Throughout human history, the act of sharing food together has suggested a level of bondedness between the people sharing the meal. Some of the significance has been lost in this day and age of American drive-throughs and eating on the run, but certainly in the Jewish culture of the Middle East at the time of Jesus, a shared meal connoted a level of intimacy between eaters. (For this reason Jesus was consistently criticized for sharing food and drink with tax collectors and sinners.) The Passover ritual that Jesus celebrated as his Last Supper included the practice of sharing food from common bowls, not unlike in various cultures and ethnic restaurants still today. In this sense, Judas can be accused of betraying not just the bond of people who eat together but a bond which should have been stronger between those who celebrated a religious feast together. The text in Matthew which identifies Judas as the one who dipped his hand into the bowl with Jesus (Mt 26:23) might also be an allusion to Psalm 41:9: “Even my bosom friend in whom I trusted, who ate of my bread, has lifted his heel against me.”
John’s gospel is slightly different in that it indicates that Jesus dipped his bread in the bowl and then gave it to Judas. There is a tradition that the host gave the dipped bread to an honored guest as a sign of affection. So perhaps John highlights this detail to even further heighten the act of Judas’ betrayal.