Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek or email to email@example.com to find out what's happening.
A while back, I asked people about cooking techniques or steps that they have been told to do but weren't really sure why. Molly Fulton responded with:
There are two reasons why meat needs to rest after cooking. The first is because, just because you've taken the meat away from the heat, does not mean that it's done cooking. It's going to heat up some more, then start cooling down. Why?
If you remember back to my kitten and elephant metaphor about temperature, when heat is transferred into molecules, they start to vibrate and otherwise move. Some molecules move easily (the kitten types), and some not so easily (the elephants). Meat doesn't transfer heat well, and so it's made of metaphorical elephants. When elephants start running, they don't stop quickly, so when the meat heats up, it keeps going even after you've removed the heat. Eventually it figures out what's going and slows down, and that's when the meat reaches its peak temperature then cools down. You want to let it cook all the way, so that's the first reason to let the meat rest.
The second reason is not so straightforward. Meat, aside from being metaphorically elephantine, is composed of lots of tightly woven strands of proteins and the like. At cold temperatures, these are very strong, so they're not likely to move or break. However, when you cook them, they lose a lot of their structural integrity. They stretch out, they become brittle and easy to break. Also, the collagen, which is connective tissue that will be woven more or less through the strands depending on the type of meat, goes from being a solid to a liquid.
When the meat cools back down again, the collagen starts firming up, and the various strands start regaining some, but not all, of their strength back. If you cut too soon, and with too dull a knife, you will break the already loosened strands, and the liquids will pour out of the meat easily. If you wait for the strands to gain back some strength, the various strands will be able to withstand some of the damage you're inflicting, and thus you'll retain a lot more liquid.
How long to rest? Well, I've been told by a chef that she rests her meats 8 minutes per pound, and she teaches others to do the same. Harold McGee mentioned in On Food and Cooking that a chef told him to rest for as long as you cooked the meat. These are probably not bad suggestions, but you may want to eat sometime before midnight, so you'll want to wait at least 8-10 minutes for something small like a steak and 20 minutes for something like a roast. If you already have your probe thermometer in, go ahead and cool it to 120°F.
These times and temperatures may vary by type and cut of meat (I'd be surprised if it didn't), but I haven't found a good source exploring the cooling process nearly as well as I've found sources exploring the heating process, so I'm not sure what those differences would be. Still, we're not talking about your entire dish being ruined, we're only trying to preserve as much moisture as possible. Use the guidelines above. If you try something on the low-end of the resting time, and you see a bunch of juice running out of your meat, you probably need to rest it longer. On the other hand, if you bit into it and it's juicy and tasty, then you're fine. The important thing is to understand what's happening so you can use your judgement properly.