Infected Batch Forensics - Brewer's Friend
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Infected Batch Forensics

Saturday, November 12th, 2011

For the first time in many years I experienced a bad batch of home brewed beer last month. I am sharing this with the community so you can avoid my mistake. It is embarrassing! Being out the time and money for the batch and having an empty keg sucks. However, I learned a couple things I can pass along. This bad batch changed my perspective on yeast re-pitching, updated my definition of what a healthy fermentation is, and helped me clean my equipment better.

The spoiled batch, which had been in the keg for about 2 weeks had the following properties:

  • Initial flavor was anti-septic (almost burning).
  • Murky appearance (different than chill haze).
  • Slippery mouth feel.
  • Finshed soapy, with the flavor getting more pronounce, to the point I spit it out. The bitter antiseptic flavor lingers on the tongue.

What the heck! Had I gotten a lax on sanitization? Was my yeast bad? Was there something in the brewing process that lead to this? I asked some home brewing experts and they attribute this to a yeast deficiency – either an infection, or bad yeast. It turns out there were two main factors, repitching of yeast and a dirty spigot in my fermentor. While I am not sure which did more damage, I have evidence of both.

Here is what the carboy looked like, note the ring:

infected home brew

If you have a goopy ring around the krausen layer you might be in trouble. The off gases from the fermentation also smelled ‘cheesy’, which tipped me off there might be a serious problem.

Beer Forensics:

For what turned out to be the spoiled batch, I opted to re-pitch yeast (Wyeast Northwest Ale 1332) from a previous batch of IPA. The harvested yeast was in the fridge in a ball jar for about 2 weeks. That IPA was already in the keg and was drinkable, but it was not my best batch ever. It had a thin finish and there were some light ‘chemical’ flavors present that come and go from sip to sip. I figured at the time, the beer was just green – and it did improve a lot after a month went by. When I kegged the IPA and harvested the yeast, it looked and smelled fine, a fresh bready aroma was present. That meant it is good to go right? WRONG!

Something funny happened with the fermentation of that IPA. One night I forgot to set the furance at 64F, and instead turned it all the way down to 58F – so it got pretty cold in the house that night. It turns out NW Ale 1332 does best between 65F and 75F. In effect, that night the yeast were really stressed. I didn’t think much of it at the time. I do recall the fermentation slowed after that. I also left the IPA in the primary for 23 days, without racking, and then harvested the 23 day old yeast cake. In retrospect I should have spent the $4-$7 for a new yeast pack. Even though that yeast cake smelled good at the time, it was no longer to be trusted given its age, and the temperature fluctuation.

I gathered two ball jars from the yeast cake. Now a month later, on inspecting the second ball jar, there is a thin line of black mold growing on top. YUCK!


infected yeast


That IPA was fermented in a plastic bucket with a spigot. The subsequent failed batch was fermented in a glass carboy. I went ahead and tore apart the spigot on the bottling bucket, and look what I found in there:

infected spigot


It had probably been over 3 years since I setup that bottling bucket and spiot. Whoops! No doubt whatever mildew / mold / germ was living in there is not good for the beer. I terminated it with extreme prejudice – a strong bleach solution!

After I soaked everything, I thought I was ready to go again, but then I noticed, inside the sealed part of the spigot, there were some faint black spots. It is hard to see in the picture, so I enhanced the second one.

infected spigot mechanism

infected spigot mechanism enhanced


The mold / mildew, whatever it is, is growing INSIDE the sealed part where the spigot rotates. There is no way to scrub that section. I am looking into getting a different type of spigot that does not have this design flaw.

Yeast Re-pitching Revisited:

All this time I had thought I was being a good sport by re-pitching yeast. That is what the pro’s do right? Well, it turns out I did not realize the risks associated. I wrote an article awhile ago that praises yeast washing, and another on yeast repitching. I have updated those articles to point out what was learned here. The time it takes to harvest and clean the yeast (15-20 minutes), plus the risk is not worth the $3 savings it offers! I should have known that…

Key Take Aways:

  • Yeast re-pitching can be risky and might not be worth the cost savings for home brewers.
  • If you are going to repitch – I would rack after about a week and save that yeast. Let the secondary fermentation finish on its own, and discard that smaller, older yeast cake. I would also be very strict about temperatures ranges and sanitization. I successfully repitched many many times, but now I am starting to see where perhaps some inconsistency came into play.
  • Tear everything apart now and then and completely clean it with PBW or a bleach solution if the materials are compatible with bleach.
  • Look for stuff growing inside what appear to be sealed parts, such as the spigot on the bottling bucket.



