Spontaneous Fermentation

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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
Lambic fermenting at Tilquin with blowoff tubes
cooling such as in a coolship where the wort is left exposed to the air and allowed to cool naturally over night and wild yeast and bacteria are introduced into the wort as it cools. Spontaneous fermentation is part of the traditional brewing process for 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 wooden barrels and/or foudres 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 (such as is seen at Cantillon), 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 which are new to the brewery and microbially clean [7] (~35 min in). We do not regard the use of well cleaned barrels but still containing native microbes from previous use to invalidate spontaneous fermentation

A native wild-microbe fermentation may also be achieved by inoculating small amounts of wort and growing up the spontaneously inoculated microbes to check for suitability. This process, called a spontaneous starter, 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 and used to ferment subsequent batches (where the organisms residing in the barrel can exert their influence on the batch) while poorly performing barrels may be discarded and removed from the brewery. This process does differ from oak barrels in that native microbes are cultured and pitched into work, rather than the additional inoculation being a result of porous surfaces which cannot be fully sanitized. 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.

There is some debate as to whether using spontaneous starters should still be termed 'spontaneous fermentation' [9]. Each brewer will have to decide for him or herself what terms to use. We recommend transparency and clarity in descriptions and process to avoid potential confusion. In a stricter sense, beers using active inoculation with spontaneous starters might better be described with terms like 'wild', 'fermented with native microbes', or a description of the spontaneous starter process rather than entirely spontaneous beers. Especially if the full wort volume was not ambiently cooled and the starter was allowed to grow for some time before mixing it in. Beers receiving additions of isolated cultures or bottle dregs are not spontaneous and are discussed under mixed-culture fermentation and commercial sour beer inoculation.

Wort Production

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 by open cooling in a coolship. Not all breweries producing spontaneously fermented beer employ these three techniques, but they are generally common among producers.

Mashing

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
First pull of turbid runnings in a homebrew turbid mash
starchy wort (which is 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 [10] passing hot mash runnings through flaked grains [11], or pulling mash runnings before full conversion without the prolonged processing of a turbid mash [11]. 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.

Hopping

Traditional spontaneous brewers use high hopping rates of aged hops in a long boil (~4 hours or more). The high hopping rates help to regulate bacterial activity and select for the desired bacteria (Pediococcus rather than Lactobacillus). Aging of the hops lowers the flavor/aroma impact the hops provide and also lowers the bitterness. The aged hops still do provide some bitterness as both oxidized alpha acids and oxidized beta acids can contribute to perceived bitterness and measured IBUs [12]. Cantillon uses hops that are on average 2-3 years old at hopping rate of 250-300g/100 L (0.334-0.40 oz/gal) [7](~49 minutes in). See also the note about Cantillon's hopping on the Cantillon wiki page, as actual hopping rates may be slightly higher than the 250-300 g/100 L quoted here. Other producers such as Oud Beersel are report using higher hopping rates [13]. The use of significantly lower hopping rates may result in less bacterial inhibition and lead to different types of bacteria present. Some lambic producers are experimenting with the use of fresh dried hops in addition to or instead of aged hops [14] [15] [16]. James Howat of Black Project Spontaneous Ales uses 0.5 ounces of aged hops per gallon of beer for spontaneously fermented beers brewed using traditional lambic techniques [17].

For hopping techniques/rates/timing, see Hops in Lambic.

Cooling

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 (8-16 hours; extended cooling times of more than a day might lead to mold growth [18]). 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 and the use of a wide shallow pan might result in cooling at a much more rapid rate than seen in traditional commercial production. 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 [19]. 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 (max nighttime minimum of about 8°C [20]) to appropriately cool the wort overnight. The ambient microbial balance may also be more favorable during this time of year (--some sources say there are more acetic acid 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, see also Alternative applications of Spontaneous Fermentation below). Whatever the root of the different resulting beers based on time of season/ambient nighttime temperature, producers do report different times of year/temperatures exerting a strong influence on the final beer [7](~39 minutes in, ~54 minutes in).

