FOOD TECH DIELECTRIC PROPERTIES

1. Presented By:-
Shahzad Saleem
14/PFT/09

2. A dielectric material is a substance that is a poor conductor of
electricity, but an efficient supporter of electrostatic field.
When a dielectric is placed in an electric field, electric charges
do not flow through the material as they do in a conductor, but
only slightly shift from their average equilibrium positions causing
dielectric polarization.

3. Dielectric property used to explain interactions of foods
with electric fields. It determines the interaction of
electromagnetic waves with matter and defines the charge
density under an electric field.
Major factors that influence these properties of agricultural and
food materials are:
 Frequency of the applied radio frequency or microwave
electric fields
 Water content
 Temperature
 Density of the materials

4.  Dielectric properties of materials are those electrical
characteristics of poorly conducting materials that determine their
interaction with electric fields.
 They determine how well energy can be absorbed from the
high-frequency alternating electric fields and thus how rapidly the
materials will be heated.
 These properties are important in any processes involving radio
frequency (RF) or microwave dielectric heating.
 They determine how well energy can be absorbed from the
high-frequency alternating electric fields and thus how rapidly the
materials will be heated.

5.  The dielectric properties of the load materials are also
important in the design of the RF or microwave power
equipment needed for the heating process.
 Dielectric properties of materials are highly correlated with the
amount of water in materials such as agricultural products and
food materials, sensing the dielectric properties can be used for
rapid measurement of their moisture content.

7.  The dielectric properties of most materials vary with several
influencing factors:
The dielectric properties also depend on the:
1. Frequency of the applied alternating electric field,
2. Temperature of the material,
3. Density,
4. Composition,
5. Structure of the material.
In granular or particulate materials the bulk density
of the air-particle mixture is another factor that influences the
dielectric properties.

8.  The dielectric properties of materials also dependent on the
chemical composition and especially on the presence of mobile
ions and the permanent dipole moments associated with water
and any other molecules making up the material of interest.

9. The dielectric properties of fresh apple tissue are shown in Fig-1 between 10 MHz and 2
GHz at four different temperatures. Both the dielectric constant and loss factor decrease
consistently with increasing frequency, but their change with temperature is more
complicated.

10. Similar frequency and temperature dependence of dielectric properties were exhibited by
fresh, uncooked chicken meat (Figure 2), but the values for both the dielectric constant
and loss factor were considerably higher than those of the fruit.

11. Values of the dielectric constant and loss factor are much lower for wheat than for the
fresh fruit or chicken meat because of the much lower moisture content and the fact that
water in the wheat is tightly bound to starch and protein.

12. Measurement of dielectric
properties in food
• Type of food
• Degree of accuracy
• Frequency
• Reflection of transmission type
• Methods
– Transmission line
– Coaxial probe
– Cavity perturbation
– Free space transmission

13. Dielectric Measurement
Method
• A microwave signal is generated at the
frequency of interest
• Signal is directed through the sample
• Changes in the signal caused by the sample
are measured
• From these changes the dielectric constant
and loss factor are determined

14. 1. Useful in detection processing conditions, or the quality of foods.
2. The major uses for dielectric properties are measuring and heating
applications.
3. Properly designed electrical instruments can be used to determined
moisture content or water activity.
4. Knowledge of dielectric properties in partially frozen material is
critical in determining the rates and uniformity of heating in
microwave thawing.
“ Dielectric behavior is important for food technologists and food
engineers to improve quality of microwave foods and to design
microwaveable foods and to develop new microwave processes. ”

15. 5. Dielectric properties are also important in the selection of proper
packaging materials.
6. In design of microwave and radio frequency heating equipment.
7. When selecting equipment for drying low moisture foods should
prevent electric field strength from exceeding a level at which flash-
over would take place.
8. RF energy is mostly used to heat or evaporate moisture from a
product; higher frequency MWs are used for defrosting & low
pressure drying

16. This presentation includes information on the influence of
important factors affecting the dielectric properties of agricultural
and food products. It includes examples showing the nature of the
influence of these factors on the dielectric properties of materials,
and thus will be helpful to those considering applications of
dielectric properties for sensing quality attributes of food products
and for those considering microwave or radio-frequency dielectric
heating applications as well.