Spontaneous Fermentation

Spontaneous Fermentation, for the purposes of this article, refers to the fermentation of wort by inoculating it in a coolship where the wort is left exposed to the air and allowed to cool naturally over night. Wild yeast and bacteria are introduced into the wort as it cools. It is the traditional method of brewing Lambic ^[1].

Defining Spontaneous Fermentation

In the most romanticized view of spontaneous fermentation, the microbes which inoculate the wort in the coolship are sourced exclusively from the ambient environment outside the brewery. Scientific publications have suggested that in the case of some producers, these microbes may be resident in the brewhouse ^[2]. This is supported by the reluctance of lambic brewers to alter their facilities (remodeling, moving, painting, etc.) and the spraying of lambic on the walls of new buildings ^[3] ^[4]. The microbes responsible for spontaneous fermentation may also be derived from the oak barrels and/or foedres which are often used to hold the fermenting beer ^[5]. Many Belgian lambic producers thoroughly clean their barrels using hot water/steam, mechanical agitation (--add Cantillon ref--), and/or burning sulfur ^[6]; however even the most rigorous cleaning likely does not fully sterilize the barrels. In the case of lambic brewers the microbes resident in barrels are spontaneous in origin, having been derived from years to decades of use in the brewery without any exposure to pitched cultures and the barrels may serve as a concentrating mechanism for the desired cultures. The role of barrels as an inoculating vessel is unclear as some producers report achieving excellent results in barrels new to the brewery and which are microbially clean (^[7] ~35 min in).

A spontaneous fermentation may also be achieved by inoculating small amounts of wort and growing up the spontaneously inoculated microbes to check for suitability. This is common in homebrew production ^[8] and allows for screening of the microbes to remove wild cultures with aggressive off flavors and/or mold. This is not unlike the potential of used oak barrels, where well performing barrels may be kept to inoculate subsequent batches while poorly performing barrels may be discarded and removed from the brewery. As different microbes survive and thrive in different environments, barrels or pre-screened and grown starters will probably not provide a complete profile of the microbes present in traditional spontaneous fermentation beers. However a combination of a coolship to inoculate the wort with ambient/brewhouse resident microbes combined with a form of pre-screening such as barrel re-use and/or spontaneous starters may provide the full microbiota present in traditional spontaneously fermented products. For the purposes of this page, beers receiving additions isolated cultures or bottle dregs are not treated as spontaneous and are discussed under mixed-culture fermentation.

Brewing Methods

(in progress)

The traditional production of spontaneously fermented beer employs a few main ---aspects-- including the production of a dextrinous wort, high hopping rates with aged hops, and inoculation of the wort with a coolship. Not all breweries producing spontaneously fermented beer employ these three techniques, but they are generally common among producers.

A dextrinous wort may be produced by different mashing procedures. The most traditional method of achieving this is through a turbid mash. With this mashing technique, unconverted starchy wort (which turbid in appearance)is pulled from the mash and heated to denature enzymes. These pulled runnings are then replaced by infusions of hot water as the mash is carried through a series of steps for conversion of the remaining grains. The starchy wort from the early 'turbid' pulls is carried to the boil with incomplete conversion, providing dextrins to sustain Brettanomyces and lactic acid bacteria in a prolonged mixed fermentation. Other methods to carry unconverted dextrins into the boil may be employed such as the addition of flour (----add source----), passing hot mash runnings through flaked grains ^[9], or pulling mash runnings before full conversion without the prolonged processing of a turbid mash ^[9]. Whichever technique is employed, the goals are the same - to provide starches which Saccharomyces cerevisiae and Saccharomyces pastorianus cannot ferment and which can feed the diverse combination of other yeasts and bacteria present.

Traditional spontaneous brewers use high hopping rates of aged hops in a long boil. The high hopping rates help to regulate bacterial activity and select for the desired bacteria. Aging of the hops lowers the flavor/aroma impact the hops provide and also lowers the bitterness. The aged ops still do provide some bitterness as both oxidized alpha acids and oxidized beta acids can contribute to perceived bitterness and measured IBUs (---include OSU talk from CBC 2015--). Cantillon uses hops that are on average 2-3 years old at hopping rate of 250-300g/100 L (33.4-40.0 oz/gal)(^[7] ~49 minutes in).

A coolship is an open vessel used to cool wort by exposure to ambient air which traditional spontaneous fermentation brewers use to both cool their wort and to inoculate the wort with ambient microbes during the open overnight cooling. Traditionally, a coolship is a broad, open-top, flat vessel in which wort cools overnight. The high surface to volume ratio allows for more efficient cooling, which is important at commercial production scales. Cantillon targets a cooled wort temp of 18-20 C (64.4-68 F) after the overnight cooling (^[7] ~50 min in). Traditional producers only carry out spontaneous fermentation between fall and spring when nighttime temperatures are sufficiently low (---add cutoff temp--) to sufficiently cool the wort overnight. The ambient microbial balance may also be more favorable during this time of year (--some sources say more bacteria in summer--), but inadequate cooling could result in similar results of enhanced acid production (similar to the effect of warm incubation in sour worting). In addition this broad, shallow design maximizes the area of wort available for inoculation with ambient microbes. On a homebrew scale, where typical batch sizes cool more quickly, a wide shallow pan is not necessary to achieve appropriate cooling overnight given appropriate nighttime outdoor temperatures. Boil kettles and similarly shaped vessels are sufficient for overnight cooling and may provide a rate of cooling more similar to that provided by coolships in commercial production sized batches ^[10]. However the aspect ratio of vessels of this size limit the inoculation of the wort by ambient microbes due to the lower surface area for a given volume compared to traditional coolships.

Homebrew 15 Gallon Coolship
Homebrew 15 Gallon Coolship
30 Barrel Coolship from Funk Factory

Microbial Succession During Fermentation

(In Progress)

Scientific research in Belgium and the US has shown a regular general pattern to the microbial succession of spontaneous fermentation beer. ^[11] ^[2] ^[12] ^[5]. This as been illustrated well by Raj Apte ^[13]. The first stage, which lasts for approximately 1 month ^[11] ^[14], is dominated by enterobacteria. Though enterobacteria contribute little in terms of gravity drop over the first month of fermentation, they may contribute aroma and flavor compounds and precursors during the initial stages of spontaneous fermentation ^[14]. Acidifying the wort to pH = 4 before cooling and exposing to ambient microbes in a coolship can eliminate the enterobacteria phase of spontaneous fermentation ^[5].

The second stage of spontaneous fermentation is dominated by Saccharomyces sp. (predominantly S. cerevisiae and S. bayanus). Most of the attenuation is accomplished during this stage, which lasts approximately 3-4 months ^[11].

The Saccharomyces dominated stage of fermentation is followed by a bacterially-dominated stage ^[11]. The main bacteria present here are lactic acid bacteria, particularly Pediococcus damnosus. Some Pediococcus strains are known to produce exopolysaccarides, resulting in "sick" or "ropey" - not all producers get this-- include (^[7] maybe ~1:10 min in?)