食品專業(yè)英語(yǔ) LESSON 10 Principles Of Food Preservation By Dr

1、Drying is one of mans oldest methods of food preservation. It is a process copied from nature; we have improved certain features of the operation. Drying is the most widely used method of food preservation. All the cereal grains are preserved by drying, and the natural process is so efficient it har

2、dly requires added effort by man. However, there have been periods in history when climatic factors were such that grains failed to dry properly in the fields. In these instances, man attempted to assist the natural action by supplying heat to the grains which otherwise would decompose. Grains, legu

3、mes, nuts and certain fruits mature on the plants and dry in the warm wind. More fruits are preserved by drying than by any other method of food preservation. The natural sun drying of foods yields highly concentrated materials of enduring quaity. yet a highly complex civilization cannot be so depen

4、dent upon the elements-they are unpredictable. Sun drying remains the greatest food preservation action. Dehydration-Artificial Drying The use of heat from a fire to dry foods was discovered independently by many men in the New and Old Worlds. Ancient man dried foods in his shelters; pre-Columbus Am

5、erican Indians used the heat from fire to dry foods. However, it was not until about 1795 that a hot air dehydration room was invented. The team of Masson and Challet in France developed a vegetable dehydrator which consisted of hot air(40) flow over thin slices of vegetables. It is worth noting tha

6、t both canning and dehydration came into being at approximately the same time, nearly a century and a half ago. Evaporation and desiccation are terms which perhaps note the same action. The term dehydration has taken the meaning in the food industry as that Process of artificial drying. Dehydration

7、vs. Sun Drying Dehydration implies control over climatic conditions within a chamber, or microenvironment control. Sun drying is at the mercy of the elements. Dried foods from a dehydration unit can have better quality than sun-dried counterparts. Less land is required for the drying activity. Sun d

8、rying for fruit requires approximately one unit of drying surface per 20 units of crop land. Sanitary conditions are controllable within a dehydration pant, whereas in open fields contamination from dust, insects. birds and rodents are major problems. Dehydration obviously is a more expensive proces

9、s than sun drying, yet the dried foods may have more monetary value from dehydration due to improved quality. The yield of dried fruit from a dehydrator is higher inasmuch as sugar is lost due to continued respiration of tissues during sun drying, and also due to fermentation. The color of sun-dried

10、 fruit may be superior to dehydrated fruit under optimum conditions of operation of both. Color development in certain immature fruits continues slowly during sun drying. This does not occur during. dehydration . In cooking quality of dehydrated foods are usually superior to sun-dried counterparts.

11、However, sun-dried animal flesh and fish can be highly acceptable. On the basis of cost sun drying has advantages, but on the basis of time to dry and quality, dehydration has merits. Furthermore sun drying can not be practiced widely due to unfavorable weather conditions in many areas where man liv

12、es and agriculture is rewarding. Why Dried Foods? Dried and dehydrated foods are more concentrated than any other preserved form of foodstuffs. They are less costly to produce; there is a minimum of labor required, processing equipment is limited, dried food storage requirements are at a minimum, an

13、d distribution costs are reduced (one carload of dried, compressed food may equal ten carloads of the fresh commodity). There are chemical and biological forces acting upon the food supply man desires. Man controls the chemical forces in dehydrated food by packaging and certain chemical additives. T

14、he biological forces are controlled by reducing the free water content and by heating. To be a suitable substrate to support growth of microorganisms, a food must have free water available for the microorganisms.By reducing the free water content, thereby increasing osmotic pressures, microbial grow

15、th can be controlled. Air-The Drying Medium Foodstuffs may be dried in air, superheated steam, in vacuum, in inert gas, and by the direct application of heat. Air is generally used as the drying medium because it is plentiful, convenient, and overheating of the food can be controlled. Air is used to

16、 conduct heat to the food being dried, and to carry liberated moisture vapor from the food. No elaborate moisture recovering system is required with air. as is needed with other gases. Drying can be accomplished gradually, and tendencies to scorch and discolor are within control. Function of Air in

