Food Dehydration Recent Advances and Approaches

Abstract

Dehydration of organic material is undoubtedly a controlled attempt to conserve or construct a novel construct that will satisfy functional devotions. Food dehydration is reviewed in light of the latest progress in food materials research. Understanding the mechanics behind the drying process is crucial in food and agricultural product dehydration. Among the most crucial steps in preserving food is dehydration. Food drying innovations include photovoltaic, thermal imaging, microwave-assisted, and comparable hybrid technologies. According to a recent study, unique food dehydration technologies might increase drying efficiency by decreasing energy usage while improving product quality. Unique drying methods reduce food component degradation and create novel items for customers. Each method’s use of specific foods will be reviewed in this chapter.

1. Introduction

Dehydrating is essential in many agricultural, food, biotechnological, mineral processing, pulp, wood, polymer, ceramics, pharmaceutical, paper, and chemical applications [1]. Perhaps dehydration is chemical science’s oldest and most versatile way of drying procedures [2]. The fruit has many valuable chemicals, making it an essential element of the human diet [3]. Since the moisture content of fresh fruits exceeds 80%, they are considered perishable foods [4]. Around 40% of postharvest losses account for total fruit output in developing nations, such as India, significantly reducing the availability of fruits to customers [5]. During the entire seasons, fruits are gathered, but due to an absence of storage [6] and preservation accommodations, the marketplaces get congested, and the fruits start decaying before the end buyers can reach them [7]. During the drying process, reduction in the moisture content and, therefore, water activity permits the microbial activity in food materials to stabilize while also controlling supplementary deteriorative processes, such as browning, enzymatic and nonenzymatic reactions, lipid oxidation, and many more. Dehydrating food helps prevent bacterial growth that causes changes in chemicals and the occurrence of spoilage and the in food by reducing the moisture content of dietary items [8]. There are several goals for dehydrating dietary products. The most obvious is food preservation by dehydration. The dehydration technique limits microbial activity and other effects by lowering the humidity level of the item [9]. This method not only preserves the food from a microbiological aspect but also preserves its flavor and nutritional properties. The process of removal of water with heat is defined as dehydration. The earliest known method of food preservation was probably dehydration [10]. Fruits can also be sun-dried, and fish and meat can be smoked using well-known traditional methods [11]. A dehydrated food item has the benefit of being lightweight, which reduces shipping costs. However, the quality of the dried product is frequently diminished because high temperature is required in most conventional drying processes [12]. Many alternative approaches must be considered for potential application in the food industry [13, 14]. Vegetable drying was first recorded in the eighteenth century. Following that [15], scenarios of a world war were inextricably linked to the expansion of the drying industry [13]. During the Crimean War (1854–1856), British troops received dry vegetables from home; during the Boer War (1899–1902), Canada sent dried vegetables to South Africa; and during World War 1, the United States shipped about 4500 tons of dehydrated vegetables [16]. Potatoes, cabbage, spinach, turnips, carrots, celery, sweet corn, green beans, and soup mixtures were among the products processed in the United States [8].

2. Dehydration of food and food products

Keeping food fresh is the best way to protect its nutritional value, but most storage procedures call for relatively lower temperatures, which are unable to preserve all over the supply chain [8]. Preprocessing techniques, such as osmotic dehydration, before freeze drying, may partially remove the water before the final drying stage [17]. When choosing a dryer, consider the material’s physical features, manufacturing capability, initial moisture content, particle size distribution, drying attributes, maximum permissible product heating rate, and explosion character traits (such as spray or fluid bed dryings) [18]. When dehydrating food, it is essential to consider factors, such as moisture content, glass transition temperature, dehydration methodologies and hypotheses, and physical and chemical changes [19]. The drying process affects the product’s chemical and physical characteristics and water content. Some characteristics that have been utilized to classify dried foods include water activity, isotherms, sorption, deterioration of microbes, enzymatic and nonenzymatic reactions, structural and physical phenomena, and degeneration of nutrient levels, perfumery, and tastes [20, 21].

