Reader and Berliner Weisse brewer Scott hit me with this recently, and it was too chewy to not repost here:
So I recently came across this thread on Homebrew Talk ( http://www.homebrewtalk.com/f163/british-yeasts-fermentation-temps-profiles-cybi-other-thoughts-221817/).
Anyway, my initial question has less to do with the subject of the thread (ferm temps) but more to do with something one of the responders noted. His premise was that for lack of a better term “head pressure” during fermentation didn’t allow full expression by some of these British yeasts. He advocated an “open” or “semi-open” fermentation. I was wondering what your thoughts on this subject matter? Would a closed environment have an off affect than an open free release of gases environment?
The short answer is … okay, it’s not that short, and it does have a few buts.
The not-short answer
Open or semi-open fermentation systems with English ale strains (or any yeast, for that matter) will produce a different result than a closed, airlocked system – I don’t think it’s fair to say the effects of closed system are totally negative for English strains (more on that later).
The ready access to atmospheric oxygen that an open, non-pressurized fermentation affords the yeast cells promotes growth, which in turn encourages synthesis of (particularly ethyl acetate and isoamyl acetate). All other things being equal, a closed system will produce a lower, less complex ester profile.
Open fermentation is a great trick to have in the playbook for tweaking the profile, not only of English ale styles, but just about anything – Weissbiers and Belgians, natch, but it’s also still practiced by some lager brewers. Anchor Steam and Sierra Nevada Bigfoot are both still done in open tanks.
Now, leaning heavily on Brewing Yeast & Fermentation by Messrs. Boulton & Quain, I arrive at a Sir-Mix-A-Lot-video level of buts:
A major environmental factor that also affects yeast behavior is fermenter geometry: it’s important to remember that when discussing these open systems, the fermentation is conducted in broad, shallow tanks versus the tall, narrow and deep modern cylindroconicals. (I’ll plug JD’s excellent experiment again here, which demonstrates the effects of geometry on flavor in a Weissbier fermentation).
In a tall, narrow tank, the fermentation tends to be more turbulent, which (again, all other things being equal) creates a faster rate of fermentation. In a broad, shallow tank, the fermentation rate tends to be slower, leading to less scrubbing of volatile aromatics by escaping CO2; it also tends to promote lower attenuation (the tradeoff is that it’s much easier to keep highly-flocculent strains in suspension and at their work in a shallow vessel than it is in a tall one).
In an open-system with a top-cropping strain (like many English ale yeasts are), the traditional method is to skim yeast from the surface; because – as Boulton & Quain write – isohumulones selectively bind to cell walls, this practice reduces bitterness levels in the finished beer.
All of that sets the stage for some fairly significant differences in sensory profile; some of the variance is probably strain-dependent (3068, for example: highly manipulatable), but overall I’m not sure the changes would be that dramatic in a closed vs. open system on a homebrew scale without also adjusting the fermentor depth … a bucket or carboy is taller than it is wide, which, I would guess, would mitigate the effects somewhat.
Speaking of depth – the headspace pressure is one of the factors attributed to ester suppression in a closed system; however, pressure is exerted on cells not just by gas in the headspace of the fermenter (which is escaping through an airlock anyway …) but by the density of the wort as well as the height of the fermentor. So depth again – shallower fermentation vessels mean less osmotic pressure on the cells. The pressure being gradually burped through the airlock during active fermentation in a closed system shouldn’t be enough to reach the levels required to actively suppress the formation of esters and other volatiles.
Boulton & Quain, everybody:
Regardless of the pros and cons of traditional versus deep fermenters, the industry has made a decision, and in the vast majority of commercial breweries beer is fermented in large-volume vessels. Thus it is reported that only 5% of total beer production is now produced by top fermentation.
… the Whitbread Brewery in the United Kingdom installed cylindroconical fermenters with capacities of 1200 hl and wort depths of 6 m. Using these vessels, ales were produced that were considered analytical and organoleptic matches for similar beer fermented in traditional shallow open vessels.
Being a good Luddite, I will go through the motions of shaking my fist and grumbling that things aren’t as good as they used to be (which reminds me, I need to order the 20th anniversary edition of Bleach … damn kids and their Skrillex). But it does raise the point that odds are good that a lot of the classic English ales we’re drinking today aren’t fermented the way they used to, and reminds us that yeast is a living organism and will adapt (or can be made to adapt) to new conditions.
Man, I need a beer and an experiment. I propose that, first, we all get a beer (or at least a coffee and an IOU to yourself for a beer); then anybody that cares to join me in conducting a split batch of some sort with an English ale strain – closed vs. open/non-pressurized, or perhaps tall and narrow vessel vs. broad and shallow. This is opportune, as I have a pack of 1469 that needs to get used. I’ll detail that brew session and results in a future post, everybody can share their results in the comments, and we can perhaps build a little compendium of flavor effects on various British strains.