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Creating Consistently Successful Kettle Sours

Souring beers is an ancient practice that has been around as long as beer itself. Before science had a good understanding of microbes and sanitation, there really wasn’t much of a way around at least some flavoring from bacterial byproducts.

But as modern brewing practices emerged, producing thoughtful sour beers has become a an artful, time-honored tradition kept by some of the oldest European breweries. Many monasteries and Old-World breweries spend years letting bacteria develop a depth of flavors.

Leave it to Americans to find a shortcut.

As sour beers have risen in popularity in a market formerly dominated by German-inspired engineering and brewing practices, a handful of American breweries brought kettle souring to the parti-gyle table. The concept is pretty simple—let bacteria produce sour flavors in wort for a few days before boiling and fermenting as usual.

Instead of spending months producing sours, brewers can achieve the same effect in a matter of days. And with the bugs being boiled off before primary fermentation, the risk of spreading an infection virtually disappears.

This is an especially attractive option for homebrewers, who may not have room to store tens of gallons of beer for a year or more or who may not want to purchase that many storage vessels. There are, of course, many pitfalls that can still befall kettle sours.

I talked to a local homebrewer who turned his love of kettle sours into a business for some advice on the practice. Matt Pennisi spent years perfecting his kettle sour before opening Durty Bull Brewing Company in Durham, NC, which specializes in sours and kettle sours. Here’s what he had to say.

Know What You’re Pitching

The most straightforward way to sour a mash is by using the bacteria already living on your grains. Start by mashing no more than 80 percent of your grain bill as normal, which will actually kill the bacteria you’re after. Once finished, drop the temperature to between 90 and 120 degrees, which can be accomplished mostly or entirely by the addition of the “cold” grain. Hold the mash at this temperature for two to four days while the bacteria grows and creates the sourness you’re after.

Of course, other undesirable critters reside in grain as well, so this approach is risky at best. So Pennisi recommends making a starter first. Prepare a starter using DME or LME at a solution of around 1.032 original gravity. Boil as normal for 10 minutes, and then split into two vessels (in case one turns too funky to use). Drop the temperature between 90 and 120 degrees, then throw a handful of grains into each, hold the temperature and wait a couple of days.
If either of the starters produces nice, clean sour flavors, pitch that into your wort rather than reserving a portion of your grain bill.

You can also use commercial strains of lactobacillus instead.

Cut Out the Oxygen

Lactobacillus does not require oxygen to get the job done, so keeping your mash free of oxygen will ensure that no unwanted critters spoil the mash and resulting beer in unwanted ways. Beyond simply covering your pot with an airtight lid of some type, you can get rid of the oxygen in any remaining headspace with some carbon dioxide.

“I would duct tape a garbage bag over the pot and leave a little escape vent,” says Pennisi. “Then I’d hook a CO2 tank up to the liquid out ball valve of my kettle and blow CO2 up through the mash to purge all of the oxygen before sealing off the rest of the bag.”

Get A Head Start On Your Kettle Sours

Another way to reduce the chances of infection is to pre-sour your mash using lactic acid. A typical mash will get your pH down into the mid-to-upper 5 range. Adding lactic acid can drop the pH below 4.4, which will stop unwanted bacteria in their tracks. But the Lactobacillus will love it. After chilling the mash but before pitching your bacteria, add 1-2 mls and get yourself a decent pH sensor to make sure you’re hitting your marks.

“This is the method we use at Durty Bull,” said Pennisi. “Since I started purging the oxygen and pre-souring the mash, I’ve never had an issue with off flavors.”


After you’ve got the desired amount of sourness in wort, take it to the burner and boil it as usual. If you’re not adding many hops, there’s no need for a full 60-minute boil. Ten to twenty minutes ought to do it. Cool your wort, transfer to a fermentation vessel, and pitch yeast like normal.

The best part about kettle souring is that you get that sourness without having to risk an infection in any of your equipment—the boil will take care of it. You also get some of those great sour flavors without having to wait months or years for a slow moving strain to do its job.