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  1. 1. Presented By:- Shahzad Saleem 14/PFT/09
  2. 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. 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. 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. 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.
  6. 6.  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.
  7. 7.  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.
  8. 8. 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.
  9. 9. 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.
  10. 10. 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.
  11. 11. 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
  12. 12. 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
  13. 13. 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. ”
  14. 14. 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
  15. 15. 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.