The Science of Gastronomy: Transcript

2017/10/28

1.0 Introduction

Welcome to this class of The Science of Gastronomy. I’m King Chow from HKUST. So when you enroll in this particular couse you must be expecting that we’re going to teach you how to cook delicious dishes. But let me remind you this course’s focus is on the science. So, hopefully in this course we’re going to introduce to you all the nitty-gritty about the science component, that how we practice it in the kitchen. Now lets look at why we start cooking. Now, if you look at the history when did we start cooking? We always say that it’s roughly between like a 2 million years ago or 300,000 years ago, that we start cooking. Why, because we have obtained archaeological evidence with all these artifacts illustrating that people at that time, in fact, we start using fire. And, we have cooking pots and all these things. So, that means roughly that time, we start cooking. But then we also need to ask ourselves, how are you different from a dog or a cat, horse or a cow? Horse and cows they eat vegetables, grasses, leaves. Dogs and cats they eat meat. So what are we? We are omnivores, we eat everything. So how’s it different? How’s it affecting our evolution. How cooking food by itself is helping us. Think about that. Do you know how much energy a dog would use for their normal function? Particularly for their brain. The reality is that they only spend about 5% of their energy for their brain to maintain their function. But how about human being? We normally use about 20 to 25%, so basically if we want to think, we are using a lot of energy. So, the ability to be able to use food, cook food specifically, to allow us to capture the energy, to allow us to digest very effectively. And utilize nutrients to support ourself, to support our development, that’s very important. So, that’s what happened and how it comes to human being. For human being that’s what we are, we utilize our brain a lot. And being able to cook foods in fact give us the advantage to capture more energy to support the development of our brain. So you look at this particular diagram illustrating on the left-hand side is the apes, on the right hand-side that’s the hominid of a human being. You may recognize that if you look at the chimpanzees, in fact the skull they have a very different shape. Look at the brain, the brain is relatively small. But then they have a really bulging jaws coming out, because they use it for eating and hunting. For human being we have a relatively flat face, but then our brain is relatively much larger in proportion. So, that’s how we are. And, what happened is that in fact after all this apes they start to develop this ability to eat cooked food. That’s another element that come into play and in fact our relative apes they are also developing too. For example in this case the chimpanzee. You may recognize that this chimpanze is like munching some of the soil after they eat some of the plants. well what happened is that because the mixture of this soil together with plants results in the generation of some anti-malarial compound. So in fact it helped them to cure disease and infection. Now, so therefore, food or the ability to eat food in a very specific way is more than just for the nutrients. Sometimes is for the health and later on we may find that becomes something like culture. Same thing happened to the bonobo. But they have is that developed skills. They want to eat the termites so what to they do? They put their saliva on the stick so that they pop into the cave where the termites are there. And they will be able to pull out and stick those termites out so that they can eat them. So, the ability to use this stick become a skill again the skill become culture too. Not just about the apes. Think about elephant. Look at this African elephant. In fact, sometimes we notice that this elephant, they would go into the cave. At the time of rainy seasons when all the plants that are running short of minerals. What they do is they go into the cave and they eat the salt directly because they want to recapture the mineral which is missing in their regular diet. Again, that becomes culture. Now having all this, we also recognize animals like the sea otter. They are eating abalone and clams. They develop this ability, they would be like floating on the water and putting all the food in front of them. Again, it is a very unique culture that they develop. So, therefore in this course we are going to cover all the science behind it. But we are also going to cover some aspect of the culture and techniques that we use for the cooking. And in order to know more about that we need to understand a little bit of the geographical location where the food is from. What kind of culture the people there have. And that help us to understand. So, collectively I think there are few important element. We want to understand how the culinary process are being done in different places. We want to know how to use certain method or technique, or tools, so that we can make good food and make improvement. And, of course, with all these, we would be able to invent our own new dishes. Now, most important of all is that we would now be able to learn about all the different culture. We can mix different kind of food. And we can use different kind of techniques, and combine them all together. And most important of all, we understand that cooking. The first rule of them is that we need to use fresh and good food, to start with. And of course we need to know how to cook them, so what kind of utensil? How to transfer the heat so you can get them cooked? And we need to have a basic understanding of the raw material, the food, the characteristic. How they contact heat, how they get cooked very easily. And of course we want to control the texture of the food. The flavor the taste and how to mix them together. And of course, at the end we want to have good food. And there are psychological aspect of it that how do we get stimulated by good food, the good taste, the good smell. And whether we should have them constantly or we should have them once a while. And most important of all, again, is that we need to be very critical about what way the people are practicing cooking. Sometimes, there’s no expert in any single field. And we simply need to learn about the rationale behind it, and we can modify it accordingly. And as a science aspect, we need to focus on experimenting. So, I hope that you’ll be able to experiment it at home in your kitchen. To learn about trying how to mix different kinds of food, with different kinds of cooking techniques together. And if you keep a good record that is like a laboratory logbook. And you learn from the experience. I hope at the end you will have fun in this course. And at the end you will be able to cook a few good dishes for yourself and your family members or your loved one. Now in this course, I’m going to tell you a lot about these theory things. But we also going to demonstrate to you some of the scientific principles, with that I need to have my assistants, Leo and Angel. And they going to be with me, helping me to do some experiment in front of the video. So that you can understand better about how the science is being practiced in the kitchen.

1.1 Energy Transfer - Topic Outline

Welcome to our first topic, energy transfer. We’ll first talk about energy transfer in general, and then narrow down on two main areas of interest, thermal properties of food and heat transfer. We’ll look at four common ways of heat transfer in cooking. Conduction, convection, radiation, and phase transition. In the discussion of thermal properties of food, we will also learn about heat capacity and heat conductance.

1.2 What is energy

Welcome back. Let’s talk about Gastronomy the next section. About cooking via the heat transfer. Now when we talk about heat transfer in cooking, there are few concepts that actually we are going to do, including the mixing of food, including heating up something using your utensils, so that we can heat up the food. We can talk about the chilling so that the food can be raised to a high temperature, and we cool it down to a low temperature. Oh, we’ll talk about freezing. Basically, you want to have the food frozen. Now all of this would involve the transfer of energy, and we are going to talk about one by one. Now energy, as we understand, can exist in multiple forms, such as chemical energy. Chemical energy usually are the energy which are stored in a substance where they keep all this molecules and atoms bound together. That’s called chemical energy. Electromagnetic energy. These are the energy which can be transmitted in wave form so that it can go from one place to the other without any medium in between. And that’s referred to as electromagnetic energy. Kinetic energy. It is very easy to understand. When you move around, you have motion, you have vibration. You migrate from one place to another place. That’s referred to kinetic energy, movement is involved. Thermal energy is very easy to understand. It’s talking about the change of temperature. So from very low temperature to high temperature. We say that that’s a increase of thermal energy. And finally, potential energy. When we have any kind of, such as a bottle, when we move it from one place to the other. Basically we would have energy stored in them in a potential form that can be used later on. That is called the potential energy. Now, all this energy form, they are interconvertible. Essentially, it’s following the law of thermodynamics. Energy can be transferred from one form into another form. Now, let’s look at actually how heat energy is being transferred. When we say that heat, when it is in atomic scale, we are talking about how this kinetic energy of a molecule or a substance when they, being transferred from one place to the other place through movement of molecules. Now, so all this would involve the change of position, change of orientation, and change of their vibration frequency. And all this results in increase or decrease of the heat energy. Now in this whole process, when some substance is changed from low kinetic energy to high kinetic energy, we call that heat transfer. Now, of course what we’re dealing with is including chilling, freezing, heating, and all these things essentially we use in cooking. Now, how about temperature? Temperature, in a way, is a very convenient way for us to measure what that kinetic energy is really is. We are referring to, what’s the average speed molecule or substance. They are moving. Now, so the higher the temperature, that means they are more active, the faster they are moving. The lower the temperature, basically they are more stagnant, they are not moving around. And of course at the, a condition of absolute zero, that is in minus 273.15 Celsius degree. What we have is that basically, the molecules or the particles, they stay without any motion. And in that situation, we call it absolute zero. When we talk about heat and temperature in actuality, we talk about how heat flow from one place to another place. So I want to introduce to you a very simple equation, not that I want you to remember it, but that highlights some of the key elements of this so called heat and temperature, and the flow of energy. Now, basically the flow of energy can be highlighted in this, marked by Q. Q is amount of energy. And t is amount of time. So therefore, amount of energy transferred per unit time, that’s referred to the rate of the heat transfer, or heat flow. And this is affected by a few parameter, including k, marked here, referring to the very characteristic property of a substance. This heat is transferring through. A refers to the area, that means how large area this energy is being transferred through. Delta T refers to the temperature difference, whether it’s going from a very high temperature to a very low temperature, or they only differ by very little. So of course, the higher the difference between the two temperature, the faster the flow of the energy. Yes. And of course, finally, L. L refers to the distance between this two source. One is the energy source. The other one is the sink where the energy is flowing to us. Now, having that, I want to highlight that when we are cooking, we usually deal with a few ways that heat is being transferred. Usually, they are done by conduction, convection, radiation, vaporization, or condensation, and sometimes we refer to that case as a phase transition. All this would involve transfer of heat from one form to the other form. Now, with that, let’s take a break, and we’ll look at each of them and see actually, what’s the principle behind them.