Moisture diffusion influences the process of drying during the rate of the first falling stage. According to Fick’s second law of diffusion, the tortuosity, intercellular space, and distortion of tissues of vegetables, as well as the structure and chemical composition of the food, all impact moisture transport during the dehydration process [22]. It is crucial to examine the compatibility of intrinsic barriers and how to alter them given the number of food items that have undergone evaporation and dehydration in command to maximize the benefits of drying and dehydration [23]. High latent heat of vaporization of water is the primary method of removing water from food was climate change. Climate change needs much energy, for example, 2.26 MJ/100°C [24]. Mechanical pressing was commonly employed before thermal drying to remove 20–30% of the water from solid food wastes. The variety of food drying procedures and equipment demonstrates the complexity of handling and processing solid foods. As well as the unique criteria for various food products. Furthermore, economic issues are a big concern, given the high amount of low-cost goods, such as skim milk [25].

Some water may be kept together by interactions between water molecules, resulting in a multilayer of water molecules. At the same temperature, this type of water, referred to as “bound water,” has a lower partial pressure than pure water [26]. Additionally, the heat of vaporization for bound water is higher than pure water during similar temperatures. A food product’s chemical composition directly affects how much bound water is present in it [27]. A product retains free moisture over its equilibrium moisture content. Only free moisture in a product may be eliminated during a specific dehydration technique [28]. The product type, temperature, and concentration of water vapor in the air influence the free moisture content of a product [28]. The bonded water in a food product is removed with considerable effort during the dehydration process [28]. Dried foods have various advantages, including improved storage stability, reduced packing requirements, and reduced transit bulk. Figure 1 shows the osmotic dehydration process in food.

Figure 1.

Schematic demonstration of the osmotic dehydration process.

3. Conventional food dehydration processes

3.1 Solar dehydration

Between the end of the 1800s and the start of the 1990s, artificial dehydration took the place of sun dehydration [29]. Vegetables were first mechanically dried in the eighteenth century, an improvement over solar drying. It is a monitored and efficient solar energy system. Solar driers may produce hotter air and lower relative humidity [30]. The oldest industrial technique still in use is probably this one. It has been used with various things since antiquity, comprising meat, fruits, plants, and fish [31, 32]. However, this strategy significant downsides bound its application to industrial manufacturing. Among all are the prerequisites for vast amounts of heavy labor inputs and space, the challenge of monitoring the drying frequency, pest infestations, and microbial contamination [33].

3.2 Tray drying

It has a simple design and can dry much stuff. The first hot air dehydrator was invented in 1795 and used to dry fruit and vegetables, such as raisins and prunes [29]. The proper operation of the tray dryer depends on an even distribution of airflow over the trays [29]. Colak and Hepbasli developed a green olive model in 2007. The energy efficiency first study of dehydrators in 1921 by Christie and Cruss [34]. At the time, heated forced-air dehydrators were used to dry prunes instead of sun use [8]. The prime problem of the tray dryer is irregular drying brought on by inadequate airflow dispersion in the drying chamber [29]. The efficiency of a tray dryer system can be increased and drying nonuniformity minimized. Due to the systems’ low operating costs, many dryer structures have been created using solar energy [8].

3.3 Smoke dehydrating

To preserve food through smoking is almost as ancient as direct-air dehydration. The two strategies are frequently employed in tandem [35]. Smoke has the additional benefit of giving attractive tastes to meals [5]. Furthermore, some of the chemicals produced by smoking have antibacterial characteristics [36]. While primarily not employed to lower the food content of moisture, the heat involved in smoke production has a drying impact [36]. Smoking always associates with fish and meat.

