Introduction

Pear (Pyrus communis) is a widely cultivated fruit in temperate regions, especially in Europe. Pears are highly appreciated by consumers due to their good taste and low calories, and high nutritional properties, such as dietary antioxidants, vitamins, minerals, and fiber. Generally, pears can be consumed fresh, as well as canned, yogurt, nectar, and syrup used in purees after processing and drying, which are widely used [1].

Drying processing reduces the water content in the product to safe levels, preventing microbial growth and enzymatic modifications [2]. In addition, it eliminates the use of expensive cooling systems for the enclosure and simplifies transportation and storage. Most importantly, it offers products with new tastes, textures, and flavors [3]. However, drying is one of the most extensive processes in energy use (12–25% of the total energy) in industrial processing [4]. Ekka et al. [5] stated that about 30% of the total energy consumed in the world is consumed by the agricultural sector, of which about 3.62% is related to the drying of agricultural products in different dryers. A large number of industrial processes directly or indirectly involve the drying process. The energy used to dry agricultural products is more than fossil fuels, which can create significant greenhouse gases [5]. These results show that the drying of agricultural products is associated with high energy consumption, but it also has high industrial importance. These results show that the drying of agricultural products is associated with high energy consumption, but it also has high industrial importance. Due to the high energy consumption in the drying of agricultural products, the production of greenhouse gases and its environmental effects also become very important [6]. The increase in the emission of greenhouse gases in the world due to the excessive consumption of fossil fuels has brought negative consequences such as global warming, the expansion of desert areas, climate changes, and the increase in the level of open water for the environment [7]. Therefore, energy and greenhouse gas analysis of dryers is very important to improve design and technical sustainability [8]. For example, drying in power plants that use lignite and sub-bituminous coal to generate electricity generation requires a much energy (about 7%) to evaporate the moisture from the coals [7, 9]

In the past decades, some researchers conducted SEC and GHG analyses for drying process of different products using various drying systems such as Seyfi et al. [10] for refractance window, convective and solar refractance window drying of aloe vera, Karimi et al. [11] for fluidized bed drying of canola seeds, Pérez-Won et al. [12] for vacuum microwave drying; freeze drying; and hot air drying and pulsed electric fields as pretreatment of Chilean Abalone, Taghinezhad et al. [13] hot air-infrared dryer with ultrasound pretreatment of blackberry, Ashtiani et al. [14] for ultrasound-assisted convective drying with Cold plasma pretreatment of goldenberry, Wang et al. [15] for solar-electric hybrid drying setup of lotus bee pollen, Fang et al. [16] for solar-electric hybrid drying of lotus seeds, and Hadibi et al. [17] for solar dryer drying of onion slices.

The most preferred method for drying agricultural products is hot air drying [18]. However, high temperature, microwave power, sample thickness, and long drying time cause adverse effects on quality parameters, such as nutritional values, shrinkage [19], color [20], and rehydration capacity of the products [21]. New drying technologies have been developed to energy consumption reduce and environmental influence [22]. Combined drying (infrared-hot air and microwave-hot air) are relatively new methods in which heat transfer is done through hot air flow and infrared or microwave radiation. In this way, these dryers have the merits of both individual methods and usually reduce the drying duration and SEC and increase the quality of the final product [23].

Several research has been done on determining the functional characteristics of different dryers and the effect of these parameters on the quality properties, drying time and energy consumption of agricultural products, such as moisture content (MC), color and RR for ‘Ankara’ pear in an electrohydrodynamic-hot air [18], MC and bulk density for pear in a ventilated drying chamber [24], drying time and SEC for soybean in a fluidized bed drying [25], drying time and SEC for cherry fruits in an air convective [26], MC, color, and water activity for pear in an air convective [27], moisture ratio, water activity, color, antioxidant activity and total phenolic content for “Rocha” pear in a tray dryer with MW and ultrasound pre-treatments [28], MC, color, SEC, RR for pear in an MW-vacuum [29], moisture ratio, RR, SEC, water activity, antioxidant activity, color, and total phenolic content for pear in an IR and mid- IR-freeze [30], and drying time and SEC for peppermint leaves in MW, IR and CV dryers [31], drying time, SEC and GHG for dog-rose [32] and Pistacia Atlantica [33] in hybrid drying, including MW, IR and CV. However, few studies have investigated the influence of simultaneous microwave and infrared powers, drying temperatures, and sample thickness on specific energy consumption (SEC), color change, rehydration, shrinkage, and GHG. In this study, (I) pear samples were dried in five different drying systems at different sample thicknesses, temperatures, microwave and infrared powers, (II) vital quality properties, such as color change, shrinkage, and rehydration of the dried pears were determined, (III) drying time, SEC and greenhouse gas emission products were obtained according to different types, and (IV) determined properties were comparatively evaluated in detail.

