Drying of Fruits and Vegetables

Introduction ......................................................................................................................................... 606 25.2 Postharvest Technology of Fruits and Vegetables ............................................................................... 606 25.2.1 World Production.................................................................................................................... 606 25.2.2 Losses ...................................................................................................................................... 606 25.2.3 Role of Preservation................................................................................................................ 606 25.2.4 Preservation by Drying ........................................................................................................... 607 25.3 Pretreatments for Drying ..................................................................................................................... 607 25.3.1 Alkaline Dip............................................................................................................................ 607 25.3.2 Sulfiting ................................................................................................................................... 607 25.3.3 Blanching................................................................................................................................. 608 25.4 Drying Techniques and Equipment ..................................................................................................... 608 25.4.1 Dehydration ............................................................................................................................ 608 25.4.2 Solar Drying............................................................................................................................ 609 25.4.2.1 Sun or Natural Dryers ............................................................................................ 609 25.4.2.2 Solar Dryers—Direct............................................................................................... 609 25.4.2.3 Solar Dryers—Indirect ............................................................................................ 609 25.4.2.4 Hybrid Systems ....................................................................................................... 609 25.4.2.5 Mixed Systems ........................................................................................................ 610 25.4.3 Hot Air Drying ....................................................................................................................... 611 25.4.3.1 Cabinet Dryers ........................................................................................................ 612 25.4.3.2 Tunnel Dryers ......................................................................................................... 612 25.4.3.3 BeltTrough Dryers ................................................................................................. 612 25.4.3.4 Pneumatic Conveyor Dryers ................................................................................... 612 25.4.4 Fluidized Bed Drying .............................................................................................................. 612 25.4.5 Explosion Puffing .................................................................................................................... 615 25.4.6 Foam Drying........................................................................................................................... 617 25.4.7 Microwave Drying .................................................................................................................. 617 25.4.8 Spray Drying ........................................................................................................................... 618 25.4.9 Drum Drying........................................................................................................................... 618 25.4.10 FreezeDrying........................................................................................................................ 619 25.4.11 Osmotic Dehydration ............................................................................................................ 620 25.4.12 Heat Pump Drying ................................................................................................................ 621 25.4.13 Ultrasonic Drying of Liquids ................................................................................................ 621 25.5 Quality Changes During Drying and Storage ...................................................................................... 621 25.5.1 Loss of Vitamins (Vitamins A and C)..................................................................................... 621 25.5.2 Loss of Natural Pigments (Carotenoids and Chlorophylls) .................................................... 622 25.5.3 Browning and Role of Sulfur Dioxide .................................................................................... 623 25.5.4 Oxidative Degradation and Flavor Loss................................................................................. 625 25.5.5 Texture and Reconstitution Behavior ..................................................................................... 627 25.5.6 Influence of Water Activity ..................................................................................................... 627 25.5.7 Glass Transition Temperature Related Changes..................................................................... 629 25.5.8 Microbiological Aspects .......................................................................................................... 629 25.5.9 Factors Affecting Storage Stability ......................................................................................... 630 References ...................................................................................................................................................... 63

25 Drying of Fruits and Vegetables K.S Jayaraman and D.K Das Gupta CONTENTS 25.1 25.2 Introduction Postharvest Technology of Fruits and Vegetables 25.2.1 World Production 25.2.2 Losses 25.2.3 Role of Preservation 25.2.4 Preservation by Drying 25.3 Pretreatments for Drying 25.3.1 Alkaline Dip 25.3.2 Sulfiting 25.3.3 Blanching 25.4 Drying Techniques and Equipment 25.4.1 Dehydration 25.4.2 Solar Drying 25.4.2.1 Sun or Natural Dryers 25.4.2.2 Solar Dryers—Direct 25.4.2.3 Solar Dryers—Indirect 25.4.2.4 Hybrid Systems 25.4.2.5 Mixed Systems 25.4.3 Hot Air Drying 25.4.3.1 Cabinet Dryers 25.4.3.2 Tunnel Dryers 25.4.3.3 Belt-Trough Dryers 25.4.3.4 Pneumatic Conveyor Dryers 25.4.4 Fluidized Bed Drying 25.4.5 Explosion Puffing 25.4.6 Foam Drying 25.4.7 Microwave Drying 25.4.8 Spray Drying 25.4.9 Drum Drying 25.4.10 Freeze-Drying 25.4.11 Osmotic Dehydration 25.4.12 Heat Pump Drying 25.4.13 Ultrasonic Drying of Liquids 25.5 Quality Changes During Drying and Storage 25.5.1 Loss of Vitamins (Vitamins A and C) 25.5.2 Loss of Natural Pigments (Carotenoids and Chlorophylls) 25.5.3 Browning and Role of Sulfur Dioxide 25.5.4 Oxidative Degradation and Flavor Loss 25.5.5 Texture and Reconstitution Behavior 25.5.6 Influence of Water Activity 25.5.7 Glass Transition Temperature Related Changes 25.5.8 Microbiological Aspects 25.5.9 Factors Affecting Storage Stability References ß 2006 by Taylor & Francis Group, LLC 606 606 606 606 606 607 607 607 607 608 608 608 609 609 609 609 609 610 611 612 612 612 612 612 615 617 617 618 618 619 620 621 621 621 621 622 623 625 627 627 629 629 630 631 25.1 INTRODUCTION From the point of view of consumption, fruits are plant products with aromatic flavor that are naturally sweet or normally sweetened before usage [1] Apart from providing flavor and variety to human diet, they serve as important and indispensable sources of vitamins and minerals although they are not good or economic sources of protein, fat, and energy The same is true in the case of vegetables, which also play an important role in human nutrition in supplying certain constituents in which other food materials are deficient and in adding flavor, color, and variety to the diet [2] After moisture, carbohydrates form the next most abundant nutrient constituent in fruits and vegetables, and are present as low-molecular-weight sugars or high-molecular-weight polymers like starch and so on The celluloses, hemicelluloses, pectic substances, and lignin characteristic of plant products together form dietary fiber, the value of which in human diet is increasingly realized in recent years, especially for the affluent society of the Western countries Virtually all human’s dietary vitamin C, an important constituent of human diet, is obtained from fruits and vegetables, some of which are rich in provitamin A (b-carotene) (e.g., mango, carrot, etc.) They are important suppliers of calcium, phosphorus, and iron Fruits and vegetables have gained commercial importance and their growth on a commercial scale has become an important sector of the agricultural industry Recent developments in agricultural technology have substantially increased the world production of fruits and vegetables Consequently a larger proportion of several important commodities is handled, transported, and marketed all over the world than before with concomitant losses calling for suitable postharvest techniques for storage and processing to ensure improved shelf life Production and consumption of processed fruits and vegetables are also increasing 25.2 POSTHARVEST TECHNOLOGY OF FRUITS AND VEGETABLES 25.2.1 WORLD PRODUCTION The present world production of fruit (excluding melons) according to Food and Agricultural Organization (FAO) was about 444.65 million metric tons (mt) in 1999 [3] China with a production of 59.5 mt (13.4%) is a leading producer of fruits in the world India, with 38.56 mt (8.7%) occupies second position, followed by Brazil (8.45%), United States (6.4%), and Italy (4.3%) ß 2006 by Taylor & Francis Group, LLC World production of vegetables (including melons) is about 628.75 mt The major vegetable producing countries were China, India, United States, Turkey, Italy, Japan, and Spain China was the largest producer accounting for about 250.0 mt (39.8%) whereas India was the second contributing about 59.4 mt (9.45%) 25.2.2 LOSSES Most fruits and vegetables contain more than 80% water and are therefore highly perishable Water loss and decay account for most of their losses, which are estimated to be more than 30–40% in the developing countries in the tropics and subtropics [1] due to inadequate handling, transportation, and storage facilities Apart from physical and economic losses, serious losses occur in the availability of essential nutrients, notably vitamins and minerals The need to reduce postharvest losses of perishable horticultural commodities is of paramount importance for developing countries to increase their availability, especially in the present context when the constraints on food production (land, water, and energy) are continually increasing It is being increasingly realized that the production of more and better food alone is not enough and should go hand in hand with suitable postharvest conservation techniques to minimize losses, thereby increasing supplies and availability of nutrients besides giving the economic incentive to produce more [1] 25.2.3 ROLE OF PRESERVATION One of the prime goals of food processing or preservation is to convert perishable foods such as fruits and vegetables into stabilized products that can be stored for extended periods of time to reduce their postharvest losses Processing extends the availability of seasonal commodities, retaining their nutritive and esthetic values, and adds variety to the otherwise monotonous diet It adds convenience to the products In particular it has expanded the markets of fruit and vegetable products and ready-to-serve convenience foods all over the world, the per capita consumption of which has rapidly increased during the past two to three decades Several process technologies have been employed on an industrial scale to preserve fruits and vegetables; the major ones are canning, freezing, and dehydration Among these, dehydration is especially suited for developing countries with poorly established low-temperature and thermal processing facilities It offers a highly effective and practical means of preservation to reduce postharvest losses and offset the shortages in supply 25.2.4 PRESERVATION BY DRYING The technique of dehydration is probably the oldest method of food preservation practiced by humankind The removal of moisture prevents the growth and reproduction of microorganisms causing decay and minimizes many of the moisture-mediated deteriorative reactions It