Tuna meat ranges from deep ruby red to pale pink depending on the species, the cut, and how the fish was handled after harvest. The rich red that distinguishes a slab of bluefin sashimi from most other fish comes from myoglobin, the same oxygen-carrying protein that makes beef red. But that color is far from static: it shifts with exposure to air, temperature, freezing, cooking, and even deliberate tampering, making tuna one of the most color-dynamic foods you can buy.
Why Tuna Meat Is Red in the First Place
Most fish flesh is white or pale because fish generally have low concentrations of myoglobin, a protein that stores oxygen inside muscle cells. Tuna are a dramatic exception. As fast, warm-blooded predators that swim continuously and sometimes cross entire oceans, tuna demand huge amounts of oxygen in their muscles. That demand drives up myoglobin levels far beyond what you find in a typical fish fillet. The oxygen-bound form of myoglobin, called oxymyoglobin, is what gives fresh tuna its characteristic bright red appearance.
A review of pigments in fish muscle confirmed that oxymyoglobin is the molecule responsible for the vivid red color in tuna and other dark-fleshed fish, in much the same way that astaxanthin determines the orange-pink color of salmon.1PubMed Central. Undesirable discoloration in edible fish muscle: Impact of indigenous pigments, chemical reactions, processing, and its prevention The key distinction matters: salmon gets its color from a carotenoid pigment picked up through diet, while tuna’s redness is intrinsic to the muscle itself. A wild salmon eating a different diet could lose its pink hue; a tuna will always produce myoglobin-rich red muscle as long as it is swimming.
Dark Muscle Versus Light Muscle
If you have ever broken down a whole tuna or examined a cross-section at a fish market, you have noticed that not all of the flesh is the same shade. A band of very dark, almost maroon meat runs along the midline of the fish, right beneath the skin. This is the dark muscle, sometimes called “bloodline” in culinary settings. Surrounding it is the lighter, brighter red ordinary muscle that makes up the majority of what people eat.
The color difference between these two zones is steep. In a comparative study of albacore, yellowfin, and skipjack tuna, myoglobin concentrations in the dark muscle were roughly ten to forty times higher than in the ordinary muscle.2J-STAGE (Nippon Suisan Gakkaishi). Comparative Studies on Ordinary and Dark Muscles of Tuna Fish – Section: Abstract Dark muscle also contained far more structural connective tissue proteins. In the kitchen, this translates to meat that is stronger-tasting, more iron-rich, and chewier. Many sushi chefs trim away much of the dark muscle for aesthetic and flavor reasons, though it is perfectly edible and packed with nutrients.
The ordinary muscle itself varies in shade across species and even across cuts from the same fish. The loin nearest the head tends to be slightly darker than the loin near the tail, and fattier belly sections (toro, in Japanese cuisine) can look lighter because intramuscular fat dilutes the visual concentration of myoglobin. When you see a piece of tuna labeled “super red” at a market, that color intensity is more about myoglobin density and freshness than about any single species trait.
How Color Varies Across Tuna Species
Not all tuna are created equal when it comes to color. Bluefin species, whether Atlantic, Pacific, or southern, tend to have the deepest red flesh. Bigeye tuna runs a close second. Yellowfin is typically a lighter, brighter red that leans slightly toward pink. Skipjack, the species most commonly found in canned tuna, has dark reddish-brown raw flesh that many consumers never see because it is cooked before it reaches the shelf. And albacore, the “white meat” tuna of the canning world, has flesh that is notably pale pink to almost beige when raw, with significantly less myoglobin than its tropical relatives.
These differences trace back to the ecology of each species. Bluefin tuna are the largest, most metabolically active, and most migratory of the group, diving deep into cold water and maintaining elevated body temperatures. All of that physiological demand pushes myoglobin production higher. Albacore, while still an impressive open-ocean swimmer, is smaller and operates at a somewhat less extreme metabolic pace. The relationship between swimming physiology and myoglobin content is consistent enough that you can roughly rank tuna species by muscle redness and arrive at the same order as ranking them by body size and metabolic rate.
