Before beef reaches consumers, it is always aged for a certain period to improve its flavor and texture. The initial aging occurs after an animal has been slaughtered, which causes its muscles to stiffen (the condition known as rigor mortis). Meat cooked in this state would be very tough; aging allows the muscles to become tender again. The biochemistry behind aging isn't fully understood, but apparently enzymes normally present in the muscles actually digest the protein in the meat, altering both flavor and texture in the process.