LOCALIDAD: CIUDAD DE LA ENCAÑADA

Wavelengths of microwave range from meters down to a millimeter, their frequencies ranging from 0.3 to 300 GHz. A typical domestic oven generates 2.45 GHz microwaves, wavelength 0.123 m, and energy of photon 1.62  10-24 _{J (10 eV). For industrial}

applications, the frequency may be optimized for the specific processes.

Percy L. Spencer (1894–1970), the story goes, noticed that his candy bar melted while he was inspecting magnetron testing at the Raytheon Corp- oration in 1945. As a further test, he microwaved popping corns, which popped. A team at Raytheon developed microwave ovens, but it took more than 25 years and much more effort to improve them and make them practical and popular. Years ago, boiling water in a paper cup in a microwave oven without harming the cup amazed those who were used to see water being heated in a fire-resistant container over a stove or fire. Microwaves simultaneously heat all the water in the bulk food.

After the invention of the microwave oven, many offered explanations on how microwaves heat food. Water’s high dipole moment and high dielectric

constant caused it to absorb microwave energy, lead- ing to an increase in its temperature. Driven by the oscillating electric field of microwaves, water mol- ecules rotate, oscillate, and move about faster, increasing water temperature to sometimes even above its boiling point. In regions where water has difficulty forming bubbles, the water is overheated. When bubbles suddenly do form in superheated water, an explosion takes place. Substances without dipole moment cannot be heated by microwaves. Therefore, plastics, paper, and ceramics won’t get warm. Metallic conductors rapidly polarize, causing sparks due to arcing. The oscillating current and resistance of some metals cause a rapid heating. In contrast, water is a poor conductor, and the heating mechanism is very complicated. Nelson and Datta (2001) reviewed microwave heating in the Hand-

book of Microwave Technology for Food Applica- tions.

Molecules absorb photons of certain frequencies. However, microwave heating is due not only to ab- sorption of photons by the water molecules, but also to a combination of polarization and dielectric induction. As the electric field oscillates, the water molecules try to align their dipoles with the elec- tric field. Crowded molecules restrict one another’s movements. The resistance causes the orientation of water molecules to lag behind that of the electric field. Since the environment of the water molecules is related to their resistance, the heating rate of the water differs from food to food and region to region within the same container. Water molecules in ice, for example, are much less affected by the oscillat- ing electric field in domestic microwave ovens, which are not ideal for thawing frozen food. The outer thawed layer heats up quickly, and it is cooked before the frozen part is thawed. Domestic micro- wave ovens turn on the microwave intermittently or at very low power to allow thermal conduction for thawing. However, microwaves of certain frequen- cies may heat ice more effectively for tempering frozen food. Some companies have developed sys- tems for specific purposes, including blanching, tempering, drying, and freeze-drying.

The electromagnetic wave form in an oven or in an industrial chamber depends on the geometry of the oven. If the wave forms a standing wave in the oven, the electric field varies according to the wave pattern. Zones where the electric field varies with the largest amplitude cause water to heat up most

rapidly, and the nodal zones where there are no oscillations of electric field will not heat up at all. Thus, uniform heating has been a problem with microwave heating, and various methods have been developed to partly overcome this problem. Also, foodstuffs attenuate microwaves, limiting their pen- etration depth into foodstuff. Uneven heating re- mains a challenge for food processors and micro- wave chefs, mostly due to the short duration of microwaving. On the other hand, food is also sel- dom evenly heated when conventionally cooked.

Challenges are opportunities for food industries and individuals. For example, new technologies in food preparation, packaging, and sensors for moni- toring food temperature during microwaving are re- quired. There is a demand for expertise in microwav- ing food. Industries microwave-blanche vegetables for drying or freezing to take advantage of its energy efficiency, time saving, decreased waste, and reten- tion of water-soluble nutrients. The ability to quick- ly temper frozen food in retail stores reduces spoilage and permits selling fresh meat to cus- tomers.

Since water is the heating medium, the tempera- ture of the food will not be much higher than the boiling point of the aqueous solutions in the food. Microwave heating does not burn food; thus, the food lacks the usual color, aroma, flavor, and texture found in conventional cooking. The outer layer of food is dry due to water evaporation. Retaining or controlling water content in microwaved food is a challenge.

When microwaved, water vapor is continually removed. Under reduced pressure, food dries or freeze-dries at low temperature due to its tendency to restore the water activity. Therefore, microwaving is an excellent means for drying food because of its savings in energy and time. Microwaves are useful for industrial applications such as drying, curing, and baking or parts thereof.

Microwave ovens have come a long way, and their popularity and improvement continue. Food industry and consumer attitudes about microwav- able food have gone up and down, often due to mis- conceptions. Microwave cooking is still a challenge. The properties of water affect cooking in every way. Water converts microwave energy directly into heat, attenuates microwave radiation, transfers heat to various parts of the foodstuff, affects food texture, and interacts with various nutrients. All properties of

water must be considered in order to take advantage of microwave cooking.

WATER RESOURCES AND THE