Cooking fish is both rewarding and a little intimidating. If you’ve ever overcooked a beautiful fillet into a dry, mealy disappointment, you’re not alone. Fish differs significantly from land animals in structure and biology, and that’s exactly why it needs to be treated differently in the kitchen.
In this blog, we’ll explore the fundamentals of how fish are built, share general tips for cooking them, and highlight what makes tuna unique.
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Understanding Fish Anatomy
Fish live a more buoyant life than terrestrial animals. Floating in water means they don’t need dense bones or strong collagen to hold themselves up, which affects everything from how their muscles are built to how their bodies react to heat. Their collagen contains fewer stabilizing cross-links, making it much weaker and quicker to break down. In the kitchen, that means fish becomes tender faster—but go too far, and it falls apart.
Fish are also mostly cold-blooded (with the notable exception of some tuna), which means their muscle proteins haven’t evolved to withstand high heat. Proteins begin to coagulate at relatively low temperatures, so cooking fish gently is key. In fact, most kinds of fish taste best when cooked at temperatures between 42 °C and 50 °C / 108 °F and 122 °F. Go higher than that, and the flesh turns dry and chalky—even if the inside would still be considered rare by meat standards.
The one exception is tuna. Tuna stands out among fish for a few key biological reasons. They swim long distances at high speeds, so their muscles are built for endurance. That gives them a high concentration of oxygen-rich red muscle fibers filled with myoglobin—similar to red meat. That’s why their flesh is darker and fattier than that of most fish.
Some species of tuna can even raise their body temperature above that of the surrounding water, which gives them extra speed and adds even more fat to their muscle, especially in the belly. This fat acts as insulation in cold water and contributes to the rich flavor we associate with high-quality tuna.
Because of these unique qualities, tuna should be treated more like red meat than delicate white fish. It holds up well to searing, grilling, or being served raw in sushi and sashimi, where precise slicing techniques bring out its best texture.
Overall, with this in mind, we recommend a few key tips when cooking fish:
- Cook fish gently at low temperatures. Most fish are cold-blooded, so their muscle proteins are designed for cool environments—not heat. Fish flesh cooks at much lower temperatures than meat; the ideal internal range is 42–50 °C / 108–122 °F. Going higher dries it out and makes it mealy.
- Choose the freshest fish possible. Because collagen breaks down quickly after death, fish that isn’t very fresh becomes fragile and prone to falling apart. Freshness is key for texture and flavor.
- Recognize the light vs. dark muscle difference. Most of the fish’s body is made up of fast-twitch white muscle fibers used for short bursts of speed—these tend to be mild in flavor and tender. Darker muscle along the midline (the “bloodline”) contains endurance fibers, which can taste stronger and cook differently.
- Slice and portion strategically. Fish can be filleted in simple or elaborate ways. Large fish like tuna are often broken down into blocks or steaks. The shape and grain of the muscle will determine the best slicing method for ideal texture, especially in raw preparations like sushi or sashimi.
- Slice to control texture. For tougher or more fibrous seafood (like abalone or cuttlefish), use thin slicing or scoring to tenderize it and control the bite.
- Match the cooking method to the fish. Because each species has different fat content and muscle structure, some are best suited to raw or gently cooked applications, while others handle searing or braising better. Lean white fish can dry out fast; fattier fish like tuna or salmon are more forgiving. For example:
- Black cod, mackerel, and sardines benefit from dry curing for 3 hours with 7 g of salt and 5 g of sugar for every 100 g of fish.
- Halibut is great when seared over banked coals on the grill.
- Snapper is delicious when drizzled with spiced oil after cooking sous vide.
- Tuna is great when seared quickly in brown butter after cooking sous vide. We often cook at 55 °C / 131 °F, after brining the fish first and finishing with an aromatic oil.
Brining Fish
Another incredibly helpful tip is to brine fish before cooking (especially when cooking sous vide).
Salt isn’t just a universal seasoning; it also has a powerful chemical ability to help fish and meat retain moisture during cooking, as long as it’s distributed evenly at the right concentration.
Simply coating the outside of a fish or piece of meat with salt won’t do the trick—unless you’re aiming for the distinctive flavor and firm texture of cured foods like corned beef or smoked salmon. At high concentrations, salt can actually cause muscle proteins to break down.
Brining, on the other hand, has a gentler, more widely useful effect. It involves soaking the meat in a dilute salt solution until the salt gradually permeates the tissue. The goal is a final salt concentration of about 0.5% throughout the meat—much lower than curing. The challenge is ensuring the interior is seasoned as evenly as the exterior. Without care, it’s easy to end up with uneven salt distribution.
Modernist brining, much like sous vide cooking, uses a long soak—up to 24 hours—in a slightly stronger solution to achieve even seasoning without the risk of oversalting. Brining fish this way not only seasons and firms the flesh but also helps preserve its delicate color by preventing the albumin in salmon from leaching to the surface and masking the beautiful orange-red pigment with an opaque whitish color. Make sure to cook your brined fish immediately.
FURTHER READING
- The Modernist Cuisine Seafood Collection
- Cooking Salmon
- Fish Brine
- Cooking Shellfish
- Best Bets for Brining and Curing Fish—Modernist Cuisine vol. 3:172
- An Illustrated Guide to the Muscle Structure of Fish—Modernist Cuisine vol. 3:20–23
- Cooking Additional Kinds of Fish Sous Vide—Modernist Cuisine vol. 3:102
- Why Slime Is the Key to Crispy Fish Skin—Modernist Cuisine vol. 3:129
- Seaweed Fish Brine—Modernist Cuisine at Home, p. 133