Sours, Coolships, Barrels, and the Aging Process


A touch of brewing alchemy.

Commercial beer is most often fermented “single culture” style, which is to say, with one of two kinds of yeast: Saccharomyces cerevisiae or Saccharomyces pastorianus. The former is responsible for most types of ale, while the latter, thanks to its ability to tolerate colder temperatures, is responsible for the low-and-slow fermentation that creates lagers. While this might sum up the yeast and fermentation connection in brewing, beer is never really that simple.

The main difference between a sour beer versus a clean beer is the use of bacteria in the fermentation process. Beer fermentation happens when yeast and bacteria convert grain-derived starches and sugars into alcohol, CO2, and other flavorful and aromatic compounds –– including acids. Because different yeast and bacteria perform this task uniquely, the species and health of the organisms in the fermentation process deeply impacts the what the final product becomes.

Using wild yeasts is the bedrock of our brewing process. It’s a gamble, sure. But that’s part of the culture of wild beer and sour beer brewing. When it’s right, it’s amazing. It might seem like inviting wild microbes into your beer to party seems to go against the grain of most brewing practices because brewing generally involves tons of sanitation—gloves, hoses, a careful and constant effort to keep unwanted microbes out of every stage of the process––especially fermentation. But spontaneous fermentation can also be done under the same controlled environment, and that’s a good thing (a great thing, really) because that leaves room for a third, slightly unpredictable kind of fermentation: the wild kind.

Harnessing yeast for the glory of flavor.

Beer made with malt, hops, water, and yeast wasn’t always classified as such. If fact, in 1516, yeast wasn’t even included in the original German Beer Purity Law (or “Reinheitsgebot”), no one knew what it was or the role it played in brewing. Brewers simply unsuspectingly passed yeast on from beer to beer when brewers would start a new batch from an old one, epically doing all the work of fermenting with no credit.

The initial fermentation of beer was done by ambient yeast—or yeast that simply lived in the surrounding environment wherever that first beer was brewed. As brewing knowledge developed, after Louis Pasteur discovered that microbes were responsible for souring alcohol and came up with the process of pasteurization, brewing was eventually regulated and yeasts were cultivated for their ability to develop consistently good beers. But it also gave brewers the knowledge and permission to experiment and go off script, to explore their wild side.  

Yeast strains like Saccharomyces but also Brettanomyces (brett) and wild microbes like Pediococcus and Lactobacillus are key players in our wild fermentation process, even though our “wild” fermentation is controlled, with wild bacteria carefully selected, cultured, and controlled as much as possible to yield a flavor profile.

Coolships and how they work.

Hundreds of years ago there was no fast way to cool down beer. In fact, brewers didn’t even really know why they had to cool wort down, since yeast had yet to be discovered. The fastest and most cost-effective way to cool the hot wort was for brewers to pump steaming liquid into large rectangular pans called coolships and expose them to cool, outdoor air (thereby also exposing them to natural bacteria in the environment). The large, exposed surface area of the vessel was a perfect mechanism for cooling the beer (relatively) quickly.

Coolship, referred to “koelschips” in Flemish, is a Belgian brewing technique that draws from time-honored brewing method dating back hundreds of years. It consists of traditional old-world fermentation techniques that expose the beer to unique microflora in open air. Because of its shallow depth, a coolship produces interesting and different flavors and character within the cooling wort at various temperature levels.

Once the benefits of yeast in brewing came to be discovered, coolship brewing took on a whole new relevance to beer’s flavor. Belgian brewers began to understand that this type of process helped to create the distinctive characteristics that would form their Lambic brews. They learned that as the wort sat in the coolship to cool, it also became a home to all the ambient yeast and microflora wafting through the Belgian night air.

Once that wort was fully exposed to this wild, invisible microflora, it was transferred into barrels for an extended stay—as much as three years. This method of fermentation, called “spontaneous fermentation,” produced a beer of singular taste: tart, funky, and unmistakably complex––a category we like to attribute to our beers at Wild Mind Artisan Ales. We are excited to be the first Twin Cities brewery to introduce local beer drinkers to coolship style beers. We believe that using a coolship adds to our house “terroir”, a term used by French winemakers when they reference the climate, soil and other local features that influence flavors and aromas.

