While whisky with a splash of water may be a popular order at the bar, scientists say they have figured out just how big this splash should be.

Adding a little water is thought to open up the flavour of the drink, but a new study suggests there is a point at which it becomes too much – about 20%.

At this point, too much water can make whiskies in the same group taste the same, it found.

However, the most discerning palates may still be able to tell a single malt Scotch and an American bourbon apart.

According to the researchers, the findings may also help explain why larger ice cubes, or balls, have become so popular, as they allow the drink to be enjoyed before it gets too diluted.

Tom Collins, a Washington State University assistant professor and senior author on the study, said: “If you want to enjoy a specific whisky, this suggests that you don’t want to dilute it by more than about 20%.

“By the time you get to 60/40 whisky to water, the whiskies are not differentiated by the panellists – they begin to smell the same, and that’s not really what you’re looking for.”

He added: “This study helps to understand why those large, square ice cubes have become so popular because you can actually enjoy the whisky before it gets diluted to the point that it’s not the same whisky.”

For the study, researchers chemically analysed how volatile compounds in a set of 25 whiskies responded to the addition of water, including bourbons, ryes, Irish whiskeys and both single malt and blended Scotches.

They also had a trained panel of 20 experts assess six of those whiskies, three Scotches and three bourbons.

Both tests found that adding a little water could change how the whiskies smelled, and after 20%, they may start to have the same aroma.

The scientists say that since smell and taste are often closely linked, this likely affected the spirit’s flavour as well.

Glass of whisky on the rocks
The findings may help whisky makers better understand how their customers will experience the drink if they chose to add water or have it on the rocks (Alamy/PA)

Working with Elizabeth Tomasino at Oregon State University to run the sensory panel, the researchers found that at 100% whiskey, the panellists could easily tell all the whiskies apart from each other.

They could still differentiate whiskies within each group at 80/20 whisky to water, but after more water was added, that changed.

The study found that while within each style of whisky the aromas became more similar, the larger grouping of Scotches, both single malts and blended, remained distinct from the American bourbons and ryes.

Chemical analysis revealed similar results showing the changes in volatile compounds that entered the area above the liquid (headspace), when water was added.

Whisky is a mix of compounds that run the scale from hydrophilic to hydrophobic, in other words, ones that are attracted to water and others that are repelled by it.

Adding water sends the whisky’s hydrophobic compounds into that space and leaves the hydrophilic ones behind, changing the aroma of the liquid, experts say.

The chemical analysis matched the impressions of the panel, the study found.

For instance, many of the Scotch whiskies started out with a smoky, peat aroma, but as they were diluted, they moved towards a fruitier aroma known as pome.

Mr Collins explained: “This happens because of the way dilution affects what’s in the headspace.

“The compounds that are associated with smoky aromas dissipate, and they were replaced by compounds that are associated with fruity aromas.”

At first American bourbons were mostly associated with vanilla and oak scents, but as more water was added, they took on more aromas of the corn and grains used to make them.

Researchers suggest the findings, published in the Foods journal, can help whisky makers better understand how their customers will experience the drink if they chose to add water or have it on the rocks.

Mark Reynier, CEO and founder of Waterford Whisky – an Irish distillery that follows the French winemaking philosophy of terroir, said: “There is often a certain fear, within whisky circles, of adding water to whisky – as though diluting somehow breaks a kind of unwritten purity law.

“That can be the case if too much is added. But adding a few drops – usually no more than three – can make the unique flavours of a whisky all the more rewarding.

“When drops of water are added, ‘viscimetric whorls’ form – these are those mesmerising spirals you can see in the glass.

“They allow the spirit to breathe: the whisky literally ‘opens up’ as compounds unclench, and complex aromatic potential – derived from different barley flavour sources – can be fully realised.”