Refreshing and complex, sour beer is a wide ranging category loved by craft beer enthusiasts and brewers. But brewing traditional sour beer takes months — even years — and risks contaminating your brewing equipment. Kettle souring makes quick sours without this risk of infecting future batches.
Homebrewing sour beers is fast and straightforward when using the kettle sour technique. By souring wort with lactobacillus in the kettle, there is no risk of contaminating your equipment. All this in a fraction of the time as traditional sour beer.
What Does Kettle Sour Mean?
Sour brewing uses a mix of bacteria to acidify beer and add a complex flavor. Lactobacillus ferments sugars in wort to create lactic acid. During this lacto-fermentation, the beer becomes sour. Traditional sour beers take months, if not years, to develop acidity and complex character. But recently, savvy brewers developed an innovative, yet simple, solution to help speed up the process.
Kettle souring is a brewing method used to make quick sour beers. The souring takes place in the brew kettle. Brewers produce a standard wort, add the souring bacteria, and let it sour for 12 to 36 hours. From there, the wort is boiled and fermented like any other typical beer. Because the bacteria is killed off during the boil, there’s no risk of contaminating fermenters or other cold-side equipment.
What Do Kettle Sours Taste Like?
The kettle sour process can be used to produce a wide variety of styles. From historical to new-age, brewers use kettle souring to produce some unique and interesting sour beers. Here’s a short overview of some of the most popular kettle sour beer styles.
Berliner Weisse
A classic German sour beer from Berlin brewed with wheat. Around 3.5% ABV, tart, and super refreshing, Berliner Weisse is probably the most commonly brewed style of kettle sour.
Gose
Similar in strength and acidity to Berliner Weisse, Gose gets an addition of salt and coriander seed. This is traditional in the style’s origin city of Leipzig, Germany. The combination of flavors sounds bizarre, but it makes a very delicious beer.
Sour IPA
A relatively new beer style, Sour IPA mixes hoppiness and tartness to create the best of two popular styles: sour beer and IPA. Generously hopped with fruity varietals, the sourness of the beer brings out the tropical notes in hops. Sour IPA is both refreshing and complex, making it a really popular style driven by innovators like Hudson Valley Brewery.
Saison and Farmhouse Ales
Saison, the effervescent and rustic Belgian specialty, is a style that lends itself very well to kettle souring. Lightly tart saison, like Hill Farmstead’s Arthur, has an added layer of elegance and complexity.
Fruited Sours
Fruit additions go hand-in-hand with sour beer. Starting with a base of a kettle soured beer, almost any fruit can be added to create some juicy and wonderful flavors. Common fruit additions to kettle sours are apricots, raspberries, cherries, and mango. Most brewers add fruit during secondary fermentation.
How to Kettle Sour
Brewing kettle sours is definitely an intermediate to advanced homebrewing technique. It’s a good idea to have a few brews under your belt, and have a firm grasp on sanitation and process.
We’ll guide you through what you need to know to brew amazing kettle sours.
Equipment
Kettle souring doesn’t require much, if any, special equipment. With some clever adaptations to your current set-up, you likely have all you need to get started.
Boil Kettle
The “kettle” in kettle sour — where the main event takes place. Almost any boil kettle will do, but there are a few things to look out for.
Volume
You have to kettle sour the full volume of wort. Diluting wort in the fermenter, like when brewing extract batches, will raise the pH of the wort and the beer won’t end up as sour as you intended.
Material
Only kettle sour in stainless steel kettles.
Aluminum is not stable at low pH. Since a kettle sour wort will have a pH below 4.0, it’s possible that the aluminum oxide is stripped from the kettle. Aluminum floating in your beer is clearly not good and not worth the health risk.
Heat source
Kettle souring takes place in 80-100F wort. Maintaining a consistent, warm temperature ensures healthy lactobacillus growth and efficient souring.
During the souring process, it’s always a good idea to insulate the kettle. Further, occasionally applying heat is also necessary. If you’re a stovetop brewer, it’s easy to turn on a burner now and then to get the temperature back into proper range. Similarly for electric brewing or propane burners. Always make sure to remove the insulation from the kettle when applying heat to avoid risk of fire.
All-in-one brewing systems, like the Grainfather, make excellent vessels for kettle souring. Simply program the thermostat and the integrated controller will keep the wort at the perfect temperature.
If it’s safe to do, you can also move your kettle to your fermentation chamber and easily keep it at a consistent temperature.
