Kettle souring is an easy, predictable way to sour a beer, the only complication is how to keep the wort warm, above 90 degrees, for 12-18 hours after pitching your lacto. Here’s the easy way to do it:
- Mash your grains to produce wort. Since you’ll be souring and the lacto can digest some dextrines, it might be prudent to mash a couple of degrees warmer than normal for the wort you’re making.
- Lauter and sparge as normal.
- Boil the wort for 10-15 minutes. DO NOT ADD HOPS AT THIS TIME!
- Chill the wort to 120 degrees F. If you’re using a thermometer or a temperature controller probe, put it in the wort at this time.
- Pitch your lacto culture. Organic yoghurt, sourdough starter, natural sauerkraut, a handfull of grains from the homebrew store, anything that can get a lactic fermentation started in your wort.
- Isolate the wort from air. You can do this by blanketing with carbon dioxide or you can cover the wort with a layer of plastic wrap.
- Use the temperature controller and a heat source to maintain 120 degrees or insulate the kettle to maintain at least 90 degrees – warmer is better.
- After 8-12 hours, measure the pH or taste the wort. You may see a pellicle, a mat or ropey mass of bacteria, on top of the wort. That’s normal. It will have a distinct sour odor as well, kind of like spoiled milk. This is also normal. It will dissipate in the boil. Take your sample from underneath the pellicle. If you’re measuring the pH, the wort will be slightly sour at above pH 4, it will be tart at pH 4 and for very sour beer, pH 3.5 is what you’re looking for. If you’re tasting, remember the wort has sugar that will offset some of the tartness.
- If the wort isn’t sour enough, let it stand hot longer. The bacteria are reproducing exponentially so check the pH/taste frequently.
- Once the wort is where you want it, boil and hop as desired – from here the process is that of a normal beer.
I like a sour blonde ale, you may want to experiment with fruits or other flavorings. Brewed to a low alcohol content, this will be a very refreshing summer beer. With a low hop content, it can be a Berliner Weisse. It’s a versatile method for producing sour beers, play with it!