I recently brewed a Berliner Weisse and used my Speidel Braumeister to kettle sour the beer. I figured I would share some of my experiences and discuss why and when using a kettle sour technique is recommendable. Let’s get to it!
First let’s start with what a kettle sour is. The Kettle souring technique takes the unfermented wort and rapidly sours it by using Lactobacillus. Once the wort has reached the wanted levels of acidity the wort is boiled, cooled down, and fermented as normal.
Kettle souring has some advantages over other souring techniques. The most obvious one is the time required to sour the beer. When I brewed my Farmhouse sour it took over 5 weeks to acidify. With kettle souring it often takes less than 24 hours to get to the wanted acidity. Another advantage is that you are better in control of the acidity because you can stop the souring at any point in time. Once you boil the wort, you kill the Lactobacillus, and thus stop the souring. Lactobacillus is also very sensitive to hop bitterness, although this differs from strain to strain. A very small IBU can already interfere with the effectiveness of some Lactobacillus strains and thus limits you in your hop usage. Kettle souring is a good way to circumvent this limitation. And finally, using Lactobacillus for mixed fermentation can lead to increased diacetyl and acetoin in your beer. A kettle soured beer will almost always taste more clean.
The advantages of the technique are quite numerous. Before we proceed to the how-to kettle sour in a Speidel Braumeister I’d like to discuss what you need before starting:
- A Lactobacillus strain (Milk the Funk has an amazing overview of strains and their characteristics)
- Recommended: a way to measure pH (pH strips, but ideally a digital pH meter)
- Recommend: some (lactic) acid to pre-acidify the wort
The brew process is relatively similar until the end of the mash. I mashed the recipe below @ 66 degrees for 60 minutes, mashed out at 78 degrees, raised the temperature to a boil for a minute to sterilise, and next cooled down the wort to 16 degrees C (61 F). I took a pH reading to get a nill reading. Ideally you will add some lactic acid to lower the acidity to about pH 4.5. The reason for this pre-acidification is to stimulate the growth of the Lactobacillus and to prevent any other bacteria and yeast to move in before your lactobacillus. The pre-acidification is not strictly necessary, but recommend. This is the moment that you pitch the Lactobacillus. I used L. delbrueckii.
I wrapped the top of my Braumeister with tinfoil and put the lid on top of the foil layer to prevent any yeast or bacteria to get in. I turned up the heater of the Braumeister to 38 degrees C / 100 F (and let it sit for about 24 hours taking regular pH readings). Please note that there are difference between strains in terms of optimal temperature or speed of acidification. Milk the Funk is the best online resource for this.
Now it’s time to wait and take regular readings. When it’s done depends on what kind of beer you are trying to create. If you want a subtle acidity aim for a high 3. something pH. If you want mouth puckering sourness aim for low 3. something pH. When you have reached the acidity you are aiming for it’s time to boil the wort. Please note that the unfermented wort can now smell a bit vegetal, like saurkraut. Please don’t worry as in my experience these will boil off.
Now boil your wort as you normally would, cool down your wort, and stick everything into the fermenter. Please note that the lower pH can cause a small lag in the start of your fermentation, so be sure to pitch enough yeast.