The Art of Tart: The Many Ways (Both Quick and Slow) to Make Sour Beer

Josh Weikert gets you started up the sour-beer mountain with a discussion of the basics of bugs, traditional and modern, and fast and slow ways of souring your beer, and how to bring your sours (stylistically) into the twenty-first century.

Josh Weikert Dec 28, 2016 - 22 min read

The Art of Tart: The Many Ways (Both Quick and Slow) to Make Sour Beer Primary Image

I remember a time—not that long ago—when sour beers were actually quite rare in the marketplace. Mostly they came from places with odd-sounding Flemish names, evoked images of head-shorn monks or country farmers making beer in ancient barns, and were enjoyed sparingly and infrequently. My, oh, my, how times have changed.

Sour beers are now, arguably, the hottest thing since a certain three-initialed beer style became king of the craft world. While IPA is still the most-consumed craft product (by a country mile), more and more breweries are producing sour beers. Their creations not only mimic the more traditional sours of Europe but also break new ground on “wild” versions of modern styles and have also sparked a renaissance for near-extinct tart beers. It’s a fun time to be alive as a beer drinker—and there’s no reason that you shouldn’t be brewing your own sours as well as drinking them.

Here, we discuss the basics of bugs, traditional and modern, and fast and slow ways of souring your beer, and how to bring your sours (stylistically) into the twenty-first century. For those who are new to this, there’s a whole new (flavor) world out there waiting for you. And for those who have been at it for a while, you may find that the methods and styles have opened up new options and doors for you to explore!

Your Souring Workhorses

There’s a saying among brewers, usually used as a cautionary tale: “Everything likes beer as much as we do.” It’s a reminder that it isn’t just your good old brewing yeast that wants to get in there and ferment; it’s everything. Ordinarily we’re invested in keeping anything except our yeast out, but when it comes to sours we actually want other things in—not everything, but a few select dinner guests that will give us some of the flavors we want (and on occasion, even an “open door” policy is desirable!). But first let’s talk about the “big three” in sour beer fermentation: Brettanomyces, Lactobacillus, and Pediococcus.



Brettanomyces (Brett) is a wild yeast that is probably the most commonly used agent in sour beers. One reason for its ubiquity is its versatility: depending on the strain, acidity, and alcohol levels and presence of other microbiota, you can bring out a massive array of flavors that range from apple and pineapple to deeper pit fruits such as plum to earthy notes of horse blanket and hay and (in extreme cases) more-intense funky flavors (one was described by a senior beer judge as “cherries rolled in cigar ash, but in a good way”). When Brett is used as the only fermenting agent, the flavors tend to be surprisingly restrained even when using a more-aggressive strain: fruit leads the way when young, and only after extended conditioning in the package do the more barnyard-y flavors emerge. In a mixed fermentation, though, strain will matter a great deal more. Brett claussenii tends to produce a less-funky fruitiness, whereas Bruxellensis and Lambicus strains tend toward the funkier flavors. Select your strain carefully and choose a fermentation process consistent with your desired flavor profile!

Lactobacillus and Pediococcus

Moving on from yeast, we turn to bacteria, specifically Lactobacillus (Lacto) and Pediococcus (Pedio). Both produce significant amounts of lactic acid (a nice, clean, sharp acidity), with Pedio taking substantially longer to do so than Lacto. The advantage of Pedio lies in its tolerance for a wider range of environments: Lacto sours quickly and well, but it is not a survivor. It requires an environment in which almost no isomerized alpha acid is present (anything more than 10 IBUs is greatly limiting) but can be used successfully in beers such as Berliner weisse, where hops are used minimally, or in sour mash/kettle souring applications (see below).

If Lacto isn’t an option, though, Pedio can produce the same kind of acidity—but it needs help cleaning up after itself. Never pitch Pedio alone. It should be used as part of a double-act with Brett to ensure that precursors and off-flavors are dealt with; my most-common tasting note when Pedio is present (absent Brett) is a combination of buttery diacetyl and grape juice. Pedio can also lead to a period of “sickness” in your beer, in which the beer becomes ropey and thick, but this should clear over time and is not a reason to dump the beer!

