Over the past 3-5 years I have noticed there have been a large number of breweries putting out sour beers. Sours have actually been around since the early 1800s when brewers would use open vats to ferment, and wild yeast, or Brettanomyces, would enter the wort creating a sour taste. A sour ale is an ale that is made intentionally acidic by adding either Lactobacillus, Pediococcus, or Brettanomyces. The pH is extremely low, usually between 3.3-3.5.
In order to create a sour in 3-4 weeks, brewers will create a kettle sour. To kettle sour a beer you will add Lactobacillus to the wort after the lautering process. It can become a bit tricky for a home brewer to do a kettle sour because the wort needs to stay between 112-120 degrees Fahrenheit for 24-48 hours. These temps help the souring process speed up, as the Lactobacillus culture is more active in this temperature range. You will also want to check the pH over the next 12-24 hours to make sure it doesn’t drop below 3.3. When it gets to this pH raise the temperature above 162 degrees to kill the Lactobacillus. A Gose is a version of a kettle sour.
A barrel sour is a little more time-consuming and tougher than your kettle sour. To create a barrel sour, the brewer will brew a very low alcohol beer, usually around 1%-2%. The reason for this is 3% alcohol and higher will kill the new Pediococcus bacteria. So in order to have a beer that is higher than 3% with Pediococcus, you must introduce the bacteria at a lower alcohol lever and then slowly increase the alcohol percentage over time. Most brewers will fill the barrel half full with the low alcohol and after 1-2 months add a higher gravity beer to the mixture. Lactobacillus can handle higher alcohol, but usually takes 6-9 months to drop the pH to around 3.3. The reason it takes this long is your barrel will be sitting at room temperature instead of at 112-120 like the kettle sour.
It is not completely necessary to use Pediococcus in the barrel sour for the fact that it’s alcohol tolerance is very low. Even if you decide to just use Lactobacillus, it will still take 6-9 months at room temperature to get the desired pH for your sour. Gueze, Flanders Red, and Lambics are all types of barrel sour ales. In my opinion barrel sour ales are more complex and a better drinking beer than your kettle sours. The main reason is barrel sours age for longer period of time and have a more of a body than the kettle sours do.