What is the significance of the title The Grapes of Wrath?

The title of The Grapes of Wrath is taken from an old song called the Battle Hymn of the Republic. There is a line in there that says:

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.

This is a hymn of freedom from slavery, which both main characters suffer (of a sort).