|The Effects of Heat|
The Effects of Heat
Foods are composed of proteins, carbohydrates (starches and sugars), and fats, plus small amounts of minerals and vitamins. Changes in shape, texture, colour and flavour of foods may occur when heat is applied to cad these nutrients. By understanding these changes and learning to control them you will be able to prepare foods with the characteristics desired. Although volumes are written on these subjects, it is sufficient for you to know the following processes as you begin your study of cooking.
The proper term for the cooking of proteins is coagulation. Proteins are large, complex molecules found in every living cell, plant as well as animal. Coagulation refers to the irreversible transformation of proteins from a liquid or semi-liquid state to a solid state. As proteins cook, they lose moisture, shrink and become firm. Common examples of coagulation are the firming of meat fibres during cooking, egg whites changing from a clear liquid to a white solid when heated and the setting of the structure of wheat proteins (known as gluten) in bread during baking. Most proteins complete coagulation at 1600F to 1850F (710C-850C).
Gelatinization is the proper term for the cooking of starches. Starches are complex carbohydrates present in plants and grains such as potatoes, wheat, rice and corn. When a mixture of starch and liquid is heated remarkable changes occur. The starch granules absorb water, causing them to swell, soften and clarify slightly. The liquid visibly thickens because of the water being absorbed into the starch granules and the granules themselves swelling to occupy more space.
Gelatinization occurs gradually over a range of temperatures-I 50F to 212Th (66C-100C)-depending on the type of starch used. Starch gelatinization affects not only sauces or liquids to which starches are added for the express purpose of thickening, but also any mixture of starch and liquid that is heated. For example, the flour (a starch) in cake hatter gelatinizes by absorbing the water from egg, milk or other ingredients as the batter bakes. This causes part of the firming and drying associated with baked goods.
The process of cooking sugars is properly known as caramelisation. Sugars are simple carbohydrates used by all plants and animals to store energy. As sugars cook, they gradually turn brown and change flavour. Caramelized sugar is used in many sauces, candies and desserts. But caramelised sugar is also partly responsible for the flavour and colour of bread crusts and the browning of meats and vegetables. In fact, it is the process of caramelisation that is responsible for most flavours we associate with cooking.
Sucrose (common table sugar) begins to brown at about 338F (1700C). The naturally occurring sugars in other foods such as maltose lactose and fructose also caramelize, but at varying temperatures. Because high temperatures are required for browning (that is, caramelizing), most foods will brown only on the outside and only through the application of dry heat. Because water cannot be heated above 212F (1000C), foods cooked with moist-heat methods do not get hot enough to caramelize. Foods cooked with dry-heat methods, including those using fats, will reach the high temperatures at which browning occurs.
All foods contain some water. Some foods, especially eggs, milk and leafy vegetables, are almost entirely water. Even as much as 75% of raw meat is water. As the internal temperature of a food increases, water molecules move faster and faster until the water turns to a gas (steam) and vaporizes. This evaporation of water is responsible for the drying of foods during cooking.
Fat is an energy source for the plant or animal in which it is stored. Fats are smooth, greasy substances that do not dissolve in water. Their texture varies from very firm to liquid. Oils are simply fats that remain liquid at room temperature. Fats melt when heated; that is, they gradually soften, then liquefy. Fats will not evaporate. Most fats can be heated to very high temperatures without burning, so they can be used as a cooking medium to brown foods.
Foods can he cooked in air, fat, water or steam. These are collectively known as cooking media. There are two general types of cooking methods: dry heat and moist heat. Dry-heat cooking methods are those using air or fat. They are broiling, grilling, roasting and baking, sautéing, pan-frying and deep-frying.
Foods cooked using dry-heat cooking methods have a rich flavour caused by browning. Moist-heat cooking methods are those using water or steam. They are poaching, simmering, boiling and steaming. Moist-heat cooking methods are used to emphasize the natural flavours of food. Other cooking methods employ a combination of dry- and moist-heat cooking methods. The two most significant of these combination-cooking methods are braising and stewing. Each of these cooking methods can be applied to a wide variety of foods- meats, fish, vegetables and even pastries. Here, we discuss only the general characteristics of these cooking methods.
Dry-Heat Cooking Methods
Cooking by dry heat is the process of applying heat either directly, by subjecting the food to the heat of a flame, or indirectly, by surrounding the food with heated air or heated fat.
