separating a liquid from a mixture by boiling it off and condensing the vapour. The best-known use of the technique is to make alcoholic drinks such as brandy; but there are other, food-related, uses; flavourings such as rosewater (see roses) and orange flower water are prepared by a type of distillation.
Distillation can also be used to produce drinking water from sea water, a process described by Aristotle in the 4th century bc. At that time the Egyptians were already distilling turpentine from pine resin, and probably also making small amounts of alcohol for medicinal use. It was only with the rise of Arab science that the distillation of alcohol was carried out on an appreciable scale. Islam prohibits the consumption of alcoholic drinks, but certainly by the time the technique reached European countries in the 14th and 15th centuries it was being used for that purpose.
The principle of distillation is that different liquids boil at different temperatures. Ethanol, the type of alcohol found in drinks, boils at 78 °C (173 °F), considerably lower than the boiling point of water at 100 °C (212 °F). Brandy is made by heating wine to a temperature between these figures in a closed vessel—a still. The alcohol boils but the water does not. The alcohol vapour passes out through a tube at the top of the still. This tube continues as a long downward spiral in which the vapour gradually cools and condenses back to a liquid. The whole process is accompanied by complications which often make it necessary to repeat the distillation at least once to achieve the required strength and purity.
Drinking water is distilled from sea water in a vacuum still, in which pressure is reduced to lower the boiling point so that less heat is required. Even so, it is an expensive process and largely restricted to rich countries such as the Gulf States.
Distilled, and thus very strong, vinegar is used for making pickles. The boiling point of acetic acid, at 118 °C (244 °F), is higher than that of water so the distillation of vinegar is a process of boiling away unwanted water.
Steam distillation is used to extract essential oils from flower petals, herbs, and spices. The materials to be distilled are mashed in water. Steam is bubbled through them to heat and evaporate the oils, which are carried away by the steam. Since the oils do not mix with water, they are easy to recover from the condensed liquid.
Destructive distillation is the heating of a solid in a closed vessel until it decomposes into other substances. Artificial ‘smoke’ flavouring is made by the destructive distillation of wood.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