1.3 How energy can be transferred through conduction

Let’s continue to talk about the different form of heat transfer. Let’s go first to conduction. Conduction, usually we are talking about thermal energy is being transferred from one molecule to another molecule, from one place to another place. Through the direct contact between these two particle or molecule so that heat energy can be transferred from one to the other as kinetic energy. So in this particular diagram very easily you can recognize that we have a lot of particles lining up in this particular plane. What we have is if we heat it up what we have is the heat is coming up into this single molecule. And when it’s being heated up, it starts to vibrate. It increases kinetic energy. And that kinetic energy is being transferred to the next molecule and make the next molecule vibrating faster. As a result the energy transfer from one particle to other particle and gradually they spread over an entire surface or entire volume. In that process, we call it heat transfer. Again using the equation that we talk about, it is very easy to understand what we are really talking about. We are talking about the transfer of energy per unit time. Q divided by t. It is affected by the property of the substance. K. In this case, K is referred to the conductance of this particular substance. And its affected by the area, the difference in temperature as well as the distance between the heating surface and the food or the target subject. Now so in this case. Q divided by t refers to how fast this particular substance can be cooked. K as I said conductance. A very specific characteristic of this food. For example steak, pork or chicken. A refers to how large is the area. This heating surface is in contact with the food. t is how hot is the surface clear above the food. How large is the difference between the two. And of course, how fast the heat can be transferred depending how far they are away from each other. The shorter they are close to each other or the thickness of this particular steak is thinner than we will say that the heat can be transferred faster through the steak. Now so that’s how we understand how heat is transferred. Now for the principle of this conduction, we need to recognize that in fact food heats up always very slowly because there is a lot of cell structure in food. Which they requires the heat energy to make them into movement into kinetic energy. And so therefore this thickness the shape of the food the area of contact the temperature difference between the pan and the food as well as how much time it takes all comes into play. And if you remember that particular equation you understand. How you can manipulate the system so that you can cook the food more effectively. Now there’s another element I want you focus on which is, how much energy is required to heat up the food. And that’s another element that you need to remember. In this case Q is the same thing, talking about amount of energy. But then there are now two other elements that we need to focus on. One is delta T. Delta T refers to difference of temperature and m and c. m means that actually how large is that particular piece of food you want to cook it up, of course. The larger it is you need more energy, the smaller it is you don’t need as much energy. And the other part which is about the heat capacity of a specific heat capacity. Again another very specific characteristic of a substance or material. And some of the substance they need to take a lot of heat energy in order to raise the temperature, some of them don’t. We need to understand for most of the food in fact we are having them cooked in water. And water in general actually has a very high heat capacity. So what I mean is that in order to raise the temperature of this particular substance which is mostly made of water, you need to use a lot of energy. Of course, there are other substances such as metal, they don’t need to use a lot of energy. You can raise the temperature very easily. So what is it that I want to show you that about the relation between conductance and heat capacity. Essentially we will say that some substance, they have very high conductance. They conduct the heat very fast. Some very slow. And on the other hand, on heat capacity some of the material, they need to take a lot of energy in order to raise the temperature. Some don’t. And we plot them all into one single graph. For example, if you look at the y axis, it is referring to conductance. You may see that in fact some of the metals such as silver and copper, which we use for making a cooking utensil, pot or so. They are usually made of metal and they have a very high conductance. What I mean is that when heat go through them they can be transferred very quickly through them. On the other hand if you talk about water, meat, fish these are mostly made of water as the major content, the conductance is very low. So what I mean is if you want to heat up water it takes a much longer time compared with the time you take to heat up the same metal. On the other hand if you look at the x axis. We’re talking about the heat capacity. Water has a very high heat capacity. Meaning that you need to use a lot of energy in order to raise it’s temperature. So combine together what it mean is that you can use very little energy and very quickly. You can heat up metal. While on the other hand for most of the food made of water, it takes a lot of energy. And it takes a long time for you to allow the heat to transfer into them and heat them up. With that in fact it has lot of implications how we cook things. For example we do stone grill. Why do we want to use stone grill? Because stone in fact they can store a lot of the energy. So that they can gradually transmit out and be passed on to the meat. If you want it to be even faster what you do is you can do the tepanyaki. Which is using a stainless steel as a surface which allows very quick heat transfer. Similarly probably you have encounter burning of your tongue when you’re trying to eat pizza. Think about that, what happened in a pizza. You’ll find that on the pizza on the surface that may be tomato that may be cheese or so. These are watery substance. In a way they have very high heat capacity. So they store a lot of heat in them so when your tongue get in contact with them. You have all this heat stored in them they will be passed on to you very quickly and therefore you get burned. On the other hand if you look at a lot of the air bubbles and all this, they have very low heat capacity. So when you’re biting onto the crust, where because there’s lot of air bubble in it, in fact you don’t get burned very easily. In order to illustrate this relationship of heat conductance and heat capacity. We are going to go to some experiments to show you how it can be illustrated.

1.4 Demonstration - Comparing heat capacity

In this demonstration, we’re trying to show you what heat capacity is really about. When we talk about heat capacity we are referring to a body of material, and if you want to change the temperature of this fixed body of material for a fixed amount of temperature, how much energy is required? For example, for a material which takes a lot of energy to raise the temperature for this fixed amount, it has a high heat capacity. But, for another body which takes in less energy to generate the same temperature change, that has a lower heat capacity. So let’s demonstrate and see how it can be visualized. So what we have here is two body of liquid. On my left it is 100 grams of water, and on my right is 100 grams of oil. Now, because of the difference of density you may see that oil has a slightly larger volume, but that’s fine because it’s fixed amount and fixed mass. Now what we’re going to do is we look at initially what the temperature of these two bodies are. Both of them right now is roughly about 23.3, 23.4 degree Celsius. Now, so we are going to transfer some energy into it. How do we do it? We are going to have on my right hand side a heating block that heated up two metal blocks to 80 degree. Now what I have done when I transfer this metal block into this body of liquid, the heat will be dissipated into this body of liquid. And as a result, the temperature of this body of liquid is going to increase. But depending on the heat capacity of this material, they’re going to have a different degree of change of temperature. Now let’s see what happens. Now it’s 80 degrees, so it’s a little bit hot, so I need to use the gloves to carry it. Now, here it goes. Now, I transfer this metal block into water. Okay. So I shake it a little bit. And then I transfer another metal block into the oil. Okay. And then I shake it a little bit. Now, it may take awhile that having these two body of metal block put into the liquid, heat will be dissipated. And it takes awhile for them to go through the process of equilibration, so I need to stir them a little bit and see how the temperature is going to be changed. Okay, now let’s look at the temperature. Okay, now both of them actually has a temperature increase. Let’s wait a while and see how the temperature continue to change. So we continue to stir it a little bit to make sure that it reaches equilibrium. After a while, so the equilibrium has been reached. So let’s look at the temperature again. So, for the body of water now is about 28 degree, 27.9. And for the oil, the temperature is 31, 31.1. So that’s a big difference in terms of temperature. So what it tells us is that water, with high heat capacity, it takes in the same amount of energy, but the temperature change would be less compared with the oil, which has lower heat capacity. The same energy results in a higher difference of temperature.

