1: Dielectric Heating

Question 1

What is the theory behind dielectric heating?

Answer 1

1. When water and salt molecules are exposed to EM, the dipoles oscillates to the direction of EM.
2. Frictional energy is generated between neighbouring molecules.
3. Components of the surrounding are haeted through conduction or convection.

Question 2

Describe the factors that affect the heating efficiency of dielectric heating methods.

Answer 2

1. Dielectric constant: Electric permittivity of a material
2. Loss factor: The ability of a material to convert EM into heat.

These factors are affected by the moisture content, temperature of food and frequency of electric field.

Question 3

Why is the loss factor of water greater than ice?

Answer 3

• In ice, the water molecules are held in a rigid lattice structure and are less able to oscillate in response to an electromagnetic field. Therefore, less energy is being absorbed and dissipated, resulting in a lower loss factor.

Question 4

Can food wrapped in paper or polymer film be heated in a microwave?

Answer 4

Yes, this is because glass, paper or polymer film has low loss factor. The low loss factor allows the microwaves to penetrate the paper without being heated.

Question 5

What are the factors that affect the dielectric constant and loss factor of food?

Answer 5

• Moisture content
• Temperature
• Frequency of electric field

Question 6

Define loss tangent.

Answer 6

• (e’‘/e’)
• The ratio between the loss factor (e’’) and the dielectric constant (e’)

Question 7

What are the advantages of dielectric heating?

Answer 7

• Rapid heating
• No surface browning/overheat
• Small and compact equipment
• Clean operation
• Low risk of cross-contamination

Question 8

How is dielectric heating applied to food processing?

Answer 8

• Thawing
• Tempering
• Dehydration
• Baking
• Rendering of fat
• Frying
• Pasteurization
• Sterilization

Question 9

What are the advantage and limitation of thawing with dielectric heating methods?

Answer 9

• More efficient than conventional thawing. In conventional thawing, the thickness of water layer increases when ice melts. Thermal conductivity of water is lower than ice and therefore water slows down. Dielectric heating does not have this problem because the heating occurs throughout the food product.
• Only suitable for small size frozen food. Water has higher loss factor than ice. Melted water may be heated and temperature increases rapidly on certain parts of the food, which would lead to uneven thawing or even cooking certain parts of the food.

Question 10

Describe the difference between thawing and tempering.

Answer 10

• In thawing, the ice in a frozen product is completely melted for cooking.
• In tempering, the frozen product is raised to a temperature of -20C to -3C to ease the pre-processing preparation
• Thawing requires higher amount of heat energy compared to tempering. This is because energy is needed to break the H-bonds between the molecules in order to melt the ice. In tempering, the ice is not melted completely.