I brew ten-gallon batches of beer split into two fermentors. The last batch that I kegged had one infected fermentor, while one remained uninfected. One bucket showed a white powdery substance covering the remnants of the krausen, as well as an oily and whitish film on the surface; the other looked normal. I understand that there are only a few organisms known to grow in beer. Is there a way to identify which bug has contaminated the brew?

Perhaps the various organisms are different colors? Perhaps the texture varies?

4 Answers 4


I can think of a couple of ways:

  1. Pull a sample and look at it under a microscope. Bacteria and yeast cells look quite different from each other. After you get an idea of what they look like, you can look around online for pictures of various souring organisms (acetobacter, lactobacillus, brettanomyces, enterobacter, pediococcus, etc.). That would get you close as far as which genus it's from, but as brewchez says, you'll need more labwork to find the actual species.
  2. Pull a sample and smell and taste it. if you smell or taste vinegar, it's probably acetobacter, a yogurt-like sourness would be lactobacillus or pediococcus, poop? probably enterobacter, barnyard/cherries/leather brettanomyces.

There's also a good chance that it's a combination of organisms, so you'll wind up with a mixture of those smells and flavors.

I have only had one accidental souring, and it actually tasted pretty good, so I just drank it pretty quickly.


I agree with baka's approach, and for the general question of "Name That Micro-Organism," his answer is best.

In your specific case, though, I would bet that your beer is not contaminated. White film or clumps happen all the time - it can, on occasion, be a lacto contamination, but usually it does not affect the flavor or aroma of the beer at all. Google it. Or read this , this, or this.

I'll admit, it's curious that it only grew in half your batch, but nonetheless, this is a common occurrence and not necessarily and indicator of spoilage.



I think people who are well versed in certain areas of microbiology may be able to tell if something was a wild yeast, bacteria or mold. However, it would require a little more lab work and inspection to determine which strain per se.

I for one am glad I can not identify the bugs by eye, as it would indicate I'd had enough infections to do so! :)


I also don't think you're infected... If your brew looked anything like mine, don't worry. Mine turned out to be one of my best.

See my similar question here: What is this white stuff on the surface of my beer?

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