7.6: Why does the apple turn brown?

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

The Science of Gastronomy

195 ratings

The Hong Kong University of Science and Technology

The Science of Gastronomy

195 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

So now we have a rainbow, I want you to play around with, to decorate your dish.

All the colors, but then there are some color we may not want to see.

Browning. You may recognize that when you're eating

vegetables. when your vegetable turns old they become

yellowish and gradually they become brown.

And sometimes for some of the fruits or vegetables that when you cut them open,

when you leave them there for awhile, they turn brown.

So this is a process we call browning. So what happens is that when all this

vegetable or fruits they were like being bitten on, or you have cut them, at the

edge where you cut them usually you see this browning, very obvious.

What's going on in fact that is referring to a situation that, that is damaged.

And when the damage occurs this browning enzyme is going to come in.

I'll tell you what the enzyme is. It will come in and it will convert some

of the components in these plant cells into a product which is dark in color.

And they are brownish, and so therefore they aggregate, and you see them

deposited in the cell. And so as a result, you can see this

particular apple on one end. Actually you have a freshly cut apple

white in color, but you leave them there for long enough.

They turn brown. Now so what is going on?

In fact what happened, browning is referred to, a condition we call

oxidation of compounds related to phenol. And these phenols.

Or we call them hydroxybenzenes. And what happen is that when the oxygen

comes in they are going to modify these two growths and as a result this product

would eventually generate this dark pigment.

Very similar to what we have as melanin, dark in color.

So, what do we do is that when we see plants for example in banana a lot of

time we leave them there for a while over a time in a few days or so they greatly

turn very dark. And not just vegetables.

Sometimes even for animal. You remember when you buy some shrimps

from the market, when you leave them there for long enough, you find that they

start to develop all this darkened area in the head.

That's also because of this phenolic component, which is being converted into

dark pigments. Now, so how can we prevent them?

It's relatively easy. We need to understand what's going on.

The browning enzyme involved in fact is an enzyme called polyphenoloxidase.

This polyphenoloxidase, its function is to add the oxygen group onto this phenol

compound. And because they need to add oxygen to

the compound, that means oxygen needs to be provided, so we have an enzyme and we

have oxygen. Add onto the phenol, the phenol becomes

dark, so it is very easy to think about how to eliminate this possibility, either

you don't allow enzyme to work or you don't allow oxygen to be available.

So if you can do that, limiting the supply of oxygen or inhibiting the

enzyme, you will be able to prevent browning.

So how can we do that? Very easy.

We notice that enzyme, these are proteins.

Enzyme can be very easily denatured by your increase, by increasing the

temperature. So we can trick this plant by heating

them. Think about that.

You, have you ever cooked soup with apples?

And after the cooking you find that apple never turned brown.

Because the enzyme has been destroyed. Okay.

So, of course, you're not going to cook the apple at this point.

What you do is that you can subject it to blanching.

Blanching means that you simply dip the apple.

And to very hot water for a very brief moment, before it get cooked.

Basically it kills the enzyme. Now so with that, of course with the

cooking sometimes it also help to generate some of the flavor of this plan

or vegetable. And at the same time it softened a little

bit the texture. So it's changing multiple thing.

If you don't want to use the boiling water you can do some other thing.

For example, we say enzymatic reaction. How can we inhibit this enzyme from

functioning? Very easy.

Because we notice enzyme. In fact you need to recognize this oxygen

and you need to recognize the substrate so that they would go through the

reaction. But then, sodium chloride, this is a very

commonly found component in the kitchen. It turns out that it bind onto this

enzyme in something which is called non-competitive way, meaning that, once

it bind with the enzyme, it doesn't want to go away.

So, if it's not going away, basically the enzyme is totally inactivated.

Even though you have plenty of oxygen around.

You have plenty of this phenolic compound around.

They are not going to become brittle. So, were if that so, you may recognize a

lot of people. After they prepared.

Or cut open the apple. They like to dip into some water, but of

course, you need to remember that when you dip it into salt water, you are

having some salty taste. So this salty taste, make sure it's not

strong, otherwise, it would generate taste and taste into action.

And, of course, there's other way that you can do.

for example, we say that this reaction, in fact, it need to take place and, it

depends on actually what kind of pH that it would alter acidity, is going to

affect the rate of this reaction. So we noticed that when you put it in a

very acidic condition, such as you try in a malic acid, tartaric acid or citric

acid, when the pH, whenever it's below 5 in fact there's been polyphenoloxidase,

it is not going to be very active. So once you put in acid.

They are not going to have any reaction going on.

So they're not going to be darkened. Now, very easily you can think about

that. You want to be acidic.

You also want to gather, get rid of the oxygen.

What would be the better way? Vitamin C.

You all know that vitamin c is a antioxidant.

It prevents oxygen from oxidizing it's product.

And at the same time it provided acidic condition.

So, next time, if you have the convenience and you have the vitamin C at

home, the best way to preserve a browning apple is to dip them into vitamin C.

And it may actually provide you additional protection for your health.

Now and of course one area that we can focus on is that well, eliminate oxygen.

So that's what the people do. a lot of people they would like to cut

the fruit and before they get brown, dip them into syrup.

And in the syrup you can protect them from in direct contact with the oxygen in

the atmosphere and, of course, a better way to do that is you put them into the

can and take a vacuum and remove all the oxygens, although you need to recognize

that whether you can really 100% eliminate all the oxygens.

If you leave them there for long enough, they can still go with the browning

process. So, with all these mashes, essentially,

we do have a color, we mean not one to see in the vegetable, browning, but there

are plenty of way for you to take care of it so that they remain very colorful.

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