Like many people, most of the tuna I eat is canned. A sale in the seafood section at my local grocery store, and my recently discovered love of grilling everything, though, had me getting ready to cook a pound of tuna loin. I’ll spoil the ending right now: it turned into the delicious meal you see here. But the side dishes weren’t the only good thing to come with this piece of fish. If you enjoyed learning about the anatomy of steaks last winter, read on.
So what’s a loin, anyway? As I learned in December, the tenderloin is a psoas or hip flexor muscle in cattle and pigs. Tender, because it’s a muscle that doesn’t get much use, and loin because of its location on the body. According to our friends at Wikipedia and elsewhere, the loin is the part of the body between the rib cage and the pelvis.
So what does that have to do with a fish, which doesn’t even have a pelvis? Well, the online resources for those of us interested in butchery and anatomy are somewhat sparse for commonly consumed quadrupeds, but even sparser for fish. Between some unverifiable (at least not easily verifiable) discussion in an online forum and an informative article on pork from Serious Eats, though, I think I’ve pieced something together.
What tuna loin and tenderloin have in common actually seems more related to the tender than the loin. A step back: the back of a fish has some strong, slow-twitch muscle close to the backbone that does the constant work of swimming. It’s darker and tougher than the premium cuts, much like the brisket and chuck are tougher parts of a cow — it’s a hard-working muscle, so it needs to be broken down more to be tasty.
The “loin” of the tuna, like the tenderloin or backstrap on cattle, gets less constant use. It’s a fast-twitch muscle, with less of the connective tissue that makes harder-working muscles tough. They’re much more desirable for eating because they take a lot less cooking to be tender.
Further research into fish anatomy will have to wait for another day, but my working theory is that “tuna loin” is simply a marketing term for a premium cut of fish. I’m not complaining — it was just as good a cut as a good steak.
Oh, and how did I cook it? I grilled it pretty hot until it seemed cooked through. (It was easy to tell because there’s a natural slit between muscles into the center) It was definitely tasty, but now that I’m armed with this information, I might do it differently next time. Perhaps a combination of lower heat and hot grill searing? I’ll keep you posted.