Effect of Selected Drying Methods and Emerging Drying Intensification Technologies on the Quality of Dried Fruit: A Review

Abstract

Drying is one of the oldest methods for food preservation that removes the water from fruit and makes it available for consumption throughout the year. Dried fruits can be produced by small- and large-scale processors, which makes them a very popular food among consumers and food manufacturers. The most frequent uses of drying technology include osmotic dehydration, vacuum drying, freeze-drying and different combinations of other drying technologies. However, drying may provoke undesirable changes with respect to physiochemical, sensory, nutritional and microbiological quality. Drying process energy efficiency and the quality of dried fruits are crucial factors in fruit drying. Recently, innovative technologies such as ultrasound, pulsed electric field and high pressure may be used as a pretreatment or in combination with traditional drying technologies for process intensification. This could result in quality improvements of dried fruits and enhanced efficiency and capacity of the production process, with a positive impact on environmental and economic benefits.

1. Introduction

The term drying usually refers to the operation by which the moisture present in a material evaporates because of heat and matter exchange between the product and the working medium. Fresh fruits have high moisture contents as they are classified as highly perishable commodities; therefore, storage at refrigerated temperatures and controlled humid conditions is required [1]. Fruits are rich sources of nutrients, including vitamins, minerals, dietary fibers, phenolics, carotenoids, etc., that are useful for human health. Drying is an alternative method for the preservation of the nutritional value of fruits, which increases their relative concentration, extends their shelf life, and minimizes packaging, handling and transportation costs [2]. In addition, drying is an alternative to expensive postharvest management and selling surpluses of fruits on the market. The drying of fruits by conventional methods, such as sun drying or open-air drying, can degrade quality and food safety. Numerous disadvantages of these technologies led to the development of new technologies, such as oven drying, microwaving, vacuuming, as well as infrared, freeze and different hybrid drying, which are being used successfully for different kinds of fruits [3,4,5]. Each drying technique depends on various factors, such as the required type of product, size, level of ripeness, structure, color, aroma, chemical composition, nutritional composition, together with expected final quality, availability of a dryer and costs.

Sette et al. [6] investigated the application of wet and dry infusion as a pretreatment to air and freeze-drying on the physical properties of raspberries. Freeze-dried pretreated samples exhibited higher firmness and lower deformability as compared to air-dried ones. Moreover, the highest volume reduction was developed after air-drying, while freeze-dried samples showed 11% shrinkage.

Color is a very sensitive parameter in terms of the influence of drying methods. Krokida et al. [7] stated that color parameters L*, a* and b* of dried banana, apple, potato and carrot were significantly affected by convective, vacuum and microwave drying techniques. On the other hand, the same samples preserved their original color after freeze and osmotic drying.

Fruits are commonly subjected to various chemical and/or physical pretreatments prior to thermal drying to shorten the drying time, reduce the energy consumption and preserve the quality of products. By modifying the properties of fruit tissue, pretreatments could increase the drying rate, inhibit the bio-enzymes, and minimize possible deterioration reactions during drying and subsequent storage [8]. Therefore, each product needs to be dried by using appropriate pre- and post-processing steps, such as osmotic dehydration, blanching, soaking, or by the use of innovative approaches, e.g., ultrasound (US), pulsed electric field (PEF), high hydrostatic pressure (HHP), cold plasma (CP) or other treatments to add satisfactory value after drying [9,10]. This review presents the effects of different drying technologies and following treatments and/or pretreatments on dried fruits by providing the most important quality aspects.

2. Drying of Fruits

There are various studies on the topic of fruit drying where researchers have discussed the drying of fruit under different methods. The most commonly reported method is convective drying [11]. The other methods used for fruit drying are osmotic and osmo-convective [12,13,14,15], vacuum [16,17], solar drying [18,19], microwave [20,21] and freeze-drying [22,23,24].

2.1. Convectional Hot Air Drying

Dehydration/drying is still the most commonly used method due to economic benefits. Widely used equipment components are chambers, belts or tunnel dryers. Having a closed atmosphere with regulated airflow and temperature makes this method more advantageous than solar drying. In addition, convective drying is a quite effective and simple method; nevertheless, it is energetically inefficient. Kalra and Bhardwaj [25] found that convective drying in comparison to solar drying is faster and more efficient for mangos, papayas and apricots. On the contrary, the disadvantages of the convective drying method and heating can cause progressive physical, mechanical, chemical and nutritional changes in products. In particular, Stamenkovic et al. [26] revealed losses of 32–40% for total phenols, 3–25% for flavonoids and 44–60% for anthocyanins in dried raspberries as compared to controls (fresh samples). Moreover, in comparison to fresh samples, L-ascorbic acid content was significantly reduced during convective drying (0.94–97.93%), whereas the losses observed by freeze-drying were around 2.36%. The main drawbacks also include the length of the drying during the last stage and the slow heating of the material. Cavusoglu [27] studied the effects of high air temperatures on the drying kinetics and quality of tomato. It was reported that the treatment of raw tomatoes with air temperatures 150 °C, 130 °C and 100 °C within short intervals could reduce drying time without degradation of product quality. To remediate this, hot air drying can be assisted with other methods (e.g., microwave, osmotic or infrared) or different pretreatments (e.g., US, PEF or CP), which can yield better quality of product.

