Expert: J. Peter Clark - 8/21/2008

Question
Good afternoon Mr. J. Peter Clark!

i just want to know what are the two types of eat penetration.
Would you kindly explain it by means of comparing those two types.

And what is the relevance of those two in cooking..

thanks a lot in advance.

i just need it in my research project.

thanks..  

Answer
Roxanne,

There really are four types of heat transfer that can apply to food processing and cooking: conduction, convection, radiation, and microwave. Conduction occurs in solids like metal, meat, bread and the interior of other non-liquid foods. It is the transfer of energy by direct contact at the molecular or atomic level - one high energy molecule, atom or small region makes an adjoining one warmer. The rate of transfer is proportional to the difference in temperature.

Convection occurs in fluids - liquids or gases - by the bulk movement of warm "pockets" or regions from one place to another. This may be caused by mechanical mixing or by differences in density - warm regions are less dense than cooler ones and so are inclined to move under gravity. The rate of convection is also proportional to the temperature difference, but may have a higher rate constant than does conduction.

Radiation occurs by transmission of energy from a hot source to a surface through a transparent medium - vacuum or gas that does not absorb the energy first. The rate is propoortional to the difference between the absolute temperatures each raised to the fourth power. The highest energy content radiation for heating is infrared and is generated by resistance heated rods or surface flames.

Microwaves heat substances whose molecules interact with the frequency of radiation applied, which, in contrast with infrared radiation, can penetrate some distance into some materials. Interacting molecules vibrate and generate heat. Infrared radiation only heats the surface; energy from there is transmitted by conduction or convection to the interior.

In practical situations heat transfer often uses a combination of these mechanisms. Many combinations are possible, especially in cooking. An oven might use convection and radiation to heat the surface of a roast and the interior is heated by conduction from the surface. A stew in a pan might be heated by conduction from an electric element and the interior by convection as it is stirred.