If I leave my mash tun unattended for a few days after brewing it smells like a dead animal covered in vomit. What is the name of the bacteria/fungus/other that excretes this smell?

  • I did this, then let my mash tun air out in the backyard. The two feet of snow that filled it that night didn't help... Commented Dec 18, 2009 at 0:51
  • Cleaning up tags. Changes badsmell and spentgrain to bad-smell spent-grain Commented Feb 10, 2010 at 17:17
  • Experienced this personally last week. Left my tun untended for a few days. Commented Feb 10, 2010 at 17:21
  • Dean. I am glad I am not the only one who lets this happen occasionally. I always regret it, but I never learn.
    – brewchez
    Commented Feb 11, 2010 at 13:42
  • I'm usually good about dumping the mash tun during the boil, but it was a late-night brewsession. Commented Feb 13, 2010 at 5:50

4 Answers 4


The answer is: Every bacteria that exists in your local area. Lacto, brett, wild yeast, and less pleasant wee beasties. Its unlikely that any bacteria on your grain survived the mashing process. Not impossible, but unlikely.

I have no doubt that its a combination of all of those factors.

My advice would be to never let anything sit around dirty. Clean equipment is happy equipment.

  • I would have assumed that :-) I am more wondering, Which microbe is causing that smell. It's very distinct. I agree clean equipment is happy equipment. Luckily, it was the mash tun, all the wort that touches it, is later sanitized during the boil.
    – Tim Weber
    Commented Dec 18, 2009 at 21:57

That smell is mostly form pedicoccus. Its a bacteria that work aerobically and it has a vomit like smell. Lacto is anaerobic and has a fairly clean aroma.

When doing sourmashes (leaving the mash for a few days at ~110-120F) there will often be a layer of nasty smelling malt on the top that can be scooped out. The mash underneath is soured and very "clean" tasting aside from the sour lactic acid taste.

  • Nope, don't think that's the one. en.wikipedia.org/wiki/Pediococcus
    – Tim Weber
    Commented Jan 5, 2010 at 16:45
  • 1
    I don't understand what in that link says its not pedio. I thought I remember reading in Zymurgy a few years back that pedio was the dominant player in this 'vomit' like smell.
    – brewchez
    Commented Feb 11, 2010 at 13:48

Just a guess, but maybe lactobacillus bacteria? Read somewhere that it can be introduced from the husks of barley...

  • All of the lactobacillus in the malt would have been killed during mashing and sparging, pasteurization temperatures. To sour a mash you have to add fresh malt after the mash has cooled down quite a bit. I add it at about 110F.
    – Tim Weber
    Commented Dec 18, 2009 at 21:54
  • I do doubt that all of it would have been killed. Its a time and temperature issue. I am sure that after a mash you could find living lacto or pedio in there.
    – brewchez
    Commented Feb 11, 2010 at 13:47

I always thought vomit smells in brewing come from butyric acid and butanoate, produced by Clostridium species. Wikipedia agrees: http://en.wikipedia.org/wiki/Butyric_acid

Butyric acid boils at 164 C, so boiling your wort won't get rid of it, but it's worth a shot. I know Papazian and Brew Your Own recommend one long sour mash at 110F, but why aren't we making our sour mashes like sourdough bakers make starters? Here's what I do to get a clean lactic sourness:

Combine ~100 mL of boiled wort with a handful of pilsner malt straight out of the bag. Give it a good shake to provide oxygen for yeast growth. After 12 hours, dilute 1:2 into more boiled wort, add half a handful of malt, and shake. Repeat until you see bubbles, then repeat a few more times. Voila, you've got an alcoholic and sour culture. The lactic acid bacteria/yeast symbiosis has grown much faster than any clostridium, enteric bacteria, or molds, and has come to dominate, even preventing clostridium growth -- but only because of the frequent dilution with fresh wort.

Lactobacillus is a facultative anaerobe, which means it can tolerate oxygen, but the yeast depends on oxygen for growth, so don't forget to shake (remember sourdough is not anaerobic).

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