Accessories
It’s best to kettle sour in kettles with a valve. You need to check the souring progress to ensure consistent kettle sours. Opening the kettle lid introduces oxygen and potential contaminants that could spoil the wort, so that should be avoided.
On top of that, a tight fitting lid is definitely necessary to provide a seal and insulation during the lacto-fermentation.
pH Meter
Periodically checking the pH of the wort is best practice to maintain a controlled and healthy souring process.
Thermometer
If your kettle doesn’t have a built in thermometer, you will need a way to monitor the temperature of the wort throughout the souring process. Pulling a sample through the valve and quickly taking a temperature reading works. You could also use a digital water-proof thermometer with a long wire probe and leave it submerged in the wort.
Ingredients
Lactobacillus
Lactobacillus (or lacto, for short) is the most common souring bacteria used in kettle sours. Specifically, lactobacillus plantarum is probably the most popular strain of lacto. The plantarum strain sours quickly and cleanly, making it a reliable choice for sour brewers.
Buying lacto cultures used to be a bit of a challenge. With the recent boom in popularity of kettle sours, these days, there are many options available.
Yeast Labs
Commercial homebrew yeast labs offer many great pure culture lactobacillus products. White Labs, Wyeast, Bootleg Biology, Escarpment Labs, and many other labs produce their own versions. Some bacteria cultures are meant for secondary fermentation and long-term souring. These can still be used, but making a starter is highly recommended to ensure proper cell count.
Probiotics
Many health food enthusiasts swear by probiotics for their positive effect on gut-flora. Luckily for brewers, this means that you can find pure cultures of lactobacillus at your local health food or grocery store. Lactobacillus plantarum is sold in capsule form. Two or three capsules usually have enough cells for kettle souring, but it’s also a good idea to make a starter.
GoodBelly is a popular manufacturer of probiotic drinks that are easy to find in most grocery stores. Their StraightShots can be pitched directly into wort and produce very reliable and quick souring. Two StraightShots is plenty for a 5 gallon batch.
Alternative Sources
Milk the Funk, one of the internet’s best resources for detailed sour homebrewing, has a comprehensive list of lacto sources on their wiki.
Lactic Acid
Lactic acid is used to give the souring process a head start. Lowering the pH of the wort to 4.0-4.3 before pitching lactobacillus helps stave off potential contamination. Plus, pre-acidifying the wort greatly helps with the beer’s head retention. It slows down an enzymatic reaction with the lacto and helps protect foam-positive proteins in the wort.
Yeast
Because of the low pH of the wort, some yeast strains have difficulty completing a healthy fermentation. You should always pitch a healthy yeast starter and ferment at a consistent temperature. Don’t be afraid to slightly overpitch to give your fermentation the best conditions to finish without the risk of off-flavors.
Malt
Any grain bill will work great for kettle sours. Wheat is often used to boost body and head retention. Feel free to experiment with dark malts as well. Slightly soured stout can be delicious and even traditional. Guinness, for instance, has a very pleasant light lactic twang.
In terms of wort gravity, kettle sour beers shouldn’t be much higher than 1.050. Some strains of lactobacillus might struggle in higher gravity wort.
Hops
It’s very important to only add hops after the wort has been soured. Hops inhibit lactobacillus growth. To ensure proper souring, only add lactobacillus to unhopped wort.
Sour beer and bitterness can be a difficult balance to achieve. Too bitter, and the beer can come across pithy and astringent. Sour beer does, however, benefit from some bitterness to keep the malt sweetness and lacto acidity in check. Aim for 10-15 IBU as a start. If necessary, consider experimenting with pre-isomerized hop extract to fine tune bitterness post-fermentation.
Whirlpool and dry hopping can pair amazingly with sour beer. There are endless possibilities for flavor combinations and innovation.
Process
Planning
Kettle souring the first few times can be a wild ride. You could be waiting anywhere from 12 to 48 hours, so you have to be ready to finish the brew at any time over a number of days.
Before you start your brewday, you need to have a solid plan to make sure the kettle souring process goes smoothly. If using a lactobacillus starter, give it at least 24 hours. On top of that, your brewer’s yeast starter will need to be ready to pitch 12-36 hours after starting the brew.
Mash
You can mash any way you’re used to: all grain, brew in a bag, partial mash, or extract. Collect the full volume of wort in the brew kettle.
Pasteurize Wort
Before pitching the lactobacillus, the wort needs to be pasteurized. This ensures that all bacteria present in the wort are killed. Without this step, unwanted yeast or other bacteria could grow during the souring process and spoil the wort.
