souring by Thermos

I was talking to a fellow homebrewer who suggested a means of producing a sour beer without using yeasts designed for this purpose.

He suggested that when lautering, to remove a thermos full and let it sit on the counter for 12-24 hours. This apparently turns sour (somehow -- by ambient yeast?) and you can dose it into your batch as desired.

I've never heard of this technique before. Has anyone tried it?

New Glarus came out with a couple sour beers in the last few years, and I thought they were pretty good so I'd definitely be interested in trying a sour beer sometime.

similar to guiness? I seem to remember reading that they do something like this. I might be wrong though, what do I know.

souring by thermos sounds reasonable, however I don't think the volume of sour liquid in one thermos added to a 5 gallon batch would sour it enough.

There are a number of threads here regarding various sour mashing techniques, mostly involving leaving your second runnings out with a handful of grain for 12-15 hours.

Grains naturally have some bacteria on them, so leaving them in warm water will cultivate any present bacteria. I don't think that just leaving it sit out and open without any grains will do the trick.

In either case, it is necessary to reboil the soured liquid to kill any of the baddies, lest they take over your entire batch and brewery.

jezter6 said:

In either case, it is necessary to reboil the soured liquid to kill any of the baddies, lest they take over your entire batch and brewery.

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I don't think this is true, maybe that batch(which may not be bad), but not your brewery. Jamil Z agrees, but advocates getting seperate tubing and bucket(any soft plastic), if you use glass its fine.
Souring the whole mash, or wort, is easy to control. When its sour enough just boil it, it can take 1-3 days. But you don't have to boil it, this is how traditional Berliner Weisse is made, without boiling all the wort. The lacto/etc. is left to do their thing throughout fermentation.