Some more industrial producers of Belgian lambic as well as smaller North American brewers employing spontaneous fermentation acidify their wort before primary fermentation. This may eliminate the enteric bacteria step [5] (see below, Microbial Succession During Fermentation). In addition it may act as a safeguard against Clostridium botulinum (the bacterium responsible for botulism) in the beer as it can grow at the typical pH range of unfermented and unacidified wort and its spores can survive the boiling process [21]. The degree of botulism risk is not known, though if any reported cases of botulism poisoning from beer exist they are not known to us. Traditional lambic producers have been fermenting unacidified and spontaneously inoculated wort for decades to centuries, which suggests that the risk, if it does exist at all, is very small when following traditional lambic production methods. Furthermore, hops have antimicrobial properties against gram positive bacteria [22] and Clostridium botulinum is gram positive [23]. The degree to which Clostridium botulinum might be resistant to the antimicrobial properties of hops is unknown. Some suggest eliminating any potential worry of botulism by acidifying your wort before inoculation [21][24]. Whether or not this protects from botulism, it will influence the final beer by preventing enteric bacteria growth. In addition, acidifying may influence the activity of Pediococcus in a spontaneously fermented beer, including the development of "sick" beer, and may therefore alter the final beer [7] (~1:10 in).

The presence of more than 2-5 ppm of dissolved oxygen (DO) in the wort might also reduce the risk of botulism [25](more references needed), however the levels of DO in wort that has been cooled in a coolship has not been well studied, and neither has the amount of DO during the first few days of fermentation. Dissolved oxygen in wort that is near boiling temperatures will be limited due to Hentry's law, but some amount of atmospheric oxygen will be absorbed as the wort cools over night [26][27][28]. Some reports of DO in wort cooled in a coolship MTF include ~4 ppm in a small coolship that was 2' x 1' x 1', and 3.6 - 3.8 ppm in wort cooled overnight in an open 10 gallon boil kettle [29]. The DO levels from a commercial sized coolship (10 BBL; 6' x 10') were reportedly 2.6 ppm after the transfer to the coolship while the wort was still hot, and 5.1 ppm after the wort cooled for 14 hours [30]. The dissolved oxygen in the wort, however, could be quickly consumed by aerobic bacteria and yeast. Additionally, some strains of C. botulinum are more oxygen tolerant than others. Therefore, DO levels should not be relied upon for preventing botulism. Instead, a timely (within 4 days has been a suggestion, however no one really knows how long it takes C. botulism to grow in anaerobic wort and produce enough botulism toxin [31]) reduction in pH below 4.6, and an increase in alcohol (decrease in gravity), are more effective measures for making sure that botulism is not a concern [25][32].

Fermentation of Spontaneous Beers

Producers of spontaneously fermented beer typically do not oxygenate their wort [33] (~27 minutes in). Visual signs of fermentation (CO2 production, krausen, bubbles, etc.) generally takes 4-7 days, although we have seen reports of up to two weeks [34][35][36][37]. Traditional producers conduct fermentation for a
Spontaneous fermentation beginning in a carboy
long time period (1-3+ years) in wooden vessels. The long fermentation process allows the different microbes present to carry out their slow metabolism of the complex carbohydrates present in the beer, developing the flavors and acidity associated with spontaneous beers [38] [39]. Extended aging in the same vessel as fermentation does not present the same sort of autolysis problems that may be found in 'clean' beers aged for long periods of time on the yeast cake. There are some ideas for why this is the case, such as the extended activity of other microbes taking up autolysis products. It is also possible that the influence of autolysis is found, but that they are expressed differently in these sorts of beers and some say that the autolysis character is an important component of the beers [40] (~28 minutes in).

The wooden fermentation vessels are frequently oak wine barrels in the 220-400 L (58-105 gal) range but other woods such as chestnut are used and the vessels may also be large tuns or foudres holding upwards of 45 HL (about 1200 gal, or about 34 bbl). These barrels provide two primary benefits for the fermentation - they allow a small amount of oxygen permeability and they provide an environment which houses some of the microbes active in the fermentation (notably Brettanomyces, which can penetrate into the wood and in some cases can metabolize compounds present in the wood such as cellobiose, which is produced from toasting of the wood) [41](~3:22 in). While a controlled micro-oxidation can be beneficial to the beer, too much oxygen exposure can lead to excessive acetic acid and/or ethyl acetate production (either from Brettanomyces or Acetobacter) [42]. In addition the barrels may provide flavor and structure from tannins and, in some cases, what they previously held.

