Does food expire/rot in Stardew Valley?
There is currently (as of March 7, 2016) no rotting or expiration mechanic.
I have a chest full of foraged goods and crops (salmonberries, a few fruits, etc.). I'm currently at Fall and the salmonberries that I harvested in late Spring are still fine.
Jack O Lanterns DO in fact rot on day one of winter. So as of this reply, there is at least one item that can rot.
Although it is not a food item, it will rot and display as a rotten item in your inventory.