Experiment: If you cook expired milk into yogurt will it still taste good?
Answer: Yes.
I had a gallon of milk that was five days past the expiration date. It smelled and tasted a little off. I did not want to waste it, so I tried an experiment - I cooked and fermented it anyway to see how it would turn out.
It worked just fine.
The milk took an extra 4 hours before it successfully ‘yogged’ and thickened into yogurt. After 4 hours of setting it was still liquid, but after 8 hours it was done. This could have been because the milk was past prime, or perhaps rather because the yogurt starter is getting old. Anecdotally I see people claiming they usually get 2 to 4 batches of yogurt out of the same store-bought starter - re-using yogurt from one batch to ferment and start the next - before the starter becomes too weak to ferment properly. This batch was the third generation of yogurt, so perhaps it is getting weaker with each use.
Overall the yogurt tastes fine! It is slightly more sour, because it had 4 more hours of setting. But otherwise no ‘off’ flavours.
So fermenting yogurt may still be a good way to use up old milk. If it doesn’t turn out: the next stop is cheese. Hopefully Sandor Katz' “Refusa” goddess would be proud.