On a homebrew scale a fair amount of attention has been paid to the topic of oxygen permeability in different fermentation vessels and closures [43] [44] [45]. It has been suggested that sealing a glass carboy with a wooden dowel or chair leg can result in similar oxygen permeability as a wine barrel. Although this was quite a clever idea for replicating oxygen exposure, this is not recommended as it can lead to breakage of the glass carboys [46]. While micro-oxygenation may be an important part of some spontaneous production it may be getting too much attention in homebrew carboy conditions [46] (see comments) relative to other controls such as temperature, microbes, and time. See the Barrel page for discussions on the barrels available to homebrewers. Since spontaneous fermentations can take several days to begin (generally 4-7 days, although we have seen reports of up to two weeks), some professional brewers and a microbiologist have recommended that carboys should be filled as close to the neck as possible to limit the initial headspace and oxygen in that headspace so as to avoid mold growth (lowering the wort pH to under 4.5 will also help prevent mold growth during the early stages of fermentation) [47][48].

Regarding fermentation temperature, commercial producers looking for balanced acidity and flavor/aroma complexity prefer cooler fermentation temperatures in the range of the high 50s to low 60s F (~13-18 C) [7] (~1:14 in). Temperature control is very important to some Lambic producers. 3 Fonteinen had temperature controlled cellars, highlighting the importance of aging temperature. Unfortunately the temperature control thermostat failed and resulted in the brewery nearly going out of business [49]. This temperature range allows slow and balanced fermentation by the diverse array of microbes present. Warming the fermentation too much results in enhanced production of acidity which is out of line with what the lambic producer is aiming for. This can be used to the advantage of the brewer when producing certain non-lambic inspired spontaneously fermented beers (see below, Alternative applications of spontaneous fermentation).

American brewers who use coolships for spontaneous fermentation have reported that the success rate for spontaneously fermented beer is around 90-80%. Brewers will often dump undrinkable beers from individual barrels or even beers from barrels that don't meet the expectations of the brewers [34].

Blending (and Dumping)

Blending is a fundamental part of traditional spontaneous beer production (and typically of wood aged sour beer production in general). In barrel aged mixed fermentation beer, and especially spontaneously fermented beer, there is a high potential for variability in different barrels/fermentation vessels, even those resulting from the same hot side process. To help create a more balanced and complex product, producers of sour beers often blend barrels (of both the same and of different vintages) together into one final product. The homebrewer can employ the same techniques and blend to reach the desired final product from beers of different vintages and different carboys/vessels of the same brew. See the blending page for more information on this topic.

Frequently a non-trivial amount of beer is dumped at spontaneous beer breweries [50] (~8.5 min in). The exact amount depends on the conditions of the brewery and the willingness of the brewer to try to blend in batches that might not taste as good and/or have mild off flavors at the expense of the overall quality of the blend, but commercial brewers have reported dumping levels of 5% (and possibly up to 15%) of total production [33] (~13 minutes in). This may be due to an imbalance in the microbes [33] (~14 minutes in) or a bad barrel resulting in off woody flavor [7] (~1:31 in) or excessive O2 exposure. In addition to the beer inside such barrels being dumped, the barrel itself is also often discarded [33] (~14 minutes in). Homebrewers who are fermenting spontaneously may expect that from time to time they will need to dump a batch.

Microbial Succession During Fermentation

Scientific research in Belgium and the US has shown a regular general pattern to the microbial succession of spontaneous fermentation beer. [51] [2] [52] [5]. This has been illustrated well by Raj Apte [53]. The first stage, which lasts for approximately 1 month [51] [54], is dominated by enterobacteria that produce large amounts of DMS which can be smelled during the early stages of fermentation (see Dimethyl Sulfide for more details). 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 [54]. 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 [51].

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 [51]. Other sources describe these as one extended maturation phase with acidification from Pediococcus and Brettanomyces growth occurring simultaneously [5] [2] [52]. Note that many scientific publications use the terminology Dekkera rather than Brettanomyces. As many of the flavor and aroma characteristics that we associate with spontaneously fermented beer are produced during this slow maturation/acidification phase, allowing sufficient aging time is important when producing spontaneously fermented beers [38] [39].

During the extended maturation phase, a beer may become "sick" or "ropey", though not all producers get this [7] (~1:10 min in) [55] (~1:44 in) [41] (~3:44 in). This is the result of exopolysaccharides, 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 exopolysaccharide 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 [55] (~1:47 in). A beer which is sick in the bottle will generally clear through the same process as a younger aging beer when given appropriate time. See the Pediococcus page for more information.

