The title of the book appears as the last line of this chapter. What does it mean?

In this chapter, Steinbeck extends his metaphor of ripening and decay among the elite business class. The wealthy owners lavished great expense to ensure that the fruits grown on their farms were ripe and healthy, impervious to disease, yet were the engineers of the eventual rot. By accumulating too much and forcing the prices of the fruit too high when others had too little, they ensured that nobody would be able to buy the fruit. They have engineered their own demise. Yet there are more important victims in this tragedy. Children die from disease, for their parents cannot afford the fruit. They are literal victims of the profit margin. All this provides the "grapes of wrath" and the title of the novel.