Heat Transfer and Cooking

Cooking food transforms it into a more nutritious and delicious form. Adding heat changes the chemistry and alters the form of food to allow our bodies to digest it easier.

While we don’t need to be able to calculate the Btu’s required to sear a steak, a little understanding of the basic concepts of heat transfer can help improve our cooking skills.

Understanding the why behind cooking food helps make the how easier. Knowing the why helps you adjust and innovate so that you can consistently cook great food.

We’ll start by going over why heat gets from your burner, grill, or oven into the food.

There are three ways heat moves from one object to another, Conductive, Convective, and Radiant Heat Transfer. We’ll discuss how we can use each of these below, but first lets go over the basics of why heat transfers from one object to another.

The Basics of Heat Transfer

The simple explanation of heat transfer is that heat moves from hot things to cooler things. Let’s get just a little more technical than that.

At the atomic level, the more heat in a substance, the faster the atoms will vibrate back and forth. What we call temperature is simply a measure of the amount of movement of the atoms.

So when two substances come in contact with each other, the atoms moving faster (higher temperature) will bump into the slower atoms causing them to move faster and increase in temperature.

When heat is transferred by two substances being in contact with each other, this is called conductive heat transfer.

There is a special case of conductive heat transfer where one of the substances is a moving fluid (air or liquid) and this is called convective heat transfer.

The third type, radiant heat transfer, does not require two substances to be in contact with each other and occurs when energy emitted from one substance as electromagnetic energy is absorbed by another. This energy transfer is most evident with really hot substances such as a fire where you can feel the heat from a distance away due to the large amount of energy being emitted.

Using Conductive Heat Transfer in your Cooking

Every time we cook something in a pan on the stovetop, we are using conductive heat transfer. Whether it be pan frying a pork chop or sweating onions, conduction of heat from the burner to your pan to the food is the mechanism being used.

The largest hindrance to conductive heat transfer is what is called resistance. If you’ve ever dealt with building materials, you may have heard the term R-value. This is the resistance value of the material, or how well it slows the speed of heat transfer.

When building a house, we want high resistance to limit movement of heat so that the furnace and A/C does not have to run as much. When it comes to cooking food, we are going for the opposite with low resistance so that we can quickly cook our foods at the desired temperature.

What creates resistance? Going back to the basics, conduction happens when atoms bump against each other, transferring their energy. So materials with densely packed atoms (think metal) will have more atoms and shorter distances to bump into each other and will transfer energy quickly, whereas widespread atoms (think air) will resist the transfer of heat.

The second factor impacting conduction is distance. The farther the energy has to travel, the longer it will take. You can notice this in different thicknesses of pans you use on the stove.

A heavy cast iron frying pan is thicker and will take longer to heat up and be slower to change temperature when you adjust the burner than a thinner pan like stainless steel. This difference in resistance and heating times is something to learn to account for when controlling the temperature in your pans.

After accounting for the resistance of heat transfer from the burner to the pan, the next area of resistance is between the pan and the food. While a food like a steak appears to be flat, at the micro level, most foods are rather bumpy and if placed directly on a pan, there will be lots of little air gaps between the food and the pan.

As mentioned above, air is a poor conductor of heat due to the wide spacing of the atoms. So in cases where we want to efficiently transfer heat to do things like sear a steak, a conductive substance between the pan and food is needed to more effectively transfer heat.

Fats such as butter or olive oil reduce the resistance and assist in cooking by replacing the air pockets with more densely packed atoms that transfer heat and also have side benefits of adding flavor and repelling water, important when browning meat or sweating vegetables.

Whenever cooking by transferring heat between surfaces in contact with each other, thinking about the factors of resistance will help you better control temperatures and cooking times to get your desired results.

Convection Uses in Cooking

Convection is a special case of conduction where heat is being transferred between an object and a moving fluid (air or liquid). So when you boil pasta or use your air fryer to make French fries, you are using convection.

Similar to conduction, heat is being transferred by atoms bumping into each other. In contrast, with convection, the moving fluid is continually replacing the atoms that have transferred their energy with new higher energy atoms so heat transfer can happen at a faster clip.

This is most evident if you have used a convection oven. With a traditional oven, you are primarily relying on conduction of heat through the air into the food. With a convection oven, a fan is circulating the air, thereby increasing the amount of heat transfer and speeding up the cooking time.

Air fryers take this heat transfer further by increasing the air speed even higher. All these options have their benefits.

When cooking something low and slow or making something delicate like a soufflé, a traditional oven is the best option. When you want to cook something fast and hot, an air fryer or convection oven will get your pizza or chicken nuggets cooked faster.

Another aspect of convection to be aware of is laminar and turbulent flow. A slow moving fluid will tend to flow in smooth straight lines (think placid river) where a fast moving fluid will churn and swirl (think white-water rapids).

Turbulent flow will transfer heat faster as the swirling will provide more opportunities for atoms to bump together. Foods that can take the turbulent movement, such as pasta, can be cooked at a rolling boil to maximize heat transfer and cook faster. Less firm foods like rice can be cooked at a simmer to still get some increased heat transfer from turbulence without breaking up their structure

Slow moving laminar flow will transfer heat slower but is important for delicate foods such as poached eggs.

Convection is a great tool in cooking and a basic understanding of how moving fluids transfer heat can help you get your foods cooked to perfection every time.

Cooking with Radiance

Every time we step outside into the sun, we can be reminded of the importance of radiant heat transfer to life on this planet. The sun emits energy in a wide band of wavelengths that are absorbed by the earth and provide us the heat that keeps the earth from being a giant ice ball, while also supporting photosynthetic energy production by plants and algae that is the basis for the web of life and the food we eat.

If you remember back to your science classes, you may recollect something about the electromagnetic spectrum. Energy can be classified by wavelength and frequency.

From the visible light that allows us to see to ultraviolet light that we blame for sunburns to radio waves that allow us to transmit and receive music to listen to as we drive down the road, the different electromagnetic bands provide us important aspects of our existence.

When it comes to cooking, we are primarily interested in the infrared part of the spectrum. Everything with temperatures above absolute zero emits infrared energy. This is what makes night vision goggles possible as they show images based on heat emitted through infrared wavelengths.

To cook something using infrared energy, we generally need a very hot surface to get sufficient heat transfer. So radiant heat transfer plays the largest role in cooking with things like grills and toaster ovens.

Radiant heat transfer generally operates at higher temperatures and is harder to control so it is primarily used to sear or brown foods.

Grills actually utilize all three methods of heat transfer with conduction of heat through the grill grate, convection of heat from the air flow up off the gas or coals, and radiant heat emitted from the hot surface to the food.

Microwave ovens are a special case of radiant heat transfer where a specific wavelength of energy is transmitted into the food. This wavelength specifically excites water molecules, and as we learned above, the faster a molecule moves the higher temperature of the substance. So microwaves can actually heat an object from within by increasing the vibration of water molecules which is nice for heating up leftovers but can also turn your food into mush if not careful.

Radiant heat transfer is a great tool in cooking and works best when it is used to brown and sear foods.

Next in the series…

Up next, we will look at how the size and shape of food impacts heat transfer and how we can use this knowledge to improve our cooking skills.

John and Sarah Gilbert farm with their family in North Central Iowa. They care for pigs, cows, and the land to bring you beef and pork you can love. They have a passion for cooking and helping others develop the skills they need to put healthy and delicious home-cooked meals on the table. They can be contacted by email and through Facebook and Instagram.