Inert Gas and Whisky OxidisationInert gas, such as argon, can be used to extend the life of opened whisky bottles considerably. Ultimately these seem to be the Royce Rolls of whisky storage solutions, albiet with a sizable caveat. Our own experiments could find no measurable difference between a whiskies which had been stored over a 1 year period in small filled bottles, and those which had been preserved using noble gasses.
The Long Read
Unlike wine whisky doesn’t continue to mature in the bottle once opened, unfortunately the chemical transformations don’t stop entirely. While the change much slower than with wine, Oxidisation will inevitably change the taste of the spirit over the course of months and years. This can be a net benefit, with oxidisation improving younger whiskies, rounding out deficiencies just as it did to the whisky maturing within the cask. Unfortunately it can also be very detrimental. The surface area of the whisky and fill level of the bottle are the main determining factors in how quickly whisky will spoil. Whisky filled to the neck will oxidise far more slowly than one over half empty. Decanting into smaller bottles and the use of vacuum pumps are common solutions to this, less commonly marbles are sometimes used to displace oxygen content. The gold standard is the use of inert gas, however this comes with an unfortunate price tag.
Why Does Whisky Oxidise?
Whisky oxidises because of chemical reactions that occur when the liquid comes into contact with oxygen. This happens throughout the whiskies time in cask, and is responsible for both additive maturation (the creation of new flavour chemical) and subtractive maturation (the removal of unpleasant flavours such as sulphur).
What is Whisky Oxidisation?
Oxidation refers to the chemical reaction that strips electrons from the whisky. Because any loss of electrons by one substance must be accompanied by a gain in electrons by something else, oxidation and reduction always occur together. As it is the whisky that is reduced the result is the loss of aromas and flavours, general flattening and a the whisky paling over time.
How Does Oxidisation Change Whisky
The reaction of oxygen in the air with the alcohol (ethanol) produces acetaldehyde, later this process forms acetic acid, the acid in vinegar. Happily this process takes place on an incredibly small and slow scale, the effect cannot be smelled or tasted immediately but the gradual build up will ultimately spoil the whisky.
When whisky is exposed to air, the oxygen molecules combine with the hydrogen of the alcohol leaving behind aldehydes. Acetaldehyde, which is coincidentally one of the primary causes of hangovers, is one of the most common aldehydes and gives whisky a tart, slightly sour apple flavour. This process will continue until it is not possible for any more hydrogen to be liberated and a new additive stage of oxidisation begins. The combining of oxygen atoms with aldehyde molecules creates acetic acid, more commonly known as malt vinegar.
The biggest challenge is that every time a bottle is opened and poured the headspace is replaced with fresh oxygen. A highly reactive element that changes the whisky little by little. A number of solutions exist:
- The ratio can be reduced via the use of smaller bottles, though these don’t look anywhere as impressive
- The bulk of the air can be pumped out, though this requires a large number of caps
- The bottle can be filled with marbles to displace the air. This unfortunately makes them rather heavy
- An inert gas can create a blanket between the oxegyn and the whisky
The periodic table identifies Helium, Neon, Argon, Krypton, Xenon and Radon as noble or inert gases. These ‘group 18’ elements all occur in a free state in the atmosphere and are generally considered inert. In reality the noble gases are not truly inert (non-reactive), Xenon tetrafluoride (XeF4) for example is produced by the chemical reaction of xenon with fluorine. Regardless the terms inert gas is often used interchangeably with noble gas, and since noble gases undergo very few chemical reactions these can in principle be considered to be inert under most conditions.
Molecular Weight & Density
If you’ve ever heard of a Zeppelin or a blimp then you know that some gases are heavier than others, put simple these dirigibles leverage the fact that their contents weigh less than the air around them to achieve lift. The same process can be used to create a seperative barrier or carpet between a whisky and the air above it.
The density of gas is proportional to its molecular weight, as air has an average molecular weight of approximately 29 g/mol any gas with a molecular weight greater than this will be denser than air. Unfortunately while most gases are heavies than air, they are also chemically reactive and not all inert gases are heavier than air. Helium and Neon 2 of the noble gases are on the list of 13 lighter gases and thus unsuitable. Radon being radioactive is likewise not an option. Krypton and Xenon are notably expensive. Thus we are left with only Argon.
Carbon Dioxide and Nitrogen and Argon
In addition to the noble gases two others are generally used as inert gases;
- Nitrogen is abundant and very cheap and shares an incredibly strong triple bond between two nitrogen atoms. The prohibitive energy required to break nitrogen apart effectively render it inert, at least so far as spirit preservation requires
- Carbon dioxide, commonly called CO2 is a carbon atom bonded covalently with two oxygen atoms. This tri-molecular gas seldom reacts with other substances so again is widely treated as an inert gas
Purified argon and nitrogen gases are most commonly used as inert gases due to their high natural abundance (78.3% N2, 1% Ar in air) and low relative cost.
Poorly Understood Chemistry
The chemistry of whisky oxidisation is not well understood among the wider population. A number of forum, english and non-english content unfortunately fall victim to the Dihydrogen monoxide parody (literally “two hydrogen, one oxygen”) with satements such as: “CO2 reacts with dihydrogen monoxide in whisky creating carbonic acid” or “CO2 permeate the whisky creating carbonic acid over time”. These three gases are as safe within whisky as they are within wine.
Many inert gas sprays, such as Private Preserve, are a combination of all three potential gases, others such as Preservintage, ArT & Winaro (pure Argon) or WineKeeper (pure nitrogen), we’ve been unable to find pure CO2 systems. These have been used over numerous bottles and using new bottles (purchased at the same time, though without guarantee of similarity in some cases). Single cask whisky, single grain and generic bourbon were all compared.
It’s important to note ahead of time that the sample size used and the results described below do not and would not meet the requirements for scientific publication. Nor were they intended to be. The participants were not aware of what they were drinking (blind), the testers however were (the experiment was not double blind), the results while they may be interesting should not be considered conclusive. Nor did they support the findings of Marcus Fan however with the above limitations understood this should not be considered to contradict the previous study.
Given Standard Temperature and Pressure (STP) we can measure the density (and thus weight) of a gas using its Gram per litre (g/L). Based on this we would expect CO2, being heavier to perform better than Argon & Nitrogen given their respective weights. We would likewise expect neccesary periodic movement (to prevent corks decay) to hinder those whiskies in larger bottles at expense of those in smaller containers.
Inert Gas Findings
All systems used demonstrated, in the main, some improvement over whiskies sat oxidising in largely empty bottles. No detrimental effect was found from the use of Private Preserve, Preservintage, or WineKeeper. Preservintage and WineKeeper performed marginally better than Private Preserve on both the single cask peated and sherry bottlings, no meaningful difference was found on single grain, or bourbon. Unsurprisingly the smaller bottles (requiring less turning and offering far smaller surface areas) performed better than any gas based solution.
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