The Science of Ingredients

What heat does to starch and sugar

Every ingredient in a recipe has a job, called its functional property. Many of those jobs come from a chemical change when the food is heated. Starchy and sugary foods give three of them.

A sauce that thickens

A cook stirs flour into melted butter, adds milk, and keeps stirring over the heat. The sauce gets thicker. What is happening?

• Gelatinisation: the starch in the flour takes up the milk and swells as it heats
• Dextrinisation: the flour is browning in the pan
• Caramelisation: the sugar in the milk is turning brown
• The milk is simply boiling away until little is left

What fats and oils do

Fats and oils give texture as well as flavour. Three of their jobs come up again and again in recipes.

Which job is the fat doing?

Three ways to get air in

Light cakes, breads and pastries rise because air or gas is trapped inside them, then the structure sets around it. That is aeration, and there are three ways to do it: - Mechanical: beating, whisking, creaming or sieving traps air by hand or machine - Chemical: a raising agent, such as baking powder or bicarbonate of soda, gives off carbon dioxide - Steam: water in a wet mixture turns to steam in a very hot oven and pushes the mixture up Recipes often use more than one at once.

How does the air get in?

Sort each example by the method that makes it rise.

• Whisking eggs and sugar for a Swiss roll
• Creaming butter and sugar for a sponge
• Baking powder in a batch of scones
• Bicarbonate of soda in a ginger cake
• Choux pastry puffing up in a hot oven
• Yorkshire puddings rising high in the tin

What happens to protein

Protein molecules are long chains folded into a shape. Cooking and preparing food changes that shape, and fruit and vegetables have a change of their own.

What changed the protein?

Sort each example by what caused the protein to denature.

• Egg white turning firm and white in a frying pan
• Chicken cooked right through in the oven
• Raw fish turning opaque in a lime juice marinade
• Meat becoming more tender in a vinegar marinade
• Egg white whisked until it stands in stiff peaks
• Whisking egg whites before folding in sugar for meringue

Keep the apple pale

A fruit salad has to wait an hour before it is served. Select the TWO things that would stop the cut apple turning brown.

• Toss the slices in lemon juice
• Keep the slices covered so air cannot reach them
• Cut the apple into smaller pieces
• Leave the bowl somewhere warm

Where does it set?

These are the steps of a quiche recipe. Tap the step where coagulation happens.

• Rub the fat into the flour and add water to make the pastry.
• Roll the pastry out and line the tin.
• Beat the eggs and milk together and pour them in.
• Bake until the filling is firm in the centre.

One sponge, five changes

A Victoria sponge uses almost every change in this module. Creaming the butter and sugar traps air (mechanical aeration), and the baking powder adds carbon dioxide (chemical aeration). The butter is soft enough to cream because of its plasticity. In the oven the egg proteins coagulate, setting the structure around the bubbles so the cake keeps its rise, while the starch in the flour gelatinises using the moisture in the mixture. The surface turns golden through dextrinisation and caramelisation. In an exam answer or an investigation write-up, name the ingredient, the change, and the effect on the finished food, in that order.

Make a white sauce in order

Put the stages of making a roux white sauce in order.

• Melt the butter in a pan over a low heat
• Stir in the flour and cook it briefly to make a roux
• Take it off the heat and add the milk a little at a time, stirring
• Return it to the heat, stirring, as the starch takes up the milk and swells
• The swollen granules burst and the sauce thickens and coats the spoon

Explain crumbly pastry

Choose the word for each gap so the sentence explains the texture.

Name the change at speed

Answer each one correctly to keep your lives.

Explain the science of a dish

Choose a dish you could make, such as a Victoria sponge, a quiche or a white sauce. Explain the job each main ingredient does, and name the scientific change behind each one.

• The dish and its main ingredients
• The job each ingredient does
• The change behind each job, such as coagulation or gelatinisation
• What would go wrong if one ingredient were left out