brings about substantial reduction in weight and volume, minimizing packing, storage, and transportation costs and enables storability of the product under ambient temperatures These features are especially important for developing countries and in military feeding and space food formulations A sharp rise in energy costs has promoted a dramatic upsurge in interest in drying worldwide over the last decade Advances in techniques and development of novel drying methods have been made available for a wide range of dehydrated products, especially instantly reconstitutable ingredients, from fruits and vegetables with properties that could not have been foreseen some years ago The growth of fast foods has fueled the need for such ingredients Due to changing lifestyles, especially in the developed world, there is now a great demand for a wide variety of dried products with emphasis on high quality and freshness besides convenience This chapter is intended to provide a comprehensive account of the various drying techniques and appliances developed and applied over the years specifically for the dehydration of fruits, vegetables, and their products Theoretical and practical aspects of drying as applied to foodstuffs in general have been covered by Sokhansanj and Jayas in the earlier edition of the Handbook of Industrial Drying [4] Therefore, discussion will be restricted to fruit and vegetable drying besides quality changes during drying and storage as specifically applied to these commodities 25.3 PRETREATMENTS FOR DRYING Fruits and vegetables are subjected to certain pretreatments with a view to improve drying characteristics and minimize adverse changes during drying and subsequent storage of the products These include alkaline dips for fruits and sulfiting and blanching for fruits and vegetables [5] 25.3.1 ALKALINE DIP The alkaline dip involves immersion of the product in an alkaline solution before drying and is used primarily for fruits that are dried whole, especially prunes and grapes A sodium carbonate or lye solution (0.5% or less) is usually used at a temperature ranging from ß 2006 by Taylor & Francis Group, LLC 93.3 to 1008C [1] It facilitates drying by forming fine cracks in the skin Oleate esters constitute the active ingredients of commercial dip solutions used for grapes They accelerate moisture loss by causing the wax platelets on the grape skin to dissociate, thus facilitating water diffusion 25.3.2 SULFITING Sulfur dioxide treatments are widely used in fruit and vegetable drying as sulfur dioxide is by far the most effective additive to avoid nonenzymatic browning [NEB] [6] It also inhibits various enzyme-catalyzed reactions, notably enzymic browning, and acts as an antioxidant in preventing loss of ascorbic acid and protecting lipids, essential oils, and carotenoids against oxidative deterioration during processing and storage It also helps in inhibition and control of microorganisms, especially microbial fermentation of sugars in fruits such as sun-dried apricots as encountered during prolonged drying It has the advantage of allowing higher temperatures, hence shorter drying times, to be used It is intended to maintain color, prevent spoilage, and preserve certain nutritive attributes until marketed Fruits for dehydration are often treated with gaseous SO2 from burning sulfur as used in the manufacture of dried apricots, peaches, bananas, raisins, and sultanas Alternatively, apple slices are generally dipped in solutions of the additive (prepared by dissolving sodium bisulfite or SO2 in water) and may receive an extra treatment with gaseous SO2 during drying Treatment of vegetables with SO2 gas is impractical Sulfite solutions are preferred as the most practical method of controlling absorption As vegetables are blanched before drying, generally the additive is incorporated at the blanching stage either in the blanch liquor if the vegetable is to be dipped or as a spray in the case of steam blanching Sufficient SO2 must be absorbed by the prepared material to allow for losses that occur during drying and subsequent storage The various methods of application of SO2 result in varying levels of uptake, which is a function of SO2 concentration, length of treatment, and time allowed for draining, size and geometry of the food, and the pH of the blanch liquor or spray Drying times in excess of 12 h for fruits and vegetables and of several days as in sun drying of fruits necessitate use of large amounts of SO2 It has been shown that only 35–45% of the additive initially incorporated is measurable after drying The subsequent loss of SO2 from dried products occurring during storage determines the practical shelf life with respect to spoilage through NEB Table 25.1 and Table 25.2 show suggested levels for SO2 in vegetables and fruits, respectively, after the completion of drying [6] 25.3.3 BLANCHING Blanching consists of a partial cooking, usually in steam or hot water, before dehydration It is intended to denature enzymes responsible for bringing about undesirable reactions that adversely affect product quality such as enzymic browning and oxidation during processing and storage The effectiveness of the treatment is judged by the degree of enzyme inactivation Thus, activity of polyphenoloxidase is followed in fruits, that of catalase in cabbage and of peroxidase in other vegetables The other beneficial effects produced by blanching include [5] reduced drying time, removal of intercellular air from the tissues, softening of texture, and retention of carotene and ascorbic acid during storage Commercially both continuous- and batchtype blanchers are employed, involving 2- to 10-min exposure to live steam Series blanching in hot water is also used, in which the solids content of the water is maintained at an equilibrium level to minimize leaching losses In addition to water and steam blanching, use of microwave energy was demonstrated to be a convenient and effective method of blanching [7] and superior in retention of ascorbic acid The texture of rehydrated, microwave-blanched freeze-dried spinach was firm, chewy, and highly acceptable Low-temperature long-time (LTLT) blanching (65–708C for 15–20 min) was found to improve the TABLE 25.1 Suggested Sulfur Dioxide Levels in Dried Vegetables Vegetable Beans Cabbages Carrots Celery Peas Potato granules Potato slices Sweet potatoes (diced) Beets Corn Peppersa Horseradish a SO2 (ppm) 500 1500–2500 500–1000 500–1000 300–500 250 200–500 200–500 Not necessary 2000 1000–2500 Destroys flavor 0.2% antioxidant BHA gives better color retention Source: From Dunbar, J., Food Tech New Zealand, 21(2), 11, 1986 With permission ß 2006 by Taylor & Francis Group, LLC TABLE 25.2 Suggested Sulfur Dioxide Levels in Dried Fruits Fruit SO2 (ppm) Apples Apricots Peaches Pears Raisins 1000–2000 2000–4000 2000–4000 1000–2000 1000–1500 Source: From Dunbar, J., Food Tech New Zealand, 21(2), 11, 1986 With permission quality (texture) of dried carrot (together with calcium treatment) [8] and dried sweet potato [9] as compared to high-temperature short-time (HTST) blanching (95–1008C for min) Because at this temperature pectin methyl esterase was active to desterify and increase the free carboxyl group of pectin, which could then form salt bridges with divalent cations to produce a firmer textured product [8] The prevalence of water blanchers in the industry necessitates the comparison of different types of blanching for their energy utilization On the basis of a theoretical requirement of 134 kg of steam per 103 kg of raw vegetables, energy efficiency of a steam blancher was estimated at 5%, a hydrostatic steam blancher at 27%, an IQB unit at 85%, and a water blancher at 60% [10] 25.4 DRYING TECHNIQUES AND EQUIPMENT 25.4.1 DEHYDRATION Dehydration involves the application of heat to vaporize moisture and some means of removing water vapor after its separation from the fruit and vegetable tissue Hence it is a combined and simultaneous heat and mass transfer operation for which energy must be supplied Several types of dryers and drying methods, each better suited for a particular situation, are commercially used to remove moisture from a wide variety of fruits and vegetables [11] Whereas sun drying of fruit crops is still practiced for certain fruits such as prunes, grapes, and dates, atmospheric dehydration processes are used for apples, prunes, and several vegetables Continuous processes, such as tunnel, belt-trough, and fluidized bed (FB), are mainly used for vegetables Spray drying is suitable for fruit juice concentrates and vacuum dehydration processes are useful for low-moisture, high-sugar fruits Factors on which the selection of a particular dryer or drying method depends include form of raw material and its properties, desired physical form and characteristics of the product, necessary operating conditions, and operation costs Three basic types of drying processes may be recognized as applied to fruits and vegetables: sun drying and solar drying; atmospheric drying including batch (kiln, tower, and cabinet dryers) and continuous (tunnels, belt, belt-trough, fluidized bed, explosion puff, foam mat, spray, drum, and microwave heated) processes; and subatmospheric dehydration (vacuum shelf belt/drum and freeze dryers) Recently the scope has been expanded to include use of low-temperature and low-energy processes like osmotic dehydration In the following sections only a few types of dryers and drying techniques of importance to fruit and vegetable drying are briefly discussed Detailed information on their design, operation, and economics may be obtained from references quoted in the relevant sections 25.4.2 SOLAR DRYING One of the oldest uses of solar energy since the dawn of civilization has been the drying and preservation of agricultural surpluses It was also the cheapest means of preservation by which water activity was brought to a low level so that spoilage would not take place It has been used mainly for drying of fruits such as grapes, prunes, dates, and figs There is no accurate estimate of the vast amount of material dried using this traditional technique Since the method was simple and originated and utilized in most of the developing countries, its acceptance created no problem But there were many technical problems associated with the traditional way of drying in the direct sun These problems include rain and cloudiness; contamination from dust and by insects, birds, and animals; lack of control over drying conditions; and possibility of chemical, enzymic, and microbiological spoilage due to long drying times The recent increase in the cost of fossil fuels associated with depletion of the reserve and scarcity has led to renewed interest in solar drying Bolin and Salunkhe [12] have exhaustively reviewed the drying methods using solar energy alone and with an auxiliary energy source, besides discussing the quality (nutrient) retention and economic aspects They suggested that to produce high-quality products with economic feasibility, the drying should be fast Drying time can be shortened by two main procedures: by raising the product temperature to that moisture can be readily vaporized, whereas at the same time the humid air is constantly removed, and by treating the ß 2006 by Taylor & Francis Group, LLC product to be dried so that moisture barriers such as dense hydrophobic skin layers or long water migration paths will be minimized Developments in solar drying of fruits and vegetables up to 1990 have been reviewed by Jayaraman and Das Gupta [13] To design a solar dryer for drying fruits and vegetables, two important stages are to be considered: to heat the air by the radiant energy from sun and to bring this heated air in contact with the material inside a chamber to evaporate moisture Solar dryers are generally classified [14] according to their heating modes or the manner in which the heat derived from solar radiation is utilized These classes include sun or natural dryers, direct solar dryers, indirect solar dryers, hybrid systems, and mixed systems 25.4.2.1 Sun or Natural Dryers Solar or natural dryers make use of the action of solar radiation, ambient air temperature, and relative humidity and wind speed to achieve the drying process 25.4.2.2 Solar Dryers—Direct In direct solar dryers the material to be dried is placed in an enclosure with a transparent cover or side panels Heat is generated by absorption of solar radiation on the product itself as well as on the internal surfaces of the drying chamber This heat evaporates the moisture from the drying product In addition it serves to heat and expand the air, causing the removal of the moisture by the circulation of air 25.4.2.3 Solar Dryers—Indirect In indirect solar dryers, solar radiation is not directly incident on the material to be dried Air is heated in a solar collector and then ducted to the drying chamber to dehydrate the product Generally flat-plate solar collectors are