Persistent pinkness in cooked ground beef patties is of considerable concern for food service establishments. Consumers view ground beef patties that are pink in the middle as being undercooked and unsafe when, in reality, these patties may be fully cooked and safe to eat. Persistent pinkness can be caused by reducing agents, pH, nitrite contamination, and/or carbon monoxide from gas ovens. Raw beef contains myoglobin, which combines with oxygen to form oxymyoglobin, which has a bright red color. Meat color is regulated (mostly) by a heme ring on the surface of the myoglobin protein. As myoglobin is heated, the protein will denature and unfold allowing oxidation of the iron molecule on the heme ring. Oxidation of the heme iron causes the formation of metmyoglobin, which is a cooked brown color. Reducing agents and the inhibition of oxygen reacting with the heme iron may keep the heme ring from oxidizing thus leaving a pink cooked color.
When the pH value in beef muscle is greater than 6.0, myoglobin is protected from denaturation, leaving oxymyoglobin or a pink color in cooked meats. High pH values may result from long-term stress to live animals prior to harvest, or during quick changes in environmental conditions, especially in the fall and spring. Bull and cow meat may have high pH values, thus cow and bull trimmings may contribute to persistent pinkness.
In addition, nitrite or nitric oxide contamination from water, spices, or processing equipment can also cause nitrosylhemochromagen to form a cured pink color. Incomplete gas combustion in gas-fired ovens may also cause a persistent pink color due to the binding of carbon monoxide or nitric oxide to the heme pigment. Ground beef patties should be heated to 160°F and monitored for proper cooking with a temperature probe rather than relying on color to assure food safety.
Note: This material was originally prepared by Pat Mies and reviewed by Jimmy T. Keeton (3/2003) for the FAQ section of meat.tamu.edu.