Heat Transfer & Food (AQA GCSE Food Preparation & Nutrition)

How heat is transferred to food

• Heat can be transferred to food using different methods:

Conduction

• Conduction is the transfer of heat energy through a solid or liquid using the vibration or collision of particles

• Solids:

  • Solid particles are aligned in rows that cannot move

  • When heat is applied, heat energy causes the nearest particles to vibrate, this then passes to the next particle and so on

  • Heat energy is transferred along the solid

• Liquids:

  • The same process occurs in liquids, however, liquid particles can move and so transfer heat energy through collisions with nearby particles

• The cooking pan will conduct the heat energy directly to the food

  • Saucepans, frying pans and baking tins/sheets are often made of metal as metal is a good conductor of heat energy

• Examples of food cooked by conduction are:

  • Sausages cooking in a frying pan

  • Butter melting in a saucepan

  • A pastry case in a pie dish

Convection

• Convection occurs when heat energy is transferred to a food through a liquid or gas

• When liquid or gas particles are heated they will rise (because liquid and gas particles can move), transferring the heat energy

• Cooler liquid or gas particles will move in to take their place and so the process continues creating a convection current

• As long as the heat source is still present this movement of particles will continue until the whole liquid or gas is heated

• Convection is used to heat a pan of water (liquid) or convection ovens rely on convection currents to transfer heat energy by gases (the air) throughout the oven space

• Examples of food cooked by convection include:

  • Poached eggs

  • Rice or pasta

  • A cake in a convection oven

Radiation

• Radiation uses heat energy, in the form of heat waves, to heat food without any particles directly touching the food

• Heat waves are emitted from a heat source, when they reach the food they are absorbed and cook the food

• Grills, microwave ovens and toasters use radiation to cook food

• Examples of food cooked by radiation include:

  • Melted cheese on top of a pizza

  • Toasted bread/bagel in the toaster

  • Meat or halloumi cooked on a BBQ grill

• Some cooking methods combine heat transfer methods to cook food such as cooking a sauce which uses conduction of heat energy through the saucepan and convection of heat energy within the sauce liquid

• It is important to keep the sauce moving by agitation (stirring or whisking) to avoid the sauce sticking to the pan and burning