used for heating the air for low and moderate temperature use Efficiency of these collectors depends on the design and operating conditions The main factors that affect collector efficiency are heater configuration, airflow rate, spectral properties of the absorber, air barriers, heat transfer coefficient between absorber and air, insulation, and insolation By optimizing these factors, a high efficiency can be obtained More sophisticated designs of flat-plate collectors are now available Imre [15] described such collectors and their efficiency 25.4.2.4 Hybrid Systems Hybrid systems are dryers in which another form of energy, such as fuel or electricity, is used to supplement solar energy for heating and ventilation 25.4.2.5 Mixed Systems Mixed systems include dryers in which both direct and indirect models of heating have been utilized (Figure 25.1) Several experimental methods were evaluated for the solar dehydration of fruits (apricots): (a) wooden trays; (b) solar troughs of various materials designed to reflect radiant energy onto drying trays; (c) natural convection, solar-heated cabinet dryers with slanted plate heat collectors; (d) dryers incorporating inflated polyethylene (PE) tubes as solar collectors; and (e) PE semicylinders either incorporating a fan blower to be used in inflated hemispheres or incorporating a similar dome used as a solar collector, the air from which is blown over fruit in a cabinet dryer [16] Method (d) was found to be cheap, 38% faster than sun drying, and could be used as a supplementary heat source for conventional dehydrators Solar drying incorporating a desiccant bed for heat storage has been used to dry fruits and vegetables [17] Hot air up to 278C above ambient was obtained in a single glass-covered collector with an airflow of about 140 kg/h and raised to 528C for airflow of 25 kg/h In the absorbent circuit, which used a double glass-covered collector, temperature differences were 10% higher Other forms of heat storage involving use of natural materials such as water, pebbles or rocks, and the like, and salt solutions or absorbents have also been used Design and construction of a dryer was described [18] to utilize solar energy in the two-step osmovac dehydration of papaya consisting of a 56-by-25-by25-cm plexiglass (3.8-cm thick) and a portable condenser vacuum unit (Figure 25.2) Solar osmotic drying had higher drying rates and sucrose uptake than in the nonsolar runs Similarly, drying rates from solar vacuum drying were about twice those of nonsolar vacuum drying Solar drying of a number of vegetables using a solar cabinet dryer fitted with three flat-plate collectors was described [19] It was concluded that use of three flat-plate collectors instead of one improved the performance of solar cabinet dryer by increasing cabinet temperature and air circulation as compared to drying using single flat-plate collector Since solar drying of fruits and vegetables is usually long because of large amount of water to be removed, Grabowski and Mujumdar [20] examined the possibility of coupling osmotic drying with solar drying for more effective drying They have also 63.5 151.2 11.6 19.0 9.0 66.0 A 45Њ B 2.4 12 14 15 28Њ 45.7 FIGURE 25.1 Dimensions of a combined-mode solar layer (A, dryer; B, solar collector) (From Bolin, H.R and Salunkhe, D.K., Crit Rev Food Sci Nutri., 16, 327, 1982 With permission.) ß 2006 by Taylor & Francis Group, LLC Vacuum pump Vacuum chamber Condenser Control panel Refrigeration system FIGURE 25.2 Components built for solar vacuum drying (From Moy, J.H and Kuo, M.J.L., J Food Process Eng., 8(1), 23, 1985 With permission.) illustrated applications of solar-assisted osmotic dehydration systems for different production scales It was observed that minimum twofold increase in the throughput of typical solar dryers was possible while enhancing the nutritional and organoleptic qualities A solar drying system consisting of eight flatplate solar collectors was designed and constructed [21] Each flat-plate solar collector had a gross area of 2.0 m2, effective area of 1.86 m2, and average heatgenerating capacity of 18.6 MJ/d (at 50% efficiency) A dehydrator of 250 kg capacity was constructed for a fruit or a vegetable (apricot, grapes, persimmon, onion, chillies, etc.) Economic analysis showed that solar drying system is very economic for dehydration of fruit and vegetable For commercial success a solar dryer should be economically feasible But, in general, solar energy systems are capital-intensive In these dryers, although operating costs are low, large investments have to be made on equipment The prime economic problem is to balance the annual cost of extra investment against fuel savings Therefore solar drying could be economical only if the equipment cost is decreased or in the event of fuel cost escalation 25.4.3 HOT AIR DRYING Currently most of the dehydrated fruits and vegetables are produced by the technique of hot air drying, which is the simplest and most economical among the various ß 2006 by Taylor & Francis Group, LLC methods Different types of dryers have been designed, made, and commercially used based on this technique In this method, heated air is brought into contact with the wet material to be dried to facilitate heat and mass transfer; convection is mainly involved Two important aspects of mass transfer are the transfer of water to the surface of the material that is dried and the removal of water vapor from the surface The basic concepts, various methods of drying, and different types of hot air dryers are discussed by various authors in review articles and books [1,2,5,22–24] To achieve dehydrated products of high quality at a reasonable cost, dehydration must occur fairly rapidly Four main factors affect the rate and total time of drying [23]: physical properties of the foodstuff, especially particle size and geometry; its geometrical arrangement in relation to air (crossflow, through-flow, tray load, etc.); physical properties of air (temperature, humidity, velocity); and design characteristics of the drying equipment (crossflow, through-flow, cocurrent, countercurrent, agitated bed, pneumatic, etc.) The choice of the drying method for a food product is determined by desired quality attributes, raw material, and economy The dryers generally used for the drying of pieceform fruits and vegetables are cabinet, kiln, tunnel, belt-trough, bin, pneumatic, and conveyor dryers Among these, the cabinet, kiln, and bin dryers are batch operated, the belt-trough dryer is continuous, and the tunnel dryer is semicontinuous 25.4.3.1 Cabinet Dryers 25.4.3.4 Pneumatic Conveyor Dryers Cabinet dryers are small-scale dryers used in the laboratory and pilot plants for the experimental drying of fruits and vegetables They consist of an insulated chamber with trays located one above the other on which the material is loaded and a fan that forces air through heaters and then through the material by crossflow or through-flow Pneumatic conveyor dryers are generally used for the finish drying of powders or granulated materials and are extensively used in the making of potato granules The feed material is introduced into a fast-moving stream of heated air and conveyed through ducting (horizontal or vertical) of sufficient length to bring about desired drying The dried product is separated from the exhaust air by a cyclone or filter Jayaraman et al [25] described a pneumatic dryer in which an initial high temperature (160–1808C for min) drying of piece-form vegetables was done up to 50% moisture, resulting in expansion and porosity in the products that hastened finish drying in a conventional cabinet dryer besides significantly reducing rehydration times and increasing rehydration coefficients of the products (Table 25.3) [25] 25.4.3.2 Tunnel Dryers Tunnel dryers are basically a group of truck and tray dryers widely used due to their flexibility for the largescale commercial drying of various types of fruits and vegetables In these dryer trays of wet material, stacked on trolleys, are introduced at one end of a tunnel (a long cabinet) and when dry they are discharged from the other end The drying characteristic of these dryers depends on the movement of airflow relative to the movement of trucks, which may move parallel to each other either concurrently or countercurrently, each resulting in its own drying pattern and product properties 25.4.4 FLUIDIZED BED DRYING The fluidized bed type of dryer was originally used for the finish drying of potato granules In FB drying, hot air is forced through a bed of food particles at a sufficiently high velocity to overcome the gravitational forces on the product and maintain the particles in a suspended (fluidized) state [22] Fluidizing is a very effective way of maximizing the surface area of drying within a small total space Air velocities required for this will vary with the product and more specifically with the particle size and density A major limitation is the limited range of particle size (diameter usually 20 mm–10 mm) that can be effectively fluidized The bed remains uniform and behaves as a fluid when the so-called Froude number is below unity 25.4.3.3 Belt-Trough Dryers Belt-trough dryers are agitated bed, through-flow dryers used for the drying of cut vegetables of small dimensions They consist of metal (wire) mesh belts supported on two horizontal rolls; a blast of hot air is forced through the bed of material on the mesh The belts are arranged in such a way to form an inclined trough so that the product travels in a spiral path and partial fluidization is caused by an upward blast of air TABLE 25.3 Process Conditions for High-Temperature, Short-Time Pneumatic Drying of Vegetables and Rehydration Characteristics of Products Moisture Content (%) Material Potatoes Green peas Carrots Yams Sweet potatoes Colocasia Plantains, raw Optimum HTST Drying Raw Cooked/Blanched HTST Treated Final Dried Temp (8C) Time (min) 82.2 71.1 89.3 76.6 73.6 80.2 80.8 83.3 72.5 91.0 78.3 78.6 83.3 83.3 59.3 38.3 52.9 50.2 53.8 54.2 58.8 4.1 3.4 4.2 3.9 5.3 4.9 4.6 170 160 170 180 170 170 170 8 8 8 Rehydration Time Rehydration Coefficient 5 2 0.94 1.06 0.50 1.01 1.06 0.98 0.97 Source: From Jayaraman, K.S., Gopinathan, V.K., Pitchamuthu, P., and Vijayaraghavan, P.K., J Food Technol., 17(6), 669, 1982 With permission ß 2006 by Taylor & Francis Group, LLC The theory and food applications of fluidized bed drying have been discussed in many textbooks and articles [5,22–24,26,27] Apart from the commercial drying of peas, beans, and diced vegetables, it is also used for drying potato granules, onion flakes, and fruit juice powders It is often used as a secondary dryer to finish the drying process initiated in other types of dryers It can be carried out as a batch or continuous process with a number of modifications The advantages of fluidized bed drying are high drying intensity, uniform and closely controllable temperature throughout, high thermal efficiency, time duration of the material in the dryer may be chosen arbitrarily, elapsed drying time is usually less than other types of dryers, equipment operation and maintenance is relatively simple, the process can be automated without difficulty, and, compact and small, several processes can be combined in an FB dryer [5] Heat transfer in FB drying could be improved by increasing gas velocity But, at higher velocities, the particles are transported out of bed and voidage in the bed increases, reducing the volumetric effectiveness of the equipment From the viewpoint of good gas-to-solid contact, this is undesirable because most of the gas passes around the layers of particles without effective contact Another drawback of conventional fluidized bed drying is that the maximum gas velocity is closely related to the physical characteristics of the food particles such as shape, surface roughness, bulk density, and firmness The maximum gas velocity controls the amount of heat delivered to the bed, since for foods there is usually a critical maximum gas temperature for processing The centrifugal fluidized bed (CFB) was designed [28,29] to overcome the limitations of piece size and heat requirements encountered in a conventional FB dryer by subjecting the food particles during fluidization to a centrifugal force greater than the gravitational force This had the effect of increasing the apparent density of the particles and allowing smooth, homogenous fluidization Smooth fluidization could be achieved at any desired gas velocity by varying the centrifugal force The other advantages provided by CFB include increasing the gas velocity to provide improved heat transfer at moderate gas temperature without the problem of heat damage, and large pressure drops across the grid supporting the bed are not needed to obtain smooth fluidization It was demonstrated to be effective for extremely high rate drying of high-moisture, low-density, sticky, piece-form foods Drive shaft