Skip to main content
Cart 0 items: $0.00
The Rare Barrel
December 20, 2016 | 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


Time Posted: Dec 20, 2016 at 8:00 AM Permalink to An Overview on Fermentation at The Rare Barrel Permalink Comments for An Overview on Fermentation at The Rare Barrel Comments (13)
The Rare Barrel
May 21, 2014 | The Rare Barrel

How long should you cellar our bottles?

Our bottles of sour beer are ready for consumption when we release them, however, aging bottles of sour beer can be a fun and exciting experience. Since sour beers are acidic (similar pH to wine) and have very little hops, they have the potential to age well over the years. A lot of our visitors enjoy cellaring sour beers, so we’re often asked, “How long should I cellar my bottle of your sour beer?” It’s a simple and great question, with one simple and one complicated answer.

The first, and simplest of the answers, is that we don’t know how long you should age them. Why don’t we know how long you should cellar your beers? Well, we don’t have many data points with our own bottles. We’ve only had beer in bottles since December of 2013, so we can’t say with 100% confidence that our beer will taste great/better in 1, 2, or 5 years. However, our bottles of sour beer should age well for years to come, if cellared properly (55F and in a dark place), which leads us to our second answer to this question. 

The second answer, which is long and a little more complicated, is that we do everything we can to preserve the long term quality of the beer and pass on the cellarmanship of that bottle from us, to you.  If you are interested in geeking out over the things we do to ensure our bottles of beer are best suited for cellaring, here are a few of the things we do to maintain the long-term quality of our sour beers...

Bottle Conditioning With Fresh Yeast – When we bottle a sour beer, we will add some fresh yeast and a very specific amount of sugar to the beer prior to packaging. Over the course of 1-3 months, the yeast will consume the sugar and create CO2, which will naturally carbonate each bottle of beer. One of the main benefits of bottle conditioning is that the fresh yeast will also absorb some residual oxygen, which can very damaging to beer flavors. Bottle conditioning with fresh yeast should reduce oxidation in the bottle. 

Oxygen Scavenging Bottle Caps – Oxygen, a highly reactive molecule, is very damaging to flavors in beer. Picture oxygen as a wrecking ball, swinging around and demolishing the flavors that the yeast and bacteria have spent so long creating. We do everything we can to prevent oxygen from coming into contact with our beers when they are aging and in the package. It’s impossible to remove 100% of oxygen from beer, but our oxygen scavenging bottle caps will continue to absorb oxygen that is in the head space of the bottle after packaging.

Complete Fermentation Before Bottling – Have you ever opened a bottle of beer, only to find the contents gushing out so fast that when all is said and done, the bottle is half empty and you have a big mess on your ceiling? Lame. Over carbonated beer is most often a result of too much fermentable sugar still in the beer when it is bottled. If a beer is bottled and there are still fermentable sugars available for yeast to consume, then the yeast will ferment those sugars, produce CO2, and carbonate the beer further. In order to prevent over carbonation in our bottles, we take regular density measurements to ensure that the yeast has stopped attenuating the beer before the beer is packaged in bottles. 

Very Thick Glass Bottles – It’s possible that a beer can over carbonate so much in a bottle that the pressure breaks the bottle. Not only do you lose all the beer you wanted to drink, but more importantly, this is very dangerous and could hurt someone. While we let our beers fully attenuate and do everything to prevent our beers from even getting close to this dangerous level of carbonation, we also use bottles that can withstand 15 Bar (over 200 psi). We don’t expect to have issues with over carbonation, but if we do that issue, we know we’re using the strongest bottles available.

Amber Glass and Oversized Labels – Light is another thing that can damage the flavors in beer and other foods. Specifically in beer, light will drive a photochemical reaction that converts iso-alpha-acids into 3-methly-2-butene-1-thiol, which has aromas that are reminiscent of a skunk. Not too appealing, right? Fortunately, iso-alpha-acids come from boiled hops, and most of our beers are under 10IBUs, so this shouldn’t be much of a problem with our sours. That being said, we use amber glass bottles and cover them with the largest label we can fit on the bottle to prevent as much light from reaching our beers as possible.

While we dont have a concrete answer as to how long you should cellar our beers, we are taking every step we can think of to ensure long-term qulaity in our beers.  We’re currently cellaring bottles of each brand, and we’ll be performing sensory analysis on these bottles for years to come. We’ll update this blog post in 10 years, when we have some concrete data around how our bottles have been aging over the years.    


Time Posted: May 21, 2014 at 11:03 AM Permalink to How long should you cellar our bottles? Permalink Comments for How long should you cellar our bottles? Comments (3)
The Rare Barrel
March 18, 2014 | The Rare Barrel

Now Serving Draft Beer at 44°F

If you've had the chance to visit our tasting room in the last three weeks, you might have noticed something different about our beer: it's warmer. We are now serving our draft beer at 44°F, which is about 4°F warmer than we were serving our beer for the first two months of being open. Why are we serving our sours at a higher temperature? It tastes better.

