It smells wrong.
Now, I believe that the tastes and smells of foods are messages from the gods. If something smells bad, that is your message from the universe that you should not put it in your mouth.
Example: What do you do to see if milk has spoiled? You smell it. And you know instantly if it has gone bad. Clear message from the gods: "Dude, do not ingest this. It will fuck you up." It is very clear.
Spoiled milk smells horribly wrong.
I don't know anyone who would go ahead and drink it anyway.
Me, I just pay attention to the subtler things. The smell of yogurt just makes me gag a little bit, in comparison to fully rancid clumpy spoiled milk. The taste of salmon. That's wrong and bad and the reason the gods make things smell and taste bad is to warn us.
I never knew why Doritos, of all things, tastes wrong. Until I found out they had MSG in them.
I'm weird about food in all kinds of ways. Yes.
Also, since a few people have assumed that the yogurt mention was because I need acidophilus - I don't! Or at least no doctor has ever mentioned it. :) No, docorion just mentioned yogurt as a delivery system for protein powder, and I had the immediate "What are you tryin' ta do, kill me?" reaction. :)