Drying is one of man's 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 hardly 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, legumes, 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 dependent 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 American 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 that 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 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 drying 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 process 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 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. 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
lives 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, and 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. The 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 growth can be controlled.
 
    Humidity-Water Vapor Content of Air

    The weight of water vapor in air may be determined from the equation:
 
    18.016 (p)
     
W = ───── ────
    28.967 (P-p)
        
    where W is the grams of water vapor per gram of air, p is the partial pressure of water vapor, and P is the total pressure.
 
    The percent saturation of air with water vapor is obtained from the equation: 

    W  

飽和百分數 = ── (100)

    Ws  

    Where Ws is the value for saturated air.
 
    The percent relative humidity of air is obtained from the equation:

    P 

Percent RH = ── (100)

    Ps  

    where Ps is the pressure of saturated water vapor at the existing temperature.
 
    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 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 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 vapor 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 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 Food─As a working figure, 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.

    The 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 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 Hardening─if the temperature of the air is high and the relative humidity of the air is low, there is danger that moisture will be removed 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 prevented by controlling the relative humidity of the circulating air and the temperature of the air.
 
    Types of Driers─There 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 product, economics, and operating conditions.
    
    The types of driers and the products upon which they are used are generally as follows:

     Drier                                        Product

     Drum drier                                   Milk veyetable juices,

                                                  cranberries, bananas

     Vacuum shelf drier                           Limited production of certain

foods
     Continuous vacuume drier                     Fruits and vegetables

     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 driers                                  Apples, some vegetables

     tunnel driers                                Fruits and vegetables


    干燥是人類保藏食品最古老的方法之一。這是從自然界學來的工藝方法，但對作業的某些方面已經作了一些改進。干燥是應用最廣泛的食品保藏方法。

    所有谷物都是通過干燥保藏的，自然干燥過程的效果很好，所以幾乎毋需人工的進一步努力。然而，歷史上也有這樣一些時期，由于氣候原因使糧食未能在田間得到恰當干燥。在這些情況下，人類就嘗試通過給糧食加熱（不然就會腐爛）以助天功。谷物、豆類、堅果和某些水果在植株上成熟并在暖風中干燥。用干燥方法保藏的水果比用任何其他方法保藏的都多。食物的自然曬干可得到品質穩定的高度濃縮的物料。盡管如此，高度綜合的文化決不能過份依賴那些不可預測的自然力。不過太陽曬干仍然是食品保藏的最廣泛的活動。

    脫水——人工干燥 

    利用火的熱量進行食品干燥是過去東、西半球上許多人各自獨立發現的。古代人在它們的棲身處進行食品的干燥，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年才發明了熱風脫水室。法國的的馬松和查理德研究小組開發了一種由熱氣流（40℃)通過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半世紀以前的同一時期內出現的。

    蒸發（evaporation)和干化（desiccation)兩詞也許指的是同一作用，而脫水一詞在食品工業中的意思是人工干燥過程。

    脫水與曬干 

    脫水意味著要控制干燥室內的氣溫條件即小環境控制。曬干收自然力的支配。由脫水設備干燥的食品能夠比曬干的食品具有更高的品質。實施干燥所需的場地較少。水果曬干所需的地面為水果莊稼土地的1/20。

    脫水工廠內部的衛生條件是可以控制的，而在曠野上，受灰塵、昆蟲、鳥類和 齒動物污染是主要問題。

    脫水干燥的費用顯然比曬干大，然而，由于質量上的改善，脫水干制食品可以有較高的售價。脫水干燥器的干果產率相對較高，因為，曬干過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于脫水干燥水果。在太陽曬干過程中，有些未成熟的水果的發色仍在緩慢進行，而在脫水干燥過程中卻沒有這種顏色的變化。

    脫水干燥食品的烹調特性通常優于對照的曬干食品。不過，曬干的動物肉和魚很受歡迎。

    就成本而言。曬干有其有點，當就干燥時間和質量而言，脫水干燥也有它的好處。另外，在許多有人生活且農業效益大的地區，由于氣候條件不利，不可能廣泛地實施太陽曬干。

    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。它生產費用低，所需勞力量少，加工設備有限，干制品貯存要求不高，運輸費用低（一車干制收縮的食品可相當于十車新鮮食品）。

    人們所需的食品供應常受化學作用和生物作用的影響。人們利用包裝和某些化學添加劑控制脫水食品中的化學作用。生物作用可通過降低自由水含量和加熱來控制。食品如果要成為支持微生物生長的合適基質，就必須有可供微生物利用的自來水。因此，通過降低自由水含量從而增加滲透壓，就能控制微生物的生長。

    濕度——空氣中水蒸氣含量 
 
    空氣中水蒸氣重量可以由下式確定：

    W= 18.01628.967   (P)(P-p)

    其中,W是每克空氣中水蒸氣的克數,p是水蒸氣分壓,P為總壓。

    含水蒸氣的空氣的飽和百分數由下式得到：

    飽和百分數=  W ×100

    Ws 

    其中Ws中是飽和空氣的W值。

    空氣的百分相對濕度由下式得到：

    RH(%)= P ×100

    Ps 

    其Ps中是現有溫度下的飽和蒸氣壓。

    空氣——干燥介質 

    食物可以在空氣、過熱蒸汽、真空、惰性氣體中干燥，可以用直接加熱干燥。由于空氣來源豐富、使用方便，并能控制食品的過熱，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的濕氣帶走。用空氣干燥不需要精密的水分回收系統，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。

    干燥過程中空氣的功能——空氣將熱傳送給食品，使水分汽化，同時，它又是將正脫水的食品中釋放出來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    蒸汽運走離開干燥室所需要的空氣量。如果進入的空氣不干燥，或者離開干燥室的空氣    沒有被濕氣所飽和，那么所需要的空氣體積就會發生改變。按一般規律，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5~7倍。空氣容納水蒸氣的能力取決于溫    度。

    標淮壓力下，溫度每升高l℃，氣體體積增加1／273。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——作為計算數據，在一般脫水溫度下，使454g    水蒸發變成蒸汽需要4490kgc熱量。實際上，汽化熱與溫度有關。

    自由表面汽化的速率——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空氣的速度增大而增大，只要不出現表面硬化，空氣的溫度越高、    溫差越大，干燥速率就越快。要除去最后6％水分所花費的時間幾乎與水分從80％降到    6％所需要的時間相同。干燥時間隨著最后水分含量接近其平衡值而迅速加長。

    表面硬化——如果空氣的溫度較高而相對濕度較低，就有可能出現這樣的危險，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部擴散離開的速度快，從    而形成表面硬化或結殼。這一不透氣的殼層(或邊界層)會阻滯水分自由擴散，這種情    形稱為表面硬化。控制循環空氣的相對濕度和空氣的溫度可防止表面硬化的出現。

    干燥器類型——食品脫水用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、所要求的終產品形式、經濟及操作條件。

    以下是一般常用的干燥器以及用這些干燥器來生產的產品：

        干燥器                                   產    品
        滾筒干燥器                            牛奶、蔬菜汁、紅莓、香蕉
        盤架式真空干燥器                      有限量的生產某些食品
        連續真空干燥器                        水果和蔬菜
        連續帶式(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