1.5 Demonstration - Comparing heat conductance

In this following demonstration we’re trying to show you what conductance is. Conductance refers to the property of a material, how effective it is in terms of the transfer of energy from one temperature to the other temperature through a proper body. Now let’s see how this demonstration is going to be done. On my right, I have an ice box where I have already prepared to use some ice cube and I’ve already put some pigment or dye in it so that they would be colored. So what I’m going to do is that I’m going to take some of this ice cube and put them into this plastic bag. Let’s see. I’m going to do that. Right. So in each of these bags I’m going to put seven cubes into it. OK. And then I seal it and put it under the surface of a plate made of different material for example, in this case this is a wooden plate. All right, so I leave them there and allow them to be in contact where the ice cube and see how heat would be conducted through this plate. And I put other ice cube on the second block. I put them on top of a plastic plate and the third one, what I do is I’m going to leave them with all this ice cube. And I leave them on this metal plate. OK, now all of them will have the same number of this ice cube. And what I’m going to do is that I’ll leave them there for a period of time, roughly about 15 minutes and I’ll see in 15 minutes how the ice would be melted because of the conduction of heat from these different plates onto these ice cubes. So we have waited for about like fifteen minutes. See what happened. So you may see that on all the three plates, you will see the ice cube they’ve melted. And so there are a lot of watery material around. So let’s see how much volume of the ice has melted. Let’s see what happened. Now this one is coming from the wooden plate. Now see actually how much water is there. All right. So we got all the solution out, and this one is from the plastic plate. So we got all the solution out. Now this one is from the metal plate. OK. So let’s see what happened. On my right is from the wooden plate. We have roughly about like the 24 mLs or so. OK, in the middle, this is from the plastic plate. The volume is about maybe 25, 26, around that. But how about on the metal plate? The volume goes to about like 30 mL. So amount of all three, in fact a metal plate conducts the heat to melt ice cube more effectively, so that more of the ice cube melted over this 15 minutes. So it suggests to us that different material, wooden plastic or metal, they have different conductance so that they transfer the heat to this ice cube at a different rate. So that tells us what heat conductance is really about.

1.6 How do we cook by convection

Okay, let’s talk about the second form of heat transfer, convection. Now in convection heat usually is being transferred involving some sort of like bulk movement of the material. So in that case we have some substance which is originally at high temperature, and they are going to move from one place to another place. And at the same time this happens, heat energy is being carried with them and be transferred when it’s in direct contact with another substance which is at low temperature. Now usually when convection takes place, it occurs in liquid or gases. Usually these are fluid like material. And inevitably we notice that when convection is taking place conduction is required. Because it takes direct contact between one hot substance and the cooler substance to be in contact so that energy can be transferred. It is very easy to understand that everybody at home they may have a heater which is using this convectional scheme. We would have this heater which is heating up. What happen is that you would have air being heat up by this heater. So when they’re heating up what you have is that the air become having lower density. Having lower density they start to rise and move to the top. At the same time it creates some sort of like a vacuum. So that the cooler air will start to fill in and be heat up by the heater again. And the hot air when it goes up gradually become cooler. They sink back so as a way what we have is that we have this convection current going on and allowing this bulk of movement of fluid like substance involving the transfer of the heat from one place to another. Now in order to show you how this takes place, I’m going to invite my assistants to do an experiment for you. Leo and Angel. Now in this particular experiment what do we have is that we will have some hot water, which is highlighted in yellow with dye and we have cold water which is highlighted in blue. Now what we’re going to do is we’re going to put this hot water and cold water together. But each time we are doing it in a slightly different way. In one case we put the hot water on top and in the other case we put the cold water on top. Let’s see what happens. Angel and Leo. Now you see, they use a card to help to hold the solution. And now they are putting on your right-hand side, we have the hot water on top and the cold water at the bottom. And on the left hand side we have the cold water on top and the hot water at the bottom. Let’s pay attention on the right hand side. Do you notice that the yellow solution in fact they remain yellow all the time? And the blue solution remain blue all the time. Basically that’s no movement whatsoever, why? Because the yellow water, they are of lighter density, they stay on top. And the blue one, colder, they remain having higher density, they stay at the bottom. And that’s no bulk movement of the solution. On the other hand if you look at the left hand side, what you see is hot water originally from the bottom it is having lower density. It’s start to move up. The cold water blue on top it having higher density. It sinks back. So you may recognize in a minute or two they get all mixed up. It becomes a green solution. Homogeneous. So that tells you how convection takes place involving bulk movement. In fact this kind of convection process, we utilize this principle in a lot of other cooking location. For example when we are baking we again utilize the density difference of air. For example, in oven you can put some sort of food in the middle. Now what happens is that you have a heater which is heating up at the bottom. So what happens when it heats up the air become heated up, they will have a lower density. They will start to move up and they will be in contact with the steak and then the heat energy is transferred from the air to the steak. And when they go to the top eventually they become cooled down and they come back. So eventually in the oven what we have is that we’ll have collision of the air molecules with the steak with the collision of the air molecule with the oven surface which is heated up. So therefore heat is being transferred from the heated surface, the metal, to the air. And again from the air to the steak by direct contact conduction. And this movement of air results in the convectional current and it helps us to heat up food in oven. But of course in oven we would have other heat transfer events going on including radiation, which we’ll talk about a little bit later. Now similarly we say that these two combine together in fact allows us to cook a steak inside a oven. Now how about in another situation such as in boiling. Boiling is very easy to observe. When you have water we say that you put a pot of water on a heated surface. The heated surface will transfer the energy by conduction onto the body of water. And on the surface when they are in contact. The energy get picked up they become hotter lower density and they start to move up. And on the surface of the water body when the density is higher, they start to sink back resulting in this bulk convectional movement of this body. And so what you have is that by this convectional current you’re going to generate a some sort of like a heat transfer and eventually the entire body of the water become heated up. Now, of course, the advantage of that is you can maintain a very stable temperature by this active movement. Now, in fact, when we talk about boiling, you need to understand that boiling in fact is defined by the condition that we would have a particular liquid. When they go to a particular temperature that the vapor pressure equals to atmospheric pressure. Then that’s the time that we call the boiling point and that’s the boiling occur. In fact, if it increases the pressure, what it means is that you need to have more energy introduced in order to allow this water to go through the vapor state. And so therefore what we notice is that if you have one atmospheric pressure, what we have as the boiling point is 100 degree. But, if you have higher atmospheric pressure that means it takes more energy to raise the body of water to that particular condition that they can generate vapor. And you’ll find that the boiling temperature can go up as high as 160-180 degree when this ten atmospheric pressure. And of course the same thing happens if we have even higher pressure applied to this liquid body. We can utilize it as a way to help us to cook things more effectively. You remember that many people would have a pressure cooker at home. What they allow you to do is that would raise the temperature inside this particular cooker. And when you boil it because the pressure keeps on being built up so therefore you need to have higher temperature to keep it boiling. So therefore you can raise the temperature above 100 degree so that allows more effective cooking of the food. Boiling in fact can also be affected by other elements such in this case is salt. We say that when you’re boiling something the temperature at the boiling point actually is 100 degree. In fact you can manipulate it by adding more sugar or salt. Because when you put salt and sugar there when salt and sugar reach the point of 100 degree. Do you think salt and sugar they would become vapor? No they won’t. They keep on absorbing more heat and yet they are not being vaporized. So what happens is that it results in keep on elevating the temperature and as a result when you have salt and sugar added to a solution the boiling point usually go up. A few degrees, sometimes 101, sometimes 102. And we are going to demonstrate that to you in a separate experiment that you’ll see later. Now, of course at the same time we can also do deep frying. In this case when we are doing deep frying we are relying on another medium, in this case oil. We use the oil as a medium with a hot surface. When the hot surface in contact with the oil, the oil becomes heated up. And when they heat up, they have lower density. They start moving up. So now let’s say you have some sort of meat in the middle. The meat will be constantly in contact with all this heated-up oil, which is moving upward. And then by direct contact conduction. Heat will be transferred to the meat. And of course, when they rise to the top the oil which is at lower temperature or higher density, they sink back. So again it creates a convectional current that allows us to heat up the meat by deep frying. Now all this, of course, it would have advantage because it allows you to generate very high temperature. Not in water 100 degree. Oil can go up to a 100, 200 degrees. Now so what happens is that at that time such a high temperature sometimes it results in dehydrating because water at that point they would all go into vapor. Because it’s higher then 100 degrees. Now so that’s what we do in deep frying. You basically put all the food which is made of water and dip it into the oil. On that I want to illustrate again, when we are boiling something in fact this convention current is heavily affected by what kind of medium they are in. Sometimes the viscosity of a medium can affect how the convention current can take place. For example, if you are making some kind of soup, which is very thick such as creamy soup or a porridge or so. They are very thick, convection will not be able to take place very easily because that requires bulk movement of the medium. On the other hand if it’s in water, this bulk movement is much easier. So in the following section we’re going to show you in another experiment that how the viscosity is going to change convectional current. And I hope that you see how it exactly occurs and what makes a difference when you’re cooking some sort of food with different viscosity.

