For food safety, ground beef should reach an internal temperature of 160 degrees F (71 degrees C). If you prefer a pinker burger, cook to your own comfort level using an instant read thermometer. The thumbprint dent in the center is the key to flat, even patties instead of puffy domes. Do not press the burgers with a spatula while they cook, since that squeezes out the juices. Store cooked burgers in the fridge for up to 3 days, and freeze raw or cooked patties for up to 2 months.