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Facts and Physics Behind Bringing Water to a Boil

Times Staff Writer

That old cliche “I can’t even boil water” may not be as much of an exaggeration as it sounds. Determining whether liquids are simmering, boiling or at the rolling boil stage is important to the success or failure of a recipe.

At sea level, water simmers between 180 and 211 degrees Fahrenheit. In this range, bubbles form slowly, rise and break below the surface (Photo 1). Maintaining a simmering stage requires careful adjustment of the heat source.

When water at sea level reaches 212 degrees, bubbles rise in a steady pattern and break on the surface (Photo 2). As the water boils harder, there is active agitation of the liquid and some is transformed to vapor (steam).

At a rolling boil stage (Photo 3), the liquid is cooking at the fastest boil possible to speed up evaporation.

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In “Food Fundamentals--Fourth Edition” (John Wiley & Sons: 1985), author Margaret McWilliams explains that boiling occurs when the vapor pressure of a liquid (pressure in a liquid to escape) just exceeds atmospheric pressure (downward pressure on the liquid).

“The pressure being exerted downward upon a pan of water must be overcome before boiling can occur. Therefore, this pressure has an instrumental role to play in determining the temperature at which boiling will happen,” according to McWilliams.

“Atmospheric pressure is influenced by the altitude; 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 elevations 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 one degree for each 500 feet of gain in elevation.”

To illustrate McWilliams’ point, at 8,000 feet water boils at 196 degrees, in contrast to 212 degrees at sea level. This explains why foods cooked in boiling liquids in the mountains require a longer time to become tender than they do at lower elevations. They are simply cooking at a lower temperature.


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