The Purification of Food in Puja

Yagna shishta asinaha, santa ha is a metaphor for giving something back. In ancient times food was not necessarily thrown in the fire, but symbolically offered to ishura. Today we do this whenever we perform puja: We bring a plate of food to the altar and offer that food to God. That food is taken as prasada, as a sanctified gift from God; it's ritually purified.