Broiling uses radiant heat from an overhead source to cook foods. The temperature at the heat source can be as high as 20000F (10930C). The food to be broiled is placed on a preheated metal grate. Radiant heat from overhead cooks the food, while the hot grate below marks it with attractive crosshatch marks. Delicate foods that may be damaged by being placed directly on a metal grate or foods on which crosshatch marks are not desirable may be placed on a preheated heatproof platter then placed under the broiler. Cooking will take place through indirect heat from the preheated platter as well as from the direct heat from the broiler's overhead heat source.
Although similar to broiling, grilling uses a heat source located beneath the cooking surface. Grills may be electric or gas, or they can burn wood or charcoal, which will add a smoky flavour to the food. Specific woods such as mesquite, hickory or vine clippings can be used to create special flavours. Grilled foods are often identified by crosshatch markings
Roasting and Baking
Roasting and baking are the processes of surrounding a food with dry, heated air in a closed environment. The term roasting is usually applied to meats and poultry, while baking is used when referring to fish, fruits, vegetables, starches, breads or pastry items. Heat is transferred by convection to the food's surface, and then penetrates the food by conduction. The surface dehydrates, and the food browns from caramelisation completing the cooking process.
Sautéing is a dry-heat cooking method that uses conduction to transfer heat from a hot sauté pan to food with the aid (if a small amount of fat. Heat then penetrates the food through conduction. High temperatures are used to sauté, and the foods are usually cut into small pieces to promote even cooking.
To sauté foods properly, begin by heating a sauté pan on the stovetop, then add a small amount of fat. The fat should just cover the bottom of the pan. Heat the fat or oil to the point where it just begins to smoke. the food to be cooked should be as dry as possible when it is added to the pan to promote browning and to prevent excessive spattering. Place the food in the pan in a single layer. The heat should be adjusted so that the food cooks thoroughly; it should not be so hot that the outside of the food burns before the inside is cooked.
The food should be turned or tossed periodically to develop the proper colour. Larger items should be turned using tongs without piercing the surface. Smaller items are often turned by using the sauteuse's sloped sides to flip them back on top of themselves. When tossing sautéed foods, keep the pan in contact with the heat source as much as possible to prevent it from cooling.
Sautéing sometimes includes the preparation of a sauce directly in the pan after the main item has been removed. Stir-frying is a variation of sautéing. A wok is used instead of a sauté pan; the curved sides and rounded bottom of the wok diffuse heat and facilitate tossing and stirring. Otherwise, stir-frying procedures are the same as those outlined for sautéing and will not be discussed separately here.
Pan-frying shares similarities with both sautéing and deep-frying. It is a dry-heat cooking method in which heat is transferred by conduction from the pan to the food, using a moderate amount of fat. Heat is also transferred to the food from the hot fat by convection, Foods to be pan-fried are usually coated in breading. This forms a seal that keeps the food moist and prevents the hot fat from penetrating the food causing it to become greasy.
To pan-fry foods properly first heat the fat in a sauté pan. Use enough fat so that when the food to be cooked is added, the fat comes one third to one half way up the item being cooked. The fat should be at a temperature somewhat lower than that used in sautéing; it should not smoke but should he hot enough so that when the food is added it crackles and spatters from the rapid vaporization of moisture. If the temperature is too low, the food will absorb excessive amounts of fat; if it is too high, the food will burn on the outside before the interior is fully cooked.
When the food is properly browned on one side, turn it without piercing it, using tongs. Always turn the food away from you to prevent being burned by any fat that may splash. When the food is fully cooked, remove it from the pan, drain it on absorbent paper and serve it immediately.
Deep-frying is a dry-heat cooking method that uses convection to transfer heat to food submerged in hot fat. Heat then penetrates the food, cooking the in tenor through conduction. Foods to be deep-fried are usually first coated in batter or breading. This preserves moisture and prevents the food from absorbing excessive quantities of fat.
Deep-fried foods should cook thoroughly while developing an attractive deep golden-brown colour. Foods to be deep-fried should be of a size and shape that allows them to float freely in the fat. Today, most deep-frying is done in specially designed commercial fryers. These deep-fat fryers have built-in thermostats, making temperature control more precise.