See also Lambic.info "Microbiology and Biochemistry" wiki page.

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 [33] (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 [56]. 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.

Jester King has put grapefruit zest in the coolship and run hot wort through ~1 lb/bbl of Sorachi Ace before the coolship[57] (~57 minutes in).

Spontaneous Fermentation versus Mixed Fermentation

Spontaneous fermentation yields the greatest diversity of microbes in the wort, including many outside of Saccharomyces, Brettanomyces, Pediococcus and Lactobacillus [2] [5] [52]. The degree to which these diverse microbes present during spontaneous fermentation are active and influence the characteristics of the final product is unknown, but brewers report in some cases upwards of 100 distinct microbes present and 24 different microbes which are active and important in producing the character of their beers [7](~36 minutes in). Spontaneous fermentation may be conducted anywhere, though the microbes present in different environments and/or at different times of the year or from different cooling rates due to different ambient night time temperatures may be better or more poorly suited for producing a good tasting final product [33] [7](~39 minutes in, ~54 minutes in). In addition, many of the microbes active in commercial spontaneous fermentation derive from the brewery environment [2], which is a benefit that the average homebrewer likely does not have. This great range in the potential of spontaneous fermentation can produce some of the most complex beers in the world, but it can also produce undrinkable products.

Some brewers may opt for the more controlled techniques of Mixed Fermentation to approach the sorts of characteristics found in spontaneously fermented beers. Mixed fermentation employs the controlled pitching of different lab sourced microbes or bottle dregs. These may be pitched all at once or staggered to control the final product. This greater degree of control can limit some of the risk of poor outcomes and can allow a brewer to better achieve the beer they want; however this approach cannot yield the same microbial diversity of spontaneously fermented beer. For this reason, homebrewers may need to decide what degree of risk they are willing to take and what sort of final product they are after to determine which technique is right for them. Many use a hybridized approach of the two, combining open cooling for spontaneous inoculation with pitching of labs cultures and bottle dregs. While this is technically not spontaneous fermentation and it may yield different results from truly spontaneously fermented beers, it can be a good balance of the benefits of spontaneous fermentation (collection of ambient microbes to express regional terroir and a greater diversity of microbes present) with benefits of mixed fermentation (some pre-screening and greater control in dominant microbes to help select for a final beer of the brewer's preference). Ultimately the brewer must decide which approach, or combination of the two approaches, is right for them with regard to the desired flavor and aroma profile, adherence to tradition, timeframe, and risk of bad beer.

