The meat of most adult mammals falls in the category of red meat, such as beef, lamb, pork, venison, goat and some others.
Not all red meats are the same color. The color variations result from differences in diet, exercise, breed, and especially by the age of animal, which can produce darker red meat as it gets older.
In the case of red meat, you probably associate a pinkish-red color with freshness. This color is a result of the protein called myoglobin, that stores oxygen in muscle cells and is purplish in color. When it is mixed with oxygen, it becomes oxymyoglobin and produces bright red color. The more myoglobin there is in the cells, the redder the meat.
When you cook the red meat, the color changes depending on what the meat’s interior temperature is. The redder the meat, the darker it will get when cooked.
Prolonged exposure to air and light will darken meat that starts out bright red. Color changes can also occur if raw meat is frozen, where it can fade or turn more of a brownish shade due to oxidation.
Even if there’s a color change, the meat remains perfectly fine to eat if stored in the refrigerator or freezer and consumed within a safe period of time. Ground meat may be refrigerated for one to two days and five days for other cuts.
Beyond color change, the best way to tell if the meat gone bad is to let your other senses be your guide. Smell it, touch it, and look at it.