The Chemistry of Adding Water to Whisky
It’s one of those recuring questions during bursts of casual confabulation with random whisky dabblers. Do you add water to your whisky? The general consensus is that a dash of the finest mountain spring juice can, on occasion, make whisky taste better. There’s less burning sensation, the aromas seem to change too, and it makes one look like one knows what one’s doing!
The chemistry behind this phenomenon is somewhat shrouded in tradition, assumptions, and anecdotal evidence, however. Once we examine whisky molecules with microscopes and stethoscopes, things are not as one would expect. Bizarre occurrences happen. Strange things. Things that will change your perspective of savouring, interrogating, or analysing a dram or two.
So with pipettes at the ready, let’s drop in.
Love water – hate water
When admiring the amber shades of a whisky glass one can get lost in the way the liquid sticks to the sides as the spirit is swirled. It’s delightful. The colours themselves draw us in with a sense of excited anticipation. It’s easy to assume that the whisky before us is a complete mixture. The result of a long journey that’s created a tipple to be praised, prized, and appreciated. But in fact that whisky in your glass is really quite incomplete.
What do I mean?
As it turns out, water and alcohol, such as ethanol, have a peculiar relationship. From a distance they appear to marry together reasonably well. It’s not like we have to shake the bottle to wake the drink. But upon very close examination, strange things occur which are stepping stones to understanding what happens when we add water to whisky.
When mixing simple alcohols like ethanol with water, the alcohol and water molecules do not mix uniformly. Instead, the alcohol molecules tend to cluster together, as do the water molecules. The different molecules connect, but not in the same way that others do. In other words, the mixture is incomplete.
Molecules that love water are called hydrophilic, and molecules that hate water are called hydrophobic. Ethanol is both at the same time. Crazy right? This is because ethanol is an amphipathic molecule, meaning it has both water-loving and water-hating parts to its structure. On the one hand it likes water, but on the other hand it doesn’t.
The findings from this 2002 study in Nature show that in a mixture of water and alcohol, most water molecules form tiny clusters, and the alcohol molecules remain tightly packed together in their own little gangs too. But the water clusters link the alcohol clusters together through hydrogen bonds. In simple terms, ethanol-water mixtures have tiny, distinct regions where either water or ethanol are more prevalent, rather than being perfectly mixed at the molecular level. Why does this matter?
Firstly we need to get a handle on a couple of terms - the interfaces and phases of mixtures.
A journey to the centre of the dram
Understanding the difference between the interface and the phases of a dram will help to clarify what happens when water is added to whisky. The interface is the boundary where a mixture meets something such as air, another liquid, or a solid, such as glass. So for example, in your dram of whisky, the interface is where the whisky meets the glass or the air above its surface. It’s important because it’s at the surface interface that aroma molecules are released into the air.
A phase is a region of space within a mixture where all physical and chemical properties are uniform. For example, in a mixture of oil and water which do not mix well, the oil represents one phase and the water another phase. Even though both are in the liquid state. The composition of the oil is consistent, and the composition of the water is consistent. But as a mixture they have distinct chemical compositions, or phases, which can be seen as the oil floats on the surface of the water.
Within ethanol-water mixtures such as whisky we have something called the bulk phase. The bulk phase is the larger, internal part of the mixture, where there is no interaction with the interface or exterior bit. Remember that in such mixtures the ethanol and water molecules like to hang about in their own groups on an extremely tiny level.
The fascinating part is that as the concentration of ethanol in the mixture changes, it also impacts where the ethanol molecules like to hang out. Either at the interface, or at the bulk phase.
Molecules that gather at the surface
This 2017 study demonstrated how at higher concentrations of ethanol (i.e. cask strength whisky at 59% abv), the ethanol molecules gathered in the bulk phase of the mixture away from the surface. Not to the point of complete separation, we’re just talking about small changes here. At lower concentrations of 27% abv, ethanol molecules preferred to hang out near the surface of the mixture, at the liquid-air interface to make it sound technical.
Do you see why this is important yet?
Remember that ethanol is amphipathic. It holds onto water molecules with a loose grip because some parts of its structure form a bond but others don’t. This is what causes it to do all this weird behaviour in your glass of whisky. As it turns out, ethanol molecules are not the only amphipathic molecules in your dram.
The most studied molecule in such contexts, and the subject of the aforementioned 2017 study, is the famous phenol - guaiacol. Yes that guaiacol. The superhero for those classic smoky and woody aromas. The very thought of which gets Islay whisky fans very excited. Guaiacol is an important component of peat smoke, but it is also absorbed into whisky from oak casks during maturation.
Like ethanol, guaiacol is also an amphipathic molecule. When more water is added to whisky guaiacol gathers at the surface of the liquid, ready to begin its journey towards your nose. After all, when we sniff a whisky it’s not the molecules at the bottom of the glass we are detecting. It stands to reason that adding water to whisky makes molecules like guaiacol more readily available as aromas by encouraging them to concentrate at the surface.
Hopefully this is all now starting to make sense in some wonky way.
