Radiation

• A process that does not require direct physical contact between heat source and the object.

• Thermal radiation: heat from the sun; Electromagnetic radiation: microwave

Principle of radiation

• All molecules are vibrating to some extent, → all matters emit thermal energy all the

time. • The hotter an object gets, the more energy it

radiates in higher regions of the spectrum.

Nina Paley [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Principle of radiation

Opensource Handbook of Nanoscience and Nanotechnology

Application of radiation in cooking

1. Grilling and broiling • These cooking methods all emit

visible light and also intense radiation of infrared energy.

By _BuBBy_ from USA (365 Days Project 188/365: Saturday Night Grilling) [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

Grilling

Broiling

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Grilling and broiling

• Advantage: rapid and thorough browning of

the surface, and so produces intense flavors.

• Disadvantage: huge disparity between the rate of

heat conducted within the food.

Examples related to food handling and preparation - Radiation

• In an oven, the heating caused by radiation can be increased by moving food closer to the walls or reduced by wrapping the food with reflective aluminum foil.

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Energy transfer can take place by phase change

Description of Phase Change

Term for Phase Change

Heat Movement During Phase Change

Temperature Change During Phase Change

Solid to liquid Melting Heat goes into the solid as it melts. None

Liquid to solid Freezing Heat leaves the liquid as it freezes. None

Liquid to gas Vaporization, which includes boiling and

evaporation

Heat goes into the liquid as it vaporizes.

None

Gas to liquid Condensation Heat leaves the gas as it condenses. None

Solid to gas Sublimation Heat goes into the

solid as it sublimates.

None

Energy transfer can take place by phase change

• Latent Heat of Condensation

– The amount of heat energy released to the environment when a gas changes its state to a liquid

– For one gram of water, the amount of heat energy released is 540 calories at a temperature of 100℃.

Energy transfer can take place by phase change

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Principle of vaporization/condensation

• When water boils, the change from liquid state to gaseous state require a large amount of energy –latent heat

• Conversely, a large amount of energy

is released when a gas condenses to become a liquid.

Phase Change of Water

By Cawang (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Application of vaporization in cooking

• Steaming – Impart energy of motion and energy

of vaporization.

Advantage: – especially quick in bringing the surface of

food to boiling point and keeping it there.

Disadvantage: – lower penetration power

Conclusion

• In cooking, heat energy can be transferred by conduction, convection, radiation and vaporization/condensation

• One kind of heat transfer usually predominates in a given cooking technique and, together with the cooking medium, has a distinctive influence on foods.