Percy Spencer invented the first microwave oven shortly after World War II by utilizing the newly discovered technology of radar. RaytheonTM then licensed patents for the home-use microwave in 1955, but these designs were too large and expensive for the average household. In 1967 the first countertop microwave oven was designed by the Amana CorporationTM which had recently been absorbed by RaytheonTM. Unlike a conventional oven, a microwave cannot directly brown or caramelize food because it does not reach the thermal energy required for the food to undergo Maillard reactions. A Maillard reaction is a chemical reaction between an amino acid and sugar which results in the browning seen in many foods such as bread and French fries. The exceptions to this rule are foods such as bacon, which are very oily. These foods can brown in a microwave because the oil can reach a temperature higher than boiling water. When cooking with a conventional oven, which creates a thermal environment for the food to be prepared in, the thermal energy has to transfer through the outside layers of the food before it can cook the inside. This leads to uneven heat transfer distribution and a burnt outside with a raw inside. However, a microwave oven’s waves penetrate through the food and are able to uniformly excite the water molecules inside. To put it simply, a conventional oven has the objective of conducting heat, but a microwave oven has the objective of creating movement in the molecules.
How a Microwave Heats Food
The most important part of cooking with a microwave is the object being heated. “Microwaves take advantage of the behavior of water when subjected to electromagnetic waves found in the microwave band.”
[i] Water is affected by electromagnetic waves due to its dipole property. This means that one side of the molecule is positively charged while the other side is negatively charged. When an electromagnetic wave, which comprises alternating electric and magnetic fields and possesses a frequency of 2.45GHz [ii], interacts with a positively charged particle the particle experiences a force in the direction of the field. Conversely the negatively charged particles would experience a force in the opposite direction. Due to the fact that the electromagnetic wave is made of alternating electric and magnetic fields the molecules are forced to rotate. The rotation of the agitated molecules causes them to rub against the material of the food, causing friction. This friction in turn causes heat and if the water molecules are evenly distributed within the food it will be heated uniformly.
The Metal Misconception
One of the common misconceptions about microwaves is that under no circumstance should metal be placed inside of a running microwave oven. Many people accidentally leave a fork on a plate and put it in the microwave oven and soon it is sparking and giving off a burnt odor. However, while this is true for most situations there are many occurrences where having metal in a microwave oven is completely fine.[iii] In fact, the inside walls of most microwave ovens are made of metal so that a “Faraday Cage” is formed. This ensures the microwaves will stay inside and not affect the surrounding environment. Even the wrapper on a Hot Pockets® sleeve has a thin sheet of aluminum in it to allow the outside of the Hot Pockets® to crisp. Many microwave ovens have removable metal grates which can allow more than one item to be cooked at a time. These metal examples have all been designed with specific shape, size and distribution properties which allow them to either absorb the microwaves as water would or safely reflect the waves as a crisping sleeve does. Theoretical heat transfer and thermal management models indicate that metals with a particle size less than 100 micrometers can actually absorb microwaves similar to water.[iiii] However, when a piece of metal that is not designed for a microwave oven is subjected to microwaves, a surplus electrical charge can build up on the surface of the object and result in arcing. Arcing is the transfer of electricity from one conductive surface to another, in this situation from the metal to the wall of the microwave. A microwave oven is just one example of a kitchen appliance which utilize engineering and plays a large role in our day to day life.
[i] Universe Today “How Microwaves Work” John Carl Villanueva November 18 2009
[ii] How Stuff Works.com “How Microwave Cooking Works” Marshall Brain 2012
[iii] McGraw Hill Education Access Science “Why can’t you put metal objects into microwave ovens?” Dinesh Agrawal
[iiii] McGraw Hill Education Access Science “Why can’t you put metal objects into microwave ovens?” Dinesh Agrawal