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Explained: Whisky cask ageing, how it works and influence on flavour

Explained: Whisky cask ageing, how it works and influence on flavour

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In our latest video Charlie Pountney talks to Richard Bayles "The Wood Expert" at Charles Edge to how cask ageing works. click on the image above to watch the video.

In the whisky industry everyone talks about water, size of stills, yeast and length of fermentation less about the casks themselves which the whisky is aged. It is estimated that around 60-80% of flavour comes from the cask maturation process so the quality and types of casks are vitally important.

By tasting through a few different whisky finishes, this video covers how cask ageing actually works and what flavours we can expect from different casks. This will help you identify flavours you might enjoy when looking to purchase a whisky and give you an insight to some of the tasting notes you might discover.


You can read the video Transcript below:

Charlie Pountney (CP): Hello and thanks for joining us at the next episode of The Edge on Whisky.  Today we are talking about whisky casks, why are they important. I'm joined by my colleague Richard Bayles.  We've got a little selection of whiskys that have all been aged in different casks so we'll be tasting a few of those as we go along. We’re going to discuss types of

casks and how they influence the flavour.


What we are hoping to get across what to look for on a label for instance that will indicate what you might like because there's a lot of choice and lots of different casks, different terms to describe them as well.


Richard, please give us a bit of your background….


Richard Bayles (RB): Hi,  pleased to be here. Well, my background, I started in Wines and Spirits at the age of 23. I spent about 10 years in the flavour industry and commodity ingredients trading.  So I worked a lot with flavour chemists understanding chemistry of flavour as well as sourcing all the ingredients and extracts such as vanilla, quinine etc.   Then I joined Ethimex, a partner company of Charles Edge London.  I'm the expert in casks apparently(!) but I have spent 10 years sourcing.

And making decisions about casks for Charles Edge London.


CP: Why are casks so important Rich?


RB: Well, they're critically important. First of all whisky wouldn't be whisky without a cask, by law.

Whisky in European Law and therefore also the Scots regulations, Scotch must be aged for a minimum number of three years in Oak Beyond that, you know it's important that the initial the initial distilled spirit is of excellent quality because a good cast cannot change the fact that you've made a bad spirit.  Casks do have a huge influence on the flavour of a finished spirit.


CP: Isn’t it something like 60 to 80% of flavour comes from the cask?


RB: Yes, it’s a figure that I've heard…. it's a bit of a factoid though.  It's one of those factors that goes around and has been going around for years.   I’m not saying it's not entirely true, but it's often misrepresented as coming from the cask.  I would say maybe 70-80% 8 of the flavour comes from cask maturation.  That’s the distinction.  Cask maturation is the resting, breathing and maturing in oak and necessarily reactions that are coming from the wood, or compounds that are coming from the wood. 


CP: If we are talking about ageing whisky in a cask what does that actually mean, and can you explain you how it works a little bit?


RB: In the context of spirits whisky, especially is cask is a wooden vessel. It is a generic word meaning any size of a container wooden container. In America they call casks ‘barrels’ whereas in the UK if you want to be a bit pedantic, a barrel is a specific size which is about 200 litres. In general parlance a lot of people say barrel instead of cask, but let's not be too pedantic. 


Casks are made of oak, one of the very few hardwoods which suitable for aging whisky because it’s porous, watertight and spirit tight.  It’s also easy to manipulate into your cask so the quality makes it almost the ideal material. And I haven't even touched upon the actual qualities of the wood itself.


Nowadays the flavours you get from oak maturation for cognac, rum or whisky are very complementary to the spirit. The reality is whisky, and all other spirits and wines have always been put in casks and so now these flavours are what we associate with the spirits themselves. 


CP: You touched a bit on the different sizes and types of casks?


RB: The most common Cask in whisky in Scotland would be the hogshead and also potentially the ASB which is the American Standard Barrel.  Hogsheads are 250 litres and a Barrel is 200 litres. They're all actually made, strange enough, from ASB barrels…to make a hogshead you take four barrels to the cooper in Scotland and they break all the staves down and basically make they use four bourbon barrels to make three 250l hogsheads.


The Scots believe the 250l hogshead is the optimum size for maturing whisky.  It has a good barrel to liquid ratio.  A small Barrel would mean a higher wood to liquid ratio as there would be more surface area touching the spirit so you would get too much wood influence. 


