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  

饱和百分数 = ── (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


    干燥是人类保藏食品最古老的方法之一。这是从自然界学来的工艺方法，但对作业的某些方面已经作了一些改进。干燥是应用最广泛的食品保藏方法。

    所有谷物都是通过干燥保藏的，自然干燥过程的效果很好，所以几乎毋需人工的进一步努力。然而，历史上也有这样一些时期，由于气候原因使粮食未能在田间得到恰当干燥。在这些情况下，人类就尝试通过给粮食加热（不然就会腐烂）以助天功。谷物、豆类、坚果和某些水果在植株上成熟并在暖风中干燥。用干燥方法保藏的水果比用任何其他方法保藏的都多。食物的自然晒干可得到品质稳定的高度浓缩的物料。尽管如此，高度综合的文化决不能过份依赖那些不可预测的自然力。不过太阳晒干仍然是食品保藏的最广泛的活动。

    脱水——人工干燥 

    利用火的热量进行食品干燥是过去东、西半球上许多人各自独立发现的。古代人在它们的栖身处进行食品的干燥，哥伦布以前的美洲印第安人利用火的热量干燥食品。然而,直到1795年才发明了热风脱水室。法国的的马松和查理德研究小组开发了一种由热气流（40℃)通过蔬菜薄片上方的蔬菜干燥器。值得一提的是，罐藏法和脱水法几乎是大约一个半世纪以前的同一时期内出现的。

    蒸发（evaporation)和干化（desiccation)两词也许指的是同一作用，而脱水一词在食品工业中的意思是人工干燥过程。

    脱水与晒干 

    脱水意味着要控制干燥室内的气温条件即小环境控制。晒干收自然力的支配。由脱水设备干燥的食品能够比晒干的食品具有更高的品质。实施干燥所需的场地较少。水果晒干所需的地面为水果庄稼土地的1/20。

    脱水工厂内部的卫生条件是可以控制的，而在旷野上，受灰尘、昆虫、鸟类和 齿动物污染是主要问题。

    脱水干燥的费用显然比晒干大，然而，由于质量上的改善，脱水干制食品可以有较高的售价。脱水干燥器的干果产率相对较高，因为，晒干过程中不断的组织呼吸作用，以及酵解作用使水果中的糖分受到了损失。

    在两者最适操作条件下，晒干水果的颜色可能优于脱水干燥水果。在太阳晒干过程中，有些未成熟的水果的发色仍在缓慢进行，而在脱水干燥过程中却没有这种颜色的变化。

    脱水干燥食品的烹调特性通常优于对照的晒干食品。不过，晒干的动物肉和鱼很受欢迎。

    就成本而言。晒干有其有点，当就干燥时间和质量而言，脱水干燥也有它的好处。另外，在许多有人生活且农业效益大的地区，由于气候条件不利，不可能广泛地实施太阳晒干。

    为什么食品要干制 

    晒干和脱水食品比其他任何保藏形式的食品都得到更大程度的浓缩。它生产费用低，所需劳力量少，加工设备有限，干制品贮存要求不高，运输费用低（一车干制收缩的食品可相当于十车新鲜食品）。

    人们所需的食品供应常受化学作用和生物作用的影响。人们利用包装和某些化学添加剂控制脱水食品中的化学作用。生物作用可通过降低自由水含量和加热来控制。食品如果要成为支持微生物生长的合适基质，就必须有可供微生物利用的自来水。因此，通过降低自由水含量从而增加渗透压，就能控制微生物的生长。

    湿度——空气中水蒸气含量 
 
    空气中水蒸气重量可以由下式确定：

    W= 18.01628.967   (P)(P-p)

    其中,W是每克空气中水蒸气的克数,p是水蒸气分压,P为总压。

    含水蒸气的空气的饱和百分数由下式得到：

    饱和百分数=  W ×100

    Ws 

    其中Ws中是饱和空气的W值。

    空气的百分相对湿度由下式得到：

    RH(%)= P ×100

    Ps 

    其Ps中是现有温度下的饱和蒸气压。

    空气——干燥介质 

    食物可以在空气、过热蒸汽、真空、惰性气体中干燥，可以用直接加热干燥。由于空气来源丰富、使用方便，并能控制食品的过热，所以，常用它作为干燥的媒介。利用空气把热传给被干燥的食品，并将食品中释放出来的湿气带走。用空气干燥不需要精密的水分回收系统，而用其它气体干燥就有这样的必要。干燥可以逐渐完成，而且可以控制烘焦和变色的趋势。

    干燥过程中空气的功能——空气将热传送给食品，使水分汽化，同时，它又是将正脱水的食品中释放出来的潮湿蒸汽带走的载体。

    干燥中所需空气的体积——将热传给食品使所含水分汽化所需要的空气量要多于将    蒸汽运走离开干燥室所需要的空气量。如果进入的空气不干燥，或者离开干燥室的空气    没有被湿气所饱和，那么所需要的空气体积就会发生改变。按一般规律，加热食品所需    的空气量是带走食品中水蒸气所需空气量的5~7倍。空气容纳水蒸气的能力取决于温    度。

    标淮压力下，温度每升高l℃，气体体积增加1／273。温度每增加15℃，空气容纳水分的能力便增加1倍。

    使食品中454g水分汽化所需的热量——作为计算数据，在一般脱水温度下，使454g    水蒸发变成蒸汽需要4490kgc热量。实际上，汽化热与温度有关。

    自由表面汽化的速率——食品的表面积越大，表面越疏松，干燥的速率就越快。干燥速率随食品上方流动空气的速度增大而增大，只要不出现表面硬化，空气的温度越高、    温差越大，干燥速率就越快。要除去最后6％水分所花费的时间几乎与水分从80％降到    6％所需要的时间相同。干燥时间随着最后水分含量接近其平衡值而迅速加长。

    表面硬化——如果空气的温度较高而相对湿度较低，就有可能出现这样的危险，即    水分从被干燥食品表面移走的速度比水分从食品颗粒潮湿的内部扩散离开的速度快，从    而形成表面硬化或结壳。这一不透气的壳层(或边界层)会阻滞水分自由扩散，这种情    形称为表面硬化。控制循环空气的相对湿度和空气的温度可防止表面硬化的出现。

    干燥器类型——食品脱水用的干燥器有多种类型，具体类型的选择取决于被于燥物    料的性质、所要求的终产品形式、经济及操作条件。

    以下是一般常用的干燥器以及用这些干燥器来生产的产品：

        干燥器                                   产    品
        滚筒干燥器                            牛奶、蔬菜汁、红莓、香蕉
        盘架式真空干燥器                      有限量的生产某些食品
        连续真空干燥器                        水果和蔬菜
        连续带式(常压)干燥器                  蔬菜
        流化床干燥器                          蔬菜
        浓缩泡沫干燥器                        果汁
        冷冻干燥器                            肉类
        喷雾干燥器                            全蛋、蛋黄、血清蛋白和牛奶
        旋转式干燥器                          某些肉制品、一般不用于食品干燥
        箱式干燥器                            水果和蔬菜
        窑式干燥器                            苹果、某些蔬菜
        隧道式干燥器                          水果和蔬菜