题目内容
Centuries ago, man discovered that removing moisture from food helps to preserve it, and that the easiest way to do this is to expose the food to sun and wind. All foods contain water--cabbage and other leaf vegetables contain as much as 93% water, potatoes and other root vegetables 80% , lean meat 75% and fish anything from 80% to 60% depending on how fatty it is. If this water is removed, the activity of the bacteria(细菌) which cause food to go bad is checked. Nowadays most foods are dried mechanically. The conventional method of such dehydration(脱水) is to put food in chambers through which hoi air is blown at temperatures of about 110°C at entry to about 43°C at exit. This is the usual method for drying such things as vegetables, minced meat, and fish. Liquids such as milk, coffee, tea, soups and eggs may be dried by pouring them over a heated horizontal steel cylinder or by spraying them into a chamber through which a current of hot air passes. In the first case, the dried material is scraped off the roller as a thin film which is then broken up into small, though still relatively coarse flakes. In the second process it falls to the bottom of the chamber as a fine powder. Dried foods ’take up less room and weigh less than the same food packed in cans or frozen, and they do not need to be stored in special conditions. For these reasons they are invaluable to climbers, explorers and soldiers in battle, who have little storage space. They are also popular with housewives because it takes so little time to cook them. The last paragraph tends to discuss ______.
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When will Linda arrive
Astronaut Jim Voss has enjoyed many memorable moments in his career, including three spaceflights and one space walk. But he recalls with special fondness a decidedly earthbound (为地球引力所束缚的) experience in the summer of 1980 when he participated in the NASA-ASEE Summer Faculty Fellowship Program. Voss, then a science teacher at West Point, was assigned to the Marshall Space Flight Center’s propulsion lab in Alabama to analyze why a hydraulic fuel pump seal on the space shuttle was working so well when previous seals had failed. It was a seemingly tiny problem among the vast complexities of running the space program. Yet it was important to NASA because any crack in the seal could have led to destructive results for the astronauts who relied on them. "I worked a bit with NASA engineers," says Voss, "but I did it mostly by analysis. I used a handheld calculator, not a computer, to do a thermodynamic(热力学的) analysis." At the end of the summer, he, like the other NASA-ASEE fellows working at Marshall, summarized his findings in a formal presentation and detailed paper. It was a valuable moment for Voss because the ASEE program gave him added understanding of NASA, deepened his desire to fly in space, and intensified his application for astronaut status. It was not an easy process. Voss was actually passed over when he first applied for the astronaut program in 1978. Over the next nine years he reapplied repeatedly, and was finally accepted in 1987. Since then he has participated in three space missions. The 50-year-old Army officer, who lives in Houston, is now in training for a four-month mission as a crew member on the International Space Station starting in July 2000. Voss says the ASEE program is wonderful for all involved. "It brings in people from the academic world and gives NASA a special property for a particular period of time. It brings some fresh eyes and fresh ideas to NASA, and establishes a link with our colleges and universities," Voss explains. "There’s an exchange of information and an exchange of perspectives that is very important." For the academic side, Voss says, "the ASEE program also brings institutions of higher learning more insight into new technology. We give them an opportunity to work on real world problems and take it back to the classroom.\ Why was the hydraulic fuel pump seal important for the space shuttle
Centuries ago, man discovered that removing moisture from food helps to preserve it, and that the easiest way to do this is to expose the food to sun and wind. All foods contain water--cabbage and other leaf vegetables contain as much as 93% water, potatoes and other root vegetables 80% , lean meat 75% and fish anything from 80% to 60% depending on how fatty it is. If this water is removed, the activity of the bacteria(细菌) which cause food to go bad is checked. Nowadays most foods are dried mechanically. The conventional method of such dehydration(脱水) is to put food in chambers through which hoi air is blown at temperatures of about 110°C at entry to about 43°C at exit. This is the usual method for drying such things as vegetables, minced meat, and fish. Liquids such as milk, coffee, tea, soups and eggs may be dried by pouring them over a heated horizontal steel cylinder or by spraying them into a chamber through which a current of hot air passes. In the first case, the dried material is scraped off the roller as a thin film which is then broken up into small, though still relatively coarse flakes. In the second process it falls to the bottom of the chamber as a fine powder. Dried foods ’take up less room and weigh less than the same food packed in cans or frozen, and they do not need to be stored in special conditions. For these reasons they are invaluable to climbers, explorers and soldiers in battle, who have little storage space. They are also popular with housewives because it takes so little time to cook them. Which of the following statements is NOT true about drying food
进程互斥是进程同步的特例。
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