Oxidation. Oxidised. Oxidisation. Oxidise…..
- (Verb) To undergo a reaction in which electrons are lost by a molecule, atom, or ion to another matter or species
- (Whisky) A loose, general, catch-all term to describe the phenomenon of a whisky’s quality slowly deteriorating in the bottle over time, after you’ve opened it.
It’s one of the most common questions and concerns you see asked in whisky forums and discussion groups: “Once I open my bottle, how long have I got until it starts to go off or deteriorate?” Or it may take the form, “I have a bottle with just one third left in it, and I left it at the back of the cupboard for a year. Now it tastes different – what’s going on?” You don’t travel far along your whisky journey until the word “oxidation” enters your vocabulary.
The entire subject is clothed in a mix of mystique, truth, and opinion – often triggering debate and robust discussion. There are two key inherent problems when discussing oxidation: The first problem is that there is widespread confusion about oxidation and what it actually is. Scientifically, there’s a very valid argument that oxidation isn’t even the right word to describe what everyone’s discussing in the first place! (More on that in just a moment.) The second inherent problem is that oxidation is both a force for good and evil, and so there are differing opinions as to its impact and merits. There are plenty of whiskies that seemingly benefit from a small amount of oxidation when they’re first opened. However, the long-term impacts of oxidation can be disastrous, seriously destroying a dram’s quality and credentials.
For the purposes of this discussion and article, we’ll follow the word’s common usage amongst the whisky community and we’ll broadly accept and define oxidation as the changes that occur to a whisky after you open your bottle and then consume it over a prolonged time period. But be advised: What’s actually happening is more than oxidation in its true, scientific sense – there are other actions taking place that have fancy scientific names but the aim here is to keep this article simple and to fall in with the commonly held terminology.
We’ll also state up front that oxidation is a real phenomenon. The whisky community is a rich, broad, and diverse tapestry of people, but if you’re part of the community that denies or refutes oxidation and its negative impacts on the liquid, you’d do well to stop reading here and go join your chums over at the flat earthers society.
So what is oxidation in a whisky? Can whisky oxidise?
There are two things inside your bottle of whisky:
1. Whisky 2. Air
Like any domestic co-habitation, there are different dynamics and powerplays between our two roommates, and there are several factors that determine how they play and get along with each other. The most critical factor is obviously volume…the roommate with the most amount of real estate will govern the space and the relationship. When you first pop the cork on your whisky and pour the first few drams, the whisky is still the alpha in the household. Over time, that relationship and dynamic changes. With each dram poured out, more air is introduced into the bottle, and sooner or later, the ratio is such that there is more air in the bottle than whisky. The micro-environment has changed, and air now has the capability to dictate the relationship and to facilitate change to the whisky. Rightly or wrongly, we’ll use the widely-adopted term oxidation to describe what might now start happening to the whisky. (For readers currently screaming at their screens and shouting, “No, it’s not oxidation, it’s just evaporation or dissipation!” – relax, we’ll come to that in a moment).
What’s happening chemically?
The air trapped inside your bottle of whisky is not pure oxygen. It’s the same air we breathe, and it’s thus roughly 78% nitrogen, 21% oxygen, and the last 1% is a mix of some pure gases (e.g. argon, neon, helium, hydrogen) plus atmospheric passengers likes carbon dioxide.
This is where the science gets interesting: Does a chemical reaction take place and do those elements in the air pro-actively interface with and transform the whisky molecules? Is there a form of corrosion taking place? The accepted wisdom is yes, although such induced organoleptic change is very minor in the big picture. The exception is when it comes to sulphur. Many drinkers report that some whiskies develop a sulphury note when the bottle is consumed over a prolonged period of time. In such instances, it’s likely that organo-sulphur complexes in the spirit are being oxidised to SO2 over time. This is true oxidation in the scientific sense of the word. (It also implies that a reduction must also be occurring somewhere, but we’ll leave that science for another day.) But, again, this isn’t the big picture…
The bigger picture is that the whisky is simply “breathing” in the presence of air. And, as the fill level drops and more air is introduced into the bottle over time, the whisky’s potential to breathe and transform is increased. One of the things that’s now taking place is a slow but sustained evaporation of sorts. Aroma and flavour molecules simply dissipate from the liquid and are lost into the headspace within the bottle. The whisky is losing aroma and flavour molecules to the surrounding air.
