Sour Beer close up WM 3

Sour beer isn’t a new concept; in fact, the earliest beers were probably all sour by today’s standards. This is largely due to the wild yeasts and bacteria that would infiltrate old-world open-container brewing systems and add their own unique flavors to the brew. Eventually, these wild flavors and sour notes were phased out as brewers improved their sanitation methods and began using closed containers and stainless steel. Keeping wild yeasts and bacteria out allowed brewers to choose specific yeast and bacteria strains, which meant the flavor and alcohol content of their beer would be more predictable.

As a general rule, having a predicable recipe is a good thing, but shouldn’t we be able to have our cake and eat it too? Over the centuries, brewmasters have worked to isolate the specific strains of bacteria and yeast that made those ancient beers so good, and leave out all the ones that made them so bad. Once upon a time, sour beers were manufactured by only a few specialized breweries in Belgium and Germany, but in recent years, small breweries all over the world have begun experimenting with sour beers. And where good beer goes (haha, Gose?) people will follow. At long last, sour beers have earned a small but thirsty following in America, and I’m proud to be among them.

Beer pouringI was inspired to write about sour beers a few weeks ago after attending a sour beer tasting event, called Sour Beer Takeover, at the Historic Sixth & I Synagogue in Washington, DC. In fact, once I started writing, I couldn’t stop–which means I’ll be doling out the sour beer love in two installments. The first installment, below, is about the science of sour beer (and regular beer too). It’s a great primer for the beer nerd in all of us. The second installment, to be posted next week, will highlight some of the amazing local brews I tasted at the event. It’ll come complete with my reviews and rankings, as well as some fun tidbits from the event. But for now, let’s get down to the science and uncover how a humble grain of barley becomes a refreshing sour beer. (Protip: you might want to pour yourself a cold one before reading on.)

Beer Science

All beer starts with grain. In most cases, that’s either exclusively barley, or a combination of grains (like barley, wheat or corn). Grains are made up of mostly starch, which is the starting material for beer. Starches are basically long chains of sugar molecules, and those sugars are what are needed for fermentation (and alcohol production) to occur. The trick of turning grain into beer is breaking those starches down into small, simple fermentable sugars, like glucose. The first several steps in brewing are geared toward separating as many of those small, simple sugars as possible.

Malted grain sm


Malting is the first step in turning grain into beer. The raw grain is dampened with warm water and allowed to sprout (germinate). Malt is defined as germinated grain. Germination triggers the production of an enzyme, called amylase, which immediately begins converting starch into usable sugars. Under different circumstances, the young plant might use those sugars to help it grow, but since this little sprout is intended for brewing, those sugars are going to go toward a nobler cause–beer. Once the sprouts have grown to roughly three-quarters of an inch, the malt is drained and roasted in a kiln, which stops the work of the amylase enzyme. Malt may be roasted until it is light, dark, or black. The roast level of the malt is what determines the color and malt flavor of the final beer. Another wrinkle that occasionally comes into play at this stage is smoke. Instead of roasting in a kiln, some malts are dried over smoking woodchips. Smoked malt is what gives Rauchbiers (smoked beers) their smoky flavor. Smoked beers are relatively rare, but some sours, including a two of the local brews I’ll mention in my second installment, are smoked to add warmth and character.

Brewing science: Starches are converted to fermentable sugars via enzymes and heat.
Flavor effects: Malting itself changes the flavor of the final beer, as is evidenced by the slightly different flavor of unmalted beers. The balance of barley to other grains as well as the extent to which the malt is roasted or smoked determines the color and final maltiness of the beer.


Once the malt is fully roasted, it’s ground into a mash and added to a tank called a mash tun. Here, it’s churned with water and heated, further breaking down starches into fermentable sugars. Longer mash time usually means more fermentable sugars. Shorter mash time means more unfermentable starches and other compounds, which give beer more body. The cooked mash is then transferred to a new vessel, where the solids are allowed to settle to the bottom.

Brewing science: Starches are converted to fermentable sugars via heat and agitation.
Flavor effects: Brews that spend more time in the mash tun tend to end up with more flavors from alcohol and other fermentation products, but have a lighter body than those that spend less time there.

Hop sm


The liquid portion of the mash, called the wort is drained off and moved to a kettle. Flavorful hops (and any other botanicals) are added at this time, and the wort is boiled. This kills any pre-existing microorganisms, such as yeasts, bacteria, and molds that might spoil the beer or introduce unwanted flavors. Boiling also helps break down some of the remaining unfermentable compounds and develops the malt and hop flavors. Once the wort is sterilized, it’s strained, cooled, and transferred to a fermentation vat. Strained wort is basically unfermented beer.