3.4 Drum dehydrating

Drum drying began 120 years ago, in the early 1900s, with Just Hatmaker developing the first drum dryer in 1902 [29]. Initially, a double drum dryer was created with feed going into the nip. It was less suitable for viscous liquids, so in 1945 a single barrel with top feed was developed to handle viscous products [29]. Feed was applied using dipping, splashing, spraying, and bottom feed rolls in a single drum. The feed’s viscosity typically dictates the feeding method [37]. Food dried with drum qualities, such as bulk density, solubility, moisture content [37], and particle size, is affected by drum dryers’ attributes, such as drying air temperature, rotation speed, feed ability, feed rate to focus, and ambient air quality [37].

3.5 Spray dehydrating

Spray dehydrating is essential to repeatable, continuous, scalable, time-saving, and economical technology for creating dry ultrafine powders [38]. Depending on the nature of the components as well as the required final attributes, numerous types of dryers can be used to dry them [39]. The spray drying procedure maximizes heat transmission and may be utilized for any substance with a liquid-like characteristic [38]. Because of its versatility and speed, it is the most commonly utilized drying technology for various heat-sensitive constituents. Spray drying has an advantage over other dehydration procedures due to its superior product quality, consistent texture, and quick rehydration [39]. Spray-drying technology has also been used in the chemical, pharmaceutical, food, cosmetic, and taste sectors. The approach has several advantages, including being fast, continuous, and repeatable [40]. As a result, it has been effectively implemented at both laboratory and industrial sizes.

3.6 Fluidized-bed drying

By forcing air through the plate’s pores, this dryer, which has a drying chamber with a saturated layer design, dries food by heating it until it becomes liquid-like [41]. Typically, crushed materials have a water content of 10–20% in a fluidized bed and 2–5% in the final product [42]. Whey, cocoa, cheese, dessert powders, potato bits, and dried powdered milk are all dried using this drying mechanism [43]. High-water-content foods cannot be dried by fluidized bed drying, but items with low humidity can be dried more gently than other methods, and the materials can be serially ground, chilled, and categorized as they are transported [43].

3.7 Freeze-drying

Although it takes hours, freeze drying has the least amount of protein denaturation as it is done at temperature changes between −30°C and − 40°C and is predicated on vaporization pressure [44]. It also costs between four and six times as much as drying. Freeze drying, correspondingly stated as lyophilization, is a popular method for producing the best quality food solids and powders [45, 46]. Because it functions at lower temperatures and beneath higher pressure, it is the desired technique for dehydrating foods with thermally delicate chemicals and lying to oxidation. Food quality varies as a result of dehydration. Due to the lack of water, oxygen-free atmosphere, and lower temperature conditions, the dehydration of fruits and vegetables through freeze-drying is the optimum method to keep an optimum bio compound content in the final goods [47]. Freeze-drying is a popular method for dehydrating plant-based goods, such as spices, fruits, vegetables, and some unusual meals. Despite its lengthy process time and hefty cost, it is favored for its best final quality [3, 48, 49].

3.8 Novel technology for food dehydration

Consumer desire for processed goods that retain the majority of the unique features of fresh plants has risen in recent years [50]. As a result, drying must be done correctly to keep the plants’ flavor, aroma, color, look, and nutritional content as possible [12]. Traditional dehydration processes need extensive drying times and high energy use, resulting in dehydrated items of poor quality [12, 51]. Novel food dehydration methods are a reaction to the newest customer expectations for high-quality dried goods that are also ecologically and economically sustainable [52]. As well as other cutting-edge new drying techniques for food dehydration, the most recent applications of microwave-assisted, solar-assisted, and various drying source-assisted hybrid drying technologies are discussed [53].

3.9 Osmotic dehydration

Osmotic dehydration is a straightforward process that permits fruits to be managed while retaining their natural properties, for example, color, fragrance, nutritional content, and texture [54]. The material derives into contact with a lower water activity mixture during osmotic dehydration when a counter-current mass transfer happens [55] from the product to the solution when water is transported. In the osmotic solution, the soluble solids are assimilated into the foodstuff reversely [55]. As soon as the osmotic pressure of a hypertonic solution is raised, tissue flows water into the solution [55].