Both our Coolship, and other sour beers, are aged in a variety of wooden barrels for several years to allow the yeasts to mature and create distinctive tastes. The beers gain complexity after they are transferred to the barrel for aging, picking up flavors from the liquid the barrel previously held, encountering microorganisms in the wood, and slowly oxidizing over time. This type of wild fermentation allows for more creativity than clean fermentation techniques and opens a world of flavors and aromas, which seemed like the perfect approach for our beers to take.

Our oak barrels and the notes they reveal.

When talking about spontaneously fermented beer, it’s better to think of the barrel not as a container, but as an environment where it evolves into something fabulous. We like to barrel our beers in oak barrels. Oak is a naturally porous material, and as such, microbes easily penetrate and find a home in the wood of the barrel which makes them ideal for the type of spontaneous brewing that we do.

Oak uniquely imparts oaky vanilla and caramel flavors into what’s inside. It also facilitates something called “oxygen ingress”, where minute amounts of oxygen seep into the barrel through the natural pores in the wood. This has a profound effect on the beer. The porousness of oak makes it ideal for the beer to pick up some of the aroma and flavor characteristics within the wood––as well as the characteristics of what that barrel previously contained.

Most of the oak found in the barrels that we use at Wild Mind Artisan Ales comes from the United States, France, or Hungary, however barrels made from oak obtained in the United States typically have the most robust flavor and are the most porous. It ages beer the fastest and imparts the most intense flavors. French oak has a tighter grain and lighter flavor so that barrel will age slower and impart lighter flavors. Hungarian is right in the middle. Red wine barrels occupy the largest amount of our barrel inventory, generally because they are affordable and are the most neutral in flavor.

Occasionally we use fresh white wine, bourbon, scotch, or other sprit barrels for some of our select batches. When barreling fresh beer, we always consider what the previous tenant of that barrel might bring to the table. There’s no getting around it; different barrels produce different beer. Steaming will knock back the microbes for a while, but we’ve found that there is no way to obliterate the impact of previous barrel residents entirely (which is O.K., we put those microbes to work for us!)

Aging beer and the difference it makes.

We age our beer for a few reasons but namely, because bacteria are slow growing and needs somewhere to live and do its work. Aging is the biggest reason why we choose to use oak barrels over stainless steel. Beer is a living product that changes over time. Most of our beers rest for 4 - 16 months in barrels to cultivate and build character. Ageing beer in oak barrels makes an excellent environment for this transformation to happen.

This long aging process allows the bacteria to add tartness to the beer over time slowly. Also, yeast needs to be exposed to a little bit of oxygen to be healthy. The natural breathability of oak barrels allows this to happen. The brett slowly goes to work on breaking down complex sugars or acids in the beer over time to dry out the beer and create complex esters. Brett adds fruitiness to beers in addition to a high acidity taste, and it can have a pretty incredible impact of keeping the freshness more intact and delaying degradation of the beer while enhancing the beer at the same time.

In addition to creating esters and flavor compounds while aging, many other good things are happening. This entire aging process is lengthy, and it is also why most sour beers take at a minimum one year. The size of the barrel also plays a role in aging. The larger the barrel is, the smaller the ratio of surface area to beer will be. This means there will be a smoother aging process with less oxygen infiltration. It is why we additionally use massive foeders as part of our aging process.

Time truly matters. Sometimes, if we have a higher alcohol or hopped beer, we may wait a little longer for the beer to mature. Beers on their own, given the right amount of time and proper conditions can gain an incredible complexity to them not typically tasted in fresher ales and lagers. Beers not only gain complexity with age and conditioning but with the beautiful flavors and aromas that the barrels provide.

We cellar our beer to pick up the fantastic flavors and aromas from these barrels and from leftover flavors of the spirits that were in them before, absorbing the flavors, aromas, toastiness and natural flavors of the oak wood barrels, vanilla, bourbon, coconut and raisin or dried fruits. Aging in barrels gives the beer time to breathe and reach their full potential of maturation until it reaches your glass.

This is the last post in our three blog series. We hope you’ve enjoyed learning more about us and what makes our Wild Minds Artisan Ales so amazing. This brewery is truly a labor of love for us. Every barrel we make is a revelation of sorts, unlike the barrel before it. We invite you to experience the difference for yourself and be a part our journey.