Fixed plenum Air supply Air discharge Rotating perforated basket (ascending side) Door for product removal Air flow Packed bed F0 > Drug force Air flow Dense fluidized bed F0 = Drug force Spouted bed F0 < Drug force FIGURE 25.3 Modified design of centrifugal fluidized bed dryer allows for lower pressure drops and better heat economy As the air velocity is increased, the degree of fluidization changes from packed to spouted (From Brown, G.E., Farkas, D.F., and De Marchena, E.S., Food Technol., 26(12), 23, 1972 With permission.) ß 2006 by Taylor & Francis Group, LLC A modified centrifugal fluidized bed dryer (CFBD) developed consisted of a cylinder with perforated walls, rotating horizontally about its axis in a high velocity, heated crossflow airstream (Figure 25.3) [29] Piece-form product to be dried was fed into one end of the rotating cylinder, moved along the cylinder in almost plug-flow manner through the hot air blast, passing crossflow through the perforated walls, and discharged from the other end of the cylinder On the downstream side (relative to the airflow) within the cylinder, the pieces were held as a fixed bed against the wall by the additive forces of frictional air drag and centrifugation At high rpm or low air velocity, the centrifugal force on any particle was greater than the drag force of the entering airstream and each particle remained fixed in place If the air velocity was increased or the rpm decreased, dense-phase fluidization was obtained on the upstream side of the bed because the drag force on the pieces was equal to or slightly greater than the opposing centrifugal force If the air velocity was further increased, transport of the particles across the cylinder occurred as in a spouted bed Centrifugal force obtained through cylinder rotational speed to give 3–15 Gs allowed the use of air velocities up to 15 m/s or higher, many times greater than can be employed in conventional FBs Carrots, potatoes, apples, and green beans dried in this modified CFB at an air velocity of 2400 ft/min and 2408F showed that a weight reduction of 50% could be achieved in less than for all items In comparison with a tunnel dryer with a crossflow air velocity of 780 ft/min, 1608F temperature, and lb/ft2 tray loading, it was shown that average drying rate in a modified CFB (air velocity 2400 ft/min) was 5.3 times the crossflow value This increase in drying rate (three times the theoretical value) was due to high efficiency of the air-to-particle contact achieved in the CFB A continuous CFBD was further designed (Figure 25.4) with a dryer surface of approximately 21 ft2 in the form of a rotating perforated stainless steel cylinder (10-in diameter and 100-in long) with an open area of 45% and Teflon-coated inside [30] The cylinder could be rotated at speeds up to 350 rpm (Fe ¼ 17.4ÂG) through a belt drive and tilted between 08 and 68 from the horizontal to help control the residence time of material that is dried Centrifugal fans with steam heaters enabled air temperatures up to 1408C Table 25.4 gives the performance data from trials for drying bell peppers, beets, carrots, cabbages, onions, and mushrooms using a CFBD [30,31] Good continuous operation was achieved for a 1-h period Feed rates and evaporation (kg/h) are given for a range of dryer sizes in Table 25.5 for cabbages, carrots, onions, and mushrooms [31] 12 13 14 12 10 11 11 12 FIGURE 25.4 Isometric view of centrifugal fluidized bed drying system (1, dryer cylinder; 2, drive pulley; 3, aspiration feeder; 4, feeder blower; 5, discharge chute; 6, air blower; 7, air discharge damper; 8, steam coil heater; 9, plenum; 10, air vent; 11, vent port; 12, recirculating duct; 13, make-up air; 14, blower intake) (From Hanni, P.F., Farkas, D.F., and Brown, G.E., J Food Sci., 41(5), 1172, 1976 With permission.) ß 2006 by Taylor & Francis Group, LLC apple and potato was reported [54] to produce a highquality product with lower freeze-drying times 25.4.11 OSMOTIC DEHYDRATION Osmotic dehydration is a water removal process that consists of placing foods, such as pieces of fruits or vegetables, in a hypertonic solution As this solution has higher osmotic pressure and hence lower water activity, a driving force for water removal arises between solution and food, whereas the natural cell wall acts as a semipermeable membrane As the membrane is only partially selective, there is always some diffusion of solute from the solution into the food and vice versa Direct osmotic dehydration is therefore a simultaneous water and solute diffusion process [55] Up to a 50% reduction in the fresh weight of the food can be achieved by osmosis Its application to fruits and, to a lesser extent, to vegetables has received considerable attention in recent years as a technique for production of intermediate moisture foods (IMF) and shelf-stable products (SSP) or as a predrying (preconcentration) treatment to reduce energy consumption and heat damage in other traditional drying processes Some of the stated advantages of direct osmosis in comparison with other drying processes include minimized heat damage to color and flavor, less discoloration of fruit by enzymatic oxidative browning, better retention of flavor compounds, and less energy consumption since water can be removed without change of phase However, products cannot be dried to completion solely by this method and some means of stabilizing them is required to extend their shelf lives Many workers have studied the different aspects of osmotic dehydration: the solutes to be employed, the influence of process variables on drying behavior, the opportunity to combine osmosis with other stabilizing techniques, and the quality of the final products The osmotic agents used must be nontoxic and have a good taste and high solubility besides low aw Sugar in different concentrations is widely used Common salt is an excellent osmotic agent for vegetables The quantity and the rate of water removal depend on several variables and processing parameters In general it has been shown that the weight loss in osmosed fruit is increased by increasing the solute concentration of the osmotic solution, immersion time, temperature, solution-to-food ratio, specific surface area of the food, and by using vacuum, stirring, and continuous reconcentration Also, to obtain the same aw reduction, time tended to decrease exponentially as the temperature is increased Several models were proposed to show the effect of concentration of osmotic solution and temperature on the rate of water loss and gain of osmotic agent ß 2006 by Taylor & Francis Group, LLC Thus, a model developed [56] for the calculation of osmotic mass transport data for potato and water activity to equilibrium in sucrose solutions for the concentration range 10–70% and solution/solids range 1–10 showed that, at equilibrium, there was an equality of water activity and soluble solids concentration in the potato and in the osmosis solution A linear relationship existed between normalized solids content (NSC) and log (1 – aw) and was given by NSC ẳ 6:1056 ỵ 2:4990 log (1 À aw ) Another model developed [57] for solute diffusion in osmotic dehydration of apple based on solids gain divided by water content M as a function of rate constant K, time (t), and a constant A was given as M ẳ Kt ỵ A A relationship was established in the form of K ¼ T1.40C1.13, where rate parameter K is related to temperature T at different sucrose concentrations C The average activation energy of the process was 28.2 kJ/mol The effects of solution concentration, osmosis time, and the osmosis temperature were studied in the osmotic dehydration of pineapple in sucrose solution [58] The solute diffusion was analyzed by Magee’s model The effect of sucrose concentration C on rate parameter K was given by power law regression equation as K ¼ 4.15 Â 10À4C1.51 at 208C An empirical equation derived based on osmotic dehydration of apple slices could predict rate of osmosis F, that is, percentage of dehydration of any given fruit slices of specific size with time T, given the concentration of sugar (% B) and the temperature as follows [59]: F ¼ 31:8 À 0:307B À (0:56 À 0:016B)t À 2:10À9:26=B À 1(T À 0:3)0:54 À 0:00425t where F is the decrease in mass %, and was valid for B ¼ 60–75%, t ¼ 40–808C, and T ¼ 0.5–4.5 h Direct osmosis of different fruits at 708 Brix sugar at atmospheric and low pressure (about 70 mmHg) revealed higher drying rates with the latter The addition of a small amount of NaCl to different osmotic solutions increased the driving force of the drying process Apple cubes submitted to HTST osmosis in sugar at 60–808C for 1–20 showed osmosis to be greatly accelerated by high temperature, as the water loss in apples after 1–3 HTST osmosis was the same as that given by 2-h treatment at ambient temperature and HTST osmosis completely inactivated the enzymes Partial dehydration of fruits and vegetables by osmosis using various osmotic agents has been employed before drying by other conventional methods, namely, hot air convection drying, high-temperature fluidized bed drying, vacuum drying, freeze-drying, and dehydrofreezing as a means of reducing processing time and limiting energy consumption besides improving sensory characteristics Osmotic dehydration has been utilized for developing intermediate moisture fruits stabilized solely by aw control with added antimycotic preservative, as well as SSP with higher aw stabilized by a combination preservation technique involving aw and pH control plus heat pasteurization, due to simplicity of the operations involved, economy, and low-energy inputs 25.4.12 HEAT PUMP DRYING Drying at low temperatures can improve quality Higher energy efficiencies are achieved because both the sensible and the latent heat of evaporation are required Drying conditions and therefore drying rate is unaffected by drying conditions Against these advantages, a number of factors limit the application of heat pump drying These include the use of electrical energy which is generally more expensive than other forms, higher capital cost and that the maximum drying temperature is limited to around 608C to 708C with currently used refrigerants Typical drying temperatures in a heat pump dryer are in the range 308C–608C It is expected that drying by this technique would improve the retention of volatile flavor, reduce the color degradation as well as the loss of heat-labile vitamins [61] OF LIQUIDS The utilization of ultrasonic energy to remove water from dilute solution of nonfatty products was reported [62] In this process the liquid is atomized ß 2006 by Taylor & Francis Group, LLC 25.5 QUALITY CHANGES DURING DRYING AND STORAGE 25.5.1 LOSS To improve the thermal economy and efficiency of conventional hot air dryer, use of heat pump technology was utilized for the development of heat pump dryer In its simplest form, the heat pump dryer passes the drying air over the evaporator of a refrigeration system This cools the air to below its dew point, condensing water vapor from the air stream This cool air is then passed over the condenser over the refrigeration system to reheat the air to drying temperature Most available heat pump dryers recirculate all the air, but nonrecirculating types are also available Both types can be highly energy efficient [60] The three major advantages of heat pump dryers are [60]: 25.4.13 ULTRASONIC DRYING through a nozzle initially and then by cavitation using ultrasonic energy An ultrasonic technique for drying of vegetables using a power ultrasound generator was reported [63] In this technique high-intensity ultrasonic vibrations were used to investigate the drying of carrot slices and effect of this technique were compared with those of conventional drying and forced air-drying assisted by airborne ultrasonic radiation Dramatic reduction in drying time was achieved maintaining the quality OF VITAMINS (VITAMINS A AND C) Fruits and vegetables are the major sources of vitamin C (ascorbic acid) and provitamin A (b-carotene) besides minerals It is, therefore, quite understandable that to determine the efficacy of dehydration techniques scientists have primarily investigated and compared the effect such techniques have on these nutrients The effect of predrying treatments, dehydration, storage, and rehydration was studied [64] on the retention of carotene in green peppers and peaches during home dehydration Carotene was completely retained in the case of green peppers In peaches, 72.7% of the carotene was retained after predrying treatment, which decreased to 37.3% after dehydration Retention of ascorbic acid during predrying treatment and dehydration depended on the nature of food Thus, in the case of green peppers, most losses occurred during storage whereas dehydration was responsible for most of the loss in the dipped peaches In general, rapid drying retained a greater amount of ascorbic acid than slow drying Thus vitamin C contents of vegetable tissue