2.2. Osmotic Drying

Osmotic drying is used to partially remove water from biological tissues by immersion in a highly concentrated osmotic solution. The driving force for the transport of moisture from the tissues into the solution is provided by the higher osmotic pressure of the highly concentrated solution. Moisture diffusion is accompanied by simultaneous diffusion of the dissolved substance from the osmotic solution into the tissue. Since the cell membrane responsible for the transport of matter is not absolutely selective, other solutions that are present in the cells can also reach the osmotic solution [28].

Osmotic drying is most often applied as a pretreatment to another process to reduce the moisture content of a product, improve its quality during storage, and reduce the total amount of energy for other subsequent processes to osmotic drying. Due to the non-selective nature of a cell membrane, components such as sugar, acids, minerals, and vitamins can be diffused in miniscule quantities from the plant material to the surrounding solution while still affecting the sensory, nutritional and functional characteristics of the final product [15]. To ensure greater stability and longer shelf life of osmotically dried products, they must be subjected to additional preservation methods, including freezing, sublimation drying [29], vacuum drying [30], convective drying, and microwave drying [31,32]. One of the benefits of osmotic drying as a pretreatment for convective drying is energy savings due to moisture transport without phase change [33]. Osmotic pretreatment also increases the sugar–acid ratios, which can be important for fruits with high acid contents. In this way, the taste of the final product is better preserved [34]. This also improves the texture, volume reduction in the material and the stability of the pigmentation during drying and storage [34,35,36]. The osmotic drying process could be enhanced by using different pretreatments, including: pulsed electric field [37,38], sonication [39,40], blanching [41,42], and microwaving [43].

2.3. Microwave Drying

Microwaves (MWs) are electromagnetic (EM) waves that are synchronized perpendicular oscillations of electric and magnetic fields in a frequency between 300 MHz and 300 GHz, with wavelengths from 1 m to 1 mm. The mechanisms of microwave heating are based on the oscillation of ions and molecules when a material is exposed to the EM waves, which causes the internal friction and conversion of kinetic energy into heat [44]. The MWs provide volumetric and rapid heating of fruits with low energy consumption [45], while the literature usually reports MW drying with applied constant power.

There are different opinions in the literature regarding the homogeneity and control of heating with MWs. While some researchers confirm a homogenous heating rate of material [17], others state it as a drawback of this type of drying [44,45]. The reasons for uneven heating with MWs are due to several factors: (i) large product size; (ii) resonance phenomena; (iii) heterogeneous material composition; and (iv) shape of product.

Dried tomato and onion with and without MW power control are shown in Figure 1 [44]. Successful application of MW drying was found with potato chips, pasta, and snacks [46]. Because of the supply of drying energy directly to the volume of a product, its internal pressure will increase, which will drive water to the plant surface, resulting in an increase in drying rates [47,48]. However, depending on the type of material, MWs in some cases cannot fully complete drying and are usually combined with hot air drying or vacuuming. So even though both MW heating and hot air drying separately have disadvantages, combining these two methods could prove very beneficial. In that sense, the application of MW to the finalized drying of banana slices (T = 60 °C) reduced hot air drying by about 64%.

2.4. Freeze-Drying

Freeze-drying is a technique, first used for the preservation of thermally sensitive biological material, that employs the principle of sublimation of frozen water. The process takes place at a low temperature, where biologically active compounds remain preserved in large quantities. Over the last decade, freeze-dried food has gained popularity, especially for fruits such as berries [49]. Hammami and Rene [50] reported that optimal freeze-drying parameters for strawberry are 30 Pa and 50 °C of a drying plate, and that under these conditions, neither off-flavors nor the off-tastes were formed. Since the key difference in the freeze-drying is actually the time it takes while using different settings; therefore, several examples of freeze-dried fruits with operational parameters are given in

Table 1. Examples of freeze-dried fruits with various operational parameters.

Dried Material	Shape and Form	Freezing Temperature	Pressure of the Chamber	Drying Time	Monitored Properties	Ref.
Raspberry	Whole	−20 ℃	1 Pa	48 h	Bioactive compounds, shrinkage, color change	[26]
Strawberries	Pieces—3.5 cm high layer	−20 ℃ and −80 ℃	15–200 Pa	60–65 h	Moisture, rehydration ratio, appearance, shape, color, texture	[50]
Saskatoon berry	Whole	n/a	n/a	24 h	Moisture, water activity, color, polyphenolic compounds	[51]
Raspberry	Whole	Frozen with liquid nitrogen	4 Pa	48 h	Water sorption, glass transition temperature (Tg), molecular mobility, texture and rehydration properties	[6]
Apple	Slices of a thickness of 6 ± 0.5mm, average diameter 72 ± 3 mm	Samples were not frozen	100 Pa	Needed to achieve MR = 0.004 840 ± 21 and 368 ± 10min, for the untreated and the pulsed electric field (PEF) treated	Moisture, rehydration, hygroscopic properties, water activity	[52]
Strawberries	5 and 10 mm slices and wholes	−40 ℃	50 Pa	12 and 24 for slices 48 for whole fruit	Moisture, color, volume,	[53]
Carrot	Cylinders with a diameter of 20 mm and 8 mm height	−35 ℃ for 48 h, 1 h in liquid N2	3–300 Pa	24 h	Volume, bulk density, glass transition temperature, porosity	[54]
Kiwi	Whole fruit (without peel)	−40 ℃	12, 20, 42, 85, and 103 Pa	n/a	Color, texture, rehydration, total phenolic content, antioxidant properties and sensory analysis	[55]
Banana	Cylinders with a diameter of 20 mm, height 8 mm	−35 ℃ for 48 h, 1 h in liquid N2	3–300 Pa	24 h	Volume, bulk density, glass transition temperature, porosity	[54]
Blackberries	Juice with carrier agents	n/a	0.0004 Pa	48 h	Moisture, thermal property, density, morphology, antiradical activity	[56]
Blueberries	Whole fruit	−35 ℃	13 Pa	n/a	Mass transfer, drying time, berry-busting, skin perforation	[57]

