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Maturation is a key aspect of the production of Whisky. Companies spend millions of pounds on sourcing oak from the Ozark region in the US for wood that will be used for the production of specific casks and the location of good bodegas in Europe is highly sought-after industry knowledge. This is all due to statistics such as 80% of a whisky’s flavour comes from the cask. However, taken literally this is not the case.
Maturation is a step in the production of whisky that is required to improve the sensory qualities of whisky and an oak cask is a tool used to achieve good maturation.
New spirit from a still is clear like gin and when it is filled into the cask it will take all of its colourings from the cask, the amount of charring the cask has endured, the cutting into the wood and charring it; the deeper the char the more colour.
The new spirit will take the majority of the colour and flavour that it can from the oak alone within the first 6-12 months. It is good to imagine a cask as a big sugar-coating tub as wood contains lots of sugar which add sweetness amongst other flavours to the spirit. The amount of flavour donated to the spirit from the cask depends on the quality of the oak. This can be highly variable even if it’s from the same forest and the same treatment of the wood, such as how long was it seasoned in a kiln or outside in the open air, or the level of toasting/charring (charring the inside of the cask at varying temperatures caramelises oaks sugars to release different flavours from cola, custard and spice amongst many others).
An interesting suggestion from the field of research in maturation is that a Sherry cask matured whisky does not necessarily get it’s Sherry flavour from the Sherry that was filled into the cask previously but from the oak. Oak typically used to produce Sherry casks is from Europe and European wood has a greater level of tannins and fruitiness – where US oak is kilned, European Oak is left outside for seasoning so the tannins can be reduced.
During and after 12 months, the oak allows for sufficient oxidation to take place. This drives away many of the undesirable characters from the spirit such as sulphur. However, over-oxidation can create vinegar! Oxidation is a key aspect of maturation, like the browning of an apple. It allows the breakdown of flavours from the cask while esters (fruity flavours from the distillation) are enhanced/uncovered. The level of oxidation depends on the cask size, where it is located in the warehouse and the strength of the alcohol when it was filled into the cask.
There is little known about maturing spirit over 10 years with exception to it generally being a smoother spirit with particular emphasis on mouthfeel being improved.
The large changes in new spirit into whisky result from the loss of some of the undesirable flavours from the original distillate with a more limited contribution from the wood (Whisky, Technology Production and Marketing. 2nd Edition, 2014. I. Russel and G. Stewart).
Last year we released a few single grain casks of Whisky, a Girvan Grain and before that an Invergordon. In the future, we will continue to release single grain casks as matured grain is exceptionally tasty and good value.
It is a different product to Single Malt and probably has more in common with gin than single malt due to the way it is produced.
Scotland only has 7 grain distilleries compared to over 120 single malt distilleries, but grain distilleries are much larger – Diageo’s Cameron bridge has a capacity of 110,000,000 a year whereas Glenfiddich, with its 13 pot still produces 13,000,000.
Grain whisky is made from different ingredients to single malt whisky. Grain can be 85-90% grain and 10 + 15% malted barley. By grain, we are referring to maize, wheat and other cereal products that are much cheaper than barley and which are usually cooked first. Single malt whisky can only be produced using malted barley. The spirit which is used to produce gin is called Grain Neutral spirit and can be 100% grain but the production of grain whisky requires adherence to legislation laid out in the Scotch Whisky Act 1998. Grain neutral spirit does not have to adhere to this act and therefore cannot be used in whisky but grain whisky could be used as a high-quality Grain Neutral spirit for gin.
Grain whisky is produced to be around 95 per cent ABV, whereas single malt is closer to 69 per cent ABV. Where in Scotch we look for a high concentration of congeners (flavour compounds) in grain the target is low congeners and greater rectification; more strength and smoothness. The reason for this is that most of the single grain whisky is used for blending; to act as a base for malts.