As mentioned, there are other amphipathic mixed-up kids in your glass too. How about vanillin, which needs little introduction but interestingly bonds with ethanol better than it does with water. Plus, fatty acids or long-chain acids. More about these shortly. But what about the molecules that are not amphipathic?
Water-hating aroma molecules
As a reminder, hydrophilic molecules interact well with water, often because they can form strong hydrogen bonds. Hydrophobic molecules do not interact well with water and tend to repel it; however, they often mix well in alcohol such as ethanol. Applying what we have discovered about how ethanol reacts in water at different concentrations, it is easy to see how other flavour-active molecules may behave also.
A joint research study between The University of Edinburgh and The Scotch Whisky Research Institute dipped a toe into this very topic. The researchers used molecular dynamics simulations to investigate how molecules related to scotch whisky flavours behaved at different concentrations of ethanol, or abv. They selected a handful of molecules with varying levels of hydrophobicity, or water-hating characteristics.
At alcohol strengths below 41% abv, both octanol and octanal were attracted to the liquid-air interface (surface) where they are likely to be detected as aromas. Octanol is associated with a characteristic waxy odour, whereas octanal smells soapy or oily. At higher ethanol strengths of 59% abv and above, they both still showed a preference for hanging about at the surface. However, a portion were attracted to the bulk phase of the solution (remember that?). This would naturally reduce their detection as odours in the headspace above the whisky.
Octanoic acid is associated with a goat's cheese aroma and sour taste. Again, at lower abvs (below 41%) it displayed a preference for concentrating near the surface interface. As the abv increased towards cask strength (59%), an even larger proportion was attracted to the bulk phase of the liquid when compared to the previously mentioned octanol and octanal.
The final molecule they studied, ethyl hexanoate, provides classic fruity aromas in whisky. Again this showed an attraction to the interface (surface) below 41% abv, with a diminishing attraction as the abv increased towards cask strength and above. At the higher strength abv it displayed a greater attraction to the bulk phase, away from the surface of the spirit.
The key takeaway is that changing the alcohol concentration of whisky causes molecules to alter their preference for either congregating near the surface of the whisky or diving into the bulk phase of the whisky, away from contact with the air. It all depends on the hydrophobicity of the flavour molecules i.e. how much they dislike water. Adding water can therefore encourage some aroma molecules to become airborne vapours more easily.
Now, it would not be proper to discuss adding water to whisky without taking a look at fatty acids would it?
Why does whisky go cloudy?
Fatty acids are the ones that are largely responsible for making whisky go cloudy, or turbid to use the technical term. As amphipathic (water-hating) molecules they tend to clump together when the concentrations of ethanol are reduced through the addition of water. However, chilling the spirit has a similar effect too.
Hence chill-filtration was created, which has the function of removing them. Chilling the whisky causes these long-chain fatty acids to join together, like when a bottle of non-chill-filtered whisky goes cloudy when it’s cold. Once the fatty acids have bonded together, they become easier to remove by pumping the spirit through a fine filter.
The most well-documented of these fatty acids are lauric acid (ethyl laureate), palmitic acid (ethyl palmitate) and palmitoleic acid (ethyl palmitoleate). These little characters are generally considered as being more mouthfeel motivated molecules, and less impactful on aroma. Hence, the exact impact of chill-filtration on flavour is not black and white. Although mouthfeel arguably has greater stakes in the whisky flavour game than taste, believe it or not.
However, here’s an interesting study that looked at the impact of ethyl palmitate in an ethanol solution. In particular, how it impacted the aroma thresholds of other molecules. The conclusions were that this fatty acid “changes the release behaviours of volatile compounds in solution, and increases their olfactory detection thresholds”.
What does this mean?
Put simply, ethyl palmitate reduced the availability of some aroma molecules in the headspace of the solution. It kept them held in the liquid. The headspace being the area above a whisky where we point our nose and have a sniff.
Similar results were mirrored in another Chinese study that sought to investigate the flavour impacts of long-chain fatty acids on their national spirit, Baijiu. In this study long-chain fatty acids seemed to lower the intensity of some fruity esters. Ethyl acetate (pear drops) and ethyl hexanoate (apple skin) were in fact reduced in the headspace of the spirit. However, ethyl butyrate concentrations were actually increased.
So while fatty acids may not smell of much themselves, they could impact how we experience aroma active molecules such as esters when water is added. But while ‘nosing’ whisky is a huge part of the experience, the molecular shenanigans of putting water in one’s glass may very well impact taste too.
How does water change a whisky’s taste?
Orthonasal olfaction is how we usually think of smelling things – sniffing through the schnozzle nostrils. But when a food or drink is in the mouth, odour molecules from it travel up the back of the throat and stimulate the odour receptors from a different direction.
This is called retronasal olfaction. It’s what we commonly relate to when we describe taste. As far as our neurological necessities are concerned the experience is separate from smelling through the nostrils, activating different areas of the brain.
Within a dram of whisky, certain aroma molecules will huddle closer to the bulk phase away from the surface. Therefore, they will not be picked up by the nose with the same intensity as others that congregate near the surface. But once in the mouth they have the opportunity to be released much more readily. Partly through the dissipation of the whisky and partly through further dilution, this time with saliva.
