Ask the Experts: Source of Unintentional Sour Flavors in Beer

Homebrew expert Brad Smith, author of the Beersmith homebrewing software and the voice behind the Beersmith podcast, discusses the causes of unintentional sour flavors in beer.

Brad Smith Mar 15, 2018 - 4 min read

Ask the Experts: Source of Unintentional Sour Flavors in Beer Primary Image

A Craft Beer & Brewing Magazine reader recently asked us the following question:

Last year I was making some great beer, but several batches ago I started getting sour flavors in my beer, and it seems to be getting worse with every batch. What can I do?

Your sour-beer problem is almost certainly being caused by bacteria. Bacteria such as Lactobacillus and Pediococcus occur naturally and will infect and rapidly sour a batch of beer if you don’t kill them off using sanitizers. These bacteria produce lactic acid and are widely used in sour-beer brewing.

Poor sanitation could be the cause, but if you have not changed your sanitation process in the past year, it may not be the problem. The fact that the problem is occurring from batch to batch and you did not have it earlier indicates that some of your equipment may be infected. Most likely one or more pieces of equipment on the “cold side” of your system are infected with bacteria, which is then souring your beer during transfer, fermentation, or storage.

I say that the “cold side” is the problem because your beer is not subject to infection during the mash, and the boil as the act of boiling will kill any bacteria. However, it is possible for your chiller, pump, transfer tubes, fermentor, or even bottling bucket or kegs to be infected.

The most likely location for an infection is plastic, rubber, or silicone. Unfortunately, even a small scratch in a plastic bucket can harbor bacteria that are very difficult to remove. While stainless-steel and other metals are largely impervious to infection if properly sanitized, it is possible for your fittings, gaskets, valves, pump heads, and tubes to harbor bacteria.

At a minimum, I recommend disassembling all of your fittings, hoses, gaskets, valves, pump heads, and other interfaces and giving them a thorough cleaning and then soaking for a bit in sanitizer. If that fails, you might want to consider replacing old hoses, gaskets, plastic utensils, and other rubber and plastic in your system to eliminate potential sources of an infection.

Finally, I want to mention that it is also possible to create sour beer if you have improperly stored ingredients, such as wet malts. In this case, an infection of the malt before brewing can produce lactic acid in the malt or mash that then can carry forward to the beer. Essentially, you are creating a sour mash. This is far less common than a cold-side bacterial infection, but it is a possibility you should be aware of.

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