I've never used frozen beef heart, but I do buy whole, fresh beef hearts at the butcher shop and process them myself. I am sure that when you buy the frozen stuff, the entire heart has been tossed into a large grinder or proccessor or whatever. When you buy a heart yourself, it is very easy to see and feel which parts are pure organ, as opposed to the bits that are sinewy, fatty, or tough skin. I chop it into little bits for my oscar, convict cichlids, and piranha. I grind it up in the food processor, along with all sorts of other goodies (usually spinach, shrimp, fish fillets, peas etc.), freeze it in a flat sheet (pressed out in a Ziplock bag), then break off pieces for my smaller fish. Everyone, including my bettas, loves it.
As for mussels, I buy them fresh, still in the shells, then shell them and feed them to my bigger fish.