Brewing science: Wild microorganisms are boiled off. Hops also have some anti-microbial effects that help sterilize the beer. A few more starches are converted to sugar via heat and agitation.
Flavor effects: Hops add their waxy, bitter, resinous, citrusy notes to the brew, along with any other botanicals (e.g. orange peel or coriander). This method of flavor infusion is similar to steeping tea. Sterilization of the wort prevents off flavors from developing during fermentation.

Wort w Hops WM sm

Principal Fermentation

This is where the magic happens! The steps up until this point have centered on maximizing the amount of sugar we can draw out of our grain. Sugars are important because they drive fermentation. Fermentation microbes need sugar in order to produce alcohol, carbon dioxide (CO2), and delicious flavor and aroma compounds. Up until this stage, the process of making sour beer has been largely interchangeable with making regular beer, but fermentation changes everything. The difference between sour and regular beers has everything to do with which strains of bacteria and yeast are chosen for fermentation. Here are the microbial MVP’s you’ll see most often in sour beers:

  • Lactobacillus (aka “Lacto”) – A bacteria which produces lactic acid, the most common source of sourness in sour beer. There are two Lacto sub-types important to sour beer:
    • homofermentative, which only produce lactic acid, and
    • heterofermentative, which produce lactic acid, acetic acid (vinegary), alcohol, CO2, and other flavor and aroma compounds.
  • Brettanomyces (aka “Brett”) – A yeast which produces alcohol, CO2, and variety of strange and funky flavor and aroma compounds. Those compounds will vary based on the specific strain of Brett used, and what the temperature and other environmental conditions are during fermentation. Examples of some sought-after Brett flavors and aromas include fruit, citrus, barnyard, vinous (wine-like), musk, and butter. Brett may sometimes produce some sourness as well, but not to the extent of Lacto.
  • Top-fermenting Saccharomyces cerevisiae (aka “ale yeast”) – The yeast used to make regular ales. It imparts no sourness, only alcohol, CO2, and regular ale-like flavor and aroma compounds.
  • Bottom-fermenting Saccharomyces cerevisiae (aka “lager yeast” or “beer yeast”) – The yeast used to make regular lagers. It imparts no sourness, only alcohol, CO2, and regular lager-like flavor and aroma compounds.

As you might imagine, regular beer is brewed using only ale yeast or lager yeast. In fact, the interesting character imparted by Brett is considered an off-flavor in regular beer. I mention them here as MVP’s for sour beer because most sour beer brewers choose to use a blend of fermentation microbes, including the regular tasting ones, to achieve the perfect balance of flavors. Principal fermentation can progress in any number of ways. Sour beers are often separated into different batches or fermentation stages because each microbe requires its own special conditions to thrive.

Yeast and Lacto Fermentation (anaerobic) vert WM sm

If the brewer decides to use more than one of these fermenting microbes, he’s likely to choose one of the following two fermentation procedures: (1) separate the sterile wort between several small vats and inoculate each one with a different microbe; or (2) put all the sterile wort into one big vat and add the various microbes one at a time, sterilizing between each change.

BrettAnna Y. Myces 4 smIf the brewer chooses option 1 (separate vats), one vat would get Lacto; another would get Brett and so on. Each beer would be fermented separately, and then blended back together at the end. This option is nice because it doesn’t require any mid-fermentation sterilization. Every batch is allowed to ferment in peace; however, it requires more space and equipment than many small breweries are able to spare for a single beer.

If the brewer chooses option 2 (single vat with one microbe at a time), he would move all of his sterile wort into one big fermentation vat and inoculate it with his first microbe–often a homofermentative Lacto. The bacteria would consume sugar and produce lactic acid until the beer reached the brewer’s desired acidity level. Lacto fermentation would then be halted by boiling off the bacteria. The newly sterilized, partially-fermented (now sour) beer would then be inoculated with a yeast to finish off the remaining sugars in the beer. For a simple, clean-tasting sour beer, the brewer might choose an ale or lager yeast. For funky fruity sour beer, he would likely choose Brett.

The CO2 produced during fermentation is collected and stored to be added back into the beer at a later stage.

Brewing science: Microbes ferment the majority of sugars into alcohol, CO2, acids, flavors, and aromas. Sour beer fermentation MVP’s are Lacto bacteria, Brett yeast, and (to a lesser extent) ale and lager yeasts.
Flavor effects: More Lacto means more sourness, more Brett means more funky fruitiness, more ale or lager yeast means more “clean” beer flavor. Brewers can control the balance of these aspects by either brewing separate small batches of single-strain beer and blending them at the end, or they can ferment their beer in stages, sterilizing the beer in between inoculations.