Bring the wort to a boil and hold for 5 or 10 minutes. At this point, you should also sterilize your wort chiller.
Chill Wort
Chill the wort down to about 80F to 100F, or the recommended souring temperature of the lactobacillus strain you’re using.
Pre-Acidify Wort
There is no formula for calculating the exact amount of lactic acid to add. Wort chemistry varies considerably based on grain bill and water profile. The only sure way to get the wort between a pH of 4.0-4.3 is by adding lactic acid, measuring the pH, and adding more until the desired pH is reached.
Usually, you’ll need between 15-25ml of food grade lactic acid (88%) per 5 gallons of wort. Start with 10ml, measure the pH, and repeat as necessary.
Pitch Lactobacillus
Once the wort is at a pH of 4.0 to 4.3, pitch the lactobacillus directly. There is no need to stir at this point.
Seal Lid
To prevent contamination and off-flavors, oxygen exposure during kettle souring must be minimized. Tightly wrapping the top of the kettle with plastic wrap is the easiest way to do this. On top of the plastic wrap, place the boil kettle’s lid for thermal insulation.
Purging the headspace with CO2 before sealing is a common practice among homebrewers. This helps guard the wort from oxygen and is an added safety to ensure a healthy souring process.
Kettle Souring
At this point, your wort has been inoculated with the lactobacillus and it’s all ready to sour. Maintain a temperature of 80F to 100F by insulating the pot and applying heat when necessary. The souring process will take anywhere between 12 and 36 hours. Choice of souring bacteria, wort gravity, and temperature all factor into the souring time.
Monitor pH
Take your first pH reading after 12 hours or souring. Continue every 6-8 hours until a final pH of 3.0 to 3.7 is reached.
If you don’t have a pH meter, you can monitor the sourness by taste. Once you feel the sourness is where you want it, you can move on to the boil step. Note that the residual sugar in the wort will make it seem sweeter than a fully fermented beer.
Boil
Boil the wort according to your recipe. A short (i.e. 15 minute) boil is recommended. Reboiling wort, as with kettle sours, can impact the quality and cause THP. This is a typical off-flavor in sour beer reminiscent of cheerios. Keeping the boil short helps minimize this risk.
Cold Side
After the boil, you chill and ferment as you would any other beer. At this point, there are no live bacteria in the wort. There’s no more risk of infection to your cold-side equipment.
Sample Recipe — Sour IPA with Sabro Hops
Kettle sours are meant to be fun, experimental, and — most importantly — refreshing. This hopped-up sour IPA should be served in a coconut with a tiny umbrella. Juicy, tart, and mouthwatering, Sabro hops amplify the tropical acidity to push this IPA into fruit juice territory.
| Final Volume | Original Gravity | Final Gravity | ABV | IBU | SRM |
| 5.5 Gallons | 1.047 | 1.009 | 5% | 17 | 3.5 |
Malt
| Amount | °L | Bill % | |
| 8 lb | 2-Row | 1.8 | 80% |
| 1 lb | Wheat | 1.8 | 10% |
| 1 lb | Flaked Rye | 2.8 | 10% |
| 10 lb | Total |
Hops
| Amount | Variety | AA | Use | Time | IBU |
| 1 oz | Sabro | 14 | Boil | 15 min | 17 |
| 4 oz | Sabro | 14 | Whirlpool at 170F | 20 min | |
| 4 oz | Sabro | 14 | Dry Hop | 4 days |
Yeast
Safale S-04 fermented at 66F for 1 to 2 weeks
Steps
- Mash, boil, and chill to lactobacillus pitching temperature, between 80F and 100F
- Pitch lacto culture and seal kettle
- Kettle sour for 12 to 36 hours, until a pH of 3.5 is reached
- Boil and add hops as per recipe
- Chill to 68F
- Pitch healthy yeast starter and ferment at 66F for 8 days or until FG is reached
- Dry hop for 3 to 4 days
- Transfer into CO2 purged keg and carbonate to 2.5 vols CO2
Final Thoughts
Kettle sours are fun to brew and delicious to drink. The innovative, yet simple, technique makes sour brewing accessible to homebrewers of any level. With proper planning, some keen eye for detail, and a bit of creativity, you’ll be impressing your friends with incredible sour beer.
Once you nail down the basic kettle sour technique, you can experiment with countless other variables. Hop combinations, fruit additions, even blending — the possibilities are endless. Kettle souring is an important and useful skill for every homebrewer to have in their repertoire.
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