
Part of the Year of the Sour Mash series.
Sour beer, the homebrewer’s final frontier. Many homebrewers spend their entire brewing careers fighting infections, but there are some that crave them; choosing to harness the organisms that would otherwise spell ruin to make something magical.
To many, the idea of making a sour beer seems like an impossible dream: the time, equipment, and expertise required ensures that many homebrewers view sour beer as a pastime for the brewing elite. My friends, do not despair, sour beer is within your grasp! There is a fast, easy method to get active making sour beer: the sour mash. Sour mashing is a technique for any beer that would benefit from a crisp, tart, or funky edge.
I am kicking off the Year of the Sour Mash with this overview of sour mashing. I will be covering many of these topics in greater depth in future posts, but in the mean time I hope to convince you that sour mashing is not something to be afraid of, but a fun process that is easy to be successful at if you follow a few techniques. Without further ado, let us get started!
What Is Sour Mashing?

lactobacillus – isis325
At its core, the goal of sour mashing is simple: create an optimal environment for Lactobacillus bacteria and a sub-optimal environment for spoiling organisms like Clostridium, Acetobacter, and mold. By optimizing the growth of Lactobacillus we harness its innate ability to convert lactose and other sugars to lactic acid, a compound with a clean, bright acidity that is both smooth and refreshing in beer. Fortunately, the optimal conditions for Lactobacillus are generally inhospitable for the organisms that produce the off-flavors and aromas described as “baby diaper” and “vomit.”
Generally speaking, Lactobacillus is split into two types:
- Homofermentative (e.g. Lactobacillus delbrueckii) – produces only lactic acid.
- Heterofermentative (e.g. Lactobacillus brevis) – produces both alcohol and lactic acid. If desired, it is possible to entirely ferment your wort with heterofermentative Lactobacillus (The Bruery does this for Hottenroth).
For simplicity’s sake I am going to be assuming our mashes will be predominantly fermented by homofermentative Lactobacillus, which I find to be the more likely strains found “in the wild” on grain husks.
Sources of Lactobacillus
The world is in no short supply of Lactobacillus bacteria: they are in the soil, on our food, in our gut and mouth, and on just about any surface you can think of. Additionally, many foods possessing a tart acidity such as sauerkraut, cheese, sourdough, kimchi, and yogurt are made with Lactobacillus. However, for the purposes of sour mashing there are two sources we are concerned with:
- Wild Lactobacillus from unmashed base malt.
- Pure cultures from sources like White Labs, Wyeast, The Yeast Bay, Omega Yeast Labs, and GigaYeast. Milk the Funk has a great writeup on the different strains available.
For the more adventuresome out there, I have heard reports from folks successfully culturing Lactobacillus from yogurt and probiotics!
Regardless of the source–much like a Saccharomyces fermentation–it is recommended to first create a starter to build up your Lactobacillus army and insure a fast and clean souring. I will be covering the creation of a Lactobacillus starter in my next post, so be sure to check back for further details.
Why Sour Mash?
As an enthusiast of sour beers (especially Berliner Weisse) I will admit to being someone that once thought making my own would be something I could never get into. I have come to realize that folks tend to make sour beer production a lot more complicated than it needs to be; sour mashing is fast and requires minimal extra equipment. Sour mashing’s appeal for me, at least initially, was that since I was using all hot-side equipment I did not have to worry about contaminating my gear. Additionally, since Lactobacillus is Gram-positive that means it is inhibited by hops, thus by reaching desired levels of sourness before the boil we are no longer restrained in our ability to use hops in our sour beers.
Otherwise, here are the pros and cons of sour mashing:
Pros
- The fastest way to create a sour beer.
- No extended period of ropy “sick” character as can happen with Pediococcus souring.
- No need for set of “dirty” equipment.
- Fine control over final sourness.
- Possible to make hoppy sour beers.
- Finished beer can be “clean.”
Cons
- Not impossible to create foul tasting and smelling wort.
- No chance for nuance from long-term sour process with diverse critters.
- Wort pH < ~3.3 interferes with Saccharomyces fermentation.
A fast lactic souring opens the door for one of the ideas I am most intrigued by as described by Michael Tonsmeire in American Sour Beers:
When paired with an aggressive pre-boil souring technique [e.g. sour mash] a 100% Brett fermentation is a good solution for making a complex sour beer without waiting as long as you would for a traditional mixed fermentation… Given the popularity of sour beers today, it is surprising that this is not a more common method.
Imagine that, a sour/wild beer ready in weeks! It is a technique my friend Peter over at Toolbox Brewing has been using to crank out some amazing tart and funky beers in as little as six weeks. The sky is the limit, folks!
Sour Mashing vs Kettle Souring
Before I get any further I need to make a distinction between a slightly similar process called kettle souring (which Michael Tonsmeire calls sour worting). The main difference between sour mashing and kettle souring is that sour mashing occurs before sparging with the mashed grains still present while kettle souring occurs after sparging with the full pre-boil volume (and frequently in the boil kettle). Additionally, sour mashing will have a moderately higher gravity because it is undiluted first runnings.