1.7 Demonstration - Increasing boiling point with salt

As discussed in the lecture, if we boil the regular tap water under the normal condition, under one atmospheric pressure, they’re supposed to be boiled at 100 degrees Celsius. So we have done this 100 degrees exactly. Now we also said that, if you start adding salt or sugar into it, the addition of salt and sugar, in fact, it’s going to make the boiling temperature goes up. So, let’s try to do it. So now, I simply put one spoon of salt into it. Now of course, when I first add the salt, the water comes down in the temperature a little bit. And it’s now it’s boiling again. It’s now back to 100 degrees. And look at what happened. Immediately 101. So if we keep this with this salt in it, in fact the boiling temperature is going to be higher than 100 degrees. So our demonstration illustrate what the point that we want to make. Recognizing that after you add the salt. The salt is going to absorb the energy. But then after the energy is absorbed it’s not going to be vaporized. So therefore the heat is being trapped and the temperature continues to go up. So now if you look at the temperature again. So now it’s 102 degrees. Illustrating the point that we want to make in the lecture.

1.8 Demonstration - Cooking medium and convection

In order to demonstrate to you how the viscosity of a solution is going to affect the convection current. We’re going to put together this particular experiment. Very similar to what we had done a moment ago. so what we are going to have is that in the middle, we have two yellowish bottles. On the left hand side, it is a solution which is having corn starch in it. So the viscosity a little bit higher. On the right hand side it is just water. Both of them are hot. And on the outside we have two blue bottles with starch solution as well just water. So we’re going to do the same thing. We put the cold water on top of the hot water. And ask just by the convection how fast they will be able to be mixed up. to generate that convection effect to transfer the heat. So Leo and Angel, please. So they’re now putting the cold water on top of the yellow water. The yellow water is hot and so therefore once they are in contact, they generate some sort of like a convection current. And they start being mixed up. Okay. And you may see that on your right hand side, that’s the solution we’ve starch on it. So you may find that the solution is much more viscous. So yes the hot solution, they’re having lower density they want to move up. But then the cold solution having a higher viscosity is blocking it from moving. So this so called bulk movement of the medium is not going to be very efficient. On the other hand on your left hand side you see that this is just water with the dye in it. And you’ll find that within just a minute or two in fact, they have all mixed up into one single solution, which is green in color. So with that as a demonstration, it tells you clearly that viscosity is going to change the way, how convection is going to take place. So remember in the future when you’re cooking any creamy soup or you’re cooking any porridge make sure that convection is not going to be very efficient. If you have a very thick medium or very thick soup you better stir it a little bit to help to cook it better.

1.9 Can radiation and phase transition be used in cooking

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.

1.10 Energy Transfer - Topic Summary

Welcome. You’ve made it through the first topic. So far, we’ve looked at the topic of energy transfer and to be more specific, the thermal properties of food and heat transfer. We learned about heat capacity and gained basic understanding of the fact that heating of different food requires different energy depending on its mass, its specific heat capacity, and the necessary change of temperature. We also talked about the factors that affect the heat conductance, such as the nature of the food, its area, temperature difference, and thickness. In terms of heat transfer, we covered the four most common ways of heat transfer in cooking. Conduction, such as stone grill, convection, for example, deep frying, radiation, such as grilling and broiling, and phase transition, most commonly done by steaming. This week we also introduced to you a number of different examples and experiments of heat transfer. Try to keep those in mind throughout the course.

2.0 Hunger and Satiety - Topic Outline

Welcome back. The following topic discusses questions such as, why do we need food? How do we choose the food we’re going to eat? And what senses are involved in enjoying your meal? In other words, we’re going to tackle the concepts of hunger and satiety. We’ll present to you two main control systems of hunger. Endocrine control and neural control. We’ll first focus on the endocrine control. Particularly on the hormones insulin, leptin and ghrelin. Which are essential part of triggering the feeling of hunger. Then, we’ll focus on the neural control of hunger. We’ll give you a basic understanding of the functions of neurons and the perception of sensation. Having talked about hunger, we’ll concentrate on understanding how satiety works. We’ll firstly talk about the sensory specific satiety and the aspects that can influence it. Such as the amount of food consumed and the amount of sensory stimulation. Then, the discussion will move to the factors that affect our choice of food. In particular, mood, labelliing, the environment. Our hedonic or innate responses. And our previous exposure to the food. As usual our videos are packed full with practical examples and experiments. We hope you enjoy it, and see you on the other end.

2.1 Why do we need food

[MUSIC] Hello student, welcome back. We’re going to continue to talk about food. And I’m going to cover about enjoyment of food and how are we going to define whether the food tastes good or bad. And what kind of parameter defines it? Now, in fact, there are a few things I want to talk about. First of all, I want to talk about when the food comes to our body. What are they? And how are they being perceived? And most of the time, in fact, these are the chemical that we take into our body. And in fact, it’s going to trigger a whole bunch of physiological response, which involves hormone. And they would regulate how we feel and whether we want to have more, we don’t want to have more. Of course, all this food, when it come in front of us, it is because of how they look, how they taste, how they smell. So therefore there’s a lot of sensory input. And so, for that, we need to look at how these sensory input is being perceived by us and how it’s going to affect the quality of this food. And finally, we want to talk about how this so called sensory-specific satiety, when we are constantly being stimulated by the same sense. How do we feel? And whether we get tired of them, or even though that we love this kind of food. But then we can continue to eat them again and again, even though we are full. Now having that in mind, I want to highlight a few other factors, which is affecting how we perceive the quality of food. In fact, a lot of time when we are eating, the surrounding atmosphere makes a lot of difference. When you’re in a very romantic restaurant or whether you’re eating with your family. The mood, what kind of mood you are in, in fact affect how you taste the food. Sometimes the environment, whether you’re outdoor, indoor, the dishes when it has been put on the table, whether they give it a very good name. Reminding you some of your memory, that is going to change your perception of the quality. And, of course, sometimes we have personal experience, and some of our responses to a particular flavor. That would all influence how we perceive food being good or bad. Now, we can ask ourself, why do we want to have food? Why do we want eating? As a biologist, a lot of times we look at this because we want to have nutrients. We eat food. What kind of food? Vegetable? Fruits? These are sweet food. And both of them, when they were taken, they’ll be converted into energy. And one type of the energy, such as a mineral, amino acid, vitamin or glucose. These are all very important, they allowed us to be incorporated, having all these nutrients for us to mature, to grow as an individual. But let me remind you, most of the time these days, when we’re eating, it’s simply not because of all this calculation of how much nutrients are there. It’s simply because we are hungry, when you’re hungry, you eat. And sometimes even though we’re not hungry, simply because we like to eat. Something that we like so much that we want to keep on eating it, it is called craving. Now, that defines at a stage when or why we want to have food. Now, let them come to the point of hunger. We say that a lot of time when you’re hungry, you want to eat food. But what is controlling it? As I pointed out earlier a lot of it is because of our biological response, it’s because of these logical states because we don’t have enough energy. We want nutrients. So therefore, we want to eat so that we can recapture some of these nutrients to allow us to function normally. But as I pointed out, a lot of time, it’s the psychological state, the social state. How we’re interacting with the environment. All this affects us. So therefore, I want you to stop thinking about the reason why we eat is because of all this integral response in our brain. And that tells us that we want to eat something. And sometimes we enjoy eating, not necessarily because we are eating the material, the food by itself It’s because we like the feeling of anticipating that some food will be available very soon. Now, so in the next section, what I’m going to tell you is that I want to dissect this process one at a time. We’re going to address how the endocrine system, how our neural system, how we feel the sensory specific satiety, when you are stimulated by the same food again and again. Whether you feel full or you become tired of it. And finally, why sometimes even though we’re not hungry, we still want to eat? Why we have this craving behavior? So we will address it in a moment.

