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 sanitation and began using closed containers and stainless steel. Over the centuries, brewmasters have worked to isolate the specific strains of bacteria and yeast that made those ancient beers so good, and avoid all the ones that made them so bad. Below is a summary of basic brewing science, with a special focus on the microbes that give sour beer its signature tang and funk. As with all things brewing, there are as many exceptions as there are rules, but this is the basic brewing science from which all beers are a variation.
Beer starts with grain. In most cases, that's barley, wheat or corn. Grains are high in starches, which are basically long chains of sugar molecules. 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.
Malting is the first step in turning grain into beer. The raw grain is dampened with warm water and allowed to sprout (germinate). Germination triggers the production of an enzyme, called amylase, which immediately begins converting starch into usable sugars. The malt is then drained and roasted in a kiln, which stops the work of the enzyme. Malt may be roasted until it is light, dark, or black, which determines the color of the final beer
Once the malt is fully roasted, it's ground 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, which means more alcohol and fermentation flavors in the final beer. Shorter mash time means more unfermentable starches and other compounds will remain in the beer, which give it more body.
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 turns some of the remaining starch into sugar. Once the wort is sterilized, it's strained, cooled, and transferred to a fermentation vat.
Principal Fermentation 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.
The CO2 produced during fermentation is collected and stored to be added back into the beer at a later stage.
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. 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, enhancing the flavor of the beer and adding carbonation.
Carbonation, Packaging and Pasteurization
These are the finishing touches. The beer may be filtered or centrifuged to improve its clarity, and CO2 (which was collected during principal fermentation) is pumped back in. The beer is then packaged into kegs, cans, or bottles. Kegs are stored and shipped as-is, but cans and bottles are pasteurized first to make sure there are no surviving microbes that might continue to ferment during storage. This allows for safe long-term storage at room temperature without the risk of dangerous gas buildup inside the bottle. Kegs are usually kept colder, stored for less time, and are sturdy enough to handle pressure changes, so pasteurization isn't required. This is part of the reason for the flavor difference between draft and bottled versions of the same beer.
Here's a list of sour beers to look for at your local store--there's a good mix of sour, funky, fruity, and mild in here, but it's by no means a complete list. Tell us your favorite beer (sour or otherwise) in the comments.
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
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4. Nummer, B. A. Brewing With Lactic Acid Bacteria. More Beer (2012). at
5. Wyeast Laboratories. What is Yeast? Yeast Fundamentals at