|How Food Cooks|
Cooking can be defined as the transfer of energy from a heat source to a food. This energy alters the food's molecular structure, changing its texture, flavour aroma and appearance. But why is food cooked at all? The obvious answer is that cooking makes food taste better. Cooking also destroys undesirable microorganisms and makes foods easier to ingest and digest.
To cook foods successfully, you must first understand the ways in which heat is transferred: conduction, convection and radiation. You should also understand what the application of heat does to the proteins, sugars, starches, water and, fats in foods.
Perhaps most important, you must understand the cooking methods used to transfer heat: broiling, grilling, roasting and baking, sautéing, pan-frying, deep-frying, poaching, simmering, boiling, steaming, braising and stewing.
Each method is used for many types of food, so you will be applying one or more of them every time you cook. The cooking method you select gives the finished product a specific texture, appearance, aroma and flavour. A thorough understanding of the basic procedures involved in each cooking method helps you produce consistent, high-quality products.
Heat is a type of energy. When a substance gets hot, its molecules have absorbed energy, which causes the molecules to vibrate rapidly, expand and bounce off one another. As the molecules move, they collide with nearby molecules, causing a transfer of heat energy. The faster the molecules within a substance move, the higher its temperature. This is true whether the substance is air, water, an aluminium pot or a sirloin steak. Heat energy may be transferred to foods via conduction, convention or radiation. Heat then travels through foods by conduction. Only heat is transferred-cold is simply the absence of heat, so cold cannot he transferred from one substance to another.
Conduction is the most straightforward means of heat transfer. It is simply the movement of heat from one item to another through direct contact. For example, when the flame of a gas burner touches the bottom of a sauté pan, heat is conducted to the pan. The metal of the pan then conducts heat to the surface of the food lying in that pan.
Some materials conduct heat better than others. Water is a better conductor of heat than air. This explains why a potato cooks much faster in boiling water than in an oven, and why you cannot place your hand in boiling water at a temperature of 212 F (100C), but can place your hand, at least very briefly, into a 400 F (200C) oven.
Generally, metals are good conductors. Copper and aluminium are the best conductors, while liquids and gases are poor conductors. Conduction is a relatively slow method of heat transfer because there must be physical contact to transfer energy from one molecule to adjacent molecules. Consider what happens when a metal spoon is placed in a pot of simmering soup. At first the spoon handle remains cool. Gradually, however, heat travels up the handle, making it warmer and warmer, until it becomes too hot to touch.
Conduction is important in all cooking methods because it is responsible for the movement of heat from the surface of a food to its interior. As the molecules near the foods exterior gather energy, they move more and more rapidly. As they move, they conduct heat to the molecules nearby, thus transferring heat through the food (from the exterior of the item to the interior).
In conventional heating methods (non-microwave), the heat source causes food molecules to react largely from the surface inward so that layers of molecules heat in succession. This produces a range of temperatures within the food, which means that the outside can brown and form a crust long before the interior is noticeably warmer. That is why a steak can be fully cooked on the outside but still rare on the inside.
Convection refers to the transfer of heat through a fluid, which may be liquid or gas. Convection is actually a combination of conduction and a mixing in which molecules in a fluid (whether air, water or fat) move from a warmer area to a cooler one.
There are two types of convection: natural and mechanical. Natural convection occurs because of the tendency of warm liquids and gases to rise while cooler ones fall. This causes a constant natural circulation of heat. For example, when a pot of stock is placed over a gas burner, the molecules at the bottom of the pot are warmed. These molecules rise while cooler, heavier molecules sink. Upon reaching the pots bottom, the cooler molecules are warmed and begin to rise. This ongoing cycle creates currents within the stock, and these currents distribute the heat throughout the stock.
Mechanical convection relies on fans or stirring to circulate heat more quickly and evenly. This explains why foods heat faster and more evenly when stirred. Convection ovens are equipped with fans to increase the circulation of air currents, thus speeding tip the cooking process. But even conventional ovens (that is, not convection ovens) rely on the natural circulation patterns of heated air to transfer heat energy to items being baked or roasted.
Unlike conduction and convection, radiation does not require physical contact between the heat source and the food being cooked. Instead, energy is transferred by waves of heat or light striking the food. Two kinds of radiant heat are used in the kitchen: infrared and microwave.
Infrared cooking uses an electric or ceramic element heated to such a high temperature that it gives off waves of radiant heat that cooks the food. Radiant heat waves travel at the speed of light in all direction unlike convection heat, which only rises until they are absorbed by a food.
Induction cooking uses a special induction coil placed below the stove top's surface in combination with specially designed cookware made of cast iron or magnetic stainless steel. The coil generates a magnetic current so that the cookware is heated rapidly with magnetic friction. Heat energy is then transferred from the cookware to the food by conduction. The cooking surface, which is made of a solid ceramic material, remains cool. Only the cookware and its contents get hot.
Induction cooking is a relatively new process, more popular in Europe than the United States. But it is gaining acceptance in professional kitchens because of the speed with which foods can be heated and the ease of cleanup. which only rises) until they are absorbed by a food. Infrared cooking is commonly used with toasters and broilers. The glowing coals of a fire are another example of radiant heat.
Microwave cooking relies on radiation generated by a special oven penetrate the food, where it agitates water molecules, creating friction heat. This energy then spreads throughout the food by conduction (and convection in liquids) Microwave cooking is much faster than other methods because energy penetrates the food up to a depth of several centimetres, putting all water molecules in motion at the same time. Heat is generated quickly and uniformly throughout the food.
Microwave cooking does not brown foods however, and often gives meats a mushy texture, making microwave ovens an unacceptable replacement for traditional ovens. Because microwave radiation affects only water molecules, a completely waterless material (such as a plate) will not get hot. Any warmth felt in air used when microwaving food results from heat being conducted to the plate.
Microwave cooking requires the use of certain types of utensils, usually heat-resistant glass or microwavable plastic. Even heat-resistant glass can shatter and is not recommended for professional use, however. The aluminium stainless steel utensils most common in professional kitchens cannot be used because metal deflects microwaves and this can damage the oven.