n/a, not available.

.

The advantages of freeze-drying, in terms of chemical and nutritional quality, over other drying methods have been reported by numerous authors. However, in comparison to other drying technologies, freeze-drying has a high energy consumption and prolonged processing time [6,26,50,51]. However, this can be reduced by using PEF treatment. For instance, Lammerskitten et al. [52] reported that PEF pretreatment of apple slices intensifies freeze-drying kinetics and thus reduces processing time by 57% in comparison to untreated samples. Despite many advantages linked with freeze-drying, high initial costs are still a limiting factor for many producers.

3. Quality of Dried Fruits

The intensity of changes in quality parameters during drying depends on the nature of material, pretreatment, used process and their parameters. The majority of these changes include color, shape, volume, density, texture, flavor, nutrients, water activity, microbiology, rehydration ability, etc. [58]. These changes and influencing factors are reviewed below.

3.1. Physical Quality

The physical properties of food products are responsible for perceived quality. Shape, appearance and color are characteristics in sight of customers and liable for product acceptability. Currently, the color of food is measured directly and non-destructively by using apparatus like colorimeters. They use several color spaces for the expression of color values. The usually used color space is CIELAB (CIE L*a*b*, color system), and this method was useful for the monitoring of color changes during the drying of quince [34], kiwifruit [59], persimmon [60,61], etc. Commonly, the most color deterioration is associated with air heat drying, while enzymatic browning can be responsible for both color changes and off flavor. To this extent, this problem can be controlled by the thermal control of drying and the addition of a sulfur dioxide, acids and osmotic pretreatments. Rodrigues et al. [61] stated that the color of osmotically treated papaya is very similar to the color of fresh fruit. The layer of sugar that forms on the surface of the fruits during osmotic drying presented a barrier to air during convective drying that prevented browning [30].

Although freeze-drying was mentioned as a gentle dehydration process, it still led to higher color changes of raspberries compared to the hot air drying [6,26,62]. Sette et al. [6] reported that increased color change was due to the decomposition of pigments; therefore, it was recommend to consider internal structure when observing color changes during drying.

Texture properties that include structural and mechanical characteristics greatly influence the quality of dried products. Freeze-dried products have superior quality, better-preserved color, flavor and appearance, a higher rehydration ratio, and are crispier in comparison to the products dried with traditional drying technologies. Conventionally dried fruits tend to be chewy and tough to bite, but with an optimal moisture content that is pleasant for consumption. Hot air drying usually has destructive influences on the structure of fruits, as drying provokes structural collapse in fruit tissues due to moisture removal [63]. However, by the selection of optimal drying parameters, excessive changes in volume in fruits can be avoided, as is the case with raspberry fruit. Hot air drying at a temperature of 70 °C and a velocity of 1 ms⁻¹ provokes volume shrinkage by 23.17%. Convective drying by an air temperature of 50 °C will lead to a total collapse, while samples dried with an air temperature of 80 °C reached a volume shrinkage of 43.13% [64]. Here, lower temperatures totally collapsed samples, while higher temperatures led to the creation of a porous outer rigid crust or shell that fixed the volume [65]. Radojčin et al. [41] reported that the shrinkage of osmotically pretreated quince cubes was proportional to the moisture losses. This was also observed by Krokida and Maroulis [66] and Lozano et al. [67] for carrot drying during the entire process. In other studies for squid flesh [68,69] as well as potato and sweet potato [67,70], it was found that the volume of removed water during the final stages of drying was higher than the reduction in sample volume. One theory proposed for the explanation of this shrinkages was the process of glass transition. However, this concept may not be useful for the freeze-drying of all biomaterials, as it was shown with the latest experimental results [17]. For some cases, the incorporation of other factors, such as mechanisms of moisture transport, structure, surrounding pressure and surface tension, was also required [71,72]. More so, puffing effect under a vacuum had significant influences on the shrinkage, structure and porosity of dried materials [73,74].

Pretreatments can enhance drying with respect to energy savings and improved quality of dried product. Namely, PEF, US or HHP are able to induce cell damages, with increased permeability and mass transfer. For example, PEF treatment with assisted hot air drying at atmospheric pressure reduced the drying time of apple up to 12% and carrots up to 8% when 10 kV cm⁻¹, 50 pulses, air temperature 70 °C, and 5 kV cm⁻¹, 10 pulses, air temperature 70 °C were applied, respectively. The specific energy of pulsed electric field treatment was 80 and 8 kJ kg⁻¹ [75,76]. Other authors also reported reductions by 5–28% for parsnips and carrots at 60 °C. The specific energy of pulsed electric field delivered was about 65 kJ kg⁻¹ for carrots and parsnips. PEF (400 V cm⁻¹) assisted the freeze-drying of potatoes expedite freezing, reduced drying by 18% (0 °C and 0.04 mbar pressure), lowered residual moisture content, decreased shrinkage, and improved the appearance of samples [77].

