Roger Patrick, age 15, of Columbia, Tenn., for his question:
WHEN DID WE START USING REFRIGERATION?
Refrigeration is the process of removing heat from a space and providing for a location where things can be made cold and kept cold. Natural ice was used for the first refrigeration. Blocks of ice were cut during the winter from a lake's surface. The ice was then stored during the summer in sawdust in ice houses, creating a cool pace to store food.
The first shipment of natural ice for refrigeration happened almost 200 years ago, in 1799. A shipload of ice went from New York City to New Orleans, La.
In 1805, six years later, a shipload of ice was sent from Boston, Mass., to the West Indies. Refrigeration was needed there to help fight yellow fever.
By 1850, railroads started carrying blocks of ice to keep vegetables cool during shipments.
The first home refrigeration unit was the old fashioned ice box. It had a apace at the top, usually lined in metal, where a block of ice was placed, and a storage space below where food was kept. Openings between the ice chest and the storage area allowed the cold air to circulate from the top to the bottom.
The old fashioned ice box worked on a simple scientific basis. The warm air in the food storage area rises by convection, to the ice compartment. There the warm air gives up its heat to the ice, causing the ice to melt and chill the air. The cooled air, being heavier, moves downward into the food space below, were it absorbes more heat and cools the food.
The heat that melted the ice is then carried away in the water. This water flows out of the icebox through a tube.
Mechanical refrigeration is used in most homes today, rather than the ice box. The first mechanical refrigerators were introduced during the 1920s.
Early mechanical refrigerators were much like ice boxes, except they used evaporators, instead of ice, and compressors in the bottom below the food space.
In today's modern refrigerator, the space for unfrozen food is kept at a temperature of between 35 and 50 degrees Fahrenheit. Above 50 degrees bacteria and molds develop quickly and food can spoil.
In the older home refrigerators, ice cube freezing sections were kept at about 20 degrees. More common today is 10 degrees for the ice cube freezing section.
For longtime storage of frozen food today, zero degrees or lower is necessary. Foods that are frozen shouldn't be kept at temperatures above zero except for short periods of time. At about 15 degrees, frozen foods lose quality quite quickly.
The storage life of both unfrozen and frozen food varies with the food.