How is the title, The Grapes of Wrath, appropriate for the novel?
The phrase "grapes of wrath" appears in Chapter 25, a non-narrative chapter about how the economy of farming leads to the act of destroying crops.
The chapter focuses on the machinations of pricing that create a situation where the crop is worth less when sold than it will cost to harvest. In order to keep prices from falling even lower, the crops of oranges and other fruit and vegetables are destroyed.
At the same time, there are people starving on the roadsides and in the migrant camps. They cannot buy land to grow crops of their own and they cannot affort to buy food. The destroyed crops become a symbol of their destitution and of the nature of their poverty - manufactured by industry.
The people become angry.
"...in the eyes of the hungry there is a growing wrath. In the souls of the people the grapes of wrath are filling and growing heavy, growing heavy for the vintage."
Instead of finding copious amounts of work and easy access to fruit like oranges and grapes, as they expected, the migrant families find hunger and hardship. People like Granpa and others dreamed of California as a promise land, a land of ease and plenty, but they find a difficult life there and must die, some of them, before the family can make it to California.
The bitterness of wine, the image of fruit that is not eaten, and the notion that this growing wrath is a natural result of circumstances are all present in the title and apply thematically to the experiences of the migrant families in the novel.
There are many biblical allusions and symbols in Steinbeck's novel, and the title is no exception. The Grapes of Wrath is one line of lyrics to the Battle Hymn Republic (google the whole song) It goes, "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....."
I think the purpose of using lyrics to the Battle Hymn Republic conveys the meaning that the dust bowl was God's fate to drive out the "grapes of wrath" to the "promise land" of California