Green = Sour Mash, Red = Kettle Sour
Folks seem to have strong opinions on which process is better, but in my mind I use the two interchangeably. They are mostly the same process with the same result so just choose the one that works best for you! It might be worthwhile to choose kettle souring if you are souring a higher gravity recipe so you are not trying to ferment ultra-high gravity first runnings.
The Process of Sour Mashing
Sour mashing requires only a small deviation from your normal routine and has three goals:
- Create an optimal environment for Lactobacillus.
- Prevent spoiling organisms from producing foul aromatics and flavors.
- Drop pH to produce desired amount of acidity/sourness.
How Do We Do That?
- Give the Lactobacillus a healthy head-start by pitching a large number of them to ensure they are the dominant organism (similar to pitching a large amount of Saccharomyces for a “clean” fermentation).
- Keep the temperature of the mash/wort at ~110ºF, the optimum temperature of Lactobacillus and above the healthy range of competing organisms (Lactobacillus optimum health range is 95ºF – 120ºF).
- Keep oxygen away, Lactobacillus is anaerobic (without air) and many spoiling organisms are aerobic (involving air).
- Get the mash pH < 4.5 ASAP, few competing organisms thrive in low pH.
Step By Step
- Mash as usual. This is exactly the same as every other mash you have done, mash high or low as your recipe requires. I have heard folks say they have had better success with souring when using a thinner mash, but the reports are anecdotal.
- Lower pH of wort to ~4.5. I recommend using food-grade lactic acid. Its helpful to have a pH meter for this, but otherwise I find adding 10-15 ml (2-3 tsp) of lactic acid after mashing to get me down to that range.
- Cool your mash (or wort) to ~110ºF.
- Pitch your Lactobacillus starter.
- Cover surface of mash/wort with plastic wrap and purge with carbon dioxide.
- Place in a warm and insulated place. I place my souring mash in my fermentation fridge (off) for insulation and keep the mash warm by using a reptile heater pad
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- Once a day or so take a sample, but try not to let much oxygen in. Temperature should be between 100ºF – 110ºF. Check the pH if you have a meter or taste the sample for desired sourness. It usually takes 2-4 days to get down to a pH of ~3.3.
Note: While a starting pH of 4.5 will typically eliminate the risk of food poisoning, use caution when tasting the results of a sour mash. Only a low pH and the presence of alcohol can guarantee your fermented product is safe to drink. - Evaluate your sour mash. It may look and smell a little gross/funky, this is fine ( my first sour mash smelled like tomato soup). A good sour mesh smells cleanly sour, if it smells a lot like vomit or makes you want to vomit, you may not want to continue. Some off-flavor will boil out or be scrubbed by fermentation.
- Finish your sparge and/or boil. A pellicle or mold may have formed, just skim them off. If only souring part of the mash, add the sour part back to the end of your regular mash.
- Boil the wort. This will sterilize the wort, making your ferment “clean.” Everything from here on requires your standard cold-side process.
Now What?
Congratulations, you now have a wort with a low pH and a bright, clean sourness! What you do next is up to you, but for the most part it should behave like any other fermentation. I have had success fermenting with Saccharomyces with pH all the way down to 3.3, but if you managed to go lower I would recommend pitching a strain of Brettanomyces as it has less trouble with low pH.
If your soured wort has a light amount of off-character do not fear, the character is frequently scrubbed out in the boil or during fermentation. However, any more than a light amount and you should consider cutting your losses. I find most folks that have problems due to 1) not dropping the pH < 4.5 fast enough; 2) letting the temperature drop too low (< ~95ºF); or 3) letting too much oxygen in. Of those, pH and oxygen are the two main offenders, so if you can get control of those you can be a little more lax with temperature.
Good luck and happy souring!
In my best Joker voice… “Why so sour?”
Awesome article, Derek!!!
Thanks! I’m secretly just doing the heavy lifting for my NHC presentation this summer. I figure by the time I get done with this series I’ll just be able to copy/paste it into my presentation 🙂
I like your style
Offtopic: what software did you use for the diagram? 🙂
I like to use draw.io because it’s free, decent, and doesn’t require registration 🙂
I’ll create my diagram then export directly to PNG.
Good information. I’ve never had issues with sour worting, but I have gotten many off flavors and a grainy aftertaste that I did not like with sour mashing. With some of the new lacto strains available commercially, why not sour kettle. I think the drawbacks are much higher with sour mashing. Check out the new lacto strain from Omega. Room temp lacto in 24 hours is very much obtainable.
I don’t have a problem with commercial pitches or sour worting, per se, but I’ll admit that I enjoy a bit of that something extra I get from wild pitches in a sour mash. By lowering the pH and keeping temps high I’ve never had a problem with off-flavors.
