Drying Technology Evolution and Global Concerns Related to Food Security and Sustainability

3.1 Rotary drying

Rotary drying is also known as tumbling dryer. It is considered the oldest continuous and the most common high-volume dryer used in industry. It has developed more adaptations of the technology than any other dryer classification. Rotary dryers stand out for their flexibility and applicability to a large number of materials, as well as their high processing capacity [24]. Large amounts of granular substantial with particles of 10 mm or larger that are not too fragile or heat-sensitive or cause any other solids handling problems are dried in rotary dryers [25]. Conventional rotary dryers have flights, which lift solids and make them cascade across the dryer section (active phase) when there is effective fluid-particle contact [26]. It is one of the many drying methods existing in unit operations of chemical engineering. The drying takes place in rotary dryers, which consist of a cylindrical casing usually made of steel plates rotating on bearings, lightly inclined horizontally. Usually, its length is 5–90 m, while its diameter is 0.3–5 m, and rotates at 1–5 [25]. It is regularly worked at a negative interior pressure to stop dust get-away. Wet feedstuff is presented into the system upper edge and the feed proceeds through it by force of rotation, head influences and shell inclines. Then the dried stuff is withdrawn at lower edge. The direct heat rotary dryer diagram is illustrated in Figure 1. Gas flow direction throughout the cylinder in the case of the solids is dictated by the processed substance characteristics. Current flow is utilized for heat-susceptible substances even with high entrance gas temperature because of the gas’s fast cooling through initial surface moisture evaporation. While for other substances, the countercurrent flow is desired for higher thermal efficiency. In the first status, the solid flow rate is boosted by gas flow, whereas it retarded in the second status [27, 28, 29, 30].

3.1.2 Combined conduction-convection heating rotary dryer

For drying the high-moisture paddy in the field conditions, a combined conduction and convection heating rotary dryer for 0.5 t/h capacity was designed and developed by Likitrattanaporn et al. [32] by using liquefied petroleum gas (LPG) as the heat source. Moreover, a trial rotary dryer proposed with a system of concurrent flow comprising two main parts, a discharge cover, and a double cylinder is illustrated in Figure 2. The forward motion of the paddy happens by rotary motion of the cylinder and inclination angle. The air is inflated within the cylinder by a suction fan positioned on the discharge cover top. A 1-hp motor with a 1–60 reduction gear was utilized for driving the rotary dryer. LPG bulb on the entry edge heats the air. The heated air stirs to another end by the suction fan. Through the forward movement, the paddy contacts the external surface for the interior cylinder. Where, the conduction heating occurs followed by a cascading activity over the interior of the outer cylinder, which causes a convection heating. After this, paddy is placed into the emptying cover and got out of the dryer. When the suction fan absorbs the humid air, comparatively less humidity is taken away through the last or third pass at 100°C and 110°C, meaning 1.5% and 1.7%, respectively. At 120°C, the humidity content of 2.1% could be extracted. Clearly, this is because there was less free water available at the third pass of drying [33].

3.2 Fluidized-bed dryers

Fluidized-bed dryers (FBDs) are used extensively for the drying of wet particulate and granular materials with sizes ranging from 50 μm to 5 mm that can be fluidized, and even slurries, pastes, and suspensions that can be fluidized in beds of inert solids [34]. Here, the product is held aloft in a high-velocity hot air stream, thus promoting good mixing and heat transfer for uniform and rapid drying (Figure 3). FBDs are commonly used in processing many products such as chemicals, carbohydrates, foodstuff, biomaterials, beverage products, ceramics, medicines in agglomerated or powder form, healthcare stuffs, fertilizers, agrochemicals and pesticides, detergents, pigments, and surface-active agents, tannins, polymer and resins, as well as products for combustion, calcination, incineration, waste management, and environmental protection processes. Fluidized-bed operation offers imperative benefits such as good materials mixing, high amounts of heat and mass transfer, as well as easy material moving [34].

The air passages across a cribriform tray from below the foodstuff in addition to check it. The fragments are consumed in one end and from above. This support pushing along pieces formerly in a dryer, which they leave at the other termination. This is fortified by the lower-humidity fragments having lesser density and mass. This system has a suitable thermal efficiency and reduces individual fragments over warming [35].

The traditional fluidized bed is made by passaging a gas stream from bottom of bed for special materials. With little gas speeds, the bed of units is packed, which rests on a gas distributor plate. The passages of fluidizing gas is done over wholesaler, which regularly spread crossways bed [35]. The pressure droplet through the expansion of bed as the velocity of fluidizing gas is improved. At a particular gas speed, a fluidized bed is formed when the whole bed weight is completely supported by gas stream. This state is identified as minimum fluidization,and the corresponding gas velocity is named minimum fluidization velocity. Pressure drop through the bed remains near the same as pressure drop at minimum fluidization even if the gas velocity is increased further. Figure 4 demonstrates numerous particulate bed regimes from filled to bubbly bed when the gas velocity is boosted. The graphs express the bed pressure drops and bed void age under many regimes [35].

By superficial gas velocities greater than the lowest fluidization velocity, typically from 2 to 4 umf, the fluidized bed is controlled. The minimum fluidization velocity could be assessed experimentally and using numerous correlations [36]. It should be noted that these correlations have limitations such as particle size, column dimensions, and operating parameters. Thus, they are valid in a certain range of criteria and operating conditions.

3.3 Drum dryers

The drum dryer is normally used to dry slurries, pastes, concentrated solutions, or viscous on rotating steam-heated drums [38, 39]. This is because of the moisture boiling off and flashing or of irreversible thermochemical transformations of their content, which take place on their first interaction with the hot drum surface [40, 41, 42]. The viscous slurry or paste is automatically spread by spreading action of both counter rotating drums into a fine sheet, which follows to warmer drum within single-drum dryers or splitting sheets on both hot cylinders in double-drum dryers. The adhering fine paste sheet is then speedily dried conductively by excessive heat flux of steam condensation in the drum. In the case of humid slurries, which generate wet slabs, the wet thin slab drying could be improved by blowing heated dry air on the sheet outward. The fine slab has heat-sensible supplies, which could be dried also at a minor temperature in the vacuum [43].

