Annabrahma Rasovishnu

A prayer or invocation offered before eating. The act of consuming food is considered to be a Yajna (sacrifice) and the offering goes to the divine authority called Jataragni, the digestive fire in the belly. It is that principle which breaks down the food consumed into a form that can be supplied to through the blood for the whole body. The creative energy in the food (Annam) is Brahma. The nourishing energy (Rasa) in the body is Vishnu. The transformation of food into pure consciousness is Shiva. If you know this, then any impurities in the food you eat will never become part of you.

This shlokam is chanted before eating food so that even while eating, our attention is focused on the food, and we eat it with an awareness of its importance.

While eating our food, we must utter the words, “Annam Brahma” (food is God); Raso Vishnuhu (the essence of food is Vishnu). Food is God. It gets into the body and provides its essence to all the parts of the body. It indeed gets transformed into blood and energy. Bhoktodevo Maheswarah (the one who consumes the food is Maheswarah). This principle teaches wisdom to the whole world. Annam Brahma, Raso Vishnuhu, Bhoktodevo Maheswarah –These three correspond to body, mind and action, respectively.
Manasyeka Vachasyekam Karmanyekam Mahatmanam
(those whose thoughts, words and deeds are in complete harmony are noble ones.)
The oneness of thought, word and deed is ritham.
They represent the Trinity of Brahma, Vishnu and Maheswara. Hence, everyone must strive for the purity of these three.

Purify the food by offering it to God.
Annam Brahma. Consider food as verily the form of Brahma.
Raso Vishnuhu. The essence of food that spreads to all parts of your body is Vishnu swarupa.
Bhokta devo Maheswarah. The partaker of food is the very embodiment of Siva principle. When man develops such sacred feelings, he becomes Siva Himself– Sri Sathya Saibaba

Also see Annapurna Stotram.