirit is not matured at its distillation strength. Instead, it is diluted generally to the mid 60% abv level. On initial inspection, this does not appear to make much sense. If you dilute the spirit, you are going to need more casks and a bigger warehouse. The reason for doing this is that the additive flavour compounds tend to be more soluble in water than they are in alcohol. Adding water extracts flavour more efficiently and develops the mature character more quickly. In addition, at the end of maturation, the product requires less dilution to bottling strength. There will therefore be less reduction of additive flavours that have entered the spirit .
Type of Warehousing
Traditionally, spirits were matured in small warehouses, referred to as dunnage warehouses. The casks were stacked on their sides, two or three high. Today, the majority of casks are held in large warehousing complexes. The main role of the warehouse is to keep the cask dry in an environment where the new make liquid can integrate with the wood in a natural way to encourage the desired product quality changes.
Cask storage / warehouse options
One option is to store the casks on their sides. This is referred to as racked storage. A honeycomb of either wooden or metal structures holds individual casks in rows and tiers. Access to these casks is by high reach forklift trucks.
The other option is to store the casks on their ends on pallets. This is called palletised storage. The palletised warehouse holds pallets of six to eight casks, tightly bound within an empty warehouse shell. They stand at a maximum of seven to eight pallets in height, depending on the warehouse’s height. Access to these palletised casks is again by high reach forklift trucks. This is the least labour-intensive process.
Palletised warehousing is more economical as you can get more casks into the same space. However, it comes at a cost in terms of maturation rate. In this orientation, the top end of the cask (about 12% of the total surface area) is not in contact with the spirit, reducing the additive and subtractive flavour reactions. It is also more difficult to gain access to specific casks, in particular those located at the back to the warehouse.
Storage Conditions
The environmental conditions in the warehouse, namely temperature and humidity, influence flavour changes. Additive flavours are extracted more rapidly at higher temperatures, while the combination of temperature and humidity affects the angels’ share. In cool, humid conditions, more alcohol is lost than water and the spirit’s strength reduces. This loss is referred to as the angel’s share, as the vapour is lost into the warehouse atmosphere. Warmer and drier conditions cause the alcohol strength in the cask to rise.
Environmental conditions depend primarily on where in the world the warehouse is located. A warehouse in Sweden will be very different to one in North America or the Caribbean. The design and construction of the warehouse also affects internal temperature and humidity. Warehouse design generally focuses on minimising factors that influence the angels’ share, as this has major cost implications. It is possible to control the conditions within the warehouse (e.g. heating or air-conditioning), but this approach is not usual practice.
Length of Maturation
Different types of maturation reaction progress at different rates. Additive flavour reactions are highest in the early stages of storage, and level off later. Other reactions, such as some of the subtractive reactions, where flavour compounds are transformed, take place at a slower rate but continue throughout maturation.
Some brands will have an age statement on the bottle, for example 10-year-old or 21-year-old, which dictates the required minimum maturation period.
Cask Management and Regeneration
When a cask is emptied, the company must decide what to do with it next. In some cases, this decision is easy. As previously mentioned, the Bourbon industry only uses its casks once, and then sells them on to other spirit producers. For companies maturing in previously used casks, the decision is more complex.
The first option is to refill the cask again. However, empty casks are not always refilled immediately. If this is the case, they need to be stored carefully. If rainwater enters the cask, the resulting damp conditions encourage moulds and bacteria to grow. These microorganisms can produce off-notes, undesirable flavours and generally sour or musty aromas. In some companies, operatives will assess the aroma of each cask before refilling. They will then put any casks containing off-notes to one side.
Each time a cask is refilled, the additive and subtractive flavour reactions decrease. A point is reached where it is no longer economical to refill the cask, as it will not give the desired mature character within the required timescale. This does not mean that the cask has reached the end of its lifespan. It can instead be put through a regeneration process (sometimes called rejuvenation). Regeneration involves removing the internal surface of the cask and re-heat treating it.