It can be used in the irreversible thermochemical transformations during the slurry’s first contact with the hot drum. This is to impart the certain required qualities together of the dried product [44]. In the case of starch slurries, to produce pregelatinized starch, the starch can be gelatinized before the sheet is dried. When the thin sheet is exposed to the high heat flux and high temperature for a short period, the pored structure is imparted to the dried slab, because of the speedy formation of vapor bubbles in the slab during drying. The pored goods are premium in immediate food constructions because they are more readily wetted and can be simply rehydrated. For this, a drum dryer is broadly utilized over the universe in pregelatinized starch fabrication for immediate food construction.

3.3.1 Types of drum dryers

The drum dryer was first patented for use in the manufacture of pregelatinized starch in Germany [45]. Number and configuration of drums, heating systems, and product removal have been considered in many experiments. The drum diameter varies from 0.45 to 1.5 m, and its length varies from 1 to 3 m. The drum wall thickness is between 2 and 4 cm. The drum dryer is classified according to the steam-heated drums number and configuration as well as the atmosphere pressure around the drying sheet.

3.3.1.1 Atmospheric double-drum dryer

This dryer type has a higher rate of production, it can handle a wide range of goods as well as it is more efficient [38, 39, 40, 46]. Across the pendulum nozzle or a header, there are several nozzles, the paste or slurry is nourished onto the tweak of two steam-heated drums counter-rotating toward each other, making a boiling pool at the tweak as illustrated in Figure 5. The feedstuff could be nourished in the drum tweaks and applicator cylinder as shown in Figure 6. Starch slurries turn into gelatine in boiling pool to form pastes, which be extra sticky. The counter-rotation of the drums spread the slurry or paste into two soft slabs on both drums that subsequently dry conductively.

3.3.1.2 Atmospheric single-drum dryers

The paste or slurry is nourished throughout a header that has various nozzles or across a pendulum nozzle onto the tweak of a steam-heated drum and highly cooler applicator cylinder counter-rotating toward each other, then making a boiling pool at the tweak (Figure 7) [38, 39, 40, 46]. Starch slurries turn into gelatine in boiling pool, making pastes extra sticky. The drum counter rotation and applicator cylinder diffuse the paste/slurry into a soft slab on the warm drum, which subsequently dries conductively. Otherwise, the slurry could be nourished by dip coating a dip or applicator cylinder in a feed plate at the dryer bottommost, then cylinder coated on the drum as illustrated in Figure 8. Also, the slurry could be nourished by dip coating the drum directly in the feed tray (see Figure 9) or scattered or marked from a feed plate as illustrated in Figure 10.

3.3.1.3 Atmospheric twin-drum dryers

The applied of slurry is by direct dip coating of the twin drums in the feed tray at the dryer bottom (Figure 11), or by splash, or spray feeders from a feed tank at the dryer bottommost (Figure 12) onto the top of the both steam-heated drums, which are counter-rotating faraway from each other [38, 39, 40, 46]. Sheets are made by cohesion onto the drum top and are held up versus gravity by their surface tension, then dry conductively. This kind of dryer is appropriate for mixtures that produce dusty stuffs.

3.3.1.4 Enclosed drum dryers

If the solvent steam other than humid discharged in the course of the drying by the drum requires to be improved, or if dried foodstuffs produce much powder, the atmospheric twin-drum dryers could be bounded in vapor or in dust-tight enclosure [38, 39, 40, 46]. The steam vapor could be retrieved by an appropriate condenser, and the dust could be eliminated using a wet scrubber.

3.3.1.5 Vacuum double-drum dryer

Heat-sensitive materials can be dried in a vacuum double-drum dryer where the dryer is enclosed in an airtight enclosure under a vacuum (Figure 13) [38, 39, 40, 46]. This dryer type is also attached to a vacuum pump, a scrubber, and a condenser. Dryer operation is like its atmospheric version excluding that there are two product basins: one is for vacuum breaking and the other is for product discharge.

3.4 Industrial spray drying systems

The development of spray drying equipment and techniques evolved over several decades. With the sudden need to minimize the food materials transport weight, spray drying was established during World War II. This method enables feed transformation from a fluid state into a dried particulate shape by feed spraying into hot drying media. It is a continual particle processing drying operation. The feed could be an emulsion, suspension, solution, or dispersion. The dried product could be in the shape of granules, agglomerates, or powders dependent on feed’s chemical and physical characteristics, and the dryer scheme, as well as the final powder properties, desired [47]. Spray drying is a suspended particle processing (SPP) operation. This method uses fluid atomization for creating droplets. The droplets are dried into single particles when transferred to a hot gaseous drying media, generally it is air. It is considered a one-step continual unit processing procedure. Nowadays, over 25,000 spray dryers are used commercially to dry products such as fine and heavy chemicals, dyestuffs, dairy products, agrochemicals, and biotechnology products as well as mineral concentrates used in pharmaceuticals for preparing with evaporation capacities ranging from a few kg/h to over 50 tons/h.

Spray drying advantages:

• Biological products, pharmaceuticals, and heat-sensitive foods could be dried at atmospheric pressure with minimum temperatures. Sometimes the inactive atmosphere is used.

• Product characteristics are considerably further effectively controlled.

• Spray drying authorizes high-capacity production within the continual process and comparatively modest tools.

• Goodstuff links into contact together with the equipment surfaces in an anhydrous condition.

• A comparatively regular, spherical particle with about the same attribution of nonvolatile compounds as in the liquid feed is produced by spray drying.

• The efficiency could be compared with that of other types of direct dryers because the operative gas temperature might range from 150 to 600°C.

Spray drying disadvantages:

• If the product has a high bulk density, spray drying fails.

• Generally, the system is not elastic. If the unit is designed for fine atomization, it might not be able to make a coarse product and vice versa.

• The feed has to be pumpable.

• Has a high initial investment compared with other continuous dryer types.

• Increasing the drying cost when the product is recovered and dust collection.

Spray drying involves of three process phases: (1) atomizing, (2) moisture evaporation and spray-air mixing, and (3) separation of dry product from the exit air. Every phase is conducted according to the operation of the dryer as well as the design or chemical and physical properties of the feed, determining the features of the final product. A spray drying procedure model is illustrated in Figure 14.