2.2 How do I know that I am full or not

To understand actually how food is being perceived by us we need to go through all the systems. First about endocrine control. What is endocrine control? In fact when food is being taken a lot of the time actually they carry nutrients, they carry energy containing material that would be digested. One of the most common one, in fact, is the carbohydrates, which will eventually be converted into glucose. And glucose has a lot to do with how we feel. Whether we feel hungry, we want our food, or we don’t have food. Sometimes we find that when we have taken some food, and after a meal, we accumulate a lot of glucose in our blood. And over time, gradually, they will decline. And when they decline, basically our body will have a system which detects the level of glucose from that. It will tell us a signal and say, well you are running out of glucose. So they start to fire the signal to our body to tell us that you should eat more food. So we feel hungry. Now let’s summarize the entire process. So what exactly would that be? Some experiment has been done to show that in fact, when we have low glucose We usually feel very weak. And when you feel weak, a lot of times you feel hungry too. So I believe that many of you have that experience. On the other hand, sometimes you know you don’t need to eat the food. Because we find that using mouse or rats in experiment, when they are hungry then you simply infuse into the gut, or in this case, the duodenum, some glucose. In fact, they would feel that they have the sense of satiety. They no longer feel hungry. So that tells us that whenever there is glucose being supplied to us, that we would feel the sense of fullness. Now, so therefore, what I want to focus on is that in fact whether we feel hungry or whether we are full in fact is an integrated hormonal control. And for this hormonal control, we are monitoring the glucose level. And glucose level, a lot of times, will be regulated by insulin. You remember that when you have high level glucose, actually that’s no good. So what we have is our body, the pancreas to be specific, they produce insulin. And the insulin would help us to take in all the glucose and convert it into something for storage. And by then, when the glucose level goes down, then you feel hungry again. But when the glucose levels stay high, you stop eating. So therefore, the hormonal control insulin, in this case, have a good correlation of the state of being full or hungry. Now let’s look at how the insulin work. Insulin is produced by the pancreas. In fact it’s something secreted into the blood and it’s circulating in your body. It will be in contact with different tissues, organs, and cells. And we know that on a cell, on the surface they have some of those so called receptors. They can sense whatever surrounds them. For example insulin as a molecule, it comes in it finds a receptor. The cells that what do they do? They know that when you have a lot of insulin that means there’s lots of glucose. We want to take in the glucose and import it into the cell so they produce a lot of the glucose transporter. Because there’s a lot of glucose, therefore, you don’t need to break down the other materials, such as the protein, lipid, or other carbohydrates. And at the same time, because there’s a lot of glucose, don’t need all of them. So what do you do? You assimilate them, you start producing glycogen and fatty acid. And because you have a lot of nutrients, that means that you are in a good shape so that it’s time for you to start growing. You will build up more DNA and you take in more amino acid to allow the cell to grow. So in a way, insulin allows us to promote cell growth. Now when I talk about endocrine, insulin is only one component of it. There’s another hormone which we call leptin. Leptin is another hormone which is produced by the fat cells, or sometimes we call that adipose tissue. This leptin, as it’s produced, is secreted again into the blood. And it will be transported to the hypothalamus, where it bind on the receptor. What does it do? When it goes to the receptor, it would tell us that, well, you are full. And so, therefore, it would inhibit your appetite and you don’t eat any more. So that means when you have high leptin, basically, it tells you that you don’t want to eat. What happen if you don’t have Leptin and you don’t have the receptor just like this popular ,mouse. It doesnt know that it is full it constantly think that its hungry, it keeps on eating so gradually it becomes very fat. Now what it means that leptin is telling you that you should not eat. So inhibit your appetite. But we certainly know that this leptin whatever level they are they’re telling you that whether you are at the state that you have low level of energy. You don’t have food. And so basically, it’s somehow using the hypothalamus as some sort of fuel gauge. That allows you to tell your body whether you have a good level of glucose in your body. And remind you that these are hormone, insulin is a hormone, leptin is a hormone. And they were secreted into the bloodstream, they travel along your body to the target site and then they trigger a response. Is it going to be very fast? The answer is no. It’s relatively slow. So what do they do is when they travel over they bind onto the receptor, trigger the production of a response. It is always a time delay. So therefore, this systemic way of handling the level of glucose or nutrients in the body is always has a delay, in terms of response. Now that become a very important element that we need to remember. Now we have the leptin telling you that you should not eat. We have another hormone which is called ghrelin, which is called a hunger hormone and it tells you that you were in a hungry state. So which tissue, which organ will be the best place for you to produce this hormone? Certainly your stomach or your gut. In the stomach, in the part of it which we call the fundus, the top part of the stomach, they are the place where they produce the ghrelin. When you are starving, when you don’t have energy, you don’t have any food in your stomach, it is thing that tells you that, well, you are hungry. So this particular part, the fundus, which produces a lot of ghrelin, and the ghrelin one is produced it would tells you that well energy is low and so therefore produce more ghrelin and then become hungry and start eating. So what I mean is that when you feel full, now your stomach is filled And you stop producing the ghrelin. And when you produce the ghrelin when you’re hungry it gives you this ravished feeling, you want to eat. Again, we call that it is a stomach defect, there’s a time delay. So what happened then, is that you may recognize that when we have high level of ghrelin, when you’re fasting, basically you want to eat more. When you have low level of ghrelin, you want to stop eating and usually when that happen it is after you have eaten a meal and you will have low level ghrelin. So what it suggests to us is that it makes sense if you eat more frequently. Not all the time makes you feel very full but you eat a little bit of it so it stimulate a little bit after sometime you produce the craving and then you go and eat. You eat a little bit and then the glucose go up and the craving comes down. So that’s how takes place and functioning in such a hormonal cycle. So let me go through them again and remind you. When we eat something, after eating it we convert them into glucose energy and we want to convert glucose into storage using the insulin. And at the same we are stimulating the production of this hormone, leptin. From the fat cell, it tells that wow, you have a lot of energy. So stimulate hypothalamus and say well, don’t eat any more, inhibit your appetite. On the other hand, if you don’t any energy, you don’t have any food, no insulin production, no leptin, no leptin receptor activated. So the appetite is not inhibited. At the same time ghrelin would no longer be blocked by the leptin. And so therefore, it will stimulate your appetite and tell you that you’re hungry. So Ana, you will start eating more. So with this full cycle, in fact we are controlling whether you feel full or you are hungry.

