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?

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:

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.

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 . 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!

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