3.4.1 New developments in spray drying

Numerous innovative designs and operational modifications have been proposed in the literature [48, 49, 50, 51] although very few are readily available commercially due to incomplete knowledge about the new systems.

3.4.1.1 Superheated steam spray drying

Although superheated steam drying was proposed one century ago, the potential of superheated steam like a drying medium was not exploited industrially for half a century. The advantages of this type of drying are reviewed by [52] as follows:

• No oxidative damage

• No explosion hazards

• Recovery of potential heat supplied for evaporation

• Ability to operate at vacuum and high operating pressure conditions

• Minimizing air pollution by closed-system operation

• Producing a product with better quality under appointed conditions

When the constraints could brief to the following:

• The higher temperature of product

• The capital costs is higher than the hot-air drying

• The possibility of air infiltration making recovery of heat from exhaust steam is difficult using compression or condensation

The initial scheme of a superheated steam spray dryer was made by Gauvin [53, 54]. The patent on superheated steam spray drying was issued by Raehse and Bauer [55]. This drying has been examined using the CFD system by Frydman et al. [56].

3.4.1.2 Two-stage horizontal spray dryer

A horizontal spray dryer has been suggested as an alternative to the conventional vertical types [57, 58] for pharmaceutical materials and heat-sensitive food [59, 60]. The low temperatures of drying permit flavor reservation, controlled porosity and density, high solubility, as well as fine goodness agglomerated products. Essential energy provisions relative to the conventional vertical spray dryer with a lower electrical loading for a specific capacity.

A suggested design is described in Figure 15. The high-pressure pump is used for feed pushing to the spray nozzles. These nozzles are organized in several configurations. Then, the spray travels horizontally from the nozzles, and due to gravity, it goes down. After that, transporting of the dried powders using a conveying strap in the bottom and carried to bag filters or cyclones. A model for the horizontal spray dryer (6.0 m × 3.0 m × 3.0 m) has been presented by Cakaloz et al. [61].

In the single-stage horizontal spray dryer process (Figure 16), it has been shown that the dwell period might be overly short for allowing the droplets for drying completely. Therefore, to overcome this constraint, a new two-stage, two-dimensional horizontal spray dryer idea is suggested by Mujumdar [58] and shown in Figure 15, which is prepared for permitting long drying periods. This is necessary for big droplet sizes and heat-sensitive products. The variance between single and two-stage horizontal spray driers is that in the single spray drier, there is a fluid-bed dryer, which is installed at the horizontal chamber bottom. Whereas, the two-stage horizontal spray drier is not commercialized yet. The CFD process could be utilized to emulate the horizontal spray dryer, which should be connected to a fluid-bed drying system. The droplet’s surface moisture is taken away rapidly from the chamber of spraying. While the inner water occupies longer, which could be extracted in a thin layer, out of a deliberation dryer in the bottom of the room.

3.4.1.3 Low-humidity spray drying

On occasion, it has to choose freeze dryers instead of spray dryers, particularly for pharmaceuticals and biochemicals drying. This is because spray drying utilizes high-temperature gas like the medium of drying and the sediments on the wall cannot be averted in feasible spray dryers. The products are subject to degradation by overheating [48]. Yet, the freeze dryer has high operating, and capital costs, as well as high energy consumption, compared with the spray dryer [62, 63].

To decrease the active compound degradation in the dried powders, a novel kind of spray dryer was proposed, such as low dew point (LDP) spray dryer. This type utilizes air at a nearby surrounding temperature of 80°C and quite minimum humidity (Figure 17). It comprises four units: the preprocessing system of drying air including heating and dehumidification; the preparation system of feed; the drying system and feed atomization including drying chamber, air disperser, and atomizer, etc., in addition to the system of product collection.

3.4.1.4 Spray freeze drying

The freeze drying (FD) is a significant drying machinery for pharmaceuticals, foods, and biochemicals, which could preserve the biological activity, aroma or flavor, etc. Freeze drying is a drying method, in which the solvent in the material is solidified at low temperatures, then sublimates directly from the solid into the vapor state under vacuum. The freeze-drying process of an aqueous solution contains three phases: product freezing, ice sublimation, and removal of solvent vapor. Nevertheless, compared with spray drying, the freeze drying is an order-of-magnitude costlier drying method because of its need for vacuum, refrigeration, and long operating times [64]. In a recent combination of both drying processes, called spray freeze drying (SFD), this process was carried out as a batch method as follows: The liquid nitrogen is atomized in a cryogenic medium to spray the droplets to freeze. Frozen droplets are scuttled in a cryogenic medium and processed out and dried in the freeze-dryer under a vacuum. A model flow slab is illustrated in Figure 18. This method is conducted in batches instead of continuously, because of the required long freeze-drying time. In addition, it is appropriate for high-significant products with low tonnages [65].

A new method called a combined atmospheric spray and fluidized-bed freeze drying (ASFBFD) process was examined. The planned schematic flow graph is described in Figure 19. It mainly comprises internal fluidized bed, drying or freezing room, liquid nitrogen cooler, internal bag filter, fan, nozzles, valves, pipes, pump, etc. [65].

The process is briefly explained as follows: The feed is transported into the nozzle from the top through a pump, where it is atomized using a nozzle. The fine spray is connected with freeze nitrogen or air immediately. Upon the material of feed, the drying medium is frozen to about −90°C using liquid nitrogen and inflated by a fan within the fluidized-bed cribriform tray. The fluidized bed is placed at the bottom of the chamber. As the temperature of the cool air is lower enough for spray freezing, the frozen atoms conserve their main conditions and go to the bottom or to the fluidized bed through gravity. Nearly a few frozen atoms might be restricted by air or nitrogen; nonetheless, they are detached from the air using the interior bag filter. Throughout the phase, the spray freezing drying is investigated continuously until the exhaustion of feed in the batch procedure. After this phase is completed, suitable drying terms are chosen, such as liquid nitrogen, which is regulated to meet freeze-drying terms. Drying actual operation and its time terms should be specified. Many investigators have already investigated many studies on atmospheric freeze drying [66, 67, 68, 69, 70, 71, 72]. These reviews showed that the atmospheric spray and fluidized-bed freeze drying grouping is a viable new procedure that needs additional R&D. Lastly, a summary of the comparison between the four drying processes, such as SD, FD, SFD, and ASFBFD, is assumed in Table 1.

