Hmmmm....I thought food grade gelatin was made from tissue other than hide....hooves, bone, cartilage, membrane, sinew, etc. I thought hides were not used in the process of making gelatin because they are worth more if sold for other purposes (leather, for example).
When I buy hide glue, I make sure it's made from hides (or skins). Fish glue is made from fish skins, rabbit skin glue is made from, well, rabbit skin, and hide glue is made from hides (cows, pigs, goats, etc.). The skin provides the best basis for glue, right? I've been under the impression that "impurities" such as bone will weaken the glue. Maybe not.
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