7.3: What gives green color to plants?

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From the course by The Hong Kong University of Science and Technology

The Science of Gastronomy

263 ratings

The Hong Kong University of Science and Technology

The Science of Gastronomy

263 ratings

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

From the lesson

Module 7 and Module 8

This week, we will look at how fruits and vegetables can enhance the quality in cooking and to learn about the properties of meat.

7.0: Fruits and Vegetables - Topic Outline0:29

7.1: What gives the rigid texture to plants?9:59

7.2: How can we manipulate the texture of plants?12:44

7.3: What gives green color to plants?12:28

7.4: Why some plants are not green in color?7:03

7.5: Demonstration – Manipulating the color of red cabbage3:37

7.6: Why does the apple turn brown?7:34

7.7: Demonstration – What is the best way to prevent browning?5:55

7.8: What are the prominent tastes in fruits and vegetables?8:32

Assignment 8 – Instruction Video2:51

Assignment 8 – Result Sharing4:23

7.9: Fruits and Vegetables - Topic Summary1:27

Meet the Instructors

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

Having vegetables and fruits in the dish,

it helps us a lot to generate a lot of different colors.

So, where does that color come from?

It is coming from the plant where they have different kinds of organelles or

small particles inside the cells which we call chloroplasts, or chromoplasts.

And of course, also in the vacuole.

Now usually this kind of color, they are pigmented molecules in these particles and

what they have is that remember what about light.

In fact, we see something which is green.

It is not because they are emitting green light, but what actually they

are is that they absorb all the blue and the red and they reflect the green.

And so when we have something which is red, they have soft green and

the blue they reflect the red.

So, we say that in these organelles they have some pigment such as chlorophyll.

The chlorophyll is green, because they absorb all the red and

blue light and then reflect only the green one.

Now and in fact, how are these organelle like?

For example, under left-hand side.

In fact, we do have some orange vegetables.

What is it?

It's carrots.

When you look at the source, they have a lot all of the small particles in them.

In fact, those are the particles,

the organelle that contributes to the orange color.

How about a green plant?

Well, it looks like this.

In fact, they are made of cells that they are densely populated with all this

green chloroplast.

That is why we see them as green.

Now so therefore, when we start talking about pigment,

I want to cover a few of them.

The first one is chlorophyll.

That usually be associate with the vegetable, which are green.

And of course, there are also other ones, such as the orange, red one,

the carotenoid.

And also we have the purple, red or the pale yellow,

such as anthocyanins and anthoxanthins.

And of course, at the end, we also have red and yellow betalains.

Now, let's look at how chlorophyll gives us a green color.

Now this seems to be a very complicated structure, but

I don't need you to really remember what the structure is like.

I simply want you to remember two things.

One is on the left-hand side,

you may find that there is a very long chain of carbons extending out.

I simply call that a tail.

That particular tail.

Because having the long chain of this carbon connected to it,

they are very hydrophobic.

And therefore, they are very fat-soluble.

And so what happen is that this particular tail allowed this molecule

to be anchored on the cell membrane, which is made of lipid.

And on the right-hand side,

what you see is that there is like a flat like a planar structure like this.

In this, we have this structure which we call porphyrin.

And this porphyrin in it, I want you to focus inside.

There is a metal ion, which is magnesium.

And in fact,

this is the magnesium that gives us the particular nature of this molecule.

Your amid, the green or reflect the green color.

Now usually, this set of chlorophyll they are found in the chloroplast that I

indicated to you earlier.

They are small particles inside the green plants.

So what they have is that they would be [INAUDIBLE] altered in terms of

their property, if you subject these green plants to the cooking.

For example, you can have this particular tail being taken out or

you can change the content of that porphyrin.

Now, n fact, if you focus on the porphyrin,

I told you that this is the magnesium iron that gave us that green color.

If you change it to the other ion such as you put a couple of salt,

you can easily change a porphyrin to give it different color.

And in fact, that is the way to help people generate a lot of different dyes.

Now what happened as I said, if this molecule during the cooking,

it get modified.

So for example, the tail is removed.

So if the tails removed as I said, the tail is required for

the anchorage of this molecule onto the membrane.

So now, it is no longer anchored on the membrane.

So now what I mean is the porphyrin becomes water-soluble.

When it's water-soluble, what can be done?

That means it will be released from the cell.

So in such a case, they would all go into the cell and what would cleave

this popular long chamber or the tail is at an acidic condition or

an alkaline condition already within itself in that.

So that when this tail has been removed, this porphyrin would go outside of

the cell and become water-soluble and they stay there.

And so also,

there was enzymes called chlorophyllase which helped to cleave that.

Now, so what happened?

If this molecule,

this porphyrin, the green part get released, what would be the outcome?

So if you're cooking some vegetable and this particular tail get cleaved and

the porphyrin goes into the solution.

So, that's on the left before you cook the vegetable and

on the right after you cook the vegetable.

You'll find that the soup turned green,

because all the porphyrin now goes into the medium.

Now, also as pointed out.

Well, using acid, we can also cleave this particular tail.

So what happen is that well, we can use enzyme and all this.

We can remove this magnesium ion from the center of the molecule.

But on the other hand, when is acidic,

this magnesium ion can also be kicked out of that porphyrin.

