Photo 2. Example of premature browning in ground beef patties both cooked to 55 degrees Celsius.
Picture courtesy of the Kansas State University Meat Science Program.

Premature browning is the only non-typical color change that represents a food safety issue for consumers because of the potential for consuming beef not cooked to the proper internal temperature. As with the internal color of the cooked patty, the color of the juice that comes out of the patty during cooking also cannot be used as a reliable indicator that the patty was sufficiently cooked. As such, proper use of a meat thermometer is the best method to assure that beef reaches a safe internal temperature (160°F for ground beef, 145°F for steaks and roasts).

The occurrence of premature browning is greatly affected by the chemical state of myoglobin and muscle pH (Warren et al., 1996; Hunt et al., 1999). If the chemical state of myoglobin is predominantly MMb in a ground beef patty, that patty has a greater likelihood of premature browning upon cooking than a patty with mostly OMb and DMb. DMb is the most heat-stable chemical state of myoglobin. More often than not, all three chemical states of myoglobin exist within a ground beef patty or on or near the surface of a steak. When MMb reaches approximately 30 to 40% of the total chemical states of myoglobin, a brown color becomes visible. 

Ground beef is manufactured from larger cuts of meat or trimmings from primals and sub-primals. When these materials are ground, oxygen is incorporated and results in a bright, cherry red color (OMb). As meat is stored, the relative proportions of the three chemical states of myoglobin continue to change. When fresh beef is ground, the degree to which the beef blooms (turns bright red, formation of OMb) is influenced by the respiration of the muscle, its reducing activity (ability to keep the iron in the reduced state) and the pH of the meat. As fresh meat is exposed to oxygen, the biochemical processes of the muscle are still active and consume oxygen thereby slowing oxygenation of the pigment and/or slowing the penetration of oxygen into intact cuts such as steaks and roasts. The iron in the heme ring of myoglobin must be in the reduced state in order for oxygen to bind. Meat with limited aging and storage time has very good reducing activity, but reducing activity declines with increased display time. When the meat’s reducing activity declines, MMb begins to accumulate.

Fresh ground beef placed in an oxygen impermeable package, for instance packaged in a vacuum-sealed chub, will have consumed all of the oxygen present in one to two days and the primary pigment will be DMb. Ground beef that is on display in a retail store that has a bright red color is likely overwrapped with an oxygen permeable film or possibly in a modified atmosphere package that contains elevated levels of oxygen. A third possibility for the bright red color would be beef packaged with an atmosphere that contains a low level of carbon monoxide (e.g. 0.4% carbon monoxide). Myoglobin has a high binding affinity for carbon monoxide. When carbon monoxide binds, the bright red pigment, carbon monoxide myoglobin (COMb), is formed (Sørheim et al., 1999). 

Regardless of the packaging system, the color will gradually deteriorate and MMb will begin to accumulate. Beef that has been stored for a considerable time or has been temperature abused will promote MMb formation. The resulting brown color indicative of MMb accumulation is associated with loss of the meat’s reducing activity or microbial growth, which increases the likelihood of premature browning. 

Even though a steak or ground beef patty is visibly brown on the outside, this doesn’t mean MMb is throughout the entire product. Steaks that are allowed to bloom before being vacuum packaged, particularly steaks cut from aged roasts, will have a spike in MMb once the meat consumes most all of the oxygen. At this point, the inside of the steak would be expected to be DMb. With additional time in the vacuum package, the surface would convert to principally DMb. The brown color associated with MMb can also occur when two oxygenated pieces of beef come in contact with one another. The interface of these two pieces creates an anaerobic environment (no new available oxygen) and a brown color will appear as the meat level of oxygen is reduced to very low levels.

Beef that has undergone extended periods of frozen storage will have less reducing activity and greater unsaturated fatty acid oxidation. Metmyoglobin reducing activity is also less at lower pH levels (Stewart et al., 1965). Unsaturated fatty acids that become oxidized can lead to free radicals (reactive substances) that oxidize the meat pigments and promote premature browning. 

If salt has been incorporated into patties or is present in the case of enhanced steaks, salt decreases metmyoglobin reducing activity allowing more MMb to accumulate (Stewart et al., 1965). In addition, salt promotes heat denaturation, or breakdown, of myoglobin.

Persistent Pink 

Beef that has some degree of red remaining after cooking can be associated with the presence of undenatured pigments or the formation of certain denatured globin hemochromes. Relative to undenatured pigments, this color situation is influenced by many of the same factors as in premature browning but on the opposite end of a given factor. The undenatured, red pigments could include OMb and DMb, and COMb. Persistent pink from undenatured pigments is promoted by a high pH level, pigments in the reduced state that are more heat stable, and in muscles containing high levels of pigmentation. 

A high pH protects myoglobin from heat denaturation (Trout, 1989). High pH values may result from long-term stress to the live animals prior to harvest. Beef from older animals (bull and cow meat) may have high pH values (6.0 or higher), thus such trimmings may contribute to the persistent pink color. When the chemical state of myoglobin is predominantly DMb, this protein will be less susceptible to heat denaturation compared to MMb.

The visibility of an uncooked color or persistent pink is influenced by the amount of pigmentation in the meat. Even at the higher end of endpoint cooking temperatures, some of the myoglobin is not heat denatured. At a high pigment concentration, the meat can appear red because a sufficient level of undenatured pigment remains. In contrast, meat with a low level of myoglobin may appear fully cooked even though the same percentage of the pigment remains undenatured.

The persistent pink associated with undenatured pigments is primarily an issue with the internal color of the product because the outer regions of the product would be more thoroughly cooked than the internal area. In contrast, persistent pink formed by reduced, denatured globin hemochromes may be an issue on the surface as well as internally in the cooked product. Pink color present on the outer surface and extending only a few millimeters into the product is described as a pink ring. Products cooked on gas-fired ovens or charcoal can produce products with a pink ring. Nitrogen dioxide may be present as fuel-combustion byproducts in gas-fired ovens leading to the formation of a pink ring (Cornforth et al., 1998). The pigment associated with this pink ring is nitrosylhemochrome. This pigment is the same one that is formed in cured meats (corned beef, beef franks, ham) as a result of the direct addition of sodium nitrite. If the pink color is within the beef product, nitrosylhemochrome may also be present as a result of unintended contamination of the product with nitrite in water that was added, seasonings or from contact with cured products. 

An internal pink color can also be formed by the reaction of various nitrogen containing substances with the denatured meat pigment. Nicotimamide-denatured globin hemochromes found in tuna and poultry (Brown and Tappel, 1957; Claus et al., 1994) can occur in beef as well. A number of amino acids present in meat can yield this pigment. For these types of reduced, denatured globin hemochromes to form, the native structure of the protein portion must be denatured to open up the binding site on the heme ring for these larger molecular weight compounds. In addition, the iron in the heme ring must be in the reduced state (ferrous iron) in order to bind these compounds. Unlike the persistent pink produced from undenatured pigments, the color of the reduced, denatured globin hemochromes tends to noticeably fade soon after exposure to air and light.