Boiling

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Boiling, a type of phase transition, is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmospheric pressure. Thus, a liquid may also boil when the pressure of the surrounding atmosphere is sufficiently reduced, such as the use of a vacuum pump or at high altitudes. Boiling occurs in three characteristic stages, which are nucleate, transition and film boiling. These stages generally take place from low to high surface temperatures, respectively.

Nucleate boiling is characterized by the incipience and growth of bubbles on a heated surface, which rise from discrete points on a surface, whose temperature is only slightly above the liquid’s saturation temperature. In general, the number of nucleation sites are increased by an increasing surface temperature. An irregular surface of the boiling vessel (i.e. increased surface roughness) can create additional nucleation sites, while an exceptionally smooth surface, such as glass, lends itself to superheating. Under special conditions, a heated liquid may show boiling delay when heated over its boiling point, by starting to boil suddenly and violently.

When the surface temperature reaches a maximum value, the critical superheat, vapor begins to form faster than liquid can reach the surface. Thus, the heated surface suddenly becomes covered with a vapor layer. Because of the vapor layer’s lower thermal conductivity, this vapor layer insulates the surface. This condition of a vapor film insulating the surface from the liquid characterizes film boiling.

Transition boiling may be defined as the unstable boiling, which occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling.

Boiling in cookery

In cookery, boiling is cooking food in boiling water, or other water-based liquid such as stock or milk. Simmering is gentle boiling, while in poaching the cooking liquid moves but scarcely bubbles.

In places where the available water supply is contaminated with disease-causing bacteria, boiling water and allowing it to cool before drinking it is a valuable health measure. Boiling water for a few minutes kills most bacteria, amoeba, and other microbial pathogens. It thus can help prevent cholera, dysentery, and other diseases caused by microorganisms.

The temperature of a substance is constant as it undergoes a phase transition. Therefore, increasing the temperature of a liquid already boiling by increasing the rate of heat transfer is impossible: it will just boil more quickly. Once it has turned into steam, water will increase in temperature as heat is applied to it. Pressure and a change in composition of the liquid may alter the boiling point of the liquid. For this reason, high elevation cooking generally takes longer since boiling point is a function of atmospheric pressure. In Denver, Colorado, which is at an elevation of about one mile, water boils at approximately 95 C. Depending on the type of food and the elevation, the boiling water may not be hot enough to cook the food properly. The boiling point is defined as the temperature at which the vapor pressure of the substance equals the pressure above the substance. Increasing the pressure as in a pressure cooker raises the temperature of the contents above the open air boiling point. Adding a water soluble substance, such as salt or sugar also increases the boiling point. This is called boiling-point elevation. However, the effect is very small, and the boiling point will be increased by an insignificant amount. On the other hand, salt or ethylene glycol can cause significant freezing point depression. Due to variations in composition and pressure, the boiling point of water is almost never exactly 212 F / 100 C, but rather close enough for cooking. Boiling is when the vapor pressure exceeds the air pressure.

Foods suitable for boiling include:

• Fish
• Vegetables
• Farinaceous foods such as pasta
• Eggs
• Meats
• Sauces
• Stocks and soups

Advantages:

• Older, tougher, cheaper cuts of meat and poultry can be made digestible
• It is appropriate for large-scale cookery
• Nutritious, well flavoured stock is produced
• It is safe and simple
• Maximum colour and nutritive value is retained when cooking green vegetables, provided boiling time is kept to the minimum

Disadvantages:

• There is a loss of soluble vitamins in the water
• It can be a slow method
• Foods can look unattractive

Boiling can be done in two ways: The food can be placed into already rapidly boiling water and left to cook, the heat can be turned down and the food can be simmered; or the food can also be placed into the pot, and cold water may be added to the pot. This may then be boiled until the food is satisfactory.

A wet pot takes longer to boil

The water on the outside of the pot actually increases the time it takes a pot of water to boil. The pot will heat at a normal rate once all excess water on the outside of the pot evaporates.

Baking

From Wikipedia, the free encyclopedia



Baking is the technique of prolonged cooking of food by dry heat acting by conduction, and not by radiation, normally in an oven, but also in hot ashes, or on hot stones.[1] It is primarily used for the preparation of bread, cakes, pastries and pies, tarts, and quiches. Such items are sometimes referred to as "baked goods," and are sold at a bakery. A person who prepares baked goods as a profession is called a baker. It is also used for the preparation of baked potatoes; baked apples; baked beans; some pasta dishes, such as lasagne; and various other foods.

Domestic ovens in North America are usually provided with two heating elements: one in the bottom for baking, using convection and conduction to heat the food; and one in the top for broiling or grilling, heating mainly by radiation. Meat may be baked, but is more often roasted, a similar process, using higher temperatures and shorter cooking times.

The dry heat of baking changes the structures of starches in the food and causes its outer surfaces to brown, giving it an attractive appearance and taste, while partially sealing in the food's moisture. The browning is caused by caramelization of sugars and the Maillard reaction. Moisture is never really entirely "sealed in," however; over time, an item being baked will become more and more dry. This is often an advantage, especially in situations where drying is the desired outcome, for example in drying herbs or in roasting certain types of vegetables. The most common baked item is bread. Variations in the ovens, ingredients and recipes used in the baking of bread result in the wide variety of breads produced around the world.

Some foods are surrounded with moisture during baking by placing a small amount of liquid (such as water or broth) in the bottom of a closed pan, and letting it steam up around the food, a method commonly known as braising.