So this is a part of the puzzle that helps to explain why some whiskies taste different to how they smell (even though it’s all predominantly aroma but we think of it as taste). This can often be experienced with cask samples of grain whisky. They can be full of butterscotch sweetness on the nose, but the same intensity doesn’t translate to the mouth. Perhaps this is due to higher concentrations of water-loving molecules in grain whisky?
Of course a big contributing factor is the simple dilution of ethanol itself. Alcohol has the ability to dominate the sensory experience and it’s a difficult hurdle to leap over at very high abvs. An effect called masking. Lowering the abv simply reduces the impact of masking and enables other odour molecules to grab the attention of our odour receptors and taste buds.
It’s more complex than molecules alone
To summarise, adding water to whisky reduces its abv, and therefore encourages some molecules to come out of hiding from the centre of the liquid, and make themselves known near the surface interface where they meet the air. This is how adding water changes the character of whisky, especially its aroma. It makes molecules either more or less readily available at the surface of the whisky. The other part is the dilution of the alcohol, thereby lessening its masking effects.
But despite all the amazing research, the chemistry of diluting a dram remains a largely turbid affair with a great deal more to be explored. Technology allows us to chemically dissect whisky and gather molecules to study like insects under a microscope. But it’s all meaningless without our sensory organs and a brain to decode it all. Other changes take place too, such as lipid oxidation, which in fact creates new odour molecules within the mouth through reactions with oxygen and saliva. But there’s an additional complication.
When it comes to experiencing flavours, we are all quite unique. For example, on average we each have around a 30% variance of olfactory genes from one another. In other words, when it comes to how things smell, each individual person has a surprisingly different set of sensory instruments to use. This is why aroma blind spots are extremely common, where people have trouble detecting certain aromas.
So when it comes to enjoying a dram, for example, we will each enjoy the same dram a little bit differently from one another. How we experience adding water to whisky will also be a little bit different for each of us. There are no hard-and-fast rules, and it’s far from being black and white, or even varying shades of golden yellow.
You may be aware that master blenders and those performing sensory assessments of whisky and new make spirit will significantly reduce the alcohol level of the samples. For tasting, 30% abv is the industry standard, while for nosing the samples are commonly reduced to 20% abv. But there are practical reasons for this that do not apply to the hedonistic enjoyment of whisky.
Those who assess samples in an industrial setting will often have a requirement to nose and taste many samples in a day. At higher abvs, even at the standard 40%, the smell and taste receptors fatigue rapidly due to the irritant effects of alcohol. After only a few samples, one’s sensory abilities become impaired. Of course, the inebriating effects of the alcohol must also be considered when one is working, using equipment, or having to drive a vehicle.
So for pure enjoyment, adding water to whisky is a very personal choice. Layer on top of this the multitude of chemical variations within a dram and it becomes a mindbogglingly complex affair. Every dram will react differently to water being added because it will have different concentrations of water-loving molecules, water-hating molecules, and those that sit on the fence such as guaiacol.
What does this mean for whisky tasting glasses?
The conversation raises interesting questions around getting the most out of your dram. If adding water causes molecules to concentrate either around the bulk phase or at the liquid-air interface, what are the implications for swirling the whisky in the glass? Does swirling reduce the masking effects of ethanol but negates a difference between the bulk phase and the surface? Or after adding water, should a whisky be swirled then left to settle? And if so, for how long? Should a dram be swirled at all? Such insights add a rather scientific dimension to something so simple as swirling – an act we instinctively do to enhance odour perception.
How about glass design? The surface area of a whisky, the liquid-air interface, is of course determined by the shape and size of the glass. The larger the surface area, the less the bulk phase as a proportion of total volume. How does this impact headspace concentrations of volatile aroma molecules? Would a larger liquid-air interface cause bulk phase molecules to sit closer to the surface?
Most whisky tasting glasses are designed to focus aromas at the headspace, like a miniature olfactory chimney. But at the detriment of surface volume. What would the impact on whisky aroma be for a wide glass, like a brandy glass, but with a narrow opening to focus the aromas? Perhaps there’s something out there already. Perhaps there’s an opportunity to experiment at home. I pass the baton to you, but please report back.
The mystery of adding water to whisky
It’s the complexity of this thing we call flavour that gives whisky a unique allure. Around the perimeter of whisky-making and dram partaking, there’s a whole load of science. From the raw ingredients, to production, maturation, blending, marketing, and of course sensory science. It all gets analysed, scrutinised, and categorised by people much smarter than myself.
But in the middle there’s a little black hole that no-one really understands. And it’s from this black hole that amazing whiskies emerge. It’s a place of science fiction magic that retains supernatural abilities in a world of data, facts, findings, and corporate spreadsheets. An antidote to actionable items. A panacea for pain points, paradigm shifts, and pushing the envelope.
We all need a little magic in our lives. Retaining a child-like enchantment in the mystery and wizardry of the world is something to be appreciated, cherished and celebrated. Perhaps, therefore, there’s an argument for leaving some stones unturned. But I hope, at least, you know understand a little of how water changes your whisky.