Regeneration reinstates some of the additive flavour compounds, namely those that come from the breakdown of lignin. The other compounds naturally present in the wood (not from heat treatment) are more difficult to regenerate. The spirit can extract these compounds from deep within the wood, so a significant internal layer has to be removed to get back to fresh, undepleted oak. As a result, a regenerated cask will not perform in exactly the same way as the original cask. If the cask is charred during regeneration, which is frequently the case, it has a renewed capacity to remove sulphur compounds.
“Mature” Flavour without the Cask
We will now consider whether it is possible to create a “mature” flavour without maturing in wood.
The legislation for many spirits categories specifies that the product has to be matured in casks. However, this is not always the case. Some products have the option of creating flavours that are similar to those arising during maturation, without using a cask. The following sections describe three key examples of the following:
- Use of toasted oak chips
- Addition of flavourings
- Filtration through char
Use of Toasted Oak Chips
Toasted oak chips are added to the spirit to boost its “mature” flavour. Usually, we do this by adding oak chips to a vat containing the spirit. These chips have been heat-treated to break the lignin down into additive flavour and colour compounds, similar to the heat treatment of the inside of a cask. The chips have a very high surface area, so compound extraction is much faster than it would be in a cask. Although oak chips give additive flavours, there are none of the subtractive reactions associated with cask maturation. Hence, the overall character at the end of the process will not be the same.
Addition of Flavourings
Flavourings are used to mimic cask maturation. These can be natural flavourings, such as vanilla, or chemical food additives. They may or may not be the same compounds that come from the wood. When flavourings are used (and allowed), they tend not to contain the full range of compounds that oak contributes, so the resulting flavour is not as complex.
Filtration through Char
The two previous examples approximate additive flavour reactions. This final example, filtration through char, simulates subtractive flavour removal. It has a similar impact to contact with char in the cask, but again the surface area is much larger and the effect more rapid. As we have already learned, a legal production requirement for Tennessee whiskey is filtration through a bed of maple charcoal. This mellows the whisky through the enhanced removal of flavour impurities.
Post Maturation Processes- Blending and Reduction
Once we have matured the spirit, there are some final treatment and processing stages required before bottling.
Blending
It is unlikely that any two casks will have exactly the same flavour, regardless of any effort to control the character of the original spirit, type of cask used and other maturation conditions. Wood is naturally variable. However, the consumer expects their spirit brand to have the same flavour every time. This is where blending comes in. Individual casks with different characteristics are mixed together in order to achieve consistency.
There are two types of blending. The first option is where casks from the same distillery are mixed together. This aims to achieve a consistent product style for that distillery, evening out any variability. The other type of blending is where spirits from different sources are mixed to give a distinctive product that has elements of flavour from each of its component parts. A good example of this is the production of blended Scotch whisky, where malt and grain whiskies, sourced from a number of distilleries, are combined. An important factor in this type of blending is inventory management, i.e. having future availability of all the necessary component parts required for the final blend. However, blending is not a fixed recipe. Individual components of the blend can be replaced, if necessary, with other components that will give a similar flavour contribution. In order to do this, you must know both the characteristics of each component, and how they come together and interact in the mixture. This is the responsibility of the company blender, who has built up this understanding of flavour through years of experience.
When blending, you should try (if you can) to carry out a sensory assessment of each of the casks that will be added to the blend. As well as checking that each component of the blend has the expected characteristics, it also allows any casks with off-notes to be identified. The blender has the expertise to know if these are going to cause problems. Some off-notes will disappear on blending, as they are masked by the other flavours in the blend. Other types of off-notes are more difficult to blend away. Good examples of these are musty aromas, which have low sensory thresholds. They can still be detected even after mixing them with a large volume of spirit from good quality casks.
Reduction
At the end of maturation, the spirit needs to be diluted to the required bottling strength. This process is known as reduction. Since a lot of care is taken throughout the earlier stages of production, we must not introduce any undesirable flavours at this point. Therefore, we should regularly check the water for off-notes. Demineralised water is used because calcium needs to be removed before the water is used for reduction. It can react with oxalic acid, which comes from the wood, forming calcium oxalate crystals. These insoluble particles are visible to the human eye, and hence undesirable in the bottled product.
