April 23, 2014

How Food Cooks – Conduction, Convection and Radiation

If you are feeling scientific and have begun to wonder how it is that the microwave or convection oven really cooks your food, or even what good old fashioned heat is doing to your meal, we’ve found your answers.  At drdavescience.com, a PhD candidate with a flair for explaining the scientific tells us how it all works. 

The science of heat

Cooking is the transfer of heat energy from some source to the food. In the kitchen there are three devices that are used to cook food: the stovetop, conventional oven, and microwave oven. Each of these devices are designed around a different method of heat transfer.

The movement of heat is so important that there is a name for it: Thermodynamics. By understanding how heat moves, we can gain insight into our everyday world. It is responsible for the weather, car engines, your refrigerator, cooking, and a host of other things that you may not have even thought about.

Heat is transferred in three basic ways listed below:

Conduction is heat transfer through direct contact. When cooking on the stovetop, the heat from the flame or electric grill is applied directly to the frying pan. This means that only the flat surface of the pan is sufficiently hot enough to cook anything and we must flip and toss around the food to cook it properly. conduction cooking a sandwich

It is important to note that most pans are made of metals, like copper, that conduct heat very efficiently and do not melt on the stovetop.

Convection is heat transfer through a fluid. The fluid can be liquid or gas and in the case of a convection oven, the fluid we care about is air.

An oven is a confined area that gets hot by flames or electric coils. The air inside is warmed to a desired temperature and, as a result, cooks the food from all directions. This method of heat transfer is responsible for pizzas, cakes, and other baked treats!

Keep in mind that ovens heat foods from the outside. The inside slowly heats up with time, and it is not uncommon to see food where the outside looks done, but the inside is uncooked. This is very important when preparing a Thanksgiving turkey, and there are special thermometers that measure the temperature of the food in the center of the turkey to show that it is properly cooked. (editor’s note:  All ovens cook by convection – where the hot interior air does the cooking.  What appliance manufacturers call “convection ovens” have an additional heating element and an extra fan to make the air circulation more efficient and effective, boosting the heat transfer from the air to the food, and thus altering the way the food cooks -faster, dryer etc..)

Radiation is the transfer of heat using electromagnetic radiation. A microwave oven uses very strong radio waves (a form of electromagnetic radiation), which are very weak and not hot. So how does it work? microwave oven

Microwave ovens work by spinning water, fats, sugars and oils inside the food. This causes friction, which then heats the food and cooks it from the inside.

Please do not be confused by the word radiation or electromagnetic radiation. In science, these terms are very general and mean a lot of things. Radiation comes from many sources, some are beneficial and others are harmful. For example, solar radiation from the Sun is responsible for heating the Earth and the light we see is a form of electromagnetic radiation.

As you know, heat is very important in the cooking process. Now you have the basic knowledge of heat transfer.

Questions and Comments About Convection Ovens

It seems lately that everyone is interested in convection cooking and that even though manufacturers have brought the technology to the simple toaster oven, many home cooks are still confused about how to use them.  The The Wichita Eagle fields some questions regarding convection oven use and has some simple advice.

I almost never use the convection option when I bake, because recipes never mention it. When I tried it — shortly after we got the new oven — the suggestion to lower the temperature didn’t work the way they said it would, and I prefer having temperatures I can rely on. What is your opinion of convection cooking? Should I keep trying to figure it out?

Unlike a traditional oven, a convection oven contains a built-in fan that intensifies the circulation of hot air, which evenly surrounds your food and speeds cooking. It is particularly good for cookies and pastries because the even heat promotes uniform browning and saves you from repositioning baking sheets midway through.

It’s also great, says cookbook author and frequent Post contributor Tony Rosenfeld, for getting that perfect crisp skin on a roasted chicken. He would know: He cooked hundreds for his book “150 Things to Make With Roast Chicken (And 50 Ways to Roast It).”

Convection baking is less successful, some say, for wetter foods, such as braises or casseroles, that you want to keep from drying out.

The degree to which the cooking is sped up depends on the model of oven and what you’re cooking. Recommendations vary, but most guides advise decreasing regular recipe temperatures by about 25 degrees and cooking times by 10 to 25 percent. One alternative is to cook at the prescribed heat but start checking for doneness early.

The best advice is to follow the instructions for your particular oven and to spend time learning what works and what doesn’t. If you like to follow recipes to a T, use the regular settings.

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