3.10 Microwave drying

More and more food items are being dried in the microwave to remove moisture [56]. Microwaves have electromagnetic radiation through frequencies between 300 MHz and 300 GHz and wavelengths between 1 m and 1 mm [57]. Microwave drying provides several benefits over traditional drying processes, including a faster dehydration rate [58]. Microwave drying has numerous downsides, including nonuniform heating, textural degradation, and a restricted microwave penetration depth into materials [59, 60]. A chamber of production in which the material is equipped, a system that measures, controls, and monitors the dehydrating time, and a system that places the product in the dehydrating chamber are the components that make up a typical MWD system [59].

In contrast to drying with hot air, microwaves result in faster drying times and better product quality [61]. When used in place of hot air, microwaves fasten the dehydrating process, reduce oxidation, and increase the dried material’s characteristics [62]. These characteristics include density, pore volume, tensile and reconstitution capabilities, color, and the number of bioactive substances present [62]. It is possible that the worth of dried product could be significantly increased with some careful consideration of variables, such as microwave power, an electromagnetic method, temp, humidity ratio, and so on [63].

3.11 Pulsed electric field (PEF)

This method of food preservation is a novel nonthermal method that employs electric current for microbial inactivation [64, 65]. It has been discovered to have little or no negative impact on the quality of food materials [64, 66, 67]. This method primarily processes liquid and semiliquid food products [64, 68, 69]. PEF-assisted drying helps preserve the dried products’ physicochemical properties, color, and bioactive compounds (Figure 2) [70, 71]. PEF-assisted drying also improves the kinetics of drying and stimulates rehydration. Moreover, also allows selective cell disintegration while keeping the quality of a product [72]. PEF pretreatment inactivates enzymes and microbes; also controls respiratory movement, which may take part in preservation [73]. Despite multiple benefits, the applicability and efficiency of PEF-assisted drying can be enhanced in the future.

Figure 2.

Proposed mechanism of PEF-assisted drying.

3.12 Ball drying

A screw conveyor transports the material that needs to be dried to the top of the drying chamber [74]. It is possible to bypass the conveyor and feed the material directly into the drying chamber; however, doing so will result in the product being fed at a more erratic pace [41]. In addition to that, heated air is constantly being blown into the space that is being examined [75]. The material is brought into contact with heated ceramic balls or other heat-conductive balls while in the dehydrating chamber. The most critical component of the dehydrating process is convection [75]. The large screw located within the chamber spins while the drying procedure is being carried out, and the rate of rotation controls the amount of time the product spends inside the chamber [75]. When the item reaches the end of the chamber, it is taken out along with the balls and then collected [75].

3.13 Ultrasonic drying

The use of ultrasound has shown promise in the inactivation of microorganisms at temperatures close to those of the human body, the enhancement of energy efficiency, the reduction of thermal deterioration of food components, and the maintenance of the site nutritional and sensory integrity of food materials [76, 77, 78]. For food dehydration, a technology using stepped-plate transducers to link ultrasonic energy to food samples directly has proven to be an extremely effective method [76, 79]. Plate radiators have a tremendous amount of surface area, which is one of the primary reasons this technology is so advantageous [76, 80]. Other benefits of this technology include the possibility for large-scale industrial applications [76, 81].

