Functional & Chemical Properties of Food (AQA GCSE Food Preparation & Nutrition): Flashcards
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Define protein denaturation.
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Define protein denaturation.
A change in the structure of a protein that occurs when the amino acid chain unfolds.
State the three conditions that cause proteins to denature.
contact with an acid, e.g. marinating in lemon juice or vinegar
mechanical action, e.g. whisking eggs or kneading dough
the application of heat, e.g. cooking meat or baking bread
Define protein coagulation.
When denatured proteins clump together, trapping water between them and changing the appearance and texture of a food.
Why does overcooked food become dry and dense?
Water is forced out of the coagulated proteins as they continue to tighten.
Explain what happens to an egg as it is fried.
The proteins denature and then coagulate, turning the liquid egg into a solid and changing its colour to white.
What causes milk to curdle?
The milk proteins denature when exposed to an acid or to heat, then clump together, giving a lumpy appearance.
Describe how curdling is used to make cheese.
Milk separates into solid clumps of protein (curds) and liquid (whey).
The whey is removed.
The curds are pressed together and processed, e.g. by adding salt, to form cheese.
Explain how gluten is formed.
Adding water to flour lets two proteins, glutenin and gliadin, move and meet.
They join together to produce gluten, and kneading builds a network of gluten through the dough.
Why is gluten important in bread making?
Gluten is stretchy and strong, so the dough can expand and support itself as carbon dioxide bubbles form during proving.
What happens to gluten during baking?
It denatures and coagulates, which holds the shape of the gas pockets in the dough and gives a well-risen loaf or cake.
Explain how a foam is formed.
Whisking or beating introduces air into a protein-containing liquid.
The mechanical action denatures the proteins, which rearrange into longer strands.
These coagulate into a mesh-like structure that traps the air bubbles.
What is the difference between egg white whisked to soft peaks and to hard peaks?
Soft peaks: the proteins are only slightly stretched, so the foam can return to its original state.
Hard peaks: the proteins are fully denatured and the change is not reversible.
True or False?
Egg whites cannot be over-whisked.
False.
Over-whisking breaks the bonds holding the protein mesh together, and the foam may collapse.
Heating a whisked foam causes the proteins to form a ＿＿＿＿＿＿＿＿ structure, as in a cooked meringue.
Heating a whisked foam causes the proteins to form a solid structure, as in a cooked meringue.
Name three foods that rely on foam formation.
Any three of:
whipped cream
chocolate mousse
whisked cake batter
meringue
Define gelatinisation.
The process in which starch molecules are released from starch granules on heating, thickening the surrounding liquid.
Describe the four stages of gelatinisation.
Starch granules are mixed with water.
Heat breaks the bonds within the starch molecules, so the granules absorb water.
The granules swell.
The granules burst open, releasing starch into the liquid and thickening it.
Name three dishes that rely on gelatinisation.
Any three of:
gravy
soups
risotto and rice pudding
some custards
How does the starch to liquid ratio affect a sauce?
The higher the starch to liquid ratio, the greater the thickening effect — so the more viscous the sauce.
Give two sources of the starch released during gelatinisation.
Particles suspended in a liquid, e.g. cornflour or flour in a roux or béchamel.
Solid foods cooked in a liquid, e.g. rice, pasta or potatoes.
What happens to a starch-thickened liquid as it cools?
The starch may set into a gel.
Define dextrinisation.
The breakdown of starch into smaller molecules called dextrins when dry heat is applied.
What effect does dextrinisation have on food?
It causes browning, and adds a crispy texture and a sweeter flavour, e.g. to bread, biscuits and pastries.
Which cooking processes cause dextrinisation?
Dry heat methods such as baking and toasting.
What determines how much dextrinisation occurs?
The length of time the food is heated — longer heating gives more dextrinisation.
Define caramelisation.
The breakdown of sugars, releasing water, when sugar or a food containing sugar is heated to a high temperature.
Name three examples of caramelisation in cooking.
Any three of:
fudge and toffee
a crème brûlée topping
browning of vegetables, e.g. caramelised onions
Water is sometimes added during caramelisation to reduce the risk of burning, because burnt sugar tastes ＿＿＿＿＿＿＿＿.
Water is sometimes added during caramelisation to reduce the risk of burning, because burnt sugar tastes bitter.
True or False?
The browning of meat when it is heated is caramelisation.
False.
Meat and fish do brown and taste sweeter when heated, but that process involves the proteins in the food and is different to caramelisation.
What is the difference between dextrinisation and caramelisation?
Dextrinisation breaks down starch into dextrins using dry heat.
Caramelisation breaks down sugars at a high temperature.
Define shortening.
The effect of fats and oils in a dough that produces a firm and crumbly — or 'short' — texture, e.g. in shortcrust pastry and shortbread.
Explain how rubbing fat into flour produces a short texture.
The fat gives the flour a waterproof coating.
When water is added, the flour proteins cannot reach it, so no gluten network forms.
The dough is crumbly and 'short' rather than stretchy and strong.
Define aeration.
Beating fat and sugar together to produce a mixture containing many small air bubbles.
What is the effect of baking an aerated cake mixture?
It produces a cake with a light and spongy texture.
Define plasticity.
The property of fats that means they soften over a range of temperatures, rather than melting at a single point.
Why do fats soften over a range of temperatures?
Fats contain many triglycerides with different melting points — saturated ones melt at a high temperature, unsaturated ones at a low temperature.
