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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 <ref>[https://www.facebook.com/JesterKingBrewery/posts/10154502699393649?comment_id=10154504389923649&reply_comment_id=10154512163043649&comment_tracking=%7B%22tn%22%3A%22R2%22%7D Thread on Jester King Brewery Facebook thread. 01/16/2017.]</ref>). 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 <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1068826853145528/ Facebook post by James Howat] </ref>. Cantillon targets a cooled wort temp of 18-20 C (64.4-68 F) after the overnight cooling <ref name="Spontaneous Sour Hour" /> (~50 min in). Traditional producers only carry out spontaneous fermentation between fall and spring when nighttime temperatures have a low of -3.9 to 8°C (25-46°F) to appropriately cool the wort overnight <ref>Conversation between Dave Janssen and Armand Debelder of [[3 Fonteinen]], July 2011</ref><ref name="Spontaneous Sour Hour" />(~39 minutes in, ~54 minutes in). 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 [[Wort Souring]], see also [[Spontaneous_Fermentation#Alternative_Applications_of_Spontaneous_Fermentation|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, for example Rob Tod from Allagash Brewing reported solventy and ethyl acetate issues in beers after they were cooled over night at warmer temperatures and aged for 2 years <ref name="Spontaneous Sour Hour" />(~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 <ref name="Spitaels et al., 2015">[http://www.sciencedirect.com/science/article/pii/S074000201500012X/ Spitaels et al., 2015]</ref> (see below, [[Spontaneous_Fermentation#Microbial_Succession_During_Fermentation|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 <ref name="James Howat presentation at NHC 2015">[http://www.ahaconference.org/seminars/wild-and-spontaneous-fermentation-at-home James Howat presentation at NHC 2015]</ref>. 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 <ref>[http://www.sciencedirect.com/science/article/pii/S0168160503001533/ Sakamoto and Konings, 2003. Beer spoilage bacteria and hop resistance.]</ref> and ''Clostridium botulinum'' is gram positive <ref>[https://en.wikipedia.org/wiki/Clostridium_botulinum/ Clostridium botulinum Wikipedia page]</ref>. 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 <ref name="James Howat presentation at NHC 2015">[http://www.ahaconference.org/seminars/wild-and-spontaneous-fermentation-at-home James Howat presentation at NHC 2015]</ref><ref>[http://suigenerisbrewing.blogspot.com/2017/01/fact-of-fiction-can-pathogens-survive.html "Fact of Fiction - Can Pathogens Survive in Beer? The RDWHAHB Edition". Bryan of Sui Generis Blog. 01/05/2017. Retrieved 01/16/2017.]</ref>. 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 (acidic conditions can trigger exopolysaccharide production in some strains of lactic acid bacteria; see [[Pediococcus#.22Ropy.22_or_.22Sick.22_Beer|''Pediodoccus'']]) <ref name="Spontaneous Sour Hour" /> (~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 <ref name="Pérez-Fuentetaja">[https://link.springer.com/article/10.1007/s10750-005-0011-1 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.]</ref>(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 <ref>[https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/respiratory-system-22/gas-laws-210/henry-s-law-1032-977/ "Henry's Law". Bouldess.com website. Retrieved 03/07/2017.]</ref><ref>[http://docs.engineeringtoolbox.com/documents/639/oxygen-solubility-water-2.png Graph of oxygen solubility in water at different temperatures. Engineering Toolbox website. Retrieved 03/07/2017.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1599584193403122/?comment_id=1599693336725541&reply_comment_id=1600361503325391&comment_tracking=%7B%22tn%22%3A%22R%22%7D 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.]</ref>. 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 <ref>[https://www.facebook.com/browse/likes?id=1605741916120683 Amaral, Justin. MTF discussion on dissolved oxygen in coolship wort. 03/07/2017.]</ref>. 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 <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1601687793192762/?comment_id=1601698063191735&reply_comment_id=1602191886475686&comment_tracking=%7B%22tn%22%3A%22R1%22%7D Coker, Ryan. MTF discussion on dissolved oxygen in wort cooled in a commercial coolship. 03/07/2017.]</ref>. 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 <ref>[http://beerandwinejournal.com/botulism/ "Storing Wort Runs the Risk of Botulism". Dr. Colby, Chris. Beer and Wine Journal blog. 04/17/2014. Retrieved 03/07/2017.]</ref>) 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 <ref name="Pérez-Fuentetaja" /><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1604630206231854/?comment_id=1605313322830209&reply_comment_id=1605352169492991&comment_tracking=%7B%22tn%22%3A%22R%2312%22%7D Bryan Heit of Sui Generis Blog. MTF discussion on dissolved oxygen in wort cooled in a coolship. 03/07/2017.]</ref>.
==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]]'' <ref name="Bokulic et al., 2012">[http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0035507/ Bokulich et al, 2012]</ref> <ref name="Spitaels et al., 2015">[http://www.sciencedirect.com/science/article/pii/S074000201500012X/ Spitaels et al., 2015]</ref> <ref name="Spitaels et al., 2014" />. 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 <ref name="Spontaneous Sour Hour" />(~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 <ref name="Beer Temple interview with De Garde">[https://vimeo.com/127084279 The Beer Temple Interviews #264 with Trevor Rogers of De Garde]</ref> <ref name="Spontaneous Sour Hour" />(~39 minutes in, ~54 minutes in). In addition, many of the microbes active in commercial spontaneous fermentation derive from the brewery environment <ref name="Bokulic et al., 2012">[http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0035507/ Bokulich et al, 2012]</ref>, 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.