Sunday, June 12, 2011
Banjo's Insight - Heat migration in food - oven vs pan
Heat migrates. That means it moves. The popular notion is that the heat moves from the heated part to the coldest part.
Douglas Baldwin, in his book "Sous Vide for the home cook" has some formulas which you can use to determine the amount of time needed for heat to migrate from the outer point to the inner point. But that's more detail than I'm trying to convey.
I'm just trying to convey the concept, so you know how to use it in your cooking.
When I first started cooking, I didn't understand why sometimes we cooked items in an oven, and other times we cooked it on the stove. I also didn't understand why we covered it sometimes, and left it uncovered other times.
If we put a big piece of meat in a pan and place it on the stove, then turn the heat up on the stove, the pan will get hot, it will transfer the heat to the outer surface of the meat where it is in contact with the meat. Soon, some oil and fluids will come out of the meat and coat the pan, which will improve the rate at which heat is transferred from the pan to the meat. Before long, the outside of the meat will be browned, and we will need to rotate the meat if our purpose is to brown the entire outside of the meat. However, if we were to cut the meat open at this point, it would be almost as cold as when we started.
So we have meat that has become much hotter on the outside where it has contacted the pan, but virtually no heat has made its way to the center of the meat.
We have applied too much heat, in too short of a time. The outside of the meat has absorbed virtually all of this heat, while almost none of it has moved (migrated) to the center.
If our goal is to heat the entire piece of meat, then we need to apply the heat at a rate that better approximates the rate the heat can be transferred from the outer surface to the inner part of the meat. Our solution here is to slow down the rate at which we apply heat to the meat. This can be done in several ways: we can reduce the heat applied to the pan, or we can move the meat into an oven.
An oven applies heat to meat in a manner different than that of a pan. A pan obtains heat directly from flame, so it can get hot really fast. An oven however, first heats the air in the oven, then the walls of the oven are heated by this air at the same time the air is heating the meat (by the way - this is why we preheat the oven; we do it to get the walls hot). Once the air in the oven, and the walls of the oven are all up to the heat, the meat can begin to absorb heat energy from the air and walls.
This energy is not transferred to the meat nearly as fast as from the pan. Think about this: you can't put your hand into boiling water ( a temperature of 212 dF at the pressure of sea level), you can't put your hand onto a pan heated to 200 dF (ouch!), but you certainly can stick it into an oven briefly, as long as your hand is only in contact with air - as soon as you touch the rack, pan, or wall, you've been burned). So you've just demonstrated to yourself that the heat isn't transferred as fast within an oven as it is on a stove top. It's possible to get the pan up to 400 - 600 dF.
So, if we've got a big piece of meat, placing it on a pan on the stove will transfer too much energy too fast to the surface of the meat - faster than it can migrate to the center of the meat. By moving the meat to the oven, we slow down the rate of heat transfer from the oven to the meat, so it more closely approximates the heat transfer rate of the meat.
We want the inside of the meat to reach the desired temperature without over cooking the outside surface of the meat.
So we cook in the pan for minutes, but cook in the oven for hours.
If we want the meat browned, we can then introduce the broiler once we have cooked the meat - this blasts the meat with a high amount of energy, which causes the surface of the meat to be browned (it's getting heated really fast, and that's heat's not really transferring into the center of the meat very quickly). So the outer surface will brown, giving us that great flavor.
We can place meat in a pan in a liquid, such as water, to increase the area of the meat that is being subjected to the heat (braising). The water spreads the area of the meat in contact with the water out considerably, as compared to the single points of meat in contact with the pan with no other liquid. So this helps transfer more of the heat energy into the meat. However, we adjust the heat so the temperature of the water is below boiling, to a simmer, which is around 185 - 200. So we are again reducing the amount of heat being applied to the meat while in the pan, from 600 dF to 200 dF, which allows for more time for the heat to migrate to the center of the meat.