Open Fermentation

Then there’s the “open fermentation” option. There are any number of wild strains of yeast and bacteria just floating in the air that might make for incredible beer: one reason for the success of the lambic breweries of Belgium was the presence of desirable wild seasonal yeast strains floating through the air in the Senne River valley. It may well be that your brewery, yard, or garage has a number of good strains available to you! You can simply let your local microbiota have a crack at fermentation and see what you get.


Be warned, though: this is also a good way to get a disgusting beer. In addition to Brett, Lacto, and Pedio, you may well be exposing your beer to mutated wild yeasts—Acetobacter (adding vinegar flavors from acetic acid) and other foreign bugs—that could produce something rank instead of something beautiful. In two open fermentations in my brewery, I’m batting .500: one was a pleasantly tart version of an English strong ale, and the other was a dubbel that tasted like I’d been aging it in burnt tires. Feel free to give it a shot, but if you roll the dice, you have to take what you get.

Traditional Souring Methods

Sour beers developed fairly organically—in fact, they probably developed inevitably. Since every beer is contaminated to some extent with something other than Saccharomyces cerevisiae, every beer will (probably) sour given enough time. In the days before sanitation became de rigueur, the solution was simply to drink the beer before it became unpleasantly sour. Some beers, though (those lucky enough to have been fermented in the Senne River valley or in barrels that imparted more beneficial than harmful souring agents), turned out to be better when soured, and so regional sour specialties cropped up. When the brewing world became sanitary, the traditional beers became “clean,” but the sour beers stuck around because…well, because they tasted great! Brewers developed traditional methods of getting the right kinds of sour into their beer, and those methods became widely known and imitated. As time moves forward, though, we have also added some newer methods to the tool kit, and while some consider these “cheating,” there’s no reason you shouldn’t use all of the options available to you to make the beer you want.

Bugs + Time

Traditionally, the path to sour beer was simple: bugs + time. Brewers would create a beer via a conventional mash, standard boil, and typical primary fermentation. These beers also tended to be produced with reasonable amounts of aged hops (one advantage being that this reduced the total IBUs, allowing such bugs as Lacto to pitch in—no pun intended). Then, upon completion of fermentation, the beers were transferred into barrels that happened to harbor healthy cultures of desirable microbiota, and over time those bugs produced the traditional sours we know and love (lambics, Flanders reds and browns, and others). All it took was time: Pedio and Brett, in particular, are sloooooooow actors, and patience is required to let them do their work.

For our purposes, nothing much changes on the “hot” side for the traditional method. Research the flavor contributions of the microbiota you plan to introduce (including traditional yeast-strain flavor profiles) and generate your recipe. Then make your beer—usually taking care to create a healthy base of long-chain sugars that traditional brewing yeast can’t consume, generally by mashing warm or directly adding dextrin malts to the beer—and start fermentation.


The differences begin with fermentation. You can either add the brewing yeast first, the bugs first, or add them at the same time and let them act concurrently. In most cases, you’ll be doing a traditional brewing-yeast fermentation first, then adding the bugs. The exception might be Berliner, in which case it is common practice to add the Lacto first to give it a “head start” and add the brewing yeast after a taste test reveals a prominent level of sourness. For most traditional sours, though, you’ll let the brewing yeast ferment the simpler sugars, then transfer to a fermentor and directly inoculate with your bug culture to let the bugs spend time chewing down the long-chain sugars that remain.

After inoculation (either in a barrel or a traditional fermentor), the only trick is to do nothing and wait! Six months is really your minimum waiting period, and many sours age well for a year or longer. The timing should be determined by taste—take a sample and evaluate for yourself. It is insufficient to rely on pH; pH is relevant to the process of producing beer (any fermentable beverage, really), but it does not equate to the perceivable acidity (flavor) of the beer. Let your tongue tell you when your level of acidity is sufficiently high.

Once you have the beer you want, you can usually package as usual without any concerns about creating gushers or bottle bombs—since souring agents work very slowly, they will likely never build up enough pressure to do so. However, if you’re paranoid (or adding fruit and want a sweet sour beer) you can filter, pasteurize your beer, or add something such as potassium metabisulfite to end your fermentation completely.

