Buttermilk - what's it like when 'off'

[hobbgoblin]

I've some buttermilk lurking in the fridge ready to make Nigella's buttermilk chicken recipe. Just noticed it went out of date yesterday. I haven't opened it yet. I would still use it despite dates if I thought it was okay - as I do with cream.

So, can anyone tell me what off buttermilk smells/tastes like as I've never sampled it straight from the container?

I don't know, but I would guess that it will taste REALLY sour if off, and probably have green fur growing on it. Probably the foil top will have blown as well.
If you taste it and it is edible, then it is probably OK. You are going to cook it anyway....aren't you?