Second Fermentation and Lagering (Resting)

After the principal fermentation, most of the heavy lifting is done, and the young sour beer can now be allowed to rest in a storage tank. The beer is cooled drastically to slow the activity of fermentation microbes and encourage large particulates to settle out of solution. As sediment gathers on the bottom of the tank, the beer becomes less cloudy. It’s allowed to rest at this cool temperature for several weeks, months, or years depending on the beer. During this resting period, known as lagering, a slow second fermentation occurs, further enhancing the flavor of the beer and adding carbonation. In certain types of beers, such as framboise, additional fruit (or another sugar source) is added before second fermentation to give fermentation microbes a fresh batch of sugar to work on.

Brewing science: Lingering sugars are fermented slowly at low temperatures. Cloudiness is reduced due to sedimentation. Flavor mellows and changes over time, and fermentation microbes have to work harder to find food.
Flavor effects: Aging allows the flavor of the beer to mellow and improve. Newly formed CO2 is trapped, improving the beer’s effervescence. Off-putting particulates settle out of solution.

Sour Beer Bottle sm

Carbonation, Packaging and Pasteurization

Now the beer is ready for the finishing touches. It may be filtered or centrifuged to improve its clarity, and CO2 (which was collected during principal fermentation) is now pumped back in. The beer is then packaged into kegs, cans, or bottles. Despite the fact that very few sugars remain in the beer, there remains the risk that fermentation might continue during storage, potentially causing a buildup of CO2 within the packaging. This isn’t a problem for beer packaged in kegs, as kegs can withstand astonishingly high amounts of pressure (about 300 psi—for context, car tires are usually inflated to about 32 psi). However, cans and bottles aren’t as strong, so they’re pasteurized after packaging to kill off any remaining fermentation microbes. Pasteurization improves the stability and shelf life of canned and bottled beers, but slightly alters their flavor. Some beers packaged in cans or bottles may remain unpasteurized if the brewery employs sterile filtration instead. Sterile filtration physically filters out yeast and any other living microbes instead of killing them. This pasteurization work-around is how some brewers are allowed to describe their canned or bottled beer as “draft beer”.

Brewing science: Final measures are taken to clarify the beer and add CO2 back in. Beer is packaged and sterilized (except keg beer) to prevent further fermentation.
Flavor effects: Filtration removes any particulates which may detract from the appearance or flavor of the beer. Additional CO2 changes the body and lightness of the beer, while increasing “carbonated” taste (slightly tangy). Pasteurization of canned and bottled beers requires heat, which slightly changes their flavor and halts fermentation.

Now let’s all crack open a nice cold Bud Light to celebrate our new-found knowledge of sour beers! Just kidding. Here are a few sours I’ve found in my local beer stores which have contributed to my appreciation of sour beers. It’s by no means a complete list, but give any (or all!) of these a try and expand your beer palate. If you have a favorite sour beer, tell me in the comments; I’d love to hear your recommendations!

Festina Pêche – Dogfish Head Brewery
The Kimmie, The Yink & The Holy Gose – Anderson Valley Brewing Company
Rodenbach – Brouwerij Rodenbach N.V.
Monk’s Café Flemish Sour Ale – Brouwerij Van Steenberge N.V.
Petrus Aged Pale – De Brabandere
Petrus Oud Bruin – De Brabandere
Petrus Aged Red – De Brabandere



1. Grossman, H. J. Grossman’s Guide to wines, beers, & spirits. (Scribner, 1983).

2. O’Brien, C., Engert, G., Zeender, N. & Roy, M. Sour Beer Takeover. (2014). at <>

3. Bennion, M. The science of food. (Harper & Row, 1980).

4. Nummer, B. A. Brewing With Lactic Acid Bacteria. More Beer (2012). at <>

5. Wyeast Laboratories. What is Yeast? Yeast Fundamentals at <>

Tagged with →  

One Response to Beer Science: Sour and Funky Beers

  1. Preston says:

    Awesome article! While I have to go with Monk’s Cafe as my favorite sour (for both flavor and sentimental reasons), I’m very keen on trying to new offerings by many of the more popular brewers–e.g., New Belgium’s “La Folie” and Allagash’s “Confluence”!

    It will be really interesting to see how the new school / mainstream brewers compare to the traditional guys who have been doing it for decades overseas!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: