There's a difference between rotten, bacteria infested food with salmonella and listeria and e. coli and spoiled food. Different bacteria, fungi, enzymes and yeasts are involved in decay. I think you'd probably be disgusted by the smell of spoiled food that has not been refrigerated. For example an apple can lie on the ground and rot. You wouldn't eat it would you? However an apple can become bruised or spoiled during transportation and you can still eat it. However, if you have not put beef or any meat in the fridge and it smells awful, you can say: "That meat has gone off, don't eat eat it. Throw it out."

Similar in meaning but a spoiled apple could perhaps still be eaten if you cut away soft or bruised areas. A rotten apple means disgustingly and completely bad. In milk or meat, spoiled is a stage from where food is at a stage where you have to decide to risk eating it right through to rotten. "The beef IS going to beCOME rotten/spoiled if you don't keep it in the FRIDGE "The beef IS going bad" ? Spoiled could also mean the food has been overcooked or burnt - still able to be eaten but not as nice.

Sort of. "Spoiled" is more general. "Rotten" 腐 is a kind of "spoiled".