Rabbi Chayim Palachi (Turkey, 19th century), in his work Kaf Ha'chayim (24:6; listen to audio for precise citation), discusses the comment of the Sages that nowadays, after the Temple's destruction, a person's table serves as an altar to earn him atonement. The Kaf Ha'chayim explains this to mean that a person achieves atonement by having the intention while eating that he eats solely for the purpose of maintaining good health to enable him to properly serve his Creator. He adds that one should verbally express this intention and say explicitly before sitting down to eat that he eats not merely for physical enjoyment, but in order to serve the Almighty. This declaration, the Kaf Ha'chayim teaches, has the power to create a new angel and eliminate the Sitra Achara (evil angel).
In addition, one must have in mind that his eating should have the effect of elevating the sparks of sanctity concealed within the food and restore them to their original place. Food contains a certain spiritual component referred to in Kabbalistic thought as "sparks," and by observing the various Mitzvot associated with eating one elevates these sparks. The Kaf Ha'chayim therefore rules that one should not remove leftover shells and bones from the table until after Birkat Ha'mazon, since the process of elevating the sparks of sanctity is not complete until one recites Birkat Ha'mazon.
This concept, of the elevation of the sparks of sanctity in food, extends so far that according to the Ba'al Shem Tov (founder of Chassidism, Poland, 1700-1760), when a gentile eats and receives energy to do a service for a Jew, the food he ate is elevated. Since that food facilitated a service for a Jew, the sparks in that food earn elevation. All the more so, then, does food achieve elevation when a person himself eats to enable him to perform Mitzvot.
Summary: A person can earn atonement through eating if he verbally expresses his intention that he eats solely for the purpose of enabling him to properly serve God. One should also have in mind that his eating should have the effect of rectifying the spiritual energies within the food. To that end, one should not remove leftover bones and shells from the table until after Birkat Ha'mazon.