Chemically speaking, the alcohols in your whisky are lighter and more volatile than the water they sit in, and these volatiles will evaporate/dissipate first and faster. Since it is the alcohols that carry the aroma/flavour molecules (Distillation 101), it stands to reason that the liquid’s flavour will change over time as these volatiles are lost. In addition to the shorter chain alcohols, the spirit will also lose the shorter chain esters, as these are also quite volatile. Esters are responsible for many of the fruity notes and aromas we detect in whisky. So…with aroma-carrying esters and flavour-carrying alcohols being lost from the spirit, this is one reason why “oxidised” whiskies are typically described as seeming flat, dull, bland, or having lost their lustre.
But it doesn’t stop there. Ethanol is hygroscopic, meaning that it will absorb water vapour directly from the atmosphere. So – in addition to the volatile ethanols leaving your whisky, the remaining ethanols will actually pull moisture out of the continually renewed headspace in the bottle each time you take the cork out and pour a dram. These two actions/forces both have the effect of reducing the spirit’s ABV as it sits in the bottle! Your whisky is simultaneously losing alcohol whilst also gaining water.
But wait, there’s more….
Further clouding the discussion, there are other influences and catalysts that can negatively impact an opened bottle during its long-term residency on your drinks shelf. These might include your local climate (i.e. prolonged hot daily temperatures, humidity, and condensation), direct sunlight exposure or just simple prolonged UV light from a lightbulb, and other external factors. (Tip: If you build or acquire a fancy drinks cupboard or shelving unit with built-in lighting, don’t put your good whiskies directly under or near the lightbulbs!) If your whisky seems worse for wear after sitting two-thirds empty on your shelf for 18 months, how do you definitively differentiate between the deterioration due to oxidation, and deterioration due to these other factors? The answer is, you can’t. But what you can conclude is that opened bottles of whisky will not remain in pristine condition indefinitely.
What about evaporation out of the bottle’s closure?
Cork stoppers or closures are not impermeable, and the reality is that evaporation (i.e. physical loss) does occur through the closure over time. It’s the bane of whisky collectors worldwide, and there is much scrutiny of older bottles at whisky auctions as the evaporative loss (ullage) is assessed and taken into account. Depending on the bottle’s storage conditions and the quality of the cork and its seal, fill levels of unopened bottles gradually go down over time. It thus stands to reason that a bottle that has been opened will be even more prone to evaporative losses through the closure since the seal/foil capsule has been broken and the cork loosened each time a dram was poured. Given that gas expands and contracts more than liquid does under temperature change, the expansion and contraction when there is more headspace in the bottle will be of far greater volume, pushing more of the volatiles out past the cork when it expands and pulling in more air when it contracts. If the room or place where you keep your whisky is subject to large ambient temperature differentials (e.g. summer versus winter), this is not good for the long-term storage of your bottles.
Evaporative losses through the closure are thus also clearly a contributor to a whisky’s quality deteriorating. It is one thing for the spirit’s volatiles to escape into the air trapped within the bottle, but if the bottle can actually breathe, then volatiles can be lost into the atmosphere and new air can enter the bottle. The process will thus be more pronounced and accelerated.
Can I prove or demonstrate oxidation?
Yes! And it’s remarkably simple: Simply pour a dram of your favourite whisky – one that you’re very familiar with – and then leave it out on the table, uncovered, and undisturbed for 24 hours. Come back to it after at least 24 hours and assess it. It will probably be a little cloudy. It will still smell familiar, but the aromas won’t shine with the vibrancy and clarity that you’re accustomed to. And it will taste….different. This, ladies and gents, is “oxidation”. (In the sense that the whisky community uses the term). There is also a more profound and long-term experiment you can do at home, but we’ve presented that further down as an Appendix, as it’s a bit more involved.
Is oxidation good or bad?
As mentioned earlier, it can be both. Some whiskies – particularly cask-strength releases with higher ABV’s – definitely seem to benefit initially from the introduction of air. Many whisky drinkers report opening a bottle and being underwhelmed by their first dram. They put it back on the shelf “to rest” and then return to it two or three weeks later, whereupon they find the whisky has dramatically improved. There’s now even a designated term for this practice, and so many drinkers subscribe to the notion of the “neck pour”. After breaking the seal when first opening a bottle, the neck pour is almost treated as a sacrifice to discard, and their enjoyment (and judgement) of the whisky commences two or three weeks later when they bring the bottle “back into service” on their drinks shelf! Of course, all that’s actually happening in reality is that the harsher volatiles in the spirit are being allowed to escape early.
However, the long term impacts of pronounced oxidation can be very deleterious on some whiskies. Whisky drinkers with a large number of open bottles often find that some whiskies get overlooked or forgotten about for long periods of time (i.e. a year or more), and if the level of whisky left in the bottle was already low, then the remaining spirit can be very flat, or subdued, or – in some cases – unpleasant to drink. In extreme cases – and many of us have encountered this – the whisky can be vile and undrinkable.
