When Good Beer Goes Bad

There are myriad ways for otherwise great beer to develop unwanted flavors, despite the best efforts of the brewers making it. Why? The answer is simple, yet more complex than you might think.

Jamie Bogner May 2, 2018 - 11 min read

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Many of us have had them—beers that didn’t taste quite right. Maybe it was notes of tart cherries, or a touch of movie-theater buttered popcorn, or an exuberant carbonation that overflowed the glass that raised questions in our minds as to whether this was the way we should be experiencing the beer. But something just wasn’t right.

The bad news for beer fans everywhere is that the things that turn good beer to bad—bacteria such as Lactobacillus and Pediococcus, wild Saccharomyces yeast strains, funky Brettanomyces yeast, and similar microbes—are absolutely everywhere in our human environment. Scientists estimate that within our bodies alone, about 100 trillion microbes—bacteria, yeast, viruses, etc.—live and thrive. Many provide beneficial effects, as Lactobacillus in particular is an important part of the gut biome and aids in digestion. But those same bacteria are also highly efficient at consuming the sugars within beer and converting them into flavors that brewers either embrace (in the case of sour and wild beer) or despise (in every other style of beer).

Given the incredible number of these microbes in our general environment, brewers everywhere work hard to control and corral them with cleaning and sanitizing processes designed to knock down their numbers. Once yeast produces alcohol, the environment within beer becomes even less hospitable for potential spoilers. Yet still, some slip through in large enough quantities to establish a foothold. Let’s dig into why and how some of these spoilers are able to evade even some of the most aggressive sanitation and testing regimens in the industry.

Raw Ingredients as Vectors

We may have a mental picture of hops, grain, and yeast as “clean” ingredients, but the microbial reality of these fundamental ingredients is anything but. Hops possess natural antimicrobial properties that discourage growth, but some bacteria still ride along with the green cones, which is why brewers for centuries have added them to beer while boiling.


The growth of dry hopping as a brewing technique (adding hops cones, pellets, or powder directly to fermenting or already- fermented beer) has led to more potential for rogue microbes to gain a foothold, but the antimicrobial properties of the hops themselves combined with the massive quantities of hops used tends to minimize the danger of creating infection this way.

Malted grain, however, brings all sorts of potential beer spoilers into the brewhouse. Lactobacillus lives in significant quantities on the husks of malted barley, so much so that many breweries create kettle-soured beer using those naturally occurring bacteria. Once again, the boiling process on the hot side of brewing kills off any living bacteria, sterilizing the wort before it’s cooled and fermented.

Yeast from commercial providers is typically tested for purity and potential spoilers, but even the biggest yeast providers have issues from time to time with rogue bacteria and yeast infiltrating their pitches of single strains. In addition, most commercial breweries harvest and re-pitch yeast, and while many test that yeast for purity, cell count, and viability, there are a number of spoilers that may exist among the “good” yeast cells in small enough numbers so as not to be visible to normal testing.

Wooden Barrels—A Cesspool of Microbial Activity

Many of us love barrel-aged beers, but the very thing that makes barrels great is also the thing that makes them a dangerous place to condition and age beer. Their permeability to oxygen allows for a gentle, controlled oxidation, and the pores in the wood allow for increased surface area for beer to extract beneficial flavors such as vanillin and tannin in addition to any latent spirits character soaked into the oak. But those same pores also provide a perfect breeding ground for bacteria and wild yeast, and in the absence of strong alcohols to kill them, those bacteria can thrive.


As a result, all barrel-aged beer—whether “clean” or intentionally sour—is “infected” to some degree. All of it.

Many brewers who produce “clean” barrel-aged beer test the barrels before they blend them together in a bright tank. That testing first can consist of plating samples from the barrel on media selected to accelerate the growth of Lactobacillus and other spoilers. If colonies form, the lab knows that something is growing in there and can work to identify it. That choice of media can make a huge difference, though, as Goose Island learned in 2015. They plated all of the barrels of Bourbon County Brand Stout and Bourbon County Barleywine that year but used media that did not foster quick growth of a particularly slow-growing alcohol-tolerant Lactobacillus strain. The result was a widespread and very expensive recall.