Drying method	Spray drying (SD)	Freeze drying (FD)	Spray freeze drying (SFD)	Atmospheric spray and fluidized-bed freeze drying (ASFBFD)
Drying period	Short	Long	Long	Medium
Final shape	Irregular or agglomerated	Cake	Porous and spherical, mono	Porous and spherical
Power requirments	Low	High	Highest	Medium
Quality dried product	Medium	Good	Good	Good
Operation	Continuous	Batch	Batch	Semicontinuous
Productivity	High	Medium	Low	Medium
Cost	Low	High	High	High

Table 1.

Drying method properties [65].

3.4.1.5 Encapsulation

Spray drying and fluid-bed drying lead to another common food application of these technologies, that is, microencapsulation. It is defined as a process by which one material or more is entrapped within alternative material [73]. This method is generally used to prevent the core material from degradation and to control releasing or separate reactive constituents in a formulation. Because the spray dryer is generally fast, available, economical and produces good-quality products [74], it enhances the most public means of encapsulation process. This procedure is simple and comparable with the one-phase spray drying technique. The coated substance is named active or core substance, and the coating substance is entitled a shell, while the wall substance is called a carrier or encapsulant [75]. The encapsulated active substance is scattered in the hydrocolloid transporter, such as maltodextrin, dextrin, Arabic gum, gelatin, and modified starch. Subsequently, the emulsifier is supplemented, and the mix should be homogenized to compose an oil-in-water emulsion, then it’s consumed by the atomizer for the spray dry. In the drying room, the aqueous stage dries, and the active substance is captured as atoms through the protein film or hydrocolloid, discharging of the active substance from capsules under specific terms. The temperature, moisture, and pressure are the main controlling factors of its release [76].

3.5 Freeze drying

Freeze drying is used for high-quality foodstuffs stabilization, certain biological materials, and pharmaceuticals such as proteins, vaccines, bacteria, and mammal cells. These substances are freeze-dried; thus, the quality of the dried product is retained [63, 79]. Freeze drying is a process in which the water or another solvent is sublimated by the direct transition of water from solid (ice) to vapor, thus omitting the liquid state, and then desorbing water from the “dry” layer [80, 81, 82, 83, 84]. As a rule, freeze drying produces the highest-quality food product obtainable by any drying method. This is due to the fact that freezing water in the material prior to lyophilization inhibits chemical and biochemical such as non-enzymatic browning, protein denaturation,, microbiological, and enzymatic reactions processes. Consequently, the content of various nutrients, smell, and taste do not change. Raw materials comprise water, ranging from 80% to 95%. The water is removed by sublimation resulting in extremely porous structure creation of the freeze-dried products, and the finalized rehydration or lyophilizing happens immediately when water is added to the substance at a later time [85, 86]. However, the water in foodstuffs could be free or bound to the solution using different powers. The free water is freezed, but the bound does not. In the freeze-drying method, the iced and some bound water should be detached. Consequently, lyophilization is an extremely complex and multistep procedure that comprises (a) freezing under atmospheric pressure phase, (b) main drying; appropriate freeze drying; ice sublimation, at decreased pressure. For example, iced water is managed, then sublimation at 0°C and absolute pressure of 4.58 mmHg could happen. Nevertheless, subsequently the water usually exists in a solution or a joint state, the substance should be cooled below 0°C to preserve water in a frozen phase. So, during the main drying step, the frozen layer temperature as showed in Figure 20 is at −10°C or lower at absolute pressures of about 2 mmHg or less. As the ice sublimes, the sublimation interface, which began at the external surface (Figure 20), dried material retreats and porous shell residues. The latent heat (2840 kJ/kg ice) could be operated using the dried substance and frozen layers, as illustrated in Figure 20. The vaporized water is conveyed within the dried substance porous sheet. Through the main drying step, in the dried layer, a large amount of nonfrozen water might be desorbed. The desorption method might affect the amount of heat that reaches the sublimation interface, and subsequently, it might affect the velocity of the sublimation front moving. The period when there are no frozen sheets is possessed to perform the end of the primary drying phase. (c) Secondary drying phase; desorption drying; drying the product to the required final humidity, this stage starts at the end of the primary drying stage, and the desorbed water vapor is transported through the pores of the material that is dried [63, 80, 87]. During the above three phases, six main physical phenomena could be distinct, which have a significant impact on the process path, the quality of the obtained substance, and its overall costs [88]. Those phenomena are as follows: the water is transitioned into ice, then the ice to a vapor phase, the water particles are desorpted from substance structures, the obtainment of sufficiently low pressure, the re-sublimation of water vapor removed from the substance on the condenser surface, and the removal of a layer of ice from the capacitor surface.

In this context, the conditions of the freeze-drying process should be selected in a way that does not melt the water. The presence of water during freeze drying may result in many changes in the composition, morphology, and physical properties of foods (e.g., shrinkage), then reducing product quality during storage [89]. The effect of freeze-drying conditions on the nutritional properties, antioxidant activities, and glass transition characteristics of different food materials can be found in the literature [90, 91, 92, 93, 94, 95]. Despite the long processing time and high cost, it is preferred for extreme-quality products. Though some damages in bio compounds could be set after freeze drying, this method is preferable to maintain nutritional qualities, particularly when operated under a vacuum. Moreover, the quality parameters such as freeze-dried products’ rehydration and porosity are favorable for manufacturing foods. Newly, the freeze-drying process condensation with innovative technologies or pretreatments permit overcoming some of these drying processing challenges [96].

