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Meat ColorJane Ann Boles and Ronald Pegg

Montana State University and Saskatchewan Food Product Innovation ProgramUniversity of Saskatchewan

Basic Meat Color

The first impression consumers have of any meatproduct is its color and thus color is of utmostimportance. The color of meat may vary from thedeep purplish-red of freshly cut beef to the light grayof faded cured pork. Fortunately, the color of meatcan be controlled if the many factors that influence itare understood.

Fresh and cured meat color both depend onmyoglobin, but are considerably different from eachother in terms of how they are formed and theiroverall stability.

Myoglobin is a water-soluble protein that stores

oxygen for aerobic metabolism in the muscle. Itconsists of a protein portion and a nonproteinporphyrin ring with a central iron atom. The ironatom is an important player in meat color. Thedefining factors of meat color are the oxidation(chemical) state of the iron and which compounds(oxygen, water or nitric oxide) are attached to theiron portion of the molecule.

Fresh cut meat surface. The meat pigment ismyoglobin.

Because muscles differ greatly in activity, theiroxygen demand varies. Consequently differentmyoglobin concentrations are found in the variousmuscles of the animal. Also, as the animal getsolder there is more myoglobin. A greater myoglobin

concentration yields a more intense color. Musclepigment concentration also differs among animalspecies. For example, beef has considerably moremyoglobin than pork or lamb, thus giving it a moreintense color.

Meat Color Reaction

Immediately after cutting, meat color is quite dark -beef would be a deep purplish-red. As oxygen fromthe air comes into contact with the exposed meatsurfaces it is absorbed and binds to the iron. The

surface of the meat blooms as myoglobin isoxygenated. This pigment, called oxymyoglobin,gives beef its bright cherry red color. It is the color

consumers associate with freshness.

Striploin steaks allowed to bloom tooxymyoglobin.

Myoglobin and oxymyoglobin have the capacity tolose an electron (called oxidation) which turns thepigment to a brown color and yields metmyoglobin.Thus, myoglobin can change from a dark purplecolor to a bright red color simply from oxygenation orto a brown color by losing electrons. The pigmentsmyoglobin, oxymyoglobin and metmyoglobin can bechanged from one to the other, depending on theconditions at which the meat is stored. Aftercooking, a brown pigment called denaturedmetmyoglobin is formed, which normally cannot bechanged to form another pigment.

Striploin steak that has been stored andmetmyoglobin has formed

Oxymyoglobin, commonly known as the fresh meatcolor, is the most desirable color for fresh meats.Maintaining this color requires that the meat surfacebe free from any contamination which would cause achemical reaction resulting in the formation of thebrown pigment metmyoglobin. Also, oxygen must beavailable at a sufficient concentration in order tocombine with the myoglobin to form oxymyoglobin.This reaction is reversible and dependent on theavailability of oxygen, active enzymes and reducingcompounds in the muscle.

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Vacuum packaged fresh meat has a dark, purplishred color because the oxygen has been removedfrom the package and reducing enzymes haveconverted the meat pigment back to myoglobin.Once the meat is taken out of the vacuum package itwill recover its bright red color, albeit for a shortertime period than its unvacuum packaged counterpart.

The change from myoglobin to oxymyoglobin andvice versa usually occurs quite readily. Similarly, thereaction that produces the brown meat metmyoglobinoccurs quite easily, but the reverse of this is moredifficult. In raw meat there is a dynamic cycle suchthat in the presence of oxygen the three pigmentsmyoglobin, oxymyoglobin and metmyoglobin areconstantly interconverted, all three forms are inequilibrium with one another.

Myoglobin

Oxygenation Oxidation

OxidationOxymyoglobin Metmyoglobin

Interconversion of meat pigments. Myoglobinwhen oxygenated is bright red in color and called

oxymyoglobin. Both myoglobin andoxymyoglobin can lose an electron (oxidize) to

form metmyoglobin

Metmyoglobin is associated with chilled meat thathas been stored too long ( ie reducing enzyme activityavailable to reduce metmyoglobin to myoglobin hasbeen exhausted), but also appears when oxygenpartial pressure is low, such as when meat piecesare stacked one on top of another. Oxygen partialpressure can also be reduced when aerobic bacteriause up the oxygen and it is unavailable to react withthe myoglobin.

Effect of Meat pH

The rate and extent that muscle pH declinespostmortem are both variable and have a greatimpact on the color of meat and meat products. Thenormal pH decline in muscles is from approximately7.0-7.2 down to near pH 5.5-5.7 over about 24 hrs.With this pH decline, whole tissue is thecharacteristic color of the species. If the pH declinesto the normal pH of 5.5-5.7 within 45 min or less, themuscle will appear very pale and soft (PSE). A verylow ultimate pH (

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Particle Size Reduction and Mixing

Air is incorporated into meat products during thegrinding process or mixing in of ingredients. Themore air that is incorporated, the more stress is putupon the natural reducing systems of meat whichhelp maintain oxymyoglobin stability and keepmetmyoglobin formation in check. Longer mixingtimes and smaller meat particles results in shortercolor shelf-life for meat products. Use of vacuummixers helps to improve color stability, but will notcompletely bring the stability back to what would beexpected in whole muscle products.

Cooked Meat Pigment

During the cooking process, myoglobin is denatured.All of the pigment is not affected at the same time orto the same extent and this is why you get reddishcolor at different end point temperatures whencooking. The cooked pigment is denaturedmetmyoglobin. It is brown and is easily recognized incooked meat products. Certain meat conditions canresult in protection of the myoglobin.