What Happens When Tuna Turns Brown
Fresh tuna starts losing its red color the moment it is cut. The culprit is oxidation: oxymyoglobin, the bright red form, gradually converts to metmyoglobin, which is brown. This is the exact same chemistry behind a steak turning gray-brown in your refrigerator, just happening faster in tuna because fish myoglobin is less chemically stable than mammalian myoglobin.
The speed of this color shift depends on temperature, exposure to air, and how the fish was killed and chilled. A piece of sashimi-grade tuna kept at near-freezing temperatures and tightly wrapped will hold its red color for days, while a slice left uncovered at room temperature can start browning within hours. Research on longtail tuna slices demonstrated that certain plant-derived antioxidants, particularly quercetin and a green tea compound called EGCG, could slow this browning by chemically converting metmyoglobin back toward its red oxymyoglobin form.3PubMed Central. Effects of Different Phenolic Compounds on the Redox State of Myoglobin and Prevention of Discoloration, Lipid and Protein Oxidation of Refrigerated Longtail Tuna (Thunnus tonggol) Slices – Section: Abstract A follow-up study found that combining EGCG with vitamin C (ascorbic acid) was particularly effective at reducing metmyoglobin levels and preserving the red color of refrigerated tuna slices.4PubMed. Effects of polyphenols in combination with L-cysteine/L-ascorbic acid: Myoglobin redox state, color and quality of refrigerated longtail tuna (Thunnus tonggol) slices
For consumers, the practical takeaway is that some browning on tuna is completely normal and does not automatically mean the fish is spoiled. A slight shift toward purple or brown at the surface while the interior remains red is just oxidation doing its thing. The real freshness indicators are smell, texture, and how long the fish has been out of the water, not color alone. Ironically, an unnaturally vivid red that refuses to fade should raise more suspicion than a little natural browning, a point we will return to shortly.
Burnt Tuna, or Why Some Tuna Turns Pale and Mushy
Occasionally a tuna carcass will yield meat that is disturbingly pale, soft, and grainy instead of firm and red. In the industry, this defect is called “burnt tuna” or, in Japanese, yake-niku. The name is misleading since no heat is involved. What happens is a drastically accelerated breakdown of muscle tissue after the fish dies, driven by factors like extreme exertion during capture, high body temperature at the moment of death, or delayed chilling.
An ultrastructural study of yellowfin and southern bluefin tuna found that burnt meat was pale or brown in color and soft, occurring in carcasses where postmortem degeneration had run unusually fast. Under the microscope, burnt muscle did not look structurally different from normal degenerated meat; the problem was simply that it had reached an advanced state of breakdown before being cooled and processed.5Journal of Food Science. Burnt Tuna: An Ultrastructural Study of Postmortem Changes in Muscle of Yellowfin Tuna (Thunnus albacares) Caught on Rod and Reel and Southern Bluefin Tuna (Thunnus maccoyii) Caught on Handline or Longline Later comparative work across yellowfin, bigeye, and skipjack tuna confirmed the pale, grainy, and exudative character of burnt tuna and identified a specific protein, creatine kinase, as a potential biochemical marker that could help detect the defect early.6PubMed. Comparative study of muscle proteins in relation to the development of yake in three tropical tuna species yellowfin (Thunnus albacares), big eye (Thunnus obesus) and skipjack (Katsuwonus pelamis)
Burnt tuna is a real economic problem in commercial fisheries. A tuna that fights violently on a line for an extended period, driving its core temperature up before it can be killed and iced, is much more likely to develop yake-niku. This is one reason the most prized sashimi-grade tuna are often killed instantly using the ike-jime technique, a swift spike to the brain followed by immediate bleeding and chilling. The faster you stop the fish’s metabolism and get the body temperature down, the better your chances of preserving that clean red color buyers expect.
