Microwave heating has gained popularity in food processing due to its ability to achieve high heating rates, a significant reduction in cooking time, more uniform heating, safe handling, ease of operation and low maintenance.
Thus, the food industry is said to be the largest consumer of microwave energy, where its application has been utilized in thawing, baking, dehydration, melting, tempering, and pasteurisation, sterilization, heating, and re-heating, etc.
Microwave (MW) energy is a form of radiation. The term radiation means that the energy is transported by the force fields of electromagnetic waves; they can radiate through a perfect vacuum and do not need any medium to transfer energy from one object to another. All electromagnetic waves have two components
1) Electric field
2) Magnetic field
When a charge (electric or magnetic) in a medium changes its position in space, the corresponding field that it produces also changes in space. These changes in electric and magnetic fields produce an oscillatory wave, which is called an electromagnetic wave. Electromagnetic waves are characterized by their frequency, velocity, and electric field strength.
Principle: Microwave energy is transmitted as electromagnetic waves and the depth to which these penetrate foods is determined by their frequency and the characteristics of the food involved. Thus, frequencies ranging from 300 MHz to 300 GHz and a wavelength of 1 mm to 1 m are associated with microwaves. However, 0.915GHz and 2.45GHz frequencies are commonly used for microwave heating because of two reasons. Firstly, they are frequencies allocated to industrial, scientific, and medical (ISM) radio bands, basically for non-communication purposes.
Secondly, the penetration depth of the microwave is greater and more effective at these lower frequencies.
Although, heating is not necessarily increased with decreasing frequency as the internal field can be low depending on the properties of the material. For this reason, 2.45GHz is widely used in domestic microwave ovens and some industrial applications while 0.915 GHz is preferred for industrial/commercial microwave ovens
Mechanisms of microwave heating:
Heating with microwave frequency involves primarily 2 mechanisms-
- Ionic polarization: When an electrical field is applied to food solutions containing ions, the ions move at an accelerated pace due to their inherent charge. The resulting collisions between the ions cause the conversion of the kinetic energy of the moving ions into thermal energy.
- Dipole rotation/Dielectric Heating: Food materials contain polar molecules such as water. These molecules generally have a random orientation. However, when an electric field is applied, then the molecules orient themselves according to the polarity of the field.
- In a microwave field, the polarity alternates very rapidly, e.g., at the microwave frequency of 2,450 MHz, the polarity changes 2.45 billion cycles per second.
- Such rotation of molecules leads to friction with the surrounding medium, and heat is generated.
It is the measure of loss of energy in a dielectric material due to slow polarization or some other dissipative phenomenon.
How does a microwave turn electricity into heat?
- Inside the strong metal box, there is a microwave generator called a magnetron. When you start cooking, the magnetron takes electricity from the power outlet and converts it into high-powered, 12cm (4.7 inches) radio waves.
- The magnetron blasts these waves into the food compartment through a channel called a waveguide.
- The food sits on a turntable, spinning slowly round so the microwaves cook it evenly.
- The microwaves bounce back and forth off the reflective metal walls of the food compartment, just like light bounces off a mirror. When the microwaves reach the food itself, they do not simply bounce off. Just as radio waves can pass straight through the walls of your house, so microwaves penetrate inside the food. As they travel through it, they make the molecules inside it vibrate more quickly.
- Vibrating molecules have heat so, the faster the molecules vibrate, the hotter the food becomes. Thus, the microwaves pass their energy onto the molecules in the food, rapidly heating it up.
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