For those that don’t know, most barrels using Scotch ageing come from the USA and they've been used already for whisky making just once because the law in in the USA to produce bourbon and Rye is that you must use a new, charred cask.  And it can only be used once. 


This came in around the 1940s, around the war, they brought in the regulation in America saying you had to use a barrel just once for bourbon so suddenly there was this this huge availability in bourbon casks for the Scotch whisky market.


CP: We're drinking this very lovely Deanston, and the reason why we picked this one is because it's got very interesting cask that it's been aged in, which is tequila casks.  Let's have a little chat about how the different casks and can influence flavour and this is maybe a good time to talk about the ageing process. 


RB: There’s ageing and finishing.  Ageing is just resting something in in a barrel for a particular time in a cask, this can be for a many number of years.  Then there is finishing which is a shorter maturation which follows the longer period of ageing.  It's a relatively new concept, maybe 20 or 30 years old which has become quite popular recently.  I think it was Bill Langston was one of the proponents or this process.  So the concept is you've got a whisky that's been aging full term either in one cast or maybe a couple of different casts which could be for 8,12,15 years and people just want to give it a little Pizzazz at the end of its life to introduce some different flavours maybe. Liven it up a bit a little.  Let's not forget the marketing angle it's uh people want to have something exciting to learn something, a different expression.  So there's been a huge increase in cask finishing over the last 10 years maybe 15 20 years.


CP:  it's a kind of manipulation of flavour if you like.  Right now, is a good time to ask the question so actually how a high level does this period of maturation in in barrels how does that actually affect Flavour? How does it work?


RB: That's a big question yeah um well I mean let's keep it as simple as we can as you can imagine.  It’s what you would call extractive processes. The hydro ethanolic liquid, which is basically whisky because it's partially ethyl alcohol and partially water, along with a few other good, tasty impurities is a kind of solvent which draws extracts out of the cask.  This happens under pressure generally through absorption / soaking to the staves. Through chemical reactions,

A lot of great flavour components that wouldn't have existed in the original distillate are extracted from the wood. 


CP: It's a bit like if you had a sponge and you poured a load of water on it, it kind of seeps in and then when you squeeze it goes out… but obviously you're not squeezing about it just happens on a natural daily basis with fluctuations in temperature of the casks?


RB: Yeah. I mean a cask although it's described as porous relatively little oxygen gets into a cask so it's a very gradual process.  A cask is almost hermetic and therefore pressure can go up and down within the cask.  If you can imagine it could just let air in and out so easily there would be no pressure build up in the cask, so it forces the liquid in and out of the staves on a daily cycle.


CP: It’s perfect makeup that it allows the water and the liquid to get in a little bit but not escape through the outside of the barrel totally by pouring all over the floor!


RB: Exactly it's amazing that wood actually and it's important to probably talk about the inside of the cask just briefly. You have the charr on the inside of the barrel that brings a different flavour compared to a uncharred virgin cask.


We've got to remember these barrels have been used once in Kentucky for example so during that three to five year process a lot of the wood extractors have already been pulled out before it ends up in Scotland. 


CP: The other thing is the oxygen can go in and out of the cask?


RB: Yeah, and just the oxidization is an important part.  Oxygenation of the liquid the heads of the cask and directly into the liquid uh creates oxidation reactions.   There will be for example acetic acid which is sort sounds awful but in relatively undetectable quantity it's fruitier unless it gets to a high level. Oxygenation gives a real boost to the ester profile through oxidation. 


CP: And all this helps to round out the flavour of course.  If you try a young whisky, it will be a lot harsher than one which has been aged.  In new casks there will be a lot more tannins drawn from oak compared to used casks.  It will have more of a clawing, astringent feeling on the back of the tongue. 


RB: The interesting thing about tannins is with oxygen coming into the casks they form longer chain tannins which you can't detect on your palate… so creating longer bigger tannins is actually the way to get rid of tannins in a sense!


OK so we’re going to try this Wormtub 10 year old sherry cask finish now….


This has been aged in a 500 litre Bodega Cask from Spain. Most Sherry casks nowadays are new and seasoned with Sherry in Spain purely for the purposes of making whisky. Traditionally sherry casks had no wood influence, they would be inert as they would have filled the cask with some Sherry or wine to draw out the tannins for a couple of years before ageing.  They didn’t want wood influencer in sherry… no vanilla no spices.  But what they do nowadays is they follow a similar process of seasoning the butt’s for maybe 18 months two years sometimes less prior to sending to Scotland. 


