lol, the things arent expanding per se…
the edges cook faster than the inside, so it contracts, making the inside pouf out in response to the squeezing. Cooked meat is compressed because the proteins are coagulating and the water/juices are being squeezed out=)
Make your burger patties dished in the middle and they will end up the same thickness throughout when you are finished=)
Let me see if i can find the link i want…<rummages on the internt>
hmm, here is quote))
Grilling Great Hamburgers
Burgers often come off the grill tough, dry, and bulging in the middle. To our surprise, making a shallow depression in the center of the patty was the first step toward a great burger.
The challenge: Upon deciding to develop a foolproof method for grilling burgers, we knew straight off exactly what we did not want: that all-too-familiar backyard burger that’s tough, chewy, and dry, with at least half of the crust torn off and left stuck to the grill. What we did want was a moist and juicy burger, with a texture that is tender and cohesive, not dense and heavy. Just as important, we wanted a flavorful, deeply caramelized reddish brown crust that would stick to the meat, and we wanted a nice flat surface capable of holding as many condiments as we could pile on. Given the vagaries of a live fire, we knew it would be a challenge to hit all of these marks.
The solution: What we first discovered is that the kind of meat you use to make a burger matters. Almost every recipe we looked at recommended chuck as the cut of choice for robust flavor and juiciness. Most recipes also suggested starting with a chuck roast or large steak and asking the butcher to grind it up as opposed to buying preground beef. Testing bore this out. It also taught us that for optimal flavor and texture, the ideal ratio of fat to lean was 20 percent to 80 percent.
Our next tests concerned burger size and shape. Too thicK-anything over 1 inch-and the burgers were tough and chewy by the time they had cooked through. What ended up giving us the best results was a “sunken” burger, with a rim about 3/4 inch thick and a center area about 1/2-inch thick. The reason for depressing the center is to eliminate the puffing that otherwise occurs when burgers cook. The thinner centers on our burgers do puff, but only to the level of the 3/4-inch outside rim, thereby producing a perfectly flat burger able to contain ketchup, onions, and more.
As for grill heat, most recipes recommended a high level to allow for development of a flavorful crust. We thought this might be too much for our relatively thin burgers, and we were right. It was too easy to overcook the burgers on a blazing hot fire. Medium hot did the job, producing enough heat to nicely sear the crust while also providing a wider margin of error during which to cook the center.
For good measure: Lots of people try to hurry burgers along as they cook by flattening them with a spatula. Unfortunately, this serves only to squeeze the juices out of the burger. We recommend that you use the spatula for flipping purposes only.