Kettle Souring is a brewing technique that allows brewers to sour unfermented wort very quickly, in most cases between 24 and 72 hours.
A "kettle" is a piece of brewery equipment called a brewing kettle that is the vessel in which the beer is soured. This is a stainless steel saccharification keg where the souring process takes place and the beer is then left to ferment in the tank. The main difference between kettle souring and traditional souring is that a steel container is used instead of a keg.
Souring beer using kettle souring technology offers brewers a number of benefits.
1. Time frame: The ability to quickly sour a beer while maintaining all the other important key flavors is an obvious benefit of kettle souring.
2. No contamination: Souring the wort and then boiling it to kill Lactobacillus acidophilus eliminates the fear of contaminating other batches or infecting any other equipment in the fermentation area.
3. Flavor control: Lactobacillus acidophilus is extremely sensitive to isomerized alpha acids (down to 2 IBUs!); this method of souring allows you to make a beer full of hoppy flavor, far more than can be achieved using traditional methods.
Before putting in the Lactobacillus, the wort must be prepared.
The wort is boiled quickly for about 10 minutes to kill any organisms that have survived so far. This process also drives out the oxygen introduced into the mixture, which helps prevent harmful bacteria from proliferating.
115°F (46°C) is the standard temperature for fermenting with lactic acid bacteria, but each brewer has their own magic temperature between 95°F (35°C) and 115°F (46°C). These temperatures provide the best combination of flavor and speed for a fast sour beer.
If a brewer uses lactic acid bacteria obtained from a commercial supplier (rather than from a natural source), it is best to follow the supplier's recommended pitching speed and temperature.
After boiling, the wort must be cooled to about 95 - 115ºF (35-46ºC) in order to incubate the lactic acid bacteria, depending on the strain. This is easily accomplished by passing the wort through a tubular heat exchanger. This not only cools the wort, but also provides disinfection (although hot wort can also prevent harmful bacteria from multiplying to some extent).
Pre-acidifying wort to pH 4.0-4.5 by using a food-grade standard acid (lactic or phosphoric) is effective in inhibiting other bacterial strains that may be present in the wort, such as intestinal bacteria. Lactobacillus does not produce proteases that break down foam proteins below pH 5.0.
This acidification also hinders the enzymatic action that Lactobacillus utilizes in breaking down proteins. This means that the resulting beer will exhibit all the head retention and body that you would expect from a beer fermented with yeast only.
During this time, CO 2 can be added to the top of the wort (or bubbled through the wort) to form a "bed". This can prevent any further aerobic bacterial action. Alternatively, another jacketed vessel, such as a fermenter or wort receiver, can be purged with CO 2 and the wort transferred to that vessel to free up your kettle for more brewing.
For fresh grain, approximately 2 liters (about 2.5 pounds or 1.15 kg) of grain per 10 buckets in a mesh bag at 115ºF (46ºC) will lower the pH from 5.2 to 3.2 in about 14 hours.
For other methods, between 100ºF-118ºF (38ºC-48ºC) may be effective. If you are unsure, start with the starting batch.
Again, if you are using lactic acid bacteria from a commercial supplier, follow their recommended pitching rate and temperature.
The target final pH should be between 3.0 and 3.5, depending on the desired sourness of your recipe.
✴ Don't forget to boil the wort again after the kettle has become sour.
✴ Do not use any hops before the final boil. Hops severely inhibit lactic acid bacteria.
✴ When you are ready to ferment, add a lot of yeast. The yeast cells will have to struggle with the acidity.
That's all we have to say about this very interesting (and delicious!) process. If you need more advice on equipment, please contact the ZYBREW team.