CP: What do they do with this Sherry then after it's been in the wood?


RB: It's a very good question because it actually called ‘sacrificial Sherry’, it goes round and round maybe two or three times four or five times and at that point it goes into the Sherry vinegar industry


Previous content of casks is very important for colour as well. The interesting thing about bourbon is that the whisky has drawn a lot of colour from the wood as it is extracted from the wood. Then in Scotch you’re drawing the bourbon whisky out of the staves because there's what we call ‘in drink’ where a tiny bit of a tiny bit of the whisky remains in the barrel.  They estimate four, five or six litres in the barrels.


CP: To what extend can you really identify that bourbon influence?


RB: It's just taken for granted that's all just in the cask influence but when you start using other casks with different prior contents you get a much more noticeable effect.  This Wormtub has been aged most likely ex-bourbon to start with… it says it's a 10 year old so for probably for eight nine yeah most likely ten years and then finished for a short while in in some oloroso casks.


The colour is fantastic! If they do say Sherry, it is usually oloroso casks.  There are certainly five key types of sherry and I'm fortunate enough to go there quite often and you can't help from being drawn into the Sherry culture!


The most commonly sherry drunk in Spain is Fino (also called) which is a white sherry wine, Manzanilla, you have Palo Cortado, Amontillado and at the other end of the scale is the highly oxidized and dry oloroso, which literally means “smelly” but that's not a good translation it probably means aromatic or scented.  Then there is an outlier but still classed as a sherry wine which is Pedro Ximenez or PX.


PX is totally different in a way because it’s extremely sweet, extremely thick, viscous but it has an important part to play in sherry blending. 


CP: So, the two main sherry casks we would tend to see in whisky finishing would be oloroso and PX?


RB: Oloroso is the most widely available and probably the most impactful of adding character during long-term maturation, adding that superb character of dried fruits. 


CP: People describe like Christmas pudding, lots of dry fruits, sticky, caramelly…


RB: Totally um it is and it's interesting how it affects whisky because if you taste Sherry on its own it does have those flavours but of course there's nothing sticky sweet about it.


CP: Now it’s the perfect time to talk about regulations because the Scottish Whisky Association control what you can age whisky in, in order to call it Scotch. You can't just chuck new make spirit in any old barrel for three years and say yes this is Scotch.  This is a good thing.  But what they have done recently was relax the restrictions a little bit to allow a little bit more experimentation and a little bit more diversity in what type of casks you can age whisky in. 


This has meant we're getting a little bit more experimentation with the likes of tequila casks.


RB: The Scotch Whisky Technical File (SWTF) is basically our submission to the European Commission to basically say we want GI (Geographical Indication) protection.  In order to achieve that you have to submit technical files which say this is how we make whisky, and this is what we're trying to protect. 


So, with regard to casks there's a section that says you can use almost any cask so long as it’s a cask that's been used previously in a traditional process.  This was to prevent people doing weird things like seasoning barrels with chili peppers.  So, the restrictions are about maintaining traditions and using traditional barrels so for example Cognac, Armagnac, sherry, Madeira, masala, rum, bourbon etc.


CP: This is Infrequent Flyers, a Craigellachie independent bottler from Scotland, it's a Madeira Hogshead finish, so can you talk us through some Madeira fortified wine fishiness?


RB: There are four styles, or regions where fortified wines are made.  The classic styles are sherry (Jerez), Port, Marsala (one of my favourites from Sicily) which is possibly the one with the most similarities to Sherry but distinct from his own right, and then finally but not least is Madeira made on the island of Madeira, the Portuguese Island.


I tend to get more flavours of figs and dark fruit, but not quite the same as you're getting from sherry. 

It's an oxidized style of wine, it's oxidized through high temperatures, so you do get stewed and baked flavours from it.   You might pick up some stone fruit I don't put words in your mouth!


CP: How long does Madeira age for in casks?


RB: It could be young, two or three years but sometimes up to typically eight or nine years when you're getting into a different kind of Madeira, and sometimes a hell of a lot longer.


Are you getting any figs or marmalade?


CP: I'm not getting marmalade I'm getting more of the prunes


RB: If you tasted the actual Madeira which is always a good thing, you probably would get that sweet marmalade influence.


CP: On the nose you can clearly get the Madeira influence from it, its rich on the palette. 


