Microwave Heating Module- 21 Lec- 21 Dr. ShishirSinha Dept. of Chemical Engineeriing IIT Roorkee
Definition Heat - a form of energy matter possesses as result of the motion of its molecules. Temperature - relative measure of hotness and coldness 1-calorie-the amount of heat required to raised the temperature of 1 gram water by 1 C 1Btu (British thermal unit)-the amount of heat required to heat 1 lbs of water by 1F Medium of heat transfer Air - roasting, broiling, baking Water- boiling, parboiling, simmering, poaching, stewing Steam- rack above water pan, waterless cookers, pressure cooker, food wrapped in foil Fat- sauteing, stir-frying, pan-frying, deep-frying Microwave ovens Convert electrical energy into electromagnetic field. This field changes the direction of positive and negative charge billions times per second Heat is created by molecular friction when dipolar molecules such as water rotate forth and back alingning themselves with the electric field or ions migrate in response to field. Consumer microwaves: 500-600 watts (low power) 600-750 watts (medium power) 700-900 watts (high power) Remember that: Most recipes are written for medium power microwave ovens. If the oven is a low power oven cooking time must be increase by 15%
If the oven is a high power oven cooking time should be reduced Cooking times for microwave recipes should be considered as approximate Approximate wattage of oven at home can be tested as follows: A one liter glass measuring cup should be filled up with one liter of cool water and temperature of water should be recorded. The cup should be placed in the center of oven and cooked at its highest setting for exactly 2 minutes. The water should be stirred with a thermometer and temperature is recorded. The increase in a temperature is multiplied by 19.5 for Fahrenheit scale or 35 for Celsius scale. The number obtained is approximate wattage of the oven. Advantages of microwave cooking: Food is cooked 2-10 times faster Reconstitution of leftovers with their flavour and texture Absence of heat in surrounding air and utensils Saving electric energy Disadvantages of microwave cooking: Air inside microwave is cool. This produces a surface cooling in the food-browned crust does not developed readily simmering and stewing to tenderize food difficult flavour does not blend well Deep-frying eliminated Limited to small quantities microbial hazard Principles of microwave heating: Nonionizing radiation -5 Quantum energy is 1.2x10 ev, energy required to break hydrogen bond is 5.2 ev
Microwave is comprised of electric and magnetic fields oriented perpendicularly Electric field play primary role in heating by promoting rotation of polar molecules. Heat is generated by molecular friction The ability to heat is dictated by: Dielectric constant-describes the material s capacity to store electrical energy. Water - 78, ice 3.2 Dielectric loss- describes the material s ability to dissipate electrical energy as heat. Water-10.6, ice-0.03 Tangenial loss refers to material s capacity to be penetrated by elictrical field and subsequently dissipate energy as heat. It is expreesed as a ratio of dielectric loss to dielectric constant Dielectric behaviour of food system is affected by free ionic salt content, moisture, and solid contents Microwave-depth of penetration The depth of penetration is the depth to which microwave energy reaches until it is 37% of its value. The depth of microwave penetration to 1% saline solution is 50% of that in water. High moisture products -greater microwave absorption. Low moisture products -may also heat well due to lower specific heat capacity and greater wave penetration. Microwave frequency-2415mhz and 915MHz. These frequencies produce waves of 0.12m and 0.33m respectively. As the wavelength increases the penetration increases Mass, density, geometry Larger products absorb more energy than smaller, but it take more time to cook these products in order for conduction to bring about temperature gradient to equilibrate. The uniformity of microwave heating is affected by: Moisture and ionic content of food-moisture and salts decrease the penetration depth. As the moisture increases the depth of penetration decreases. The specific heat of various food components
Product density Product shape Load volume Presence of air within product makes product not only a good insulator, but also reduces dielectric constant. Deeper penetration results in shorter baking times. Objects with corners show localized heating due to multidirectional distribution of microwave energy. Round objects heat more uniformly The uniformity of microwave heating Specific heat-amount of heat gained or lost by a unit weight to obtained desired change in temperature w/o a change in state. Oils vs water -oils heat quicker as less energy is required to reach specific temperature Temperature- as temperature increases less energy is absorbed. Lethality of microwaves: Thermal effects -microwaves reduces the number of bacteria by traditional effect exerted by heat. Athermal effects-attributed to the intrinsic nature of micowaves Cooking techniques: Arrangement Stirring and rotating Covering to retain moisture and prevent fat spattering Standing time Cooking utensils: Must be able to transfer microwaves Selection should depends on the time of cooking Shape of utensils should prevent overcooking at the edges.
Glass, china - test pour 1/2 cup of water into dish. Cook one minutes on the highest setting. If water warm and dish cool, the dish safe to use. Paper goods -readily absorbs moisture, prevents sogginess, good for short cooking Plastics-dish-washer safe good for short cooking. Prolonged cooking (steam) cause softening Straw and woods - only good for short time. Slight amount of water in woods may cause warming of wood and cracking Metal containers-should not be used as the reflect waves from food and cause arcing -damage magnetron Heat susceptors-disposable made of metalized polyester laminated with paperboard. Paper absorb energy reflected from metal and becomes hot enough to brown or crisp the food by conduction. Safety rules: Always check a new oven for shipping damage and read manufacture manual Never operate oven with an object caught in the door or with door that does not close properly Do not tamper with safety locks or operate empty oven keep the door seal clean and free of grease without using cleanser or steel wool. Check periodically for wrapping, pitting or tearing Serviced the oven by qualified servicemen do not used recycled paper goods avoid the use of cold storage containers or letting plastic wrap to touch food as chemicals may migrate into food use caution when heating baby foods in jars or formula in bottle as hot spots can occur and cause serious burns.
References http://en.wikipedia.org/wiki/microwave_oven http://www.comsol.co.in/multiphysics/microwaveheating/ http://www.ecw.org/prod http://www.industrialmicrowave.com/ http://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&v ed=0cfwqfjae&url=http%3a%2f%2fscholar.lib.vt.edu%2ftheses%2favailable% 2Fetd-12232002-135250%2Funrestricted%2Fthesis.pdf&ei=1CHzUqaLDomMkwWVr4DoBQ&usg= AFQjCNFYrnmP2sxwCmZJc94XtGJ2khJfYw&sig2=51yok_uyfharftnPlTBWHQ http://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&v ed=0cfqqfjad&url=http%3a%2f%2fwww.ecw.org%2fprod%2f433-1mh.pdf&ei=1chzuqaldommkwwvr4dobq&usg=afqjcnhqfgyna8yskoefhx wcrobqiyvh-a&sig2=nm18z6zhy3hyxfwbnawy_g http://www.liv.ac.uk/~mimi/chapter3.pdf http://www.bmskorea.co.kr/bms_product/makerimage/notice/milestone/microwave_ heating.pdf