low-pressure distillation , or vacuum distillation, is a high-efficiency alternative to traditional distilling. Distillation at normal atmospheric temperatures is an energy-intensive process. Under normal pressure at sea level, ethanol boils at 78.4° C, but if one lowers the pressure by 50 percent (to half an atmosphere), ethanol will boil at 63° C. This means less energy is required to achieve the same goal, making production more affordable and environmentally friendly. See environmental impact of distillation.
In a low-pressure distillation setup, the system is sealed, and a pump is used to reduce the internal pressure. The distillation system operates in the same manner as a normal atmospheric still, just at a lower temperature and pressure.
An added benefit of a low-pressure distillation unit is the capability of improved fractionation to produce a purer distillate. The heavier organic compounds found in fusel oils, like propanol and amyl alcohol, are not as volatile as ethanol. The boiling point of these fusel compounds at half an atmosphere requires slightly more energy than normal to boil, resulting in easier fractionation. These slight changes in the volatility of the fusel oils may seem insignificant, but over the course of a long distillation run this makes a significant improvement in the purity of the distillate. The lower heat required also serves to preserve some delicate aromatic compounds that break down at higher temperatures, making it useful for botanically derived or flavored spirits.
Because of the complex nature of low-pressure distillation, it is best suited for large-scale spirit production (although the countertop Rotovap vacuum still is used by many high-end cocktail bars to produce small quantities of unique ingredients; see
See also fusel oil.
McGee, Harold. “A Chill at the Still to Keep Flavors Fresh.” New York Times, December 1, 2009.
Willkie, Herman F., and Joseph A. Prochaska. Fundamentals of Distillery Practice. Louisville, KY: Jos. E. Seagram & Sons, 1943.
By: Darcy O’Neil