As any all-grain brewer knows, a mash is a mixture of hot water and grain. A “sour mash” is a mash that has acid-producing bacteria in it.

Most people associate the term with whiskey from the southern United States. But the technique can also be used in homebrewing. A brewer can make a small sour mash to lower the pH of his main mash when brewing ordinary beers. Or he can make a large sour mash and brew a sour beer.

Using a small sour mash to lower your mash pH should not affect the taste of your finished beer any more than adding a small amount of lactic acid. Using a large sour mash will add a definite crisp tang to the beer.

For most beers, the pH of the main mash should be between 5.2 to 5.4. The pH scale measures the acidity of a solution and ranges from 0 to 14. A solution with a pH of 0 is very acidic; a solution with a pH of 14 is very basic. Measuring mash pH is easy. You simply use a hand-held digital pH meter or use pH test strips, which are available at homebrew shops.

If your brewing water is low in carbonates and has enough calcium, the mash pH should establish itself within this range. But using water with high carbonate levels will result in a high mash pH. A pH over 5.6 can decrease extraction efficiency and allow tannins to leach into the wort during the sparge.

There are many ways to lower the pH of a mash. The most straightforward solution is to add lactic acid, calcium sulfate (gypsum) or calcium chloride. Sour mashing takes a bit more time and effort, but there are a variety of reasons to try it. Sour mashing is a traditional way of lowering mash pH; if you’re a purist, it complies with the Reinheitsgebot. Some brewers feel it’s more “natural” than adding straight acid.

Personally, I don’t worry about the Reinheitsgebot or about being “natural.” There’s no such thing as unnatural lactic acid. Chemical companies produce lactic acid from bacteria just as sour mashers do. Homebrewers should try a sour mash simply because it’s a fun, new brewing experience.

There are some disadvantages to making a sour mash. First, it requires extra time and energy. You’ll need at least a two days for the bacteria to produce enough lactic acid to be valuable. More importantly, you’ll be actively growing a “harmful” bacteria. Lactobacillus species are unwanted contaminants for almost all styles of beer. But if you clean and sterilize your brewing equipment as usual, you should not encounter any problems.
 
First, the Basics

Sour mashes are a source of lactic acid. The acid is produced by the bacteria Lactobacillus delbruckii. The brewer introduces L. delbruckii by adding malt to a small mash and allowing it to grow at a high temperature in a sealed container for a few days. These conditions favor the growth of L. delbruckii while inhibiting the growth of other, unwanted organisms. L. delbruckii ferments sugars from the malt, producing lactic acid.

Beers made with sour mash do not have bacteria in the final product. This is because L. delbruckii can’t survive the heat of the main mash (often between 150-158° F).

L. delbruckii is homofermentative, thermophilic and anaerobic. Homofermentative means that L. delbruckii produces only a single product — lactic acid — when they ferment. Hetero-fermentative species in the genus Lactobacillus produce acetic acid, ethanol and carbon dioxide, along with lactic acid, during fermentation. They may also produce diacetyl. Thermophilic means heat-loving; L. delbruckii survives at temperatures as high as 131° F and thrives at temperatures between 95 to 120° F. Anaerobic means without oxygen; L. delbruckii can live and reproduce in the absence of oxygen.

Lactobacillus species are used in the manufacture of many types of foods.  L. acidophilus and other bacteria are used to make yogurt.  Other species of Lactobacillus are used in sauerkraut and pickles.

The most common contaminant of a sour mash is Clostridium butyricum. This anaerobic bacteria produces butyric acid, a foul-smelling compound that turns the mash rancid. Sour mashes infected with Clostridium should be thrown out. Luckily, these bugs are inactive above 112° F. Keeping the sour mash above this temperature will inhibit Clostridium.