3.2. Chemical Quality

Applied higher temperatures and prolonged exposure to oxygen could ultimately decrease vitamin content during the drying of fruits [78]. Nevertheless, one study concluded that applied solar drying even at lower temperatures (24.5–40.3 °C) could significantly (p < 0.05) reduce the vitamin content in the fruits [79]. Due to dehydration of the pieces to a moisture level of 10% or even lower, the relative increases in the concentrations of the mineral content, total acidity, carbohydrates and total sugars were significantly higher in the dried fruits as compared to the fresh samples. These findings broadly supported the work of Suna et al. [80], who established a clear positive correlation between the dry matter and mineral elements in different fruit varieties during and subsequent to drying. An increased temperature had negative effects on certain chemical profiles. For instance, comparisons of hot air drying (AD), freeze-drying (FD), and refractance window drying (RWD) exhibited significant changes in the content of vitamins B and C in blueberries, cherries, cranberries and strawberries [81]. FD samples, except blueberries, showed higher levels of thermolabile vitamin C in comparison to AD and RWD. Moreover, in AD samples the lowest content of vitamin C was observed. Vitamins B2, B6 and total vitamin B contents were thermally unstable during the drying of berries, and their lowest content was determined in the AD samples. The stability of the B vitamins in the FD and RWD samples was heavily dependent on the berry type. In particular, blueberries and cherries showed significantly higher total vitamin B during FD processing than the RWD, whereas for cranberry and strawberry, the RWD displayed a significantly greater total vitamin B retention than the FD samples [81]. The operating conditions of convective infrared drying could significantly affect fluctuations in nutrient elements in fruits such as strawberries [82]. A higher drying air temperature increased the concentrations of chemical elements such as N, P, K, and Mn, while the contents of Zn and Ca were reduced. By applying the various infrared power, it was found that concentrations of N, P, and K increased, while the contents of Ca, Mg, Fe, Mn and Zn were lowered. Further, different drying air velocities increased concentrations of N, P and K and decreased contents of Ca, Mg, Fe and Zn, hence it is important to optimize the drying process to ensure the best chemical compositions of a dried product.

3.3. Nutritional Quality

A recent study aimed to explore the optimal conditions for the convective infrared drying of strawberries with the aim to achieve a high nutritive product [82]. Drying experiments were conducted under: (i) constant drying air temperature (80 °C) and velocity (2.0 m s⁻¹) at various infrared powers (100, 200, 300 W); (ii) constant infrared power (200 W) and drying air velocity (2.0 m s⁻¹) at various air temperatures (60, 80 and 100 °C); and (iii) constant infrared power (200 W) and drying air temperature (80 °C) under various air velocities (1.0, 1.5 and 2.0 m s⁻¹). The authors reported a decrease in total phenols, total anthocyanins and antioxidant activity as a result of higher temperature, while 60 °C was found to be the optimal value with respect to the nutritive quality. Higher levels of total phenolics and anthocyanins were determined at 1.0 m s⁻¹, whereas increasing the velocity from 1.0 m s⁻¹ to 2.0 m s⁻¹ promoted higher values of antioxidant activity. Moreover, increasing the infrared power from 100 W to 300 W increased antioxidant activity, total anthocyanin and phenolic contents. Consequently, the authors concluded that the range of 200 W–300 W is a preferable setting for the processing [82].

