exBEERiment | Impact Open Fermentation Has On An American Pale Ale

Author: Paul Amico

Fermentation is the seemingly magical part of the brewing process where yeast convert sugary wort into alcohol and carbon dioxide (CO2), thus turning it into the much beloved beverage called beer. Whereas modern brewers, aware of the negative impact certain microorganisms can have, tend to keep their fermentation vessels sealed, open fermentation was commonplace among brewers in a number of historical brewing regions.

As the name suggests, open fermentation involves fermenting beer in an unsealed vessel where the top is exposed to the environment. In addition to allowing for easy yeast harvesting, this method is purported to encourage the development of certain esters and phenols while also allowing undesirable fermentation byproducts to readily off-gas. Some have even claimed that the lack of backpressure in an open vessel leads to less yeast stress and thus healthier fermentations overall.

Of the hundreds of batches I’ve brewed over the years, I’ve never attempted an open fermentation, as my concern about oxidation has always outweighed my curiosity. However, with a couple past xBmts indicating open fermentation may have an impact on more characterful beer styles, I began to wonder how it might affect an American Pale Ale and put it to the test!

| PURPOSE |

To evaluate the differences between an American Pale Ale fermented in a closed vessel and one fermented in an open vessel.

| METHODS |

For this xBmt, I designed a simple American Pale recipe using the LUPOMAX version of one of my favorite hop varieites.

Blunted Affect

I started off this brew day by collecting the full volume of filtered water for a single 10 gallon batch, which I adjusted to my desired profile.

After flipping the switch on my Clawhammer Supply 240v controller to get it heating up, I weighed out and milled the grain.

With the water properly heated, I incorporated the grains then turned on the pump to recirculate the sweet wort. When the hour long mash rest was complete, I hoisted the grains out of the kettle then set my controller to heat the wort up.

I then prepared the kettle hop additions.

The wort was boiled for 60 minutes with hops added at the times listed in the recipe, after which it was quickly chilled with my CFC during transfer to identical sanitized fermenters.

With a refractometer reading showing the wort was at my target 1.054 OG, I placed the fermenters in my chamber and pitched a pouch of Imperial Yeast I22 Capri into both batches.

At this point, I sealed one fermenter and draped a sanitized Brew Bag over the other one.

The beers were left to ferment at 70°F/21°C for 6 days before I returned to add the dry hop additions.

After another week, I took hydrometer measurements showing the open fermented beer finished 0.004 SG points higher than the beer fermented in the sealed vessel.

At this point, I proceeded with transferring the beers to CO2 purged kegs, which were placed in my keezer and burst carbonated overnight before I reduced the gas to serving pressure. After a week of conditioning, they were carbonated and ready for evaluation.

| RESULTS |

A total of 23 people of varying levels of experience participated in this xBmt. Each participant was served 1 sample of the beer fermented in a sealed vessel and 2 samples of the beer fermented in an open vessel in different colored opaque cups then asked to identify the unique sample. While 12 tasters (p<0.05) would have had to accurately identify the unique sample in order to reach statistical significance, only 7 did (p=0.69), indicating participants in this xBmt were unable to reliably distinguish an American Pale Ale fermented in a sealed vessel from one that was fermented in an open vessel.

My Impressions: Out of the 5 semi-blind triangle tests I attempted, I correctly identified the odd-beer-out 3 times, which isn’t consistent enough to indicate I could reliably tell them apart. I perceived these beers as having similar levels of classic Cascade hops character with a unique and tasty juiciness from the I22 Capri yeast.

| DISCUSSION |

Open fermentation is a practice that’s as old as brewing itself, and while the earliest brewers likely did it as a means of unwittingly inoculating wort with yeast, some modern brewers contend it has other benefits. Still commonly employed by brewers of traditional British, Belgian, and German ale, fermenting in open vessels is said to encourage the formation of esters and phenols appropriate in those styles. Interestingly, tasters in this xBmt were unable to reliably distinguish an American Pale Ale fermented in a sealed vessel from one that was fermented in an open vessel.

Considering past xBmts where tasters were able to distinguish open fermented Saisons and British Golden Ales from ones fermented in sealed vessels, while such was not the case in a similar comparison with Czech Premium Lager as well as this American Pale Ale, it seems plausible open fermentation has a larger impact when more characterful yeasts are used. However, there’s also the possibility something else is at play, for example, the hop character in this American Pale Ale being strong enough to cover up any differences caused by the fermentation environment. One of the more curious outcomes of this xBmt is that the closed fermentation beer had a higher FG than the batch fermented in a sealed vessel, as some posit backpressure actually leads to lower attenuation when using certain yeast strains.

Prior to this xBmt, I’d never performed an open fermentation, and given both these results as well as my own inability to tell these beers apart, I see no good reason to continue using this method, other than for future experimentation, of course. While I’m open to the idea that open fermentation can have a noticeable impact on certain types of beers, I’m happy enough with the beers I make in sealed vessels, which has the added benefits of reducing the risks of both contamination and oxidation.

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

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