What Canning Does to Tuna Color
If you have only ever eaten canned tuna, you might be surprised to learn how dramatically different the raw product looks. Canning involves precooking the fish at high temperatures and then sterilizing it inside the can, both of which destroy the myoglobin pigments that give raw tuna its red hue. The result is the familiar beige-to-tan product most people associate with the word “tuna.”
Even among canned products, color varies by species. A study measuring color changes during the canning process found that albacore suffered the greatest loss in color intensity, followed by skipjack and then yellowfin.7International Journal of Food Science & Technology. Colour changes in tuna during canning and colour improvement by chemical modification of haeme proteins This is somewhat counterintuitive since albacore starts out the palest of the three, but the researchers attributed the relative loss to differences in how each species’ myoglobin responds to heat denaturation. The end result is that canned albacore (“white tuna”) is very pale, while canned skipjack and yellowfin (“light tuna”) retain slightly more color, though neither looks anything like the raw product.
Consumer expectations around canned tuna color have commercial consequences. Canners sometimes sort fish by initial color grade, and flesh that starts too dark can end up in lower-tier products. The gap between the vivid red slab at a sushi counter and the pale flakes in a sandwich is purely a matter of heat processing and protein chemistry, not a sign that different animals are involved.
Carbon Monoxide Treatment and Color Fraud
Here is where tuna color moves from science into controversy. Some processors treat tuna with carbon monoxide gas, which binds to myoglobin and locks it into a stable, cherry-red form that resists browning for weeks. The treated fish looks almost impossibly fresh, holding a vivid red color long after untreated fish would have turned brown. In the United States, carbon monoxide treatment of tuna is legal and classified as “generally recognized as safe” (GRAS), though the fish must be labeled. In the European Union, it is banned, with the narrow exception of the Netherlands, where a specific cold-smoking method is permitted for domestic sale only.8PubMed Central. Carbon Monoxide Residues in Vacuum-Packed Yellowfin Tuna Loins (Thunnus Albacares) – Section: Discussion
The concern is straightforward: if color is the main way consumers judge freshness, and carbon monoxide makes old fish look fresh, then the treatment masks exactly the signal buyers rely on. A piece of CO-treated tuna could be well past its prime in terms of bacterial growth and histamine formation while still looking bright red. Regulators who ban the practice argue it creates an unacceptable food safety risk. Those who allow it counter that proper labeling and temperature monitoring are sufficient safeguards.
Carbon monoxide is not the only way tuna color gets manipulated. Researchers have documented the use of nitrite salts, nitrate salts, and vegetable extracts from beetroot and radish as coloring agents applied to yellowfin tuna slices, all of which enhanced the red appearance of the meat.9PubMed. Smartphone-based colorimetric study of adulterated tuna samples These treatments are considered adulteration rather than approved processing, and detecting them is an active area of food-fraud research. One study used a smartphone camera and color-analysis software to distinguish beetroot-treated tuna from untreated tuna with about 98 percent accuracy, suggesting that even simple digital imaging tools could eventually help regulators and consumers identify tampered fish.10LWT. A fast and non-invasive imaging procedure to fight red tuna fraud – Section: 3. Results and discussion
Freezing and Long-Term Storage
Temperature management after harvest is arguably the single biggest factor in how tuna looks when it reaches your plate. Sashimi-grade tuna is typically flash-frozen at extremely low temperatures, often around minus 60 degrees Celsius, to preserve color and texture. Standard home and commercial freezers operating at minus 18 degrees Celsius do a noticeably worse job.