Associating different drying technologies with the nutritional quality of fruits is a very common approach in the literature. For instance, freeze-dried, microwave-vacuum dried and osmo-microwave-vacuum dried cranberries were compared with respect to the changes in the bioactive compounds during drying [83]. Higher retention of polyphenols and antioxidant capacity was achieved by microwave-vacuum drying and osmo-microwave-vacuum drying as compared to the freeze-drying, although no significant differences were observed in polyphenolic contents among microwave-vacuum dried and osmo-microwave-vacuum cranberries. Another study aimed to evaluate the influences of three drying treatments, namely hot (AD), FD, and RWD on the stability of polyphenols and antioxidant capacity in four berry samples [81]. In comparison to AD and RWD, FD samples retained a higher total phenolic content. However, the authors did not find any statistically significant differences between the FD and RWD cranberry samples. The authors explained an observed trend with a lower temperature applied during freeze-drying as compared to the one applied during the RWD, which might stabilize flavonoids to a greater extent in the FD samples. A recent study highlighted the drying process by intermittent ohmic heating (IOH), which requires lower energy consumption while offering products with advanced quality as compared to intermittent air drying (IAD) and air drying [84]. Approximately 70–80% of polyphenolic compounds were retained in IAD dried litchi fruit (Litchi Chinensis Sonn.), while for AD samples the retention was only 60%. The potential explanation could be attributed to the reduced oxygen exposure in IAD and consequent phenolic preservation [84]. The pretreatments of fruits prior to drying may also influence the quality of the final dried product. To that end, Stamenković et al. [26] researched the impact of convective drying of fresh and frozen raspberries at operating conditions: air temperature (60, 70, and 80 °C) and air velocity (0.5 and 1.5 m s⁻¹). Freeze-drying was used as a control procedure to compare the obtained results. The exposure time to higher oxygen levels during the convective drying induced a greater reduction in vitamin C than elevated temperatures. It seems that the degradation of vitamin C in berry fruits does not depend on temperature for ranges between 80–90 °C. Therefore, it was concluded that the presence of oxygen plays a crucial role in vitamin C deterioration [85]. Although convective drying reduced the total anthocyanin in a range of 44–60%, the authors still concluded that red pigments were better preserved in dried raspberries than in the initial fresh state. In general, freeze-drying resulted in better preservation of nutritional quality, whereas the optimal conditions for the convective drying of raspberries were set at an air temperature of 60 °C and an air velocity of 1.5 m s⁻¹. This was calculated with regard to the equivalent of a freeze-drying procedure. Nguyen et al. tested the convective microwave-assisted drying of fruits abundant with thermally sensitive bioactive compounds at lower temperatures (of up to 30 °C) [86]. The authors dried bitter melon (Momordica charantia L.) at various microwave power densities (1.5, 3.0, 4.5 W g⁻¹), drying temperatures (20, 25, 30 °C), and air velocities (1.0, 1.2 and 1.4 m s⁻¹), while monitoring the impact on phenolic compounds and corresponding antioxidant activity. As the drying process involved coupled influences of heat and mass transfer, the obtained results revealed that the higher air velocity along with extended drying time initiated higher losses in total phenols and antioxidant capacity. Total phenols and antioxidant activity reached maximum at a microwave power density of 3.0 W g⁻¹. A further increase to 4.5 W g⁻¹ led to significant losses, as polar bonds in phenolic structures become increasingly weak due to the effect of microwave irradiation with possible residual enzymatic reaction. Currently, novel research on food dying processes is mostly oriented towards advanced technologies that provide energy-savings and are able to produce high quality products. Here, focus is mainly given to solar-assisted drying with the perspective to endorse sustainability in the food industry [78]. For example, five different solar drying methods were investigated for drying mangoes and pineapples, namely open sun drying (OSD), black-cloth shade (BCS), white-cloth shade (WCS), a conventional solar dryer (CSD), and an improved solar dryer (ISD). The drying processes were conducted outdoors, with a mean daily temperature and relative humidity of 26.8 °C and 26.7%, respectively. The fruits that were dried under the OSD, WCS and BCS methods revealed a significantly higher decrease in total phenols than the CSD and ISD methods [79]. The authors explained this trend with increased temperatures and prolonged drying times during the OSD, WCS and BCS methods. These finding were consistent with previously published results that attributed additional losses of phenols during drying to the direct exposure to ultraviolet solar radiation [87].

Another study documented the effects of solar convective drying over 12 months of storage on the bioactive compounds in sweet cherry samples. The operating conditions were 60, 70 and 80 °C during 8, 6 and 4 h, respectively [88]. A reduced drying time positively affected preservation of the total phenols, total flavonoids and total anthocyanins, while the drying temperatures showed no significant differences on this outcome. After the drying process, total flavonoid content decreased by 24% and slowly continued to decrease during the storage, with a final loss of 30% after the 12 months of storage. Moreover, dried samples exhibited 2-fold higher antioxidant activity from the initial values (18.32% vs. 37.64%). It is possible that these results were influenced by the shorter time of drying and the formation of a novel constituent(s) with antioxidant activity (e.g., Maillard reaction products) that could be continuously formed and released during an extended storage time.

3.4. Sensory Quality

Sensory evaluation is a method to assess the perceivable qualities of a food product using untrained (consumer) or trained panelists. Consumer panelists are recruited when the focus of the studies is on the acceptance (or liking) of certain sensory attributes. Trained panels receive thorough sensory training according to the relevant ISO standards [89,90,91]. Additionally, their performance is continuously monitored in order to ensure data validity and quality [92].

One of the key elements of fruit drying is the effect of the different drying methods on the sensory aspects of the fruits. Drying is performed by removing the water content of the fruits, which directly affects sensory and chemical properties. Traditional drying techniques, e.g., convective drying (CD), use high temperatures and a high content of oxygen in the drying agent, which deteriorate color, aroma and texture properties [93]. Several authors reported that convective drying preserves flavors well in quince [94] and jujube fruits [95]; however, the presence of a measurable off-flavor intensity (burnt flavor) has also been reported. Additionally, CD provided darker color compared to other drying methods. Papers have also reported that lower drying temperatures produce better accepted products [96]. Freeze-drying (FD) has been identified as one of the best drying methods when it comes to preserving the sensory and nutritional quality of fruits [17]. On the contrary, FD is identified as one of the most expensive methods; therefore, there is a continuous search among researchers to find affordable methods which provide the same (or as close as possible) quality as FD. Vacuum-microwave drying (VMD) was shown to cause intermediate color changes, positive effects on texture and low intensities of off-flavors [97]. VMD has been used to dry several fruits, e.g., quince [94], jujube fruits [95], cranberry [97], etc. However, it must be noted that since microwave drying methods replace thermal energy with electric energy, textural attributes change significantly. Crisp texture results from puffing due to expansion when water evaporates within the product, and it is also related to porosity [98]. Heat-pump drying (HPD) also shows promising results on the nutrient content and sensory aspects of fruits. HPD has been shown to consume 22 to 40% less power compared to electrically heated dryers [17]. When HPD and modified atmosphere heat-pump drying (MAHPD) were compared to vacuum drying and freeze drying, it turned out that MAHPD led to better physical properties, such as reduced shrinkage, decreased firmness and more porous structure of the materials, which resulted in quicker rehydration. When using inert gas, the color of the heat pump dried food proved to be similar to vacuum or freeze-drying [99].

