Have you ever eaten a hamburger that is brown in color and actually undercooked? What about one that is still pinkish in color and safe to eat? If you are scratching your head thinking this is not possible, purchase and use a meat thermometer and see what happens. Michigan State University Extension indicates that using a meat thermometer is the only safe way to determine doneness of meat. Color is not an indicator of doneness, especially in ground meats. Ground meat needs to be cooked to 160 degrees Fahrenheit, whole muscle meats to at least 145 F, and poultry to 165 F.
Myoglobin breaks down during cooking and causes meat to be brownish in color when cooked to well done (170+ F). Meat at lower degrees of doneness such as rare (140 F) and medium rare (145 F) has not fully denatured and still provides some red or pinkish red color to the cooked meat. For the most part, this holds true for whole muscle fresh meat purchased at retail, however this is not always the case. The presence of oxygen and the state that the iron attached to the myoglobin protein can determine what cooked color actually appears.
Premature browning is what happens when the inside of a ground beef patty is completely brown but not cooked to a high degree of doneness (around 130-135 F). This can pose a significant food safety problem, especially for consumers who rely on color as an indicator of doneness in ground beef. Ground beef that has a higher amount of oxidized myoglobin in the metmyoglobin form is more apt to result in premature browning. Other factors that can contribute to premature browning include exposure to oxygen during packaging, pH, freeze-thaw dynamics of patties, the length and temperature meat was stored at, type of muscles that were used in grinding and the addition of salt to the meat. Since metmyoglobin is brown in color, it is not surprising that hamburger containing metmyoglobin will be brown in patties regardless of the endpoint temperature. The important part to remember is that cooked ground beef must be cooked to an internal temperature of 160 F for one second, regardless of its internal color.
Persistent pink is when portions of fully cooked meat have red or pink appearance to them. Persistent pinking is found on the inside of a ground beef patty and when not all of the myoglobin breaks down during cooking. This occurs more when the iron on the myoglobin protein is in the reduced state and is purplish (deoxymyoglobin) or red (oxymyoglobin) in color, if the meat has high myoglobin content (from cows or bulls), and if muscle pH is higher. Increasing the endpoint temperature of this kind of meat may decrease the pinkness, but it may never turn completely brown. Persistent pink also can occur in the first few millimeters of the surface of a meat product. This is most common in meat grilled on charcoal or gas and shows up as a pink ring. The gas nitrogen dioxide may be present during the cooking as the fuel combusts. This creates a pigment from myoglobin called nitrosylhemochrome. The pink ring that forms near the surface of the cooked meat often fades or disappears after it is exposed to air or light. The bottom line regarding persistent pinking in cooked meat is that it is safe to consume providing the endpoint temperature is confirmed with a thermometer.
The majority of the original research conducted on premature browning and persistent pinking was conducted at Kansas State University.