When you cook a hamburger, the color and temperature of the inside of the burger determines its level of doneness. A medium well burger straddles the line between a rare burger and a well-done burger. The inside of the burger remains moist and juicy, but it's also cooked through. A medium well burger has different degrees of doneness, depending on how pink the inside remains.
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According to O Chef, a medium well burger has a slightly pink center and reaches an internal temperature of 150 to 155 degrees Fahrenheit. The inside of the burger is warm to the touch and cooked through. The lower temperature indicates more pink, while a higher temperature has no pink inside the burger. Once the burger develops a grey interior, it has reached the well-done stage. It is also possible for the burger to still be slightly pink and even red inside, which means that it's still in the medium rare stage.
For a one-inch thick burger, you need to cook it for at least four minutes on each side to reach the medium well stage of cooking. The time frame holds true for burgers cooked under the broiler, in a pan on medium heat and burgers cooked on a hot grill. If you're cooking outside over low heat, then it should take longer for the burgers to reach medium well. Burgers typically need five minutes or more on each side for rare.
Testing the Burger
Test the doneness of your burger, using either a meat thermometer or your finger. Insert the thermometer into the inside of the burger, making sure that it reaches the middle of the meat. Wait five to 10 seconds for the proper temperature to appear. If you're worried about poking a hole in the meat and the juices escaping, then use the finger test. Lightly press your finger against the top of the burger at the middle of the meat. If the meat is firm to the touch, but still has a little give, then it should be at medium well.
The U.S. Department of Agriculture warns against eating meat cooked to an internal temperature of less than 170 degrees Fahrenheit. A temperature this high indicates that the burger is well done and not medium well. the Department of Agriculture recommends the higher temperature because it reduces the risk of salmonella, food poisoning and other health risks.