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The Rare Barrel
December 20, 2016 | Production, Sour Beer | The Rare Barrel

An Overview on Fermentation at The Rare Barrel

Hello internet! As the Research & Development Coordinator at The Rare Barrel, my role is to track our beer from the day it is brewed until it is packaged, ensuring that barrels are harvested at the correct point and that we always have beer to blend, fruit, or package. I also aid in blending, and work to develop new approaches and procure different yeast, bacteria, and secondary additions for our sour beers. I’ll provide a brief intro to what fermentation is, how we procure the yeast and bacteria to make our sour beer, how we attend to the different stages of sour beer fermentation, and how we use fermentation to carbonate our beers.

What is fermentation?

Beer fermentation is all about yeast and bacteria converting grain-derived starches and sugars into alcohol, CO2, and other flavorful and aromatic compounds, including acids. Different yeast and bacteria perform this task uniquely, and the species and health of the organisms greatly affect the end product. A primary difference between sour beer and clean beer is the use of bacteria in this fermentation process. Bacteria such as Lactobacillus and Pediococcus are able to convert starches that yeast cannot, and a byproduct of these conversions are a bulk of the acids that make our beer sour.

Yeast and bacteria and where we get them


The Rare Barrel is unique in the number of different yeast and bacteria that we use. In addition to using single strains of Saccharomyces cerevisiae from many diverse brewing traditions (Belgian, French, English, American, etc.), we also use a wide variety of Brettanomyces yeast (claussenii, brux drei, lambicus, anomala, brux, etc.), Lactobacillus (brevis, delbrueckii), and Pediococcus (damnosus). Like most commercial breweries, we procure most of our microbes from commercial yeast labs (we work with several). At times we will pull barrels we like from our cellar and use those as inoculants. Occasionally we will get a pitch of yeast from another local brewery (yeast can get pretty expensive, and swapping yeast is a relatively common practice in the brewing industry). We also have a few cultures that have been grown up from the dregs at the bottom of some of our favorite commercial bottles. Propagation from bottles is done in a very homebrew fashion, where we carefully “decant” (and drink) the beer sitting atop the dregs, and add a small amount of sterile wort to the bottle. We then gradually step this up into a 5 gallon carboy, then a 15 gallon brink, our 5 BBL project fermenter, and then into a 30 BBL batch.
There are a number of different ways that we approve microbes for production, and there are no hard-and-fast rules here. Jay draws on his experience at The Bruery, and his four years guiding the program at The Rare Barrel. A number of us in production are avid homebrewers, and have extensive experience with many different strains and conditions, so we definitely have opinions. Additionally, we have a “menagerie” of about twenty different microbe blends at The Rare Barrel, kept in flasks and carboys with our wort. Tasting and evaluating these samples in a democratic fashion drives some of our decisions as to what should be trialed at a larger scale.

Scaling up! Stages of fermentation at The Rare Barrel


While we have done a few experiments with primary fermentation in barrels, we start nearly all of our fermentations in our stainless conical tanks. It varies whether primary fermentation will be performed by a single strain of Saccharomyces, multiple Saccharomyces strains, a blend of Saccharomyces and Brettanomyces, or a full mixed culture with numerous strains of Saccharomyces, Brettanomyces and bacterias. Our wort is produced off-site, transported in stainless totes to our facility, and then pumped into our fermenters past an oxygenation stone. Active fermentation produces a fair amount of heat, and depending on the yeast we may decide to control fermentation temperature with the glycol jackets, or to simply let the temperature “free rise.” Fermentation temperature plays a very important role in driving different alcohols and esters, phenols, and other precursors for biotransformation during secondary fermentation.
After primary fermentation subsides (about a week or so), we will transfer the beer to oak barrels. Sometimes we will add an additional inoculant to the beer prior to racking out of the tank; in other cases we will rack onto a slurry of yeast and bacteria from past batches; and in others we will rack without further amendation into rinsed, steamed and ozoned barrels. This decision is typically made by looking at our “cellar balance” and deciding where the project fits -- whether it is intended to be a portion of a particular blend or brand, or if we are just adding to the variety of our blending stock.
After the beer is in barrels, we monitor and wait. I guess we’d call this the “secondary fermentation” phase, where the microbes that function on longer timelines (typically Brettanomyces and the bacterias) are doing most of the work, and the beer is gradually souring. We track gravity and pH on most of our batches on a monthly basis, although some get tested weekly to ensure all is well. We are getting pretty good at predicting when a particular batch will be ready to harvest and either fruit or blend for a final product. If the beer receives a sugary secondary ingredient (fruit, for instance), it will be returned to the barrel with a shot of fresh yeast for another round of fermentation, where we monitor and wait once again.