Materials and methods

Sample preparation

Pear fruits (Pyrus communis L.) were hand-picked from an orchard located in Sardasht City, West Azerbaijan Province, Iran. All pear samples were regularly stored in the refrigerator at a temperature of 3–5 °C to reduce the rate of respiration and physiological and chemical changes. The large size of this figure made it possible to prepare cubic samples with almost the same quality. Prototypes were prepared to measure initial moisture content (MC) with dimensions of 0.4 × 3 × 3 cm. These samples had no core and, skin. Forty g of samples with three repetitions were used to determine the initial MC. Pear samples were placed in an oven (Memmert, UFB 500, Germany) to measure initial MC. The initial MC was measured by the standard [34] method (24 h at 70 °C) and using Eq. (1) [18]. The initial MC of pear slices was 6.11 on a dry basis. Before performing the drying test, the samples were placed at a temperature of 23 °C and a relative humidity of 30% for one hour to reach the same equilibrium conditions [17, 35].

$$MC_{d.b} = \frac{{m_{t} - m}}{m}$$
(1)

Dryers

To perform the experiments, the laboratory dryers available in the department mechanical engineering of biosystem of Mohaghegh Ardabili University, Iran, which were previously designed and built by the research team, were used. For more access to the details of these dryers, refer to their main sources [36, 37]. Dryers have sections of airflow generation (fan), hot air temperature control system, microwave power control system, and infrared lamp control. The air passes through 3 heating elements before entering the dryer compartment for heating. 2 thermostats were applied to control the temperature of the hot air entering the dryer compartment in the hot air flow path in the dryers. Also, two magnetrons (LG2M246, China) were used to generate microwaves and 4 lamps were used to generate infrared power. Of course, it should be mentioned that the rotary part of the dryers was not used in this research. After applying the desired settings on each dryer in each experiment, 100 g of prepared samples were placed in a thin layer on the mesh tray inside the dryer. The tray was connected to a digital scale (GF-600, Japan) that took the mass data of the drying samples at 3 min intervals, which was used to draw the kinetic curves of moisture loss and drying time. The dryers were turned on for about an hour before drying experiments to achieve stable conditions before each drying period.

Convective dryer

To investigate the effect of dryer air temperature on the product, three temperatures, and three thicknesses were selected to dry pears in a convective air dryer. According to the conducted studies, the air temperature of the dryer was 50, 60, and 70 °C [38, 39]. The thickness of the samples used for drying pears were 2, 4, and 6 mm. This thickness range has been used and suggested in past studies [40, 41]. Also, the air velocity in these experiments was chosen as 1 m/s.

Infrared dryer

A total of 100 g of sliced pear samples were placed on a perforated tray (30 × 30 cm) inside an IR dryer containing four 250 W IR lamps. In this research, the IR power was chosen at three levels of 500, 750, and 1000 W, and the thickness of the samples was 2, 4, and 6 mm. Also, the distance of the sample from the lamp was kept constant at 10 cm. Then the mass and time data were measured during the drying process until reaching the final moisture content.

Microwave dryer

A microwave device (Sharp R-I96T, Thailand) was used, and after placing the samples in it at powers of 270, 450, and 630 W and thicknesses of 2, 4, and 6 mm, they were dried until reaching a specific weight. During the process of changing the weight of the samples, it was recorded every 30 s through a digital scale (GF-600, Japan). This device has an internal chamber with dimensions of 54 × 34 × 45 cm3 cubic and is made of a rotating glass plate with a diameter of 28 cm. The microwave power can be changed in the range of 90–900 W.

