Is this yeast starter infected?

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Beerandgranite

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Dark spots and dark layer on the bottom
 

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Beerandgranite

Beerandgranite

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Does it smell ok? I've had black specs in mine, and it was fine. You just don't want a sour smell.
It doesn’t smell sour, it tastes incredibly dry and slightly tart but not rancid or sour. I ran this starter on a stir plate at 75 +/- degrees for 48 hours, so I’m wondering if the warm temp could be contributing to the flavor I’m picking up
 
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Beerandgranite

Beerandgranite

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A tart taste is a pretty good indicator of contamination.
I’m wondering now if what I am tasting is acetaldehyde? The tartness definietly has a green apple characteristic, just without the sharp or sour character. Could a warm fast starter create such a situation?
 

pvtpublic

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No, unfinished fermentation would be to blame for acetaldehyde. I wouldn't worry about that in a starter at all. Acetaldehyde is an intermediate product of ethanol production. As in, it's the fermentables not fully fermented. Same with diacetyl. Cold crash your starter, decant and pitch! The fermentation will clean any acetaldehyde right up.
 

McMullan

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All you have there is some debris settling out and one or two chuncks of coagulated DME or something, not an infection. Despite popular opinions it's really not unusual for a starter to taste a little bit weird. It's a starter after all.
 
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