Fermentation also occurs during our bottle conditioning process

While we “force carb” our draft beers in a brite tank using a CO2 stone, all of our bottled beers are naturally carbonated in the bottle. This means (you guessed it) more fermentation! Prior to bottling, it is essential that all active fermentation processes have come to completion, and the beer is absolutely stable with no residual sugars remaining. To carbonate our bottled beer, we add a calculated solution of dextrose and a yeast--just enough to add the level of carbonation that we target. If there were any sugars remaining in the beer, carbonation would increase potentially leading to gushers or even bottle-bombs. We are extremely cautious with packaging, and it is not unusual to delay bottling if we notice an anomaly.
After bottling, we allow the beer to rest and re-ferment for a minimum of 9 weeks before it is put up for sale. We have weekly check-ins on the progress of the conditioning, and will delay a release if we notice anything out of the ordinary. Bottle conditioning is a fermentation process, so the beer does go through some of the same phases that we notice in primary fermentation, although because dextrose is such a simple sugar it is consumed at an increased rate.
Our beers continue to evolve in the bottle even after they are a packaged. The yeast and bacteria that were present in primary and secondary stages of fermentation--especially Brettanomyces and Pediococcus--remain active and continue to slowly transform the more difficult to digest starches, creating new flavors, aromas and textures. While bottles are certainly ready to drink when we release them, they are a living product and change over time.
I hope this provides an interesting glimpse into how we think about the various stages of fermentation at The Rare Barrel. Post up any comments you might have and I’ll be glad to follow up on any questions!
Stay Sour!
Aaron M. Wittman
R&D Coordinator



Ludwig Mayer's Gravatar
Ludwig Mayer
@ Dec 20, 2016 at 9:21 AM
Aaron, Can you elaborate on your use of bacteria that produce only lactic acid (homo-lactic) versus bacteria that produce a mixture of lactic and acetic acids (hetero-lactic bacteria)?

Ben's Gravatar
@ Dec 20, 2016 at 9:55 AM
Also, do you kettle sour any of your beers? Or is the souring taking place in stainless (primary) or in the barrel?

Taylor's Gravatar
@ Dec 20, 2016 at 9:59 AM
Rare Barrel beers have a distinct, almost clean tartness that's been pervasive throughout your beers that I've tried. Curious if this is an intentional goal of your blending process and what factors you attribute it to (mash temp, yeast/bacteria strains, etc)? Are there any plans to diversify flavor profiles to encourage more brett-forward esters in some of your blends, or do you see this being a "house" character that identifies your end product?

AJ Moseley's Gravatar
AJ Moseley
@ Dec 20, 2016 at 10:03 AM
Hey Aaron,

Thanks so much for the post! I was curious about your process of bottle conditioning. When you guys have decided that the uncarbed beer has reached terminal and is ready to go into the bottle, do you calculate cell counts for YEAST additions when moving onto conditioning, or is it only pertinent to measure the sugar because there will already be enough active yeast before bottling?

Thanks for any insight!


Jerad's Gravatar
@ Dec 20, 2016 at 2:17 PM
Aaron, are you at liberty to share which strain you guys bottle condition with?

Aaron Wittman's Gravatar
Aaron Wittman
@ Dec 20, 2016 at 2:22 PM
Hey Ludwig -- Great question. We don't specifically differentiate in our treatment of homolactic or heterolactic cultures, or the beers/barrels that contain either or both. Instead, our strategy is to control acetic acid production through limiting oxygen exposure.

Heterolactic cultures produce acetyl phosphate, which is anaerobically reduced to ethanol; or, in the presence of oxygen, is reduced to acetic acid. As you probably guessed, this isn't the only pathway to acetic acid in our beers -- other microflora we use have the potential to create acetic acid as well. In addition to the heterofermentative Lacto, perhaps the most prominent acetic acid producer at our brewery is Brett Drei. In the early days most of our beers were made with Drei, and we learned quickly that the barrels would have a limited shelf-life, after which we would be on the lookout for acetic acid or ethyl acetate (perfume / nail polish).

Admittedly, as barrels are actually porous and get moved around with a forklift, we cannot entirely eliminate exposure to oxygen, but we have developed some strategies: First -- we track and know the ages of all our barreled beer, as well as the attributes of the cultures that we've worked with. If we have a batch that has a large proportion of Brett Drei, we know that the sweet spot for those barrels will be 8-10 months after brewday, so we look to blend those for package or further aging before they hit the danger zone. If a batch has a low flocculating saison yeast, hopped wort, and a diverse mixed culture of Sacc, Brett, and Bacteria, we know it may be more stable over a longer period. Second -- we purge, purge, purge with CO2. We purge our tanks and hoses before racking. We purge barrels before filling. We push our beer with CO2. We have CO2 flowing into the tank when we fruit. Third -- when adding fruit to a beer, and when moving a beer to barrels, we like to have some amount of active fermentation occurring. Lately when we fruit a beer we will rack actively fermenting beer from a recent brew into the tank to "attack" the fructos and take up any oxygen that might have snuck in. Ideally these beers will have healthy Brett, which is known for its oxygen scavenging abilities. Fourth -- we bottle condition. Refermentation in the bottle aids in taking up any oxygen that might have snuck in during the bottling process.