At a certain scale, plating or otherwise testing barrels can grow to be a logistical nightmare. “Our barrel program has grown to the size where a few years ago, we realized we couldn’t test every barrel,” says Avery Brewing’s Andy Parker. “And it would be pointless anyway because you can’t test for everything. The idea of plating 1,000 oak barrels every month would be insane. We did that in the beginning, back when it was 50–60 barrels at a time, but even then it was, in my opinion, ineffective—because we could test for some things, but not everything.”

Another means of testing is Polymerase Chain Reaction (PCR) testing. This testing protocol measures DNA of various organisms within the beer sample to identify potential spoilers, and manufacturers such as Pall Corporation have developed straightforward systems to identify common beer spoilers by their DNA. But that “common beer spoilers” qualifier is the key here—the cost-effective PCR systems used by breweries are not open-ended systems that identify everything in the beer. Testing for every potential spoiler would be impossible, not to mention that brewers today are discovering new spoilers they didn’t know they needed to test for.



The solution that Avery and Goose Island both settled on to ensure that live microbes wouldn’t spoil their beer is pasteurization.

“We have a pasteurizer now, and it’s a really good tool for non-sour barrel-aged beer,” says Parker. “We now pasteurize all non-sour barrel-aged beer, after doing extensive testing to make sure it didn’t affect the flavor of the beer. In blind sensory stuff, no one could tell the difference between the pasteurized and non-pasteurized barrel-aged beer, so that made us feel confident about it.”

Still, there are common misconceptions about pasteurization that need to be addressed. “Pasteurization is not sterilization,” says Jon Carpenter, a Kathinka Labs consultant and experienced brewer who has workedfor Anheuser-Busch, Dogfish Head, Golden Road, and Alchemy & Science. “Pasteurization is an approach developed 150 years ago to cause a logarithmic reduction in certain types of microbes. No one should confuse the two—sterilization is much different than pasteurization.”

Louis Pasteur in the 1860s discovered that it was unnecessary to kill all of a potential spoiler in wine or beer and that by targeting a specific amount of time at lower levels of heat, you could kill enough of the spoiling agent that it would not produce flavor changes within the lifespan of the beer or wine. But even this doesn’t kill every active microbe in a beer, and some living things still get through.


“You’ll see quite a blend [in finished beer],” says Mary Burge, lab director for Kathinka Labs. “Some of the beer we test will show Saccharomyces present, there’s Brettanomyces present, there’s Candida present, there’s Zygosaccharomyces present. More often, we see the Saccharomyces- Candida blend, because Candida is everywhere around us.”

Changing Our Thinking About “Infection”

Many who use the term “infection” do so without understanding what it means or the impact it has.

“We need to separate the issue of food safety from sensory impact,” says Carpenter. “We’re talking about how beer may change from a sensory perspective. So if I keep a tight span of control over a beer [that may show the presence of potential spoilers], only serve it in my taproom, have my bartenders and staff trained on what the beer should taste like and what we accept as true-to-brand, and stay vigilant that it adheres to our internal standards, then it is not a problem.

“We’re not, and no one should ever be, talking about lowering their food-safety standards. But when a brewer releases a beer in their taproom knowing that it has a slight anaerobic microbe count in it, we know what’s growing in there is not unsafe for people; it might just change the flavor. So what we can do as conscientious individuals is make sure that the only place the beer is served is where we have control to make sure that if it does change, we can stop giving it to the customer.”

Moving past the binary concept of beer as “clean” or “infected” is one that’s important for consumers. It’s not a question of whether there are things in a beer that could take the flavor in an unintended direction—they’re there in every beer, including many beers that have been pasteurized. The question, rather, is how much is in there and whether that amount is enough for that microbe to gain a foothold in the hostile alcoholic environment?

“It’s a crazy microbial world out there,” says Parker.

Jamie Bogner is the Cofounder and Editorial Director of Craft Beer & Brewing Magazine®. Email him at [email protected].