Over time breads become hard in a process known as going stale. This is not primarily due to moisture being lost from the baked products, but more a reorganization of the way in which the water and starch are associated over time. This process is similar to recrystallization, and is promoted by storage at cool temperatures, such as those of a domestic refrigerator.

Effects of cooking

Proteins

Edible animal material, including muscle, offal, milk and egg white, contains substantial amounts of protein. Almost all vegetable matter (in particular legumes and seeds) also includes proteins, although generally in smaller amounts. These may also be a source of essential amino acids. When proteins are heated they become de-natured and change texture. In many cases, this causes the structure of the material to become softer or more friable - meat becomes cooked. In some cases, proteins can form more rigid structures, such as the coagulation of albumen in egg whites. The formation of a relatively rigid but flexible matrix from egg white provides an important component of much cake cookery, and also underpins many desserts based on meringue.

Liquids

Cooking often involves liquids, both added in order to immerse the substances being cooked (typically water, stock or wine), and released from the foods themselves. Liquids are so important to cooking that the name of the cooking method used may be based on how the liquid is combined with the food, as in steaming, simmering, boiling, braising and blanching. Heating liquid in an open container results in rapidly increased evaporation, which concentrates the remaining flavors and ingredients - this is a critical component of both stewing and sauce making.

Fat

Fats and oils come from both animal and plant sources. In cooking, fats provide tastes and textures. When used as the principal cooking medium (rather than water), they also allow the cook access to a wide range of cooking temperatures. Common oil-cooking techniques include sauteing, stir-frying, and deep-frying. Commonly used fats and oils include butter, olive oil, sunflower oil, lard, beef fat (both dripping and tallow), rapeseed oil or Canola, and peanut oil. The inclusion of fats tends to add flavour to cooked food, even though the taste of the oil on its own is often unpleasant. This fact has encouraged the popularity of high fat foods, many of which are classified as junk food such as hamburgers or convenience fried cereal snacks.

Carbohydrates

Carbohydrates used in cooking include simple sugars such as glucose (from table sugar) and fructose (from fruit), and starches from sources such as cereal flour, rice, arrowroot, potato. The interaction of heat and carbohydrate is complex.

Long-chain sugars such as starch tend to break down into more simple sugars when cooked, while simple sugars can form syrups. If sugars are heated so that all water of crystallisation is driven off, then caramelisation starts, with the sugar undergoing thermal decomposition with the formation of carbon, and other breakdown products producing caramel. Similarly, the heating of sugars and proteins elicits the Maillard reaction, a basic flavor-enhancing technique.

An emulsion of starch with fat or water can, when gently heated, provide thickening to the dish being cooked. In European cooking, a mixture of butter and flour called a roux is used to thicken liquids to make stews or sauces. In Asian cooking, a similar effect is obtained from a mixture of rice or corn starch and water. These techniques rely on the properties of starches to create simpler mucilaginous saccharides during cooking, which causes the familiar thickening of sauces. This thickening will break down, however, under additional heat.

Food safety

If heat is used in the preparation of food, this can kill or inactivate potentially harmful organisms including bacteria and viruses. The effect will depend on temperature, cooking time, and technique used. The temperature range from 5°C to 57°C (41°F to 135°F) is the "food danger zone." Between these temperatures bacteria can grow rapidly. Under the correct conditions bacteria can double in number every twenty minutes. The food may not appear any different or spoiled but can be harmful to anyone who eats it. Meat, poultry, dairy products, and other prepared food must be kept outside of the "food danger zone" to remain safe to eat. Refrigeration and freezing do not kill bacteria, but only slow their growth. When cooling hot food, it shouldn't be left on the side or in a blast chiller (an appliance used to quickly cool food) for more than 90 minutes.

Cutting boards are a potential breeding ground for bacteria, and can be quite hazardous unless safety precautions are taken. Plastic cutting boards are less porous than wood and have conventionally been assumed to be far less likely to harbor bacteria.[1] This has been debated, and some research have shown wooden boards are far better.[2] Washing and sanitizing cutting boards is highly recommended, especially after use with raw meat, poultry, or seafood. Hot water and soap followed by a rinse with an antibacterial cleaner (dilute bleach is common in a mixture of 1 tablespoon per gallon of water, as at that dilution it is considered food safe, though some professionals choose not to use this method because they believe it could taint some foods), or a trip through a dishwasher with a "sanitize" cycle, are effective methods for reducing the risk of illness due to contaminated cooking implements.

Cooking

From Wikipedia, the free encyclopedia



Cooking is the act of preparing food for ingestion. The term cooking encompasses all methods of food preparation including non-heated methods. It encompasses a vast range of methods, tools and combinations of ingredients to alter the flavor or digestibility of food. It is the process of selecting, measuring and combining of ingredients in an ordered procedure in an effort to achieve the desired result. Factors affecting the final outcome include the variability of ingredients, ambient conditions, tools, and the skill of the individual doing the actual cooking.

The diversity of cooking worldwide is a reflection of the myriad nutritional, aesthetic, agricultural, economic, cultural and religious considerations that impact upon it.

Cooking often requires applying heat to a food, which usually, though not always, chemically transforms it, thus changing its flavor, texture, appearance, and nutritional properties. There is archaeological evidence of roasted foodstuffs, both animal and vegetable, in human (Homo erectus) campsites dating from the earliest known use of fire, some 800,000 years ago. Other methods of cooking that involve the boiling of liquid in a receptacle have been practiced at least since the 10th millennium BC, with the introduction of pottery.