Spontaneous Fermentation

Spontaneous Fermentation, for the purposes of this article, refers to the inoculation of wort for fermentation with local ambient microbes. This is commonly achieved by use of open cooling such as 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, especially if the barrels/foudres have not been thoroughly cleaned ^[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 of 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 processes and goals including the production of a dextrinous wort, high hopping rates (usually 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 ^[9] passing hot mash runnings through flaked grains ^[10], or pulling mash runnings before full conversion without the prolonged processing of a turbid mash ^[10]. 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 ^[11]. Cantillon uses hops that are on average 2-3 years old at hopping rate of 250-300g/100 L (3.34-4.0 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. 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 sufficiently low nighttime outdoor temperatures. Boil kettles and similarly shaped vessels are sufficient for overnight cooling for most homebrew batch sizes and may provide a rate of cooling more similar to that provided by coolships in commercial production sized batches ^[12]. However the aspect ratios of these sorts of vessels may limit the inoculation of the wort by ambient microbes due to the lower surface area for a given volume compared to traditional coolships. 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 (8°C ^[13]) to appropriately 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).

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. ^[14] ^[2] ^[15] ^[5]. This as been illustrated well by Raj Apte ^[16]. The first stage, which lasts for approximately 1 month ^[14] ^[17], 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 ^[17]. 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 ^[14].

The Saccharomyces dominated stage of fermentation is followed by prolonged and gradual acid and flavor development accompanied by the final points of attenuation. In some descriptions this is split into an "acidification phase" which is dominated by lactic acid bacteria (LAB), primarily Pediococcus, and a "maturation phase" driven by Brettanomyces ^[14]. Other sources describe these as one extended maturation phase with acidification from Pediococcus and Brettanomyces growth occurring simultaneously ^[5] ^[2] ^[15]. Note that many scientific publications use the terminology Dekkera rather than Brettanomyces. ----continue this

During the extended maturation phase, a beer may become "sick" or "ropey", though not all producers get this ^[7] maybe ~1:10 min in? (also Vinnie on the sunday session and maybe lambic summit). This is the result of exopolysaccarides, which some Pediococcus strains are known to produce. These exopolysaccharides can be broken down by other microbes present in the beer relieving the beer of its "sickness" (this exopolysaccaride breakdown is generally attributed to Brettanomyces). A beer may also become "sick" in the bottle during bottle conditioning. This is likely due to enhanced Pediococcus activity from additional fermentable sugar, in the form of simple sugars or beer which has not completely attenuated yet. --sources for this--. A beer which is sick in the bottle will generally clear through the same process as a younger aging beer wen given appropriate time.

Alternative applications of Spontaneous Fermentation

Much of the above discussion has focused on spontaneous fermentation as applied to lambic and lambic-inspired brewing. Some brewers are applying spontaneous fermentation to yield beers quite different from lambic-oriented brewers. A notable example of this is De Garde, whose entire lineup of beers are cooled in a coolship and don't see pitched yeast ^[18] (excepting perhaps a bit of pitched yeast in some beers for bottling conditioning). De Garde produces a range of spontaneous beers including beers similar to berliner weisse by warm incubation after spontaneous inocculation ^[19]. By manipulation of parameters such as grist, hopping levels and incubation/fermentation temperatures, a diverse range of beers of spontaneous fermentation can be produced outside of lambic-inspired beers.