3.5.2 Industrial freeze dryers

The main types of industrial freeze dryers include the following:

3.5.2.1 Tray and pharmaceutical freeze dryers

The hugest amount of industrial freeze dryers in process are of the vacuum batch style with freeze drying of the food stuff in trays. There are two main styles, depending on the type of condenser used. The first style showed the condenser plates in the same chamber and near the tray-heater assembly. The second style is representing the condenser in a separate chamber linked to the first by a wide, in over-all, butterfly valve. This latter type of the factory is permanently used in pharmacological industries, but it can also be used for freeze-drying foods. To reduce product contamination risk, especially in the pharmaceutical industries, a new freeze-dryer plant concept has been developed. The system, as illustrated in Figure 21, has two doors: a little one is for charging the stuff before drying, when the full door, which is opposed to the little door, is for discharging the stuff after drying. The condenser is positioned on the floor, which is under the first floor, where the drying chamber is placed. The freeze dryer shelves are lowered to the drying chamber bottom and then lifted one by one to a location in line with the filling machine. The charging of foodstuff is prepared under a laminar flow of sterilized air; the little door is unlocked only for each plate filling and then is directly closed [87].

3.5.2.2 Multi-batch freeze dryers

The freeze-drying system in a batch factory is usually programmed and organized to minimalize the drying time and enlarge the factory production. With a single-batch factory, the load on the several systems will be very variable during the drying rotation. The product flow and handling operations will be alternating because of the batch process specifically. This means that optimum use of supplies will not be probable in a sole-cabinet batch system. To extent this weakness, an industrial freeze-drying factory is built with numerous batch cabinets instructed to operate with staggered and overlying drying rotations. Each cabinet can be filled with materials from the same system, and they are assisted by the similar central system for vacuum pumping, tray heating, and condenser refrigeration. Although, the procedure is individually organized in each cabinet from a unattached control panel. This builds probably the simultaneous different foodstuffs production, which rises the operation flexibility of the factory. With only two cabinets in operation, an important part of the batch weakness may be excluded; for example, with four cabinets, an excellent leveling of loads will be accomplished. A huge amount of industrial freeze-drying factories operate today in this style as multi-cabinet batch factories [81, 99, 106].

3.5.2.3 Tunnel freeze dryers

The process in the tunnel freeze dryer (Figure 22) takes place in a large vacuum cabinet into which the tray-carrying trolleys are loaded at intervals through a large vacuum lock at one end of the tunnel and discharged similarly at the other end. The drying conditions are carefully controlled in a number of sections of the tunnel by temperature-pneumatic controllers [81]. The plates of vapor constriction fit closely inside the channel walls yet permitting the trolleys to passage through two locations in the tunnel main unit, and gate valves turn off the locks from the main unit. Thus, the tunnel is divided into five autonomous process zones. When trolley is not moving during the period, a tray-lifting scheme causes all trays to sit in each trolley on heaters below the top. The heaters consume flat-top surfaces and ribs under which vacuum steam passes. They are cantilevered in couples from both sides of the tunnel. Vacuum steam heating has numerous benefits, including high latent heat of condensation and temperature control operating pressure. The cooling system comprises a great aqua ammonia absorption freezer instead of a compression factory. Because of easiness with the refrigeration, load can be diverse by controlling the feed of oil to the boiler that heats the absorber.

The whole capacity of the tunnel freeze dryer can be boosted as it increases volume of business. Large business factories for cottage cheese and coffee processing have been set up in this way. The tunnel freeze dryers have the same benefits of factory capacity operation that can be attained as in multi-batch factories. On the other hand, the flexibility for simultaneous production of materials or in swapping between products is missing.

3.5.2.4 Vacuum spray freeze dryers

The vacuum spray freeze dryer system is illustrated in Figure 23. It has been industrialized for tea infusion, coffee extract, or milk. The product is applied by spraying from a sole jet upward or downward in a cylindrical tower of about 3.7-m diameter by about 5.5-m high [81, 107]. The solutions are solidified into little particles by evaporative freezing. In the tower, a frozen helical condenser is coiled between the internal wall and central hopper, the latter accumulating the partially dry powder as it drops freely to the tower bottommost. This is in turn associated with a tunnel where the drying technique is accomplished on a stainless-steel belt migrant among radiant heaters. The dried material passes into a hopper that feeds a vacuum padlock, allowing alternating product removal for packing. The whole system operates under a vacuum of about 67 Pa. In the initial evaporation, the diameter of frozen particles obtained by spraying into a vacuum is about 150 mm and loses about 15% moisture, and there is no sticking of these particles.

3.5.2.5 Continuous freeze dryers

Continuous freeze dryers are used for products drying in trays and for agitating bulk material drying. When the product is handled in trays, then the most delicate treatment of the product is attained. The food material is placed in the tray. Hence, it is not exposed to scratch, and it falls in contact with surfaces only that completely meet required hygiene values. While agitating a crushed product, more efficient heat transfer to sole product particles can be realized. Thus, a significant decrease in the heating surface is achievable. Although, scratch of the product by agitation increased the production of water vapor per unit, the heating surface tends to bring little product particles with vapor stream away from the bulk bed of product and cause product loss. Any system problems for water vapor elimination to retrieve the product loss may more than counterbalance the advantage of the higher heater surface load [108]. The heat transfer to the product and the trays is by radiation, which is the easiest mode to safeguard a correct as well as consistently distributed heat transfer to the material during the practice. The radiant heat is formed by horizontal heater saucers that are gathered within temperature zones. Each tray remains for a fixed time in each temperature zone for drying time minimized [108].