We had casually talked about serving our beer at a warmer temperature for a little while, but it wasn’t until we had our Beer Week dinner at Mikkeller Bar SF that we realized we needed to serve our beer at a higher temperature. Throughout our five course beer pairing dinner at Mikkeller Bar SF, we tasted five of our sours that were served between 45-55°F, and decided that we needed to make this happen.

Why couldn’t we serve our beers warmer before? Originally, we were using straight CO2 to push beer through our lines. With our old set up, our serving temperature range was limited to 38-42°F. The closer we'd get to 42°F, the more problems we'd experience with foaming, because the CO2 would come out of solution. We could potentially have increased the CO2 pressure to keep CO2 in solution, but that would have resulted in us unintentionally carbonating our beers more, which we did not want to do.

How do we serve our beers at 44°F now? We needed to do the following four things to serve our beer at 44°F:

  1. Increase the temperature on our cooler to 44°F
  2. Increase the applied pressure on our draft system to keep CO2 in solution (CO2 wants to come out of solution at higher temperatures)
  3. Install flow-regulated faucets to provide more restriction to the draft system and counter act the higher pressure on the draft system.
  4. Push beer though the lines with a blend of CO2 and nitrogen, instead of straight CO2

The last part of the puzzle is crucial to the new set up. We recently installed a nifty Green Air Supply, which extracts nitrogen from the atmosphere, then blends the nitrogen and CO2 in a ratio that is calculated for our system. The ratio of nitrogen and CO2 is specific to our serving temperature, draft line resistance, elevation above sea level, and a few other things. Since nitrogen is an inert gas, and doesn’t dissolve into our beer and “carbonate” the beer like CO2 does, we are able to apply more pressure to the whole system, which allows us to serve beer a few degrees warmer.

Sounds like a whole lot of work just to be serving our beer a few degrees warmer, right? We don’t think so. In our tasting room, the tulip glass is the final “package” of our product. Our sours take a very long time to make, and we take great pride in doing everything we can to maintain quality throughout the entire production process. When we find a better way of doing something, we’re going to adapt and improve our processes.

Next time you visit, feel free to leave the mittens at home and let us know what you think about the new serving temperature. 



Time Posted: Mar 18, 2014 at 8:07 AM Permalink to Now Serving Draft Beer at 44°F Permalink Comments for Now Serving Draft Beer at 44°F Comments (1)
The Rare Barrel
October 2, 2013 | The Rare Barrel

Draft Beer is Ready… Save the Dates!

Our first draft beers are ready to be served! Starting on October 17th, we’ll have a few events around the Bay Area to give you a sneak peak of our sour beers.  

Here are the details on the first four events scheduled:


Thursday, October 17th, 5-9pm, City Beer Store, 1168 Folsom St #101, San Francisco

Friday, October 18th, 5-9pm, Beer Revolution, 464 3rd St, Oakland

Sunday, October 20th, 12-5pm, Sour Sunday @ Jupiter, 2181 Shattuck Ave, Berkeley

Saturday, October 26th, 4-7pm, Livermore Saloon (by The Beer Barron), 2223 1st Street, Livermore


Each of the craft beer spots listed above will be serving two of our oak-aged sours on draft, accompanied by The Rare Barrel crew. Come by, say hi, and get a sneak preview of some of the beers we will be serving in our tasting room, which will hopefully be open by the end of November. We will be having a few more events over the next couple of months if you can’t make any of the three listed above.  

Our dream of serving our first sour beers has been years in the making, so we couldn’t be more excited to share our first sour beers with you!

Stay Sour, 

Time Posted: Oct 2, 2013 at 4:23 PM Permalink to Draft Beer is Ready… Save the Dates! Permalink Comments for Draft Beer is Ready… Save the Dates! Comments (3)
The Rare Barrel
January 22, 2013 | The Rare Barrel

What is sour beer?

As we sit here and drink our 3,000,000th glass of sour beer (rough estimate), our conversation inevitably leads us to talk about the flavor profile, acidity, yeast and bacterial characteristics, and ingredients… you know… the kind of stuff everyone ponders about when they’re enjoying a sour beer, right?

Well, probably not. While we geek out over the sour beers we drink, we also realize that a lot of our friends still aren’t familiar with what sour beer actually is… yet.  Let me take a moment and give you a quick rundown on sour beer:

Sour beer…

…is a term used to encompass many different beer styles, some of which include lambic, gueuze, Flemish red, and Berliner Weisse. The first three styles mentioned are traditional to Belgium, while the last is traditional to Germany.

…has an extremely wide range of complex flavors. Some of the more common descriptors are sour, acidic, tart, dry, yoghurt-y, and fruity.

…is the oldest style of beer in the world. This wasn’t really by choice though. For thousands of years, beers were “accidentally” sour because humans didn’t even know that yeast and bacteria existed! If beer wasn’t consumed quickly enough, it would become sour.

…becomes “sour” from Lactobacillus and/or Pediococcus, which are both bacteria. “You’re putting WHAT in my beer???” someone might ask. Yup, bacteria! You may have already been introduced to these bacteria, if you enjoy yogurt, kefir, milk, cheese, miso, sauerkraut, or sourdough bread. Lactobacillus (Lacto) or Pediococcus (Pedio) are responsible for the flavors produced in these foods.

