We partnered with Lavatools to talk meat temperatures, so you can hit whatever you're aiming for no matter what you're grilling this summer.
My first college textbook was, by happy accident, Harold McGee's On Food and Cooking. Peculiar for a journalism and art history student, you might say—and I'd agree with you, but am pretty thankful for the deviation (and the guidance counselor who suggested the course).
On Food and Cooking tagged along with me every Monday morning of my first semester, as our class worked its way through sections on eggs, seeds, doughs, spirits, chocolate, and vegetables. Its scientific approach to cooking made an impression on me, and looking back, the way it told the story explaining our relationship to meat, and cooking it, felt revelatory. (So much so that I abstained from eating it for several years.)
In On Food and Cooking, McGee writes that we cook meat for "four basic reasons: to make it safe to eat, easier to chew and to digest, and to make it more flavorful." The results of applying heat to meat vary wildly across protein types—a very basic example of this would be comparing eating rare or raw beef to chicken. You wouldn't eat a slice of raw chicken, but you definitely might eat a steak cooked rare.
This also applies to pork, or lamb, or fish, as you can see above. Every meat looks a little different—and tastes different—at different temperatures. And while that might be obvious to point out, I think it's the preface for questions that are less likely to be considered: Why is ground meat a little riskier to eat rare than its steak and chop relatives? What are the small differences to look for that distinguish a medium to well-done fillet of salmon? What's going on in there at each phase that can be distinguished by temperature? And so on.
McGee includes in his chapters on meat and fish a fascinating chart that details the effects that heat has on a piece of meat or fish's proteins, color, and texture. Basically, it's a doneness scale. A few details here on rare, medium, and well-done:
Meat is designated rare, according to McGee, when cooked to between 120°F and 130°F/50°C—in general, the meat is becoming firmer and opaque at this stage, and the protein is coagulating. Fish cooked to the same temperature, on the other hand, is already past this point; it has begun, and continues, shrinking, is pretty opaque, and is exuding juices when cut into.
The USDA recommends 133°F to 145°F/65°C for rare, but McGee classifies this range as medium. (The USDA says that 155°F/70°C and above is medium.) This is when meat begins to shrink and and starts to exude juices, turning from red to pink. All of the proteins and connective tissue collagen is contracting at this point, too; the meat is starting to become more dense, less chewy. For fish cooked to between 133°F to 145°F, sheets of muscle start to separate, taking on that much-sought-after flaky nature. The closer fish gets to the end of this spectrum, the more it will continue to shrink, firm up, and then become fragile and fibrous. The USDA recommends cooking pork and ground meat to at least 145°F for consuming to kill any lingering microbes.
(But McGee does offer an interesting aside on making a safer rare hamburger: Blanch your meat. Before grinding at home yourself, he says to bring a big pot of water to boiling, submerge pieces of meat for 30 to 60 seconds, and then pat dry. This step kills bacteria and only cooks the outer few millimeters of the meat, and the grinding helps disperse those cooked bits among the rest of the meat.)
The temperature that McGee recommends for for well-done—155°F and up—is also lower than the USDA-approved 170°F/75°C. At 155°F, meat continues to shrink, becomes stiff, leaves behind little juice, and is starting to turn grey-brown. When fish gets up to this temperature, you're looking at it getting progressively more firm, dry, flaky, and fragile—and probably not in a good way.