Percolation is a relatively recently invented method of extracting essential oils from plants. It can be described as resembling distillation, with the difference that the steam is produced in a steam generator above the plant material, and percolates down­wards through this. The steam and vapours are collected into a pipe which passes through a series of cooling tanks, each one of which is progressively colder than the one before. The distillate is collected and drawn off at the end of the process in a similar manner to that used in distilling.

This method is as yet not widely used, but is valuable for extracting essential oils from woody or tough material, such .as the seeds from plants of the Umbelliferae family (Aniseed, Dill, Fennel, etc.) Using normal distillation methods, these take up to 12 hours to extract, but can be obtained in 4 hours using percolation. The shorter time involved also means that the plant materials are in contact with the steam for shorter periods, and this produces a better quality oil.

See also DISTILLATION.