Combined microwave-convective (MW/CV) dryer

To perform experiments on drying pears in MW/CV and IR/CV methods, first, the samples were cut into pieces of 3 × 3 cm and 4 mm thickness. After preparing the dryer and adjusting it to the desired temperature and microwave powers, pear pieces were placed in a special tray and placed in the dryer chamber. Pear samples of 100 g were spread evenly on the tray to absorb microwave energy uniformly. Experiments were performed at three temperature levels of 50, 60 and 70 °C and three MW power levels of 270, 450, and 630 W.

Combined infrared-convective (IR/CV) dryer

Pear slices were used for irradiation by IR/CV system immediately after cutting. In this dryer, it is possible to change the airflow speed, air temperature, distance, and lamp power. The speed of the airflow was 1 m/s and the distance of the samples from the lamp was 10 cm. To dry the cut samples for the IR/CV system, variables, including hot air temperature at three levels of 50, 60, and 70 °C and infrared lamp powers at three levels of 500, and 750, and 1000 W were used.

Drying time

The time taken to reduce the MC of pear pieces during the drying process with different dryers from the initial moisture content [6.11 (d.b.)] to the final MC of 0.11 (d.b.) was known as the drying period [42].

Shrinkage (SR)

The amount of shrinkage of the samples was measured by the liquid displacement method (toluene). First, 10 dried pear samples were randomly selected and placed inside a glass pycnometer containing toluene, and the amount of fluid displacement, which is the volume of the pieces, was recorded [43]. This process was done in 3 repetitions for each test. Finally, the shrinkage rate of the product was obtained from Eq. (2) [44, 45].

$$SR(\% ) = (1 - \frac{{V_{f} }}{{V_{i} }}) \times 100$$
(2)

Rehydration ratio

One of the quality indicators used to evaluate the dried product is the rehydration ability. After finishing the drying process of pear slices, 4 samples were taken out of the dryer and immersed in 100 mL of distilled water at 20 °C for one hour [34]. The rehydration ratio of dried samples is obtained from Eq. (3) [46]:

$$RR = \frac{{ \, R_{r} \, }}{{ \, R_{d} }}$$
(3)

Color properties

The drying process affects the surface properties of food and as a result changes the ability to reflect light and the color of food [47]. The drying temperature has a significant effect on the color of agricultural products. Measurement of color changes is used to predict chemical changes and product quality [48]. A calorimeter (HP–200, China) was used to check the color of the dried samples. Fresh and dried pear slices were randomly measured from different parts (5 points). The measurement results were expressed based on L*, a*, and b* indices. The following equation was used to determine the color difference created from the fresh pear sample (ΔE) during the drying process [49].

$$\Delta E = \sqrt {(L_{o}^{*} - L_{t}^{*} )^{2} + (a_{o}^{*} - a_{t}^{*} )^{2} + (b_{o}^{*} - b_{t}^{*} )^{2} }$$
(4)

where, the subscripts o and t correspond to the color of fresh pear and the color of dried pear, respectively. L*, a*, and b* indicate brightness (0 ~ 100), red/green (−60 ~  + 60), and yellow/blue (−60 ~  + 60), respectively [50, 51]. Fresh pear tissue was used as a reference. An increase in the value of ΔE indicates more color changes between the reference sample and the dried sample.

Specific energy consumption

The energy used in the drying process is vital for production processes in the industrial and domestic sectors. SEC represents the specific energy consumption (J) to remove one kilogram of water from the product. SEC during drying using CV (SECCV) is obtained by Eq. (5) [52]:

$$SEC_{CV} = \frac{{A.V_{a} .\rho_{a} .C_{a} .\Delta T.t_{CV} }}{{m_{CV} }}$$
(5)

To calculate the SEC in the drying methods using MW (SECMw) and IR (SECIR), Eqs. (6) and (7) were used, respectively [53].

$$SEC_{MW} = \frac{{P_{MW} .t_{MW} }}{{m_{MW} }}$$
(6)
$$SEC_{IR} = \frac{{P_{IR} .t_{IR} }}{{m_{IR} }}$$
(7)

Energy consumption in combined methods of MW/CV (SECMW/CV) and IR/CV (SECIR/CV) was obtained from Eqs. (8) and (9), respectively [32].

$$SEC_{MW/CV} = SEC_{MW} + SEC_{CV}$$
(8)
$$SEC_{IR/CV} = SEC_{IR} + SEC_{CV}$$
(9)

Greenhouse gases

Energy is used in various forms in industries, one of the most common types of which is heat energy, which requires a lot of money to provide. One of the main uses of thermal energy is to use it to dry agricultural products. The main source of energy used in industrial dryers is fossil fuels, which are a crucial source of energy in the world. In addition, these fuels are the main cause of the world’s environmental problems, including atmospheric pollution and global warming. To calculate the number of greenhouse gases and pollutants per kilowatt of electricity produced in different power plants, the coefficients related to the production of different gases were used, which were obtained by certified organizations [54]. Figure 1 shows the average coefficients per kilowatt of electricity production in Iran’s power plants and different fuels [55].

