Microwave heating

Microwave processing of materials is a technology that can provide the material processor with a new, powerful, and significantly different tool to process materials that may not be amenable to conventional means of processing or to improve the performance characteristics of existing materials.

Microwaves are electromagnetic waves in the frequency band from 300 MHz to 300 GHz. The commonly used frequency for microwave heating is 2.45 GHz.

The principle of microwave heating is related to the polar characteristic of molecules. During the microwave heating, polar molecules such as water molecules try to orientate with the electric field. When dipolar molecules try to re-orientate with rapid changing alternating electric field, the heat is generated by rotation, friction an collision of molecules.

The main characteristics of microwave heating:
*Penetrating radiation
*Rapid heating
*Controllable field distribution
*Selective heating of materials
*It is self-limiting

Microwave heating for pasteurization and sterilization are preferred for the primary reason that they are rapid (as they do not depend as much on the diffusional limitations) and therefore require less come-up time. They can approach the benefits of high temperature-short tine processes – thermal degradation of the desired components is reduced.

The ability of microwaves to penetrate and heat the material from within provides the possibility of rapid and volumetric heating without overheating the surfaces. This can lead to an increase in product yield since the surface degradation is minimized due to the cooler surface compared to conventional heating methods.
Microwave heating

Microwave Processing – How it works?

Microwave Processing – How it works?
Introduction
Microwave energy, usually generated by a magnetron, occurs as alternating current at either 915 or 2450 megahertz. Both frequencies are authorized by the federal government for use with microwave ovens. Because water molecules are polar (have positive and negative ends), they tend to oscillate as they try top align themselves alternately between the positive and negative charges of the microwave energy.

Function
When the frozen foods are exposed to microwave energy, the liquid water in them (all frozen foods contain some water in the liquid stat) begin to heat due to the friction created by high speed oscillations of the water molecules. Most of the water in frozen foods is in the ice state and is not affected by the microwave energy, until heat generated by liquid water molecules is conducted to the ice, melting it, and thereby producing more liquid water to become involved in the heating process.

History
The first application of microwave energy in industry occurred during the 1070s when cooperative experiments between the Gloucester Laboratory, a USDC facility, and the Raytheon Co. resulted in the introduction of a continuous microwave tunnel in the seafood industry, for thawing or tempering of shrimp and fish blocks. In tempering, the product is not quite thawed yet lends itself to further processing such as cutting or coating with a breading. Microwave Processing – How it works?

Microwave in food processing

Microwave in food processing
Microwave and radio frequency heating refers to the use of electromagnetic waves of certain frequencies to generate heat in a material.

Typically, microwave food processing uses the 2 frequencies of 2450 and 915 MHz. Of these two, the 2450 MHz frequency is used for home ovens, and both are used in industrial heating. It is worthwhile to note that outside of the United States, frequencies of 433.92, 896 and 2375 MHz are also used.

There is not much commercial use of these frequencies for food pasteurization or sterilization, although they are used in baking and other processes in the food industry.

Heating with microwave and radio frequency involves primarily 2 mechanisms-- dielectric and ionic. Water in the food is often the primary component responsible for dielectric heating.

Due to their dipolar nature, water molecules try to follow the electric field associated with electromagnetic radiation as it oscillates at the very high frequencies. Such oscillations of the water molecules produce heat.

The second major mechanism of heating with microwaves and radio frequency is through the oscillatory migration of ions in the food that generates heat under the influence of the oscillating electric field.

Microwave and radio frequency heating for pasteurization and sterilization are preferred to the conventional heating for the primary reason that they are rapid and therefore require less time to come up to the desired process temperature.

This is particularly true for solid and semi-solid foods that depend on the slow thermal diffusion process in conventional heating. They can approach the benefits of high temperature-short time processing whereby bacterial destruction is achieved, but thermal degradation of the desired components is reduced.
Microwave in food processing