Whenever possible, buy portions that are cut while you watch. Precut portions may be days or weeks old, and their large exposed surfaces inevitably develop rancid flavors from contact with air and plastic wrap. Exposure to light in the dairy case also damages lipids and causes off-flavors in as little as two days; in addition it bleaches the annatto in orange-dyed cheeses, turning it pink. Pregrated cheese has a tremendous surface area, and while it is often carefully wrapped, it loses much of its aroma and its carbon dioxide, which also contributes the impression of staleness.