(AKA – Spontanious fermentation. AKA – Open fermentation) – is letting yeast in the environment around the hot wort ferment the beer naturally. There are two types of fermentation: wild or single-culture. Single-culture means one strain of yeast is introduced to eat up the sugars in wort, converting them to alcohol, and creating a more controlled fermentation. Sour beers, on the other hand, undergo wild or mixed fermentation, meaning multiple types of yeast and bacteria work together symbiotically to turn sugar to alcohol, and create a spontaneous fermentation. Within the category of wild ales, beers can be subject to either controlled or open-air fermentation. Controlled fermentation means brewers use multiple types of yeast and bacteria, but have selected exactly which strains enter the tanks so they can control the fermentation and the outcome of the beer.
Open-air fermentation is exactly what it sounds like – Tanks or koelschip (Coolship), are left partially or fully open, to allow bacteria and wild yeast to drift in naturally from the environment. Open-air fermentation is what makes some Belgian beers taste so characteristically Belgian. Coolship beers are crafted using a traditional Belgian method of spontaneous fermentation. Hot, un-fermented wort is cooled overnight using outside air temperature in a traditional, large shallow pan (coolship). During the cooling process, naturally occurring microflora from the air inoculates the beer, and in the morning, it is typically transferred into French oak wine barrels or steel tanks where the entire fermentation and aging takes place, which can last anywhere from a few months to a few years. This is due to the nature of the fermentation; with so many strains of bacteria and yeast, it takes time for all those microbes to do their work. Typically what occurs is one strain of bacteria or yeast will have a surge of activity and then go dormant followed by another strain surging and then going dormant and so on until all the microbes have done their work. This type of fermentation is very unpredictable, but can add a lot of complexity to a beer, and truly capture the environment in which it was brewed in.