Videos

See Also

Additional Articles on MTF Wiki

External Resources

References

  1. The Mystery of Lambic Beer. Jacques De Keersmaecker. Aug 1996. Retrieved 05/05/2015.
  2. 2.0 2.1 2.2 2.3 2.4 Bokulich et al, 2012
  3. Cantillon Facebook post 5-February-2015
  4. Modern Brewery Age Weekly 23-October-2009 Article by Peter Reid with Frank Boon, accessed 7-May-2015
  5. 5.0 5.1 5.2 5.3 5.4 5.5 Spitaels et al., 2015
  6. Conversation between Dave Janssen and Steven Sonck of De Cam, winter 2014
  7. 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 The Sour Hour Episode 11 with Rob Tod and Jason Perkins from Allagash, Jean Van Roy from Cantillon, and Vinnie Cilurzo from Russian River
  8. The Mad Fermentationist Spontaneous Starters, accessed 7-May-2015
  9. MTF Facebook thread about naming
  10. Burgundian Babble Belt discussion
  11. 11.0 11.1 Flat Tail on the Brewing Network, ~1:04 in
  12. Understanding How to Control Flavor and Aroma Consistency in Dry Hopped Beer. Dan Vollmer, Dan Sharp, Dr. Tom Shellhammer (Oregon State University). Oral presentation at the 2015 Craft Brewers Conference
  13. Conversation between Dave Janssen and Gert Christiaens of Oud Beersel, 19-September-2015
  14. Cantillon Iris
  15. Cantillon Facebook Page post 22-Sept-2015
  16. Conversation between Dave Janssen and Jean van Roy of Cantillon, 17-Sept-2015
  17. Howat, James. Facebook live video stream. 12/23/2016. ~5:30 minutes in.
  18. Thread on Jester King Brewery Facebook thread. 01/16/2017.
  19. Facebook post by James Howat
  20. Conversation between Dave Janssen and Armand Debelder of 3 Fonteinen, July 2011
  21. 21.0 21.1 James Howat presentation at NHC 2015
  22. Sakamoto and Konings, 2003. Beer spoilage bacteria and hop resistance.
  23. Clostridium botulinum Wikipedia page
  24. "Fact of Fiction - Can Pathogens Survive in Beer? The RDWHAHB Edition". Bryan of Sui Generis Blog. 01/05/2017. Retrieved 01/16/2017.
  25. 25.0 25.1 Influence of Limnological Conditions on Clostridium Botulinum Type E Presence in Eastern Lake Erie Sediments (Great Lakes, USA). Alicia Pérez-Fuentetaja, Mark D. Clapsadl, Donald Einhouse, Paul R. Bowser, Rodman G. Getchell, W. Theodore Lee. 2006.
  26. "Henry's Law". Bouldess.com website. Retrieved 03/07/2017.
  27. Graph of oxygen solubility in water at different temperatures. Engineering Toolbox website. Retrieved 03/07/2017.
  28. Bryan of Sui Generis blog. MTF discussion on dissolved oxygen in wort cooled in a coolship, and the accuracy of DO meters. 03/02/2017.
  29. Amaral, Justin. MTF discussion on dissolved oxygen in coolship wort. 03/07/2017.
  30. Coker, Ryan. MTF discussion on dissolved oxygen in wort cooled in a commercial coolship. 03/07/2017.
  31. "Storing Wort Runs the Risk of Botulism". Dr. Colby, Chris. Beer and Wine Journal blog. 04/17/2014. Retrieved 03/07/2017.
  32. Bryan of Sui Generis Blog. MTF discussion on dissolved oxygen in wort cooled in a coolship. 03/07/2017.
  33. 33.0 33.1 33.2 33.3 33.4 33.5 The Beer Temple Interviews #264 with Trevor Rogers of De Garde
  34. 34.0 34.1 [http://comeandbrewit.libsyn.com/2016/page/2/size/25 "Episode 34- Sour Beer 102", Come and Brew It podcast. Interview with James Howat from Black Project Spontaneous Ales. 01/07/2016 (~40 minutes in).
  35. Conversation with Caleb Buck on MTF about spontaneous fermentation. 11/24/2015.
  36. Conversation on MTF with Dustin Carver on how long his spontaneous fermentation took to start. 12/14/2015.
  37. Conversation on MTF with Mark B. Fry on how long spontaneous fermentation took for him. 01/13/2016.
  38. 38.0 38.1 Van Oevelen et al., 1976. Synthesis of aroma components during the spontaneous fermentation of lambic and gueuze
  39. 39.0 39.1 Spaepen et al., 1978. Fatty acids and esters produced during the spontaneous fermentation of lambic and gueuze
  40. Rudy Ghequire from Rodenbach on the Sour Hour
  41. 41.0 41.1 Vinnie Cilurzo of Russian River on the Brewing Network's Sunday Session, 17-January-2010
  42. Yakobson, Chad. Pure Culture Fermentation Characteristics of Brettanomyces Yeast Species and Their Use in the Brewing Industry. Pure Culture Fermentation Discussion. 2011.
  43. Raj Apte's oxygen permeability table
  44. Better Bottle closure study
  45. Dan's video discussing airlocks and fermenters
  46. 46.0 46.1 Mad Fermentationist $8 homebrew barrel
  47. MTF post regarding mold growth in homebrew spontaneous fermentations. 03/06/2016.
  48. MTF post regarding limiting headspace to prevent mold growth. 10/19/2016.
  49. lambic.info 3F
  50. Sour Beer Panel at the Firestone Walker International Beer Fest
  51. 51.0 51.1 51.2 51.3 Van Oevelen et al., 1977
  52. 52.0 52.1 52.2 Spitaels et al., 2014
  53. Raj Apte Concepts of sour Beer, 2004
  54. 54.0 54.1 Martens et al., 1992
  55. 55.0 55.1 Recording of Vinnie's talk at NHC
  56. MTF facebook conversation with screenshot of brief De Garde process, March 2014
  57. The Sour Hour #14: with Jester King, pt. 1