The quotation from John Steinbeck’s The Grapes of Wrath appears just after a description of the waste and destruction of food when people are starving. Carloads of oranges have been dumped, but people spray them with kerosene so no one can eat them. Camp guards prevent starving people from collecting potatoes that have accidentally fallen in the river. Pigs are slaughtered but then covered with quicklime and buried so no one can eat them. Yet all the while, these migrants are starving, literally to death. They cannot get jobs; they cannot afford to buy food. Yet no one gives them anything, for the owners are greedy. If they cannot sell their products, they refuse to share their products.

Here is where this quotation comes in. The people are becoming angry. “In the souls of the people the grapes of wrath are filling and growing heavy, growing heavy for the vintage.” The reference to the grapes of wrath comes from the biblical Book of Revelation 14:19-20. God has become angry at the corruption of the earth, and He tells the angels to collect the harvest and throw it into the “great wine press of the wrath of God.” The time of reckoning has arrived; punishment is at hand.

The people in Steinbeck’s novel have also reached the extent of their patience. Their wrath is growing and swelling, and the suggestion is that pretty soon they will harvest that wrath. The results are implied by the next verse in Revelation: “And the wine press was trodden outside the city, and the blood flowed from the wine press, as high as a horse's bridle.” The people are ready to exact their vengeance for the abuse they have suffered, for the loved ones who have starved to death, for the anguish they continue to experience as they strive to meet their basic needs with no help, deprived of food that could easily have been given to them but for the greed of others.

Steinbeck wrote The Grapes of Wrath during the Great Depression to critique what he considered the failures of the capitalist system. A radical at that time, Steinbeck did not believe the New Deal or the Democratic Party went far enough to address systemic injustices in US society.

In writing "the grapes of wrath are filling and growing heavy ... for the vintage," he is using a metaphor that compares the wrath or anger of the many poor people in the United States at that time to grapes growing on the vine. Just as grapes come to fruition by being exposed to water, sun, and nutrients in the soil, the "grapes" of the anger of the common people are being fed by unemployment, hunger, unfairness, and lack of opportunity. The implication is that the "fruit" of this suffering will be an uprising or revolution.

This statement is based on fears at the time that capitalism itself had failed. The aftermath of the stock market collapse of 1929 had shown that the private sector was unable to self-correct on its own and steer the economy back on course. The Russian Revolution was only twenty years old when Steinbeck wrote his novel, and people were well aware that economic collapse had, in part, sparked that revolution. Steinbeck, in his statement, is openly playing on the fears of the middle and upper classes, encouraging them to change the system or end up with a revolution.

19 So the angel thrust his sickle into the earth and gathered the vine of the earth, and threw it into the great winepress of the wrath of God. 

20 And the winepress was trampled outside the city, and blood came out of the winepress, up to the horses’ bridles, for one thousand six hundred furlongs.

Revelations: 14:19-20

A Biblical allusion as well, the title, The Grapes of Wrath recalls the final acts of God for justice. Said Steinbeck himself:

I want to put a tag of shame on the greedy bastards who are responsible for this [the Great Depression].

The oppressed such as the Okies are "ripening" in their understanding of their oppression. The fruit of their anger is ready to be harvested--the "grapes of wrath" are fully ripe. At the end of Chapter Twenty-Five, harvests of potatoes are thrown into the river, and crates of oranges are dumped after being sprayed with kerosene. And,

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.

Greatly angered at the purposeful spoilage of food in order to drive prices up, the oppressed migrant and other migrant workers, then, begin to organize and effect justice. This growing anger at their disenfranchisement is what Steinbeck felt would foment the lower classes to revolt against their capitalist oppressors.

Thought of by his wife, the title was approved by Steinbeck because the novel itself, he declared, is a "kind of march" as it is in the American tradition of revolution. In fact, Steinbeck's short lyrical chapters of exposition are punctuated with the narrative chapters in a short one, longer two beat. The intercalary chapters were called "pace changers" by the author and were designed, he said,

"to hit the reader below the belt [because] with the rhythm...of poetry [which can]--open him up and...introduce things...which he would not or could not receive unless he were opened up."

As Steinbeck himself said, The Grapes of Wrath is, in tone and scope, "symphonic." It is, indeed, like the words of the "Battle Hymn of the Republic" and the powerful passage from Revelations, both impressive and stirring.

John Steinbeck, at the end of chapter 25 of The Grapes of Wrath, the following quote is found: "In the souls of the people The Grapes of Wrath are filling and growing heavy, growing heavy for the vintage." The idea of the grapes of wrath came from Julia Ward Howe's "The Battle Hymn Republic." In the song, the grapes of wrath are being trampled by the Lord. 

In the novel, Steinbeck's reference to the grapes of wrath growing heavy for the vintage refer to the continuing struggle the people are facing. As their struggles grow, their souls become heavy. Metaphorically, Steinbeck is providing readers with an image of a grape so full that it is about to burst. The characters are so burdened by inhumane treatment, hardship, and death that they feel as if they are about to burst from the weight they carry. The reference to the vintage illustrates the time when they can be "pressed" or relieved of their weight (to live lives with fewer burdens).