17、Drying-Air conveys heat to the food, causing water to vaporize, and is the vehicle to transport the liberated moisture vapor from the dehydrating food. Volume of Air Required in Drying-More air is required to conduct heat to the food to evaporate the water present than is needed to transport the vap

18、or from the chamber. If the air entering is not dry. or if air leaving the dehydration chamber is not saturate4 with moisture vapor, the volume of air required is altered As a rule, 5 to 7 times as much air is required to heat food as is needed to carry the moisture vapor from the food. The moisture

19、 capacity of air is dependent upon the temperature. The volume of a gas at standard pressure increases l/273 in volume for each. 1 rise in temperature. Each 15 increase in temperature doubles the moisture ,holding capacity of air. Heat Required to Evaporate 454g of Water from FoodAs a working figure

20、, 4400 kgc are required to change 454 g of water to vapor at common dehydration temperatures. The heat of vaporization is actually temperature dependent.Rate of Evaporation from Free Surfaces.The greater the surface area, the more porous the surface, and the higher will be the drying rate of food. T

21、he drying rate increases as the velocity of air flowing over food increases. The higher the temperature of air end the greater the temperature drop, the faster the rate of drying will be, providing case hardening does not develop. Almost as much time may be consumed in reducing the final 6% moisture

22、 as is required to bring the moisture content of 80% down to 6%. The drying time increases rapidly as the final moisture content approaches its equilibrium value.Case Hardeningif the temperature of the air is high and the relative humidity of the air is low, there is danger that moisture will be rem

23、oved from the surface of foods being dried more rapidly than water can diffuse from the moist interior of the foods particle, and a hardening or casing will be formed. This impervious layer or boundary will retard the free diffusion of moisture. This condition is referred to as case hardening. It is

24、 prevented by controlling the relative humidity of the circulating air and the temperature of the air.Types of DriersThere are many types of driers used in the dehydration of foods, the particular type chosen being governed by the nature of the commodity to be dried. the desired form of the finished

25、 product, economics,and operating conditions.The types of driers and the products upon which they are used are generallyas follows: Drier Product Drum drier Milk veyetable juices, cranberries, bananas Vacuum shelf drier Limited production of certainfoods Continuous vacuume drier Fruits and vegetable

26、s Continuous belt (atmospheric) Vegetables drier Fluidized-bed drier Vegetables Foam-mat driers Juices Freeze driers Meats Spray driers Whole eggs, egg yolk, blood albumin and milk Rotary driers Some meat Products usually not used for food Cabinet or compartment driers Fruits and vegetables Kiln dri

27、ers Apples, some vegetables tunnel driers Fruits and vegetables第十課 食品的干燥保藏原理 干燥是人類保藏食品最古老的方法之一。這是從自然界學(xué)來(lái)的工藝方法，但對(duì)作業(yè)的某些方面已經(jīng)作了一些改進(jìn)。干燥是應(yīng)用最廣泛的食品保藏方法。 所有谷物都是通過(guò)干燥保藏的，自然干燥過(guò)程的效果很好，所以幾乎毋需人工的進(jìn)一步努力。然而，歷史上也有這樣一些時(shí)期，由于氣候原因使糧食未能在田間得到恰當(dāng)干燥。在這些情況下，人類就嘗試通過(guò)給糧食加熱（不然就會(huì)腐爛）以助天功。谷物、豆類、堅(jiān)果和某些水果在植株上成熟并在暖風(fēng)中干燥。用干燥方法保藏的水果比用任何其他方法保藏

28、的都多。食物的自然曬干可得到品質(zhì)穩(wěn)定的高度濃縮的物料。盡管如此，高度綜合的文化決不能過(guò)份依賴那些不可預(yù)測(cè)的自然力。不過(guò)太陽(yáng)曬干仍然是食品保藏的最廣泛的活動(dòng)。脫水人工干燥 利用火的熱量進(jìn)行食品干燥是過(guò)去東、西半球上許多人各自獨(dú)立發(fā)現(xiàn)的。古代人在它們的棲身處進(jìn)行食品的干燥，哥倫布以前的美洲印第安人利用火的熱量干燥食品。然而,直到1795年才發(fā)明了熱風(fēng)脫水室。法國(guó)的的馬松和查理德研究小組開(kāi)發(fā)了一種由熱氣流（40)通過(guò)蔬菜薄片上方的蔬菜干燥器。值得一提的是，罐藏法和脫水法幾乎是大約一個(gè)半世紀(jì)以前的同一時(shí)期內(nèi)出現(xiàn)的。蒸發(fā)（evaporation)和干化（desiccation)兩詞也許指的是同一作用，而

