Food Science Friday: Heat Transfer and Cooking

By Brian Montgomery

Cooking makes food taste better. It also destroys harmful microorganisms and makes food easier to eat and digest. But did you ever wonder what exactly is going on when you put a burger on a grill or throw a potato in the microwave? In this first installment of our new "Food Science Friday" series, we'll talk about the three main methods of heat transfer involved in cooking.

You probably remember from science class that heat is a type of energy. When something gets hot, its molecules have absorbed energy and start vibrating rapidly and bouncing off of each other. When they collide with nearby molecules, the heat energy gets transferred. When we cook food, we transfer heat through one or a combination of the following three methods: conduction, convection, or radiation.

Conduction

This is perhaps the easiest method to understand, because conduction heating happens when heat is transferred from one thing to another through direct contact. For example, the flame of the gas burner on your stove touches the bottom of the frying pan, conducting heat to it. Then the pan conducts heat to the egg you just cracked into it, which cooks the bacon.

Bacon frying in a pan via conduction heating

Some materials conduct heat better than others--for example, boiling a potato cooks it faster than in an oven because water is a better heat conductor than air. Conduction does happen in all cooking methods in some form, because it moves the heat from the food's surface to its interior.




Convection

Convection happens when heat gets transferred through a fluid or a gas. To get a bit more technical, convection combines conduction, and a mixing effect when the molecules in a fluid (air, water or fat) move from a warmer area to a cooler one. There are actually two types of convection.

Convection heating

The first, natural convection, happens when you place a pot of soup on a burner and heat it. The molecules in the bottom of the pot warm and rise while the cooler, heavier molecules sink. This circulating current distributes the heat throughout the liquid.

Mechanical convection happens when you stir something, either with a spoon, or in a convection oven that uses a fan. Because the stirring circulates heat more rapidly, cooking times are shortened and the food heats more evenly. Even non-convection ovens still rely on the natural circulation of air to transfer heat energy to whatever you're baking or roasting.

Radiation

Radiation or radiant cooking is different from the first two methods because it doesn't require any contact between the heat source and what you're trying to cook. What actually happens is heat waves or light waves strike and penetrate the food. Infrared radiation cooking uses some sort of heating element (usually electric or ceramic) heated to a very high temperature.

The element then gives off waves of heat that cook the food. These waves actually travel at the speed of light in any direction (unlike convection heat that only rises). Examples of infrared cooking include toasters and broilers, or even the glowing coals in a fire.

Microwave cooking uses the special type of waves generated in a microwave oven. Here, the waves penetrate the food, agitating its water molecules. That creates friction and heat. This energy is spread throughout the food by conduction (in solids), and convection (in liquids).

Foods are made of some combination of the following components: protein, carbohydrates (starch, sugar), water, and fat, plus vitamins and minerals. In the next Food Science Friday post, we'll look at what actually happens to the shape, color, texture and flavor of foods when each of these components are heated!

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