Question for the micro-biologists

[fa50driver]

I had a bottled beer turn sour after about 8 weeks at 80 F temps - and thought it was an isolated incident with my DE filter. I started plating all of my beers on LMDA including the beer known to have the contaminant. I plated the known contaminant twice (1ml on LMDA, incubated up to a week at about 80F, aerobically) and both times it showed no growth (I'm assuming that I can plate correctly.) I just noticed another bottled beer starting to sour (an IPA at about 65 IBU's and 7% ABV) that showed no growth on LMDA either, so now I'm a bit lost and I'm sending everything to the lab since I can't verify anything on my end... The beers tend to sour almost overnight - IE they will taste fine for several weeks, and within about one week will turn noticeably sour. No ropiness, rings in the bottle, sulfer order, or anything other than a very citrusy sourness that continues to get stronger over time.

Any guesses to what could be souring the beers post fermentation that doesn't show positive growth on LMDA?

[BenBailey]

Try plating one of the bottles after it has gone sour.
How are you plating? Membrane filter? Pour Plates? Streaking? If you are just streaking the plates, you may not have enough of the bacteria in one loop to show any growth. If you are putting a sample of beer in the plate and then pouring agar on it, you agar may be hot enough to kill all the organisms.

It sounds like you may have a strict anaerobe, which all beer obligatory beer spoilers are. The fact that it takes some time before the beer begins to spoil may indicate that the infection has to wait for the oxygen to become chemically bound in the beer before they can grow. You won't see the infection with your current method because you are incubating aerobically. There are some cheap ways to incubate anaerobically. The oldest method is the lighting a candle in a bell jar and letting the candle burn all the oxygen. A slightly more expensive method (but still cheaper than an anaerobic incubator) is the Gaspak system that BD makes. https://us.vwr.com/store/catalog/pro...mber=90003-642

Hope that helps. Good Luck!

[fa50driver]

Thx for the info on the anaerobic incubator - I'm probably going to start adding that to my sampling protocol.

Yes, I am plating the soured beer - basically 1ml on LMDA aerobically. Not using a filter yet, mostly because it is on order still, but the beer is unfiltered so .45 micron is probably going to clog pretty quick.

I sent some samples to the lab, but the results were few organisms on them, one of which was a wild yeast that tended to outgrow everything else. Many indicators point towards Lacto, but still trying to figure it out. All yeast is now getting tested before pitching along with a dose of Chlorine Dioxide.

[sw341034]

Might seem basic, but have you looked at a sample under the scope? Should be able to tell a lot just by taking a quick look.