4.1 Infrared dryers

Conserving energy and achieving the best quality of dried products have become the most important factors that determine the usefulness and success of operating any drying unit. Where heat is transmitted through the drying unit in three forms conduction, convection, and radiation. In this regard, when the temperature of the fresh material increases, the molecular motion gains more energy; as a result, it causes changes in the structure of the material as well as its chemical properties to increase the shelf life and improve the quality [2]. Drying agricultural products, one of the most important processes of food processing and preservation, is a high priority in achieving food security. Where, drying process aims to improve the stability of food products, by reducing water activity, which leads to a decrease in microbial activity and thus limits physical and chemical changes during the storage of dried products [109]. The common conventional approach to the drying process is using convection heat transfer by transferring heat from hot air to the target product by convection, and as a result, the water evaporates into the air also by convection. However, this conventional drying method was characterized by many related disadvantages such as time consumption, low efficiency, unfair exposure to high temperatures, quality variation, in addition to high energy consumption [110]. These disadvantages of traditional drying by convective led to the innovation of other drying technologies to overcome these drawbacks such as drying by microwave, infrared (IR), osmotic drying, fluidized bed, and hybrid drying methods (integrating two or more drying methods). IR drying technique is one of the most important modern drying technologies compared with traditional drying because it is characterized by short process time, uniform temperature, high heat transfer coefficient, good quality of final products, improved energy efficiency, and safe as mentioned [111, 112, 113]. Therefore, [114, 115, 116, 117, 118, 119] concluded that drying by IR heating is a promising method to achieve high-quality dried products and is suitable for fruits, vegetables, grains, and other high-value products. IR heat derives from the IR radiation that lies between the visible light (Vis) spectrum and the microwave band along the electromagnetic spectrum as depicted in Figure 25. IR drying technology idea is based on making changes in the electronic, rotational, and vibrational states of the atoms and molecules of the fresh material when exposed to IR radiation within the wavelength range of 780–106 nm.

IR band is divided into three regions, near-IR (NIR) is the first region whose wavelength ranges from 750 to 2500 nm to 0.75–1.4 μm at temperatures between 400 and 1000°C, the second region called mid-IR (MIR) with wavelength ranges from 2500 to 25,000 nm to 1.4–3 μm at temperatures between 400 and 1000°C. While the wavelength of the third region (far-IR, FIR) ranged from 25 × 103 to 106 nm ≃3–1000 μm at temperatures above 1000°C [120, 121]. After IR radiation penetrates the fresh material surface, IR rays move and vibrate the constituent molecules of the fresh material through the frequency of the IR band within a frequency of 60 × 103–150 × 103 MHz, which results in friction between the molecules and leads to rapid internal heating [114, 122, 123, 124]. There are many sources of IR radiation according to their operational power, whether they are electrically heated or gas-fired generators to generate IR energy. The most common electrical heated sources are reflector-type IR incandescent lamps (incandescent vacuum lamp, gas-filled lamp, and tungsten halogen lamp), IR emitter-type quartz tubes, ceramic, and radiant panels as shown in Figure 26. While the traditional IR sources used for heating are electric heaters that produce infrared radiation in a range of 1100–2200°C and gas-fired generators, which consist of a perforated metal plate heated by gas flames until the temperature of the metal plate rises and infrared energy is radiated at a temperature range of 343–1100°C [110, 125, 126]. IR radiation sources can provide different wavelengths ranging from short to long wavelengths according to the voltage value applied to the IR emitters. The efficiency is another significant factor in the evaluation of both electric IR heaters and IR gas heaters, where the efficiency of electric IR heaters ranges from 40 to 70%, more than the efficiency of IR gas heaters, which ranges from 30 to 50%, and emit medium to long IR. In addition, Sheridan and Shilton, 1999, mentioned that the appropriate IR wavelength for industrial heating ranges from 1.17 to 5.4 μm, which corresponds to temperature values from 260 to 2200°C, and found that the best efficiency of heat transfer by IR radiation source occurs when the heater turns red. The idea of electric infrared emitters is based on passing an electric current to an electric heater through a high-resistance wire such as nichrome wire, iron chromium wire, and tungsten filament. When the metal wire is heated to the glowing stage and the temperature rises to 2200 K, it will emit NIR with a wavelength between 0.7 and 1.4 μm. Accordingly, the Incandescent lamp type for producing IR radiation is classified as a short-wave IR emitter, while the quartz tube type is classified as a medium-IR wave emitter [126].

Many studies have reported that organic matter and water are the main constituents of fresh material or foodstuffs, which is based on the absorption of IR radiation significantly, especially at Mid and Far IR [113, 127, 128]. Where, the water absorption spectral coefficient was noted at 3 (MIR) and 6 (FIR) μm different wavelengths as shown in Figure 27 and concluded that these wavelengths are considered suitable to be fixed in large-scale IR dryers for food products that generally contain 90% water. Confirming this, both [129, 130] mentioned that food efficiently absorbs IR radiation at wavelengths greater than 2.5 μm through a change in the state of vibration of the vibrating mechanism, which leads to a high temperature of the product being dried.

Several studies have proved that IR heating as a non-traditional drying method has many advantages and benefits such as uniform heating, short processing time, high heat transfer rate, high efficiency (80–90%), low energy consumption, low cost characterized, and improving final product quality in addition to it could be used as an application to measure water content in food products. Nowak et al. [113, 131, 132, 133] observed that far-infrared drying helps retain sensory quality in products such as sweet potatoes, grapes, Cordyceps militaris, and mangoes. Also, [134] investigated the effects of both hot air temperature and the IR drying method on the kinetics of persimmon slices, and the results found that the logarithmic model was the best model fitted to the experimental IR drying. In this regard, [135] studied the combined hot air at temperature levels (of 55, 65, and 75°C) and the IR drying method at radiation lamp power levels of 150, 250, and 375 W, on the persimmon fruits’ moisture loss kinetics. The study finds that the drying time was reduced by 36% when increasing the drying hot air temperature from 55 to 75°C, while the drying time was reduced by 68.4% with increasing IR radiation power from 150 to 375 W. Nowak and Lewicki [113] dried the apple slices with IR radiation and by convection under equivalent conditions and reported that the heat-irradiated apple slices evaporated much more water than that not heated by IR energy until 80% of water is removed. Sun et al. [136] mentioned that the IR drying method combined with hot air as a pre-drying method can save 20% of drying time as compared with the IR drying alone throughout the drying of a thin layer of apple pomace. Chen et al. [117] conducted a comparative study between traditional hot-air (HA) and innovative drying methods of short- and medium-wave infrared radiation (SMIR) for drying jujube slices. The results find that the jujube slices dried by SMIR were of better color, higher retentions of vitamin C, total flavonoids content (TFC), and cyclic adenosine monophosphate (cAMP) content than the HA drying method, in addition to shorter drying time and higher drying efficiency. Also, the effects of the IR drying method on carrots were studied by [137]; the results pointed out that increasing IR drying time caused dramatic changes in the water state in dried carrots. Moreover, [138] used a combined drying method of IR and freeze drying to produce high-quality dried bananas at reduced cost, and the results showed that the dried banana samples were of a better color, higher crispness, and higher shrinkage compared with those produced by using regular freeze drying. As well, [139] achieved a considerable moisture reduction and higher drying rates in drying bananas with IR drying compared with hot air drying in the early stage.

