Understanding the Cooking Process

The key concept to cooking our steak is heat. Heat itself is not a form of energy, rather it is a transfer of energy. The heat produced by our fuel sources is what cooks our steak, and the physics involved is both beautiful and appetizing.


When we use our grill, the first step is to preheat it to a constant temperature. When our fuel undergoes combustion, heat is released into our enclosed grill. However, the heat doesn't simply stay confined. The second law of thermodynamics states that heat is always transferred from a hotter to a colder system. As heat buildups in our enclosed grill, it also being released to the cooler, surrounding environment. The heat escapes through leaks in the seal of our enclosed grill, designed holes in the grill itself, and by the surface of the grill enclosure which conducts the heat which is then released to the environment. The hotter our grill gets, the heat escapes due to the temperature difference between the the enclosure and the environment. Eventually, our grill will not get any hotter and it has reached an equilibrium, where the heat loss to the environment is equal to the heat gain by our fuel source.

                        http://www.thedailymeal.com/cook/how-make-restaurant-quality-burgers-your-backyard-barbecue-slideshow/slide-8                                                                    http://dc-cdn.s3-ap-southeast-1.amazonaws.com/dc-Cover-rkk7kibkdqvknsqp4qbv2tsq01-20160315015750.Medi.jpeg

As our steak is presumably colder than our grill, heat will be transferred to it once we put it on. There are three different heat transfer processes, and all are being used to cook our steak.

Conduction is the heat transfer due to objects being in contact with one another. While this seems straight forward, an analysis at the particle level is quite interesting. The "hot reservoir" (which in this case is our grill grates) contain particles with a higher kinetic energy than the "cold reservoir" (the raw meat). When we put our steak on the grill, these faster mover particles collide with the slower moving particles of our steak, transferring kinetic energy. This energy speeds up the steak particles, which is why it gets warmer. Thus the heat transfer is simply the transfer of energy from physical collisions between our hot and cold reservoirs.
Convection is the transfer of heat by the movement of fluids, such as air or water. The majority of the energy that cooks the steak is due to convection heating. As our grill reaches its preheated temperature, the air inside of it is circulating around and carries thermal energy with it. This fluid of hot air surrounds our steak and transfers energy in a similar manner as conduction heating.
Radiation is the heat transfer that occurs due to electromagnetic wave propagations. Unlike the physical collisions between molecules which transfer energy in convection and conduction, the electromagnetic waves excite the steak particles and cause them to speedup without an interaction of matter. The radiation emitted is due to the electromagnetic waves from the grills flames. The flames we see are really just heated up electrons, which emit electromagnetic waves in the form of light and energy as they drop back to a lower energy level. This EM wave emission propagates through our steak, transferring energy which helps to cook it.   


"How Would You Like That Cooked?

We all have a preference with how we like our steak cooked, based upon how much pink/red is present. The meat is initially red due to the presence of the protein myoglobin, which is responsible for storing oxygen. When the myoglobin is exposed to oxygen the iron oxidation level is in the +2 state. This causes the iron to be attracted to O2 molecules, which gives the red appearance. As we cook our red meat, the iron changes to a +3 oxidation state, which causes it to turn brown. The reason why white meat isn't red is due to a smaller amount of hemoglobin present in the meat. However, whether red or white meat, if we burn the meat it turns black. This is because we have burned away the chemical structure of the meat and now all that remains is carbon which is responsible for the black color.