Let's talk about the third method of the heat transfer, radiation. Radiation in general, it involves no direct contact of the heating surface and the target subject. So in this case, physical contact is not required. Now the most important part of it, in fact, radiation occurs all the time. When you go out in the sun, heat is being felt, right? So what happens is that the heat from sun, goes through radiation and hit you, you feel the warmth of it. The same thing we are applying the thermal radiation. We can use it for the cooking the same thing that we apply, for example, in microwave. We are relying on this electromagnetic wave to transmit the energy. All the molecules involved are vibrating to some extent. And when they're vibrating, one point they are emitting the thermal energy, but at the same time, because they're vibrating, they're also creating some sort of electromatic wave. Which is been passed on and been sent out to its surroundings, and that's what we call radiation. And for whatever subjects which is in contact by this radiation, then the heat will be transferred. And then once they pick up this particular radiation energy, they would increase their kinetic energy, and they become heated up. So therefore, radiation can occur over a wide range of electromagnetic wave, such as microwave, radio wave, or even ultraviolet. In all the situation, using different wave length, energy is being transferred from one source to another without going through any intermediate medium. How do we apply radiation in cooking? Now, I can easily illustrate by using grilling and broiling as an example. What's grilling? Grilling is what you have is a food, steak, you put it there and then you have a hot surface, such as charcoal, at the bottom. They do not need to have direct contact, but then the charcoal, because it's at higher temperature, it generate the radiation and transmit the heat directly onto the steak. How about broiling? Broiling, what you have is that you don't even need to have the heating surface at the bottom of it. The heating surface can be from anywhere, as long as they are having high temperature they generate the radiation. And this radiation pass on the energy. And when it be in contact with at the steak, energy is transferred, and therefore, the steak get heat up. Now, with that, you can have a very effective way of heat transfer without going through any kind of intermediary medium. Now, that's the advantage of it. But, of course, there's also a disadvantage. Because, what you have, is that when you have the surface being heat up, they become browned, and that's generating the so called browning effect. But at the same time, what happens if you look underneath the steak? The first layer underneath it, they are not in direct contact of this radiation. So therefore, they will not be able to get to a high temperature. And how do they get that heat up? It's only by conduction, when the surface gets heated up. They will be in contact with the lower layer and by conduction they will heat up too. So what happens is that again, relied on the rate of conduction in order to have an entire piece of steak get cooked. Now, that essentially is what we are trying to do when we are using the oven. And the heat is caused by radiation, as we talk about earlier, that is coupled together with conduction, with the air going through this conductional current. Now, one thing that we take pay attention to is, when this is heated up by radiation, radiation is by the wave form, it can be reflected by some soft surface, such as aluminum foil. So therefore, you may notice that people they try to wrap up their food with aluminum foil to avoid having extensive heating on the food. So that would help you to slow down the process of heating. On top of it, let's come to the final part of heat transfer. We're talking about phase transition. It is a very busy table, but then the bottom line is the following. We say that when you have some food which is in solid form, when you heat it up to a certain state they become liquid form. And when you heat up even further the liquid form can become the vapor form, gas form. At the transition between from the solid form to the liquid form, there's a phase that the temperature is not rising, and yet there, is a transformation of the shape and the property of the material, we call that phase transition. The same thing happened when some substance is moving in the form from liquid into gas. At a certain point they would maintain the temperature, but then their physical property change from liquid to become gas. And in that part in fact a lot of energy is taken up. Think about that. In fact, you can go through the reverse phase transition. You can go from gaseous form, when it comes down to become liquid, what it means is that, there will be a stage the temperature is not changing, but it is capable of releasing a lot of energy. So, if we can utilize that energy, we can cook very fast. So, that's the phase transition. So essentially, we're talking about, at that transition point, it is using the so called the latent heat. Because heat or energy is required to result in the transformation of a state of physical state. So what we notice is that if you want to change one gram of water heat it up right at the stage from liquid into the gaseous form, steam. It takes up 540 calories, the same happen when you go from steam to become water, both at 100 degree. They are going to release 540 calories a very hot amount of energy. In such a situation, you would utilize the energy released in this phase transition, and as long at that point you have some solid food material next to it the energy can be absorbed. That means they got cooked, they got heated up. Using this, in fact, we can use it from the liquid to gases form transition. Or we can also use it to thaw things, such as, going from the solid to liquid, or from liquid to the solid form. So this large amount of energy can be utilized, which is right at this point, condensation from steam to become liquid. Or from liquid to become solid, energy transfer, huge amount, that's how we utilize it for cooking, such as in steaming. We cup a part of water, raise the temperature to be steam, so that you have a lot of steam in that volume. Now let's say if you're steaming a fish, you put the fish inside, what happened? The fish is relatively cold, and so when it encounters the steam, this cold surface results in the condensation. That's the steam immediately getting converted into the liquid form. When they come down to the liquid form they release a lot of energy. So what happens? The fish you want the steam on the surface. It got cooked very quickly, because it absorbed a lot of energy. Now remind you, the same thing like radiation, it absorb all the energy on the surface. But in order to have the heat energy transferred to the deeper into the fish, we relied on conduction, that means through direct contact of the material, the surface of the fish to the deeper layer of the fish. All these that I've talk about, essentially touch upon the four different ways of heat transfer. Conduction, convection, radiation, and using the phase transition to illustrate how heat can be transferred from one, into another target. So what you have is that you need to understand how you play around with these four method, so that you can cook your food effectively. Using the right amount of time, and right amount of cooking material, and the right utensil. With all this, it allows you to make a good dish.
