Given cooking is essentially one big, creative, science experiment, I figured it about time to get in to some details about the science of cooking, heat and the different techniques we use for cooking. Lots of this has been mentioned in passing elsewhere (such as when I wrote about humidity in cooking, we went through some science about braising, and when I wrote about bullet smokers we touched on convection), so this is going to be a run down across these topics.
Ultimately, most of this article is going to be about energy, and the transfer of energy (think back to high school physics lessons). Cooking is not just about temperature, that would be far too simple, right? It’s about energy. The easiest way to understand this is that you can braise short ribs in a simmering pot (liquid will be less that 100C) and have them perfectly cooked in three hours, but cooking the same short ribs in an oven at a higher temperature of 10C/225F you might be cooking in excess of eight hours to get them to the same level of doneness.
This is where the energy science comes in. Cooking, especially when we bring live fire and the variety of techniques that brings in to the mix, is all dependent on the techniques we use to get the heat energy into our food.
We’ll start by looking at the three main techniques for cooking (we won’t get into the fourth, microwaving, though) and then take a look at why there is the differences.
1. Types of cooking: Convection
All I can think of when I hear the term “convection” is sitting in class looking at diagrams explaining how radiators work. Heat rises from the radiator on one wall, moving across the top of the room and then back down again at the other side of the room, causing a constant convection current around a room. A constant movement of hot air, circulating in a consistent fashion.
Regardless of whether you have the same childhood memories, the same applies to cooking, and this is basically how an oven works. There is a heat source (and maybe a fan), that heat source heats the air that rises to the top of the oven, which then subsequently cools and lowers to the bottom, and this continuous process causes a flow of air around an oven allowing it to keep a fairly constant temperature. This same technique can be seen in kamado BBQs or bullet smokers, with the domed lid further aiding the circulation of heat. The heat energy in the molecules in the air collide with the molecules on the surface of the food and some of the heat energy is transferred.
With a consistent heat source, we can create a consistent temperature and level of heat energy in the oven or BBQ. This is why fire control in a BBQ (kettle, bullet, kamado etc) is so important - once we have set our air vents to a level that should keep the heat source (the fire) consistent, and we can just leave convection to do its thing. This is why we have a divider in a kamado BBQ or a water pan in a bullet smoker (that serves another purpose, as well) - it blocks the direct heat from the heat source, leaving our food to be cooked just by the convection current of hot air circulating in the grill.
1.1 Convection in liquid: Braising
Convection doesn’t only occur in the air though, convection currents also form in the same way in liquids, so when something is braised it is also being cooked using convection. The same way a piece of meat might sit in the oven, with the heat creating convection currents around it and the hot air cooking it, we can think of a piece of meat braising the same way, except instead of being surrounded by hot air, it’s a hot liquid.
If you have ever cooked low and slow and braised something, then you will know that braising is a far quicker method. You can braise a couple beef short ribs in just a few hours, and they will be beautifully fork-tender and soft, however, if you have tried slow cooking them in an oven (or BBQ) without a braising liquid then you could easily be looking at 8 plus hours cooking time. This comes back to the same principles of thermal mass we discussed earlier, you might be cooking in the BBQ or oven at a higher temperature (110-130 degrees) than the braising liquid (boiling point for water 100 degrees, so you can probably assume a temperature approaching that whilst braising), but despite the lower absolute temperature, the liquid has more energy stored in it, so can transfer more energy to the meat being cooked.
The final section of this article we will get a lot more into the energy dynamics at play here, and the differences between gas vs liquid convection.
2. Types of cooking: Conduction
This one is another topic with very vivid memories of secondary school science, not to mention lessons learned playing with electric fences as a child. Conduction is the transfer of heat energy through a conductor (metal, for example). When we cook a steak in a frying pan, we are benefiting from conduction. Our pan heats up on the hob, the heat energy transfers throughout the base of the pan and finally when the steak is added, the heat energy is then transferred to the steak through direct contact with the pan. That is, the heat is conducted to the steak through the pan. Unlike convection, where the food is cooked on all sides through contact with hot air (or liquid, in those cases), the food is only cooked on the surface of the food that is in direct contact with the pan itself.
A frying pan will be capable of holding even more energy than water (and can reach much higher temperature, given water’s boiling point of 100 degrees), so when a steak (or any object) lands in the pan there is potential for a much larger, and faster, transfer of energy to the meat, allowing it to cook much faster.
That larger potential for fast energy transfer makes it ideal for hot-and-fast cooking techniques, like cooking a steak for example, where you want to quickly sear the outside to form a nice deep brown crust before the centre is overdone. However, this fast energy transfer is not so good for low-and-slow cooking of tougher pieces of meat, because of the much higher temperatures involved.
