These are protein sources that don't even have to originate from a food source. Sources like bird feathers, beaks/bills, hair/fur/scales and other different animal parts that are put into an acid/base bath to shred the proteins into unrecognizable pieces so the body does not react to the protein source.
In the stomach, the body will normally perform natural hydrolysis of the food it intakes with stomach acid and then further with the pancreatic enzymes and gall from the liver for fats. Shredded protein molecules that are broken before reaching the body's own system to break down food and subjecting the highly processed ingredients again disrupts the natural process of digestion turning food into molecular forms it can recognize.
Hydrolyzed proteins are often made for pets with severe food allergies. Anyone who knows about Mad Cow disease, it’s caused by Prions, which are damaged protein molecules folded incorrectly in the brain and spinal cord. No one knows how Mad Cow Disease started or why, but we do know that Prions cannot be destroyed, even in temperatures that can melt lead and are extremely contagious, and contaminate surfaces very easily.
Somehow hydrolyzation doesn’t sound like a good idea...