are greatly reduced during a slow sun-drying process, whereas dehydration, especially by spray drying and freeze-drying, reduced these losses The effect of sun drying on the ascorbic acid content of 10 Nigerian vegetables showed that there was 21–58% loss depending on the nature of the vegetables [65] Oxidative changes would be expected to be minimum in freeze-dried samples as freeze-drying is a low-temperature process operating under vacuum A study [66] of the changes in quality of compressed carrots prepared in combinations of freeze-drying and hot air drying showed that value of ascorbic acid ranged from 15.97 mg/100 g for the totally airdried samples to 33.39 mg/100 g for the totally freeze-dried samples (Table 25.7) In the case of carotenes also the totally hot air-drying treatment had TABLE 25.7 Effect of Drying Treatment on Ascorbic Acid and a-Tocopherol of Dehydrated Carrots (mg/l00 g Dry Weight Basis)a TABLE 25.8 Effect of Drying Treatment on Carotene Content of Dehydrated Carrots (mg/100 g Dry Weight Basis)a Treatment (% Moisture) Treatment (% Moisture) Ascorbic Acid a-Tocopherol Fresh Totally freeze-dried Totally air-dried Freeze-dried (30%), mist plasticized (10%), air-dried Freeze-dried (10%), air-dried Freeze-dried (20%), air-dried Freeze-dried (30%), air-dried Freeze-dried (40%), air-dried Freeze-dried (50%), air-dried 85.28 33.39 a 15.97 d 3.41 3.45 a 0.04 f 32.76 a 27.71 b 16.78 cd 16.38 cd 20.38 c 17.49 cd 2.98 b 1.42 c 1.13 d 1.10 d 0.96 d 0.55 e a Means within columns followed by the same letter are not significantly different at the 5% level according to Duncan’s multiple range test Source: From Shadle, E.R., Burns, E.E., and Talley, L.J., J Food Sci., 48(1), 193, 1983 With permission the lowest value (34.16 mg/100 g) and totally freezedried samples had the highest value (70.37 mg/100 g) (Table 25.8) The effect of blanching, various drying methods (sun, vacuum oven, and hot air oven), and drying temperature (33–608C) on ascorbic acid content of okra was investigated [67] Blanching solution resulted in slight loss in ascorbic acid but led to more retention during dehydration Vacuum dehydrated sample retained more ascorbic acid at each of the dehydration temperature than those from hot air oven Vacuum microwave drying of carrot was compared to air-drying and freeze-drying on the basis of aand b-carotene and vitamin C content Total losses of a- and b-carotene during drying was 19.2% for airdrying and 3.2% for vacuum microwave drying samples Loss of vitamin C content was substantial due to blanching [68] The effect of blanching and drying methods on the b-carotene and ascorbic acid retention in three leafy vegetables, i.e., savoy beet, amaranth, and fenugreek showed [69] that the most suitable method for blanching was thermal treatment in water at 95 + 38C followed by potassium metabisulfite dip and drying at low temperature for the retention of ascorbic acid as well as b-carotene The retention of ascorbic acid and b-carotene was reported to be 15.0%, 49.7% for savoy beet; 40.5%, 98.5% for amaranth, 54.6%, 91.5% for fenugreek after blanching, and 7.5%, 39.7%; 30%, 48.5; 49.7%; 85.1%, respectively after low-temperature drying ß 2006 by Taylor & Francis Group, LLC Fresh Totally freeze-dried Totally air-dried Freeze-dried (3%), mist plasticized (10%), air-dried Freeze-dried (10%), air-dried Freeze-dried (20%), air-dried Freeze-dried (30%), air-dried Freeze-dried (40%), air-dried Freeze-dried (50%), air-dried a-Carotene b-Carotene Total Carotene 14.4 15.66 a 6.67 e 52.06 54.71 a 27.50 f 66.20 70.37 a 34.16 f 10.61 d 40.47 e 51.08 e 12.81 b 49.40 b 62.21 b 11.73 c 44.49 d 56.22 d 11.42 cd 47.22 c 58.68 c 11.02 cd 44.89 d 55.91 d 10.52 d 40.23 c 50.81 e a Means within columns followed by the same letter are not significantly different at the 5% level according to Duncan’s multiple range test Source: From Shadle, E.R., Burns, E.E., and Talley, L.J., J Food Sci., 48(1), 193, 1983 With permission In general it is difficult to compare the losses in vitamins during dehydration because retention of vitamins depends on the nature of foods, predrying treatments given (sulfuring, blanching methods), and the conditions of drying (techniques, time, and temperature) 25.5.2 LOSS OF NATURAL PIGMENTS (CAROTENOIDS AND CHLOROPHYLLS) Color is an important quality attribute in a food to most consumers It is an index of the inherent good qualities of a food and association of color with acceptability of food is universal Among the natural color compounds, carotenoids and chlorophylls are widely distributed in fruits and vegetables The preservation of these pigments during dehydration is important to make the fruit and vegetable product attractive and acceptable Both the pigments are fatsoluble although they are widely distributed in aqueous food systems Carotenoids are susceptible to oxidative changes during dehydration due to the high degree of unsaturation in their chemical structure The major carotenoids occurring in food are carotenes and oxycarotenoids (xanthophylls) Leaching of soluble solids during blanching had considerable effect on the stability of carotenoids of carrots during drying and subsequent storage [70] Carotenoid destruction increased with increased leaching of soluble solids Investigation of the effects of water activity, salt, sodium metabisulfite, and Embanox-6 on the stability of carotenoids in dehydrated carrots shows that carotenoid pigments were most stable at 0.43aw and addition of salt, metabisulfite, and Embanox-6 helped in stabilizing carotenoids in dehydrated carrots (Table 25.9) [71] Sulfur dioxide was found to have a pronounced protective effect on carotenoids of unblanched carrots during dehydration [72] Dehydrated, sulfited, unblanched carrots contained about 2.9 times more carotenoids than dehydrated unblanched carrots that had not been sulfited (Table 25.10) Treatment with SO2 gave additional protection to carotenoids of blanched carrots during dehydration and effectiveness of SO2 increased with an increase in SO2 content The importance of chlorophyll in food processing is related to the green color of vegetables Many studies have been made on the changes of chlorophyll during processing and storage but little is known about the pigment behavior in low-moisture systems such as dehydrated vegetables Generally, it was found that chlorophyll was quite stable in low-moisture systems Degradation of chlorophyll depended on temperature, pH, time, enzyme action, oxygen, and light The most common mechanism of chlorophyll degradation is its conversion to pheophytin in the presence of acid Although the pathways of this degradative reaction are well-known, a method for its stabilization is not well-established Water activity has been shown to have a definite influence on the rate of degradation of chlorophyll in freeze-dried, blanched spinach puree [73] At 378C and an aw higher than 0.32, the most important mechanism of chlorophyll degradation was conversion to pheophytin At aw lower than 0.32, the rate of pheophytin formation in spinach was low The rate of chlorophylla transformation was 2.5 times faster than chlorophyll-b The study of the degradation of chlorophyll as a function of aw, pH, and temperature in a spinach system during storage showed that even in the dry state the elimination of a magnesium atom and transformation of chlorophyll into pheophytin was very sensitive to pH changes [74] Effect of temperature on the rate of chlorophyll-a degradation at water activity 0.32 and pH 5.9 is shown in Figure 25.6 25.5.3 BROWNING AND ROLE OF SULFUR DIOXIDE One obstacle always encountered by the food technologists in the dehydration and long-term storage of ß 2006 by Taylor & Francis Group, LLC dehydrated fruits and vegetables is the discoloration due to browning Browning in foods is of two types: enzymatic and nonenzymatic In the former, the enzyme polyphenol oxidase catalyzes the oxidation of mono- and ortho-diphenols to form quinones that cyclize, undergo further oxidation, and condense to form brown pigments (melanins) In the dehydration of fruits and vegetables, blanching destroys the causative enzymes and prevents subsequent enzymatic browning Sulfur dioxide and sulfites act as inhibitors of enzyme action during preblanching stages The presence of SO2 retards browning of dehydrated fruits and vegetables, especially when the enzymes have not been heat-inactivated (e.g., freeze-dried products) NEB, also known as Maillard reaction, describes a group of diverse reactions between amino groups and active carbonyl groups, leading eventually to the formation of insoluble, brown, polymeric pigments, collectively known as melanoidin pigments The basic reactions that lead to the browning are well documented in the literature These reactions are sometimes desirable but in many instances are considered to be deleterious not only due to the formation of unwanted color and flavor but also due to the loss of nutritive value through the reactions involving the a-amino group of lysine moieties and other groupings in proteins It is a major deteriorative mechanism in dry foods and is sensitive to water content It is influenced by the types of reactant sugars and amines, pH, temperature, and aw The addition of sulfites during the predrying step is the only effective means available at present controlling NEB in the dried fruit and vegetable product Sulfite is considered to be a safe additive to incorporate into fruit and vegetable products up to certain permissible limits However, recently there are reports on the hypersensitivity of a few individuals to the ingested sulfite Numerous attempts are therefore made to find alternative means to prevent browning reactions Among various treatments studied, such as addition of SO2, cysteine, CaCl2, trehalose, manganese chloride, disodium dihydrogen pyrophosphate, oxygen scavenger pouch, and so on, the only ones that effectively retarded the formation of undesirable pigment in dried apples during storage were oxygen scavenging and sulfur dioxide [75] Apples stored in oxygen scavenger packages darkened slower than those stored under regular atmospheric conditions, exhibiting a different initial induction period (Figure 25.7) The effectiveness of sulfite in controlling the family of diverse reactions, leading to browning is probably due to the number of different reactions that sulfite can enter into with reducing sugars, simple carbonyls, a-, b-dicarbonyls, b-hydroxycarbonyls, ß 2006 by Taylor & Francis Group, LLC TABLE 25.9 Effects of NaCl, Na2S2O5, and Embanox-6 on Total Carotenoids, TBA Value, and Nonenzymic Browning in Air-Dried Carrots Storage Period (Months) Control Salt Treated Salt ỵ Metabisulfite Treated Salt ỵ Metabisulfite Embanox-6 Treated Carotenoids (mg/g) TBA Value NEB Carotenoids (mg/g) TBA Value NEB Carotenoids (mg/g) TBA Value NEB Carotenoids (mg/g) TBA Value NEB 1120 505 316 222 0.12 0.92 1.38 1.50 0.08 0.14 0.21 0.28 1137 669 416 308 0.12 0.83 0.92 1.05 0.06 0.10 0.15 0.24 1114 691 449 353 0.10 0.64 0.78 0.92 0.05 0.08 0.18 0.22 1135 827 620 408 0.09 0.28 0.46 0.58 0.05 0.09 0.14 0.18 TBA value, mg of malonaldehyde per kg substance; NEB, nonenzymic browning reported as optical density at 420 nm Source: From Arya, S.S., Natesan, V., Parihar, D.B., and Vijayaraghavan, P.K., J Food Technol., 14, 579, 1979 With permission TABLE 25.10 Effect of Concentration of SO2 on Carotenoid Content of Dehydrated Carrot of 5% Moisture Content during Storage at 378C Blanching Time (min) 0 5 5 Initial SO2 Content (mg/g) Carotenoids Remaining (%) Carotenoid Content after Dehydration (mg/g) 1723 2325 2330 1584 2357 9621 Storage Time (d) 464 1296 1360 1350 1202 1298 1308 1380 60 120 180 300 440 68.0 87.5 92.5 88.7 77.5 80.5 87.0 89.9 51.1 76.5 85.0 79.4 62.5 67.4 76.1 80.0 43.0 69.4 79.4 71.1 56.1 60.5 68.6 73.1 36.2 62.6 69.0 61.7 50.2 54.0 58.5 62.8 33.1 55.5 62.0 55.0 48.2 50.2 52.0 54.0 Source: From Baloch, A.K., Buckle, K.A., and Edwards, R.A., J Sci Food Agric., 40, 179, 1987 With permission b-unsaturated carbonyls, and with melanoidins [76] So far there is no practical substitute for SO2 as a means of controlling NEB, although lowering pH, dehydration to very low water activity, separation of active species, and addition of sulfhydryl compounds might have limited applications [6] 25.5.4 OXIDATIVE DEGRADATION AND FLAVOR LOSS The acceptability of dehydrated fruit and vegetable products is highly dependent upon their flavor attributes Loss of desirable flavor is the limiting characteristic for most dehydrated products The natural 100 Residual chlorophyll-a (%) 50 20 T ЊC 38.6 46.0 58.7 10 5 10 15 Time (days) FIGURE 25.6 Degradation of chlorophyll-a in spinach as a function of temperature (aw ¼ 0.32; pH ¼ 5.9) (From Lajolo, F.M and Marquez, U.M.L., J Food Sci., 47, 1995, 1982 With permission.) ß 2006 by Taylor & Francis Group, LLC 20 L* Initial–L* Stored Control SUL 10 O−SCV SUL/O−SCV 0 10 15 Storage (weeks) 20 FIGURE 25.7 