To describe a sour beer, we often use the English term "Sour", or "Sour Beer". But what exactly is a "Sour"?
There are several ways to brew a sour beer. We brew our sours using "kettle souring". After boiling, we cool the beer between 30 and 45 degrees Celsius and then add the Lactobacillus or lactic acid bacteria. These are the same bacteria that are used to make yogurt and cottage cheese, for example. The Lactobacillus bacteria now start working fairly quickly to lower the pH, which is the acidity of the beer. It depends on what pH you want to achieve as a brewer, so how long you let the bacteria work and how acidic you want the beer to become.
We discovered the Lactobacillus bacteria that we use in our sour beers in a Finnish sourdough bread. We have collected and researched sourdough dough from different regions in Finland. We isolated the Lactobacillus bacteria from these different sourdough doughs and cultured them. After a series of tests, we chose a bacterial culture that gave us the best aromas. So we use a unique lactic acid bacterium with a character that we think fits well with the beer. Our brewer Ronald de Waal has lived in Finland for 30 years and in that time has learned a lot about the Finnish traditional sourdough bread culture. He finds this a challenge to apply this to brewing sour beers.
Lactobacillus gives a very mild pleasant sourness to the beer. Very different from, for example, the harsh and more aggressive acidity that you get from acetic acid bacteria. You will find acetic acid bacteria in the Belgian Gueuze beers, so you notice the difference in acidity of the different bacteria.
If you go back in history, all beers used to be sour. There was no knowledge then, but also no means to clean properly. Beer was filled into wooden barrels. We can assume that lactic acid bacteria had free rein and soured the beer at that time. People were used to that sour taste then, so it was part of the taste of that time.
Only much later, when we were able to cool beer and cultivate brewer's yeast, did the quality of beer improve and the lactic acid bacteria were seen as undesirable. For the last hundred years we have done everything we can to keep Lactobacillus out of the brewery. Until sour beer was rediscovered and we now purposely add lactic acid bacteria to the wort to obtain the desired sourness.