Grain whisky is distilled using continuous distillation whereas single malt whisky is doubled distilled in batches. Grain Neutral spirit is also distilled using continuous distillation. Continuous distillation was invented by Robert Stein in at Kilbagie Distillery in response to the growing demand for gin in the early 19th Century. Aeneas Coffey, who visited the distillery, was an exciseman from Dublin and after making a few ‘amendments’ to the design he patented it… Robert Stein would later take his design to his cousin’s distillery at Cameronbridge owned by John Haig.
The ‘Coffey still’/Patent still is an incredible piece of kit which utilizes two columns to strip the alcohol from the Wash (this is the grain mixed with water after fermentation) and then rectify it to increase the alcohol level and purity. Once complete, the spirit that is to be made into whisky will be reduced from 95% to 63% and filled into casks to be matured for a minimum of 3 years like whisky.
If the spirit was to be used for gin it would be infused with botanicals and possibly re-distilled depending on the desire of the gin house but fundamentally the original distillation process is the same for gin and single grain whisky.
When I first started drinking whisky, peaty whisky was not enjoyable and as recently as two years ago I still didn’t like peat. This changed when I went to Feis Islay in 2018 and I tried so many different peaty whiskies that it finally clicked, like when you start drinking beer and it’s not great and then one day it’s good and it works. Funnily enough, the main reason I’ve found that people turn off from whisky is that when they were young they either had a bad experience with a heavily peated dram or a substandard blend.
The sort of flavours you would expect from peat are medicinal and smokiness. To obtain peat flavour, the malted barley is dried using heat from peat during the killing stage of production.
Even Prince Charles cares about your peaty dram - Prince Charles stocking the peat kiln at Laphroaig. Image is taken from The Distiller Blog
The temperature and length of time the barley is kilned with peat will determine much of the intensity of the flavour; high temperatures reduce smokiness while wetter barley absorbs more of the flavours.
Peaty whisky is an acquired taste for sure but although Islay is famous for peat many other mainland distilleries create peaty releases from Glenturret, Macallan (although they are sold under different brand names) and Ben Nevis amongst others.
In fact, most distilleries that I approach for new spirit will provide quotes for peated new spirit too so we’re going to see much more of it coming from other locations in addition to Islay.
There are 6 main sites for peat in Scotland and it has been identified that each site has a different flavour characteristic. The reason for this is that peat is a decomposition of vegetable matter so if you imagine that each of these 6 peat sites had a different makeup of trees and vegetation then it makes sense that when this vegetation decomposed that the different plants, flowers and trees would have different flavour characteristics.
Peat PPM is generally acknowledged as the following:
Lighted peated malt is 1 – 5 PPM
Medium peated malt is 5-15 PPM
Heavily peated malt is 15-50 PPM
If you would like to learn more about peat and the different flavours from the different bogs then check out this blog about Peat Terroir.
Visiting a distillery can be great fun and viewing the stills is always a highlight because in your mind, or at least in my mind, you recognize this is where that distillery’s unique spirit is created.
Regardless, if folk suggest that it takes place during fermentation or it’s more to do with barley strain or to do with the casks – a great deal of responsibility lies on those elegant copper pots.
So how do they do it and what influences the spirit character?
First and foremost, the still, pipes, condensers and connecting pipework are made of copper. Copper is an incredible reactive metal which removes undesirable flavours such as sulphur while enhancing desirable compounds from the original barley – the amount of time the spirit is in contact with copper enhances these virtuous qualities, so it makes sense to want to have as much copper contact as possible. The relative amount of copper contact used in the process will determine the end spirit
Stills come under three broad shapes – Ball, Lampglass and Plain
Depending on the height and shape of the still will determine the reflux. Reflux is the condensation of liquid into vapour and its return to the boiling liquid; the effect of reflux is to increase the alcohol strength in the distillate, creating a purer distillate with greater separation of volatile components and the richer the spirit will be.
Therefore, longer and narrow necks with upwardly directly lyn arms lead to greater reflux. However, a longer and narrower still does not provide adequate copper contact so a balance needs to be struck.
The size of the still is very important; distilleries in Scotland can have anything from a 2,200 spirit still at Edradour to Glendiffich which has 10 spirit stills each with a capacity of 4,500 litres. The larger a still the less copper to spirit contact and so the meatier and more sulphur the end distillate.
