In almost every legal definition of whisky, there are two factors—the first is a strict control on what one can and can’t do to the whisky from fermentation to bottling. Secondly, there’s the essence of what makes whisky: the law simply states “it must taste like the attributes generally associated with whisky.”
Home maturation experiments are an excellent way to learn about flavour evolution and balance. However, there are things that home maturation experimentations can’t simulate. I’m not an expert on this topic (besides trying it once), but I do ask a lot of questions from people who are. Here’s what I’ve learned:
Barrels as Semipermeable Membranes
The barrel itself is a semipermeable membrane. Warehouses where whisky matures have their own environment that interacts with these barrels. The best proof of this is simply looking at alcohol vs water evaporation. Alcohol is a more volatile molecule, and given most environmental conditions, it will evaporate at a faster rate than water. However, that’s not always the case. In Kentucky, unlike in Scotland, the whisky is bottled at a lower percentage and the climate is dryer. Under these conditions, water evaporates at a faster rate.
Kentucky warehouses are dry, especially on the upper levels. In areas like the caribbean where rum is commonly produced, the humid air keeps the evaporation of water and alcohol at a similar rate. In Scotland, alcohol evaporates first. Scottish air is wet, and the seasonal changes are not as dramatic as compared to Kentucky. In Scotland, the older the whisky, the less alcohol you’ll find in the barrel.
As Nicolas Villalon (brand ambassador for Edrington Group and microbiologist) told me, oak does act as a semipermeable membrane through the staves, and therefore filtration of the molecule does occur based on the molecules properties and the climate outside the barrel. There are many unknowns to this process, but some conclusions can be reached as to when alcohol concentration can increase inside the barrel:
- High Heat
- Low humidity
- Unsoaked barrels (virgin oak)
- High-Proof (High enough to breakdown and dissolve the natural wood sugars)
- Not too high (greater alcohol content will make ethanol evaporation more likely before water)
The charring of the barrel has a great effect on the flavour of the whisky. This part can be easily reproduced by using wood chips or smaller charred barrels for the home. However, these larger environmental factors are impossible to reproduce in glass jars, and difficult to control during the barrel maturation process in the home environment.
For further reading, Michael Veach has an excellent article about “What Happens To Whisky In The Barrel” that goes into greater detail about wood extraction and larger environmental factors. In home experiments the increasing and decreasing of a decanter with whisky won't replicate the pressure achieved from these larger environmental factors. Another idea often tried at home is placing the jar of oak chips on the dryer to shake more flavour from the chips. If we've learned anything from sending whisky into space, there's likely going to be some further flavour extraction from the oak, but it's not likely to produce a fine whisky.
Balancing Oxidation and Oak Extraction
There’s another component to barrel maturation that’s missing in the home experiment. It has to do with oxidization. Oxidation breaks down complex unpleasant flavours and big alcohol molecules into smaller, more palatable molecules. Briefly aged whisky tastes like bark, and while charring helps by (essentially) caramelizing vanillin found in oak, it’s not enough.
Studies have shown that it takes about six months before the complex sugars from the oak start to break down and smoothen out to flavours that we generally like. Smaller barrels, however, extract flavour from the oak at a faster rate because there’s more surface area to interact with the whisky. In the situation of using smaller barrels, you have wood extraction going at a faster rate without the balance of oxidization that we’re accustomed to from industrial-sized whisky barrels.
That’s why, with at-home barrel maturation, it’s recommended that one oxidizes the whisky outside of the wood chips to help breaking down the molecules further. It’s a good strategy. It also reduces the alcohol content. If you’re doing this right, you’ve likely already filled the barrels with higher-proof white dog, so a little reduction in alcohol is all right.
Barrel maturation at home is a good experiment to perform. I’ve done it, and there are entire groups dedicated to this process. Like with any craft, it’s not likely to be successful on your first go, and it will require frequent tastings to see how the product is maturing. There are, however, larger elements at play that are difficult to time and simulate in a home environment.
For these reasons, while obtaining a dark spirit is easy, getting to the complex essence of matured whisky in home experiments is much more difficult.
Many thanks to Nicolas Villalon, Stuart MacPherson (Master of Wood for Macallan), and Ian Mccallum (former Master Blender for Bowmore) for checking over my 'science' bits.
This is a great resource if you’d like to start on a whisky aging experiment at home: http://www.whiskyhead.com/whisky-aging.htm