4.4: Why do orange and lemon smell different?

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En provenance du cours de The Hong Kong University of Science and Technology

The Science of Gastronomy

251 note

The Hong Kong University of Science and Technology

The Science of Gastronomy

251 note

This course introduces a number of basic scientific principles underpinning the methodology of cooking, food preparation and the enjoyment of food. All topics covered have a strong basis in biology, chemistry, and physics application. Among others, they include the consumption of cooked food, the physiological and evolutionary implication of the senses, geographic and cultural influences on food, and the rationale behind food preparation. We will also discuss issues such as coupling of senses to improve sense stimulation; altering flavor by chemical means; and modification of the coloration to improve the appearance of dishes. Following the video demonstrations of the scientific principles of cooking, you will learn to recognize the key ingredients and their combinations for preparing good healthy food. At the end of this course, you will be able to: - appreciate the scientific basis of various recipes; - develop your own recipes by integrating some of the scientific principles into new dishes; - recognize the influence of the material world on human perception from the different senses; - appreciate the art of integrating science into cooking and dining. Important Note: This course is not designed for people with special dietary needs such as vegetarian, diabetic, and gluten-free diets. If you feel uncomfortable with any part of the assignments or activities of this course, you can substitute some of the ingredients or ask friends and family members to help with the tasting of your assignments. Alternatively, you may skip that specific assignment provided that you have fulfilled all other qualifying requirement to pass the course. Course Overview video: https://youtu.be/H5vlaR0_X2I

À partir de la leçon

Module 3 and Module 4

This week, we will talk about how flavor and aroma of food affects our perception of taste of food.

4.0: The Sense of Smell - Topic Outline0:27

4.1: What are we smelling?9:57

4.2: Demonstration – Factors affecting diffusion rate4:37

4.3: How do I smell coffee?10:52

4.4: Why do orange and lemon smell different?9:41

4.5: Why can’t I taste the “strawberry”?11:08

4.6: Demonstration – How does aroma affect our taste?4:35

Assignment 5 – Instruction Video2:24

Assignment 5 – Result Sharing1:39

4.7: Smelling good makes it tastes better!14:03

4.8: Demonstration – It is all about congruency!3:25

4.9: The Sense of Smell - Topic Summary1:01

Rencontrer les enseignants

King L. Chow
Professor
Division of Life Science
Lam Lung Yeung
Associate Professor
Department of Chemistry

What do we have? Why do orange and lemon they smell

different. That's what we're going to look at.

In fact, we need to remember for all the molecules that we deal with, they have

some sort of asymmetry. And in Chemistry, we use the term to call

chirality. It refers to molecules that they are

mirror image of each other. So, think about that, we have two hands,

they look alike, but actually they are mirror image of each other.

The same thing when we're dealing with some molecules, we can have this

particular molecule and this particular molecule.

In fact, they have the same composition. But in terms of how they are formed, they

are simply mirror image of each other. So, with that in fact, do we have the

ability to tell them apart? So, that's exactly what it is about,

orange and lemon. Now, with all these molecules being

mirror image of each other, it tells us that molecules have one form versus their

mirror image form. And in the living world, in fact, a lot

of molecules they have such kind of unique property.

Such as when we are working with sugar. We find that most of the sugar that we're

dealing with, in fact, they are right handed.

We call the dextro sugar. And for most of the amino acids that make

up proteins, these are left handed, we call levo.

Alright? So, if we are made of amino acid,

protein, and with all these chemicals, so that means we are also asymmetric in

terms of our structure. So, if they are asymmetric

would we be able to tell them apart such as lemon and orange.

So here it is, we say that proteins are made asymmetric because they're made of

chiral amino acid. So, here what it is?

Molecules can be mirror image of each other, so we need to able to tell them

apart. When these molecules, they are odor

molecules, we need to use our receptor. And this receptor should be able to

interact with this molecule in one way, but if I'm providing a mirror image of

it, then it would not be able to recognize.

How to make an analogy of that? You all have gloves.

Think about that. If you have a glove which is for your left

hand, call that glove is a odor receptor, receptor.

Now, if you put your right hand into that glove, do you think you'll be able to

fit? The answer is, no, it won't fit.

So, essentially, the detection of a particular odor by the receptor is just like your hand

and the glove. They need to match when they are the same

format, whether it's right-handed or left-handed.

Now, so, what happened between lemon and orange?

In fact, they emit the same kind of molecule due to the biochemical synthesis

they produce the same kind of chemical called limonene.

But, lemons, they produce one version and oranges produce the mirror image of the

same version. So, what happen is that certainly you

have that experience when you smelled lemon and orange.

They are certainly different. And you can tell them apart.

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.

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