2.3 How do I know that the food is good

We have just talked about how the endocrine system control whether you feel full or hungry. Now in fact for the perseption of the food quality. The endocrine system comes a little bit later. The first aspect of really perception really comes from the nervous system. So I’m going to talk about the neural control. Now first of all let’s do a review of nervous system and how we perceive a particular sensation. In fact in a nervous system, there are two main classes of cells. The first group, we call that the nerve cells or we call neuron. These are cells which is specialized for electrical signal transmission. So electrical signal, remember that it’s just like the electrical wire that we have at home. Whenever you have a signal come in, quickly the electricity goes to the other side. So therefore actually they would respond very quickly over a long distance. The second group of cell we call the glial cells and these sometimes we refer to them as supporting cells. And they are not capable of transmitting electricity but this kind of they, they become to shield, to shield off this electric wire. Now what happen is that when we have a neuron. This is the neuron indicated here as the green cell. It will have different kind of processes. The short one here is illustrated as dentress the inputs. And the very long one here we call it the axons. These are the output. And usually when you have one neuron this neuron is going to be connected to another neuron. The axon should be connected to the dendrites. And all the support cells are here, wrapping around this particular axons. Kind of to protect them. And to make sure that the electricity is being transmitted within this particular wire. Now having that, we have these dendrites and axons coming all together. And in our brain we have like billions of these cells where nervous system neurons and glial cells support cell, all put together. They help us to process all the signals input. And the way that they process it is that whenever the dendrites get activated, they would trigger some sort of like an action potential. Think of it as the transmission of electrons across this wire so that they can all go to the end of it so that it will lead to the release of some chemical to be connected to another neurons. Okay. So that is what we refer to the process of acting action potential transmitted from one place to another. Now perception of sensation involves multiple steps. We need to have the stimuli and what kind of stimuli are we talking about? Temperature, pain, cold, visual sensation, sound, how things smell, odor and whether they have specific taste, whether they’re sweet or bitter. And for each of them, in fact they require a very specific type of receptor to perceive them. For example on our tongue we use receptor which would not allow us to hear. In our ear we can hear something but they won’t allow us to see. So these are specific receptors, corresponding to specific sensations. And when they are all input, they would all go to our brain and get integrated together in our cerebral cortex. And they would integrate a signal and tells us what we perceive. Now having that, the sensory input that we are talking about are so called taste, smell, sight, touch and sound. And in fact for whatever food we eat a lot of this stimulation will be activated. And so what are they sensing, taste we are referring to whether are salty, sweet or sour. How they smell some aroma? How do they look? What color do they have? What temperature? What texture? And sometimes what’s the sound that they can produce? All this combined together, it gives us an integration of all the senses in our brain. So the perception of all the sensation, the important part of is these physical stimulation. Essentially it come to our brain. It gives us a conscious perception of what exactly is in the outside world. Now later on I’m going to talk about another aspect which is about this human body on the subconsciousness. And a lot of experience we accumulate. And collectively it tells us whether our sensory stimulation will give us a satisfaction of eating certain food. Now when we talk about sight, color, visual appearance, all this is telling us what the food is like. Think about that if, in the Asian world, you are a monkey, you are an animal. And you simply walk around. When you want to see food and you want to find food, what’s the first thing you do? You want to see them. You can see them from a far. Whatever color it is, what kind of shape it is. So therefore sight become the first element when you try be in contact with food that you want to have it activated. And certainly we’re going to do some demonstration with you later on that how does visual perception in fact allow us to descrimate what is a good food and what is a bad food. How about the second element. The second element is that when you have some food around you do you immediately put them in your mouth? Not necessarily. Usually you smell them. you have them maybe a few meters ahead of you but immediately you can tell that the food is around because you can smell a particular odor. So therefore our olfactory function, smelling it is the second thing that come. And finally you put it into your mouth and you can taste it. Now so all these would collectively give you a sense of what the flavor is. Well, after you see the food, you smell the food and you taste the food. Now the food is in your mouth. We would evoke another type of sensation which is the texture. So when the food is in your mouth you’ll be biting on it, the teeth will be in contact with it, your tongue and buccal cavity will be in contact with the food. And when you crunch on it the food will be broken down and so all this it tell you what the texture of it is. Whether it’s hard or soft whether it’s chewy. What size it is. Now that become another important sensation that we want to stimulate. And of course sound. When you’re biting on something, if they are very hard or very very crunchy, certainly you will hear the sound. And that sound need to be somehow consistent with what the food is. So later on we’re going to show you another demonstration how the sounds and their consistency with the texture of the food need to be considered when you are having certain dish prepared. Now of course we talk about this neuro input. These are using electrical signal. We’ve also talk about endocrine system which is having a lot of delay. Now how do they integrate together? In fact we find that this neuro input sometimes they are not simply standing alone by themselves. They do have impact on how we process the food. And how our hormonal and enzyme system in our body will be responding and influencing our satiety. How’s it, I can use two examples to illustrate that. For example, we say that, when we have a individual, which is stimulated or exposed to some stimulus. of this sensory input. And if you feed them with some carbohydrates or so. You’ll find that the one which who’s subjective to this stimulus. They would have a higher level of glucose. And at the same time it results in a significant increase of the insulin. Suggesting that the neural input somehow would be associated or is stimulating or triggering some response in the endocrine system. So that they would respond more effectively. The same thing. We can have individual who are taking some fat capsules that simply swallow them into the stomach. But then one is being fed with some full-fat cream cheese. Some fat and the other one not. So again feed, feeding them with these crunches as a sensory stimulation would result in the production of all the needed enzyme to convert these fats into the tri-glycerol more effectively. So again it suggests that the neural input, the stimulus in our body using our sensory organ. Your mouth, your olfactory function, the sight and sound. All these somehow can be coupled together with the endocrine system. So if you want to prepare a good dish, you need to consider not just about the sensory input. You need to consider the endocrine system. Combined together, you would feel that the dish is delicious.

2.4 Demonstration ? Can you see the differences

2.5 Demonstration - Sound and texture - How does the experiment work

In this following experiment, we’re trying to show how different sounds will be matched with the food and affect the satisfaction of that particular dish. Now, what we’re going to do is we’re going to use these potato chips. Essentially, every single piece is identical. What we’re going to do is we’re going to ask the participants to try three different pieces. And when they’re eating it, each of them, we are going to show them a sound. At the end, we’ll ask them to tell us which particular matching of sound with the food would give them the best satisfaction. From the result, we’ll be able to tell how the sound and the matching of food would generate different level of satisfaction. The first one. Are you ready? OK, the second one. And the third one. OK, so now you can take the headphones off. So among the three sounds, which one gave you the best satisfaction when you are trying the potato chip?

2.5 Demonstration - Sound and texture - Let’s hear other people’s views

So, among the three sounds, which one give you the best satisfaction when you are trying the potato chip? Hm, I thinks, it’s the first one. The first one? Hmm. Okay, thank you. All right. The first one. So the second one. The third one. Maybe the second one is better. The second one. I think the second one. It’s the second one. So you see, based on the experiment with all the students, we found that most of the student, they found that the second sound matches best with the food, the potato chips. So it turns out that the first and the last sound in fact, they are not really related to potato chips. But then whenever you eat something which is having a matching sound, they always find that, they give them the best satisfaction. So that’s how sound, in fact, can effect the quality of the food, and how you feel about it.