Fig. 1
figure 1

Mean pollution rate of power plants by fuel typ

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Statistical analysis

Statistical analysis of the data obtained from the effect of drying temperature, MW power, IR power, and thickness of pear slices on drying time, color, shrinkage and RR, SEC, and GHG using a factorial experiment in the form of a Tukey HSD (honestly significant difference) multiple comparison tests were performed SPSS software v20.0. In order to test the significant differences in the present study, the Post Hoc test was performed with the Tukey HSD test at a 95% confidence interval. Significant differences are shown here for both between and within groups.

Results and discussion

Drying time and SEC

Table 1 presents drying time and SEC. Effects of drying temperature and thickness on drying time (DT) and SEC were highly significant (P ≤ 0.01). The shortest drying time among groups for different thicknesses and drying conditions was determined from 70°C for CV, 1000 W for IR, 630 W for MW, 1000 W and 70 °C for IR/CV, 630 W and 70 °C for MW/CV. In addition, the shortest drying time among all dryers was 35.00 min in 70 °C air temperature at MIC 630 W. The longest drying time was obtained at 6 mm thickness at CV 50 °C with values of 618.33 min. It was about 13.25 times among the systems. It was determined that the shortest DT was in the MW/CV system and the longest DT was in the CV system.

Table 1 Drying time and SEC in different condition
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The highest SEC values for CV, IR, and MW were at 50 °C, 500 W, and 270 W at 6 mm thickness, with values of 267.61, 204.64, and 87.03 MJ/kg, while the lowest SEC values were 94.54, 85.36, and 28.33 MJ/kg at 70 °C, 1000 W, and 630 W at a sample thickness of 2 mm, respectively. EL-Mesery and Elabd [53] compared CV, MW and IR dryers to calculate the drying energy of okra. They found values that ranged from 2.75 (in MW dryer) to 56.93 MJ/kg (in CV dryer) while drying okra and their values showed a decreasing trend with an increase in air temperature, MW power and IR power within the range of 40–60 °C, 200–600 W, 1000–4000 W/m2. The MW energy penetrates into the pear samples. Then it produces a polarity in water molecules of the material and heats it up. As a result, it significantly improves the heating rate and uniform heat distribution of the material. Consequently, it reduces the required time and energy of the process [56]. Moreover, the increasing IR power, reasons a rise in evaporation rate while the drying time and the SEC decreased for the drying of pear [57]. Abbasi et al. [58] used different levels of MW power 450, 600 and 750W and with thicknesses (5, 7, 9, and 11 mm) for drying of Myrtus communis L. in an MW dryer and found that the SEC values varied between 6.98 and 18.13 MJ/kg for all the drying conditions. They stated that by increasing the MW power and decreasing the thickness of the samples, the SEC decreased. They pointed out that the transfer of moisture from the inside to the surface of thinner samples will be done faster, which will reduce the drying time and specific energy consumption.

Çetin and Sağlam [23] reported similar drying times for apple drying at 200 W (27 min) microwave, 70 °C convective (480 min), 100 W + 60 °C (120 min) hybrid (MW + CV) dryers. Polat and İzli [18] indicated drying time for 'Ankara' pear samples at different air velocities, voltages, and constant temperatures in electrohydrodynamic-hot air between 90 and 150 min at 1.5 m/s, and between 80 and 130 min at 2.5 m/s, respectively. The moisture firstly removes away from the surface of the sample and secondly towards the surface (rest of the moisture). Araujo et al. [59] reported SEC values of pear drying between 10.28 and 20.79 kWh/kg, at IR dryer. Taskin et al. [29] found SEC values for “Deveci” pear between 1 and 1.9 MJ/kg in hybrid (MW- vacuum). Antal et al. [30] Calculated SEC value for pear drying 4.5 and 339.3 MJ/kg in hybrid dryer. The present findings are also consistent with the values of 6.68 to 55.93 kWh/kg applied for edamame [60], 2.75–6.83 MJ/kg in an MW dryer and 15.9–56.93 MJ/kg CV dryer for okra [53] and 4.32–318.42 MJ/kg for Roman chamomile [61].

