rotary evaporator (rotovap) is a laboratory-grade instrument that uses a vacuum to distill a solvent. A rotary evaporator has an evaporation flask, water bath, condenser, vacuum pump, and collecting vessel. Each part contributes to precise and controlled evaporation and condensation. This, and their relative expense, is why rotovaps are usually found in science and medical labs, where they are used to separate dangerous compounds from a given solvent.
With the spread of molecular gastronomy and experimental bartending since the beginning of the twenty-first century, chefs and bartenders have harnessed the power of rotary evaporators to impart flavor to liquids. A rotovap’s vacuum allows liquids to be evaporated at lower boiling points because evaporation occurs through pressure at lower temperatures. This is useful to those working with flavor, because heat can damage or pull apart delicate volatiles. The rotating evaporating flask, immersed in a heated water bath, increases the surface area of the solvent, gently speeding up distillation. Through forced convection, the mixture is combined and heated evenly to promote stable evaporation. The resulting gas is then condensed into a liquid and collected in a separate vessel. Each component of the solvent is separated based on its specific volatility, while the parts of the original solvent that continue to distill remain in the evaporating flask. This process separates a solvent according to a strict control of temperature, air pressure, and rotation.
A chiller can reduce the temperature of the coils in the rotovap distillation in order to condense the evaporating liquid more efficiently. The lower the temperature, the better the condensation—otherwise liquid begins to evaporate out. The best results are yielded when small molecules are not allowed to escape. This way, distillates preserve a fuller aroma and flavor spectrum.
Altering the pressure alters the boiling point of the liquid inside the evaporation flask. When the pressure is lowered, the liquid in the evaporation flask more readily evaporates because less pressure compresses it. The vacuum pump and its controller come into play. A vacuum controller stops and starts the pump so that a specific pressure, and therefore boiling point, can be preset by the operator. Since most liquids contain compounds with a variety of different boiling points, common practice is to lower pressure gradually over time, so that different layers of the liquid are stripped individually.
A peristaltic pump may also be used to cut the heads and tails from the rotovap vacuum still without breaking the seal and thereby losing an enormous amount of volatiles. This allows for parts of the liquid to be pulled off to see where the best concentration of flavors is, without compromising the final flavor of the distillation.
The rotovap can be used to create infusions, concentrations, and hydrosols and also to clarify liquids for bespoke ingredients for cocktails in bars. Beyond producing ingredients that are not available commercially, the rotovap is most useful for protecting the integrity and intensity of an ingredient’s flavor, allowing its delicacy to be respected. Beverage alcohol is an excellent medium for carrying flavor and more volatile aromas. The purest and freshest flavors can be extracted from ingredients by removing the water and replacing it with a solvent like alcohol, gently and at low temperatures. Hydrosols can be made by adding solid ingredients into the evaporation flask with water. This makes the most of delicate ingredients, such as herbs and flowers. A rotovap can also strip an aged spirit of the compounds responsible for its barrel notes; these can then be added to a different spirit, giving the impression of aging and “de-aging” spirits. Removing alcohol and water from a given spirit will leave it with only its most flavorful essence, which can then be added to other spirits or ingredients to overhaul their flavor profile.
See molecular mixology.
Arnold, Dave. Liquid Intelligence. New York: W. W. Norton, 2014.
By: Tony Conigliaro