2.6 I love chocolate but I don?t want to keep eating it, why

Assignment 1 - Instruction Video

After talking so much about sensory-specific satiety, let’s do a home assignment. I hope that in this particular assignment, you directly involved in the experiment, and you’ll be able to experience what sensory-specific satiety is. Now, let’s look at how we are going to do it. What we are going to do is that we are going to set up different meals, each meal I call it an experiment. And in this experiment, they would have ten items of food to be tried. So, what you do is that you take the first piece, eat it, wait for three minutes, and then you eat the second piece, wait for another three minutes, and you eat the third piece, so on, so forth until you eat the last piece. So basically, the entire experiment roughly takes you about 30 minutes, okay? Now, I want you to do the following, is when you are taking this food, for every single item after you swallow it. You need to give a ranking about how pleasant the food is. And I want you to rank it from one to ten. Ten being the best and one being the worst. Now, so what are those experiments and what are those meals. This is the following, in experiment one, or meal A. We have nine piece of the wine gums. So, you try the first piece, three minutes, the second piece, three minutes, all the way until the ninth piece, and then you go for the last piece after you finish all the rest, you try one piece of chocolate. So, meal number one. Meal B in this case, you try nine pieces of crackers. And these crackers will be taken one, three minutes, the second one, three minutes, the third one, another three minutes, all the way till it goes to the tenth one. And then you try the last one as a piece of chocolate. How about meal C? You’ll have four wine gums at the very beginning. And then after you finish the four wine gum, after each with three minutes separating them, you try the fifth one as the crackers. And then you try the crackers, and then the crackers, and then the crackers; until the last piece you try the chocolate. How about meal number D? That’s the fourth experiment. I want you to try as the first piece being the chocolate all the way till the end, so that means you have ten pieces of chocolate. So remember, after every single piece, I want you to do a ranking and tell how satisfied you are, how pleasant is that particular food item. With that I want you to fill it out in a table. We are going to provide you enter, entry of all this data. So, after the first item, I want you to rank whether it’s bad or very good. Second item, whether it’s bad or very good. And after ten items, enter all your data. And we’re going to compile all your information from your classmates, and we’ll summarize it, and see what kind of response you have. And I hope that in the process you come to realize what sensory specific satiety is really about, when you try the same food again and again. You don’t necessarily feel very good about it.

2.7 What factors can affect my choice of food

Sometimes when we eat food, we know that we’re not eating it because of the nutrition. Sometimes we’re eating it because we like it, and somehow I would use the term we’re addicted to it. So, let’s come to talk about craving, and see how it’s related to use of nutrients. And in fact at one point a French chemist pointed out that well, if you know all the food is made up of all these nutrients, minerals, vitamins and other things, so why can’t we simply take pills? And therefore we would be able to satisfy our need for biological function. And yet we know that we are not eating food simply because of the nutrients or the energy because we want something more. What’s that something more? Now, we know that human, we are the living creatures. We have a lot of very sophisticated sensory organs associated with us. We have our ear. We can hear very specific sound wave. We have our eye, we can see all the different kind of color. We have our tongue, we have our nose, we can smell, we can taste. We can sense the world around us. Why do we need all these? Because in a way, we want to see how the world is like, and food is certainly one aspect that we want to see the world. So we want to have all the senses being stimulated. And when they’re stimulated we feel satisfied. Now, so therefore what I want to point out is that craving. I call it sometimes some level of addiction. Is not synonymous with the so-called increased eating. It’s not just you’re eating a lot. Craving means that there are two aspects of it. We have a very strong or intense desire. We want to eat something, we love it. And we are in love with a particular food, a very specific one. So in all these, in fact, we don’t need to be hungry. We can be totally in, after a meal we’re full, and still we want to eat it. That’s what we call craving, and it is a very subjective experience because we are after a particular taste. Now let me illustrate that to you. Many of you probably have the experience. Well, chocolate. How many of you like chocolate? Raise your hand. Craving research, in fact, they find that a lot of episodes of so-called craving in human beings is associated with chocolate. How come we like to crave chocolate so much? Here it goes. Let me try to analyze a little bit for you. Well, many of you are in love and we know that when we are in love we’re going through three different stages. The first stage we call the falling in love stage. And in fact chemically we say that is the amphetamine stage. Meaning that actually our nervous system is somehow stimulated by amphetamine, which give you the sense of satisfaction, euphoric feeling. And when you’re falling in love, at this stage, you can’t think logically. So that’s what happens. When you get beyond that we call that the lasting love stage, which we call the endorphin stage. It’s just like the people who go running, jogging. They like it, they run the marathon. They feel very good after running. They don’t feel tired, and the simply they feel that they are very intimate with each other. They become very dependable, they want to be with each other. And the final stage is called the bonding stage, which we say that is the oxytocin stage. That’s the level of oxytocin that gives us arousal so that we feel very relaxed. We like to be with each other and we feel that we are bonded. So, think about that. Under all these stages, you are in love. What that’s to do with chocolate? In fact, in chocolate we have this compound called phenyl ethylamine. Some of them, they have ananadamide, similar to something like the marijuana. In a way it is anti-depressant. So, what it is? It’s this compound, in fact they can be addictive. What did they do, they act on our nervous system in the reward center there is a place which we called almond body, when they’re being stimulated we feel very satisfied, we feel we want it. An what happen is, when you eating chocolate, you feel rewarded, don’t you feel good? Now you’re not eating the chocolate because you want to have the sugar, not because you want to have the milk. It’s because you want to feel rewarded. So that’s why we develop this craving behavior. So therefore, a lot of times we want to feel rewarded. And if you looked around all these factors that affect how we like one food or the other, the mood, the name of the dishes, the environment, a plate of flavor. That gives you hedonic responses or the kind of exposure. What are they doing? They’re trying to give you a sense of reward. And when they are all coupled together, we associate that with you are in a very comfortable space. Very satisfied. And so that’s why you like it. You will see that as a rewarding process. So you love it. And so you go for the food. So therefore, we say that mood and food sometimes they are so certain. When you are in a happy mood, sometimes we eat more food, when we eat healthy for two. And when you’re in a sad mood, sometimes you just like a junk, you like put in a lot of things but not less assured because that you love that particular of food. So that’s how mood and food are being associated. The other element is that, well sometimes let’s look at the menu. When I show you this menu A, it clearly tells you what kind of foods they are. Red beans with rice, seafood filet with chicken, grilled chicken. So plainly describing exactly what that food is. Do you see this kind of menu very often in the restaurant? No. Let’s look at that. Most likely in the restaurant you’ll see some menu like the one on the right, which is traditional Cajun red beans with rice. Succulent Italian seafood filet or you have home-style chicken Parmesan. So all these, what is it? All the circled words, they are adjective words that describe what the food is like and it give you some sort like imagination to what kind of things they are. Succulent homestyle, does it remind you of something? Now, so therefore with all these description, it kind of like make you feel that they are better. So one example is, we call that the magic of the menu. When you have the food, you have all this descriptive name. And usually with the descriptive name, they are rated better. Why? Because with all these description it entices you to buy the food. They link the food with some of the appealing images, experience. And sometimes they need you to expect something that you remember that you have experienced in the past. Now, so all this makes you feel that the food’s quality is better. Now a lot of this label in fact try to stimulate you stop thinking about some geographical label. It represents some stuff like a culture whether is Korean food, Vietnamese food, Chinese food, Italian food or French food. It tells you some type of a style, sometimes it gives you a nostalgic label while telling you what traditional it is home style grandma cookie. So that reminds you of what is in your history. Sensory label, succulent want something taste good, it’s delicious. And sometimes the brand, and because we’ve eaten something associated with a brand so whatever is produced by that particular brand name it must be good. So therefore it kind of gives you that perception, or in fact they are playing deception. So, also environmental factor it plays a very important role in the process. You remember that in a lot of restaurants that you go to, a lot of them they put a lot of attention on the decoration, the lighting. They want it to be very soft, very calm, quiet. It gives you a feeling that while all the waiter or waitress they are treating you very well, and they give you a good name of the dishes, describe it very well. All this creates a belief that the entire restaurant, they’re simply catering for you. They have the dish especially prepared for you. Again, deception. And sometimes we use so-called the halo effect. They pair with you to tell you that, well, if they give you some good experience on one aspect. It give you a false expectation that everything else is all well prepared for you. So this particular kind of experience, we call it the halo effect. And so next time, when you go to a restaurant think hard and say, how they toy around with all these tactics. Now, in fact, what we need to understand is that if you look at our development, in fact how we perceive the whole world. In fact, we go through three different stages. The first stages is very primitive, how you taste something, you smell something, you see something. And after this learning process, in fact, something got etched into our brain and we remembered it. And the next stage is about language. And the next stage when you’re in the college, then you go for all these higher cognitive function. But when we talk about food, mostly the sensors that we try to stimulate, is focusing on the ones which experienced by you at the early stage of your cognitive perception. So when exposed to this kind of food at early stage of our childhood. In fact, they give us a long lasting memory and we call that, in fact, it’s called conditioned learning. You kind of associate some of the culture, some kind of food with certain feeling. And it is much easier to modify that when you are a child than when you are an adult. So what happens is that if a child is brought up in a particular culture, they tend to like the kind of food provided in that particular culture. Now, very easily I want to show a few example you remember the one you called movies? You like to have popcorn why? Because when you were young when you go to movies, you always carry the popcorn or a soft drink. Well, when do you have cookies? Usually it’s the leisure time and you have nothing to do. You have a drink and you want to have cookies. So, it’s a good time. A very extreme example. We say that, there are some World War II veterans that were somehow locked up in the war in China. And at that time they were eating Chinese food. So even though when they go back to the states, whenever they eat Chinese food, they remind them of the war time experience. So Chinese food tastes bad to them. Now, that’s how we associate certain experiences with how we perceive whether the quality of the food is good or bad. So, another example that I can ask you to start thinking about. When you eat are particular meal and if there are some good stuff in it and there is some plain rice spaghetti or so which part would you going to take first? Do you finish all the spaghetti and rice and then eat all the sauce and meat and so? Or do you finish all the good stuff and then finish the rice later? That is experience because a lot of times it is related to in your childhood how you experience whether the food is readily available. And we find that, at least in the Chinese culture, we have a lot of people, if they have a lot of brothers and sister, they are short of food. The tendency is that they always go for the good stuff first and then they finish the rice later on. But, if in modern days, we have a lot of family where only one or two kid, they have plenty of food. Then usually, they don’t have a such a preference whether they eat the rice first or eat all the other dishes first. Now, that tells you how the experience impacts our choice of them. Now, of course, all these so-called hedonic responses to the flavor and how are we going to interpret whether we like them or not? There’s a lot of experience that comes into play. When we’re eating food, foods by themselves have intrinsic value associated with them. For example, have you ever tried to give a baby something sweet? I believe that all babies like sweet. So if you give it to them, they all like it. Have you ever tried and give a baby something which is bitter, or something which is sour? Try it at home again if you have the opportunity. I’m sure that all of them would hate it. They would reject it. And the same thing, like trigeminal stimulation, what is it? It’s like something in the Japanese food we have the wasabi, when you are cooking things with onion, well, you kind of feel like it’s very pungent and you don’t like the feeling. And so these are in general very unpleasant. How about salt? In fact, we are all longing for some minerals. So that we can have it incorporated to help us to function normally because of supply of the nutrients. So salty things we like them in general. So these are something called innate response. But of course, some of them can be acquired. That goes with your experience. Now, you can talk about all this hedonic responses. Yes, this pungent smell, the trigeminal stimulation, we can have it acquired. You remember that people like to eat Japanese sushi. And when you’re eating sushi, you put wasabi in it. Why do we like it? Sometimes, people simply like that kind of experience that they almost like after eating a sushi, they almost like breakdown and cry. And but that feeling they like it and they become an acquired experience and from that point onward they would like a particular food. So what I want to conclude with that, is that we need to understand that whether we see food as good or bad there are few elements that we need to think about. One, some physical stimulation. How do they look, how do they sound, how do they feel, the texture, the taste and smell? These are very physical and a lot of the time there is a lateral association with nutrients and the quality of the food. All these come in becomes something very conscious. On the other hand, we say that a lot of time, whether we like a food or not, largely is dictated by whether we feel rewarding or not. And that can be easily influenced by, all this emotional association. Our conditional learning experience, and activating of the craving circuits that we want to be rewarded, and those act together on this end. And they’re acting subconsciously detecting like whether we feel that it is good food or not good food, and of course we trigger this hypothalamus. The neuroendocrine organ that allows us to control whether we want to take in more food or we don’t want to take in more food. And so, from this point on what I hope that you understand, in fact what we’re talking about, as how we perceive food as having good quality or bad quality. It’s not just by itself as good or bad, is the integration of all these together. Only when we can stimulate all these button appropriately, you’ll be able to prepare a good dish that everybody likes.

