Transcript Why do we cook food?

Functional and chemical
properties of protein
By the end of this lesson students should be able to:
• explain the term denaturation
• explain the term coagulation
• explain the term gluten formation
• explain the term foam formation.
Denaturation
A change in the structure of the protein.
Long chains of amino acids that make up proteins unfold from their coiled state.
Protein can be denatured by:
1. Acid
- Marinating
Soaking meat and vegetables in seasoned acid or salty (lemon juice, vinegar) liquid before cooking
-
Curdling
Milk develops a lumpy appearance as protein clump together when acid, heat or enzymes are added
Occurs naturally when milk ‘goes off’
Used in cheese-making to separate protein curds away from liquid whey and then pressed into cheese
2. Mechanical action
- Whisking = egg white
Protein stretches into strands and forms a structure for air to be captured
Tiny air bubbles are held together by a mesh of protein
Continued whisking produce a foam
-
Kneading
= bread-making
Protein in flour becomes stretchy which creates a permanent change
-
Pounding, cutting up, mincing of meat
Change physical structure and makes easier to eat
3. Heat
-
Variety of cooking methods
Continued heating leads to setting/coagulation
How an acid can curdle milk
Resources required
250ml milk
1–4 tsp lemon juice whisk
saucepan J cloth or similar
Process
1. Gently warm the milk in a saucepan. Heat speeds up the process.
2. When the milk releases steam, stir in the lemon juice. Whisk the milk to incorporate the lemon
juice thoroughly.
3. The more lemon juice you add, the larger your curds will be and the faster they will form. For
smaller ‘grains’ of curds, use a smaller amount of lemon juice.
4. Remove the saucepan from the heat and let the acidic milk stand, uncovered, for 5–10
minutes at room temperature. Do not stir the milk during this time.
5. Strain by pouring the contents of the saucepan through the J cloth.
6. The curds will be trapped in the J cloth and the whey is the watery liquid that passes through.
Questions
1.
Explain why the curds develop.
2.
What will speed up the process?
3.
Explain how you can achieve larger curds.
Coagulation
• Coagulation is when the protein in food sets. This occurs when the protein
is heated. If protein is heated too much it will become hard and tough and
difficult to digest
Effect of heat:
Heated protein foods will coagulate in different ways.
•
Meat = proteins shrink as they coagulate. Heating meat proteins in a liquid will change the
structure of the muscle fibres. The protein collagen will change to gelatine.
•
Egg = white becomes solid and turns white as it coagulates at 60°C.Egg yolk becomes solid
and will eventually become dry as it coagulates at 70°C.
•
Wheat = protein in wheat is called gluten. Gluten will set and give cakes, breads and biscuits
their firm structure.
•
Milk = milk proteins will coagulate and form skin on heated milk, just below boiling point.
•
Cheese = Fat melts. Protein denature and then coagulate. If over-heated it become rubbery.
Cheese will brown too.
Syneresis results from the over-coagulation of egg protein. Water is pushed out of the egg. This
can be seen in overcooked scrambled eggs.
Gluten formation
• General name for all proteins in flour.
• Two proteins called gliadin and glutenin in the flour absorb
water to form gluten
• Glutenin gives the dough strength and elasticity
• Gliadin binds the dough together in a sticky mass
• Strong flour (or bread-making flour) contains larger amounts of
both these proteins
• As the dough is kneaded gluten continues to form, creating a
framework and causes the dough to rise.
• Yeast produces carbon dioxide gas, which is trapped in the
gluten framework and causes the dough to rise.
• When gluten is heated in oven it sets or coagulates, giving the
bread a springy light texture
A test to find gluten
Resources required
50g strong plain white flour
50g plain flour
50g self-raising flour
3 squares of muslin cloth about 10cm square
3 tie closures
Process
1.
Mix the strong white flour with enough water to make a soft dough.
2.
Knead each dough ball for 15 minutes.
3.
Put the dough ball into the centre of a piece of cloth and fasten the mixture into the cloth with a tie closure.
4.
Run cold water over the dough and squeeze it gently to wash out the starch.
5.
Continue the washing for about 5 minutes to ensure all the starch is washed out. A ball of gluten will
remain. Put the ball of gluten to one side.
6.
Repeat the process with each of the other flours, saving the gluten balls.
Questions
1.
Which mixture contains the most gluten left?
2.
Which mixture contains the least gluten?
3.
The best bread contains a lot of gluten, so which flour would be best?
Foam formation
• A foam is when a gas is spread throughout a liquid. Eggs are
excellent at foam formation. Whisking eggs produces a gasin-liquid foam.
• Whisking makes the protein in the egg white unravel and
denature. This allows tiny bubbles of air to be incorporated
into the egg white, making an egg white foam.
• Eventually the egg white will stand in soft peaks. When you
heat the foam, the tiny air bubbles expand and the egg
protein coagulates around them. Examples of foods that rely
on this property are meringues and soufflés.
Foam formation
Critical points to remember when whisking egg whites to make
a foam
• Use fresh eggs because they are slightly acidic and will produce a more
stable foam.
• Tiny amounts of fat will stop the foaming of egg whites.
• Use metal or glass bowls because fat clings to plastic and it will reduce
foam formation.
• Use a fine wire whisk to ensure that the maximum amount of air is
incorporated into the egg white quickly.
• Avoid over whisking because the air bubbles in the foam will join
together and the foam will leak water. When water leaks out of the foam
it is called syneresis.
• Adding vinegar makes the mixture slightly acidic and gives a good foam.
Concept fan