FOOD ENGINEERING: HEAT TRANSFER IN FOODS
Diagram
Conduction and convection: two ways heat moves into food
Conduction heating
Heat travels through the food itself, from hot molecule to cold molecule, with no bulk movement of the food.
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Convection heating
Heat is carried by a moving fluid, such as hot air, steam or hot water, flowing past or through the food.
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In real cooking
Most food heating uses both: convection carries heat to the surface, and conduction carries it from the surface to the centre.
Figure 1: Heat moving through a solid compared with heat carried by a moving fluid. Drawn in our own style from Karwa, Heat and Mass Transfer, Springer, 2016, and Incropera, Fundamentals of Heat and Mass Transfer, Wiley, 2009.
Introduction
Conduction heating and convection heating are the two main ways heat moves into food, and they work by different mechanisms. In conduction, heat passes through the food itself, from one part to the next, without the food moving as a whole. In convection, heat is carried by a moving fluid, such as air, water or steam, that flows past the food and gives up its heat at the surface.
The core difference is movement: conduction needs no movement of material, only a temperature difference inside the material, while convection needs a moving fluid to carry the heat. In a real oven or a real heat exchanger both act together, but one of them usually sets the speed of heating.
Quick comparison
Table 1: Conduction heating compared with convection heating
| Aspect | Conduction heating | Convection heating |
|---|---|---|
| How heat moves | Through the material itself, molecule to molecule | Carried by a moving fluid to the surface |
| Needs a moving fluid | No | Yes: air, water, steam or oil must flow |
| Where it acts | Inside the food, from surface to centre | At the surface, between fluid and food |
| Speed depends on | Thermal conductivity of the food | Fluid speed and the heat transfer coefficient |
| Temperature pattern | A gradient inside the food, hot outside and cooler inside | Surface warms fast, then conduction takes over inside |
| Best for | Solid foods and thick foods | Liquid foods and surfaces exposed to a fluid |
| Food example | Heat reaching the centre of a thick meat patty | Hot air in an oven warming the surface of bread |
| Heating rate | Slow in foods, because foods conduct heat poorly | Faster when the fluid moves fast |
| Control | Set by food thickness and conductivity | Set by fluid temperature and flow speed |
| In a heat exchanger | Heat passes through the metal wall by conduction | Heat passes from hot fluid to the wall by convection |
Scientific explanation
Definition
Conduction is the transfer of heat through a material by direct contact between its parts, driven by a temperature difference, with no bulk movement of the material. Convection is the transfer of heat by the bulk movement of a fluid, which carries heat from one place to another.
Working principle
In conduction, faster moving molecules pass energy to slower moving neighbours, so heat flows from hot to cold through the solid or still liquid. Fourier’s law describes it: the heat flow is proportional to the thermal conductivity, the area, and the temperature gradient. In convection, the moving fluid brings hot fluid to the surface and carries cooled fluid away, so the surface is always met by fresh hot fluid. Newton’s law of cooling describes it: the heat flow is proportional to the heat transfer coefficient, the area, and the temperature difference between surface and fluid.
Apparatus
The books name no single apparatus that measures conduction against convection in food. Heat exchangers, ovens, water baths and hot plates are the equipment in which the two modes are seen, and thermocouples placed at the surface and at the centre of a food show the two stages of heating.
Measurement procedure
To compare the two, temperature is recorded at the surface and at the centre of the food while it heats. The surface temperature follows the fluid temperature quickly when convection is strong, while the centre temperature lags, and the lag shows how slowly conduction carries heat inward. The books name no single standard test for this comparison in foods.
Factors affecting readings
Conduction depends on the thermal conductivity of the food, which changes with water content, fat content and temperature, and on the thickness of the food. Convection depends on the speed of the fluid, on whether the flow is natural or forced, and on the properties of the fluid. A thicker food heats more slowly by conduction, whatever the convection outside.
Formulas and calculations
Fourier’s law of conduction: heat flow q equals minus thermal conductivity k times area A times the temperature gradient dT/dx. In symbols: q = -k x A x (dT/dx). The minus sign shows heat flows from hot to cold.
Newton’s law of cooling for convection: heat flow q equals the heat transfer coefficient h times area A times the temperature difference between surface and fluid. In symbols: q = h x A x (Ts – Tinf).
Worked example: a food surface of area 0.01 square metres has a temperature gradient of 100 degrees C per metre and thermal conductivity 0.5 W per m K. Conduction gives q equal to 0.5 times 0.01 times 100, which is 0.5 W. For convection at the same surface, with h equal to 10 W per square metre K and a surface to fluid difference of 50 degrees C, q equals 10 times 0.01 times 50, which is 5 W. The numbers are an illustration of the two laws, not values from the books.
Industrial applications
- Conduction heating: the centre of a thick patty or a can of food, where heat must travel through the food itself.
- Conduction heating: a hot plate or a contact grill, where heat passes from the hot surface into the food by contact.
- Convection heating: an oven, where hot air carries heat to the surface of bread and cakes.
- Convection heating: a water bath or steam jacket, where hot water or steam carries heat to a vessel wall.
- Convection heating: liquid foods in a heat exchanger, where the flowing liquid is heated by convection at the wall.
Advantages and limitations
- Conduction heating, advantage: heats the inside of solid food evenly once the surface is hot, with no fluid to control.
- Conduction heating, limitation: slow in foods, because most foods are poor conductors of heat, so the centre lags behind the surface.
- Convection heating, advantage: fast surface heating that can be raised by moving the fluid faster, and it suits liquids well.
- Convection heating, limitation: it only heats the surface, so the inside still depends on slow conduction, and it needs a fan or pump to move the fluid.
FAQ
Which is faster, conduction or convection?
Convection at the surface is usually faster, because the moving fluid keeps bringing fresh heat. Inside the food, conduction is the only way heat moves, and it is slow.
Can a food be heated by conduction alone?
Yes, for example on a hot plate, where heat passes from the plate into the food by contact.
What is the heat transfer coefficient?
A number that says how well a moving fluid gives up heat to a surface. It rises when the fluid moves faster.
Why does the centre of a thick food heat slowly?
Because heat must travel through the food by conduction, and foods conduct heat poorly.
Do ovens use conduction or convection?
Both. Hot air heats the surface by convection, and heat then travels to the centre by conduction.
References and further reading
- Fellows, P., Food Processing Technology: Principles and Practice, Woodhead Publishing (FTN book on disk).
- Karwa, R., Heat and Mass Transfer, Springer, 2016 (Google Books page read: Ch. 2, One-Dimensional Steady State Heat Conduction, p. 7, and Ch. 7, Convective Heat Transfer, p. 380).
- Incropera, F. P., Fundamentals of Heat and Mass Transfer, Wiley, 2009 (Google Books page read: Introduction to Conduction and Introduction to Convection).
