A Brown Burger Can Still Be Undercooked. Here’s the Check That Matters.
Food
A burger’s center can change color before it reaches a safe temperature. It can also stay pink after it has been cooked safely. The USDA recommends checking ground beef with a food thermometer and cooking it to 160°F (71°C).
For a thin patty, insert the thermometer from the side so the tip reaches the center. Check the manufacturer’s instructions to see how much of the probe needs to be inside the food for an accurate reading. If one burger is thicker than the others, give it its own check.
Keep the cooked burgers on a clean plate. Use fresh utensils or wash those that touched the raw meat before handling food that is ready to eat.
When planning a barbecue, put the thermometer out with the tongs before you light the grill. It’s easier to remember the temperature check when the tool is already beside you.
Source
USDA FSIS: Can Cooked Ground Beef Still Be Pink Inside?