In the hybrid systems, the highest SEC values for IR/CV and MW/CV were 119.02 and 67.35 MJ/kg at 500 W –50 °C and 270 W –50 °C, and the lowest values were 45.71 and 20.25 MJ/kg at 1000 W –70 °C and 630 W –70 °C, respectively. Higher SEC is directly related to longer drying times, temperatures, and powers. The present findings showed that combined (MW/CV and IR/CV) drying consumes less energy than single (MW, IR and CV) drying methods. Such a case was attributed combined energy consumption of both temperature and microwave power outputs [62]. Many authors, including EL-Mesery and El-khawaga [63], Kaveh et al. [64], Wang et al. [65] and Ye et al. [57] for drying tomato, green pea, chrysanthemum, and mint showed that combined drying requires less energy consumption compared to single dryers, which is consistent with the results of this research.

Shrinkage, rehydration ratio (RR) and color properties

Table 2 presents the shrinkage, rehydration, and color properties of the dried pear. The effects of drying temperature and thickness on shrinkage were significant (P ≤ 0.01). The highest shrinkage values of CV, IR, and MW were in 2 mm thicknesses, and 50 °C (72.53%), 500 W (68.63%), and 270 W (66.21%), while the lowest values were in 4 mm thicknesses and 60 °C (54.49%), 750 W (53.05%) and 450 W (49.79%), respectively. The results showed that when the thickness of the samples increased from 2 to 4 mm, the shrinkage decreased, but when the thickness of the samples increased from 4 to 6 mm, the shrinkage increased, which means that the thickness of 4 mm had the lowest amount of shrinkage in all conditions. Also, according to the results, the thickness of 2 mm had the highest amount of shrinkage because the difference in water content between the center and its surface is less. Therefore, the shrinkage of the material is more intense [58]. Also, crust development, which hinders moisture loss, occurs slower at lower thicknesses and has a greater impact on the pear sample shrinkage.

Table 2 Shrinkage, rehydration and color properties
Full size table

The lowest shrinkage among the individual dryers (CV, IR, and MW) appeared to occur under MW drying. The lower shrinkage of pears during the MW drying process may be due to radio frequency drying, which causes higher internal pressure at the beginning of the drying stage and faster evaporation, resulting in cell swelling [53]. Similar results were obtained by Kaveh et al. [66] for green pea. EL-Mesery and Elabd [53] reported that shrinkage of MW samples was found to be significantly less than CV and IR dried samples.

In the hybrid systems, the highest shrinkage values for IR/CV and MW/CV dryers were obtained as 63.47% and 60.50% at 500 W –50 °C and 270 W –50 °C, respectively. The lowest values were obtained as 47.39 and 45.34% at 750 W –60 °C and 450 W –60 °C, respectively. Similar to the present study, Guiné [24] stated the shrinkage value of small slices of pear pulp with the value of 75.00% in a ventilated drying chamber. Differences may vary depending on the drying system and product shape [54, 67]. This result agrees with Witrowa-Rajchert and Rzaca [68] in drying of apples using, CV, IR/CV, and MW/CV. An et al. [60] observed that the shrinkage of the edamame dried with MW/CV was lower than CV and MW, which was consistent with the conclusion of this study.

Rehydration is associated with changes in the structure of plant cells and tissues, resulting in collapse and shrinkage during drying [69]. In this study, the lowest RR for non-hybrid systems was recorded in the samples dried at 2 mm thickness and 50 °C (2.80), 500 W (2.97), and 270 W (2.85), while the highest values were in 4 mm thicknesses and 60 °C (3.54), 750 W(3.66) and 450 W (3.77), respectively. Liu et al. [70] reported the RR values of pear slices as 2.66 in single hot air drying at 45 °C. Antal et al. [30] stated that the RR values of pears dried in mid-IR- freeze dryer were between 2.48 and 3.47. According to the results obtained in all thicknesses, the RR in the CV dryer was less than other dryers (MW and IR). EL-Mesery and Elabd [53] reported that MW dried okra pods had the best rehydration property. CV samples formed a sealed structure, and the RR of the sample is weak [71]. The reason may be that the longtime of exposure to heat in CV destroyed the inherent elasticity of cells, thus decreasing the RR. Similarly, the inferiority of CV in improving the RR was also reported in dried sweet corn [71] and taro slices [72]. In addition, by increasing of thickness from 2 to 4 mm and temperature from 50 to 60 °C, the RR increased, but by increasing of thickness from 4 to 6 mm and temperature from 60 to 70 °C, the RR decreased.

