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 distance so we can sense them. We'll talk about actually how those molecules they are asymmetric. They are chiral molecules with different shape so that they will elicit different kinds of response. We also talked about how come sometimes when we smell something for long time we would develop fatigue, and of course how the solubility of this odor molecule in water or oil is going to affect 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. Think 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're still far away. So, if they give out some of the smell that allow 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 or the same species. Now, all these odor molecules, in fact, they have one common property because they are airborne. So that means that usually they would be very volatile and they are having a very small molecular weight. Ah, usually, maybe 100 to 150. Now, these are the kind of molecules that can be easily go into the air and defuse over a distance. Now, I use 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. These aroma molecules 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 will have very high concentration of the red molecules but, on the other side between the barrier. In fact, we will have a very low concentration of the red molecule, but in reverse. On one end, we have a 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 happened if diffusion occur. What it will do is that the high concentration molecules, they would move to the region with low concentration. So what you see is that in this particular animation, you see the red balls all go over to the side with no red ball and the white balls they would all go towards the side with no white balls. And as the result at the end, they would even out and they would become very homogenous. 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 this 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 them most. The first one is kinetic energy. 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 they have more energy temperature refers to kinetic energy. That means they move more more quickly. And as a result, they would be able to travel for a 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 particular 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 an opposite charge. And as a result, they aggregate together and they become clumps and aggregates. And they don't diffuse very easily. And finally, the property of the medium that these molecules travel through is also very important. Think about smell. These are aroma. And these aroma molecules, they need to travel through the air and if the air is dense, or actually is of very low density or so, then the air, this aroma molecule will be able to travel through it with varies 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 diffusion rate? So that we're going to show you. Now another important property of this aroma is about the 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 form very easily. Thing about that, we're smelling something in the air if all this molecules they are in a solution in lets say in a soup. Or in some sort like a sauce, how can you smell it? The molecule must be able to come out of the soup or come out of the sauce. Now, 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 the ice cream, so what you have is that this molecule that 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, can you usually smell ice cream with a particular 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 odor molecules around is one thing. We also need to think about we smell 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, 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 can't recognize or differentiate one versus the other. So, that is so called a recognition threshold. Well 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 Acetaldehyede, a very fruity smell. Or it is the amine, which is a very fishy smell. Or something like a rotten egg hydrogen sulfide. Very pungent an irritating smell. So these are all smells? but if you look at what they are pay attention here. Molecular 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 at very low concentration such as 0.004 part 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, 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 ten folds higher concentration in order to be able to tell what exactly it is. Now, these are very important features that we need you recognize. Small molecules, we can detect something and we can differentiate them. As one versus the other. Now, so these about the property of the aroma.
