Curiously, too much oxygen has the same effect on meat as too little oxygen - both conditions can give it a brownish tinge. When myoglobin overoxidizes, it is transformed into a brown pigment called metmyoglobin. Therefore, a raw, red meat will turn brown if it is exposed long enough to the air. This color change can occur even if meat is wrapped in plastic or paper, because both allow air to pass through. However, refrigeration does slow the browning process.
