Similarly, when we cook something these kind of aroma molecule, they also have
mirror image of different versions. Such in this case, it is very easy for
you to tell that it's mint because of this particular molecule being produced.
But if the same molecule is produced as a mirror image, it will smell like the
caraway. Alright.
So with that we need to understand when we now have a particular receptor,
it's sensing a molecule that comes in. They bind on top of each other.
So, what do they do? The same thing as we learn from the
taste. They act for a receptor.
So, this is the receptor, that here, they would be able to bind onto this aroma
molecule. They go through a series of signal
transaction process and, at the end, what they would do is that they will elicit
the release of calcium inside the cell. So, we call that is, excitation.
And this calcium signal would be able to pass on in this neuron as impulse along
the axon, and then dries and so, and it will send to afar.
Now, so that's how we perceive. Now, think about that, when we
perceive a particular odorant molecule. Sometimes when we have a prolonged
exposure to it, we'll find that well, we no longer detect a smell.
What happened is that in fact, it is called olfactory fatigue.
It's simply defining this particular kind of response.
And this kind of process of odor perception, they would be able to act, be
activated again, only when this odor molecule has been removed.
And later on they come on and bind to it again, and reactivate it.
Now, having that, we say that when these olfactory cells, or the receptor, being
exposed to odor molecule for prolonged period of time, they're binding site they
are, if they're not clear of this molecule.
They cannot bind any new molecule. And without any new molecule, they cannot
excite, and they cannot sense the signal. And so therefore, you simply, you don't
sense the same smell again. Now this, in fact, involve multiple
mechanisms. Some of them involve that the binding of
this particular odorant molecule by this receptor results in some of the
modification of the receptors. So, they change the shape, so they cannot
bind the same thing again, and some of the time, in fact, these
occupied receptor. They would be internalized so that they
would not be able to be put on the surface of the cells to perceive the
molecule again. Or sometimes when they are excited, it
would lead to the cell dying, such as, you remember that at one point we were
talking about the chili, the chili pepper, the capsaicin acid activating its
receptor, resulting in the cell been killed.
Now, let's now come together and look at, well what happen when you have this
volatile compound as this odorant molecule when they're in contact with
the odor receptors, what do you do? Basically, what you do is that if this is
some of the food, let's say, well, there is a lemon in front of me.
What it is that it's going to generate some of this odorant molecule or the
aroma that comes out of it, so what I do, I come in, I sniff, and then I'm testing
at the headspace around it. What kind of molecule is round?
That works. Sometimes we can smell without using our
nose. What we do is that we eat some food into
our mouth. And what we do?
When we are trying to swallow it, it creates some sort of like a vacuum inside
your oral cavity. And then, in this vacuum some of food, if
they have some odorant molecule, they come out of it.
And then, when we breathe this molecule they come out from the nasal cavity out.
And so therefore, these aroma molecule will be in contact with the olfactory
epithelium. So, for that, we would be able to sniff.
We can eat and masticate and result in the release of this molecules so that we
will be able to smell them from the back of your oral cavity and then to the nasal
cavity. Now, very importantly, we say that, well,
in that case how can we tell, actually, what kind of molecule it is and what kind
of smell it is? and we need to understand that for the
world around us. In fact, it's not made of a single smell
or a single molecule that tell us what is smelled.
a lot of the food in fact, they are made up of multiple components.
you cook a dish with some beef, you have some vegetable, you have some chicken or
so, and sauce. Each of them is going to release some of
this aroma molecule and we should be able to tell them apart very simply look a
siuation like a cola drink. In fact don't think of it as a single
compound. We can use method which is called gas
chromatography. What it allows us to do is to pass this
aroma molecule into a column with resin. That is going to retain some of this
aroma molecule and allow them to pass it through with a different rate.
And by the time that they come out, we try to monitor what kind of molecules
they are. So, here are two cola drinks and based on
this gas chromatography profiles you may find that in fact cola drinks, they smell
very much the same. But, in fact, when you look at what are
the components that come out of it, clearly they are very sharp bands, which
are present in one drink but absent in the other.
And, in fact, it defines why we prefer one drink better than the other.
It's because of these kinds of different profile of individual aroma molecules
presence in the drink.