3.5. Comparison of Drying Methods

As it has been introduced, there are several techniques available for drying. Additionally, different settings of the given drying methods also have a significant effect on the final quality of the fruits. When it comes to comparison, novel statistical techniques provide substantial help to compare different methods. The key problem here is that the comparison of drying methods is not always clear, and sometimes there are contradictory results. For example, freeze-drying is considered to be one of the best drying methods for biological samples when dealing with nutritional and sensory quality variables. However, when technological parameters such as energetic demands and maintenance costs are considered, freeze-drying is far from the best alternative. In such cases, when different variables assess the performance of the methods differently, multicriteria decision making (MCDM) methods can provide valuable help. MCDM methods have been developed to compare multiple methods/products based on multiple aspects [100]. When it comes to comparing methods, models and products, the sum of ranking differences (SRD) method is one of the most widely applied in food science [101]. It has been successfully applied to compare horticultural products [102], sensory attributes affecting overall liking [103], energy drinks [104], the nutritional value of insect species [105] and raspberry drying [106], just to name a few.

A typical SRD input matrix consists of the comparable methods in the columns and the measured variables in the rows. The comparison of the methods is completed based on the rows. The last column of the input matrix serves as the reference (or benchmark) column. The reference can be set as minimum, maximum, average and user defined (e.g., golden standard). This is really important, as the data values in each column are ranked in increasing magnitude and these rankings are compared to the ranks of the reference column. Finally, the absolute differences between the rank-variables and the rank-reference columns in each case are calculated and summed. These values (SRD values) give the ordering of the variables. The smaller the SRD value, the better (or the more consistent) the variable. The procedure above is explained in detail in one of the recent works. The visual representation of the method is provided by Figure 2 [107].

A recent example of using the SRD method for the comparison of drying methods based on several measured parameters was published by Stamenković and co-workers in 2020 [107]. A comparative experiment was conducted in order to identify the most suitable process parameters for convective drying that may be considered as alternatives to freeze-drying, which is a widely used preservation method for raspberries even though it is a costly and energy-consuming method. Twelve convective drying regimens were applied with a combination of three influencing factors: air temperature (60 °C, 70 °C, and 80 °C), air rate (0.5 and 1.5 ms⁻¹), and stage of raspberry (fresh and frozen). The final product, a dried raspberry, was assessed for chemical, physical, and mechanical properties and rehydration capacity. SRD showed that the convective drying of fresh raspberries proved to be more similar to freeze-dried raspberries than the convective drying of frozen ones. Fresh samples dried at 60 °C air temperature and 1.5 m s⁻¹ air flow proved to be the most similar to the reference freeze-drying method. Such analyses can help practitioners to develop cheaper and simpler drying methods that could replace costly and energy-consuming methods but keep the same quality of the dried products.

4. Unconventional/Emerging Drying Intensification Technologies

Many different strategies can be applied in order to enhance the drying process and/or to improve dried food properties. These strategies usually consider the modification of drying parameters or material properties. The approach that is based on changing the parameters of drying, such as temperature, flow rate and humidity, is usually sufficient to enhance the first period of drying, which is governed by external mass transfer resistance. In turn, intensification of the second stage of drying can usually be achieved by the introduction of a pretreatment step that will change the material properties, for example, its dimensions or integrity of cellular structure. A reduction in dimensions, which can intensify drying kinetics to a great extent, is not always possible. The rupture of cellular structure can be thermally achieved, e.g., blanching [108,109]. Additionally, this can be done by non-thermal methods such as high hydrostatic pressure (HHP), cold plasma (CP), ultrasound or pulsed electric field (PEF) treatment [110]. Non-thermal treatments, in principle, allow better preservation of thermo-sensitive compounds and are linked with lower energy consumption in comparison to thermal based technologies. Currently, existing publications show that, among non-thermal pretreatment technologies, US and PEF are the most promising for dehydration intensification. It has to be emphasized that the utilization of non-thermal methods as a pretreatment before drying does not have to result in better outcomes than the implementation of unconventional drying techniques. The decision about the potential implementation of a pretreatment method before drying should be preceded by deep studies in relation to the desired technological aim. Such analysis, in addition to a literature review, should also include optimization and economical studies.

