After going through different tastes, we wonder whether we'll be able to taste something very different. We ask, can we taste the temperature? Okay, here it is, what's the temperature? We say that temperature, somehow, is related with the spicy taste. Why do we like to have foods which cause us pain? In fact, the spicy taste is something that's related to pain. Because the spicy foods, such as the chili pepper, paprika, all these, in fact, they have a chemical which is called capsaicin. Capsaicin actually binds with a receptor, which we call the VR1, the capsaicin receptor, and when it is activated we feel pain. That's how when people hit you, you feel pain because of this receptor being activated. Capsaicin, very interestingly, it is a molecule which is fat soluble. So you need to wonder, well, when you are eating something which is spicy, how come, all this spicy food, they, always, they are coming in together with some fatty food? Because they need to have the fat to dissolve this capsaicin. Now, what would you do, then, in that case, if you have eaten some spicy food and you feel very hot and you feel the pain in your tongue? So, what do you do? You want to take away capsaicin, so what would the best way to take away the capsaicin? By taking something with lipid, something with fat, not just drinking water. So, the best way to do it is to drink some milk where they carry lipid. Now, what happened is that, in fact, for this particular capsaicin receptor, when it's activated, it can be activated by the spicy food capsaicin. Also, it can be activated because the temperature increase. What it leads to is, with this activation, they are going to allow the neurons to start firing. And when they fire, it will sensitize this response and they feel that it's very hot. Now, all this process involve is the transmission of the signal from this neuron to the brain. On the other hand, we think about, well, you can sense high temperature, the spiciness of it. Can we sense something which is the opposite, for example, something we call icy cool? The situation is that when you're eating something with menthol. Now menthol, in fact, it can be perceived or read by another receptor. Forget about what the name is, TRPM8, this is a receptor which is sensitive to temperature. Anything that when the temperature is below 26 degree, this particular receptor will be activated. And when they're activated, they will send out a signal to the neuron and all the way to the brain. Now, then you'll wonder, well, what happen if I don't have such a receptor? Does it mean that I won't feel whether it's cool or not cool? The answer is, well, not really. People have done some experiment to have animals with this particular gene mutated. Would they sense whether it's cold or not? The answer is that once it dropped below like 15 degree, they can tell that it's cold. So what it mean is that this receptor for sensing this icy cool menthol feeling, in fact, is only sensing a range of the temperature. Below that there must be some other receptor for us to perceive whether it's cold or not cold. Now, all of these related were of the channel, and when it occur or it activated, you sense that, actually, that something too cold. And so they need to have other receptors to tell you that you are in danger, that it is at a point that it's too cold, more than just a regular range for food. Well, now, knowing that for the sensation of this spicy food and this menthol is relying on the two receptor, these receptors, they are ion channels. Now, the interesting thing is that these two channel, when they are firing, in fact, they would interfere each other. The most interesting part of it is, in fact, we know that by drinking alcohol, alcohol would have an effect on this two channel. And the effect is quite opposite, ethanol, in fact, it can potentiate activity of this spicy receptor. So what happen is that if you are drinking alcohol, you may find that some of the spicy food, they are even more spicy. When this ethanol is added, it would inhibit the effect of this cold receptor, so, again, because of this opposite effect. So think about that in the future when you're eating something which is spicy or that is cool, whether you want to put alcohol together with it. Now so, therefore, we need to know sometimes the temperature matters in terms of the perception of taste. Now, we need to ask ourselves for a large proportion of human beings, we find that the perception of temperature is important. And sometimes this temperature can also cross regulate and interfere the perception of other tastes. Let me give you a few example, and you can do experiment at home. The first one is, well, have you ever tried to drink some warm water? Try that, when you put your tip of your tongue in 35 degree warm water, you'll find that, in fact, you can develop a slight sense of sweetness. Why, because at such a temperature, in fact, your receptors for the sweetness will also be sensitized. At the same time, when you cool down, let's say you cool it down to 5 degrees Celsius. You'll find that actually your tongue will perceive some sense of sourness. So that's why when you're drinking cold water, that people will find that it's a little bit sour. Now so, therefore, what happen is that you need to bear in mind that sometimes you don't necessarily need to put sugar in a particular solution for you to drink. Changing the temperature will make a difference. You don't need to put acid to generate a sourness, cool down the temperature, it helps. So, certainly, you can try that at home and see whether it works. Now, this change is largely because of the activation of this receptor for the taste by the temperature. And it's because of the cross talk of some of this receptor for the temperature. High temperature or low temperature, they would interfere each other. And, of course, we would wonder, well, would it really matter when we are cooking something? Can we simply play around with this temperature and change the taste? I would say very few cooks, they would like to utilize this method to create taste. But then you need to bear in mind that by moderating the temperature some food when you are serving it at a higher