I'm sure I can come up with other strategies we employ, but I think you get the gist. Thanks for reading!

Mike Donnelly's Gravatar
Mike Donnelly
@ Dec 20, 2016 at 4:44 PM
Hey Aaron - great article. Even better surprise seeing your byline at the bottom. Glad to see that you're kicking the hell out of it in CA.

Merry Christmas!

Jonathan Owen's Gravatar
Jonathan Owen
@ Dec 21, 2016 at 6:29 AM
Excellent survey of your process! It sure sounds complicated. Never had your beer, but love Jolly Pumpkin. Will stop by next time I'm in town.

Aaron Wittman's Gravatar
Aaron Wittman
@ Dec 21, 2016 at 11:08 AM
Thanks for all the replies!

Hi Ben -- We have not kettle soured any of our beers. We use stainless steel totes to transport chilled wort from a brewery to our fermentation facility. Once here, we pump into our conical fermenters and ferment with whatever microbes we have chosen for that batch. Sometimes we will start with saccharomyces or brettanomyces by itself, sometimes sacc and brett, and sometimes a full mixed culture with bacteria. The beer sits in stainless for a little over a week, and then is transferred to barrels. Typically the beer has been inoculated with souring bacteria by the time it hits the barrel, but it still takes a few months in the barrel for tartness or sourness to develop.

Hello Taylor -- We do not have a particular house character that we target or blend toward--rather we try to combine batches and blends to create the best beer that we can. It’s difficult to attribute characteristics of our beers to a single variable such as mash temp, microbes, etc.; but these are all variables we look at when brewing to create new flavor and aroma profiles. Over the last year and a half or so our yeast and bacteria “portfolio” has increased dramatically, and we have also been developing a number of new techniques. Most releases since mid-2016 include multiple strains of sacc, multiple brett strains, and numerous lacto and pedio cultures -- this is a trend you will continue to see from us going forward. Another variable that we have been intensely experimenting (with great results) are hopping rates and times. Also, what I call “staggered pitching”, where we add additional microbes after primary fermentation has subsided. The only overarching “house character” that we target is “sour”, and even then we try to remain intensely vague in our definition of what that means.

Hello AJ and Jerad-- I thought I’d address both your questions together as they both pertain to our bottling procedure. We have a few metrics we use to determine the suitability of a beer for packaging, including fermentation trends, gravity stability, and finishing gravity. We also pay attention to pH, which can drop without affecting gravity, letting us know that homofermentative bacteria is still in play. Most importantly we rely on sensory evaluation. At times all instrument-derived indicators might suggest a beer is stable, but it just doesn’t taste or smell ready. This is particularly important in beers refermented with fruit, or soaking in spirit barrels, etc. When we determine that a beer is ready to package, we will rack the beer to a brite tank to get a more precise volume, and calculate our bottling sugar and yeast additions off of that measurement. We target anywhere from 2.2 - 2.7 vols CO2, depending on the beer. All calculations only account for the yeast we add for bottling -- we don’t rely on any residual yeast from primary or secondary fermentation. We currently use Maurivin Platinum as our bottling yeast, but are in the process of trialing other yeast to see how they might affect specific bottled beers.

Hey Mike! Great to hear from you -- Hope all is well in upstate! Hit me up if you’re in the Bay Area -- I’d love to catch up.

Sean White's Gravatar
Sean White
@ Dec 31, 2016 at 11:44 AM
Hi Aaron, Great post! What are your thoughts on cold crashing to drop out most of the sach after primary, vs. racking to barrels "warm"?

Aaron Wittman's Gravatar
Aaron Wittman
@ Jan 5, 2017 at 10:47 AM
@Sean -

We don't cold crash at The Rare Barrel, except to force carb & keg - any yeast (saccharomyces or brettanomyces) in suspension goes into the barrel. Because our beers have brett in them, and go through a long, slow secondary fermentation, we aren't too worried about (and haven't experienced) any autolysis related off-flavors. My understanding is that any sacc that might die or go dormant during this secondary aging period essentially become a nutrient source for the brett.

I'm not sure if or how bacteria interacts with dead or flocc'ed sacc cells. That might be something interesting to look into, for sure.

Kalle's Gravatar
@ Apr 13, 2017 at 10:56 AM
Hello. When you bottle, do you cold crash before even if you bottle condition to get less yeast (you will always get some if you do not filter/pasteurize it) or when doing bottle condition you only move to stainless to add yeast/priming and dump out the extra yeast in the bottom?

Great post and great answers, thanks!

Aaron Wittman's Gravatar
Aaron Wittman
@ Apr 14, 2017 at 10:37 AM
Hey Kalle -- We do not cold crash or fine our beers prior to bottling. Given the amount of time in barrels most of the yeast and bacteria flocculate out of the beer, and typically the beer is pretty clear coming out of barrels. As you mention, because we do not filter or pasteurize TRB bottles do contain residual yeast and bacteria from fermentation.

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