Do not look for food that spoils, but for the food that endures to eternal life, which the Son of Man will give you. On him God the Father has placed his seal of approval.
At Jacob's well Jesus had offered water as a metaphor for eternal life; now he offers bread. He is using what is around him - Jacob's well and the young man's barley loaves - as illustrations. Preachers would do well to illustrate their sermons with what is contemporary and immediate in order to get their message across. But here as with the water listeners can get hold of the wrong end of the stick.
We learn later that the whole 'bread of life' discourse took place in the synagogues at Capernaum, so perhaps it began here at v 27.
Jesus does not call himself the Messiah of the Christ in order not to get caught up in the movement to force him to become king, but the term 'son of man' becomes increasingly loaded in the text.