In the hybrid systems, the lowest shrinkage values for IR/CV and MW/CV dryers were 2.95 and 3.08 at 500 W –70 °C and 270 W –70 °C. The highest values were 3.88 and 4.25 at 750 W –60 °C and 450 W –60 °C. In the present study, the RR of the dried products decreased at higher temperatures. The reason for this is the collapse of the cellular structure through the drying process and the prevention of water diffusion during the rehydration process [73]. Using a hybrid dryer has a better absorption coefficient compared to all single dryers. EL-Mesery et al. [74] and Nalawade et al. [75] also obtained similar results for garlic and spearmint.

Color is a critical qualitative attribute of foods and is related to internal characteristics and product development [76]. Total color change (ΔE) is an attribute frequently used to measure color in the drying processes of foodstuffs. The highest ΔE color change value was obtained from 6 mm thicknesses and 50 °C (29.35), 1000 W (33.41), and 270 W (32.30), while the lowest value was obtained from 4 mm thicknesses and 60 °C (18.29), 750 W (23.02) and 450 W (18.16), respectively. Consistent with the present findings, Polat and İzli [18] reported ΔE values between 19.06 and 21.12 for dried ‘Ankara’ pear samples in electrohydrodynamic-hot air. Önal et al. [28] reported the ΔE color change values for 'Rocha' pears dried at the microwave (MW) at 50–60 °C between 3.86 and 23.41, respectively. In contrast to the present study, Domínguez-Niño et al. [27] found color change ΔE values for pear in CV drying at different conditions ranged from 6.36 to 15.00. By observing Table 2, it can be seen that the color of the sample CV dryer is lower than IR and MW. Because enzymatic browning reactions and non-enzymatic browning reactions were significantly weakened in CV [77]. Ismail et al. [78] used vacuum sun, MW, IR and CV to dry fresh okra and found HA-processed okra had lower color changes compared to vacuum sun, MW, and IR processed samples also reported similar results. The results of our study were consistent with the study of Osae et al. [79] who found that color change of CV dried ginger slices was less than Freeze, IR, and MW dried ones.

The highest color change values in IR/CV and MW/CV hybrid dryers were 31.14 and 24.02 at 2 mm thickness at 500 W –50 °C and 270 W –50 °C. The lowest values were 17.64 and 15.51 at 750 W –60 °C and 450 W –60 °C at 4 mm thickness. The changes in color during drying can be attributed to Maillard reactions, color pigment degradation, and enzymatic browning [80]. From the total color difference ΔE, the MW/CV group had the lowest ΔE value, followed by CV, and MW, IR/CV and IR had the largest ΔE values. This indicates that MW/IR was closest to the color of fresh pear slices. Although MW/IR used microwaves and air temperature as the heat source, continuous microwave heating may lead to a large diffusion of internal moisture to the surface, However, due to the short drying time and physiological and biochemical reaction time [81], the color difference between the dried sample and fresh pear slices was small.

Greenhouse gases for CV, IR, and MW dryers

Figure 2 shows the results of GHG emissions during the drying of pear samples using CV. The maximum value of CO2 was determined with the value of 280.45 kg/kg water from the GT-GO power plant at 50 °C (6 mm thickness). Moreover, the lowest amount (42.54 kg/kg water) was achieved from the CC-NG power plant at 70 °C (2 mm thickness). The GT-GO power plant (1.55 kg/kg water) had the highest amount of NOx (50 °C and 6 mm thickness), and the GT-NG power plant (0.18 kg/kg water) released the lowest NOX emissions (70 °C and 2 mm thickness).