Why?

Because when something is acidic, such as vinegar or some citrus food or

so, then these kind of acidic medium, they will have a lot of hydrogen ions.

This hydrogen ion can easily come in.

They kick out the magnesium.

And when magnesium kicks out, they lost their particular color.

And so even though it's water-soluble in the acid,

even though the tail has been cleaved, the porphyrin now goes in the solution.

But since they don't have the magnesium associated with it.

So therefore, they are still relatively clear.

They are not giving any specific color.

Now, so to use that as an illustration,

you remember that I was showing to you that how a vegetable.

When I put it in an acidic or the tap water or in the alkaline condition,

how did they look like?

You remember that when I said that whoa, if you put it in tab water,

they look normal.

That's what we see most of the time in the kitchen.

But if you cook them in the vinegar, you may notice that this polygala plant,

in fact, they are more yellowish.

Why is that?

No matter whether this porphyrin from the chlorophyll is released from the plant or

whether they stay inside the plant, the magnesium iron has been kicked out.

So therefore, a lot of the chlorophyll they lost their color.

So, they are no longer as green.

And if you remember that when I showed you this

particular vegetable, I cooked it in vinegar.

I show you that, it's still very rigid.

But on the other hand, you notice that its color is relatively yellowish and

this yellowish color is because of the acid of the hydrogen ion

goes into the porphyrin to kick out the magnesium.

Now having that, they will say, well,

look at the odd courses that would lead to

the dulling of the color and the answer us yes.

Sometimes when we cook vegetable without water, such as stir-fry or sometimes you

can try that, you can cook vegetables by simply putting them into the microwave.

These plants when they are raised to a temperature higher than 60-degree,

what they find is that these kind of like chloroplast, they would be damaged.

The chlorophyll would then be exposed to whatever soup in the vacuole inside

the cell and remember what happened in the vacuole.

Usually, they are very acidic.

So this own natural acid, when they encountered chlorophyll,

they also have the hydrogen ion to come in to take out the magnesium.

So therefore, color will be dulled.

And more than that, in fact, sometimes when we freeze certain vegetable,

we put them in to make the pickled vegetables or so.

Dehydration or even for the vegetable that they are dying, or they're deteriorating.

Most of the time, it will lead to the damage of the chloroplast and

expose all this chlorophyll to the natural acid inside the cell.

And in all those cases, you'll find that plants or

vegetables when they get old, they also turn to be yellowish.

Now so therefore, understanding this is important for

us to know how we will be able to keep certain vegetables,

green when you want to present them in a dish.

So now, I'm going to introduce to you a few tricks which can be used in a kitchen.

For example, we can see that when you try to cook some vegetable,

try not to put the cover on.

Why is that?

Because when you're cooking them if you take off the cover,

these plants when they are being damaged, they will release the acid.

Some of this acid, in fact, they're volatile.

So if you open the cover, essentially this acid.

When it comes out, they can easily vaporize and

then they would not be staying with the plant.

And if they're not staying in the plant,

they don't have enough hydrogen ion to compete for the magnesium.

And so therefore, this porphyrin group, they're still green.

We also say that we can cook this plant in the alkaline water.

Because in the alkaline water, they have fewer hydrogen ion.

They're not acidic.

So therefore, they don't have the hydrogen ion to compete with the binding of

this porphyrin with the magnesium.

So as a result, then you keep them green and you remember that a moment ago that

we showed how you coat the vegetable in the baking soda solution.

That certainly they are still very, very green.

Now of course, when we cook them in alkaline,

you understand that the texture get change.

They become very mushy and also this compile cooking sometimes

destroy the vitamins in the plant including the oxidation of the vitamin C.

And so therefore,

maybe alkali will actually destroy some of the vitamins or nutrients.

More than that, in fact, sometimes we say, if you want to cook vegetable,

you dip them into a pot with a large amount of water.

Why large amount of water?

Because when you have large amount of water, so given for the vegetable,

they release some of the acid.

But they would be quickly diluted away by the water medium.

And so therefore, they don't have enough of hydrogen ion around.

So on the other hand, there are other tricks too.

We say that well, you can cook the plants for

a very short period of time within five-minute.

You noticed that a lot of people when they cook plants,

they will simply put the vegetable in boiling water for a short period of time.

And that, we call it blanching.

And so the blanching process,

what it allows you to do is that you get plant cooked.

But you don't leave it there for long enough, so

that the coloration would take place.

Now of course, we can also say that these are really practices that when you cook

vegetables, don't leave them there for long.

Whenever it's ready, serve them right away or sometimes put them in ice, so

that you prevent the heating process to continue while you're taking that

dish to the table.

And of course, we often,

we recommend people try not to put acid together with vegetables.

For example, when you're trying salad.

Even though you want to put it,

simply put it right before you eat it instead of leave it there for an hour.

And sometimes, the way that we help to eliminate that possibility is that we even

coat the vegetable with some oil.

So, think about that.

When you're having all the salad dressing, a lot of them, in fact,

they have very rich content of various type of oil such as olive oil or so.

So, that would have to shield the vegetable from direct contact with

the acid.

So with all this, essentially you'll have some good ways to keep this

vegetable green and they look beautiful.

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