The type of condensers will influence the character of the spirit. The condenser takes the refluxed/vaporized spirit and feeds it back into the boiling liquid. It can be in two forms; traditionally a worm tub has been used which is located on the outside of the distillery and is made up of a spiralling copper coil which the vapour passes through and is cooled by outside water which surrounds the copper pipes cooling it – the worm tub is meant to produce a meaty flavour.￼
The shell and tube heat exchangers are another type of condenser which is located in the still house and which cools the cooper pipers by passing water through vertical tubes. The copper in the still slowly degrades over time and will need to be replaced every 15 or so years; the age of still will have an influence on the quality of spirit.
As an independent bottler, cask management and a good wood policy are essential. It is the only way I can potentially add value because I can’t control the distillate.
It is a very important step in the production of good Scotch and 50-70% of the bottled spirits flavour will come from the cask. Cask maturation provides colour, it removes undesirable flavours and the spirit extracts desirable flavours from the oak. Oak contains many sugars! Each year we lose about 3% to evaporation (the Angel’s share) and each year the alcohol level will reduce and if it falls below 40% alcohol it cannot be called whisky.
In Scotland, around 90% of Scotch is matured in ex-bourbon Hogshead casks. Prior to this Sherry Butts were used but the industry changed to bourbon casks in the late 1930s when the US decided that bourbon casks could only be used for the maturation of bourbon once and so this provided the Scotch whisky industry with lots of cheap used oak casks to mature whisky.
Where bourbon matured whisky typically provides that light golden colour spirit with vanilla and coconut flavours, Sherry matured whisky is darker in colour, with more raisin and cherry flavours.
A cask can be used multiple times until it is exhausted which is usually after 40 years. However, the intensity of flavour reduces with each refill. First fill casks are highly desirable, but in my experience when using first fill Sherry casks, it is better to use them for finishing the first time as the oak contains so much sherry flavours that if left in there for a prolonged period of time the spirit can be overpowered. Wine and Port casks are particularly susceptible to overpowering spirit if not carefully managed.
The provenance of the actual oak used to make up a cask is proving to be an interesting field of research. It has been suggested that the fruity flavours derived from a sherry cask are not necessarily due to the fact that the cask previously contained sherry. The reason is also that European Oak is different from bourbon cask oak which is taken from the United States, using a different type of oak which possesses different flavours.
Last year I visited a bodega in Europe and it is very much like visiting a lumber yard in Scotland, which I did with my Dad as a kid because he was a joiner.
Essentially the bodega will allow the oak to dry outside which has been shown to produce greater colour in the spirit and make for higher quality Whiskies. A company will pay a premium for casks that are constructed from oak that has been left outside to be dried naturally.
Charring is a key process in the construction of a cask. Charring is the process of heating the inside of the cask to burn/char the inside layer, this allows the cask to absorb spirit while degrading a layer within the cask which allows for the release of lignin and thereby vanillin which is a vanilla flavour, the process also adds colour. Typically, a distillery can choose the amount of charring they want and the deeper the charring the more intense the colour. This is why bourbons achieve so much colour so soon as the barrels used are heavily charred as is custom in the US.
The size of a cask will determine the amount of impact the wood will have. The larger the cask the less spirit to wood contact and therefore the less extraction. For example, 50-litre Octaves enjoy a huge amount of wood to spirit contact which results in lots of flavour being transferred to the spirit in only a matter of months. However, the drawback is that too long in this cask and the wood can overpower the whisky. The alcohol strength at filling will impact the extraction of flavour from oak, if too high or low the extract level is very low. For this reason, whisky is usually filled into casks at 63% to 70%.
Generally speaking, the higher the outside temperature, the quicker the oak extraction. Fortunately, Scotland is the ideal temperature to mature whisky so that it is delicately balanced but if the temperature rose significantly then the spirit would retain lots of oak character while retaining the drawbacks associated with an immature spirit such as being strong, pungent and sulphury. In the US and India and other hot climates, the alcohol level will actually increase because the water evaporates quicker than the alcohol.