Great timing and can’t wait for your NHC presentation. Might be a dumb question, but why skim the pellicle if you have one? My last sour had one unexpectedly and developed a nice cherry pie aroma but according to some developed into plasticy distracting phenolic character.
I suppose you don’t *have* to skim it off, but I don’t think it gets you anything either.
Really looking forward to the NHC presentation. This is something i would like to do soon… always afraid of the horrendous smells of stagnant mash…
Great stuff here. I followed your guide except for lowering ph to 4.5 in the mash. Tomato juice is the perfect description for the taste at 26 hours (3.7ph). Soured my entire mash in a 72qt cooler filled to the brim.
Hi, wondering what is the best yeast to use? Any dry ones that would do the trick?
I suppose it depends on what kind of beer you’re making after 🙂
You’ll need a yeast that is low pH tolerant, I’ve had good luck with WLP001, WLP011, and WLP090. Though generally speaking, I hear many Saison strains are pretty pH tolerant as well.
As for dry strains, I’d check out US-05 or Belle Saison
This is a well written guide to sour mashing. I’m just getting into it myself and found this to be invaluable. Hope you don’t mind a couple questions:
1) I’ve had a WLP 677 Lacto starter sitting at 110 for about 36 hours. The pH dropped below 4.0 within the first 24 hours. However, there is a slight Bandaid aroma. I plan to pitch it in 5 gallons of wort for a smoked Gose. Would you risk using the starter and hope the aroma doesn’t carry over or gets boiled off, or would you cut your losses and try again?
2) I would really love to see an article about your process for purging the oxygen with CO2. How much CO2 to use? Does the vessel need to be air-tight or just covered after purging? Do you purge again after taking readings?
Thanks again!
Thanks!
1) How slight? Pour yourself a sip of the starter (as cleanly as you can)–how does it taste? A small bit is maybe okay, but you don’t want to ruin a whole batch by using a bad starter.
2) I might write up an article some day, but it’s pretty simple: I take a bottle of carbonated water and top off my starter/kettle/whatever. The most important thing, IMO, is keeping O2 out than purging at the start. Air tight is best, but I’ve had good luck just covering my mash/kettle with plastic wrap–make sure it’s literally touching the surface. That’ll keep enough O2 out to stop most off flavors.
Thanks for the response. I decided to not risk it and dumped the starter down the drain. The bandaid smell only increased as time went on (4 days).
A tough call to make, but I’ve never regretted dumping a starter over dumping a full batch!
When you make a lacto starter do you put it on the stirplate? Or just add lacto to a vessel of wort and let it sit air tight?
No stir plate, you want it air tight since Lacto doesn’t need O2.
Hey, I tried to do this sour project… I made the starter with great succes, all clear, pH 3,4 – no mould or foul smells. Pitched it to the wort kept it the right temperature 48 hours – only to find the ph had only droped 0,2 – how can that be?
Hmm… how long was it between making the starter and pitching? I’ve heard stories of lacto dropping the pH so low that they kill themselves, but I’ve never experienced it myself.
Was the wort you pitched into hopped? Hopped wort would also prevent the lacto from growing/souring.
Really enjoyed the write up. I’m new to souring so bare with me. I broke a 5 gal carboy while brewing so decided to take 5 gal of my wort and put it in a bucket to see if I could get some wild spontaneous fermentation going. I left the lid open outside for a few hours then closed it up and left it outside w a blowoff. First few days were nice and sunny so I’m assuming temp got pretty high in the bucket. Next two weeks were 40/50s cold and rainy. I checked on it today, opened the lid, and drew a sample. Aroma was nice, kinda like applejuice, ph was high at 4.7, and wort was still clean looking, looked like a small pellicle was sorta forming/foaming at the surface of the wort and there are no signs of mold or other funky growth. I got brave and tasted it, had a funky after taste almost like medicine. Question is, what the hell do u think is going on in this bucket? Did it start to ferment possibly but crashed cause of the low temps? What’s with the ph? Should I just let it keep doing its thing? Should I take a big enough sample to check the gravity or just wait?
Chances are if you hopped the wort you won’t get much bacterial action/souring, but there’s a good chance you’ll get some wild yeast action! If the aroma is nice and there’s no mold I’d say let it ride a bit and see how it goes, there’s a good chance it will keep slowly chugging away.
Have you done a gravity reading? That would be the best way to tell if you’re getting some traction. Otherwise, I’d say just stash it away somewhere warm (the back of a closet works) and forget about it for a month or two.
Damn, I forgot about the kettle hops, maybe that’s what is inhibiting it from going crazy. I just can’t figure out where the apple juice smell is coming from? I guess I’ll just bring it inside and take a gravity reading and let it go. I’d be interested to know what exactly is in there. Thanks for the reply!
Yeah, I definitely find some interesting aromas coming out when I “go wild,” it’s one of the best parts I think!