Lesson 3: Heat Transfer/The thermodynamics of cooking


This is where being a physician really helps in my understanding of cooking methods. The reason is it uses both physics and chemistry. Most people believe that their need for physics and chemistry was long over when they left high school. The truth is that we use both every day. Cooking is a prime example of use of the combination of the two.

All of the food that we eat is combination of fat, water, sugar and protein. The definition of cooking is the transfer of heat to food to change it to achieve a desired flavor, tenderness, texture or overall nature from a solid to a liquid or a liquid to a gas.

For this lesson we are only going to discuss two types of cooking heat, conductive and convective. Conductive is a direct source of heat to the food, while convective is where heated air circulates around the food to cook it. In another lesson we are going to talk about radiation cooking, which is the transfer of heat by direct exposure to a source of light or radio wave energy. This is more commonly known as inferred, halogen and microwave cooking methods. Just to pique your interest, microwave cooking is accomplished by excitation or vibration of food molecules by the use of radio waves. These radio waves cause the molecules in the food to vibrate at a fast speed that cause the release of heat and thereby, cooking the food. Typically, from the inside out. Radiation cooking

Just to keep things clear and in prospective, induction is the same as stove top cooking . This method causes the conduction/convection principle to boil water and cook food. Induction cooking utilizes a copper coil is placed under a smooth cooktop and an alternating current is sent through the coil creating a rapidly changing electromagnetic field. Electrons in steel or cast iron pots on the cooktop above the electromagnet are jostled by the rapidly changing magnetism, but they resist and the resistance heats the pot. The pot then transfers the heat to the food without the cooktop or the air around it getting hot. Induction is the most efficient way to convert energy to heat on a cooktop, and it very responsive to the control knob, but its downside is that it does not work with aluminum, glass, or copper pots.

Here is a lexicon for you. Conductive heat burns, while convection dries out food. You must decide what type of heat you want to use to cook your food. Essentially, how fast do you want the molecules in the food to move?

The best method to cook the food really depends on the food you want to cook. For example, tender meats, such as veal and fillets are cooked quickly on medium-high heat, while tougher cuts of beef and pork are cooked a lower temperature for long periods of time in order to break down the fibers. Where a high/rapid of these cuts of meat would toughen the meat even more because the fibers would fuse and harden. This is the art and science of convective heat for roasting and baking.

Thickness is more important than weight

The temperature of your food moves slowly upward during cooking, but the thickness of the meat is a major factor in how long it takes to get the center to the desired temperature. A thin steak cooks faster than a thick steak. And a 5" thick prime rib that weighs 8 pounds will be done in the same time as a 5" thick prime rib that weighs 12 pounds. So any recipe that says "cook your steak for three minutes per side" without specifying the thickness of the steak, is seriously flawed. Likewise any recipe that says, "Cook your roast 30 minutes per pound" is suspect. You will learn that oven temperatures vary and that you need to know when to check your food for doneness is the key to successful cooking mastery than following a recipe to the letter. You will learn how to do this in a subsequent lesson.

Boiling water

We all know from high school chemistry that water boils at 212 degrees F or 100 degrees C. But that is water. Not all things have same boiling point. Oil, depending on what type it is has a much higher boiling point (500 degrees F). Remember, when something reaches it boiling point it starts to evaporate its liquid into a gas. That why you can boil away an entire pot of water. When you mix a liquid with a solid, once the liquid evaporates all you have are the solids left in the pot. Hence, the burnt pot.

When a recipe states to reduce the liquid by half, what is saying is to evaporate the liquid so you only have halve of what you originally put into the pot that remains behind. That is the simple principle why a sauce thickens when reduced. It essentially leaves the solids behind that cannot evaporate. By the very nature of physics and chemistry what has happened is that there is now less liquid for the solids to swim around, which appears that a thickening of the contents has occurred. You have altered the mixture by virtue of conduction and convection cooking.

Cooking things longer over a course of time and temperature also brings out the flavor hidden in food because the cooking temperature is changing it chemically. Sometimes there are foods that need to be prepared quickly conserving all the flavor value or the texture you find most desirable. Cooking with a wok is a prime example of this type of cooking method to cook food fast to conserve the vitamins and the crunch factor of the vegetables. Therefore, wok cooking requires the use of tender cuts of meat, chicken, pork, fish, etcetera as the cooking time is quick. The use of stew meat, as a tougher cut of meat, would not work and the meat will become very tough to chew. Get the idea?

Using a pressure cooker allows food to cook rapidly using high heat and steam pressure to cook the food. Therefore, you can put tough cuts of meat into a pressure cooker and it will tenderize the meat due to all that heat, steam and pressure. This is a great example of a combination of conduction and convective cooking technique. As you get more advanced in this cooking school you will learn how to use the wok and pressure cooker as essential cooking methods.

When we cook, no matter if it is conduction or convection the outside of the food cooks first before the inside even has a chance to cook. Whereas, in microwave cooking the microwaves penetrate the food and the water and solid molecules move more quickly heating the food from the inside out. The faster the molecules vibrate the hotter the food becomes. Different molecules vibrate at different rates and also, depending on the cooking method cook differently.

Now that you understand the principles of heat transfer, deciding on how to cook what foods is the next lesson.

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