Got an accidental spontaneous fermentation

So I prepared about a quart of 1.040 basic wort to use for a starter. I pitched about half of the wort and let the other half sit, sealed in a mason jar on the counter. I intended to pitch it at high krausen to build the starter.

When I opened the wort to add to the starter, I noticed a funky, yeasty smell. So i unscrewed the cap a bit to allow off-gassing and i let it ride. Over the next week, a slow fermentation occurred and the yeast are now falling out of suspension and forming a cake.

Would wild yeasts ferment and fall out of suspension so quickly?

I have used this jar in the past for yeast washing, so I'm wondering if maybe it is just some leftover yeast from a past strain (although yes, I did sanitize, twice) or even a bread yeast that was in the air (SWMBO uses bread yeast regularly)

Thoughts anyone?

I guess I can build it a bit more and make a one gallon batch of beer to see what happens

Cool. I'd give it a shot... bread yeast, an old strain or wild yeast are all possibilities. I'd step it up again and make a small batch. If it turns out well, you'll have enough for 5 gallons.

After brewing, I'll often leave a small amount of wort in a jar on the counter covered with sanitized foil. It gives me a good indication of how effective my sanitation was on that batch. Eventually it always starts to ferment, but well after the brewers yeast has started doing its job in the main batch. If the day comes when the sample jar starts around the same time as the main batch, I'll know I'm in trouble.

What I'm saying is that I'd be suspect of the quality of the starter wort you used even in the first half. Your pitch should have over-powered any wild yeast infection, but . . .

You probably have wild yeast. Go ahead and taste a sample of the liquid. If the taste is to your liking, then by all means brew with it.

AnOldUR said:

After brewing, I'll often leave a small amount of wort in a jar on the counter covered with sanitized foil. It gives me a good indication of how effective my sanitation was on that batch. Eventually it always starts to ferment, but well after the brewers yeast has started doing its job in the main batch. If the day comes when the sample jar starts around the same time as the main batch, I'll know I'm in trouble.

What I'm saying is that I'd be suspect of the quality of the starter wort you used even in the first half. Your pitch should have over-powered any wild yeast infection, but . . .

Click to expand...

I sort of gave the reader's digest version in the original post. They progressed at very different rates. the starter wort in question was actually the third wort I prepared. The starter already had over a quart at this point and was fermenting happily. It wasn't until the actual starter had been going for nearly a week that my unpitched wort started to get a smell...then it was another week before a noticeable yeast cake formed.