Proteins - Food Properties

Question 1

What is a denatured protein molecule?

Answer 1

The chemical bonds have been broken and the protein molecule has unfolded and changed shape.

Question 2

What are the four causes of denaturation?

Answer 2

Heat
Acid
Air bubbles
Mechanical agitation

Question 3

Give an example of heat causing denaturation.

Answer 3

Frying an egg

Question 4

Give an example of acid causing denaturation.

Answer 4

Adding lemon juice to cream in a lemon flan.

Question 5

Give an example of air bubbles causing denaturation.

Answer 5

A whisked sponge mixture.

Question 6

Give an example of mechanical agitation causing denaturation.

Answer 6

Whisking egg whites for meringue.

Question 7

What is coagulation?

Answer 7

The joining together of lots of denatured protein molecules.

Question 8

Why does coagulation happen?

Answer 8

Denatured protein molecules are larger and take up more space, so they knock into each other.

Question 9

What happens as the denatured protein molecules coagulate?

Answer 9

They trap and hold water from the food in pockets between them.

Question 10

Why might eggs be used when making a beef burger?

Answer 10

The proteins in the eggs are used to hold other ingredients together due to their ability to coagulate.

Question 11

What happens if a food containing protein is over cooked?

Answer 11

The coagulated protein molecules tighten up and squeeze out any water they were holding.

Question 12

What is added to baked products, e.g. bread, to give it a light an open texture?

Answer 12

A raising agent.

Question 13

How does a raising agent create the desired texture in baked products?

Answer 13

Introduces lots of gas bubbles, which will expand when the mixture is baked in the oven.

Question 14

Why are baked mixtures able to stretch and rise?

Answer 14

The protein called gluten.

Question 15

How is gluten formed?

Answer 15

It is made up of two different proteins: glutenin and gliadin, which combine when liquid is added to flour to form a dough.

Question 16

What is plasticity in dough?

Answer 16

The dough can be stretched and shaped during kneading.

Question 17

What is elasticity in dough?

Answer 17

It will shrink back when you stop stretching and shaping it.

Question 18

Why is dough elastic?

Answer 18

Long gluten molecules in the dough are coiled and bend in different places along their length. As you knead and stretch the dough, the coils and bends straighten out. When you stop, they relax and gradually return to their original shape.

Question 19

Why is plasticity an ideal quality for bread-making?

Answer 19

The dough needs to be able to stretch when the CO2 bubbles produced by the yeast make the dough expand and rise.

Question 20

Why would you leave dough to rest before you bake it?

Answer 20

Resting allows the stretched gluten molecules to relax, which is important if you are using the dough to line a flan case. As the pastry rests, the stretched coils of gluten gradually return to their original shape.

Question 21

What would happen if you didn’t leave the dough to rest in a shortcrust pastry recipe?

Answer 21

If the dough has not been rested, the gluten molecules remain stretched and the heat of the oven will cause them to shrink rapidly. The filling will then leak out of the flan.

Question 22

How are foams formed?

Answer 22

When gases (usually air) are trapped inside a liquid to form a gas-in-liquid foam.

Question 23

Why can egg white hold so much air?

Answer 23

Egg white protein can stretch.

Question 24

How are the air bubbles trapped in the gas-in-liquid foam?

Answer 24

The action of the whisk rotating very fast.

Question 25

What does the action of the whisk do (apart from trapping the air bubbles)?

Answer 25

Causes the egg white protein molecules to denature by breaking the bonds that hold them together.

Question 26

After the protein molecules in the meringue foam denature and coagulate, what do they do?

Answer 26

They surround the air bubbles and make a wall around them. This holds the air bubbles and water in place so the foam is stabilised.

Question 27

What will happen if there is any egg yolk or traces of fat in the bowl when making a foam?

Answer 27

The foam will not form properly.

Question 28

What happens if you over whisk the egg whites when making meringue?

Answer 28

The foam will begin to collapse and become watery and loose.

Question 29

Why would the foam become watery and loose?

Answer 29

Over-whisking makes the coagulated protein molecules bond together too tightly and they squeeze out the water they were holding.

Question 30

How does a gas-in-liquid foam become a solid foam?

Answer 30

When you heat it in the oven, more protein molecules in the egg white coagulate and more water is driven out. The foam sets.

Question 31

How is a whisked sponge mixture made?

Answer 31

The eggs and sugar are whisked together until a gas-in-liquid foam is made. The air bubbles produce a light, spongy texture, which sets in the oven.