How long does it take for oxidation to be noticeable?
This is where the subject again gets cloudy, and for several reasons. Firstly, “noticeable” is a subjective quantity. Drinker A might detect changes in the whisky that Drinker B does not detect. (Or, possibly, Drinker B might simply not be fussed about any perceived change and dismiss the concept.)
Furthermore, what, and how much, is noticeable? Is it the whisky now seeming a little “flat”? If so, how do you quantify that? Is it not as bright or vibrant as you recall it being six months earlier when you last had a dram from the bottle? (As an aside, can you really trust your memory’s flavour recall on something you last tasted 6-12 months ago?) This is where having an unblemished reference dram comes in handy! What other forms might noticeable deterioration take? Is the whisky now somehow unpleasant? Is there an off-note or does it leave a bitter/sour aftertaste on the finish? Is the liquid now different in appearance? All of the above blemishes are possible, and their extent and severity will be a function of how long the bottle is left unfinished, and the ratio of air to whisky in the bottle. But it all comes down to your personal ability to detect the organoleptic changes.
So in terms of timeframes, is there a definitive guideline or rule of thumb we can hang our hat on? Perhaps. The legendary Jim McEwan, whilst on a promotional tour for Bruichladdich back in 2005, shared his opinion and recommendation that once a bottle’s fill level is down to half-empty, then you’ve got six months to finish the whisky before you might start to detect noticeable deterioration. Read that again, slowly, and note the subtle nuances of what is being asserted. Some might debate the merits of such a statement but, if it’s good enough for Jim, it’s good enough for Whisky & Wisdom. And I’ve preached and shared Jim’s tip at hundreds of tasting events I’ve presented at in the years since.
Are some whiskies more prone to oxidation than others?
In a word, yes. Intuition suggests that the whiskies that are more robust, bigger, and bolder will be better equipped to stand up to the air in the bottle. But that’s the wrong way to think about it, because it’s not about “standing up” to the air. Rather, the more robust whiskies will have other things going for them, such that the sustained loss of volatiles is less apparent to your nose and palate. Younger whiskies with more (and harsher) volatiles; some smoky whiskies; higher ABV or cask-strength whiskies; or those with pronounced cask influence (e.g. wine finishes, etc) will all have pronounced features and characteristics that better mask oxidation. Thus, it’s not necessarily a case of a whisky being better equipped to resist oxidation, it’s more a case of the whisky having enough tricks or complexity to disguise the impacts of oxidation.
On this basis, some might immediately assume that peated whiskies are best equipped to counter oxidation, since the peat, smoke, and medicinal characters will always be the dominant and obvious features on the nose and palate. However, there’s an irony here: The phenol chains and compounds in the whisky that carry the peat and smoke are not necessarily more volatile, but they do actually auto-oxidise and they’ll be amongst the first to be lost from the spirit. (This is a key reason why the perceptible peat and smoke in Islay whiskies reduces with time, i.e. why there are less phenols in an 18yo whisky then there were when it was a 10yo. It’s why young Islay whiskies are peatier and smokier than older Islay whiskies, regardless of which distillery you’re dealing with and their respective ppm levels of phenols.) To quote the chemistry textbook: “Phenols are easily oxidised, but their products are often complex. This oxidation may occur with air alone (auto-oxidation) or with other oxidising agents” (Reference here.)
Peated whiskies thus typically demonstrate and display higher extents of change or “drift” over time. (Drift describing how much an oxidised whisky changes and departs from its original condition and character.) Punters returning to a near-empty bottle of Islay malt after a year or two of neglect may smell some smoke on the whisky and assume it hasn’t oxidised, but they’ll get a shock if the dram is compared to a pristine reference sample.
Meanwhile, in contrast to the robust whiskies, some whiskies are particularly delicate and fragile and do not do well at masking the impacts of oxidation. These are typically older whiskies (e.g. 25-30 year age statements or more) or whiskies bottled at 40% ABV. Interestingly, sherry cask matured whiskies appear to suffer from oxidation more readily than bourbon-cask matured whiskies.
Allowed to oxidise, older whiskies typically display more pronounced change and deterioration in shorter time periods. Drink these quickly. Whilst it’s tempting to savour old/rare/expensive bottles and enjoy them over a prolonged time period, (i.e. saving it for special occasions only) the reality is that such an approach may not do the whisky any favours in the long run. If the bottle is opened and then sustained over a year or more, the drams you pour towards the end of the bottle will not be the same quality as the ones you paid for and enjoyed at the start of the bottle!
