Lecture 4 - Microwave processing

Question 1

What is a microwave?

Answer 1

It is an electromagentic wave of frequency between 300 MHz (megahertz) and 300 GHz (gigahertz)

Question 2

What are the 2 microwave frequencies that are commonly used in the food industry?

Answer 2

1. 915 MHz
2. 2450 MHz

Question 3

Why might one use a frequency 2450 MHz instead of 915 MHz to heat up the food?

Answer 3

Higher frequency means that more electromagnetic waves hit the food per second, and there will be more movement of molecules, leading to faster heating

Question 4

Why does the food industry use 915 MHz more often than 2450 MHz?

Answer 4

Lower frequency, f = longer wavelength, λ (f and λ inversely proportional)

• With a longer λ, penetration depth is deeper, thus more uniform heating throughout the food, lower chances of cold spot formation, leading to higher food safety.

look at handwritten notes / AY18/19 exam paper

Question 6

How is food heated up by microwaves?

Answer 6

• Microwaves are a form of electromagnetic energy.
• When food is exposed to microwaves, the energy causes polar molecules (like water) and ions in the food to align and oscillate rapidly with the alternating electric field of the microwave.
• This back-and-forth movement generates friction between the molecules, which produces heat.
• The heat is then transferred throughout the food, warming it from the inside out.

Question 7

Can a food with no ionic species and no polar molecules be heated up by microwave energy?

Answer 7

No

Question 8

What is the Dielectric constant (ε’)?

Answer 8

How much electric energy can be stored in a material (but is not converted into heat)

Question 9

What is the loss tangent?

Question 10

What is the Loss tangent (tanδ)?

Answer 10

How much electric energy is lost by the material.

Question 11

What is Dielectric loss factor (ε’’)?

What is the equation linking ε’’, ε’ and tanδ?

Answer 11

Dielectric loss factor (ε’’) is the electrical energy that is lost by a material and is converted into heat.

ε’’ = ε’ x tanδ

Question 12

What is one technological challenge that is faced in microwave heating?

Answer 12

Non-unifomity in heating.

Question 13

Assume I put 100 packets of RTE pasta into a microwave oven. How does the problem of non-uniform heating occur in terms of :

• intra-pacakge uniformity

Answer 13

Intra-package uniformity : betweem each pack of RTE pasta.

• Some pasta packets are nearer to the microwave supply while some are further away from the microwave supply.
• Thus some packets may be overcooked and some packets may be undercooked as the electromagnetic waves are not evenly distributed.

Question 14

Assume I put 100 packets of RTE pasta into a microwave oven. How does the problem of non-uniform heating occur in terms of :

• inter-pacakge uniformity

Answer 14

1. Due to uneven heating, there could be hot and cold spots, where the food nearer to the centre of the packaging remains cooler than the food nearer to the edges, leading to uneven cooking.
2. RTE pasta is a composite food with different ingredients like cheese, meat, noodles, sauce. Since the dielectric properties of the food components are different, the different food components heat up at different rates, leading to uneven heating

Question 15

Components of microwave oven

What is the function of the power supply in a microwave oven?

Answer 15

It converts commercial power into a high voltage which is needed by the magnetron

Question 16

Components of microwave oven

What is the function of the magnetron in a microwave oven?

Answer 16

It utilises high voltage electrical energy to generate microwave radiation

• by combining electric and magnetic fields to produce high-frequency microwaves

Question 17

Components of microwave oven

What is the function of a waveguide in a microwave oven?

Answer 17

It is a hollow tube used to reflect the microwave energy into the oven.

Question 18

Components of microwave oven

What is the function of a stirrer in the microwave oven?

Answer 18

A stirrer is a fan-like device that rotates inside the microwave oven to help distribute microwave energy evenly throughout the oven cavity by disrupting the wave patterns

Question 19

Components of microwave oven

What is the function of the oven cavity in a microwave oven?

Answer 19

It is the metal chamber where the food is placed.

Question 20

When heating liquid foods, there is a problem of uneven heating, therefore you cannot directly heat liquid foods in an oven.

Why is there a problem of non-uniform heating in liquids if you just place it in the oven?

Answer 20

The electric field distribution is not even. Since the liquid isnt stirred or moved, the liquid may heat unevenly, where different parts of the liquid experience different energy intensities.

• this may create hot and cold spots in the liquid food, affecting food safety and quality

Question 21

To overcome the problem of uneven heating from microwaves in liquid products, the liquids can be pumped through a tube and an applicator surrounds the tube to supply microwave energy for more even heating.

How does the applicator allow for more uniform heating?

Answer 21

The applicator is designed such that the microwaves are directed and reflected towards the tube, ensuring the microwaves are hitting the tube (and the food in the tube) in all directions