Effect of in-package oxygen scavenger on dried apple darkening during storage at 308C (DL* of ¼ observable change) (From Bolin, H.R and Steele, R.J., J Food Sci., 52(6), 1654, 1987 With permission.) flavor constituents are subjected to much variation and loss during predrying operations, drying, and storage Conditions generally responsible for the destruction of natural flavors include rough handling, delay in processing, exposure to light, high temperature, and certain chemicals Flavor retention is especially important in products in which the principal flavor constituents are volatile oils, as in onions Flavor defects in dehydrated products were, however, not solely due to volatile losses Chemical reactions, especially oxidation and NEB, greatly contributed to flavor deterioration In general, freeze-dried products had more preferable flavors than air-dried ones except in the case of onions, for which an air-dried product had a stronger flavor due to entrapment of volatile oils by shrinkage Leeks and celery showed similar behavior Staling and off flavors developed during storage of both air-dried and freeze-dried vegetables The degree of change was mainly related to temperature of storage and moisture content of the dried vegetables Air-dried peas (6–7% moisture) developed off flavor at 158C after 15–18 months At about 208C, shelf life was reduced to 9–12 months and at 378C the period was 2–3 months Comparatively, freeze-dried vegetables were much more sensitive to storage conditions because the highly porous texture allowed easy entry of air and stale flavor developed rapidly For example, freeze-dried carrots developed off flavor after month in air at 208C At 308C the oxygen level had to be reduced to 0.1% to give a storage life of months [77] The absence of oxygen was essential for satisfactory storage of freeze-dried fruits and vegetables ß 2006 by Taylor & Francis Group, LLC Excellent retention of fresh flavor quality was achieved in a series of freeze-dried foods of plant origin in zero oxygen headspace, using an atmosphere of 5% hydrogen in nitrogen with palladium catalyst [78] Vegetable items took up oxygen chiefly as a function of pigment content Those with a high carotene content (sweet potatoes, spinach, and carrots) underwent a fairly rapid uptake during the first 15–40 weeks and had consumed all available oxygen at the end of year Lesser-pigmented vegetables with a lower lipid content (green beans and potatoes) showed a slow, steady uptake Two fruit items, peaches and apricots, displayed a very slow uptake, using only 30–50% of available oxygen during year One of the major causes of degeneration of flavor in dehydrated potato products was the Maillard reaction This aminocarbonyl reaction of reducing sugar and amino acid resulted in the formation of many volatile compounds Thus, flavor deterioration in potatoes during the explosion-puffing step was attributed to NEB In the puffing gun, potatoes at 30% moisture were subjected to a temperature condition conducive to NEB, which resulted in the formation of volatile aldehydes On the other hand, dominant, rancid off flavor that developed during the storage of dried potato products was due to autoxidation of potato lipids [79], giving hexanal as a major volatile product The use of BHA alone or with BHT effectively retarded the autoxidation of explosion-puffed potatoes, keeping oxidative off flavors below threshold levels for up to 12 months in storage as compared to months for air-packed samples without antioxidant The incorporation of a scavenger pouch packaging system (H2–palladium catalyst), although very effective in antioxidative effect, was severely limited because of pinhole leaks 25.5.5 TEXTURE AND RECONSTITUTION BEHAVIOR The problem of hot air drying, which is still the most economical and widely used method for dehydrating piece-form vegetables and fruits, is the irreversible damage to the texture, leading to shrinkage, slow cooking, and incomplete rehydration Many commercially dehydrated vegetables exhibit a dense structure with most capillaries collapsed or greatly shrunk, which affects the textural quality of the final product The possible causative factors suggested by different workers are loss of differential permeability in the protoplasmic membrane, loss of turgor pressure in the cell, protein denaturation, starch crystallinity, and hydrogen bonding of macromolecules Texture of air-dried vegetables deteriorates during storage if the product is exposed to high temperature or if inadequately dehydrated Even the freeze-drying technique has failed to produce an acceptable dehydrated product from celery Damage generally occurred during freezing, drying, storage, and reconstitution Water removal affects many aspects of cell structure; histological studies were generally carried out to assess the membrane integrity Pedlington and Ward [80], in studies on air-dried carrots, parsnips, and turnips, observed several changes, including a loss in the selective permeability of cytoplasmic membranes of cell responsible for maintaining turgidity and crisp texture of vegetables They found loss of water to result in rigidity of cell walls and to their slow collapse by the stresses set up by shrinkage of neighboring cells Jayaraman et al [81] studied the effect of sugar and salt to the texture of dehydrated cauliflower They found that in treated, dehydrated florets there were 80% intact cells as compared with 0% in the untreated, dehydrated florets due to tissue collapse resulting in disruption of cell walls and loss of cell integrity Khedkar and Roy [82] found a higher reconstitution ration in cabinet-dried raw mango slices as compared with sun-dried slices; this was due to less rupture of cells during cabinet drying (36.4%) than sun drying (67.3%) Different dehydration techniques were tried to improve the rehydration behavior of dehydrated pieceform fruits and vegetables Generally, it was observed that the greater the degree of drying, the slower and less complete was the degree of rehydration Dehydration techniques used to improve the rehydration qualities of dehydrated fruits and vegetables include those aimed at reducing the drying time or involving use of additives like salt and polyhydroxy compounds such as sugar and glycerol as a predrying treatment ß 2006 by Taylor & Francis Group, LLC Dehydrated carrots puffed and dried in a CFB unit absorbed 1/2 parts by weight of water and appeared completely rehydrated in whereas the unpuffed controls absorbed 1/2 parts and still had hard centers [29] Jayaraman et al found rehydration ratio, coefficient rehydration, and reconstitution time of HTST pneumatic-dried vegetables to be much superior to those of directly cabinet-dried samples [23] The effect of additives on the rehydration qualities of dehydrated vegetables was studied by Neumann [83] and Jayaraman et al [81] A combined predrying treatment of sodium carbonate and sucrose (60%) produced the best rehydrated celery, with a rehydration percentage of 71% and the dices were well filled out with texture remaining tender to firm [83] Similarly, a presoaking treatment in a combined solution of salt and sugar at 48C for 16 h before cabinet drying markedly increased the rehydration percentage of cauliflower and reduced the shrinkage as compared with control without treatment [81] The study of the rehydration ratios of forced airdried compressed carrots after partially freeze-drying to different moisture levels showed the drying treatment significantly affected rehydration ratios in all cases [66] The sample that was freeze-dried to 50% moisture, compressed, and then air-dried had the highest ratio and was the quickest to rehydrate In comparison, the totally freeze-dried and hot air-dried compressed carrots showed much lower values of rehydration ratios These observations were supported by scanning electron microscopy (SEM), which showed collapse of cellular structure and tissue coagulation to act as a barrier for rehydration Levi et al [84] observed that pectin, one of the major cell wall and intercellular tissue components, played a significant role in the rehydration capacity of dehydrated fruits 25.5.6 INFLUENCE OF WATER ACTIVITY During the last three decades water activity, aw, has played a major role in many aspects of food preservation and processing It is defined as the ratio of the vapor pressure of water P in the food to the vapor pressure of pure water P0 at the same temperature (aw ¼ P/P0) Next to temperature, it is now considered as probably the most important parameter having a strong effect on deteriorative reactions The effect of water activity was studied not only to define the microbial stability of the product but also on the biochemical reactions in the food system and its relation to its stability It has become a very useful tool in dealing with water relations of foods during processing It is now well known that microorganisms cannot grow in the dehydrated food system when the water ß 2006 by Taylor & Francis Group, LLC 100 50 Residual chlorophyll-a (%) activity range is less than or equal to 0.6–0.7, but other reactions, enzymatic and nonenzymatic (e.g., lipid oxidation, NEB, etc.) that cause change in color, flavor, and stability continue during processing and storage Water activity has become the most useful parameter that can be used as a reliable guide to predicting food spoilage or to determine the drying end point required for an SSP The relationship between equilibrium moisture content and water activity, known as the sorption isotherm, is an important characteristic that influences many aspects of dehydration and storage It can be constructed graphically or derived mathematically The shape of the isotherm generally determines the storage stability of the dehydrated product This concept is used to establish product specifications for the effective drying, packaging, and storage of foods Adsorption isotherms of potatoes were of sigmoid shape and were affected by drying method, temperature, and addition of sugar [85] The freeze-dried product absorbed more water vapor than the vacuum-dried materials The sorption isotherm prepared from fresh and freeze-dried Thompson seedless grapes indicated a hysteresis loop at both the upper and lower moisture level [86] The isotherm sun-dried grapes were slightly lower than that of vacuum-dried grapes Both lipid oxidation and NEB are greatly influenced by aw [87] Autoxidation of lipids occurs rapidly at low aw levels, decreasing in rate as aw is increased until in the 0.3–0.5 range and increasing thereafter beyond 0.5aw Most rapid browning can be expected to occur at intermediate aw levels in the 0.4–0.6 range Whether or not it is minimized at the lower or upper portion of this range depends significantly on the specific solutes used to poise aw, the nature of the food (especially amino compounds and simple sugars that might be present), as well as the pH and aw of the product Interestingly, at aw levels that minimize browning, autoxidation of lipids is maximized The kinetics of chlorophyll-a transformation was studied as a function of time at different water activities at 38.68C (Figure 25.8) [74] For aw > 0.32 the most important mechanism of chlorophyll degradation was the transformation into pheophytin; this had a first-order dependence on pH, water activity, and pigment concentration Carotenoids in freeze-dried carrots were relatively more stable in the range of 0.32 to 0.57aw [71] The maximum stability was near 0.43aw (corresponding to an equilibrium moisture content of 8.8–10%) Increase in the rate of carotenoid destruction was greater at lower aw than at higher aw 30 20 aW 0.11 0.32 0.52 0.75 10 12 Time (days) 16 20 FIGURE 25.8 Degradation rate of chlorophyll-a in spinach as a function of time at different water activities (pH ¼ 5.9; temperature 38.68C) (From Lajolo, F.M and Marquez, U.M.L., J Food Sci., 47, 1995, 1982 With permission.) The kinetics of quality deterioration in dried onion flakes (NEB and thiosulfinate loss) and dried green beans (chlorophyll-a loss) were studied as a function of water activity and temperature and empirical equations and mathematical models developed that successfully predicted the shelf life of the dried products as a function of temperature and aw (Table 25.11 and Table 25.12) [88] Above the monolayer (aw, 0.32–0.43) for onion, increasing moisture contents resulted in greater reaction rates for browning and thiosulfinate loss Very little browning was observed over a storage period of 631 d at 208C and aw ¼ 0.33, whereas all other samples stored at 30 and 408C and aw ¼ 0.43 and 0.59 deteriorated to unacceptable levels within this time period Similarly, in the case of green beans, the destruction of chlorophyll-a (pheophytinization) was found to be the principal factor responsible The dried green beans were considered unacceptable when more than 30% loss of chlorophyll-a was observed the concentration at which the dull olive-green color began to predominate Since conversion of chlorophyll-a to pheophytin is an acid-catalyzed reaction, the availability of water was essential and therefore aw could be expected to influence the rate of chlorophyll loss TABLE 25.11 Actual (and Predicted) Shelf Life (Days) of Dried Onion Flakes Based on Browning and Thiosulfinate Loss at Different Temperatures Browning aw 0.32 0.43 0.56 Thiosulfinate Loss 208C 308C 408C 208C 308C 408C >631 (4778) 593 (600) 183 (190) 474 (472) 83 (69) 31 (33) 59 (63) 22 (21) 17 (17) >631 (1619) 631 (585) 298 (288) 369 (306) 136 (139) 84 (82) 66 (55) 40 (38) 27 (29) Source: From Samaniego-Esguerra, C.M., Boag, I.F., and Robertson, G.L., Lebensml-Wiss U.