State three uses that rely on the plasticity of fats.
spreading
shortening
creaming, i.e. beating with sugar to make an aerated mixture
Oil and water do not mix, so they are described as ＿＿＿＿＿＿＿＿.
Oil and water do not mix, so they are described as immiscible.
Define emulsion.
A mixture formed when oil and water are made to mix together.
What is the difference between an oil in water and a water in oil emulsion?
Oil in water: tiny droplets of fat spread through water, giving a liquid.
Water in oil: tiny droplets of water spread through fat, giving a solid.
Define emulsifier.
A substance that prevents fats and water from separating, allowing a stable emulsion to form.
Explain how an emulsifier stabilises an emulsion.
Its molecules have two regions.
The hydrophilic region is attracted to water.
The hydrophobic region is attracted to the fat droplets, so the two liquids are held together.
Which emulsifier is found in egg yolk, and what is it used to make?
Lecithin, which allows mayonnaise to be made from egg yolk, oil and vinegar.
True or False?
Butter is an oil in water emulsion.
False.
Butter is a water in oil emulsion, along with margarine. Milk, salad dressing and mayonnaise are oil in water emulsions.
Why is butter solid at room temperature but olive oil liquid?
Butter is high in saturated triglycerides, which have a high melting point.
Olive oil is high in unsaturated triglycerides, which have a low melting point.
Define enzymic browning.
The browning of cut fruit and vegetable surfaces when they come into contact with oxygen in the air, sped up by enzymes.
Define enzyme.
A protein found in living organisms that acts as a biological catalyst, speeding up the rate of chemical reactions.
State two ways of slowing enzymic browning.
blanching — boiling briefly so the enzymes denature
adding an acid, e.g. lemon juice on sliced apple, which also denatures the enzymes
Why is fruit or vegetables blanched before freezing?
Freezing alone only slows enzyme activity, whereas blanching destroys the enzymes, so it has a greater effect.
How does blanching stop enzymic browning?
The high temperature causes the enzymes to denature, so they can no longer speed up the browning reaction.
Define oxidation in fruit and vegetables.
The loss of water-soluble vitamins when fruit or vegetables are exposed to the air or to heat.
Which vitamins are lost through oxidation?
The water-soluble vitamins — the B group vitamins and vitamin C.
The longer a fruit or vegetable is exposed to the air, the ＿＿＿＿＿＿＿＿ vitamins will be lost by oxidation.
The longer a fruit or vegetable is exposed to the air, the more vitamins will be lost by oxidation.
State four ways to reduce vitamin loss by oxidation.
minimise the time between cutting and cooking
boil the water first and use a pan lid to reduce cooking time
do not overcook
serve immediately after cooking
True or False?
Adding lemon juice to sliced apple prevents it browning.
True.
The acid in lemon juice denatures the enzymes responsible for enzymic browning.
Why does using a lid when boiling vegetables help preserve vitamins?
It reduces the cooking time, so there is less opportunity for oxidation of water-soluble vitamins.
Which two processes must be considered when preparing fruit and vegetables?
Enzymic browning and oxidation, as both reduce food quality or nutritional value.
Explain how a raising agent makes a mixture rise.
Bubbles of gas are introduced into the mixture.
Heating makes the gas expand, so the mixture rises.
The air pockets left in the cooked mixture give a light, airy texture.
Name the four types of raising agent.
chemical
mechanical
steam
biological
Which gas do chemical raising agents produce?
Carbon dioxide.
What conditions does bicarbonate of soda need to release carbon dioxide?
It must be heated in the presence of a liquid and an acid.
Why is bicarbonate of soda used only in strongly flavoured bakes?
It is alkaline, which gives a soapy flavour. Strong flavours such as soda bread, gingerbread and fruit cake mask this.
What is baking powder made from, and why?
Bicarbonate of soda and cream of tartar.
The acidic cream of tartar neutralises the alkaline bicarbonate of soda, removing the soapy flavour.
Baking powder is already present as an ingredient within ＿＿＿＿＿＿＿＿ flour.
Baking powder is already present as an ingredient within self-raising flour.
How do mechanical raising agents work?
They trap air within a mixture, which then expands during cooking and makes the mixture rise.
Name six mechanical methods of trapping air in a mixture.
whisking
beating
folding
sieving
creaming
rubbing in
How does sieving act as a raising agent?
It breaks up the particles of flour or icing sugar and traps air between them.
What is creaming, and which product uses it?
Beating fat and sugar together to trap air in an aerated mixture, used to make cake mixture.
How does steam act as a raising agent?
Heat makes the water in the mixture boil and turn into steam, which rises and puffs the mixture up.
Name two mixtures raised by steam.
Yorkshire pudding batter
choux pastry dough
Why is yeast called a biological raising agent?
Yeast is a living organism, and it produces gas by respiring.
Describe how yeast raises a bread dough.
The dough is left in a warm place to prove.
The yeast respires using sugar as fuel, producing carbon dioxide.
The gas makes the dough rise, and baking expands it further.
Define fermentation.
The anaerobic process in which yeast, having run out of oxygen, produces alcohol as an additional waste product.
State the three conditions yeast needs to work as a raising agent.
liquid, so a gluten network can form
warmth, around 25–29 °C, so the yeast respires fast enough
sugar, to fuel the yeast
True or False?
Bread contains alcohol after baking.
False.
Baking kills the yeast and any alcohol produced during proving evaporates.
Name three foods made using a biological raising agent.
bread
doughnuts
croissants
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