Post Maturation Processes and Treatments - Caramel, Filtration, Filling Temperature
Caramel Addition
Caramel is a colouring agent, not a flavouring (as many people assume). In fact, the flavour of caramel is quite unpleasant. At high levels, it is quite burnt/acrid. It is generally added to matured spirits to achieve consistent colour from batch-to-batch. The levels needed in order to achieve this are not enough to give flavour. In an earlier section of this lesson, we looked at methods that give the flavour impact of maturation without using a cask. Similarly, adding caramel could give the appearance of a product that has been matured in wood.
Filtration
The consumer does not want to find any particles in their bottled product. If the spirit has been matured, there may be particles of char and wood fragments present. These are easily removed by passing the spirit through a coarse filter.
More problematic are the small compounds that cause haze (cloudiness). Haze results from the presence of long chain esters, which are produced by the yeast during fermentation. These esters can pass through the process into the spirit. Long chain esters are particularly insoluble at lower alcohol strengths and low temperatures, for example if your storage is a cold room or refrigerator. If they are not removed, a bottled product will turn cloudy when chilled. Chill filtration reduces haze, giving a clear, bright product. This process takes advantage of the fact that these esters come out of the solution at a reduced strength and low temperature. The spirit is reduced with water and chilled (generally 2-4˚C), encouraging haze formation. The spirit is passed through a fine filter to remove this haze. The filter does not remove significant amounts of colour or flavour, as these compounds are much smaller than the long chain esters. Having said this, some spirits producers prefer not to chill filter for perceived quality reasons. In this case, the product tends to be bottled at a higher alcohol strength, of 46% abv or above. This reduces the likelihood of haze formation.
Filling Temperature
The volume of a liquid is partly related to its temperature. For instance, if we take 1,000 L of 50% abv solution at 4.4˚C (40˚F) and warm it to 15.6˚C (60˚F), we end up with 1,009 L of 50% abv solution. That is almost a 1% difference, which may not sound like much, but 1% adds up quite fast!
Most countries have legislated fill levels that are predetermined at a given temperature. For example, in the U.S. the legislated volumes are based on a temperature of 15.6°C (60°F). If the processed spirit is not at this temperature, then true fill levels may be incorrect. For most spirit bottle sizes, the U.S. allows a plus or minus 2% error. In the above example, we would only be out by half of that (assuming all of our other measurements are correct), so it would seem that we are ok. So why should we care? In this example, where we are adding 4.4˚C spirit into packaging, it means that we are overfilling the bottle by 1%. Put another way - for every 100 bottles, we are losing one that could be sold to a customer. Extrapolate that out to large production scales with say 1,000,000 bottles, this translates to 10,000 bottles that we cannot sell.
From this, it becomes clear that prior to filling, the spirit should be attemporated to the required legislated value. Otherwise, fill volumes may be incorrect, money could be lost and the company penalised.
We should note that filling temperature is generally not directly regulated, meaning that you can fill bottles at 10˚C should you choose to. The distiller should however be aware of the effect that temperature has on volume, because as we mentioned earlier, volume is regulated. Should your regional regulatory authority decide to audit your filling processes, they would likely have a standard temperature that they use for the basis of fill volumes. For instance, in the UK and EU, the volume of a spirits bottle is determined with 20˚C as the basis temperature. Therefore, when it is time to fill the bottles, a filling temperature of 20˚C becomes the easiest to work with, as it allows a quick assessment of fill levels prior to case packaging and sale. Further to this, an increase in temperature in a sealed bottle could create a build-up of pressure, causing the cork to pop out, or inconsistent fill-heights may result which are less attractive to the consumer.
Key Takeaways
Oak species dictate chemical extraction: American white oak (Quercus alba) is rich in sweet lactones and vanillin, while European oak (Quercus robur) delivers deeper tannins, dried fruit characters, and higher wood extractives.
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