-Tech., 24(1), 53, 1991 With permission 25.5.7 GLASS TRANSITION TEMPERATURE RELATED CHANGES 25.5.8 MICROBIOLOGICAL ASPECTS Glass transition is a second-order phase transition that occurs over the temperature range at which amorphous solid materials are transformed into viscous, liquid state [89] The amorphous state of foods may result from a rapid removal of water from food solids that occur during such processes as extrusion, drying, and freezing The temperature, water content, and time-dependent changes, which are the problems in manufacture and storage of powders and other low moisture foods, can be reduced by not exceeding their critical values based on Tg determination [90] The Tg can be applied in evaluating proper temperature and TABLE 25.12 Actual (and Predicted) Shelf Life (Days) of Dried Green Beans Based on ChlorophylI-a Loss at Different Temperatures a Temperature (8C) w 0.32 0.43 0.56 20 30 40 >637 (962) 478 (452) 150 (148) 273 (282) 143 (146) 61 (56) 86 (84) 45 (38) 25 (26) Source: From Samaniego-Esguerra, C.M., Boag, I.F., and Robertson, G.L., Lebensml-Wiss U.-Tech., 24(1), 53, 1991 With permission ß 2006 by Taylor & Francis Group, LLC humidity conditions of agglomeration and in reducing quality changes occurring with dehydration The collapse of the dehydrating foods during freeze-drying, stickiness of the product during spray drying, caking and agglomeration of the powders during processing, and storage are some of the properties that are related to glass transition temperature del Valte et al [91] studied the relationship between shrinkage during drying and glass rubber transitions of apple tissue Their work demonstrated that infusion of sugar during osmotic dehydration at high solute concentration brought about some protection against shrinkage This was reflected by a 20–65% increase in volume of samples treated with 50% sucrose and maltose solutions as compared to air-dried control However, reported data did not indicate that structural collapse could be reduced by diminishing the difference between drying temperature and glass transition temperature Dried samples remained in the rubbery state and shrunk during subsequent storage Drying is the oldest method of preserving food against microbiological spoilage Since presence of water is essential for enzymic reactions, the removal of water prevents these reactions and the activities of contaminating microorganisms present Removal of water increases the solute concentration of the food system and thus reduces the availability of water for microorganisms to grow There is a lower limit of water activity for specific microorganisms to grow; for complete microbiological stability, water activity of the system should be below 0.6 Drying, however, is also an effective means of preserving microorganisms in a viable state, even though their numbers may be reduced and a proportion sublethally damaged [92] Survival during and after drying will depend upon the physicochemical conditions experienced by microorganisms, such as temperature, aw, pH, preservatives, oxygen, and so on The survival of food spoilage organisms may give rise to problems in a reconstituted food item, but survival of foodborne pathogens must be viewed much more seriously With a view to minimize organoleptic changes in foods during drying, time and temperatures are kept as short and as low, respectively, as feasible The process of drying, whether by freeze-drying, hot air drying, solar drying, or by high temperature (e.g., spray or drum drying) is not per se lethal to all microorganisms and many may survive The more heat-resistant organisms are the more likely survivors (e.g., bacterial spores, yeasts, molds, and thermoduric bacteria) Thus there is a strong possibility for microbial growth, including pathogens, before the aw of the product falls below the critical level for each organism Vegetables, because of their greater proximity to soil and lower acidity and sugar content as compared with fruits, predominately have more bacterial populations A majority of the species has been found to be common for soil- and waterborne bacteria of the genera Bacillus and Pseudomonas Some workers have found other types of bacteria such as coliforms and bacteria of the genera Achromobacter, Clostridium, Micrococcus, and Streptococcus from different dehydrated vegetables Factors that influence markedly the microbial population of dehydrated vegetables include the microbial quality of fresh produce; the method of pretreatment of the vegetables (peeling, blanching, etc.); the time elapsed between preparation of the vegetables and start of the dehydration process; the time involved in the dehydration of the vegetables; the temperature of dehydration; the moisture content of the finished product; and the general level of sanitation in the dehydration plant [93] Blanching, if sufficient to inactivate enzymes, would reduce the contamination of the fresh produce to an insignificant figure Reduction in total count during blanching was found to be greater than 99.9% Coliforms and enterococci are commonly used as indicators of unsanitary conditions in food processing Clarke et al [94] isolated enterococci from 18 out of 35 dehydrated vegetable samples They found coliforms in 18 and enterococci and coliforms in 15 samples Statistical analysis showed a positive correlation between number of enterococci and coliforms The predominant species recovered from enterococci was Streptococcus faecium (60%) and from coliforms was Aerobacter (56%) Fanelli et al [95] surveyed a number of commercially available vegetable soups and found that the maximum total number of bacteria was less than 50,000 per gram and the mean and median total bacterial numbers were very low The numbers of coliforms, yeasts, molds, and aerobic species were also low Dehydrated onion, which is an important commercial flavoring ingredient, is not blanched before dehydration Its microbiology was therefore extensively investigated Total plate count (TPC) was less than 100,000 per gram in 76% of the slices from the belt dryer and only 52% of the sample in the tunneldried product [96] In both cases, the average bacterial spore count was 12,000 per gram Many workers have variously reported the presence of Bacillus, Pseudomonas, Aerobacter, Lactobacillus, and Leuconostoc species [97] Exposure to ethylene oxide gas was found to be effective in reducing the relatively high ß 2006 by Taylor & Francis Group, LLC TPC, but future application of this gas is in doubt because of its toxic hazards Alternatively, the application of gamma radiation at a level 0.2–0.4 Mrad was suggested to sterilize onion powder without any detrimental effect 25.5.9 FACTORS AFFECTING STORAGE STABILITY The shelf life of dehydrated fruits and vegetables depends on many deleterious reactions, which in turn depend on the specific nature of the food materials, storage conditions, and nature of packaging The undesirable changes that occur are due to off flavors, browning, and loss of pigments and nutrients as enumerated above Knowledge of the causes of these reactions is highly necessary to improve the shelf life of the dehydrated products Villota et al [98], in their review on the storage stability of dehydrated foods, discussed the factors mainly responsible for deterioration, that is, moisture, storage temperature and period, oxygen, and light They compiled the literature data on storage stability of several dehydrated products, which included dehydrated fruits, vegetables, and fruit and vegetable powders, based on method of drying, additional treatment, storage conditions, time required for appearance of earliest defects, and the state of other factors at times of unacceptability Moisture content is a very important parameter influencing the stability of dehydrated foods It has been suggested that the optimal amount of water for long-term storage corresponds in most dehydrated foods to the Brunauer–Emmett–Teller (BET) monolayer value On the other hand, items such as freezedried spinach, cabbage, and orange juice were reported to be more stable at a zero moisture content, whereas items like potatoes and corn had maximum stability at the monomolecular moisture content It appeared that optimal moisture content could not be predicted with precision on the basis of theoretical considerations Another important factor affecting storage stability of dehydrated foods is temperature and period of storage Generally, the storage stability bears an inverse relationship to storage temperature, which affects not only the rate of deteriorative reaction (enzyme hydrolysis, lipid oxidation, NEB, protein denaturation), but also the kind of spoilage mechanism It is well established that elimination of oxygen by packing in an inert atmosphere such as nitrogen contributes to extending the storage stability of many dehydrated products However, in certain products like spray-dried powders, in which a large surface area is exposed to air during processing, some entrapment of oxygen occurs in the final product and packing under inert atmosphere results in a very little improvement Storing in zero oxygen headspace, using an atmosphere of 5% hydrogen in nitrogen with a palladium catalyst, is reported to result in superior quality retention Further, since oxidation of lipids and vitamins like ascorbic acid, riboflavin, thiamine, and vitamin A and loss of pigments such as carotenoids and chlorophyll are initiated or accelerated by light, adequate packaging needs to be provided to protect such dehydrated foods from light REFERENCES D.K Salunkhe and B.B Desai, Postharvest Biotechnology of Fruits, CRC Press, Cleveland, OH, 1984 D.K Salunkhe and B.B Desai, Postharvest Biotechnology of Vegetables, CRC Press, Cleveland, OH, 1985 FAO Year Book Production, Food and Agricultural Organization of the United Nations, Rome, 1999 S Sokhansanj, and D.S Jayas, Drying of foodstuffs In: Handbook of Industrial Drying, 1st ed (A.S Mujumdar, Ed.), Marcel Dekker, New York, 1987, p 517 D.K Salunkhe, and H.R Bolin, Developments in technology and nutritive value of dehydrated fruits, vegetables and their products In: Storage, Processing and Nutritional Quality of Fruits and Vegetables (D.K Salunkhe, Ed.), CRC Press, Cleveland, OH, 1974, p 39 J Dunbar, Use of sulphur dioxide in commercial drying of fruits and vegetables, Food Tech New Zealand, 21(2):11 (1986) N.M Quentzer and E.E Burns, Effect of microwave steam and water blanching in freeze-dried spinach, J Food Sci., 46(2):410 (1981) S Mohamed and R Hussein, Effect of low temperature blanching, cysteine-HCl, N-acetyl-L-cysteine, Na-metabisulphite and drying temperature on the firmness and nutrient content of dried carrot, J Food Preservation Process., 18(4):343–348 (1994) L.F Moreno-Perez, J.H Gasson-Lara, and E OrtegaRivas, Effect of low temperature-long time blanching on quality of dried sweet potato, Drying Technology, 14(7–8):1839–1857 (1996) 10 J.L Bomben, Effluent generation, energy use and cost of blanching, J Food Process Eng., I(4):329 (1977) 11 L.P Somogyi and B.S Luh, Dehydration of fruits In: Commercial Fruit Processing, 2nd ed (J.G Woodroof and B.S Luh, Eds.), AVI Publishing, Westport, CT, 1986, p 353 12 H.R Bolin and D.K Salunkhe, Food dehydration by solar energy, Crit Rev Food Sci Nutri., 16:327–354 (1982) 13 K.S Jayaraman and D.K Das Gupta, Dehydration of fruits and vegetables—recent developments in principles and techniques, Drying Technology, 10:1 (1992) 14 T.A Lawland, Agricultural and other low temperature applications of solar energy In: Solar Energy Handbook (J.F Kreider and F Kreith, Eds.), McGrawHill, New York, 1981, p 18.1 ß 2006 by Taylor & Francis Group, LLC 15 L.L Imre, Solar drying In: Handbook of Industrial Drying, 1st ed (A.S Mujumdar, Ed.), Marcel Dekker, New York, 1987, p 357 16 H.R Bolin, C.C Huxoll, and D.K Salunkhe, Fruit drying by solar energy, Confructa, 25(3–4):147 (1980) 17 J.V Carbonell, F Pinega, and J.L Pena, Solar drying of food products III Description of a pilot dryer and evaluation of a flat plate collector, Revista de Agroquimica y Tecnologia de Alimentos, 23(1):107 (1983) 18 J.H Moy and M.J.L Kuo, Solar osmovac dehydration of papaya, J Food Process Eng., 8(1):23 (1985) 19 K.S Jayaraman, D.K Das Gupta