temperature, some food you are serving it at a low temperature, it makes a difference in terms of how they taste. A little bit more sweet, or a little bit more sour. Now, the important element then is we want to talk about if you want to perceive the taste, in fact, there are a whole bunch of parameters you can play around with. We can talk about the effect of saliva, we can talk about how to change the concentration of the taste molecules. Whether we would be able to suppress it to make it less perceived. And we can make different kind of taste together to generate something called mixture suppression. Or sometimes we generate something called sensory adaptation. And even more interesting, we say that when you try something as a mixture, when you take away some of this mixture component, in fact, you would perceive a particular taste very differently. Now, we're going to go through that one at a time. Let's first talk about what is saliva. Saliva is actually produced and secreted by our saliva gland, which is located inside our head. And the basic secretory unit is something in the gland which we call acini. This is a cluster of cell, which they are capable of producing a lot of the material that I'm going to cover a little bit later. And this material is going to be secreted, and what did they contain? They have water in them, they have electrolytes, they have mucus, and they have various type of protein including enzyme, and will flow into your buccal cavity. So, what happen is that when they are going into your mouth, they will do a few things. First of all, let's look at what these salivary glands they are producing. In fact, we have three different salivary glands in our head. We have the parotid gland, we have the submaxillary or mandibular gland, we have the sublingual gland. What do they do? The parotid gland actually is somewhere here, at this position, so what they produce is something very watery. Water, okay, very watery with a little bit of protein. Protein, we say serous, and, a lot of time, actually they carries enzymes. For the mandibular gland or submaxillary gland, what they do is, they produce something, it's serous and mucous. What is mucous? Mucous, actually, is mostly glycoprotein, they give you a very mucous, slimy like feeling. And, again, they do have protein, and this protein, they are enzymes. And also, the sublingual gland, they are secreting, predominantly, the mucous, glycoprotein, to give you some feeling about the texture inside your mouth. So, you can certainly use your tongue now, and try to sense how this kind of product is like. So, where are they located? They're located, parotid gland, here, and the sublingual gland is right under your tongue, and the submandibular one is around this area. So, we have three different glands, so what do they do? Now, the saliva is very important. You remember that we say a lot of this taste molecule, they are water soluble, they're non-volatile. And they need to be dissolved into the water because they are also polar. It's very easy, they are soluble, then they go into the water. So what this saliva allowed me to do is that, they would have the transport, the taste substance to the taste receptor. What it mean is that you're eating some sweet, you're eating some meat, amino acid, glucose. They need to come out, they need to dissolve into a watery medium before they would go into the taste bud and touch the sensory cell. Now, so that's important for the transportation, the second part is that they acts as a solvent. They want to dissolve the taste molecule, and also they serve for protection. You remember they are having mucus, they are proteinaceous. So what I mean is that they lubricate the surface so that it protect the taste bud from rupturing or, in contact, having the abrasive contact of the food with the taste cell. And also, with the water there, in fact, the facilitate, the binding of this taste molecule with the receptor better because the taste receptor, they need to contact external environment. And the taste molecule, when they come in, they need to have some sort of like a fitting. And that provides an environment for this kind of binding. So, this is the purpose of the saliva, particularly for the long watery part. It provides the lubrication and the binding purpose so that it would allows us to put all this food originally maybe in one big piece, but then, during the mastication, we would be able to move them and smoothen them to allow them to move around in your oral cavity. It solubilizes dry food, because sometimes when we eat food such as biscuits, originally they're very dry. You need to have some watery component to come in to soften them. And, of course, it is also for the oral hygiene. Knowing that, in fact, we have three gland that continue to secret all the saliva into your mouth. What it allows to do is that it help to constantly flush away all the food debris and keep our mouth clean. Many of you have come across dragon breath. If you don't have any active movement of your saliva, in fact, debris will accumulate and it would develop a bad smell. Now, they also initiate some starch digestion because when they are in a serous form, that means they have a lot of protein. These protein are enzymes, and these enzymes help us to digest some of the component. Many of you have encountered that if you're eating something which is rich in carbohydrate, keep masticating it or biting it in your mouth, you'll find that after a while you find that they taste is a little bit sweet. Why, because some of the sugar has been converted into simple sugar such as glucose so they taste sweet. So that means digestion's started to take place right in your mouth. And, of course, saliva serve other purposes in other animals too. Such in the ruminants, they need to somehow provide the buffering of the solutions of all the food when they're regurgitated. Sometimes we all use the tongue to release the heat, such as all the dogs will do. Now having that, we are going to go to another demonstration, which is to show you what's the importance of how saliva is going to affect the perception. And I hope that this demonstration to illustrate the function of saliva well.
