Transferring Heat in the Kitchen

Let’s get back to basics, shall we?

I’ve been reading the novel Lessons in Chemistry after receiving it for Christmas, and it’s inspired—potentially—my next 12 columns. The book itself is great and checks all the boxes for me (strong female character, a little romance, a little humor), but it really reminded me how much I loved chemistry and food science in college.  The truth is, cooking basically IS chemistry, and a little understanding of what’s going on will go a long way in your kitchen. 

I’d like to do a whole series on the art of food preparation, starting with one of the most basic principles in both chemistry and cooking: heat transfer.

In chemistry, heat transfer is thermal energy moving from one system to another.

In cooking, heat transfer is adding heat—energy—to a food, which causes changes to the food itself.

First, let’s review some principles from grade-school science:

-Everything is made up of molecules.  The more energy (heat) you add to the system, the faster they vibrate, which then transfers energy to another system. 

-Density measures how many molecules are in a specific thing.  The denser something is, the more energy it will contain.  For example, water is denser than air, which means water contains more energy than air at the same temperature.  This is why you can put your hand into an oven at 212 degrees for half a minute before it feels too hot, but you’d get a scalding burn if you put your hand into a pot of water at 212 degrees.

There are three types of heat transfer: conduction, convection and radiation. 

Conduction is the transfer of heat from one surface to another.  This is the type of cooking that occurs when you use a skillet or pan on the stove, or the cooking of the underside of a pizza or brownies in a pan in the oven.  Heat is being transferred from the skillet or pan directly to the food by touching it.

Convection is the transfer of heat through a liquid (like boiling water) or a gas (like hot air in an oven) to cook something.  Convection is not as efficient as conduction, but using a fan to keep the air moving (most ovens these days feature a ‘convection’ setting) improves its efficiency a bit.  Convection cooking includes steaming, boiling, frying and baking.  Remember that water is denser than air?  This means that boiling something will cook it faster than steaming or baking.

Radiation is the transfer of heat using electromagnetic waves.  This is the type of cooking that happens under the broiler or over a fire (s’mores!).  Radiation is the least efficient way of cooking, as the heat decreases quickly as the food is further from the heat source.  This is why broiling is rarely the ONLY way you cook food, but rather done to crisp or brown a surface after cooking has completed.

In reality, we often use all three of these methods of cooking simultaneously and don’t even know it.   An example would be cooking something on your grill at home.  The grill grate is hot and actually sears the bottom surface of the food (conduction), the bottom also gets cooked by the heat from the coals (radiation), and the top of the food is cooked when you close the lid for a few moments and hot air builds up inside the grill (convection). 

Additionally, we also use microwave energy transfer to cook our food.  Microwaves cook quickly by activating particles in your food (like the water) and causing them to vibrate.  Unlike the three methods above that only transfer heat to the outer surface of the food (which then must cook and spread the heat to the interior of the food), microwaves actually penetrate a short distance into the food itself.  This is why it’s always a good idea to stir food after it’s been microwaved, as the food is often unevenly heated.

Up next month: how bread rises!