4.1. Ultrasound

Ultrasound can be described as a cyclic sound pressure with a frequency that is inaudible to humans (>20 kHz). In food processing, ultrasound can be used for the enactment of traditional technologies or to replace them. Ultrasound can be utilized either as low-frequency but high-energy or as high-frequency but low-energy assays. The first one is usually associated with the facilitation of different unit operations such as extraction, freezing and thawing, emulsification and homogenization or drying, while the second one is mainly associated with control, analytical and diagnostic procedures [111]. The application of low-frequency but high intensity ultrasound causes different phenomena depending on the type of the medium where they propagate. The application of ultrasound in the fluid systems results in cavitation and microstreaming, which intensifies mass and heat transfer but can also lead to the formation of free radicals and reactive oxygen species. Cavitation bubbles that are collapsing may also erode and degrade the surface or structure of the materials that they contact [112,113]. When ultrasound propagates through solid-like material, for instance food matrix, cyclic compression and expansion of material can occur—such behavior is called sponge effect and it can lead to the formation of micro-channels, which facilitate mass transfer between the treated material and its surroundings [114,115]. Ultrasound can be applied using direct and contact methods or indirectly using ultrasound baths [105]. In the case of drying, ultrasound can be used not only prior to drying, but also during the process [116].

The literature about the effect of US on drying kinetics is ambiguous and the effects of pretreatment depend strongly on food matrix (Figure 3). There are reports that indicate that sonication can reduce the drying time of apples by 11–40% in comparison to untreated material [117,118], and there are articles which demonstrate that US pretreatment has no effect on process course or that it can even extend drying, as it was reported for carrots [119].

Moreover, drying kinetics seem to depend not only on the type of raw material but also on the parameters of US. The influence of sonication time is one of the most studied issues. It was found that the relation between the time of sonication and drying reduction is not linear. For instance, the sonication of 20 min of apple tissue reduced air drying better than the treatment of 30 min [118]. Similar findings were reported for other raw materials, such as pineapples or parsley leaves [120,121].

The possibility of process intensification by sonication was also exemplified by other drying methods, such as microwave assisted air drying or vacuum drying. It has been demonstrated that microwave and ultrasound assisted air drying reduced processing time by 79% in comparison to traditional, convection processes. In addition, samples produced with the assistance of US exhibited higher porosity and better reconstitution properties than untreated material [122].

The vacuum drying of nectarine with sonication was 50% shorter than the control process, plus US treated samples demonstrated higher retention of phenols and smaller changes in color. The authors of this study stated that there is a synergistic effect of vacuum drying and ultrasound treatment [123]. The positive addition of ultrasound during drying on rehydration and color retention was also demonstrated for purple-fleshed potatoes [124].

4.2. Pulsed Electric Fields

PEF is an electro-based technology since it involves electric fields for its application. PEF treatment of food depends on the exposition of material into short-lasting pulses characterized by high electric field intensity that varies from 0.1 to 50 kVcm⁻¹, depending on the desired technological effect [125,126]. PEF treatment results in a rupture of cell membrane continuity due to a phenomenon and a process of electroporation [127]. The electroporation can be irreversible or reversible depending on the induced transmembrane potential of the cell that it treated, which in turn depends on many different factors. Among them are the cell diameter and external electric field, which are the most important [128]. Figure 4 presents the SEM images of apple tissue treated by PEF at different parameters with indicated ruptures in cellular structure.

The majority of PEF applications in food processing involves irreversible electroporation. Such PEF treatment can be used to enhance extraction, juice pressing, freezing, osmotic dehydration or drying [129]. However, there are some data which demonstrated that reversible electroporation could also be applied for the improvement of drying [130]. It is worth noting that the effectiveness of PEF treatment has been proved on an industrial scale for winemaking, juice preservation or in potato processing [131,132,133].

As a contrast to ultrasound, the vast majority of scientific publications show that PEF pretreatment facilitates mass transfer during drying. Drying reduction by PEF prior to water removal varies from 2% to 57% in comparison to untreated material, as it was reported for apples and basil leaves, respectively (Figure 3). The effect of PEF on drying depends on many different factors which are related to the material properties and processing parameters: electric field intensity, energy input, number of pulses, pulse width and geometry or drying methods [134].

For instance, the intensification of drying depends on the cell disintegration index (CDI) of the material (which varies from 0 to 1, for untreated and hypothetical totally disintegrated samples). Here, the effective water diffusion coefficient of apples subjected to air drying was equal to 1.044, 1.090 and 1.252 m² s⁻¹, for untreated samples, samples with CDI = 0.33 and with CDI = 0.88, respectively [76].

A higher water diffusion coefficient of PEF pretreated samples was also reported by Ostermeier et al. [135] for the two-step convective drying of onion tissue. Further, PEF treatment was also characterized by 14.5% higher pyruvic acid content and a 47% higher rehydration coefficient.

The exposition of the material to PEF treatment was also demonstrated as an efficient method for freeze-drying improvements. Wu et al. [136] reported that the application of 30 pulses at an electric field intensity of 1 kV cm⁻¹ reduced freeze-drying time by 22.5% in comparison to untreated apples. The higher reduction in freeze-drying time of 31.5%, as compared to intact material, was reported for potatoes treated by 45 pulses at 1.5 kV cm⁻¹. A very interesting approach for the utilization of PEF in the freeze-drying process was demonstrated by Lammerskitten et al. [52]. In this case, the authors did not freeze the apple slices before freeze-drying using a freezer, rather the freezing occurred inside the freeze-drying chamber as a result of a pressure drop during the initial phases of freeze-drying. Such treated material kept its original shape (low drying shrinkage) and it was characterized by high crunchiness index, high porosity, and had similar chemical properties to the untreated material [137]. Similar findings were also reported by Fauster et al. [138] for freeze-dried strawberries and bell peppers. Some of the research papers indicated that PEF can also improve vacuum drying, similar to how it improves air and freeze-drying. As it has been reported by Liu et al. [139], PEF pretreatment of carrots reduced vacuum drying time by 33–55% and improved the retention of carotenoids.

