Once air entered into an opened can the metal can become tainted and the food takes on a slightly metallic taste which is very unpleasant. When we had a cat I didn't even like to store opened pet food in a can in the fridge for longer than a few hours because animals have very sensitive taste buds and ours would only eat tinned food when it was fresh from a newly opened can. I used to have to transfer the remainder into a plastic lidded container otherwise I was continually throwing half the contents away because the cat wouldn't touch it.