Food preparation

Temperature of Water

 

 

Freezing Point

Winter in many parts of the world brings freezing temperatures. The temperatures of winter can turn water into ice when its freezing point is reached. The lower temperature decreases water’s kinetic energy, or the energy associated with motion, which slows the movement of the water molecules until they finally set into a compact configuration. Heat of solidification occurs when at least 80 calories of heat are lost per gram of water. Unlike other substances, water expands and becomes less dense when completely frozen, which is why ice floats. The  expansion of frozen water ruptures pipes and containers filled with liquid. It should come as no surprise, then that it also ruptures the cells in plants and meats, diminishing the potential food’s textural quality. Pure water freezes at 32oF (zero degrees C) but adding anything else the water changes its freezing point. The addition of solutes such as salt and sugar to water lowers the freezing point. Adding too much, however, slows the freezing process. Thus, frozen desserts made with large quantities of sugar take extra time to freeze.

Melting Point

If removing heat from water causes it to turn into ice, then returning the same 80 calories (0.08 kcal) of heat to a gram of ice will cause it to reach its melting point and turn it back into water. While the ice absorbs the 80 calories (0.08 kcal) of heat, there is no rise in temperature. This latent heat does not register because it was put to work in moving the molecules of water far enough apart to change the physical structure of the solid ice into liquid water.

Boiling Point

Bubbles start to break the surface of water when it reaches 212°F (100°C) at sea level. This is its boiling point. The water will not get any hotter, nor will the food cook faster, no matter how much more heat is added, and this is why a slow rolling boil is often recommended. Keeping the temperature at a slow rolling boil is also more gentle on the foods and results in less evaporation.
The point at which water boils is reached when the pressure produced by steam, called vapor pressure, equals the pressure of the atmosphere pushing down on the earth. At this point the natural pressures of the atmosphere are not strong enough to push back the expanding gases of boiling water. Water requires 540 calories (0.54 kcal) of energy per gram to boil and vaporize. This heat of vaporization is quite a bit higher than the 80 calories (0.08 kcal) needed to melt ice. Serious burns can result from human exposure to steam because the amount of heat required to produce it is so high.

Increasing the elevation decreases the boiling point of water. At sea level, water boils at 212°F (100°C), but drops 1°F (-17°C) for every 500-foot increase in altitude. Water boils in the mountains at lower temperatures than at sea level because there is less air and atmospheric pressure pushing down on the earth’s surface. Steam has less resistance to overcome, and therefore occurs at lower temperatures. For example, at 7,000 feet v.-azer boils at 198°F (92°C). People at even higher elevations, such as on Mount Everest, could put a hand in a pan of boiling water and find it quite comfortable. Recipes are usually modified for elevations above 3,000 feet because the lower boiling temperature might affect ingredient actions and reactions.

Artificial changes in atmospheric pressure can be achieved by pressure cookers as well as by special equipment used only in the commercial food industry. A pressure cooker speeds up heating time by increasing atmospheric pressure to 15 pounds, thus water temperatures up to 240°F (116°C) can be achieved.

Hard versus Soft Water

Most water is not pure water, but contains dissolved gases, organic materials, and mineral salts from the air and soil. The minerals in water determine whether it is hard or soft water. Hard water contains a greater concentration of calcium and magnesium compounds, while soft water has a higher sodium concentration. The temperatures at which water freezes, melts, or boils remain constant regardless of whether it is hard or soft water. How can people tell if water is hard or soft? Hard water leaves a ring in the bathtub, a grayish sediment on the bottom of pans, and a grayish cast in washed whites. Although permanently hard water cannot be softened by boiling or distilling, it can be converted by a water softener, which works by exchanging sodium for calcium and magnesium.