A study tracking bluefin tuna muscle stored at both minus 18 and minus 55 degrees Celsius over 180 days found meaningful differences in quality across cuts. The fattier belly portions, prized as toro, deteriorated faster regardless of storage temperature because their high fat content accelerated lipid oxidation, which produces off-flavors and contributes to browning. Leaner loin portions held up better, especially at the colder temperature.11PubMed Central. Effects of Frozen Storage Temperature on Water-Holding Capacity and Physicochemical Properties of Muscles in Different Parts of Bluefin Tuna For home cooks, this means that tuna from a standard freezer will never look quite as vibrant as the stuff behind the glass at a high-end sushi bar, and that is normal. It also means that fatty cuts, while delicious, are the first to lose their visual appeal in storage.
When tuna thaws, its color can shift rapidly. Exposure to oxygen at the surface creates a brief bloom of bright red as metmyoglobin re-oxygenates, but this fades quickly as oxidation catches up. A piece of thawed tuna that looks beautiful for the first twenty minutes and then starts browning is behaving exactly as the chemistry predicts. If you are serving raw tuna at home, cutting and plating it close to serving time makes a noticeable difference in appearance.
The Rainbow Shimmer on Raw Tuna
If you have ever noticed an iridescent, rainbow-like sheen on a slice of raw tuna and worried it meant something was wrong, you can relax. That shimmer is a purely optical effect, not a sign of spoilage or chemical treatment. Research on yellowfin tuna found that the reflectance spectra of these iridescent patches showed multiple interference peaks that shifted depending on the angles of illumination and viewing, the same physics behind the colors on a soap bubble or an oil slick.12Food Research International. Muscle iridescence in yellowfin tuna (Thunnus albacares)
The cause is the internal structure of the muscle fibers themselves. Tuna muscle contains repeating bands of protein at regular intervals, and when these bands at different depths within a thin slice reflect light simultaneously, the waves interfere with each other and produce color. The researchers noted that this mechanism was identical to the iridescence previously documented in beef. The shimmer is more visible on cleanly cut surfaces where the muscle fibers are sliced at a consistent angle, which is exactly how sashimi is prepared. A rougher cut scatters light in too many directions to produce the effect.
This rainbow effect tends to alarm consumers who associate unusual colors on meat with spoilage, but it has nothing to do with freshness, bacteria, or safety. It is a structural optical phenomenon baked into the architecture of the muscle, visible whenever the geometry of the cut and the lighting happen to line up.
Reading Tuna Color at the Store
Given everything above, judging tuna quality by color alone is more treacherous than most shoppers realize. A few guidelines can help you navigate the fish counter with more confidence.
- Expect species-appropriate color: Bluefin and bigeye should be deep red; yellowfin should be a slightly lighter, brighter red; albacore will be pale pink. If someone is selling you “bluefin” that looks like albacore, something is off.
- Slight browning is normal: A thin oxidized layer at the surface of otherwise red meat is routine and does not indicate spoilage. Smell is a far better freshness indicator for tuna than surface color.
- Suspiciously vivid red is a warning: If a piece of tuna is uniformly candy-red with no variation in shade and no signs of any surface oxidation, it may have been treated with carbon monoxide or another color-fixing agent. Check the label for terms like “filtered smoke” or “tasteless smoke,” which are industry euphemisms for carbon monoxide treatment.
- Pale, soft, and grainy is a defect: Flesh that looks washed-out and feels mushy likely suffered from the burnt-tuna condition. It is safe to eat but will have poor texture and flavor.
- Rainbow sheen is harmless: An iridescent shimmer on the cut surface is an optical effect of muscle structure, not a sign of anything wrong.
Frozen tuna sold for sashimi at mainstream grocery stores has almost always been held at ultra-low temperatures and may have been treated with carbon monoxide. Fresh, never-frozen tuna from a reputable fishmonger is less likely to be treated but will show more natural color variation. Neither choice is inherently unsafe as long as the fish has been stored at proper temperatures throughout the supply chain. The real risk with tuna is histamine formation, which accelerates when the fish sits too long above refrigerator temperature and which you cannot see at all, regardless of how red the flesh looks.