You're not likely to stop it. When you open and cut it, it will "bloom" to red. this is myoglobin (sp) as the oxygen depletes from the myoglobin, it will brown. You see this in store bought meats all the time, especially beef. If you open a pkg of steaks that have been shingled in the pkg. You will find that the area where they are over-lapped is brown. If the ground beef is made into a tight loaf, it be brown in the center, less so if it is loose formed. If the pkg's of beef are stacked too high, the weight will cause the bottom pkg's to brown. All of this is due to oxygen depletion of the myoglobin.
Pork will also do this, but the color change is less dramatic.
Nothing wrong with it, just a visual thing.