I found this definition for "animal digest" from the USDA website:
animal digest. A powder or liquid mixture of breakdown products from animal tissue resulting from the controlled action of one or more enzymes. Digest contains only trace amounts of feathers, hair, horn, or teeth. Digest must name its animal source (such as beef, chicken, or pork digests). Regulate as rendered and processed protein under Animal Waste and Related By-Products on page 3-2-1
Here is the referenced document "Animal Waste and Related By-Products on page 3-2-1."
With all the complexity involved in "animal digest" as a rendered and processed protein and the fact that at least one major dog food manufacture does not name its animal source of "animal digest" (on the label) as presented in the above definition, I will be sure to feed "animal digest" free foods to my animals from this point forward.