首页
登录
职称英语
You should spend about 20 minutes on Questions 1-13, which are based on Reading
You should spend about 20 minutes on Questions 1-13, which are based on Reading
游客
2024-01-08
5
管理
问题
You should spend about 20 minutes on Questions 1-13, which are based on Reading Passage 1 below.
Astronaut ice cream, anyone?
Freeze-drying is a technique that can help to provide food for astronauts. But
it also has other applications nearer home.
Freeze-drying is like suspended animation for food; you can store a freeze-dried meal for years, and then, when you’re finally ready to eat it, you can completely revitalise it with a little hot water. Even after several years, the original foodstuff will be virtually unchanged.
The technique basically involves completely removing the water from some material, such as food, while leaving the rest of the material virtually intact. The main reason for doing this is either to preserve the food or to reduce its weight. Removing the water from food keeps it from spoiling, because the microorganisms such as bacteria that cause spoiling cannot survive without it. Similarly, the enzymes which occur naturally in food cannot cause ripening without water, so removing water from food will also stop the ripening process.
Freeze-drying significantly reduces the total weight of the food because most food is largely made up of water; for example, many fruits are more than 80-90% water. Removing this makes the food much lighter and therefore makes transportation less difficult. The military and camping-supply companies freeze-dry foods to make them easier for an individual to carry and NASA has also freeze-dried foods for the cramped quarters on board spacecraft.
The process is also used to preserve other sorts of material, such as pharmaceuticals. Chemists can greatly extend pharmaceutical shelf life by freeze-drying the material and storing it in a container free of oxygen and water. Similarly, research scientists may use freeze-drying to preserve biological samples for long periods of time. Even valuable manuscripts that had been water damaged have been saved by using this process.
Freeze-drying is different from simple drying because it is able to remove almost all the water from materials, whereas simple drying techniques can only remove 90-95%. This means that the damage caused by bacteria and enzymes can virtually be stopped, rather than just slowed down. In addition, the composition and structure of the material is not significantly changed, so materials can be revitalised without compromising the quality of the original.
This is possible because in freeze-drying, solid water — ice — is converted directly into water vapour, missing out the liquid phase entirely. This is called ’sublimation’, the shift from a solid directly into a gas. Just like evaporation, sublimation occurs when a molecule gains enough energy to break free from the molecules around it. Water will sublime from a solid(ice)to a gas(vapour)when the molecules have enough energy to break free but the conditions aren’t right for a liquid to form. These conditions are determined by heat and atmospheric pressure. When the temperature is above freezing point, so that ice can thaw, but the atmospheric pressure is too low for a liquid to form(below 0.06 atmospheres(ATM))then it becomes a gas.
This is the principle on which a freeze-drying machine is based. The material to be preserved is placed in a freeze-drying chamber which is connected to a freezing coil and refrigerator compressor. When the chamber is sealed the compressor lowers the temperature inside it. The material is frozen solid, which separates the water from everything around it on a molecular level, even though the water is still present. Next, a vacuum pump forces air out of the chamber, lowering the atmospheric pressure below to 0.06 ATM. The heating units apply a small amount of heat to the shelves in the chamber, causing the ice to change phase. Since the pressure in the chamber is so low, the ice turns directly into water vapour, which leaves the freeze-drying chamber, and flows past the freezing coil. The water vapour condenses onto the freezing coil in the form of solid ice, in the same way that water condenses as frost on a cold day.
The process continues for many hours(even days)while the material gradually dries out. This time is necessary to avoid overheating, which might affect the structure of the material. Once it has dried sufficiently, it is sealed in a moisture-free package. As long as the package is secure, the material can sit on a shelf for years and years without degrading, until it is restored to its original form with a little hot water. If everything works correctly, the material will go through the entire process almost completely unscathed.
In fact, freeze-drying, as a general concept, is not new but has been around for centuries. The ancient Incas of Peru used mountain peaks along the Andes as natural food preservers. The extremely cold temperatures and low pressure at those high altitudes prevented food from spoiling in the same basic way as a modern freeze-drying machine and a freezer.
Questions 1-5
Complete the notes below.
Choose NO MORE THAN THREE WORDS from the passage for each answer.
Write your answers in boxes 1-5 on your answer sheet.
Uses of freeze-drying
food preservation
easy 【R1】______of food items
long-term storage of 【R2】______and biological samples
preservation of precious 【R3】______
Freeze-drying
is based on process of 【R4】______
is more efficient than 【R5】______ [br] 【R2】
选项
答案
pharmaceuticals
解析
Paragraph 4: The process is also used to preserve other sorts of material, such as pharmaceuticals.
转载请注明原文地址:https://tihaiku.com/zcyy/3344806.html
相关试题推荐
Somepeoplebelievethattheyshouldbeabletokeepallthemoneytheyearn
Advertisingaimedatchildrenshouldbebanned.Towhatextentyouagreeor
Questions27-30Foreachquestion,onlyONEofthechoicesiscorrect.Writethe
Questions27-30Foreachquestion,onlyONEofthechoicesiscorrect.Writethe
Questions27-30Foreachquestion,onlyONEofthechoicesiscorrect.Writethe
Questions27-30Foreachquestion,onlyONEofthechoicesiscorrect.Writethe
Questions27-30Foreachquestion,onlyONEofthechoicesiscorrect.Writethe
Questions27-30Foreachquestion,onlyONEofthechoicesiscorrect.Writethe
Questions14-17Thetexthas9paragraphs(A-I).Whichparagraphdoeseachofth
Questions14-17Thetexthas9paragraphs(A-I).Whichparagraphdoeseachofth
随机试题
U.S.EducationSecretaryArneDuncanappealedFridayforanewgenerationof
[originaltext]Duringastateofdeeprelaxation,severalphysiologicalchan
故宫(theImperialPalace)也称紫禁城,位于北京市中心,占地面积达72万平方米,是中国也是世界现存规模最大、最完整的古代皇家建筑登(compl
《归去来兮辞》的作者是()A.陶渊明 B.张衡 C.谢灵运 D.鲍照
组织型纤溶酶原激活物(t-PA)激活PLG变为PL需A.纤维蛋白B.尿激酶型纤溶
下列关于摄影测量叙述不正确的是()A.摄影测量是基于影像的量测和
不引起淤血的是( )。A.肝硬化 B.左心衰竭 C.下肢深静脉血栓形成
下列关于汗液的叙述,错误的是A.主要成分为水分 B.渗透压高于血浆 C.Na
感知风险和分析风险是()的两个环节。A.风险识别 B.风险估计 C.风险评价
下列最符合心脏压塞诊断的是( )。A.心浊音界向两侧扩大 B.胸闷、气短症状
最新回复
(
0
)