Why is Curtis so revolted by the contents of the protein bars?

When Curtis and company arrive in the protein bar factory car, they learn the horrible truth of the ingredients:

Curtis gasps in horror and tells the artist specifically not to draw what he sees.

Sure, this is kinda gross, but the contents of the protein bars are eaten around the world today and we later learn that

Why would Curtis have such a visceral reaction to the contents of the protein bars, and why would he be so afraid of people learning the truth?