…usually ages in wood barrels. The wood creates a perfect environment for the souring microorganisms to grow and ferment over a long period of time.

…takes a long time to make. In general, sour beers will age in barrels for anywhere from 6-36 months before they are ready to be consumed. At least that’s how long our sour beers will age.

…is commonly fermented with fruits. Cherries and raspberries are among the most traditional fruits fermented with sour beer, but apricots, peaches, figs, blackberries, and strawberries also make for great ingredients in sour beers. Fruit was traditionally added to beer as a way to preserve the fruit, but now fruits are added because they just taste amazing in the sour beers!

…is commonly fermented with Brettanomyces (Brett), which is a kind of yeast that is most commonly known for its funky flavors that are found in saisons and farmhouse ales. Brett will add a small amount of acidity to a beer, but won’t add enough sourness to make a beer sour by itself.  

…is a great beer for you to try if you enjoy wine. Some of characteristics they share are acidity, dryness, and fruit notes.

…can be spontaneously fermented, which means that the wort is exposed to the open air, which allows the natural yeast and bacteria in the air to fall into the wort, and ferment it into beer. By definition, lambics and gueuzes need to be spontaneously fermented. 

…is usually blended. For example, a brewery might have 10 oak barrels with sour beer. Once the sour beers are ready to be consumed, the brewery will blend some (or all) of those barrels back together to create the final product. Each of those 10 barrels will be slightly different, so it is up to the blender to decide what barrels and proportions are blended to make the final product. 

We originally were going to include an additional 39 descriptions of what sour beer is, but instead we decided to drink another sour beer and thought you would probably have more fun doing the same.


Time Posted: Jan 22, 2013 at 2:11 PM Permalink to What is sour beer? Permalink Comments for What is sour beer? Comments (1)
The Rare Barrel
November 19, 2012 | The Rare Barrel

Why all sour?

“Why all sour?” is a question we are commonly asked, and a question we love to answer. It basically boils down to our love of sour beer, not being able to get enough of it, and using the challenges of sour beer production to our advantage.

Firstly, sour beer is our favorite style of beer.  Like many who are passionate about craft beer, we were first enamored by the citrusy, piney, and resiny flavors of hops, and the bready, toasty, and sweet flavors from malt.  After countless dances with malt and hops, we were introduced to our first sour beer. Sour beer was different, intriguing, and new. While it may not have been love at first sight, there was something captivating about this style. Whether it was the pleasant tartness of a Berliner Weisse, the deep complexity of a Gueuze, or the refreshing raspberry notes of a Frambiose, we found unique flavors in sour beer that kept us coming back for more.  Before we knew it, sour beer became our favorite style of beer.  However, there was a problem: where could we find more sour beers?

As we tasted our way through the amazing sours that some of the German, Belgian, and American craft breweries made, we quickly realized that there weren’t as many options for sour beer as there were for other styles of beer. If  we were in the mood for an IPA, we wouldn’t have just one choice, but a plethora of choices at a craft beer store or bar.  But when it came to variety in sour beer, our options weren’t has plentiful. Many stores didn’t  sell sour beer, and the ones that did would have only a couple of options . When it came to bars, we were ecstatic to find even one sour beer on draft. So why aren’t there more options when it comes to sour beer? From the production challenges to consumer tastes, breweries are usually inclined to experiment with sour beer on a small scale, if at all. With only a handful devoting a large chunk of their time to these styles, finding sour beer has become a difficult task for those who love it. We hope we can contribute to making this process of finding sour beer a little easier.

While most look at the challenges of sour production and decide against pursuing it, we’ll be embracing these challenges. Sour beer production has been studied less than other areas of brewing.   By focusing completely on sour beer, we can concentrate on learning as much as possible about the reasons certain flavors are produced in sours, instead of worrying about keeping up with the constant needs of production of styles with shorter fermentation times. Through focus, we hope to gain a better understanding of the beer, and in return, reflect that understanding in the flavors of our beers. Another big concern that many breweries have when it comes to making sour beer is cross contamination. This occurs when the microorganisms used to make sour beer find their way into beers that brewers don’t want to be sour. We plan to reduce this concern by producing only sour beer. However, we will still take great care to ensure that our beers are made using the same techniques that other breweries use to prevent unwanted yeast and bacteria from affecting beers they aren’t intended to be in. With our entire focus on sours, we can also craft a cellar around the production of sour beer. In doing so, we will partner with local breweries by renting time on their brewhouse to brew our wort, then bring it back to our barrel house for fermentation, barrel aging, blending and packaging. This allows us to forgo building a brewhouse (…for now), which would only be used sparingly for these beers which take so long to age.

It’s been said that constraint breeds creativity. By adopting a singular focus on crafting sour beer, we plan on thoroughly exploring flavors through conventional sour beer production techniques, as well as a few more experimental techniques. We are ecstatic so share this journey with you. Follow our progress on this blog, as well as our Facebook and Twitter pages to learn more! 



Are you 21 or over?