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So last night I checked on my porter, 48 hours on fermentation with Wyeast 1968, bubbling away happily. My ferm chamber is a small chest freezer.

Got a blowoff tube running from the top of the bucket down to a quart or so of StarSan (leftovers from brew day). The sanitizer foams up, of course, and with all the bubbling and krausen coming down the tube a nasty pool of brown gunk had formed on the floor of the freezer. I grabbed a rag and reached down to mop it up...

And that's when I nearly died. My sinuses screamed in pain and I had to jerk my head out of the freezer. There was a knife-sharp effect on my nose by something in the air in the freezer.

I wouldn't say it's a bad smell, per se. There wasn't really a scent. It's that my nose hurt. A lot.

I managed to clean up the mess by pinching my nose shut before diving in, but man, what an experience.

What was that "smell"? Was it just CO2 buildup in the closed chest and my nose was reacting to a complete lack of oxygen? Could I possibly have an infection, in my beer or in my nose? Any good theories here?

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It's the CO2 - I've had the same thing happen sniffing the top of a regular bubbler airlock. The CO2 enters the nose and dissolves quickly in the small capillaries creating carbonic acid, which stings. There's no real danger here since you don't stick about long enough to breathe in much CO2.

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    Yep, I've had the same thing happen when I overzealously whiff my fermentation chamber. Nose burns, eyes water, swear words fly.
    – JoeFish
    Jul 10, 2012 at 20:59
  • Winner, winner, chicken dinner! +1's to everyone who said CO2, and green checkmark to mdma for the bonus biology lesson. My assumption was CO2, but having never experienced this I didn't know what to think. I feel like Columbus with my new "discovery". Thanks, all!
    – Galapagos Jim
    Jul 10, 2012 at 22:11
  • I always thought CO2 would just suffocate you without any warning signs. Was I wrong?
    – Markus
    Jul 11, 2012 at 7:22
  • @Markus, if you're completely surrounded by only CO2, such as in a confined space, then yes, you will asphyxiate from lack of oxygen. But that's not what's happening here - we're talking about a small amount of CO2, with plenty of O2 still being available form the surroundings.
    – mdma
    Jul 11, 2012 at 8:42
  • @markus you are thinking CO, this is CO2
    – brewchez
    Jul 12, 2012 at 2:16
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Yep, it was more than likely CO2.

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  • I guess a good reason to open the freezer slooooowly and stand back for a minute or two...
    – FrustratedWithFormsDesigner
    Jul 10, 2012 at 18:51
  • Yes, I think I'll open the lid slowly and run the fan for a bit before I stick my head down there again. :P.
    – Galapagos Jim
    Jul 10, 2012 at 22:14
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I've never experienced a "knife-sharp" smell, but I'm guessing it was just CO2. Maybe that was your brain's way of telling you that you were going to suffocate unless you get some fresh air pronto.

Breweries often use large 55-gallon drums half-filled with water as their blowoff tube airlock, and on more than one brewery tour I've heard the guide say that people will stick their heads in the bubbling drum to get a good whiff of the beer and will pass out.

I doubt you'd have a wild-running infection already.

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    Not a reaction from about to suffocate - a sudden excess of CO2 manifests itself as a lack of oxygen, resulting in a headache, thumping ears. Breathing CO2 through the nose hurts because of the carbonic acid created.
    – mdma
    Jul 10, 2012 at 19:09
  • @mdma: Very interesting.
    – Hank
    Jul 10, 2012 at 20:16

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