Chapter 2: Cooking Methods and Materials
2.1 Cooking Methods
Cooking methods can be characterized into two main types: dry heat cooking and moist heat cooking. Dry heat cooking heats foods in the absence of water, and includes methods such as baking and roasting, broiling, and pan-frying. Moist heat cooking uses water to heat food, and includes boiling, simmering, braising, and steaming.
2.1.1 Dry Heat Cooking
Ovens are used in many dry heat methods. Electric ovens use two heating coils, located at the top and bottom of the oven. The bottom coil is used for baking and roasting; the top is used for broiling. The heating coils are simply resistive elements which are heated by passing an electric current through them.
In traditional ovens, when bottom coil is heated, the air inside the oven is heated primarily by conduction and natural convection. The heat is then transferred to the food, which is heated by the natural convection current. In a convection oven, the heat transfer is enhanced by the use of a fan. The fan creates forced convection within the oven, which not only heats the food faster but also encourages even distribution of heat.
Heat transfer inside an oven is actually more complicated than simply conduction and convection. The heating element emits a considerable amount of radiation which also contributes to the heating of the food. In addition, the walls of the oven become heated as well, emitting their own radiation. Broiling takes advantage of the radiation from the coil to rapidly heat the top of the food. Broiling uses only the top coil, which suppresses the natural convection current since the hot air is blocked by the ceiling of the oven. The radiation from the top coil (which is often set to a very high temperature) heats the surface of the food to high temperatures, promoting browning.
Cooking on the stovetop utilizes conduction through a pan to heat the contents. In dry heat cooking, heat is conducted directly from the pan to the food, as shown in Figure 2.1. The heat is generated by resistive elements on electric stovetops.
Figure 2.1 Dry heat cooking on the stovetop
2.1.2 Moist Heat Cooking
Moist heat methods use water in various states to heat food. The most common state is boiling, where water is heated by conduction through a pot on stovetop, and the heat is transferred to the food through convection (Figure 2.2). Water boils at 100 °C at sea level; therefore, boiling allows the food to cook at a constant temperature of 100 °C. Although this provides a convenient, consistent cooking medium, one major disadvantage is the lack of browning and the flavors that accompany the browning reaction. Browning of food occurs at temperatures above ~150 ºC, which cannot be reached by moist cooking methods. The boiling point of water can be raised slightly to reduce the cooking time. Impurities, such as salt, increase the boiling temperature by a few degrees. Pressure cookers also effectively raise the temperature of boiling water by increasing the pressure inside the pot. The relationship between pressure and boiling point is discussed in greater detail later on in the course.
Figure 2.2 Moist heat cooking
Various states of water are used for cooking, and there are special names for some of them. Poaching uses warm water just before boiling. Simmering refers to cooking in water when it has just begun to boil. Boiling uses vigorously boiling water. Steaming uses the water in vapor phase to heat the food. The temperature of steam is typically a few degrees hotter than the temperature of boiling water.
2.1.3 Other Methods
Deep frying is classified as a dry heat method, although the food is heated indirectly by convection through the oil. Oils can reach a higher temperature than water; this is why the food can be browned during frying but not in boiling. Typical frying temperatures range between 180 and 200 °C.
Microwave has become a popular cooking appliance, mainly because of its ability to heat foods quickly. It uses electromagnetic radiation to agitate water molecules, which then produce heat due to friction. You will study more about microwave ovens later on in the course.
2.2 Cooking Materials
Most cooking containers are metal or ceramic, due to their ability to withstand high temperatures. Common metals are aluminum, copper, tin, stainless steel, and cast iron. Ceramics include glass, porcelain, earthenware, and stoneware. In addition, non-stick coatings and enamel coatings may be used on the surfaces to improve the properties.
Thermal conductivity, density, and specific heat are three very important factors in determining what cooking material is best suited for the kitchen. A high conductivity material tends to have a more even temperature distribution than one with a low thermal conductivity. A material with low specific heat requires less thermal energy to heat; therefore, it heats faster than one with a high specific heat. Thermal diffusivity (α) is the combination of the three properties:
where k is the thermal conductivity, ρ is the density, and cp is the specific heat. Thermal diffusivity measures the effectiveness by which a material conducts thermal energy with respect to its ability to store thermal energy. A material with high α is characterized by a quick response to the changes in surrounding temperatures. A material with low α takes longer to reach a steady state condition, but is excellent at retaining heat once heated.
In addition to the effect of material properties, the thickness (mass) of pot also changes the heating characteristics. Thicker pots tend to allow more uniform heat distribution; however, they take longer to heat. The size and placement of the heating element with respect to the pot is also of concern.
Copper has the highest conductivity among all common cooking materials. Copper pans are highly treasured due to their ability to distribute heat evenly. However, copper tends to oxidize, leaving a black tarnish on its surface which also reduces its conductivity. To prevent oxidation, copper pots need constant cleaning and polishing. In addition, copper can be toxic if it diffuses into food in large amounts. Copper pans are often lined with tin to overcome these difficulties. The high cost of copper pans keep them from being used in majority of households.
Aluminum pans have the second highest thermal conductivity, next to copper, and are available at much lower cost. Aluminum is non-toxic and non-reactive, and also lightweight. Anodized aluminum has a thin coating of aluminum oxide (Al2O3) on the surface. Aluminum oxide forms as a result of aluminum reacting with oxygen at high temperature, and is a very hard ceramic suitable for protection from scratches.
Stainless steel is an alloy of iron, chromium, and nickel, and is desired due to its strength and resistance to corrosion. However, it is a relatively poor conductor, and often must be lined with copper or aluminum to improve the heat distribution.
Cast iron is typically iron alloyed with a small amount of carbon to increase strength. Cast iron pans are very prone to corrosion, and must be “seasoned” to prevent rusting. Seasoning a pan involves coating it with oil, then heating at moderate heat for a few hours. This fills up the small pores on the surface of the pan. Cast iron pans are generally heavy duty, allowing even heating despite the low conductivity and an ability to retain heat well.
Ceramic is usually a compound of metallic and non-metallic elements. They are poor conductors of heat, but are excellent at retaining heat once heated. They are very resistant to corrosion and are non-toxic. Due to their ability to retain heat, dishes made of thick ceramic materials will keep foods warm longer than metallic serving dishes of comparable shape and size.
A common problem with most ceramics is their tendency to crack due to thermal stress. Since they are such poor conductors of heat, there may be a large temperature difference between one side of the pan and the other. In such case, the warmer side expands more than the other, causing the pan to crack. Ceramic pans are seldom used on stovetops for this reason; ovens allow even heating from all sides, preventing large thermal gradients.
2.2.3 Coating Materials
Two common materials used for coating are Teflon (non-stick coating) and glass enamel. Teflon is a polymer which covers small roughness on pan surfaces and forms a very smooth finish, preventing food from sticking. Teflon coated pans are popular for low fat cooking; however, the coating is easily scratched by metal utensils. Glass enamel is formed by fusing glass powders to pan surfaces. The enamel improves the chemical resistance of the pan and prevents corrosion; however, they may crack when exposed to sudden changes in temperature.
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©2002 Mechanical Engineering Department*Tufts University*Medford, MA 02155