A vessel used to contain the biochemical process of fermentation. It is typically made of steel although smaller vessels are made of glass. It can be operated as a batch process, a fed batch process, or continuously. The fermentation process is usually sufficient to create natural turbulence and mixing. However, the extent of mixing can be enhanced using a mechanical stirrer such as an impeller. To avoid a mechanical stirrer, an air-lift or loop fermenter uses an internal or external draft tube up which a small flow of air is admitted to circulate the fermenting medium. In the whisky industry, the vessel is called a washback and is traditionally made of wood.