Leftovers

Is there an official Church policy on praying for leftovers? At what point do we need to repray over a piece of food? Say an important piece of business takes me away from the dinner table right after the family has said grace. When I come back 30 minutes later, nobody would expect me to pray over my food again. Yet, most of us would agree that it is completely appropriate to pray again over that same food 24 hours later. Do food prayers reset after midnight? Once something goes into the fridge? The microwave?

Surely Bruce R. McConkie had some thoughts on this. Any chance he wrote about it in Mormon Doctrine?