Everyone knows that oil floats on water. That’s because oil is nonpolar, and water is polar. These terms describe the arrangement of charges in their structure: nonpolar means a molecule has no net charge, polar means it does. The polarity of a molecule influences the way it interacts with other molecules. We tend to think of two ‘like’ substances, be they nonpolar like two different oils, or polar like water and vinegar, as mixing completely, but that’s not entirely true in all cases.
When water and ethanol mix, they do so partially, dispersing in a way that depends on their concentration. Add some small molecules into the mix, and where we’d normally expect them to diffuse evenly throughout the solution (like tea diffuses out of a tea bag when brewing) we get instead certain clusters of the molecule.
Most flavour molecules are small and hydrophobic, and prefer to hang out with ethanol rather than water, since its bigger surface allows more interaction.
Since water and ethanol are not uniformly distributed throughout the glass, this leads to hot spots of flavour and aroma. Studies have found that ethanol and its associated flavour molecules are found evenly distributed when the concentration of your beverage is above 59%.
But at concentrations lower than 45% (like the 40% of most commercial whiskeys) ethanol is preferentially found at the liquid-air interface, or the top of your cup. Flavour molecules found here are more easily inhaled and immediately tasted upon sipping, leading to an overall better whiskey experience!
This effect is heightened if the whiskey is further diluted with water (down to about 27% in most cases). The flavour molecules are found even more preferentially at the liquid-air interface, and become more likely to evaporate and contribute to that pleasant smokey smell you love.
With this in mind, I think I’ll pass along some science next time someone gives me a judgemental look for ordering a scotch on the rocks. Though, since coldness can decrease taste, we really should be adding room temperature water. Maybe I can get used to that.
Cheers! To the science of sipping!
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