Mixed culture starter behaviour - acetic

[p_p]

Hi, I am looking for an explanation on what's going on with my starter.

Long time ago I decided to brew a beer using dregs from a bottle I like (not a lambic, but lambic-type character). I made a starter on a stir-plate using dregs from 3 bottles, stepped up to the equivalent of what you get in a vial and pitched to a 5 gal batch. Turned out excellent.

Now, I have been doing the same thing ever since (not quite getting the same results, but none the less) ... What I have noticed is that while in the stir-plate, the starter froths with a think foam and smells funky, but if I stop the stir plate, within hours the starter becomes very acetic.

I am just wondering, could that be the brett spitting acetic acid or is it bacteria? Why would it happen *after* I stop the stir-plate?

The whole reason I started plating yeast was finally find out what's in these dregs. I managed to isolate one of the bugs, white, circular, entire, convex colonies. Did a mini ferment on 15ml of sterile wort (no airlock) and bingo, straight up the acetic character that I get in the starter ...

What other tests can I run? (no microscope yet)

Any thoughts?
Thanks guys.

 

[RPh_Guy]

Lots of microbes including Brettanomyces produce acetic acid in the presence of ethanol and oxygen. That's expected from such a mixed culture with aerobic propagation.
Acetic acid is there whether or not you stop the stir plate. Acetic acid is volatile, so perhaps if you are tasting the sample, it might be more acetic without the stir plate because less of it is being volatilized.

FYI aerobic propagation is not generally recommended for mixed cultures. Semi-aerobic (like an orbital shaker) or anaerobic are better propagation methods. Less acetic acid and THP are produced, and more esters are produced.

FYI dregs are not generally recommended for primary fermentation because many commercial sours contain bottling yeast ... Bottling yeast probably isn't a great primary strain.

If you're looking to actually identify a particular microbe... From my understanding, genetic testing is the only definitive way to identify (i.e. running PCR and gels for comparison). ... Although you can enrich certain media for particular microbes.
I'm not a microbiologist but I have limited experience in this arena.

A basic test is to see if your isolate ferments wort. If it does, then it's yeast. If it only forms acid, then it's bacteria.

Hope this helps

 

[p_p]

I helps a lot, thank you very much.

How about this: I made a saterter following the same procedure, but this time I hoped the s**t out of it. My understanding is that acetic acid producing bacteria is not inhibited by hop and therefore I was expecting the same results as I had before, however, in this ocassion the starter has not seemed to have developed the acecit smell/taste it usually does .. Now I am confussed.

I have managed to issolate whatever is that is producing the acetic acid, I'll probably try a mini ferment using hops and just this bug.
Need to get hold of a microscope!

Cheers.

 

[RPh_Guy]

Did you test the FG or pH of either starter with the isolate?

 

[p_p]

I have not. Only relied on "tastes like vinegar and makes me pucker" vs. "clean and all right".
I could have made a mistake, the experiments have been far from scientific .. but, is it possible to inhibit acetic producing bugs with hops?

 

[RPh_Guy]

Yes, gram positive bacteria can produce acetic acid.

Acetic acid is not a particularly useful indicator. Also, if you use the culture, you'll want to protect the beer (or whatever) from oxygen regardless, so acetic acid shouldn't really be big a concern anyway.