Time plus bugs equals sour. Easy, but long.


Quick (and Cheat) Souring

Who has years to wait for their beer? Well, actually, quite a lot of people, but others want a faster result, and luckily there are at least two process-based solutions available: sour mashing and kettle souring (also known as sour worting). There’s also straight-up cheating: just add your acids directly. The advantage of these methods is obvious: they’re fast. The disadvantage is that they’re really only good for producing a relatively clean Lacto character; if you want the more-complex funk of something such as Brett, I’m afraid you’ll just have to wait. It should also be noted that sour mashing in particular can go badly wrong: Acetobacter and/or Clostridium can grow in a sour mash, producing flavors akin to vinegar and…well, vomit. The risk of those can be limited, however, and quick (or cheat!) souring methods give you another way to add sourness to your beers, all without the months of barrel watching that traditional methods often require.

Sour Mashing

The process for sour mashing and kettle souring are highly similar, and the goal of both is identical: to give your Lactobacillus a head start relative to your brewing yeast, create a very pro-Lacto (and anti-everything-else) fermenting environment, and lock in the sourness in your beer first.

Sour mashing is done in a variety of ways, but the simplest is to mash as usual, first, then allow the mash to cool to between 110–120°F (43–49°C). At that point, you can add either a pure culture of Lacto or simply add a pitch of unmashed base grain (which is lousy with Lactobacillus) and hold at temperature for 2 to 4 days. While doing so, you should do everything you can to reduce the amount of oxygen in the mash: flush with carbon dioxide or nitrogen and press plastic wrap over the grain bed. When drawing samples for tasting, introduce as little air as possible! Failing to follow this advice could allow aerobic bacteria (such as the aforementioned Acetobacter and Clostridium) to grow and ruin the flavor. You can hold the temperature by using an electric blanket, adding small doses of boiled water, or through the use of a hot water bath (if using a kettle). Once you reach your desired level of acidity, just sparge as usual and boil away! In the event you do get a funky sour mash, it’s your call. Some actually like the funk that develops. Others have had success by skimming off the top layer of the mash. But the best tip is probably to simply toss it and start again.

Kettle Souring

Kettle souring is the same basic process, just in your kettle instead of in your mash vessel. Mash, lauter, and sparge as you ordinarily would. Allow the wort to cool to the same temperatures (110–120°F/43–49°C) you would in a sour mash (again, these are Lacto-friendly temperatures!) and pitch your Lacto culture. You should likewise ensure that you protect your wort: keep the kettle covered and flush with carbon dioxide or nitrogen to guard against unwanted intruders. Once you reach your desired sourness, you can simply proceed to the boil as usual! You can also kettle sour post-boil, but if you do so, be sure to minimize your IBUs (less than 10) so that you aren’t creating a Lacto-inhibiting environment.


Whether you are mash or wort souring, it is also a good idea to adjust the pH in the mash/wort to 4.5 using food-grade lactic or phosphoric acid. Doing so will promote Lacto activity while also creating an environment that is more hostile to the kinds of bugs you aren’t trying to encourage!

And while we’re on the subject of just directly adding acid to your beer… You can also just directly add acid to your beer.

Spiking Your Beer

Lactic acid can be purchased from most homebrew supply providers and can be used to directly spike your finished beer (or wort). This allows you to dial in an exact level of lactic acidity. To some purists, this is cheating—and such people often will also say that the resulting lactic sourness is artificial and won’t produce a great beer. I can categorically say that such people are wrong. The first Best of Show beer I ever produced was a Berliner weisse that didn’t end up quite as sour as I’d hoped it would, and so I added a direct spike of lactic acid to it to bring up the perceived sourness. The resulting beer was the best that day and remains one of my favorite beers to come out of my home brewery.

