After a number of successful brews, I think I finally have a contaminated batch. I've always been good about sanitation (or so I thought) but I've steadily progressed from paranoid germophobe to a bit more relaxed.

I realize that it's not possible for anyone to definitively say what went wrong - but I'd still like to learn from this experience. As you'll see, this wasn't one of my better brew days! Wasn't a true disaster...but it was close.

My question: Of all the possible sources of contamination, are there any that stand out as more likely than others?

My observations:

  • Possible pellicle in the carboy, but nothing dramatic. Just a faint white skin over part of the surface.
  • Carboy smelled okay (not foul) before bottling
  • A sample tasted okay - not great, maybe a bit "thin" with less flavor than expected, but not obviously horrible
  • REALLY BAD smell in the yeast cake after bottling. Much like that anaerobic bacteria smell from leftovers that are forgotten in a sealed container. The yeast cake also had more of a rubbery "mat" texture than a typical yeast sediment texture.

Here's how the brew process went. It didn't go quite like I wanted!

  • First time with a partial-volume boil. Started with 2.5gal in a 6gal kettle.
  • After the boil, added 3gal bottled distilled water to the kettle before transferring. (Metal didn't get hot enough to kill bugs on the sides of the kettle above the boiling wort, before adding water? Bugs in the distilled water or on the containers?)
  • Siphoned warm wort to glass carboy (carboy and tubing were sanitized, but maybe not completely??)
  • Wort did NOT get chilled like normal. I wasn't able to use my chiller, and assumed adding 3gal refrigerated water would be enough - but it only took it to about 100F.
  • Figured this was too hot and would stress the yeast, so I loosely covered the mouth of the carboy with plastic wrap and ran some errands in town. (plastic wrap wasn't sanitized, but I figured it would be okay at the time)
  • After about 3 hours, it had only cooled to 90F. I was getting nervous and pitched the yeast anyway.
  • Pitched 2 packets of dry yeast - one came with a kit and I didn't know how viable it would be.
  • In that 3 hours before pitching, the yeast was hydrating in pre-boiled water in a sanitized cup covered with a sanitized plate. But it still foamed over the edge and almost pushed the plate off! (still too exposed, maybe the foam-over picked up something from underneath the cup?)
  • 2 WELL-hydrated yeast packs + 90F wort = MASSIVE yeast activity in just a few hours.
  • In the morning, the wort had foamed up completely and fouled the airlock, which I replaced. (yes I sanitized the airlock & rubber stopper, but was a little lax about it, not expecting this to be as big a deal).
  • Just overnight, the gravity was halfway to FG!

My best guess is that it went wrong in the 3 hours before pitching the yeast, when the wort was still very warm - and any bugs were then out-competed by the massive amount of yeast activity? All this yielding a not-horribly contaminated brew where most of the nastiness was buried in the yeast cake?

Between contamination and bad temp management, I don't expect this to be a good one to share, but ... had to happen sometime!

  • What are the ingredients? Bottle conditionned or kegged? Priming?
    – Philippe
    Nov 27, 2019 at 17:49

1 Answer 1


What makes you think you've got contamination? A slight skin over part of the surface isn't necessarily an uncommon thing. I guess it depends how it looked. You wouldn't happen to have any photographs?

Anyway it sounds like you pitched enough healthy yeast and it took off pretty quickly, so it is difficult to know for sure, IF you've got any contamination, where it might have come from exactly. As you know there are dozens of possibilities. However these are the most likely ones:

1) Transfer tubing. How long have you had yours? In my opinion, it's a good idea to replace all tubing about once per year, or after a couple dozen batches, whichever comes first.

2) Refrigerated water. How was this treated? Was it preboiled, or purchased from a store? If purchased it is probably fine. If you jugged and refrigerated it on your own, it could be contaminated if you didn't sanitize the container(s).

3) Pitching warm about 90 F. There is a tiny bit of risk here. Also be sure to sanitize the outside of the yeast packet before using, and even the scissors used to open the pack.

Those are some things if you hadn't thought about before, might be worth some improvements on future batches, whether you have real contamination in this batch or not.

Regards and happy brewing.

  • Thanks dm - appreciate the voice of expertise. Dec 5, 2019 at 21:50

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