Acetobacter, an acetic acid-producing bacteria, can also infect sour mashes. These aerobic bacteria grow on the surface of the mash if oxygen is present and can convert alcohol to acetic acid at pH values as low as 4.5. Keeping the sour mash tun tightly closed to seal out oxygen will inhibit the growth of Acetobacter. It can also be inhibited by raising the temperature above 122° F. (Temperatures above 120° F will stun L. delbruckii, but they will survive unless temperature exceeds a high of 131° F.)

Some heterofermentative Lactobacillus species grow in the same conditions as L. delbruckii. The presence of these bacteria may be detected by the buttery smell of diacetyl. Heating the sour mash above 140° F should kill them, but then you will need to re-inoculate the mash with more L. delbruckii.

How to Make a Sour Mash

To make a sour mash you’ll need malt, water, a spoon, a thermometer, a pot and a vessel to hold the sour mash (a sour mash tun). An insulated cooler will work as a sour mash tun. It should be just big enough to hold the sour mash and have as little extra headspace as possible. L. delbruckii naturally resides in malt, so you don’t need a culture of the bacteria (although cultures are available from Wyeast).

Making a sour mash is simple. You’ll need to start two to four days before brewing day. A small sour mash should be between 5 to 15 percent the size of the main mash. (For example, if your recipe calls for 10 pounds of grain, you should make your sour mash with 0.5–1.5 pounds of malt.) For a sour beer, your sour mash should be at least 20 percent the size of the main mash or larger.

Step One: Mashing

Crush the pale malt and mash it in the normal fashion. A single infusion mash held at 150–158° F for one hour will suffice. More elaborate mash regimes are not necessary. The mash will produce sugars for the bacteria to ferment.

Step Two: Inoculating

Cool the mash to about 122° F and add a handful of crushed (unmashed) malt. This malt will contain any surviving  L. delbruckii. The “cold” malt should also drop the temperature to 120° F.

Step Three: Incubating

Keep the mash in a sealed container and hold it between 95 and 120° F for two to four days. The mash temperature will drop over time, so you will likely need to boost the temperature once a day.

To boost the temperature of your sour mash, boil some water for 15 minutes to de-aereate it. Stir this water into the sour mash until the temperature is about 120° F. When stirring, try not introduce any air into the mash. Seal the container again as quickly as possible. (You may lose some liquid from overflow when adding water. This is fine.) Don’t peek at the mash other than when you are boosting the temperature. Each time you open the lid, oxygen gets in.

Step Four: Monitoring

The mash should smell sour, but clean. L. delbruckii does not produce any smelly by-products.  You shoudd never use a sour mash that smells off in any way.

If you do notice rancid smells, diacetyl smells or growth on the surface of the sour mash, skim it and heat to 140° F. Cool the sour mash back down to 120° F and re-inoculate with a handful of crushed malt. If the smell persists, discard the sour mash. Even if the sour mash does not smell, it is a good idea to skim the top layer, which often contains any aerobic contaminants present, before using it.

Step Five: On Brewing Day

Stir the sour mash into the main mash. The pH of the sour mash should be around 4. Add half of the sour mash, then check pH.  Keep adding until pH is 5.2. (Follow pH meter instructions with care. You might have to cool down the mash to get specific readings).

If you are making a sour beer, add only a small amount of the sour mash — 5 to 15 percent of the main mash — to the main mash. Keep the remainder of the sour mash separate until after starch conversion is complete. Adding the whole large sour mash to the main mash will lower the pH below 5.2. and result in lower yields. Once the main mash is finished, add the rest of the sour mash. After mashing, proceed with recirculation, run-off and sparging as you normally would.

If you are experimenting for the first time, make a larger sour mash than you need and hold it for just two days. It’s easier to maintain a larger sour mash for two days than managing a smaller sour mash for four days. There is less work involved and less opportunity for contaminants to grow. A sour mash that is infected with unwanted organisms could taint your beer with off flavors. The smell and appearance of the sour mash should indicate the presence of any growth. When in doubt, throw it out!

Chris Colby is a contributing writer for Brew Your Own. He lives in Bastrop, Texas.