Rotary evaporator. Credit: Geni/Wikimedia Commons

Molecular gastronomy—the use of scientific techniques to create exotic cuisine—is becoming a household term. But what about molecular mixology?

An article in the December Physics World (free registration required) explores how bartenders are using scientific equipment and techniques to create new cocktails, and how many long-established tricks in the mixologists' book are firmly rooted in physics and chemistry.

One key to tasty tipples is that the physical properties of ethanol—the chemical form of alcohol in booze—permit the delivery of "flavours that are impossible to achieve using water alone," according to the article's authors, Naveen Sinha and David Weitz of Harvard University. Aromatic compounds such as those that provide "the caramel notes of rum or the oaky smell of bourbon" tend not to be soluble in water, but they play much nicer with alcohol, Sinha and Weitz explain:

Water molecules are polar and so prefer to be near other polar molecules to minimize their interaction energy. This encourages non-polar molecules, such as the aromatics, to leave the liquid phase and vaporize into the surrounding air, where they contribute to the aroma of the drink. The presence of ethanol mediates this polar/non-polar interaction and allows high concentrations of aromatics to remain in an aqueous solution.

A familiar application of ethanol's aromatic amenability is the making of an infusion—soaking herbs or pepper, for instance, in vodka for days to leach the flavors out into the liquor. But clever culinarians have concocted a way to accelerate this method, creating infusions on demand. In a process devised by Dave Arnold of the French Culinary Institute, a relatively inexpensive nitrous oxide–pressurized cream whipper turns vodka and coffee grounds into coffee-infused vodka in about a minute. Sinha and Weitz explain how the technique works:

What happens is that nitrous oxide, which is also in the canister and under high pressure, dissolves in the vodka. The high pressure of the liquid displaces any air bubbles in the coffee grounds. When the pressure is released, the nitrous oxide rapidly bubbles out of the solution, just as when a can of carbonated drink is opened. Releasing these bubbles draws flavour molecules from the coffee grounds into the vodka...

A higher-tech, and far more expensive, approach is to draw out aromatics by distillation using a lab device called a rotary evaporator. By lowering the pressure in a vessel of liquid, the rotary evaporator, which costs thousands of dollars, draws out aromatic compounds at lower temperatures than would be needed for ordinary evaporation. The evaporated aromatics can then be captured and added back to liquor as flavoring. London bartender Tony Conigliaro uses a rotary evaporator to make spirits such as blackcurrant vodka and gin spiked with silver-needle tea, according to a 2009 article in the New York Times.

And Sinha and Weitz note that rotary evaporators can separate desirable from undesirable flavors—drawing off the rich aromatics of a chili pepper, for instance, while leaving behind capsaicin, which lends peppers their heat, "yielding a liqueur that retains all the flavour of the chillies but without any nasty burn."

It seems doubtful that rotary evaporators will ever become standard equipment in bars and restaurants. But it's nonetheless encouraging to know that experimenters such as Arnold and Conigliaro are out there, pushing the boundaries, using the tricks and tools of science to make a more flavorful world—one exotic cocktail at a time.