I have read that rigor mortis is associated with an important pH chemical change in the animal which improves the tenderness of the meat, hence you want rigor mortis to occur before freezing. I have also heard that there are bad effects which occur if you freeze the meat too soon. I would think that freezing the animal 48 hours after the animal died would be sufficient time for such things to occur. I have a book on butchering that addresses these topics and a John Barneses (Spelling?) article on the trouble with freezing too quickly. I'll try to consult these authorities tonight and add more info tomorrow.