Boiling is extremely active agitation of a liquid, at which time some of the liquid changes to the vapor state. This occurs when the vapor pressure (pressure in a liquid to escape) of a liquid just exceeds atmospheric pressure (downward pressure on the liquid). The temperature at which boiling occurs is the boiling point, which is 212°F (100°C) for water at sea level. Interestingly, pure water at sea level will remain at 212°F (100°C) regardless of how fast the water is boiling. The heat can be turned higher, and the water will boil more rapidly than before, but the temperature will remain constant.
lukewarm Approximately body temperature; about 100°F (40°C).
scalding temperature used to loosen fruit skins and perform other similar functions; about 150°F (65°C).
simmering Range of temperatures between 180°F (82°C) and 211°F (99°C); bubbles form and rise, but rarely break the surface; more gentle heat treatment than boiling.
boiling Active agitation of liquid and transition of some liquid to the vapor state; occurs when vapor pressure just exceeds atmospheric pressure.
vapor pressure Pressure within a liquid for individual molecules to escape from the liquid; varies with the temperature of the liquid and with dissolved substances.
atmospheric pressure
Pressure of the atmosphere pressing downward on the surface of a liquid; varies with elevation.
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section two | food preparationTo change water from the liquid state into the gaseous state (steam) requires input of a considerable amount of energy, called the heat of vaporization. In fact, to convert a gram of boiling water into steam requires 540 kilocalories. Heat of vaporization requires almost seven times more energy than is involved in the heat of solidification.
When water is being heated, the temperature will rise quickly until the boiling point is reached, but there will be a lag before boiling actually begins because of the tremendous heat input needed to supply the energy needed for vaporization (steam) to occur. The situation is reversed when steam condenses into water, as occurs when steam comes in contact with skin. The serious burns resulting from contact with steam reflect the fact that 540 kilocalories are given off onto the skin when steam is converted back into water. This is one of the reasons to be careful to avoid contact with steam.
atmospheric Pressure The pressure being exerted downward upon a pan of water (atmospheric pressure) must be overcome before boiling can occur. Clearly, this pressure has an instrumental role to play in determining the temperature at which boiling will happen. Altitude modifies atmospheric pressure; in the mountains there is less atmosphere above the surface of the ground than there is at sea level. Consequently, atmospheric pressure is lower at high eleva- tions than at low ones. This means that boiling will take place more easily at a high elevation than at sea level; in other words, the temperature at boiling is lower on a high mountain than it is at the ocean. In fact, the boiling temperature of water drops 1°F for each 500 feet of gain in elevation (1°C for each 960 feet of gain).People who live at an elevation of 8,000 feet boil vegetables at a temperature of 196°F (91°C) in contrast to a temperature of 212°F (100°C) at the ocean. This is why vegetables or other foods cooked in boiling water in the mountains require a longer time to become tender than they do at lower elevations. A remarkable illustration of this drop in the temperature of boiling is noted in people’s accounts of expeditions on Mt. Everest, where the extremely high elevations and the consequently low atmospheric pressure cause such a decrease in the boiling temperature of water that it is possible to reach directly into a pot of boiling water.
A partial vacuum can be created in special equipment to modify the effect of atmospheric pressure. Sometimes this is done in food processing to reduce the temperature of boiling so much that water can be evaporated without causing a cooked flavor to develop. This is desirable when making orange juice concentrate. In processing the concentrate, a partial vacuum reduces the atmospheric pressure above the orange juice, causing the original orange juice to boil at a cool temperature. This technique promotes the evaporation of water, yet avoids actually “cooking” the orange juice.
Pressure saucepans and pressure canners artificially increase atmospheric pressure, result- ing in a higher temperature for boiling, which shortens cooking times. Commonly, 15 pounds of pressure will be generated within this very tight system, and this raises the cooking tem- perature to about 240°F (116°C). The elevated temperature significantly shortens the cooking
heat of vaporization
Energy required to convert boiling water into steam; 540 kilocalories per gram of water.
Industry InsIght
aFGp
The fact that fish do not freeze while living in the icy waters of the Arctic and Antarctic oceans has piqued the curiosity of food researchers since the middle of the 20th century. Considerable research has led to the isolation of a family of glycoproteins called antifreeze glycoprotein (AFGP), which is credited with keeping these fish from freezing to death by lowering the freezing point of their tissues dramatically. Two Antarctic fish, Trematomus borchgrevinki and Dissostichus
mawsoni, and a northern fish, Boreogadus saida, produce
glycoproteins.
Possible applications for AFGPs in food products are being explored now. AFGPs not only lower the freezing point but they also retard recrystallization, which helps to block the growth of large ice crystals. Because of these favorable qualities in determining ice crystal size, AFGPs might be useful as an additive in ice cream bars to help prevent the growth of large ice crystals; small ice crystals contribute to the smooth texture.
http://meetings.aps. org/Meeting/MAR07/ Event/58977
—Abstract of a paper presenting research on the effects of antifreeze glycoproteins on freezing temperatures.
factors in food preparation | chapter four
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time needed to tenderize a food (Figure 4.11). Commercially pressur-ized equipment accomplishes this same increase in temperature of the cooking medium.
Vapor Pressure The temperature of the water influences the vapor pressure, or the energy of water molecules trying to escape the liquid system. At room temperature, vapor pressure is quite low, but it increases rather rapidly as water temperature rises toward the boiling point. With adequate heat, vapor pressure will just exceed atmospheric pressure, and some of the water molecules will begin to escape into the air above the pan. Substances that form a true solution in water reduce the vapor pressure of the solution. For boiling to take place, additional heat must be supplied to raise the vapor pressure to the point at which vapor pressure just exceeds atmospheric pressure. The net result of the addition of a solute to water is to raise the boiling point. The greater the con centration of solute, the higher is the temperature of the boiling solution.
This effect is utilized in the preparation of candies, which are boiled to specified final temperatures. For example, fudge usually is cooked to 234°F (112°C), a temperature well above the 212°F tem- perature of boiling water. Several minutes of active boiling is required before the thermometer will finally reach the desired temperature because it is necessary to evaporate much of the liquid to concentrate
the sugar sufficiently. It is this high concentration that successfully reduces the vapor pressure of the solution sufficiently to cause boiling to occur at 234°F (or even higher in some other candies with still greater con centrations). Thus, the temperature is a reliable indicator that boiling sugar solution has reached the desired point. This subject is discussed in some detail in Chapter 9.
The molecules of sugar, as pointed out, are small enough to go into solution. Molecules smaller than 1 millimicron are able to form true solutions and reduce vapor pressure. Salt (sodium chloride) ionizes when it is placed in water, resulting in the formation of two ions (sodium and chloride) from a single molecule of salt. As a consequence, salt has twice as great an effect on vapor pressure as sugar. However, the unpalatable quality of a salty solution makes this effect of little practical use in cookery.
Many substances in foods are much larger than a millimicron. Proteins, for example, are large molecules that are unable to go into solution but form a colloidal dispersion. Therefore, they have no appreciable effect on the boiling point. Gelatin, starch, gums, cornmeal, and countless other food items can be added to boiling water in varying concentrations, but they will not cause a measurable change in the boiling point because they form coarse suspensions, not true solutions.