Finally, as mentioned higher up, true oxidation can occur if there are sulphur compounds in the spirit, as they can oxidise to SO2 over time, resulting in the whisky taking on a sulphury note as you slowly get to the bottom of your bottle. This implies that the distilleries that produce a sulphury newmake might be more prone to such unwanted change over time. Examples would include Dalwhinnie, Mortlach, Craigellachie, and Benrinnes and – perhaps – all the other distilleries that still use wormtub condensers. It also implies that any whisky with spirit in it that was matured in a sulphur-tainted sherry cask might also be more prone to this form of oxidation.
What can you do to combat oxidation?
As established at the start of this piece, oxidation is driven by the ratio of whisky to air in the bottle. So your primary weapon against oxidation is to simply maintain whisky as the dominant roommate in the house. Some people do this by transferring the whisky into increasingly smaller bottles as they work their way through their purchase. Another trick is to progressively fill the bottle with small glass beads to take up volume and push the liquid back up to the top, thereby reducing the air’s headspace. Others take a leaf out of the wine drinker’s playbook and pump argon gas into the bottle. Argon – an inert, noble gas – is heavier than air, and it settles on top of the whisky to form a barrier between the liquid and the air above.
Other tricks known to slow the onset of oxidation (or the evaporation side of things) are to store your whiskies at a very low temperature, i.e. in your refrigerator. (The same advice and trick is known amongst the cologne/perfume industry to keep their products intact). And, of course, many whisky drinkers use parafilm tape wrapped around the top of the bottle to slow or prevent evaporation losses through the bottle’s closure.
Whisky & Wisdom subscribes to and endorses the old school strategy combatting oxidation: Simply drink the contents of your bottle in a timely fashion before oxidation becomes an issue.
When whisky storage goes wrong…
As discussed above, there are other factors and influences that can contribute to or accelerate the deterioration of your opened bottle of whisky, leading to a pronounced perception that your whisky has oxidised. Much of this revolves around how your bottle is stored. So what happens if you store your whisky in a way that is contrary to the accepted guidelines and wisdom?
My whisky acquaintance and colleague, David Tjeder, wrote about this in a fascinating article published on ScotchWhisky.com in 2018. As reported, a Swedish whisky enthusiast decided to “punish” some bottles of whisky and deliberately subjected them to various long-term storage conditions that were designed to explore the negative outcomes of poor bottle storage. Whilst not specifically described as such, it was, in essence, an experiment that tested and explored oxidation.
The findings from the experiments were profound, telling, amusing…and conclusive! I commend the article to you, and if you’re genuinely interested in the long-term results of what we’re describing here as oxidation, you should read David’s article.
The final wrap
So…the answer to the question, “Does whisky go off in the bottle?” is yes. It’s just a matter of time. But time is relative, isn’t it? Every individual whisky and each individual drinker will have their own calibration and reference point to what constitutes “off”, and the timeframes involved are obviously highly variable for each circumstance, situation, fill level, and conditions. In short, there are a lot of different parameters at play. But these can all be nullified with a single masterstroke: Once you pop the cork, consume your whisky quickly.
Cheers,
AD
With thanks to my long-term whisky pal and collaborator, Matthew Fergusson-Stewart (Whisky Molecules) for adding valuable input.
PS…if you enjoyed this article, you might also like our other “explainer” articles:
The complete guide to peat and peated whisky
The whisky lover’s complete guide to sherry
The complete guide to oak, casks, & whisky maturation
The 1980’s heavy metal guide to whisky
The complete guide to non chill-filtered whisky
APPENDIX – A long-term experiment to demonstrate oxidation
Yours truly has undertaken some long-term tests with oxidation. I have now, on three separate occasions, investigated and experimented with long-term storage of opened bottles. If you have the time and inclination, you can try the following:
- Buy three bottles of whisky from the same supplier on the same day, ensuring they’re from the same production batch. (Thus ensuring they are essentially identical).
- Open the first bottle and pour out 50-100ml or so until the fill level sits below the bottle’s shoulder. Put the closure back on.
- Open the second bottle and pour out 80% of the contents, leaving just 20% in the bottle. Put the closure back on.
- Leave the third bottle unopened. This will be your reference bottle.
- Now set aside the three bottles somewhere for long-term storage. For one of my experiments, I left the bottles for two years; the time frame was 18 months for the other two undertakings.
- Once your pre-determined time-frame has elapsed (or your patience runs out and curiosity gets the better of you!), open the three bottles, pour a dram from all three and compare how they differ to one another.