and N Babu Rao, Solar drying of vegetables-quality improvement using cabinet and multi flat plate collectors and pretreatments In: Drying of Solids (A.S Mujumdar, Ed.), Oxford & IBH Publishing Company, New Delhi, India 20 S Grabowski and A.S Mujumdar, Solar assisted osmotic dehydration In: Drying of Solids (A.S Mujumdar, Ed.), Oxford & IBH Publishing Company, New Delhi, India 21 M Ahmad and A.S Khan, Design and construction of a solar grain and fruit drying system, Agricultural Mechanization in Asia, Africa and Latin America, 28(4):62–66 (1997) 22 W.B Van Arsdel and M.J Copley, Food Dehydration, Vol 2, AVI Publishing, Westport, CT, 1973 23 S.D Holdsworth, Dehydration of foodstuffs—a review, J Food Technol., 6:331 (1971) 24 J.G Brennen, Dehydration of foodstuffs In: Water and Food Quality (T.M Hardman, Ed.), Elsevier Applied Science Publishers, London, 1989, p 33 25 K.S Jayaraman, V.K Gopinathan, P Pitchamuthu, and P.K Vijayaraghavan, Preparation of quick cooking dehydrated vegetables by high temperature short time drying, J Food Technol., 17(6):669 (1982) 26 M Karel, Dehydration of foods In: Principles of Food Science Part II, Physical Principles of Food Preservation (M Karel, D.R Fennema, and D.R Lund, Eds.), Marcel Dekker, New York, 1974, p 309 27 D.R Heldman and R.P Singh, Food dehydration In: Food Process Engineering, 2nd ed., AVI Publishing, Westport, CT, 1981, p 261 28 M.E Lazar and D.F Farkas, Centrifugal fluidized bed drying—a review of its application and potential in food processing In: Drying’80, Vol (A.S Mujumdar, Ed.), Hemisphere, New York, 1980, p 242 29 G.E Brown, D.F Farkas, and E.S De Marchena, Centrifugal fluidized bed blanches, dries and puffs piece-form foods, Food Technol., 26(12):23 (1972) 30 P.F Hanni, D.F Farkas, and G.E Brown, Design and operating parameters for a continuous centrifugal fluidized bed dryer (CFB), J Food Sci., 41(5):1172 (1976) 31 M.W Cannon, New dryer for vegetables, Food Technol New Zealand, 13(9):28 (1978) 32 J.L Baxeires, Y.S Yow, and H Gilbert, Study of the fluidized bed drying of various food products, LebensmWiss U.-Technol., 16:27 (1983) 33 J.F Sullivan and F.C Craig, Jr., The development of explosion puffing, Food Technol., 38(2):52 (1984) 34 W.K Heiland, J.F Sullivan, R.P Konstance, J.C Craig, Jr., J Cording, Jr., and N.C Aceto, A continuous explosion puffing system, Food Technol., 31(11):32 (1977) 35 M.F Kozempel, J.F Sullivan, J.C Craig, Jr., and R.P Konstance, Explosion puffing of fruits and vegetables, J Food Sci., 54(3):772 (1989) 36 R.E Berry, C.J Wagner, O.W Bisset, and M.K Veldhuis, Preparation of instant orange juice by foam mat drying, J Food Sci., 37:803 (1972) 37 R.V Decareau, Microwaves in the Food Processing Industry, Academic Press, Orlando, 1985, p 79 38 J Youngswatdigul and S Gunasekaran, Microwave vacuum drying of cranberries I Energy use and efficiency, J Food Process Preservation, 20(2):121–143 (1996) 39 T Funebo and T Ohlsson, Microwave assisted air dehydration of apple and potato, J Food Eng., 38(3):353–367 (1998) 40 M Bouraoni, P Richard, and T Durance, Microwave and convective drying of potato slices, J Food Process Eng., 17(3):353–363 (1994) 41 D.G Prabhanjan, H.S Ramaswamy, and G.S.V Raghavan, Microwave assisted convective air drying of thin layer carrots, J Food Eng., 25(2):283–293 (1995) 42 H Fung, J Tang, D.S Mattinson, and J.K Fellman, Microwave and spouted bed drying of frozen blue berries: the effect of drying and pretreatment methods on physical properties and retention of flavour volatiles, J Food Process Preservation, 23(6):463–479 (1999) 43 K Masters, Spray Drying Handbook, 4th ed., George Godwin, London, 1985 44 K.S Jayaraman and D.K Das Gupta, Preparation and storage stability of some instant fruit flavoured milk and lassi beverage powders, Beverage and Food World (India), 16(4):15 (1989) 45 M Valenzuela-Garcia, G Quizano-Ruelas, G CamerenaGomez, R Martinez-Antunez, and V FarnandezRamirez, Spray dried vegetable juices, Technologia de Alimentos, 30(6):34–36, 38–39 (1995) 46 J.A Kitson and D.R MacGregor, Drying fruit purees on an improved pilot plant drum dryer, J Food Technol., 17(2):285 (1982) 47 M Manlan, R.F Mathews, R.P Bates, and S.K O’Hair, Drum drying of tropical sweet potatoes, J Food Sci., 50(3):764 (1985) 48 M.F Kuzempel, J.F Sullivan, J.C Craig, Jr., and W.W Heiland, Drum drying of potato flakes—a predictive model, Lebensm.-Wiss U.-Technol., 19(3):193 (1986) 49 J Lorentzen, Freeze drying: the process, equipment and products In: Developments in Food Preservation, Vol (S Thorne, Ed.), Applied Science Publishers, London, 1981, p 153 50 G.J Malecki, P Shinde, A.I Morgan, Jr., and D.F Farkas, Atmospheric fluidized bed freeze drying, Food Technol., 24(5):93 (1970) 51 K.E Yassin and H Gilbert, Atmospheric freeze-drying of foods in a fluidized bed of finely divided absorbent ß 2006 by Taylor & Francis Group, LLC 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 In: Proceedings of the Sixth International Congress on Food Science Technology, 1:208–209 (1983) G.A Bell and J.D Mellor, Further developments in adsorption freeze drying, CSIRO Food Res Quarterly, 50(2):48 (1990) G Donsi, G Ferrari, R Nigro, and P-di Matteo, Combination of mild dehydration and freeze drying processes to obtain high quality dried vegetable and fruits, Food Bioproducts Process., 76(C4):181–187 (1998) G Donsi, G Ferrari, R Nigro, and P-di Matteo, A combined technology for the production of dried vegetables: osmotic dehydration/freeze drying, Italian Food Beverage Technol., 15:9–11, 17 (1999) C.R Lerici, D Mastrocola, A Sensidoni, and M Dalla Rosa, Osmotic concentration in food processing In: Preconcentration and Drying of Food Materials (S Bruin, Ed.), Elsevier Science Publishers, Amsterdam, 1988, p 123 A Lenart and J.M Flink, Osmotic concentration of potato I Criteria for the end point of the osmosis process, J Food Technol., 21(2):307 (1984) T.R.A Magee, A.A Hassaballah, and W.R Murphy, Internal mass transfer during osmotic dehydration of apple slices in sugar solution, Irish J Food Sci Technol., 7(2):147 (1983) M.S Rahman and J Lamb, Osmotic dehydration of pineapple, J Food Sci Technol (India), 27(3):150 (1990) K Videv, S Tanchev, R.C Sharma, and V.K Joshi, Effect of sugar syrup concentration and temperature on the rate of osmotic dehydration of apples, J Food Sci Technol (India), 27(5):307 (1990) P Britnell, S Birchall, S Fitz-Paine, G Young, R Mason, and A Wood, The application of heat pump drier in Australian food industry In: Drying ’94, Vol B (V Rudolph and R.B Keey, Eds.), 1994, pp 897–903 R.L Mason, P.M Britnell, G.S Young, S Birchall, S Fitz-Paine, and B.J Hesse, Development and application of heat pump dryers to the Australian food industry, Food Australia, 46(7):319 (1994) J.S Cohen and T.C.S Young, Progress in food dehydration, Trends in Food Science & Technology, 6:20–24 (1995) J.A Gallego-Juarez, G Rodriguez-Corral, J.C Galvez Moraleda, and T.S Yang, A new high intensity ultrasonic technology for food dehydration, Drying Technology, 17(3):597–608 (1999) T Desrosier, T.G Smyril, and G Paquette, Retention of carotene in green peppers and peaches after a home dehydration process, Can Inst Food Sci Technol., 18(2):144 (1985) A.A Adenike, Ascorbic acid retention of stored dehydrated Nigerian vegetables, Nutrition Reports Int., 24(4):769 (1981) E.R Shadle, E.E Burns, and L.J Talley, Forced air drying of partially freeze dried compressed carrot bars, J Food Sci., 48(1):193 (1983) U.E Inyang and C.I Ike, Effect of blanching, dehydration method and temperature on the ascorbic acid, 68 69 70 71 72 73 74 75 76 77 78 79 80 81 colour, sliminess and other constituents of okra fruit, Int J Food Sci Nutr., 49(2):125–130 (1998) T.M Lin, T.D Durance, and C.H Scaman, Characterization of vacuum microwave, air and freeze dried carrot slices, Food Res, Int., 31(2):111–117 (1998) P.S Negi and S.K Roy, Effect of blanching and drying methods on b-carotene, ascorbic acid and chlorophyll retention of leafy vegetables, Lebensml-Wiss.U.-Tech., 33(4):295–298 (2000) A.K Baloch, K.A Buckle, and R.A Edwards, Effect of processing variables on the quality of dehydrated carrot II Leaching losses and stability of carrots during dehydration and storage, J Food Technol., 12:295 (1977) S.S Arya, V Natesan, D.B Parihar, and P.K Vijayaraghavan, Stability of carotenoids in dehydrated carrots, J Food Technol., 14:579 (1979) A.K Baloch, K.A Buckle, and R.A Edwards, Effect of sulphur dioxide and blanching on the stability of carotenoids of dehydrated carrots, J Sci Food Agric., 40:179 (1987) F Lajolo, S.R Tannenbaum, and T.P Labuza, Reaction at limited water concentration Chlorophyll degradation, J Food Sci., 36:850 (1971) F.M Lajolo and U.M.L Marquez, Chlorophyll degradation in a spinach system at low and intermediate water activities, J Food Sci., 47:1995 (1982) H.R Bolin and R.J Steele, Nonenzymatic browning in dried apples during storage, J Food Sci., 52(6):1654 (1987) D.J McWeeny, M.E Knowles, and J.F Hearne, The chemistry of nonenzymatic browning in foods and its control by sulphites, J Sci Food Agric., 25:735 (1974) D.H Palmer, A.W Taylor, and M.K Withers, Flavor, texture and color of air dried and freeze dried vegetables In: Proceedings of the First International Congress on Food Science Technology, 4:37 (1965) S.J Bishov, A.S Henick, J.W Giffee, I.T Nid, P.A Prell, and M Wolf, Quality and stability of some freeze dried foods in ‘‘zero’’ oxygen head space, J Food Sci., 36:532 (1971) R.P Konstance, J.F Sullivan, F.B Talley, M.J Calhoun, and J Craig, Jr., Flavor and storage stability of explosion puffed potatoes: autoxidation, J Food Sci., 43:411 (1978) S Pendlington and J.P Ward, Histological examination of some air dried and freeze dried vegetables In: Proceedings of the First International Congress Food Science Technology, 4:55 (1965) K.S Jayaraman, D.K Das Gupta, and N Babu Rao, Effect of pretreatment with salt and sucrose on the quality and stability of dehydrated cauliflower, Int J Food Sci Technol., 25:47 (1990) ß 2006 by Taylor & Francis Group, LLC 82 D.M Khedkar and S.K Roy, Histological evidence for the reconstitutional property of dried/dehydrated raw mango slices, J Food Sci Technol (India), 17(6):276 (1980) 83 H.J Neumann, Dehydrated celery; effects of predrying treatments and rehydration procedures on reconstitution, J Food Sci., 37:437 (1972) 84 A Levi, N Ben-Shalom, D Plat, and D.S Reid, Effect of blanching and drying on pectin constituents and related characteristics of dehydrated peaches, J Food Sci., 53(4):1187 (1988) 85 G Mazza, Moisture sorption isotherm of potato slices, J Food Technol., 17:47 (1982) 86 H.R Bolin, Relation of moisture to water activity in prunes and raisins, J Food Sci., 45:1190 (1980) 87 J.A Troller, Water activity and food quality In: Water and Food Quality (T.M Hardman, Ed.), Elsevier Applied Science Publishers, London, 1989, p 88 C.M Samaniego-Esguerra, I.F Boag, and G.L Robertson, Kinetics of quality deterioration in dried onions and green beans as a function of temperature and water activity, Lebensml-Wiss U.-Tech., 24(1):53 (1991) 89 Y.H Roos, Glass transition related physicochemical changes in foods, Food Technol., (10):97–100 (1995) 90 Y.H Roos, M Karel, and J.L Kokini, Glass transitions in low moisture and frozen foods: effects on shelf life and quality, Food Technol., 50(11):95–108 (1996) 91 J.M del Valte, T.R.M Cuadros, and J.M Aguilera, Glass transitions and shrinkage during drying and storage of osmosed apple pieces, Food Res Int., 31(3):191– 204 (1998) 92 P.A Cribbs, Microbiology of dried foods In: Concentration and Drying of Foods (D MacCarthy, Ed.), Elsevier Applied Science Publishers, London, 1986, p 89 93 R.H Vaughn, The microbiology of dehydrated vegetables, Food Res., 16:429 (1951) 94 W.S Clarke, Jr., G.W Reinbold, and R.S Rambo, Enterococci and coliforms in dehydrated vegetables, Food Technol., 20(10):113 (1966) 95 M.J Fanelli, A.C Peterson, and M.F Gunderson, Microbiology of dehydrated soups I A survey, Food Technol., 19(1):83 (1965) 96 J.M Sheneman, Survey of aerobic mesophilic bacteria in dehydrated onion products, J Food Sci., 38:206 (1973) 97 H Heath, The microbiology of onion products, Food Flavg Ingr Proc Packag., 5(1):22 (1983) 98 R Villota, I Saguy, and M Karel, Storage stability of dehydrated food: evaluation of literature data, J Food Quarterly, 3:123 (1980) ... size and geometry of the food, and the pH of the blanch liquor or spray Drying times in excess of 12 h for fruits and vegetables and of several days as in sun drying of fruits necessitate use of. .. solar drying of fruits and vegetables is usually long because of large amount of water to be removed, Grabowski and Mujumdar [20] examined the possibility of coupling osmotic drying with solar drying. .. Salunkhe, and H.R Bolin, Developments in technology and nutritive value of dehydrated fruits, vegetables and their products In: Storage, Processing and Nutritional Quality of Fruits and Vegetables