4.2 Microwaves dryers

Microwaves (MW) are a form of electromagnetic beam, as are radio waves, ultraviolet radiation, X-rays, and gamma-rays. MV is located in the electromagnetic beam range between radio and infrared bands as shown in Figure 28. MW has wavelengths of about 30 cm to 1 mm, and frequencies ranging from about 1 GHz to nearly 300 GHz [140]. Microwaves have many applications such as communications, radar, astronomy, and remote sensing, and the most famous application for most people is cooking. Where the MV rays are absorbed by water at certain frequencies.

This property of MV is useful in cooking. Water in the food absorbs microwaves, which cause the water to heat up, then cook the food. Therefore, MV was used as a heating system in several industrial applications such as food, chemical, and materials processing, for example, cooking food and drying fruits and vegetables in both batch and continuous operations. MW radiation as a drying technique is based on the passage of microwaves through the material causing a molecule oscillation [114, 141], which leads to the volumetric heating of the material. MW volumetric heating (MWVH) is a way of using MW to penetrate uniformly throughout the volume of the product, thus delivering energy evenly into the body of the material. Hence, equally heated the entire volume of a flowing liquid, suspension, or semisolid. Conversely, the traditional thermal processing methods rely on conduction and convection from hot surfaces to deliver energy into the product. A comparison study between MW and convective drying methods is shown in Figure 29. Alibas and Yilmaz [142] studied the effects of both these two drying methods on the drying kinetics, thermal properties, and quality parameters of orange slices. The results show that the MW drying processes were completed between 16 and 136 min depending on eight different microwave output power levels between 90 and 1000 W. On the other hand, in convective drying processes completed within the range 460–3120 min, at four different drying temperatures of 50, 75, 100, and 125°C. As well, the energy consumption was measured, and it observed that the MW drying method’s energy consumption was very low at high and low powers. Finally, it is concluded that the most suitable drying method is MW drying at medium powers of 350 and 500 W by considering both drying and quality parameters.

Microwave drying technology is characterized by low energy consumption and short processing time, more uniform and energy efficient, making it an attractive source of thermal energy. However, some results indicated that microwave radiation alone is not sufficient to complete a drying process with high quality. Therefore, it is recommended to combine techniques, such as forced air or vacuum, to further improve the efficiency of the MW process [114, 141, 143, 144, 145, 146, 147, 148]. Accordingly, several studies on drying using MW of various fruits and vegetables attached with an auxiliary system such as convective and vacuum methods reported that is more efficient than both MW and conventional drying techniques individually. As well, both [141, 148, 149, 150, 151, 152, 153] mentioned that the MW drying technique is widely used in incorporation with hot air-drying systems. Where, the hot air removes water in a free state from the product surface, while the MW radiation removes water from the inner of the product. Furthermore, it is concluded that the MW-hot air combination drying systems not only increase the drying rates but also better retain the quality of the final products. In this regard, Sharma and Prasad [154], modified and developed a 600 W, 2450 MHz MW oven into an MW-hot air drier as shown in Figure 30. In order to explore the possibility of using a combined microwave-convective drying technique for processing garlic and assessment of the quality of the finished product. The results showed that the combined MW-hot air drying resulted in a reduction in the drying time and an extent of 80–90% in comparison to conventional hot air drying and a superior-quality final product. Alibas [155] studied the chard leaves quality characteristics during drying by MW, convective, and combined MW-convective. The results showed that the drying periods lasted 5–9.5, 22–195, and 1.5–7.5 min for MW, convective, and combined MW-convective drying methods, respectively. Furthermore, the optimum drying period, color, and energy consumption were obtained for the combined MW-convective drying method. The optimum combination level was 500 W MW applications at 75°C.

Also, [156] studied the influence of MW-convective drying on chlorophyll and the color of herbs. The findings proved that the MW with auxiliary convective is a promising technique permitting the obtainment of dried material of high quality, additionally processing short time that can be an economic factor and incentive for the application of that method of drying on an industrial scale. In this regard [157] used different drying methods for Pistacia Atlantica seeds to study the impact on drying kinetics and quality properties. The results indicated that the MW drying method has afforded higher moisture removal in a shorter period compared with the traditional drying methods. Moreover, it was found that the essential oil composition was not considerably influenced by the MW drying method, and the texture quality is appropriate. Furthermore, [153] studied the effect of convective and vacuum MW drying on the bioactive compounds, color, and antioxidant capacity of sour cherries. It is found that in case of an increase in air temperature during convective drying as well as the increase in material temperature during VMWD deteriorated all the quality parameters of dried product. However, VMWD turned out to be much better than convective drying and competitive with a freeze-drying method. As it turned out that the best quality of the dried product and its more attractive color were found at VMWD at 480 W, followed by drying at MW power reduced to 120 W, which corresponds to anthocyanins content. As well, [158] dried pumpkin slices using convective and vacuum-MW drying methods to determine the drying kinetics, drying shrinkage, and bulk density, as well as to measure the color and carotenoid content of pumpkin slices dehydrated. They find that the vacuum-MW method has approximately tenfold shortened the time of pumpkin slice drying as compared with the convective method. Considering the viewpoint of color and carotene content, the vacuum-MW drying method was more effective than the convective method. Moreover, when use was made of vacuum-MW method, the dried products had a more attractive color. Sutar and Prasad [159], used microwave vacuum drying as shown in Figure 31 to study the effect of vacuum in microwave drying operation and track the kinetics and moisture diffusivity of carrot slices. The results find that with the increase in microwave power density, the drying rates were increased and proved that the optimum model to predict the drying behavior of carrot slices’ overall process conditions was the Page model. The combination of convective and vacuum-microwave (VMW) methods for drying kinetics and quality of beetroots was investigated by Figiel [160]. Where, convective drying with 60°C hot air and integration between convective pre-drying (CPD) and VMW drying method at 240, 360, and 480 W were used to dehydrate the Beetroot cubes.