3.14 Solar-assisted

Since ancient times, people have relied on the sun’s heat to dry out food. Solar energy works by increasing the temperature of the product being dried, which causes an increase in vapor pressure [82]. Vapor pressure is the driving force behind the moisture transfer process [83]. Traditional methods of preserving perishable vegetables include sun dehydration, which involves exposing the vegetables directly to the sun to absorb its radiation [8]. Grapes, prunes, and figs, among other dried fruits, have been preserved using this method to an extensive degree in recent years [8]. Some have been dried using hot-air dehydrators rather than sun drying in recent years because the fruit dries faster with the former, and problems caused by inclement weather are avoided with the latter [84]. Because there is a good chance that future energy limits will be reached, the availability of fossil fuels and natural gas used in dehydration may be severely restricted. If this occurs, a different energy source will be necessary to heat the air used for drying the goods [8]. Heat source from solar energy for hot-air drying appears appealing due to the fact that fruits are typically grown in regions that are warm and sunny, as well as the fact that they are dried and harvested through a time in the year when there is an abundance of solar radiation [8].

3.15 Infrared-assisted

The use of hybrid drying methods, such as infrared-freeze drying, has proven to be successful in increasing the efficiency of the dehydration process [85]. The principle behind infrared (IR) dehydration is that IR radiation from a heat source raises the temperature of the object being dried, which helps the moisture evaporate [86]. During the evaporation of water, infrared rays can pierce to a greater depth in the wet sample, which causes the sample’s temperature to rise without simultaneously heating the air around it [87]. As the moisture content of the materials decreases and diffuses out into the air, the rate of water diffusion through the material will increase, and the radiation characteristics of the samples will change [88]. Numerous industries frequently use infrared heating because it is widely acknowledged to deliver superior final product quality in addition to countless energy efficiency and cost-effectiveness than convective heating. Several of these industries include [89]. The advantages and disadvantages of common drying techniques are presented in Table 1.

Methods	Advantages	Disadvantages
Freeze drying	Polysaccharide does not lose activity	Long processing cycle
Oven drying	Easy to operate	Incomplete drying
Smoking	Added flavors	Difficult to control, slow
Vacuum drying	No oxidation	Low efficiency and high cost
Drum drying	Continuous	It may require modification of the liquid
Spray drying	Fast drying	Easy to decrease the activity of polysaccharides
Ball drying	Rapid, continuous, and comparatively low temperature	Loss of product integrity, difficulty to control
Microwave vacuum drying	Even drying	Low efficiency and low cost
Ultrasound	Reserves antioxidant properties and bioactive compounds	It will affect the antioxidant activity
Pulsed-electric field	Highly operative for refining drying rates Short-time handling	Can affect the structure and activity of nutrient
Microwave	It can improve the yield	Human hazards

Table 1.

Advantages and disadvantages of common drying approaches of food dehydration.

4. Hybrid drying technologies

The primary objective in the development of hybrid drying technologies is to slow the deterioration of products while simultaneously producing goods with the desired level of moisture content. When using various drying methods to produce quality-dried goods, some of the most important factors to consider are the characteristics of food quality criteria [90]. Figure 3 displays an overall Classification of hybrid drying.

Figure 3.

Hybrid drying technologies scheme of classification.

The term “hybrid drying technique” refers to the practice of combining two or more distinct drying methods in such a way that they cooperate to reduce the amount of time required and the amount of energy used for drying while maintaining the majority of the product’s quality characteristics, such as its flavor, nutrition, color, fragrance, and texture [56]. It has been demonstrated that combined effect and combination drying processes that have been optimized use a low specific amount of energy [91]. The product’s characteristics required to be dried play a role in the decision-making process for selecting an appropriate drying method [92]. Efforts are being made to advance hybrid drying technologies to mitigate the disadvantages of more conventional drying procedures, reduce the rate at which products deteriorate, and ensure that end products have the appropriate residual moisture level [56]. The term “hybrid drying” refers to a category of drying processes that include not only those that use multiple modes of heat transmission but also those that use two or more drying phases to achieve the desired level of dryness, product quality, drying time, and production process. A more common definition of hybrid drying is the effective integration or clever combination of two or more conventional drying methods [93]. This one factor may result in the development of an entirely new breed of hybrid drying techniques.