The ultimate pH of the muscle is one of theseconditions. The ultimate pH of meat or meatproducts will affect how the meat color changesduring cooking. If the meat has a high pH, it willhave to be cooked to higher end-point temperaturesto get the same visual degree of doneness as onewith normal pH. Frequently, complaints of this hardto cook defect are associated with a high pH of the

meat or meat product. This meat appears raw incolor, dark red to purple, long after appropriatecooking temperatures have been reached.

Cooked beef roast. The myoglobin is denaturedand oxidized to form denatured metmyoglobin.

Cured Meat Color

Curing of meat has been done for centuries for thepreservation of meat products. Today curedproducts are made for their unique flavour andtexture. Cured products have a pink pigment that isrelatively stable. To form this pigment, sodium nitriteis either rubbed onto the surface or injected into themeat with a needle injector. The nitrite, when addedto water forms nitrous acid and nitric oxide, and

which penetrate the meat and combine with themyoglobin to form nitric oxide myoglobin. Thispigment is not stable until after cooking when thefinal cured pigment, nitrosylhemochrome, is formed.The cooked pigment is more stable, but is stillsensitive to oxygen presence, temperature and light.This is why most cured products are vacuumpackaged in special UV protective films.

Many problems can occur in the curing of meatproducts that can result in the development ofstrange colors. One of the most common is theoxidation of the pigment to form a green or greycolor. This is usually caused by metal contaminationfrom moulds or smoke sticks. This sometimeshappens if the moulds are old or they have bewelded improperly with solder that contains copper oriron.

Cured cooked meat pigment.

Pinking of Uncured Cooked Products

Sometimes a pink color can form in uncured cookedmeat products. This can be caused by many factorsand should not be confused with the hard to cookphenomena caused by high meat pH.

Cooked beef or other meats can becomecontaminated with the curing salt nitrite from contactwith a cured product, incomplete cleaning of utensilsused with cured products or water contaminated withnitrites. It takes only small amounts of nitrite todevelop cured meat color. Although 50 parts permillion are necessary to maintain the pink color incooked ground beef, pink color can show up withlevels as low as 5 parts per million - that is 5milligrams per kilogram of beef.The cured meat color can be on the surface if thecontamination occurs during the cooking process butwill be throughout the meat product if it occurredearlier.

A pink color can also be formed in slow cooked meatproducts that have not been contaminated withnitrite. It is caused by specific conditions thatpromote interaction of natural meat pigments andnitrogen containing constituents of meat. This color

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is actually desired in products such as Texasbarbecue.

Surface pinking, also termed pink ring can occur ifgas ovens or barbecue grills are used to cook meatproducts. Incomplete burning of the gas orcontaminates in the gas result in the formation ofnitrogen dioxide and nitric oxide. Nitric oxide is theactive form of nitrite that yields the pink color. Inroast beef this is generally considered a serious colordefect because consumers might associate it withundercooking.

Pink ring around cooked pork. From Cornforth,D.P., J. K. Rabovitser, S. Ahuja, J. C. Wagner, R.Hanson, B. Cummings, and Y. Chudnovsky. 1998.J Agric. Food Chem. 46:255-261.

Naturally occurring nitrates found in water andvegetables can be converted to nitrites duringthermal processing and can cause a pink color toform. Carrots and cabbage are examples ofvegetables that have naturally occurring nitrates thatcan contribute to pinking of cooked meats. This

occurrence is rare because nitrates take longerperiods of time to be converted to nitrites which inturn yield nitric oxide that forms the color pigmentnecessary for the pink color. This color is usuallyseen in slow cooked soups or stews.

Another cause of pink color is the presence ofcarbon monoxide (CO). Carbon monoxide combineswith natural pigments in meat to produce a dark redcolor in raw and cooked meat. Minute amounts ofthe gas may come from dry ice, or carbon dioxidefreezer tunnels used in hamburger production and itwill affect color. Carbon monoxide, which has agreater affinity for myoglobin than oxygen, bindsalmost irreversibly to the raw color pigment.However, this occurrence is relatively rare.Researchers have shown that if CO is used inmodified atmosphere packages a dark red colordevelops and it remains after cooking.

Iridescence in Processed Meat Products

Iridescence is a common problem in sliced roastbeef and ham products. The dominant color isfrequently green and consumers sometimes confusethis with green myoglobin pigments associated with

microbial growth. The iridescence of meat productsis produced by a combination of the angle ofincidence of the light on the muscle fibres and thewetness of the surface. If the fibres are pulledslightly out of alignment during slicing, the lightstrikes the fibre at an angle scattering light whichappears as the rainbow or greenish color on thesurface of the meat. Addition of phosphate seems toexacerbate the problem by increasing the amount ofwater that is retained by the product.

Color is the single most important factorof meat products that influencesconsumer buying decision and affectstheir perception of the freshness of theproduct. Knowing the factors that affectcolor is important to understandingproblems when they occur. Everyonedealing with meat products should have

a working knowledge of the color. Wehope this article has helped!

About the Authors

Jane Ann Boles is a Meat Scientist at Montana State University.Ronald B. Pegg is a Meat Chemist at the University ofSaskatchewan and is the program coordinator for theSaskatchewan Food Product Innovation Program.

For more information contact :Jane Ann BolesDepartment of Animal and Range SciencesMontana State University119 Linfield Hall

Bozeman, MTPhone (406) 994-7352The Saskatchewan Food Product Innovation Program is funded

by the Saskatchewan Agriculture Development Fund