How Everything Works
How Everything Works How Everything Works

Question 632

How does food cook? — KJ, Irving, TX
There are two parts to this question: how does thermal energy (or heat) reach the food and what does that thermal energy do when it arrives. I'll start with the first part, but first let me define thermal energy as a form of energy associated with the random jittering about of the atoms and molecules in a material. The hotter a material is, the more average thermal kinetic energy (energy of motion) each atom has—in effect, the more vigorously the atoms and molecules jiggle. Thermal energy naturally tends to flow from hotter objects to colder objects, so that when you put cold food on a hot stove or in a hot oven, thermal energy will flow toward the food. This moving thermal energy is called heat.

There are three main mechanisms for heat transfer: conduction, convection, and radiation. Heat that flows via conduction is being passed from atom to atom inside a solid or liquid. In metals, conduction is greatly assisted by mobile electrons (the same electrons that allow metals to carry electricity) that carry heat between atoms far away from one another. Conduction is important on the stovetop, where the food touches the pot and the pot touches the hot stovetop. Heat that flows via convection is carried by a moving gas or liquid. Convection is important in an oven that's heated from below so that hot air rises to touch the food. Heat that flows via radiation is carried by electromagnetic waves (forms of light). Radiation is important in an oven that's heated from above (as in a broiler) so that thermal radiation travels downward to the food's surface.

Once the heat arrives at the food, it raises the food's temperature. As the food becomes hotter, chemical reactions begin to occur and molecules begin to change shape. Thermal energy makes it possible for chemical bonds within and between the molecules to come apart so that new bonds and new molecules can form. Water and other small molecules evaporate more and more rapidly until the water begins to boil. Sugar molecules rearrange to form caramels and carbon. Protein molecules rearrange and stiffen. These molecular changes, together with the increased temperature of the food, are what we associate with cooking.


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