Fig. 2
figure 2

The effects of temperature and thickness variations in convective (CV) dryer on CO2 and NOx

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GHG (CO2 and NOx) of dried samples at higher drying temperatures were less than those dried at lower drying temperatures. At higher temperatures, due to the higher thermal gradient, the rate of moisture evaporation from the product increases and the product dries in a shorter period of time [10]. Many researchers have shown that the GHG decrease with increasing air temperature, among them Beigi et al. [82] for drying paddy, Fabani et al. [83] for drying onions, Taghinezhad et al. [13] for drying blackberry and Karimi et al. [11] for drying canola seeds. CO2 and NOX emissions decreased proportionally with an increase in Rehydration decreased proportionally with an increase in thickness samples from 2 to 6 mm. Seyfi et al. [10] a comparison between GHG of Aloe vera gel drying at different drying thicknesses (1 to 3 mm) investigated which the results showed a downward trend with decreasing thickness.

As shown in Fig. 3, the GHG formed by an IR dryer decreases with increasing IR power and with decreasing sample thickness. The GT-GO (214.46 kg/kg water) and SP-HO (209.75 kg/kg water) power plants for 500 W and 6 mm thickness had the highest CO2. The minimum value of CO2 (38.14 kg/kg water) was when NG was used as fuel in the CC at an IR power of 1000 W and a product thickness of 2 mm. The maximum of NOX was 1.18 kg/kg water in a dryer working at IR power 500 W and 6 mm thickness when powered by a GT-GO. The minimum NOX was 0.16 kg/kg water from a GT-NG power plant with a dryer operating at 1000 W and 2 mm thickness. Any increment in infrared power resulted in a reduction in GHG. The rise in IR power reasons an increase in the temperature of the pear sample and then the rate of evaporation [84]. The increasing of rate evaporation causes a decrease in the drying period and energy consumption ultimately GHG (CO2 and NOX) emissions [32]. Similar findings were reported by Kaveh et al. [64] for green peas and Motevali et al. [85] for Aloe vera. Motevali and Koloor [32] concluded that increasing IR power reduces SEC and GHG (CO2 and NOX) emissions. They also stated that IR waves can penetrate the sample and cause an increase in the temperature of the product and more evaporation of its moisture [32, 86].

Fig. 3
figure 3

The effects of IR power and thickness variations in infrared (IR) dryer on CO2 and NOx

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Figure 4 shows GHG levels for MW treatment. As shown, the greatest CO2 levels were recorded for the SP-HO (89.21 kg/kg water) and GT- GO (91.21 kg/kg water) power plants when drying at 270 W with a sample thickness of 6 mm. The lowest CO2 level was 12.75 kg/kg water in the CC-NG power plant at 630 W MW power with a sample thickness of 2 mm. The highest NOX emissions (0.50 kg/kg water) were found for the GT- GO at 270 W and 6 mm thickness. The lowest NOx (0.05 kg/kg water) belonged to the GT- NG for drying at 630 W and a sample thickness of 2 mm. Additionally, the increments in MW power led to reduced CO2 and NOX emissions, while the increases in sample thickness led to increasing CO2 and NOX emissions. This indicates that the high MW power results in less energy required for moisture removal of the pear samples leading to a decline in GHG emission [33]. Petković et al. [87] also showed that reducing thickness and increasing MW power and leads to decreased CO2.

Fig. 4
figure 4

The effects of power and thickness variations in microwave (MW) dryer on CO2 and NOx

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Greenhouse gases for IR/CV and MW/CV dryers

Figure 5 shows the emission levels of GHG for a hybrid IR/CV dryer. The maximum amount of CO2 emissions was from the SP- HO (121.99 kg/kg water) and GT-GO (124.73 kg/kg water) power plants for drying with an IR power of 500 W at 50 °C, while the minimum result (20.57 kg/kg water) was observed in the CC- NG for drying at 70 °C using 1000 W. The GT-GO had the maximum NOX of 0.69 kg/kg water when drying at 50 °C and 500 W, while the GT- NG power plant had a minimum NOX of 0.08 kg/kg water at 70 °C and 1000 W. The lowest CO2 and NOx was obtained for blackberry drying in combined IR/CV dryer 42.53 kg/kg water and 0.18 kg/kg water [13], which was higher than the values of this study because pear drying requires less drying time and less energy for drying. However, the lowest CO2 for Aloe vera [85] and green peas [64] was 4.72 and 16.5 kg/kg water, respectively, which was lower than this study due to less time and less energy for drying them.