A slightly more subtle option is to use acidulated malt (sometimes called sour malt). This is generally 2-row pale malt that is simply sprayed down with a mist of lactic acid. It is generally assumed that for every 1 percent of your grist that is replaced with acidulated malt, the pH of the batch (at 5 gallons/19 liters) will be reduced by 0.1. Some Berliner weisse recipes call for as much as 10 percent acidulated malt to add the desired acidity, but I usually use it as a way to add a zing of bright tart flavor to beers, such as Gose or Kölsch, that benefit from a slight acidic note but are not explicitly defined by their sourness.


There are many, many paths to the top of the sour mountain. Which brings us to…

A Question of Style

Traditional sours include three distinct beers/families: Berliner weisse, the Flanders sours, and lambics. Berliner (dubbed by Napoleon as the “Champagne of the North” for its lightness and effervescence) is a pale, low-alcohol wheat beer with a prominent (or even dominant) level of acidity. The best examples also have a pleasant wheat/grain background flavor, and this beer should be drinkable in quantity.

The Flanders ales (Flanders red and brown, or oud bruin) are less tart and more funky, but also exhibit much more malt character and fruitiness to go along with the sour heritage.

Then there are the lambics…ah, lambics. Lambics are the Cadillac of sour beers. In a straight lambic, sour and funk are both clearly present, and they often exhibit specific “house” character (which can take the form of fruity, barn-y, or even honey-like characteristics) that arises from the local microbiota and barrels. Lambics can also be “fruited,” which can create astounding complementary flavors as well as increase their complexity (peach is my personal favorite). They’re also usually served uncarbonated, but their native acidity takes up the palate-scrubbing slack that is missing from the carbonic acid present in carbonated beers.


A related style, gueuze, is made by blending one-, two-, and three-year-old lambics and serving them at high levels of carbonation. All are traditional to the Belgium/north Germany region, and while the number of traditional producers has shrunk, the styles remain popular, and many breweries in other regions of the world imitate them.

We also have a number of sour styles that have traditional roots but have only recently been brought back from the brink of extinction. These include Gose, a lightly sour beer akin to Berliner weisse, but with a briny flavor in support; and Kentucky Common, a style of beer that has come to be interpreted as being slightly sour (perhaps as an homage to the sour-mash whiskeys of the region), though (according the BJCP) there is no contemporary evidence that the beer was intended to be sour.

Tradition, though, is hardly the end of the sour beer–style discussion.

Wild Ales

In the modern craft and homebrewing world, “wild” ales are becoming increasingly common. The name is a bit of a misnomer: these are almost never the result of spontaneous open fermentations. However, they are often the result of taking a traditional style and adding the contributions of the same microbiota we have discussed thus far. As a brewing strategy, you should aim for a high final gravity for a bug culture to chew on, save your hops for dry hopping to ensure that your IBUs aren’t limiting your bugs, and allow sufficient time for the slower-fermenting bugs to contribute meaningfully to the beer.


Wild ales may be the result of 100 percent Brett or Lacto fermentations, mixed-culture fermentations, spirit- or wine-barrel aging, the addition of specialty ingredients, and even blending. Modern brewers have taken sour and funk and simply added them to the flavor arsenal, and you can see new interpretations every day on bottle lists around the world. These beers can be as simple or complex as you like: the choice is yours.

Use It All

Souring agents give brewers more options than ever before. Rather than confining ourselves to the few “local” bug environments that have historically produced good sours, we can now use cultures of those same bugs in new and varied ways to enhance beers from a wide range of style families and flavor profiles. Get creative!

Sour brewing accommodates the patient brewer or the impatient tinkerer. It has potential rewards for the technician who plans every step of the way as well as the free spirit who throws open the barn doors to catch whatever comes in. And while there’s nothing new under the (Belgian) sun, there’s no denying that pushing the limits of our existing flavor palates is both fun and rewarding. Raise a glass of something sour and wild, and whatever the result, know that you’re part of the past, present, and future of brewing, all at the same time.

From Berliner Weisse to Gose and points in between, quick souring is rapidly becoming the time-constrained brewer’s choice for building pleasant tartness on a schedule. In Craft Beer & Brewing Magazine®’s online course, Quick Souring Methods, Funkwerks Cofounder Gordon Schuck explains how to use Lactobacillus bacteria, experiment with sour mashing, test acidity levels, and more. Sign up today!