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
4.4: Why do orange and lemon smell different?
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From the course by The Hong Kong University of Science and Technology
The Science of Gastronomy
195 ratings
From the lesson
Module 3 and Module 4
This week, we will talk about how flavor and aroma of food affects our perception of taste of food.
Meet the Instructors
King L. ChowProfessor
Division of Life Science
Lam Lung YeungAssociate 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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