29、脫水一詞在食品工業(yè)中的意思是人工干燥過(guò)程。 脫水與曬干 脫水意味著要控制干燥室內(nèi)的氣溫條件即小環(huán)境控制。曬干收自然力的支配。由脫水設(shè)備干燥的食品能夠比曬干的食品具有更高的品質(zhì)。實(shí)施干燥所需的場(chǎng)地較少。水果曬干所需的地面為水果莊稼土地的1/20。 脫水工廠內(nèi)部的衛(wèi)生條件是可以控制的，而在曠野上，受灰塵、昆蟲、鳥類和 齒動(dòng)物污染是主要問(wèn)題。 脫水干燥的費(fèi)用顯然比曬干大，然而，由于質(zhì)量上的改善，脫水干制食品可以有較高的售價(jià)。脫水干燥器的干果產(chǎn)率相對(duì)較高，因?yàn)椋瑫窀蛇^(guò)程中不斷的組織呼吸作用，以及酵解作用使水果中的糖分受到了損失。 在兩者最適操作條件下，曬干水果的顏色可能優(yōu)于脫水干燥水果。在太陽(yáng)曬干過(guò)程

30、中，有些未成熟的水果的發(fā)色仍在緩慢進(jìn)行，而在脫水干燥過(guò)程中卻沒(méi)有這種顏色的變化。 脫水干燥食品的烹調(diào)特性通常優(yōu)于對(duì)照的曬干食品。不過(guò)，曬干的動(dòng)物肉和魚很受歡迎。 就成本而言。曬干有其有點(diǎn)，當(dāng)就干燥時(shí)間和質(zhì)量而言，脫水干燥也有它的好處。另外，在許多有人生活且農(nóng)業(yè)效益大的地區(qū)，由于氣候條件不利，不可能廣泛地實(shí)施太陽(yáng)曬干。 為什么食品要干制 曬干和脫水食品比其他任何保藏形式的食品都得到更大程度的濃縮。它生產(chǎn)費(fèi)用低，所需勞力量少，加工設(shè)備有限，干制品貯存要求不高，運(yùn)輸費(fèi)用低（一車干制收縮的食品可相當(dāng)于十車新鮮食品）。 人們所需的食品供應(yīng)常受化學(xué)作用和生物作用的影響。人們利用包裝和某些化學(xué)添加劑控制脫水

31、食品中的化學(xué)作用。生物作用可通過(guò)降低自由水含量和加熱來(lái)控制。食品如果要成為支持微生物生長(zhǎng)的合適基質(zhì)，就必須有可供微生物利用的自來(lái)水。因此，通過(guò)降低自由水含量從而增加滲透壓，就能控制微生物的生長(zhǎng)?？諝飧稍锝橘|(zhì)食物可以在空氣、過(guò)熱蒸汽、真空、惰性氣體中干燥，可以用直接加熱干燥。由于空氣來(lái)源豐富、使用方便，并能控制食品的過(guò)熱，所以，常用它作為干燥的媒介。利用空氣把熱傳給被干燥的食品，并將食品中釋放出來(lái)的濕氣帶走。用空氣干燥不需要精密的水分回收系統(tǒng)，而用其它氣體干燥就有這樣的必要。干燥可以逐漸完成，而且可以控制烘焦和變色的趨勢(shì)。干燥過(guò)程中空氣的功能空氣將熱傳送給食品，使水分汽化，同時(shí)，它又是將正脫水的

