HOW TO COOK MEAT TO RARE, MEDIUM, OR WELL DONE
One of the initial challenges for chefs that learn to cook the meat to the proper degree of doneness is to know when to stop cooking. It is easy to cook a well done piece of meat, but what about the rare, medium rare, medium, or medium well? It's not so obvious anymore. The way that professional chefs use to judge when the meat is properly cooked is usually by touching it. During the cooking process, depending on how high temperature has developed inside the meat, the meat will become more and more resistant to pressure that you can apply with your finger. Rare beef feels very soft, almost like pressing on your cheek, medium feels more resistant, like pressing on your chin, and well done is very resistant, almost like some tough cartilage. The trick here is to learn to distinguish between all the different feelings that you get when pressing on the meat. And this you can only learn by practice.
I propose that the next time you want to cook a beef fillet or steak, do this pressure testing every 30 seconds or so to experience how the meat changes. You can actually cut the meat into smaller pieces and cook them for different time intervals and then judge their pressure and appearance inside. Just bear in mind that the pressure test will be slightly different after the meat is removed from the heat since it continues to cook for a short while after that, but you should get the right feel for the procedure anyway.
How will the meat look like inside when cooked to these different degrees of doneness? Bear in mind to look in the center of the meat, not on the edges that are usually browned in high heat! Also look at the juices that flow from the meat - the more red the less cooked the meat is.
Rare - is almost as if nothing has happened, the interior is very shiny and red
Medium rare - is approaching the pink color
Medium - is pink with some traces of light pink on the borders
Medium well - is light pink, almost cooked through, with some traces of grey on the borders
Well done - is not pink at all, just brownish grey
Or like on the picture below from left to right.
All these stages of doneness are different because of the temperature that changes the texture of the meat. Rare is approached at 57 C/135 C. The next stages happen in increments of about 6-8 C/10-13 F, with well done at 77 C/170 F. Medium well is a little below well done.
Remember that the different cuts of meat will behave differently when cooking, so you need to learn how to cook each one of them properly. For example, pork meat is becoming more opaque when medium, which is what you should look for instead of following the pink-ness as in beef, veal or lamb (described above).
When it comes to cooking poultry, they are approached even more differently - you simply cook until they are done, not too little and not too much that is. The technically best temperature for poultry is around 80 C/175 F, because it's not only good for the interior texture and moisture but also good for killing the possible salmonella microbes or other dangerous bacteria.