The story of the meeting of Alma and Amulek is intentionally cast to remind the reader of the story of Elijah and Zaraphath. The events are grossly structured to remind the reader that the Lord deals with his children in similar ways, even on different continents. Even as Elijah was commanded to enter a city and find someone who would give him to eat (see 1 Kings 17:8-16), so Alma is commanded to find a person in the same way.
The point of the Old Testament story and Alma’s experience is to show how the Lord performed the miracle of matching two complete strangers in a strange city, when the Lord wanted the two to meet. It is not surprising that the offering of food would be the point that declares the person to the prophet, for the sharing of food is a very intimate experience, and an important ritual in virtually all societies. Even in modern America, the invitation to share a meal implies an acceptance of the other person. There is an assumption of some type of relationship when we invite someone to break bread with us.
So it was with both Elijah and Alma. While the “code” was given, the precise nature of that recognizable act was more important than a password to the right person. The act of offering food not only identified the person, but also identified the sympathy of that person with the prophet, and with the Lord. This was not just a charitable person, but a companion in the gospel, and this important connection is part of what was symbolized by the offer of food.
Of course the story of Elijah continues with the miracle of the replenishing food supply, a theme not present in Alma’s story. This simply indicates that what we have in Alma is not a copy of the story, but a paralleling of the theme from the story in 1 Kings. The parallel is given to highlight the divine call of Alma, a theme that is underlined by the parallel to the stories on the plates.