The results showed that the VMFD method significantly reduced the total time of drying and decreased drying shrinkage in comparison with the convective method. Furthermore, the VMM-treated samples exhibited lower compressive strength, better rehydration potential, and higher antioxidant activity than those dehydrated in convection. Also, it is found that increasing the MW wattage and decreasing the time of CPD improved the quality of beetroot cubes dried by the combined method. Additionally, [161] optimized the drying process of Polygonum cuspidatum by using MW-vacuum drying and pretreatment methods as shown in Figure 32.

Where, a microwave vacuum drying system is designed and built that consists of a microwave drying unit, a power and temperature control unit, a moisture condenser, a vacuum pump, a vacuum manometer, and a PC-based data acquisition unit. The pretreatment methods were blanching for 30 s at 100°C, drying at 60°C, microwave pretreatment methods, and followed by microwave vacuum for 200 mbar at 50°C. Finally, it is concluded that it can be used to scale up the microwave vacuum drying system to a commercial scale.

5.1 Instant controlled pressure drop method (détente instantanée controlee, DIC)

Major technical innovations in fruit and vegetable dehydration include pre or post-treatment under high or low pressure. At present, the instant controlled pressure drop method known by its French abbreviation DIC (détente instantanée contrôlée) was innovated as a solution to overcome the issues of drying shrinkage/collapse, to obtain a better quality of the dried plants. The DIC is characterized by its ability to handle a broad range of food products and is considered one of the newest innovative drying methods, regardless of its sensitivity to heat. The DIC processing method was developed, defined, and studied by [162]. DIC as a drying method is focused on exposing the product to high-pressure saturated steam for a few seconds and then followed by an abrupt pressure drop toward a vacuum. Then hot air-drying processing took place after DIC treatment as a conventional drying method. As well, [163] mentioned that DIC processing is based on fundamental studies concerning the thermodynamics of instantaneity. Where, DIC system consists of six main parts processing vessel, vacuum tank, pneumatic valve, steam generator, air compressor, and vacuum pump as shown in Figure 33. The DIC work idea depends on exposing a partially dried product (usually the humidity is close to 30% db) to a high temperature/short time (HTST) such as a vapor pressure (P < 1.0 MPa) at high temperature (below 180°C) for a short time (less than a minute). Then this HTST stage was treated with a sharp pressure drop to a vacuum (within the pressure range of 3–5 kPa, and ∆t from 10 to 60 ms). This leads to a mechanical influence represented in a severe pressure drop in a very short time. As a result, automatic evaporation of a part of water inside the product and at the same time a cooling operation were stimulated, which stopped their thermal degradation and gives a controlled expansion of the product [164, 165, 166, 167, 168].

Hence, Figure 34 accurately describes the different stages of the work idea of the DIC processing unit as follows:

1. An elementary vacuum is implemented to minimize air resistance and simplify the steam diffusion within the product, then rapid heating is happen.

2. Secondly, injected saturated steam into the vessel until it reaches the highest pressure, then it is fixed.

3. Pressure and time parameters are given depending on the product treatment.

4. The pressurization needs to be followed by a sharp decompression till reached the vacuum.

5. Atmospheric air is injected to come again to atmospheric pressure, after the vacuum point, for sample recovery.

According to, [169], over the past 33 years, this technology has continued to expand its food applications and improve its characteristics on an industrial scale. DIC technology has continued over the past 33 years, to expand its food applications and improve its characteristics on an industrial scale. Since the DIC implementation has shown a quantum leap in quality enhancement and reducing the cost in the food industry generally, this is achieved by reducing the drying time of fruits and vegetables and enhancing the essential oils extraction, vegetable oils, and antioxidant elements. Additionally, it eliminates plant microorganisms and germs, which cause contamination, and reduces non-food ingredients and allergens. As well as it provides strong decontamination eliminating vegetative microorganisms and spores and reducing non-nutritional and allergenic components. It is known that convective air drying is one of the most industrial food drying operations applied. However, it has some disadvantages such as shrinkage and collapse of the product structure, low kinetics, nutritional losses, long drying periods, and microbial contamination. To overcome these disadvantages, the convection drying method has been enhanced by combining it with the instant controlled pressure drop technology (DIC), which has obtained excellent results, to improve the drying process and the total quality of dried products [170]. This integration has been termed “Swell-drying,” which involves a hot airflow for the pre-drying stage coupled with a DIC texturing stage. A swell-drying operation generally consists of a first hot airflow pre-drying stage until a specific water content (between 0.20 g and 0.50 g H2O/g of solid material), then followed by a DIC texturing stage (100–900 kPa during a few seconds, followed by an abrupt controlled pressure drop), and a final air-drying stage until the weight is stable. In this regard, [171] applied the DIC technology to the quality characteristics of cell wall polysaccharides of apple slices and studied their relationship to the texture. The results showed that it is possible to get apple chips with a crisp texture and excellent honeycomb-like structure by coupling convective air drying with the DIC technology (swell-drying). Also, the samples showed an excellent rehydration ratio to a homogeneous porous structure and a large specific surface area. As well, [172] applied the swell-drying technology on fresh banana pieces and studied the effect of dehydration kinetics, water and oil holding capacity, and nutritional characteristics. Drying kinetics showed that DIC technology increased the effective water diffusivity by 23%. Moreover, the water holding capacity increased by 290% under high-pressure conditions under DIC treatments, reaching 7.8 against 2.0 g H2O/g db, while the oil holding capacity was 0.60 against 1.30 g oil/g db for non-textured samples. Finally, it is noted that the DIC treatment inhibited the transformation of banana starch to reduced sugar. Additionally, concerning the strawberry fruits, the swell-drying technology is an excellent processing method to produce strawberry snacks with a high crispness behavior and a high rehydration capacity in less time than convect air-drying conventional method [173]. As pointed out by [174, 175], the DIC treatment has a significant impact on strawberries’ drying and rehydration kinetics, increasing the effective diffusivity, as well obtain the highest levels of phenols, flavonoids, and anthocyanins and antioxidant activities were achieved at 350 kPa for 10 s.