I conducted the above on separate occasions between 2005 and 2013 with Glenlivet 12yo, Lagavulin 16yo, and Glenmorangie “The Original”. The results were consistent across all three tests: On each occasion, there was a marked, discernible difference in aroma, character, and the flavour between the three “identical” bottles. Both the Glenlivet 12yo and the Lagavulin 16yo displayed considerable “drift”, whereby the bottles with 20% left were remarkably different from their respective reference bottle. The Glenlivet 12yo (which was allowed to oxidise for two years), in particular, was unpleasant to drink, with a particularly bitter/sour note on the palate. And, confirming the theory discussed above, the Lagavulin 16yo, whilst still being smoky and medicinal, was weak, thin, and had developed a metallic soapiness. If you need convincing about oxidation, try this test for yourself!
Thanks AD on yet another excellent and informative article. What you’ve outlined makes logical sense and is backed up with both science and experience. Too right; revisit open bottles more often as the volume decreases and the moral of the story is: don’t have too many open at one time, unless you’re going to get through them in a timely fashion!
Andrew, as a preferer of sherry (and other fortified) matured and finished whiskies, from time to time I come across match-strike, sulphur taint or sulphur candle residue, or other kindred remnants of the transfer process of a cask from it’s origins to it’s next life in a distillery ‘somewhere’. You I know, have a razor-sharp sulphur canary resident. I have a good one too, but some occurrences/variants, I do not mind. What I have found, is that one can leave their dram in the glass uncovered for say 20-60mins and that volatile has combined with air or other molecules and become, softer, less pronounced OR has dissipated entirely. Maybe just once on my whisky journey have I found it become more pronounced. This ‘delayed-gratification’, should not be overlooked as a method of enhancing your appreciation and/or getting the most from your dram even in the unfortunate circumstance of sulphur-taint and the like. I would be grateful to hear your opinions of the affects of oxidization on sulphur-tainted whisky. Thanks.
The issue of the shelf life of the whiskies has of course also been on my mind for some time. In an initial period, I had the bottles half-full and less with the vacuum seal. This was until I had a visit from a friend who works in the whisky industry in Scotland. You’re sucking the flavours out of the bottle – was his horrified comment.
As a result, I started a long-term experiment in November 2015. Three identical 35cl bottles “filled” with 6-8cl Glenfiddich 12 each, plus a full small sample bottle and the original bottle. The three 35cl bottles were capped as follows:
. Cork stopper
. Vacuum
. Inert gas (argon) Filling and cork stopper .
The bottles were stored in a cellar at a practically constant temperature of approx. 18 degrees and shielded from light.
In February 2017, a first comparison was made with the following result:
. Original bottle, sample, inert gas and cork are roughly equivalent – with the inert gas, the alcohol appears more present and needs more time for the aromas to develop.
. Vacuum seems a bit chemical and bitter
Then rest in the 35 cl bottles closed again in the same way and continuation of the experiment.
Next comparison in February 2019:
. Original bottle performs best
. Cork closure takes some time but appears closest to original bottle.
. Sample a little more sweet and a little less fresh
. Argon gas needs more time to develop and then comes close to version with cork stopper – but a little less brilliance.
. Vacuum closure dull and undrinkable
I have been running another long term test since June 2019 with the same experimental set up but with a cask strength whisky. No comparison created yet.
I have around 200 bottles open, so I am always approaching people on this subject. Some time ago I had a conversation with a bar owner who has a few hundred whisky bottles in the bar, most of them old and rare bottlings that he has had open for some time. His approach to the question of shelf life: remove the cork once or twice a year, let some air in and close it again !
I have only experienced a real change in the taste of a whisky once. This was with a 70 cl bottle, which I deliberately left for about two years with about 5-6 cl of whisky in it. After this time, the whisky was dull.
In a nutshell: I am very sceptical about the vacuum seal. Inert gas is one possibility, but I wouldn’t promote it as a long-term method – you have to refill with inert gas after each consumption anyway. And argon cartridges are not very cheap. A well-sealing cork and away from heat and light is best in my view – and enjoy a sip now and then !
And: at best fill into smaller bottles – usually not very sexy – but if the content is more important than the appearance…
Translated with http://www.DeepL.com/Translator (free version)
It’s a great article!
My one issue with it is the use of the term Oxidation. I think mentioning in passing that “Oxidation” is not oxidation is not enough.
As oxidation is a chemical reaction of a very specific nature, the use of it for other phenomena is wrong and misleading, and this was an opportunity to help eradicating this wrongful term, which, instead, perpetuated it.
Great article, really thorough. Thanks for sharing, Ed