[MUSIC] In this following series of lectures, we're going to talk about how we smell and what are we smelling. Now, what are we going to focus on? We're going to talk about how come we can smell, what actually is aroma and how they diffuse over a distance so that we can sense them? We'll talk about actually how those molecules, they are asymmetric. They are chiral molecule were of different shape so that they would elicit a different kind of response. We also talk about how come sometimes when we smell something for a long time, we develop fatigue? And, of course, how the solubility of this odor molecule in water or oil affects our perception of this flavor. And, of course, a very important element of it, we want to understand how taste and aroma, they would be able to interact with each other. And a lot of times we take advantage of this interaction to make a good dish. Now, let's look at what an odor is. In fact, we smell a particular aroma, an odor molecule, because we want to sense our world. The thing about that, when we talk about taste, something is in your mouth already. But before you have that food in your mouth you want to understand what it is before you put it in your mouth. So sometimes we develop a sensation to be able to capture some of the information about the food. When they are still far away, so if they give out some of the smell that allows you to differentiate them. So therefore, we are sampling the quality of air, we are looking for potential smell of food. Maybe actually we want to stay away from danger, or to identify individuals of your same species. Now, all these odor molecules, in fact they have one common property because they are airborne so that means the use of it they would be very volatile and they are having a very small molecular weight. Small molecular weight, I'm referring to a range of usually less than 200, usually maybe 100 to 150. Now, these are the kind of molecules that can be easily go into the air and diffuse over distance. Now, I used the word diffuse or diffusion. It is a very important process for the spread of aroma. Why? Because, in fact, it represents how molecules they will be able to travel from one region this aroma molecule go to another place in a long distance. Usually, why do they do that? They go through a process called diffusion that they go from an area which is having very high concentration to a very low concentration. Now, what we have in the following animation that we'll show you that on this particular graph, we will have two type of molecules. On one end, you have very high concentration of the rat molecules but on the other side, between the barrier, in fact, we would have a very low concentration of the brown molecule. But in reverse, on one hand, we have the very high concentration of the white molecules and on the other side they would have a very low concentration of the white molecules. So what happen if diffusion occur, what they will do is that, the high concentration molecules, they will move to the region with low concentration. So what you see is that in this public animation, you see the red balls all go over to the side with no red ball and the white ball stay all go toward the side with no white balls and as a result, at the end, they would even out and they would become very homogeneous. That's what diffusion is about. Well, in fact, there's a lot of factors that would affect the rate of diffusion. Or, how fast these concentration of a particular molecule, they would move over to the lower concentration part and reach equilibrium. So there are mostly four factors that is influencing the most. The first one is about kinetic energy. You recall that kinetic energy refers to the temperature. So what I mean is that if you have higher temperature, the molecules would have more energy. If there's more energy, temperature refers to kinetic energy. That means they move more quickly and as a result, they would be able to travel for longer distance and they diffuse faster. The second element of it is the size of the molecule. We find that the smaller the molecule they are, the lighter they are, they can travel through a medium, much faster than molecules of a larger size or high molecular weight. And, of course, the charges of the molecule matters, too. Why? Because when a molecule or a particle is carrying charges, they would easily be attached to a particular surface of the opposite charge. And as a result, they aggregate together and they become and aggregates. And they don't diffuse very easily. And finally, the property of the medium that these molecule travel through, is also very important. Think about smell, these are aroma. And this aroma molecule that need to travel through the air. And if the air is dense or actually is a very low density or so then this aroma molecule, they would be able to travel through it with berries, too. And later on, we're going to show you some examples that if the medium is very viscous, or the medium is less viscous, how it affects the seclusion rate. So that, we're going to show you. Now, another important property of this aroma is about this so-called volatility. Now, as I pointed out, volatility refers to the property of these molecules that whether they would be able to convert into a gaseous foam very easily. Think about that. We're smelling something in the air, if all these molecules, they are in a solution. Let's say in a soup or in some sort of sauce. How can you smell it? The molecule must be able to come out of the soup, or come out of the sauce. So therefore, if something is volatile, when you raise the temperature, they would go into the gaseous form very easily. So that would help them to travel far and you would smell them very easily. And, of course, when you have low temperature, such as, as cold as ice cream, so what you have is that these molecules, basically, they are getting trapped and they don't come out of whatever food very easily. So think about that, when you try to smell something good, do you usually smelled ice cream with a popular flavor or do you usually see a hot dish being brought in front of you and then you can smell it? Of course, hot dish is much easier for you to sense what kind of odor it has. Now, on the other hand, having the older molecules around is one thing. We also need to think about we smelled them because we can detect them, so what exactly are we detecting? We have two concepts, one is about the detection limit. What I mean is that how much, or how many molecules do we have there, so that we would be able to detect it or to sense it, the other one is that, yes, you can sense it but can you tell what exactly it is? You can sense there's something there but you can't recognize or differentiate one versus the other so that is a so called recognition threshold. Now, if you look at that, I try to show you in this particular graph, there are a whole bunch of aroma molecules. So these are like acetaldehyde, which is a very fruity smell. Or it is the amine, which is a very fishy smell. Or something like a rotting egg. Hydrogen sulfide. Very pungent, irritating smell. So these are all smells, but if you look at what they are, pay attention here. Molecule weight, they are all very small from 30 something to 100 something. Very small molecule. Remember, below 200. The second thing is that the threshold of detection, you may find that, well, in fact, we can detect this kind of molecule when they are very low concentration. Such as syrup on full parts per million. Really very dilute. We can tell that there is something. But when we ask, would you be able to recognize what exactly it is? Then, you can detect it at that low concentration, but you can't tell what it is until it reach a level at least 50 fold higher. The same thing happened for the other fishy smell. So you need to have a higher concentration to be able to tell them apart. The same thing happened here for hydrogen sulfide, you need to have at least tenfold higher concentration in order to be able to tell what exactly it is. Now, these are very important features that we need to recognize, small molecules, we can detect something and we can differentiate them as one versus the other. Now, these about the property of aroma.
