How to tell when brewed coffee has spoiled?
it depends what you mean by stale:
if you mean green floaty spots - well, that takes a few days
if you mean, no longer desireable... then that's a little more relative. For best results, keep it off a burner (like anything with an open top and a hotplate underneath)
1. they probably don't brew that well to start
2. they keep cooking your coffee while the liquid evaporates
3. best are sealed airpots that are in good condition - ie, the stainless steel should look like stainless steel, not crusted with black crap.
even in an airpot like that though you're going to notice a steady decline. I'd say an hour at the most for best results - but it depends on your expectations. Some people microwave their coffee - and some of us find that idea revolting.
Smell is key for me. If it smells great - good. If it doesn't, it's time to grind a fresh pot.
Short reply is "if it's over an hour old". I often do let mine go longer than that, but then I have a certain tolerance for what tastes like old pencil shavings. This is on a hot tray; in a Thermos or other insulated container it tastes better longer, but that fresh-brewed bloom is a fleeting thing.
For the record, I brew mine in a Chemex, mostly Peet's French Roast, which is not that aromatic to begin with and stays interesting longer (at least to my taste).
re: Will Owen
I think you've answered a different question than the one I was (trying) to ask.
I want to know at what point coffee actually spoils -- i.e., in that if you drink it you would get medically sick. The same way if you were to ingest spoiled milk.
I am not asking about stale versus fresh coffee, which ultimately only affects taste and quality and not whether the coffee can make you medically ill. I agree that coffee is best consumed freshly brewed and hot.
I've had mold grow in the coffee filter when I've gone out of town for an extended period of time and forgot to clean it before leaving, but I've never seen spoiled coffee (assuming there was no milk in it). I think it would evaporate before it spoiled, unless for some reason it was in a container exposed to mold/bacteria. Black coffee just doesn't seem to me to be a hospitable environment for mold/bacteria.
I used to work with a guy who would fill his coffee cup up with coffee just before he left work for the day, weekend or whatever. He left it uncovered on a corner of his desk until he got back, then sipped on it for a couple of hours. He never got sick, or washed the cup for that matter. According to him it never spoiled.
Mom brews a full pot in the morning, has a cup or two, then leaves the house. She turns the burner off but leaves the coffee to sit. When she gets home at night she'll microwave a cup before bed. The next morning she does the same, and repeats until the pot is empty. Never is the coffee covered or refrigerated. Sometimes it takes her three days to kill the pot. Never has it made her sick.