Author: Matt Del Fiacco

While sealing the fermentation vessel and affixing an airlock or blow-off tube to it is standard practice these days, it’s relatively new within the brewing world. Historically, fermentation occurred in loosely covered or even entirely open containers, leaving the beer exposed to whatever might be floating around the brewery, a thought sure to incite anxiety in some modern brewers.

However, open fermentation is still practiced today by brewers around the world, not many, but those who do often note it as positively influencing beer characteristics, some citing the lack of back-pressure as the primary factor. One example is Anchor Brewing who takes pride in being “the only American brewery that still employs open fermentation on a production scale,” which they believe contributes to their signature character.

Slightly more common among brewers in the UK, open fermentation is believed to encourage a more characterful ester profile, an important component of English ale, and also lead to improved attenuation. In fact, the revered Samuel Smith’s Brewery still use the Yorkshire Square to ferment in, a unique vessel that leaves the fermenting beer exposed to the environmental air.

With no experience open fermenting and as a lover of traditional English ale, I jumped at the opportunity to put this variable to the test!

| PURPOSE |

To evaluate the difference between a British Golden Ale fermented in a closed vessel and one fermented in an open vessel.

| METHODS |

With claims that traditional English ales were fermented in open vessels, I went with a classic British Golden Ale recipe for this xBmt.

Flamel

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5 gal 60 min 29.3 IBUs 3.8 SRM 1.042 1.010 4.2 % Actuals 1.042 1.011 4.1 % Fermentables Name Amount % Golden Promise (Simpsons) 5 lbs 60.15 Pale Malt (Weyermann) 2.75 lbs 33.08 Wheat, Torrified (Thomas Fawcett) 9 oz 6.77 Hops Name Amount Time Use Form Alpha % Magnum 9 g 60 min Boil Pellet 12 Cascade 34 g 15 min Boil Pellet 5.5 Cascade 62 g 0 min Boil Pellet 5.5 Yeast Name Lab Attenuation Temperature House (A01) Imperial Yeast 74% 62°F - 70°F Notes Water Profile: Yellow Balanced in Bru’n Water Spreadsheet Download Download this recipe's BeerXML file

Imperial Yeast A01 House is a classic English ale strain I thought was appropriate for this particular variable, so I got a starter of it going ahead of time.

Having collected my water the night before brewing, I started my brew day off by adjusting it to my desired profile and turning on the elements to heat it up.

I then proceeded to weigh out and mill 2 identical sets of grain.

When the water was at strike temperature, I mashed in and turned on the recirculation pump before checking the mash temperature.

Each batch was mashed for 60 minutes, after which I removed the grains and let them drip into the kettle to reach my target pre-boil volume.

While waiting for the wort to heat, I weighed out the hops.

In between hop additions during the 60 minute boils, I prepared the fabric filter I’d be using to keep critters out of the open fermentation vessel by boiling it to reduce the risk of contamination.

Once each boil was complete, I transferred the wort through my counterflow chiller directly into sanitized fermentation kegs.

Hydrometer measurements showed that both batches reached the same OG.

After placing the filled fermentors next to each other in my chamber, I sealed both and let them cool to my desired pitching temperature of 64°F/18°C. This took about 2 hours, after which I evenly split the yeast starter between the batches and set the controller to my desired fermentation temperature of 68˚F/20˚C. One vessel was sealed as usual with the keg lid and blow-off tube attached to the gas post, while I simply placed the fabric filter over the opening of the other fermentation vessel.

The next morning, active bubbling was observed in the blow-off vessel for the sealed batch and a quick peek under the cover revealed the open fermentation beer had developed a solid kräusen.

Activity in both batches had subsided after 5 days, so I sealed up the open fermentation batch and attached a blow-off tube to reduce the risk of oxidation. After another 2 days, I took hydrometer measurements showing the open fermentation beer had a slightly lower FG than its closed fermentation counterpart.

While the surface of the closed fermenation beer looked normal, the open fermentation beer had what I only hoped were large yeast rafts floating on the surface.

Gelatin fining was added to each beer before I sealed them up, purged the headspace several times with CO2, and put them back in my cool chamber to cold crash. A few days later, I racked the beers under pressure to sanitized serving kegs then placed them in my kegerator and burst carbonated overnight before reducing the gas to serving pressure. I allowed the beers to condition for a week before serving them to participants.

| RESULTS |

A total of 24 people of varying levels of experience participated in this xBmt. Each participant was served 2 samples of the closed fermentation beer and 1 sample of the open fermentation beer in different colored opaque cups then asked to identify the unique sample. At this sample size, 13 tasters (p<0.05) would have had to identify the unique sample in order to reach statistical significance, which is exactly the number that did (p=0.03), indicating participants in this xBmt were able to reliably distinguish an open fermented British Golden Ale from one fermented in a closed vessel.

The 13 participants who made the accurate selection on the triangle test were instructed to complete a brief preference survey comparing only the beers that were different. A total of 7 reported preferring the open fermentation beer, 3 liked the closed fermentation beer more, 1 reported having no preference despite noticing a difference, and 2 tasters perceived no difference between the beers.

My Impressions: The difference between these beers wasn’t as stark as I thought it would be, but out of 5 semi-blind triangle tests, I was able to identify the unique sample 4 times. I enjoyed these beers and perceived both has having a fairly mild malt character with a classic Cascade hop profile, though I noticed what seemed to be slightly less bitterness in the closed fermentation batch. Different enough to tell apart when focusing, but again, not nearly as much as I expected.

| DISCUSSION |

Open fermentation is a method believed by some to lead greater expression of esters, hence the reason some choose to rely on this approach for notably ester-forward beers such as British ale and even Bavarian Hefeweizen. Given results from a prior xBmt comparing Saisons fermented with the Dupont strain in either closed or open vessels, I didn’t personally think the beers in this xBmt would be noticeably different, but of course, I was wrong. The fact participants were able to reliably distinguish a British Golden Ale fermented in a typical closed vessel from one fermented in an open vessel suggests something about open fermentation has an impact.

One concern I had about the open fermentation beer was contamination– even though it was covered with a fine mesh bag, it seemed plausible something could get in there and have its way with the beer. While it’s possible this is what led to the significant result, I tasted these beers many times and never once detected any signs of contamination. Added to the data showing a majority of participants who were right on the triangle test endorsed the open ferment beer as their most preferred, it seems the more plausible culprits are likely differences in pressure or exposure to oxygen.

Not only were the beers unique enough for tasters to tell apart, but the method of open fermentation also had the more objective effect of encouraging greater attenuation. In talking with people after completing the triangle test, many noted the same thing I experienced in regards to the open fermentation beer being slightly more bitter than the closed fermentation beer, which perhaps the lower FG of the open fermentation beer contributed to.

This being just one point of data, I remain curious to explore the way open fermentation can shape the character of a beer, particularly with other yeast strains and beer styles. While I won’t be adopting open fermentation as my go-to approach, the significant results of this xBmt suggest it may very well be a valuable method for brewers looking to impart certain unique characteristics to their beer.

If you have any thoughts about this xBmt, please do not hesitate to share in the comments section below!

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