Few topics are at the heart of what we teach or what people ask most meat scientists about than meat color. Meat color is an important part of meat science, and for many people in the industry, it is one of the areas where problems occur most frequently. Meat color is a complex topic, and one can find in-depth textbook chapters on it. So, writing a brief article on it is quite daunting.
We will address the basics of meat color along with some of the more frequent questions and concerns we have heard over the years. Understanding the factors that affect meat color may be helpful to those who are responsible for product quality, shelf life and/or customer service where even slight changes in color may trigger a negative response from customers or consumers.
Basic meat color
The core component of meat color is the pigment, myoglobin. Most people are familiar with hemoglobin, which carries oxygen from the lungs to our cells. Myoglobin is the heme-iron-containing protein in muscle. Myoglobin serves to store the oxygen that is brought to muscle through the blood supply for when it is needed. Some muscles will have more myoglobin than other muscles.
Locomotive muscles do more work than support muscles, therefore, locomotive muscles will have more myoglobin than do support muscles. In poultry, comparing dark meat, thighs, to white meat, breast, is an obvious example of this difference. Myoglobin is a pigment, so the more myoglobin present in a muscle, the darker the color of the meat will be. Also, as animals increase in age, myoglobin loses its affinity to bind oxygen making meat from older animals darker than meat from younger animals. This is quite evident when comparing the color of meat from mature cows to meat from veal calves.
Myoglobin contains a porphyrin ring that has iron at its center. Iron is the element where oxygen binds when it is transferred from the hemoglobin in the living muscle, and in the case of meat, oxygen binds to myoglobin through the process of oxygenation or “blooming,” resulting in the familiar bright cherry red beef color or the grayish pink pork color. Myoglobin in this form is called oxymyoglobin, and when there is no oxygen available, the compound is called deoxymyoglobin.
Meat in the deoxymyoglobin state will be light to dark purple in color depending on the species, which is what meat looks like when it is first cut or when it has been vacuum packaged for some time. Exposing meat to oxygen, through the blooming process, causes it to become the bright pink or red color consumers are accustomed to seeing in traditional polyvinyl chloride (PVC)-wrapped packages in retail stores. Going from the deoxymyoglobin to oxymyoglobin state is a short process occurring in a matter of minutes.
It takes time to reverse the blooming process. The transition from oxymyoglobin to deoxymyoglobin, which happens in vacuum packaged products, is a much longer process. The oxygen has to be consumed by the muscle or otherwise removed for this transition to occur. This is where one of the main issues with meat color occurs. When there is no oxygen present, the myoglobin pigment is in the deoxymyoglobin state, and when there is atmospheric oxygen present, the pigment is in the oxymyoglobin state. When there are low partial pressures of oxygen present – too much for the formation of deoxymyoglobin and too little for oxymyoglobin to form – the iron in myoglobin will oxidize and the brown pigment, metmyoglobin, will appear. This transition is easy to observe when beef steaks are first vacuum packaged where they may be initially red, followed by the development of the brown color, and ultimately, they become purple in color. The type of muscle, its postmortem age, and even quality grade may impact the transition from purple to red with a stop through brown. If steaks are frozen when they are in this brown stage, the steaks are likely to remain there making them less appealing to most customers.
Why is the interior of the meat brown in color when the exterior is red? This is a very common concern by purchasers of fluff-packaged PVC-overwrapped ground beef. Many of these calls/emails over the years have focused on the thought that unscrupulous retailers have taken fresh ground beef and layered it over the old ground beef, so that consumers would be tricked into purchasing old product. This is definitely not the case. The answer to this phenomenon is that while the exterior of the ground beef is exposed to atmospheric oxygen resulting in the full development of oxymyoglobin, the interior portion with the low partial pressures of oxygen introduced through the grinding process encourage the formation of metmyoglobin. Therefore, the outside is red and the inside is brown.
Premature browning/persistent pinkness
Two other color problems occur in ground beef. These problems are caused by different conditions and result in different quality/safety concerns. The first condition is referred to as premature browning. When high-oxygen (approximately 80 percent) modified atmosphere packaging began being used in case-ready programs, it was discovered that ground beef, when cooked, would begin to turn brown at temperatures far lower than those degrees of doneness most commonly related to safe consumption. It was discovered that this high concentration of oxygen caused some oxidation of the myoglobin to occur, and when the cooking process started, ground beef would turn brown well ahead of the normal process most consumers associate with fully cooked beef. This is one reason why USDA’s cooking guidelines urge consumers to use thermometers rather than cooked meat color to assure that proper lethality temperatures are met.
The second condition is called persistent pinkness. This problem is found in ground beef patties that are adequately cooked to appropriate internal temperatures or combinations of time and temperatures to ensure safety, but the patties still have pinkish-to-red-colored centers appearing that they have not been cooked thoroughly or correctly. Persistent pinkness is most often associated with patties that have been manufactured from beef that has higher-than-normal pH, and with the seasonality associated with the occurrence of dark cutting beef, persistent pinkness usually follows along. This condition is troublesome because the patties have been cooked to appropriate lethality temperatures, but they appear to be undercooked, which causes consumers to complain and often return hamburgers to the counter at quick-service restaurants.
Another concern that we hear about relates to color changes after meat is cooked. We expect that adequately cooked meats will have the characteristic browned color. What we do not expect is that once cooled, red color that resembles a rarer degree of doneness sometimes occurs. The compounds responsible for development of the brown color are somewhat reversible, especially under refrigeration, and create a red/pinkish color. So, when one slices chilled roast beef, there can be a red/pinkish color, even if it was thoroughly cooked before chilling.