On a physical level, you could say it's the freshness and health of the mineral/plant/animal in question. Meaning, how nutrient dense it is, how ripe, and how recently it has been taken from the source.

Of course, there are fermented foods that can be considered "spoiled in a good way," where the molecules of the food transforms into other substances, some beneficial and some toxic.

From a metaphysical level, you can say that food doesn't really nourish us at all so much as it is a carrier of love. That is the truth behind the saying about home cooking that "the secret ingredient is love." Food tastes better and nourishes better when it is a vehicle for universal love.