Why Did Steinbeck choose the title, "The Grapes of Wrath"?

When I began reading this story the title really bugged me. I kept thinking, "What do Tom Joad, a turtle crossing a road, or dust have to do with grapes or wrath?" Granpa finally mentions the fruit in chapter ten when he dreams of the abundance of grapevines that exist in California. I immediately thought, "Granpa's dream has everything to do with the title!" I should have known that Steinbeck is much more clever then that. I finally did a Google search and read the complex reason behind Steinbeck's title decision. It turns out that the title was pulled directly from, "The Battle Hymn of the Republic." The Hymn goes like this... "Mine eyes have seen the glory of the coming of the Lord: He is trampling out the vintage where the grapes of wrath are stored; He hath loosed the fateful lightning of His terrible swift sword: His truth is marching on. (Chorus) Glory, glory, hallelujah! Glory, glory, hallelujah! Glory, glory, hallelujah! His truth is marching on." "The grapes of wrath" stand for the selfish ways of the landowners and banks which lead to the suffering of the angry migrant population. I was not able to understand this interpretation of the title until I had read the majority of the novel because the ending is where the reader can finally understand just how selfish the landowners could be. For example, in chapter twenty-five it is revealed that the landowners produced an abundance of oranges; instead of giving the extra fruit to the starving migrants, they sprayed kerosene on them so they could die and not be stolen.