Principle of convection

• In convection, heat is transferred by the bulk movement of molecules in a fluid from a warm region to a cooler one.

• The fluid can be a liquid or gases, something mobile.

• Very often, it is a process that combines conduction and mixing:

Warm air

Cool air

Fluid with higher temperature expands and becomes less dense, and the buoyant force causes it to rise. Fluid with lower temperature then flows to take the place of the rising warmer fluid, resulting in the circulation of fluid.

Animation - convection https://spark-public.s3.amazonaws.com/scigast/resources/week01/SCIGAST1.6animation-convection.swf

Baking • ∵the density of air is much less than

that of water

→ collisions between hot molecules and food are much less frequent in the oven than in the pot

→ heat energy is less readily transferred to the food

→ longer time is required for cooking

Application of convection in cooking

• Baking, Boiling, Deep-frying 1.Baking • Heat is transferred by the combination

of radiation from the walls and hot-air convection.

Boiling

2. Boiling • Water is dense enough so that

its molecules constantly collide with the food and rapidly transfer their energy to it.

• Advantage: constant boiling temperature

What is boiling? - The effect of PRESSURE

By Marcusroos [Public domain], via Wikimedia Commons

What is boiling? - The effect of PRESSURE

• Boiling defines the phenomenon that occurs to a liquid: – at a temperature when the vapor pressure

= pressure of surrounding environment

– lower the surrounding pressure easier to boil, i.e. boiling temperature is lower

– higher the surrounding pressure boiling at higher temperature

What is boiling? - The effect of SALT

• Salt and sugar dissolved in water absorb heat energy but cannot turn into a gas

At water’s normal boiling point (100 ℃) there

are a smaller proportion of molecules with enough energy to escape from the liquid and form a bubble of vapor.

Boiling point therefore becomes higher.

Deep Frying

3. Deep Frying • Used oil as a medium for

convection. • Oil has a higher boiling point

than water, leading to dehydrating the food surface and browning.

Deep Frying

By Kwiki (Own work) [Public domain], via Wikimedia Commons

Examples related to food handling and preparation - Convection

• Water will cool faster than the same volume of a thickened soup because of less resistance to the convection currents in water.

– water > chicken soup > creamy soup > thick onion soup > porridge.