유전가열, dielectric heating, 전자레인지 원리

 

 

극히 파장이 짧은 전파는 식품의 내부에 침투하고 물분자 회전에 의해 열이 발생하므로 단시간에 깊은 부분에까지 고르게 가열할 수 있다. 전파에 의한 음식물의 가열 원리를 유전가열(dielectric heating)방식이라 불린다.

물 분자는 수소와 산소 원자로 이루어져 있는데, 수소 원자 쪽이 양전하를 띠고 있고 산소 원자 쪽이 음전하를 띠고 있는 극성 분자이다. 음식물에 마이크로파를 쪼이면 이와 같은 극성 분자는 전자기파의 전기장이 양과 음으로 진동할 때 분자가 양과 음의 방향을 바꾸며 매우 빠르게 회전하여 전자기장을 따라 정렬한다. 분자의 회전에 의해 분자들이 서로 밀고 당기거나 충돌하는데 이러한 운동에너지가 음식물의 온도를 높이게 된다.

전자레인지에서 만들어지는 전자기파로 전기 쌍극자인 물 분자는 매우 강하게 회전한다. 여기서 물 분자가 가장 흡수를 잘하는 공명 주파수는 9,000MHz대이지만 이 주파수에서는 표면의 물 분자가 모두 흡수하여 겉만 타게 된다. 따라서 음식 속에 있는 물분자까지 전자파가 전달되도록 하기 위하여 가정용 전자레인지의 전파의 주파수는 보통 ISM B 대역인 2,400~2,500MHz(2.4 GHz - 2.5 GHz)로 맞춘다. 이러한 단파장의 전파를 일으키기 위해 마그네트론을 사용하며, 소비전력이 크다. 가정용은 소비전력이 700W 또는 1kW(제품에 따라 ±5%오차가 있음.)이며 1.2 kW 정도의 고주파 출력을 발생한다. 가열시간이 짧아도 되기 때문에 단시간에 끊어지는 타이머가 장치되어 있다.

전자레인지는 흔히 발생하는 주파수가 물 분자의 고유 진동수로 공명하여 가열시킨다는 오해가 있지만 위의 설명에서와 같이 실제로는 공명 현상을 이용하지 않는다. 실제로 공업용 전자레인지의 경우 915 MHz에서 작동하기도 하며, 이 대역에서도 정상적으로 음식물이 가열된다.

 

 

Since radio waves with an extremely short wavelength penetrate into the inside of the food and generate heat by rotation of water molecules, it can be evenly heated to the deep part in a short time. The principle of heating food by radio waves is called the dielectric heating method.

Water molecules consist of hydrogen and oxygen atoms, which are polar molecules with positive charges on the hydrogen atom and negative charges on the oxygen atom. When microwaves are applied to food, these polar molecules rotate very quickly in the positive and negative directions when the electric field of electromagnetic waves vibrates positive and negative, aligning them along the electromagnetic field. The molecules push and pull or collide with each other by the rotation of the molecules, and this kinetic energy increases the temperature of the food.

Water molecules, which are electric dipoles, rotate very strongly with electromagnetic waves generated in microwaves. Here, the resonance frequency that water molecules absorb best is in the 9,000 MHz range, but at this frequency, all water molecules on the surface absorb and burn only the outside. Therefore, in order to transmit electromagnetic waves to water molecules in food, the frequency of propagation in a household microwave oven is usually set to 2,400 to 2,500 MHz (2.4 GHz - 2.5 GHz), which is the ISM B band. A magnetron is used to generate the propagation of such a short wavelength, and the power consumption is high. For households, power consumption is 700W or 1 kW (there is a ±5% error depending on the product) and generates a high frequency output of about 1.2 kW. Since the heating time is short, there is a timer that cuts off in a short time.

Microwaves have a misunderstanding that frequencies that are commonly generated resonate with the natural frequency of water molecules to heat them, but as described above, they do not actually use resonance. In fact, an industrial microwave oven operates at 915 MHz, and food is heated normally in this band.