Four out of five American homes have at least one microwave oven. Because they heat food very rapidly, microwave ovens are extremely popular. A microwave oven is a device that generates microwaves from electricity. Microwaves are electromagnetic energy that have an electric and a magnetic component. Other forms of electromagnetic energy are radio waves, sunlight and electricity.
Scientists refer to microwave energy as radiation because, similar to other radiation energy, it travels through space. Microwave radiation is often called non-ionizing radiation to distinguish it from other forms of radiation, like x-rays, which are called ionizing radiation. Ionization refers to breaking atoms or molecules into two electrically charged groups.
Microwave energy, like all electromagnetic radiation, travels in a wave pattern. The waves are reflected by metals, pass through air, glass, paper and plastic, and are absorbed by food. Most microwave containers are designed to transmit microwave energy without reflecting or absorbing it, and thus are made of paper or plastic. The microwaves will travel through the container to the food.
When food is exposed to microwaves, it will absorb that energy and convert it to heat. Food composition (mainly water content) is a key factor that determines how fast it will heat in a microwave environment. The higher the water content of a food, the higher its loss factor thus, the faster it will heat. Solutes, like sugar and salt, also influence the loss factor of foods exposed to microwave energy. For foods to heat in a microwave oven, the electromagnetic waves must penetrate the food. There are limits to the depth of their penetration. This makes container geometry a very important factor in heating foods by microwave energy.
Food composition does not only influence the loss factor, but also penetration depth. For example, when salt is added to water, it will change its microwave heating characteristics in two different directions. On one hand, it will increase the water's loss factor causing the water to heat faster. On the other hand, salt will decrease the penetration depth of microwaves into the salt/water solution, decreasing the heating rate. If salt is added to water to enhance its loss factor and, at the same time, the container geometry changes to minimize the drop in penetration depth, the heating rate will be greatly increased.
There are increasing numbers of food scientists who are dedicated to developing microwavable foods that can heat quickly and evenly while maintaining high quality. Heating foods evenly in a microwave oven is difficult at best, particularly with solid foods of different composition. A frozen dinner tray is an example of a solid food with varying composition. Have you ever tried to warm a jelly-filled donut in a microwave oven? If you have, you would have noticed that the filling gets extremely hot while the cake part is hardly warm. This is because the jelly and the cake parts both have different loss factors and therefore will absorb the microwave energy at variable rates and heat to totally different temperatures. A simple way to avoid this problem is to let the food stand for a short period with the microwave oven off. This will permit heat to travel from the hot regions of the food to the cooler ones, through a process called conduction. Eventually, the food will equalize at a temperature somewhere between the hot and the cold regions.
Understanding how food composition influences microwave heating is a skill needed by those developing microwavable foods. Developing new microwavable meals and snacks is a growing segment of the food industry. Aside from using microwave energy to heat meals at home, there have been many attempts to utilize microwave energy to heat food at processing plants. Microwave energy is also utilized in the textile and paper industries to remove moisture from the products. We are likely to hear more about microwave energy application for the future.