Reading the Everything I Want to do Is Illegal book, the quick chilling instituted by the USDA may actually contribute largely to the fact of why leaner, grass fat meat is tougher. A more fat, grain-fed carcass would chill slowly, allowing the tenderizing chemical released into warm, freshly killed meat to be released for a while before the carcass is chilled all the way. However, for a grass-fat carcass, it is chilled much quicker, thereby canceling the release of the natural tenderizer. Fat doesn't necessarily mean tender. How the meat is processed and cooked also play a very big factor.