Open Fermentation is the name given to fermentations that take place in vessels that are “open” to the environment in which they are situated. Open fermentations were the traditional method of fermentation before closed or lidded fermentation vessels were introduced. Open fermentation vessels were once used in all breweries for all types of beer including lagers as well as ales. Today, open fermentations usually take place in specially constructed rooms with smooth, easily cleaned surfaces (often tiled) to minimize the risk of microbiological contamination, and they must have a good air flow to remove carbon dioxide gas that is given off during the fermentation process. Early fermentation rooms were designed with good natural air extraction, which was later replaced by more sophisticated extraction and air conditioning systems, often including cooling systems for lower temperature lager fermentations, or heating systems in breweries in colder climates.
Nearly all modern breweries now use closed fermentation vessels, which have many advantages. Closed vessels are easier to clean using modern automated cleaning equipment, and no carbon dioxide from the fermentation need enter the immediate environment. Carbon dioxide can also be collected and used later in beer processing. Closed vessels can be positioned outside the main brewery buildings and closed vessels can also be built stronger to become pressure vessels which can be used to retain natural carbon dioxide from the fermentation process. While the flow of carbon dioxide (CO2) off the surface of the liquid protects the beer while fermentation is very active, as soon as this activity ceases the beer must be quickly moved into closed vessels to avoid oxidation and bacterial contamination. Then the vessels often must be cleaned by hand, though some larger breweries have devised ingenious systems to clean open vessels automatically.
All this being said, many brewers feel that open fermentation has advantages as well, and this may be why it remains relatively common in the UK, Germany, and Belgium. One obvious advantage is the brewer’s ability to actually see the fermentation and gauge its progress. The vessels used for open fermentation tend to be broad and shallow, and this shape encourages the formation of esters that are often found desirable, especially in beer styles featuring yeast fermentation character, such as weissbier. Top-fermenting ale yeasts often form a thick floating mat on top of the liquid toward the end of fermentation. Open fermentation allows the easy collection of that yeast, which tends to be healthy and unencumbered by dead cells and protein sediment. The vitality of open-fermenting yeast has long been noted, and it forms the basis for the famed Burton Union fermentation system.