  1. 9 Responses to “Infected Batch Forensics”

  2. Thanks for sharing. A nice reminder of what can happen and what to watch out for.

    By mike on Nov 12, 2011

  3. Hey, first thank you for posting this and sorry you lost a batch. For all the talk and worries about infected batches, it often seems like the boogy man that’s always lurking but not easily seen so I appreciate you sharing pictures.

    I would recommend that you take some time using fresh yeast and then reconsider if you want to repitch again. It’s probably not worth the effort to save $4-7 bucks but not many parts of the brewing process are if only thought of that way.

    One of the things I love about brewing is purely being able to do it all. I like that I can reuse the yeast even though it takes extra time and is an added risk for a bad batch. The same can be said about brewing all-grain since more can go wrong and it takes longer but it’s worth the risk for how much I enjoy that part of the process. Cheers and I hope you have enough on tap to hold you over til your next good batch.

    By Gabe on Nov 13, 2011

  4. So, probably be more than 1 problem here. Interesting what you say about the temperature drop in the previous brew; I had a failed brew (off-flavours) last August, which used re-pitched yeast. The previous brew (as with your’s, an IPA) was fine, but I checked my records after reading this: the temperature of the IPA was 28C @ pitching, a tad high so fermenter was placed in refrigerator -a little too long- down to 10C. Removed from fridge and brewed out at ~20C. The harvested yeast was stored ~3 weeks and pitched in another ale which was undrinkable – same carboy for both batches, which has been used several times since with good results.
    I’ve repitched various yeasts many times over, including Wyeast 1084, which I’ve been re-cycling for about 5 years now. The failed yeast here was a dried yeast (Safale SO4) but never had this problem before, so looks like that wild temperature flutuation in initial brew could be quite significant for future brews.

    By David A on Nov 16, 2011

  5. Sorry about the bad batch. It is frustrating to have that happen. After reading your article, I took all my faucets and boiled them to sanitize. The heat softened the plastic to make disassembly of the faucet a snapp. I cleaned all the parts, soaked them in sanitizing solution and re-assembeld. As far as repitching, I also do it. After a stuck fermentation due to yeast that was collected from the bottom of the fermenter and not viable anymore, I started to collect my yeast from the starter flask. Right before I pitch, I decant the liquid and mix the remaining liquid with the yeast that settled on the bottom (I turn of the stir plate 24 hours before pitching to let the yeast settle). I then decant some of the yeast into a vial and pitch the remainder into the wort. The yeast is active and fresh and viable from the starter, and so far I have not have any problems with this collection method.

    By Holger B. on Jan 4, 2012

  6. Simply open one/pour of your brews. I currently bottle. I open a bottle, pour off >80% of the beer into a drinking glass. Then swirl the beer bottle to re-suspend the yeast. Then pitch this liquid out of the bottle into a new starter. I imagine you could do this with a keg as well.


    By Mike M. on Jan 13, 2012

  7. I also had a bad batch and here follows the story.I started a brew with a new packet of Mutons gold and when fermentation finished and the beer racked ,the fermenter was cleaned after the yeast at the bottom was collected and another batch was brewed and the collected yeast was pitched. The first batch was infected and undrinkable and the was drinkable although not one of my best beers that I have made. Conclusion , you should be able to wash infection out of your yeast

    By Gert on May 21, 2012

  1. 3 Trackback(s)

  2. Nov 12, 2011: Racking Tutorial with a Built In Spigot | Brewer's Friend
  3. Nov 12, 2011: Money Saving Tips – Repitching Yeast | Brewer's Friend
  4. Jun 11, 2012: Yeast Washing 101 | Brewer's Friend

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