首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
You should spend about 20 minutes on Questions 1-13, which are based on Reading Passage 1 below. Astrona
You should spend about 20 minutes on Questions 1-13, which are based on Reading Passage 1 below. Astrona
admin
2014-12-30
15
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】______
【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://kaotiyun.com/show/v3NO777K
本试题收录于:
雅思阅读题库雅思(IELTS)分类
0
雅思阅读
雅思(IELTS)
相关试题推荐
Whatisthetotalnumberofdifferent5-digitintegersthatcontainallofthedigits2,5,4,3,7andinwhichboththeunits
Mostpeoplechoosealawyeronthebasisofsuch______considerationsashiscost,hisfieldofexpertise,andthefeeshechar
A、Theinfluenceofvolcaniceruptionsonglobalweatherpatternsisalmostcompletelyunderstoodatpresent.B、Scientistshaveo
Thedistinctionbetweenmakingartandthinkingandwritingaboutitshouldimplyneitheramutualexclusivenessnorahi
Diseaseisafluidconceptinfluencedbysocietalandculturalattitudesthatchangediachronicallyinresponsetonewsc
Whiletheambitiontodrawanimmediateconclusionis______,itisnotnecessarilycorrect,forthesearchforthetruthdepends
AlthoughmostpeoplewhoacquireWestNilehaveno______andthosewhodonormallysufferlittlemorethanflu-likeillness,iti
Becausemodernartisneithercompletelyacceptednorrejectedbycritics,theirultimateevaluationsofitremain______.
Someteacherscomplainedthattheschoolboardwas______,focusingonshort-termgoalswhileignoringthelong-termbenefitso
随机试题
促进肾盂鳞状细胞癌发生的最常见因素是
负荷试验(药物或运动)与超声造影的关系是
布一加综合征的适应证不包括
对该患者的诊断是选用治疗方法为
用四环素族类药物可致恒牙发生四环素牙的时期是
某新建排灌结合的泵站工程,共安装6台机组(5用1备),设计流量为36m3/s,总装机功率2700kW。泵站采用肘型进水流道,平直管出水流道,下部为块基型墩墙式结构,上部为排架式结构。某施工企业承担该项目施工,签约合同价为2900万元,施工过程中有如下事件:
为降低工程成本,提高竞争力,施工组织设计必须努力做到()。
某公司生产甲产品,一季度至四季度的预计销售量分别为1000件、800件、900件和850件,生产每件甲产品需要2千克A材料。公司的政策是每一季度末的产成品存货数量等于下一季度销售量的10%,每一季度末的材料存量等于下一季度生产需要量的20%。下列说法中正
下列关于急性乳腺炎的治疗,说法正确的是()。
新闻事业联系群众的方式。(中国传媒大学,2008年)
最新回复
(
0
)