Although the number of publications in the field of PEF and drying is growing, the research should also focus on the combination of PEF with other unconventional drying methods, such as infrared drying or microwave vacuum drying. Although some of the drying techniques in combination with PEF are well tested, optimization studies, using advanced experimental planning methods, are needed. Such an approach could address the questions related to the potential modification of drying parameters, such as temperature, in order to get the best possible quality and economic outcomes. Moreover, there is a gap in knowledge about the effectiveness of PEF pretreatment before drying for pilot and industrial scale processes enhancement and the sustainability aspects of its utilization.

4.3. High Hydrostatic Pressure

High hydrostatic pressure (high pressure processing, HHP, HPP) is one of the oldest and most popular non-thermal food processing methods. However, it is used mainly for preservation purposes since it inactivates microorganisms but keeps low molecular weight substances (like vitamins) intact. HHP is also used in industrial scale. It has been reported that in 2015, more than 300 units of HHP were operating all over the world [140]. The utilization of HHP, like PEF or US, can also modify the cell membrane permeability and thus it can enhance water transfer during dehydration processes [141]. HHP pretreatment has been demonstrated to reduce the drying time of vegetables such as carrots, green beans and potatoes [142] or fruits such as apples [143,144] and pineapples [145]. This method has also been demonstrated as effective in the intensification of drying of ginger—processes preceded by HHP treatment (10 min, 100–400 MPa) were characterized by much higher moisture diffusivity (2.84–6.09 × 10⁻⁹ m² s⁻¹) than the reference operation (2.03–4.87 × 10⁻⁹ m² s⁻¹). Moreover, HHP pretreatment also increased the extractability of oleoresin and 6-gingerol from dried material [146]. HHP pretreatment has also been reported to increase the antioxidant activity of osmodehydrated strawberries [147]. However, the effect of HHP treatment applied prior to drying depends on the quality changes of dried material and depends strongly on matrix type. It has been reported that HHP may result in undesirable color changes like the darkening of tissue, as it has been reported in the case of garlic [148]. One of the main drawbacks of HHP treatment is cost of the processing and batch (or quasi-continuous) operating mode. Some studies report the costs of HHP to be three times higher than the costs of PEF treatment [149].

4.4. Cold Plasma

Plasma is considered as a fourth, quasi-neutral, like gas, state of matter. It is, in fact, an ionized gas which is a mixture of anions and cations, electrons, free radicals, molecules in an excited state and non-ionized molecules [150]. The presence of very active chemical molecules, such as free radicals or reactive oxygen species, makes plasma a potential tool for the decontamination of food and food contact surfaces. Indeed, most of the literature data about possible plasma utilization in food processing deal with preservation and microbial quality [151]. However, plasma application can also modify the surface properties of materials subjected for treatment, like some of the polymers [152]. The modification of the surface by cold plasma treatment was also reported for food products, such as black pepper seeds [153]. Recently, cold plasma has been reported as a pretreatment method for drying enhancement. Such an approach is related to the aforementioned possibility of modification of the surface and structure by plasma application by the physical and chemical processes—plasma can etch large cavities into the structure of material, which facilitates subsequent moisture removal during drying [154]. The time of wolfberry drying was reduced by 50% when processes were preceded by cold plasma treatment. Moreover, the plasma treated dried material exhibited better reconstitution properties and higher retention of phenolics in comparison to the untreated material. The authors of this study stated that in addition to the alteration of the surface, the cellular structure was disintegrated as well due to cold plasma treatment [155]. The acceleration of drying by cold plasma treatment has also been reported for shitake mushroom [156] or corn kernel [157] drying. Nevertheless, the data about the impact of plasma radiation of food before drying are limited, but the method seems to be very promising, especially for the facilitation of drying of peel containing raw materials such as chili pepper [158]. Moreover, since plasma consists of very reactive chemical molecules, research should also focus on the chemical property changes and safety aspects of such treated food. Another important issue related to the utilization of this method is the possibility of its scale-up ability.

5. Conclusions

Drying provides extended shelf life, reduced transportation costs and minimized losses for various foods, and it is an indispensable part in the food processing industry around the world. Recent literature is focused on applying advanced technologies for drying intensification to improve conventional drying performances with respect to product quality and energy savings. Combinations of drying methods/hybrid drying and advanced pretreatments are useful for optimal results for both product quality and environmental impacts. Therefore, the right selection of drying methods and mathematical optimizations (modeling) of the process can reduce energy consumption, operational costs and provide superior quality products. Thermal drying techniques, such as hot air, have significant adverse effects on shrinkage, color, and textural properties, but they are economic. Furthermore, microwaves due to volumetric effect increase drying rate and reduce drying time and energy consumption, with the final quality close to hot air drying. Introducing vacuuming during drying will cause the avoidance of thermal and oxidative stress, with positive repercussions on product quality. In conclusion, combinations of advanced and conventional techniques have the potential to overcome inherited disadvantages of single technologies, while improving the economic outlook of food manufacturing.