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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. 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>. 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 <ref name="Spontaneous Sour Hour" /> (~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 <ref>Conversation between Dave Janssen and Armand Debelder of [[3 Fonteinen]], July 2011</ref>) 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|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 <ref name="Spontaneous Sour Hour">[http://www.thebrewingnetwork.com/membersarchive/sourhour2015_05_wildfriendship.mp3 The Sour Hour Episode 11 with Rob Tod and Jason Perkins from Allagash, Jean Van Roy from Cantillon, and Vinnie Cilurzo from Russian River]</ref>(~39 minutes in, ~54 minutes in).
Traditional producers of spontaneously fermented beer conduct fermentation for a long time period (1-3+ years) in wooden vessels. The long fermentaiton fermentation process allows the different microbes present to carry out their slow metabolism of the complex carbophydrates present in the beer, developing the flavors and acidity associated with spontaneous beers (-------). The wooden fermentaiton 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 foeders 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 a environment which houses some of the microbes active in the fermentation (notably Brettanomyces, which can penetrate up to ---- (ref) 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) (----ref---). While a controlled micro-oxidation can be beneficial to the beer, too much oxygen exposure can lead to excessive acetic acid production (either from ''Brettanomyces'' or ''Acetobacter'') (----ref).
On a homebrew scale a fair amount of attention has been paid to the topic of O2 permeability in different fermentation vessels and closures <ref>[http://www2.parc.com/emdl/members/apte/GingerBeer.pdf Raj Apte's O2 permeability table</ref> <ref>[http://www.mocon.com/assets/documents/PPS_Article_highq.pdf Better Bottle closure study</ref> <ref>[https://www.youtube.com/watch?v=boLqmFIzUZ0&list=PLibE2BjPG_8H0IZe4fS2FD4uidCFhgzBn&index=4 Dan's video discussing airlocks and fermenters </ref>. It has been suggested that sealing a glass carboy with a wooden dowel or chair leg can result in similar O2 permeability as a wine barrel. While this was quite a clever idea for replicating O2 exposure, this is not recommended as it can lead to breakage of the glass carboys <ref>[ref name="Mad Fermentationist Oak">http://www.themadfermentationist.com/2007/02/8-homebrew-barrel.html Mad Fermentationist $8 homebrew barrel]</ref>. While micro-oxygenation may be an important part of some spontaneous production it may be getting too much attention in homebrew carboy conditions <ref>[ref name="Mad Fermentationist Oak">http://www.themadfermentationist.com/2007/02/8-homebrew-barrel.html Mad Fermentationist $8 homebrew barrel ]</ref> (see comments) relative to other controls such as temperature, microbes, and time. See the [[Barrel]] page for discussions on the barrels available to homebrewers.
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) <ref name="Spontaneous Sour Hour">[http://www.thebrewingnetwork.com/membersarchive/sourhour2015_05_wildfriendship.mp3 The Sour Hour Episode 11 with Rob Tod and Jason Perkins from Allagash, Jean Van Roy from Cantillon, and Vinnie Cilurzo from Russian River]</ref> (~1:14 in), -----Armand ref. 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).