Any non-Betweenlands food item that passes through the Betweenlands Portal or is picked up from the ground or another inventory in the Betweenlands by a player becomes Rotten Food. It is a consumable item, but it is not recommended for consumption since it restores nothing and instead inflicts Poison II () and Hunger II () for 10 seconds. If it is returned to the Overworld it will revert back to the original food item; which item this was can also be viewed in the Rotten Food's tooltip. It should be noted that Rotten Flesh does not turn into Rotten Food (for obvious reasons). This also happens to outside potions in the form of Tainted Potions.
Note that any Betweenlands food can be affected by Food Sickness.