According to Lacademie, raw tuna that has spoiled will appear off-colored, usually with a milky look or with shades of brown. Meanwhile, healthy raw tuna is a deep red or a light pink color. If your fish has started to change color or you see some sort of growth (like mold) toss that fish in the trash.
As a general rule, tuna that has darkened in color but still retains a reddish hue is safe to eat. However, if it has no reddish hue whatsoever and is just dark, it would be best to discard and not consume.
Fresh tuna, depending on its fat content, ranges in color from pale pink to deep red. (The more fat, the lighter the color.) Tuna tends to oxidize quickly, which cause it to turn an unappetizing grey or brown.
Whether fresh, or frozen then thawed, raw tuna turns a brown colour that is not appealing to consumers. The brown colour — called “chocolate” in the tuna industry — does not mean that the tuna is not fresh, but just as for beef, consumers look in raw tuna for a red colour to indicate freshness.
According to Ty Mahler, co-founder of Sushirrito, fresh, premium-grade tuna will show a deeper red color and often have a softer, more buttery texture.
Also unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red myotomal muscles derive their color from myoglobin, an oxygen-binding molecule, which tuna express in quantities far higher than most other fish.
Albacore tuna is noticeably white compared to yellowfin and skipjack, which are somewhere between tan and pink. But the colors aren't the only difference—they have distinct flavors, too. Albacore is firm and meaty, with a light, mild flavor. Yellowfin and skipjack are a bit softer and have a richer, more intense taste.
Tuna steak from the grocery store should only be consumed raw if it is labeled sushi-grade or sashimi-grade. While this is still not a guarantee against parasites, it means the fish was caught, cleaned, and frozen quickly while still on the boat and is the best option for sushi or sashimi.
When it comes to tuna, its colour is going to play a primary role when determining if it's truly sushi grade. Avoid tuna that has a glowing, plastic and almost transparent red to it. Anything that looks too vibrant has been chemically treated to give off an illusion of freshness.
Should I Wash Sashimi Before Cutting It? The short answer is, “Yes!” Raw fish needs to be carefully prepared. It is best to run your cut of fish under cold water once or twice and then pat it dry before slicing.
If you see any signs of mold, or any green, brown, or dark spots on your tuna salad, you should toss it out without even tasting it. Another sign is a foul smell. Tuna fish has a fairly strong smell to begin with, so if you smell something off, it's best to toss it out rather than take the risk.
“These constituents are mineral elements, richly supplied by the sea water in which the fish or shellfish live. These crystals can easily be mistaken for glass or plastic, although they more closely resemble crystals of salt.”
However, if the tuna steak tastes sour, mushy, or oily, it has most likely spoiled, and should be discarded immediately – do not swallow, as consuming spoiled tuna can lead to illness.
Just like a beef steak, tuna steak can be served from extremely rare to well done. Consider cooking times depending on how you prefer your tuna to be served. The centre of the steak should still be pink – be careful not to overcook it or the fish will be dry.
Ideally, you want the tuna steak to be about 2cm/¾ in thick so they are slightly pink in the middle when cooked. If they are a different width, adjust your cooking time accordingly.
Carbon monoxide treated fish, (aka tasteless smoke, filtered wood smoke, CO treated) refers to a process by which tuna loins are treated with the gas to prevent oxidation and thereby alter the cosmetic appearance of the meat.
- Meat Color
The tuna should be bright red, shiny, and translucent. The core and tail sample should have this color. The bloodline from the samples should be a darker red than the meat. It should not be black or brown.
What causes the shiny, rainbowlike appearance on raw tuna and beef? The rainbow effect is caused by the reflection of light off muscle fibers, technically known as double refraction or birefringence. It occurs when the muscle fibers are cut crosswise and can be observed for several days after slicing the meat.
White Tuna (Albacore) – The only species of fish that can be called “White Tuna” is Albacore. It is white to light pink in color and has a fairly firm texture.
Overall, light tuna is considered to be slightly healthier than white tuna, and certainly more budget-friendly.
Fish aggregating devices (FADs) can have negative effects on vulnerable species. Look for canned tuna labels that say pole-caught, pole-and-line-caught, troll-caught, FAD-free, free school, or school-caught to be sure you're getting a more ocean-healthy product.
The look of your sushi can also be a good indicator of its freshness. Fresh fish should look bright, shiny, and translucent.