Food in a microwave oven is heated by radiation. Microwave energy penetrates directly into the food to produce heat. Microwaves are a form of electromagnetic radiation, the waves being generated by a magnetron vacuum tube that converts electrical energy received by an amplifier into microwave radiation. The Federal Communications Commission has assigned two frequencies, 915 and 2,450 MHz, for radiation in microwave ovens. Both these frequencies are above the visible light range. The 2,450 MHz frequency has a shorter wavelength than the signal at 915 MHz; hence, it does not penetrate as deeply into the food mass as does the longer wavelength at 915 MHz.
Microwaves generate heat in food because of the electrical nature of water molecules. Water is a dipolar molecule: One portion of a water molecule carries a
positive electrical charge, while the other part carries a negative charge. The microwave energy penetrating the food is characterized by its very rapid alternating of electrical charge. This constant change causes the water molecules to vibrate very actively, and the resulting vibration generates heat.
Unlike other forms of cookery, this energy is generated within the food rather than traveling from the surface toward the interior. Once the food begins to heat because of the dipolar nature of the food in relation to the microwave energy, conduction also occurs, helping to equal- ize the temperature throughout the food. The standing time recommended in many microwave oven recipes acknowledges the importance of allowing conduction to contribute to the overall cookery mode.
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section two | food preparationAlthough these are distinctly different types of heating, most foods are heated by a combination of at least two of these cooking techniques. For instance, a broiled steak will be heated by conduction as well as by radiation. A soup being heated in a pan will be heated by conduction and convection.
Microwave ovens also utilize the infrared frequency range (Figure 4.14) for a unique way of heating foods. In microwave ovens, a magnetron tube is used to generate waves of frequency above the length of visible light, specifically either 915 or 2,450 megahertz (MHz). These waves, called microwaves, penetrate as much as an inch into a food, causing rapid vibra- tions of individual molecules of water or fat. It is these vibrations that cause the food to begin to heat rapidly.
For food to be heated by microwave energy, there must be some water or fat present, although a large quantity is not essential to success. Fat becomes very hot even more quickly than water does in a food being heated by microwaves. Thus, butter or other fats can be melted very quickly in a microwave oven. Similarly, meats tend to be overheated quickly by microwave cookery unless the slices are relatively thin and the progress of the microwaves into the meat is monitored with extreme care.
Microwave ovens must be operated only when water or food has been placed in them, for the microwaves introduced into the cavity must have a substance to absorb them. Otherwise, the waves will be bounced right back toward the magnetron tube, causing irreparable damage to the tube. An empty pan or dish will not absorb the microwaves any better than the empty cavity. Even if food is present, microwaves will arc back toward the magnetron if metal is placed within the unit. Consequently, water or food must always be in a microwave oven before it is turned on or operated. Also, metal should not be placed in a microwave oven because it will cause arcing.
microwave oven special type of oven that is able to heat food by sending waves of 915 or 2,450 MHz from a magnetron tube directly into foods, where water and/or fat molecules vibrate and heat foods.
http://www.colorado.edu/ physics/2000/microwaves/ index.html
—Interactive site explaining heating by microwaves. http://www.fda.gov/ Radiation- Emitting Products/ResourcesforYou RadiationEmittingProducts/ Consumers/ucm142616.htm —Information on cooking food safely using microwave ovens.
magnetron tube tube generating microwaves in a microwave oven.
microwave Form of electromagnetic energy; 915 and 2,450 MHz are the assigned frequencies for microwave ovens.
0.01 0.1 1 10 100 1,000 10,000 100,000
Wavelength (λ) in millimicrons (log scale) λ Gamma rays X-rays Ultraviolet Infrared Visible 400 700
Violet Blue Green Yellow Orange Red
400 500 600 700
Figure 4.14
Frequencies assigned to microwave ovens (915 and 2,450 MHz) are above visible light. Courtesy of Plycon Press.
SuMMary
Food preparation is done most successfully when the appropriate equipment is available. Basic preparation equipment includes measuring cups and spoons, mixing bowls, an electric mixer, a variety of utensils, a cooking thermometer, and a variety of cutlery. A set of pans, a large pot, frying pans, and appropriate baking pans are necessary for cooking and baking.
Accurate measurements are essential for preparing recipes successfully. Dry ingredients are measured volumetrically using graduated measuring cups or spoons. Liquids are measured in clear glass measuring cups.
The risk of a fall in the kitchen can be reduced if spills are wiped up as soon as they happen. Any mats or rugs in the
factors in food preparation | chapter four
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kitchen should lie flat and have a backing that prevents slipping.Care needs to be exercised when plugging in or using electri- cal appliances. Food should be cut by pushing the knife blade away from the body. Burns can be prevented when handling hot pans by using hot pads (not towels). Safety of the food is dependent upon the sanitation practices of food handlers and on proper temperature control.
Freezing and boiling temperatures are influenced by the presence of sugar. Sugar dissolves and lowers the freezing point of ice creams and other foods high in sugar. Sugar solutions boil at elevated temperatures because sugar lowers vapor pressure. Although salt also alters boiling temperatures, it is used in such small amounts that the effect is insignificant in food preparation.
Heat control is important in food preparation and storage. Moderate heats include lukewarm, scalding, and
simmering. These can be achieved by use of a double boiler or a very low heat setting on a range.
High elevations and/or partial vacuums will cause water to boil at a lower temperature than normal. The converse is provided by pressure saucepans or pressure cookers, which reach elevated temperatures, thus reducing cooking times. Deep-fat frying is another technique that provides fast cook- ing because temperatures in fat can be about 375°F (190°C) or even a bit hotter.
Food may be heated by conduction, convection, radia- tion, or a combination of these methods. Radiation is quite fast, resulting in reduction of cooking time compared with either convection or conventional ovens; microwaves cause water and fat molecules within a food to vibrate rapidly, resulting in heat generation and rapid heating of food from within.
StuDy QueStIONS
1. Describe the way to measure flour and explain the rationale
for the technique.
2. What is the best way to measure 3/4 cup of rice
volumetrically?
3. What is the most accurate way of measuring (a) 3 teaspoons
of baking powder, (b) 5 1/3 tablespoons of sugar?
4. What is the recommended method for measuring 1/2 cup
of shortening? Describe a second method sometimes used
to measure solid fats in the home. Why is this method less desirable than the first?
5. Why does the temperature rise when a sugar solution is
boiled actively for several minutes?
6. How can atmospheric pressure be modified? What
influence does atmospheric pressure have on the boiling point of water?
7. Describe the ways in which heat is transferred into foods.