Great story about Grape Nuts.
Mixed with yeast (one cup per 2,000 pounds) and water, the flour turns to dough, gets chopped into 10-pound loaves and sent into a huge oven — 1,610 loaves at a time. “Now it gets interesting,” Mr. Vargas said at his workstation, watching the loaves emerge from the oven and catapult into the darkness. An instant later, they hit the fan — a whirling high-speed shredder that rips them to smithereens.
So that’s what they are: effectively, compacted croutons.