RB: I'm getting a slight mustiness from the cask but in moderation I like it.  It's not a fault don't get me wrong but if it was in a higher concentration, it could be heading in that direction.


CP: How are distilleries sourcing these casks?


RB: Strangely enough smaller distilleries are more likely to go direct to bodegas in my experience.  Sometimes they'll have success and sometimes they'll find it extremely difficult.  But these are the kind of guys that are spending large amounts of money on one or two pallets of casks. 


The larger distilleries are typically too big to muck around doing little sourcing missions so

they'll come through people like me.  We move tens of thousands of barrels around the world now because we are known as the specialists, We go out there to source them and because our sister company is a major player in the realm of bulk Spirits we've worked with a lot of distilleries

and wineries to supply them alcohol or buying their Spirits.  That gives us that sort of access and relationships to be able to pop down and select some their finest fast casks which very few companies have got that luxury. That’s why a lot of the multinationals as well as medium-sized Scottish distillers rely on us for what we call speciality casks.


You do get some guys jumping in their cars and going down to the bodegas and buying two casks, this is great stuff but it's not a sustainable, scalable model.


CP:  I'm really enjoying this Madeira finish.


RB: It’s so well integrated yeah.


CP: So our last one is Pedro Ximenez we've saved this one to last um because of the richness that

yeah hopefully going to take from it.  It looks bold and dramatic! This is Old Perth which is a brand from Morrison Distillers using Pedro Ximenez casks. 


Whilst we're pouring this, we can talk about what to look for on a label when we're talking about casks.  How do you know what's good, how do you know what to buy? There are key things to think about such as finishing, obviously the type of cask is important and the size as well.  There is information which we can get from the bottle which will indicate to us whether that's going to be interesting to us and whether we'll like it or not. 



Well, if you're lucky enough to get a single independent bottling that gives you all that information then, great.  Much of the time you won't get all that information I can give you a little bit of a steer on certain things to avoid and look for.  First of all, I mean there are some good exceptions I'm trying to think of which ones like the Laphroaig Quarter Cask.  This is a quarter the size of a sherry butt (500L) and since in Scotland are casks are loosely based around the butt you have a Hogshead (half a but so 250L) and the quarter cask is half a Hogshead, or a quarter of a Sherry butt.  So its roughly 125 litres but not precisely.  It's anything between sort of 100 130 litres. 


Something to be aware of though is normally if it's a craft bottling, and not to be rude, I'd avoid these smaller casks as they're often out of balance in my opinion.  I've tasted a few of them, with too much wood influence coming through.  But the Laphroaig Quarter Cask was excellent.


CP: I like the Tobermory Ledaig peated whiskies finished with red wine casks which are super juicy with really punchy berry flavours.  They are quite young, smooth, and bright. 


RB:  I'm a big fan of full-term maturation sherry.  If you put a good quality whisky in a sherry cask whether it's a Hogshead or even a blend of the two you can’t go wrong.


CP:  We've got a 12 year old sherry butt single cask Bunnahabhain bottling which is completely aged in a sherry butt and it's just dark, gorgeous and delicious.  So, you're looking for maybe full term sherry maturation?


RB: Ah it's a personal thing I suppose but you can't really go wrong.  There are some great examples like the famous Macallan or Arran… they've brought out a great sherry finish I tasted they are not a huge producer but they do some very good full-time sherry maturation whiskies.


CP: Maybe we can't give the answers here because it's very dependent on what you like isn’t it.  We've tried to talk about the flavour influence of casks, but everything is going to have some degree of bourbon influence. 


So we're looking at finishes and then working out what you like really isn't it? Do you like more fortified wine finishes? We've talked about Madeira being plummy, the tequila that we had was lighter and slightly grassier.  Sherry finishes are quite sticky sweet you know Christmas cake….


RB:  Yeah I think you're right, this is personal taste I mean it's all about personal taste but my advice would be go out and get a Madeira finish, a sherry finish and explore.  Or you could go down the road into peated whisky and start exploring in that direction.  If you don’t like peat, then definitely expand your horizons by switching it up with different cask finishes.


CP: Very good okay we've come to the end I think! I really hope we've given you some interesting facts information on barrels.  Rich, as you can probably see is a font of knowledge when it comes to casks and barrels. Please give us a like and subscribe to our channel and we'll hope to see you on the next on the next podcast.

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