I've been making beef jerky using various marinade recipes I found on the WWW. The results have been mixed, but always lack that one distinct flavor that seems so prevalent in store-bought jerky. If I had to describe it, I'd say "protein-y". It's not a spice I recognize, and doesn't seem to be present in any combination of soy sauce, Worcestershire, etc.
Recently I found an old seasoning packet that came with my dehydrator, took a little taste, and recognized that same flavor immediately. Unfortunately there is no ingredient list printed on the packet. Where is that flavor is coming from?