Distillation is the technique of process of separating a liquid out within a mixture, by refining volatile substances by heating them until they vapourise and then cooling (condensing) the vapour back to their original state; traditionally, distillation was carried out using equipment called a still. It relies on the fact that different liquids boil at different temperatures. The most common use of distillation is the making of spirit alcohols - eau de vie and uisge-beath (whisky), literally waters of life - but also is used in making rosewater and orange flower water, or even making potable water from sea water. While Aristotle knew about distillation, it was the great period of Arab science that was the real early developers of the technique.
Distilling spirits relies on the separation of alcohol and water because the alcohol is more volatile than water and boils at a lower temperature, 78oC (172oF) for alcohol versus the much hotter 100oC (212oF) for water. Depending on the relative proportions of alcohol and water, the mixture will boil at an intermediate temperature between these points and the vapour will contain a mix of both alcohol and water. The alcohol in for example brandy boils and its vapour evaporates out of the top of the still; this tube continues into a long spiral shaped tube in which the vapour cools, condense into a liquid and is collected in a container. The first run through will produce a distillate of 20 - 40% alcohol in a basic still. A second distillation will increase the alcohol to 70%, deepening on the spirits being made and the methods used. Further distillation can get an alcohol of 97.2% purity as a maximum by volume. The only benefits for the chef of an increased alcohol content is the type of drink made, i.e. liqueurs, and for preserving fruit.
When making spirits, e.g. brandy, rum and whisky, there are other considerations than simply their alcoholic strength. Fermented marshes contain varying amounts of colatile substances. One is methyl alcohol, which is poisonous, but is made from woody cell walls and pectin when fruit mashes are fermented. The levels of methyl alcohol formed are low, so are usually too small to be harmful, but the first run through (the heads or foreshots) may contain more, hence why they are thrown away. Other substances include the congenerics which make up fusel oil. The congenerics are a mixture of higher alcohols and other substances that are less volatile and mainly come over in the last distillation. Although the congenerics are largely removed from vodkas, some are retained for flavour in other spirits.
Distillation is, also, used to make:
- Distilled Water: this is water that is virtually free from mineral matter and contain little dissolved air. It is safe to drink, but has a dull and flat flavour. Water distilled from sea water is used in some oil-rich, but waterless, countries, but overall is of no genuine culinary purpose. Drinking water is distilled in a vacuum still to extract potable water from sea water.
- Flower Waters: these are delicately flavoured distillations of water with flowers, mainly orange and rose flower waters. In this case, water is distilled with the flowers in (roughly) a 1:1 ratio through a still, then the distillate is run through a second distillation in a cohabating column which enriches the flavours by about ten times; it is the volatile oils from the petals that are being concentrated into the water.
- Distilled Vinegar: distilled vinegars are strong vinegars that rely on the high boiling point of acetic acid of 118oC (244oF) versus water's boiling point. In effect, distilled vinegar is made by boiling off water.
In addition to flower waters, steam distillation is used to extract essential oils from flowers, herbs and spices. The ingredients to be extracted a mashed in water, then steam is bubbled through them to heat and evaporate off the essential oils, which are carried away by the steam. Since the oils do not mix with water, they can be easily extracted. This is the basis for many favours in processed foods, e.g. curcumin from turmeric.