32、食品中釋放出來(lái)的潮濕蒸汽帶走的載體。 干燥中所需空氣的體積將熱傳給食品使所含水分汽化所需要的空氣量要多于將 蒸汽運(yùn)走離開(kāi)干燥室所需要的空氣量。如果進(jìn)入的空氣不干燥，或者離開(kāi)干燥室的空氣 沒(méi)有被濕氣所飽和，那么所需要的空氣體積就會(huì)發(fā)生改變。按一般規(guī)律，加熱食品所需 的空氣量是帶走食品中水蒸氣所需空氣量的57倍?？諝馊菁{水蒸氣的能力取決于溫 度。 標(biāo)淮壓力下，溫度每升高l，氣體體積增加1273。溫度每增加15，空氣容納水分的能力便增加1倍。 使食品中454g水分汽化所需的熱量作為計(jì)算數(shù)據(jù)，在一般脫水溫度下，使454g 水蒸發(fā)變成蒸汽需要4490kgc熱量。實(shí)際上，汽化熱與溫度有關(guān)。 自由表面汽化的

33、速率食品的表面積越大，表面越疏松，干燥的速率就越快。干燥速率隨食品上方流動(dòng)空氣的速度增大而增大，只要不出現(xiàn)表面硬化，空氣的溫度越高、 溫差越大，干燥速率就越快。要除去最后6水分所花費(fèi)的時(shí)間幾乎與水分從80降到 6所需要的時(shí)間相同。干燥時(shí)間隨著最后水分含量接近其平衡值而迅速加長(zhǎng)。 表面硬化如果空氣的溫度較高而相對(duì)濕度較低，就有可能出現(xiàn)這樣的危險(xiǎn)，即 水分從被干燥食品表面移走的速度比水分從食品顆粒潮濕的內(nèi)部擴(kuò)散離開(kāi)的速度快，從 而形成表面硬化或結(jié)殼。這一不透氣的殼層(或邊界層)會(huì)阻滯水分自由擴(kuò)散，這種情 形稱為表面硬化?？刂蒲h(huán)空氣的相對(duì)濕度和空氣的溫度可防止表面硬化的出現(xiàn)。 干燥器類型食品脫水用

34、的干燥器有多種類型，具體類型的選擇取決于被于燥物 料的性質(zhì)、所要求的終產(chǎn)品形式、經(jīng)濟(jì)及操作條件。 以下是一般常用的干燥器以及用這些干燥器來(lái)生產(chǎn)的產(chǎn)品： 干燥器 產(chǎn) 品 滾筒干燥器 牛奶、蔬菜汁、紅莓、香蕉 盤架式真空干燥器 有限量的生產(chǎn)某些食品 連續(xù)真空干燥器 水果和蔬菜 連續(xù)帶式(常壓)干燥器 蔬菜 流化床干燥器 蔬菜 濃縮泡沫干燥器 果汁 冷凍干燥器 肉類 噴霧干燥器 全蛋、蛋黃、血清蛋白和牛奶 旋轉(zhuǎn)式干燥器 某些肉制品、一般不用于食品干燥 箱式干燥器 水果和蔬菜 窯式干燥器 蘋果、某些蔬菜 隧道式干燥器 水果和蔬菜rodent a 咬的,嚼的動(dòng)嚙齒目的醫(yī)侵蝕性的 n嚙齒動(dòng)物（如兔鼠等）m

35、onetarya錢的,貨幣的,金融的fermentation發(fā)酵（作用）humidity濕度scorchvt vi 烘焦,燒焦（某物）之表面,使萎使枯焦,萎,枯temperaturedrop 溫度差chamber室,小室impervious 不可滲透的,透不過(guò)的,穿不過(guò)的不受影響的,不受干擾的 無(wú)動(dòng)于衷的,不接受的 1) the yield or dried fruit from a dehydrator is hieher inasmuch as sugar is lostdue to continued respiration of tissues during sun dying，and also due to fermentation. inasmuch assince， 這一短語(yǔ)引出的從句inasmuch as sugarto fermentation 是原因狀語(yǔ)從句。該狀語(yǔ)從句中，due to continueddrying和due to fermentation 是兩