5. Changes in a structure after dehydration of food substance

Various dehydration processes can potentially alter the qualities of foods with a high moisture content [94]. Because moisture removal from a material frequently results in changes to the material properties, these changes play an essential part in the design and prediction of heat and mass transfer processes that occur during dehydration [95]. Additionally, these changes help determine how the methods used to dehydrate products influence the quality properties of dried goods [45]. During the drying process, structural and thermos physical properties such as mass-volume-area-related parameters involved in heat transport are among those that change [96]. Most of these qualities indicate shifts on scales ranging from microscopic to macroscopic in the chemical composition and the structural organization of dried items [97]. Micro and macrostructure observations of the surfaces and cross-sections of fruits and vegetables are frequently included to generate information on the influence of dehydration procedures and circumstances on the textural features of dehydrated samples [56]. These observations can be made at different scales. In addition, morphological parameters are recorded so that an analysis can be performed to determine how the drying processes and environmental factors affect the size and shape of the samples. Drying methods can change primary food attributes: bulk characteristics, flow property, moisture, appearance, aroma, structure, rehydration ability, nutrients, and volatile chemical retention [12]. In terms of nutritional properties, oxygen, extreme heat, and cell injury are common opponents of bioactive component retention during processing. As a result, dehydration can have an effect on the stability of important chemicals in plant-based diets [98]. Phenolic substances may be susceptible to enzymatic breakdown due to polyphenol oxidase activity [98].

The various dehydration techniques unaffected the fundamental structure of polysaccharides [99]. However, different dehydration strategies may change crude polysaccharides’ output, protein concentration, and ash concentration [4, 100]. The elimination of vitamin C and carotenoids throughout dehydration procedures is influenced mainly by water concentration and temperature. It is also hypothesized that the structural and thermal physical characteristics of fruits and vegetables change during the drying process based on the chemical composition, the physical organization of the structure, the phase distribution of the system, the internal and exterior pore space represented by the porosity, and other factors [101].

Certain foods subjected to heat treatment have the potential to exhibit an increased total phenolic content [102]. There is a possibility that the drying processes that hasten the breakdown of cellular components are to blame for the rise in the total phenol concentration in samples that have been dried [103, 104, 105, 106]. It is possible that the rise can be attributed to the heat-induced breakdown of complex phenolic tannins, which leads to an increased amount of phenolics being extracted [107]. Additionally, the increase in total phenolic content might be explained by the production of Maillard reaction products, which would result in the synthesis of new phenolics from precursors during heat treatments. Compared to conventional dehydration, the color loss that occurs during novel-assisted dehydration is significantly less [108].

6. Future prospective

Osmotic dehydration can extract juice from osmotically concentrated fruits in which juice is ejected from pre-concentrated fruits using osmotic dehydration. It essentially allows the production of high-concentration juice without heat, preserving the nutritional and organoleptic properties of the juice.

7. Conclusion

Dehydration is a simultaneous mass transfer procedure that primarily stimulates the movement of water particles from the meal, resulting in a final item with high sensory characteristics and physiological qualities. In the processing of dried foods, dehydration of food is one of the utmost significant alternative food preservation and treatment techniques. Numerous food dehydration processes are necessary to produce high-quality dried foodstuffs at a low cost. This study discusses recent developments in energy-efficient drying technologies for dehydrating food, including solar, infrared, microwave, and other assisted drying techniques. A new dehydrating method uses less energy and preserves the dried product’s chemical, color, taste, flavor, and appearance components. Innovative food drying technology can contribute to environmental protection.

Acknowledgments

We thank Food and Nutrition Society Gilgit Baltistan, Pakistan, for giving free access to journals. The authors also want to acknowledge the support of Guangdong Provincial Key Laboratory of Intelligent Food Manufacturing, Foshan University, Foshan 528225, China (Project ID: 2022B1212010015).

Conflict of interest

There is no conflict of interest.