Fig. 5
figure 5

The effects of power and temperature variations in infrared-convective (IR/CV) hybrid dryer on CO2 and NOx

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Figure 6 shows the GHG [CO2 and NOX] emissions for using the MW/CV dryer to dry 1 kg of the pear sample. It presents that the CO2 is reduced with rising MW power and air temperature. Accordingly, its maximum CO2 in ST-HO power plant (69.03 kg/kg water) was at 50 °C and 270 W, and its minimum values in CC-NG power plant (9.11 kg/kg water) were at 70 °C and 630 W. The GT-GO released its greatest quantities of NOX (0.39 kg/kg water) when drying at 50 °C and 270 W. As a result, the GT-NG provided the lowest amount (0.04 kg/kg water) for drying at 70 °C and 630 W. Considering that the energy consumption at 630 W and 70 °C was lower than the other drying conditions, this led to a reduction in greenhouse gas emissions. As the drying temperature increases and MW power, the thermal gradient will increase, which can improve the moisture released from pears and reduce the drying time. Hence, GHG emissions will be lower [61]. In the study of Kaveh et al. [33] the amount of CO, and NOx in the drying of the Pistacia Atlantica in an MW/CV dryer was between 2.70 and 29.68 kg/kg water and 0.01–0.16 kg/kg water, respectively. Motevali and Koloor [32] showed that the amount of CO2 (from 0.09 to 2.91 kg/kg water) and NOx (from 0.002 to 0.016 kg/kg water) decreased by increasing drying temperature and MW power for drying dog rose by MW/CV dryer. At higher temperatures and IR power, the GHG emission decreased due to the heating effect of IR radiation, as the increase in IR power reduces energy consumption; this, in turn, shortened the drying time [32, 33, 74]. In addition, rapid evaporation reduces drying time and SEC, which in turn decreases GHG emissions [31, 32].

Fig. 6
figure 6

The effects of power and temperature variations in microwave-convective (MW/CV) hybrid dryer on CO2 and NOx

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Compared with other fossil fuels, NG is a cleaner and less polluting resource. The results of burning NG, GO, and HO as fuel in various power plants are analyzed. It has been shown that NG could significantly decrease GHG (CO2 and NOx) emissions from power plants. According to Figs. 2–6, the highest emission production of the GT power plant under all conditions was due to burning diesel fuel. Results from GHG (CO2 and NOx) emissions of the CC power plant (Figs. 2–6) revealed that, compared to other power plants, this plant has lower GHG (CO2 and NOx) emissions. In all dryers, the highest pollution level was at the lowest temperature, MW power and IR power. The highest emissions in the CV dryers occurred at 50 °C. The reason for the high CO2 and NOx emission in CV dryer is the need for high energy due to the use of both conduction and convection heat transfer [64, 84]. The study results showed that the MW/CV dryers could have the lowest energy consumption and GHG (CO2 and NOx) emissions. The microwave penetrates the product in an MW and MW/CV dryer, polarizing the moisture content. This causes a rapid rise in temperature and a rapid fall in humidity, which lowers SEC and GHG emissions [64].

Conclusion

The present study investigated the drying time, energy, shrinkage, rehydration, color, and greenhouse gas emission parameters of pear drying in five various dryers, CV, IR, MW, IR/CV, and MW/CV. Based on the study, it was obtained that the SEC showed a decreasing trend at increasing temperature-power and decreasing sample thickness. It was found that shrinkage values were higher at low sample thicknesses and temperature powers. It was also determined that the rehydration rate of the mid-drying parameters was higher, and the color change was less in the groups. The CV dryer operating at 50 °C with a sample thickness of 6 mm produced the highest GHG emissions, specifically CO2 and NOX. Using a higher sample thickness can increase greenhouse gas emissions. In conclusion, SEC and GHG emissions can be minimized when the MW dryer operates at 630 W power. The present findings can be used in dryer design and to produce quality products. It can be seen from the experimental observations that compared with MW/CV and CV, MW, IR and IR/CV equipment operating at MW/CV-630 W –70 °C brought relatively lower energy consumption and less greenhouses gas (CO2 and NOx) emissions on the basis of faster drying rate and higher drying efficiency. The results of the current study corroborate that MW/CV equipment is a more suitable choice for energy saving, emission reduction and quality assurance of food processing.