This method is also hugely dependent on the materials in question - this is why cast iron has such ability to sear food, and why you should investigate the materials of a saucepan if you have particular purpose in mind!
3. Types of cooking: Radiation
Radiation is a method of cooking where heat waves directly penetrate the food being cooked. A heat source, such as a gas or electric grill, or burning coals in a BBQ will omit heat waves, and food placed directly above or below that will be cooked via radiation.
The main methods of cooking that use this are a traditional kitchen grill (or as our American friends call them, a broiler) and cooking on a BBQ directly above hot coals, which is the more traditional form of UK BBQ. Radiation cooking is, perhaps more obviously, utilised in microwaves, but as mentioned, I won’t get into microwaves just yet!
The closer to the heat source, or the higher the temperature of the heat source both increase the speed of energy transfer and heating of the food being cooked. Unlike convection heating, where hot air molecules collide with the food to transfer the energy, radiation is direct waves of energy.
When you read BBQ recipes or instructions and they talk about indirect or direct cooking, they are simply referring to whether we are cooking with convection or radiation. Indirect cooking, where a divider or some other mechanism is blocking the direct radiation from the fire, means that your grill will be cooking the food just using the convection method. Direct cooking, where food is placed directly above the heat source (fire) is being cooked by radiation.
If you place food above a direct heat source (fire) and close the lid on your grill, then you will be benefitting from both radiation and convection techniques. With the lid closed, the fire will still create convection currents, which will cook the food on all sides, but the radiation from below will mean greater cooking to the underside. To rectify this you would need to flip the food regularly to cook it evenly, as the radiation will be far stronger than the convection, but regular flipping would mean regular opening of the lid, which probably defeats the point anyway! The main exception to this is when you are cooking with a rotisserie. This method allows a gentler all round cooking with convection (good for larger pieces of meat) whilst also getting a good, even sear on the meat.
When the heat source is low enough, or far enough away from the food being cooked such that it is able to cook low-and-slow, you are most likely no longer utilising radiation as the cooking method, but instead resorted back to convection cooking.
4. Physics: Let’s talk about energy!
I mentioned at the start that cooking is a lot about energy, and there are lots of interesting nuances where this comes into play. Specifically, the materials we use and their capacity to hold and transfer energy.
All objects have the capacity to store energy. If I once more take you back to my school days, learning about energy, I remember a stretched elastic band stores potential energy. This potential energy can then be converted into kinetic energy - movement - when it is released. Energy can be transferred (in our elastic band example, the energy is transferred from potential energy to kinetic energy), but it doesn’t really just go away, it just changes form. This is applicable when we think about heat and the energy retained in different things - be it meat, pans, braising liquids or BBQs.
If we heat something up, then the heat energy is transferred from the heat source (the fire, the oven etc) into the object being heated up. That object then stores that heat energy until it is released. A really simple example of this process in action is frying something in a pan:
- You put a frying pan on a hob, the hob is a heat source and transfers energy to the pan
- The pan gets hot and is storing some of that energy (it becomes hot to touch)
- You put a steak in that pan, the energy in the pan is transferred to the cold steak which then heats up and cooks
Alternatively, imagine cooking a turkey in the oven. The heat energy produced by the oven is transferred to the turkey, when the turkey is cooked and you remove it from the oven, it will be hot to touch - this is because the turkey is now storing the heat energy itself and releasing it back out through the surface of the bird. That is, the turkey has now become a heat source full of the stored heat energy (transferring some of that energy to your fingers if you touch it).
All objects have what is called a thermal mass, this is an object’s ability to absorb and retain heat energy. Pieces of food, saucepans and BBQs are no exception, they will all have a thermal mass. Depending on the properties of that object, that energy may be retained for some time or lost relatively quickly. If the object being heated up is simply the air (e.g. the air circulating inside a preheated oven) then that energy is lost quite quickly, whereas a saucepan of water will retain the heat energy for a lot longer.
Why is this important? Because how different things retain or transfer energy is widely applicable in cooking - from what material to look for in a saucepan, to what happens to a piece of meat once it is cooked, to the roles of water pans and ceramics in BBQ.
This idea is exactly why:
- Braising is a faster method of cooking (liquid holds more energy than air, for example)
- Cast iron pans get such a good sear on a steak (cast iron has a good thermal mass)
- Kamados can hold such a steady temperature and cook so fuel efficiently (the ceramic walls have a good thermal mass, which allows them to store up the heat energy and slowly release it at a constant rate during the cook)
As my old physics teacher loved to joke, Physics is Phun. Turns out, when it comes to BBQ and cooking, he wasn’t wrong!
Photo by House Method on Unsplash