To deep-fry food, first heat the fat or oil to temperatures between 325F and 375F (160C-190C). Slowly place the food in the fat, where it should float freely. Use tongs to turn it if necessary. When the food is clone, remove it from the fat, drain it on absorbent paper and serve it immediately.
Moist-Heat Cooking Methods
Cooking with moist heat is the process of applying heat to food by submerging it directly into a hot liquid or by exposing it to steam.
Poaching is a moist-heat cooking method that uses convection to transfer heat from a liquid to a food. For poaching, the food is submerged in a liquid held at temperatures between 160F and 180F (71C-82C). The surface of the liquid should show only slight movement, but no bubbles.
The flavour of the poaching liquid strongly affects the ultimate flavour of the finished product, so stock, court bouillon or broth is generally used. Poaching is most often associated with foods that do not require lengthy cooking to tenderize them, such as eggs or fish.
To poach food, first bring the poaching liquid to a boil in a suitably shaped cooking vessel. Add the food to be poached either by placing it directly into the liquid or by lowering it into the liquid using a specially designed rack. Adjust the heat as necessary to maintain the desired temperature throughout the cooking process. Do not allow the liquid to reach a boil since the agitation will cause meats to become tough and stringy and will destroy tender foods such as fresh fruit or fish. The liquid used to poach food is sometimes used to make an accompanying sauce.
Simmering is another moist-heat cooking method that uses convection to transfer heat from a liquid to a food. For simmering, the food is submerged in a liquid held at temperatures between 1857F and 2057F (850C-960C.) Because simmering temperatures are slightly higher than those used for poaching, there should be more action on the liquids surface, with a few air bubbles breaking through. As with poaching, the liquid used for simmering has a great effect on the foods flavour. Be sure to use a well-flavored stock or broth and to add mire-poix, herbs and seasonings as needed. Simmered foods should be moist and very tender.
Boiling is another moist-heat-cooking method that uses the process of convection to transfer heat from a liquid to a food. Boiling uses large amounts of rapidly bubbling liquid to cook foods. The turbulent waters and the relatively high temperatures cook foods more quickly than do poaching or simmering. Few foods. however, are cooked by true boiling.
Steaming is a moist-heat cooking method that uses the process of convection to transfer heat from the steam to the food being cooked. The food to be steamed is placed in a basket or rack above a boiling liquid. The food should not touch the liquid; it should be positioned so that the steam can circulate around it. A lid should be placed on the steaming pot to trap the steam and also create a slight pressure within the pot that speeds the cooking process. The liquid used to steam the food is sometimes used to make a sauce served with the item.
Another type of steaming uses a convection steamer. Convection steamers use pressurized steam to cook food very quickly in an enclosed chamber. Convection steamer cooking does not result in a flavored liquid that can be used to make a sauce.
Combination Cooking Methods
Some cooking methods employ both dry-heat and moist-heat cooking techniques. The two principal combination methods are braising and stewing. In both methods, the first step is usually to brown the main item using dry heat. The second step is to complete cooking by simmering the food in a liquid. Combination methods are good for less tender but flavourful cuts of meat.
Braised foods benefit from the best qualities of both dry- and moist-heat cooking methods. Foods to be braised are usually large pieces that are first browned in a small amount of fat at high temperatures. As with sautéing, heat is transferred from the pan to the food mainly by the process of conduction. Vegetables and seasonings are added, and enough sauce or liquid is added to come one third to one half way up the item being cooked. The pan is covered, and the heat is reduced. The food is then cooked at low heat, using a combination of simmering and steaming to transfer heat from the liquid (conduction) and the air (convection) to the food. This can be done on the stovetop or in the oven. A long, slow cooking period helps tenderize the main item. Braised foods are usually served with a sauce made from the cooking liquid.
Stewing also uses a combination of dry- and moist-heat cooking methods. Stewing is most often associated with smaller pieces of food that are first cooked either by browning them in a small amount of fat or oil, or by blanching them in a liquid. Cooking is then finished in a liquid or sauce. Stewed foods have enough liquid added to cover them completely and are simmered at a constant temperature until tender. Cooking time is generally shorter for stewing than for braising because the main items are smaller. Blanch-to very briefly and partially cook a food in boiling water or hot fat; used to assist preparation (for example, to loosen peels from vegetables, as part of a combination cooking method or to remove undesirable flavours.