Assignment 2 - Instruction Video

Now it’s the end of week one, let’s do an assignment. And this assignment would involve all of you, all of the students of this course. We’re trying to do a worldwide mapping of the taste map of our tongue. Let’s look at what happened. On this particular map, I’ll indicate to you that this is the tongue. On the tongue, that’s the very inner area I call it a. And on both sides at the back, I call it b. More towards the front I call it c and the tip of your tongue is d. So I want to understand which part of this area on the tongue, in fact, is used for sensing certain tastes. Okay, I need to have my assistant to come in, Leo. So what you are supposed to do is that you are going to use one of these solutions. For example, this one. This is one of the solutions. And what I’m going to do. We follow a procedure like this and you can prepare different kinds of solutions. For example, if you want to have a sweet solution, you would have sugar. Two teaspoons of sugar in 100 ml of water. For sourness, we use either lemon juice or vinegar. We can use water as the blank. We can have some table salt in water to have the salty solution. We can have some cocoa powder put in water to represent bitterness. Now, what we need is, we need to use a cotton swab. This particular solution represents one of these. So what are we supposed to do? What we’re going to do, is we’re going to use this cotton swab. We dip it into the solution. And then what we do is we ask my assistant to stick his tongue out. And then I’ll try to put this particular solution on a particular spot. Let’s say this time I want to try the tongue’s tip, here. And then what I want you to do is then after you try it, you would rank, well, is this one sweet? Is this one salty? Is it sour or bitter? And if it’s salty, what level of saltiness is it? So rank it with the grade of one through four, four being the strongest. And then you write that down. Now, and then I go back and use another swab, dip it, and then put it in. And now I put it in a second position, for example, the very interior part of the tongue or on the side of the tongue. Then you would have a score for each particular spot for the same solution. And you rank them according to the intensity. Then after you have done all this what you’re going to do is take all this data and tabulate it on a table. So what you’re supposed to do is then afterward on the course website you are going to be prompted where some of the questions. So what you need to do is to write down if you’re trying the first solution, for example. Whether it’s a sweet solution, one, two, three, and four accordingly and you rank them. If you put it on the position a, b, c, and d, what’s the intensity of a particular taste? Now, after we’ve collected all those, we will be able to integrate all the data from thousands and thousands of students’ input. And we’re going to have a taste map with the input from all of you. And then next week, I’ll tell you what the taste map of human beings is.

2.8 Hunger and Satiety - Topic Summary

Nice work, you’ve reached the end of our second topic. In the first part of this topic, we’ve learned about two main control systems of hunger, Endocrine control and Neural control. In terms of the endocrine control, we explained how different levels of the hormones insulin, Leptin and ghrelin, trigger or suppress the feeling of hunger. And in terms of the neural control of hunger, you’ve now gained a basic understanding of the functions of neurons and their mechanism for transmission of signals. As part of the neural control we also discussed how sight, taste, odor, touch, and sound, form our perception of sensation, and we established that sensations are the conscious and subconscious response to sensory stimuli. In the second part of this topic we focused on Satiety, we talked about the sensory specific satiety and the factors that affect it, such as the amount of food consumed and the sensory stimulation. We also explained how the sensory specific satiety is a strategy for survival. A strategy that promotes variation of food, in order to enhance energy intake, maintain nutritional balance, and help with meal planning. Then the discussion moved to the factors that affect our choice of food. In particular, mood, labelling, the environment, our hedonic responses, and our previous exposure to the food. We explained in detail that we don’t choose food only to satisfy our need for energy. We choose specific food because they make us feel a certain way, because the name of the dish is attractive. Because of the halo effect, because of our innate preferences, and of our overall previous experience.