The Last Supper celebrated by Jesus and his disciples is linked to the Jewish Passover celebration, which contains a variety of symbolic acts.
During this meal, Jesus proclaims that one of his disciples will betray him.
When Jesus had thus spoken, he was troubled in spirit, and testified, “Truly, truly, I say to you, one of you will betray me.”John 13:21, 26-27
Jesus answered, “It is he to whom I shall give this morsel when I have dipped it.” So when he had dipped the morsel, he gave it to Judas, the son of Simon Iscariot. Then after the morsel, Satan entered into him. Jesus said to him, “What you are going to do, do quickly.”
Why did Jesus dip a “morsel” and give it to Judas?
There are many different interpretations of this act, but one possibility is that Jesus was performing a part of the Jewish Passover that recalls the betrayal of Joseph in the Old Testament.
This is what D.T. Lancaster suggests in his article, “Last Seder: A Jewish Reading of the Last Supper.”
After the first cup, participants in the Passover Seder wash hands and then take part in a ritual called karpas. The ritual involves dipping a green vegetable twice into red wine vinegar. (In modern seders, salt water often substitutes for the wine vinegar.) The meaning of the ritual is obscure, but according to some opinions, it represents the betrayal of Joseph whose brothers dipped his coat in goat’s blood—the event that initiated the descent into Egypt. It may also represent dipping the hyssop into the lamb’s blood.
Shlomo Riskin also explains this ritual for an article on Israel Times.
Interestingly enough, there is a custom in many Yemenite communities to dip the karpas vegetable into the charoset, a mixture of wine, nuts and sometimes dates, which the Jerusalem Talmud says is reminiscent of blood. Hence, just as the brothers dipped Joseph’s cloak of many colors into the blood of the goat claiming to their father that Joseph had been torn apart by a wild beast; we dip our karpas into the charoset.
Everything in the Gospel is there for a reason, even a small minute detail of Jesus dipping a piece of food and giving it to Judas.