I found it! I was just Googling muckrakers again and found a list of the muckrakers and their works. The Octopus by Frank Norris is the book.. I do remember reading it in college or soon after. The summary on Google says it's based on the wheat industry in California. The summary includes the scene I described of the man suffocating in grain. It was a gripping book but that one scene has stayed with me.

Thank you all so very much for all your ideas and suggestions. Your efforts encouraged me to persevere. So glad this is solved!