Effect of Infrared-Combined Hot Air Intermittent Drying of Jujube (Zizyphus jujuba Miller) Slices: Drying Characteristics, Quality, and Energy Consumption Dimensions

Abstract

The objective of this research was to investigate the effect of infrared-combined hot air intermittent drying (IIRHAD) on energy consumption, drying characteristics, and the quality of jujube slices. The water content of jujube slices decreased from 0.267 g/g to 0.05 g/g during the experiment, and the infrared heating plate’s temperature was fixed at 70 °C while the hot air temperature was fixed at 50 °C. Nine different intermittent ratios were used to dry jujube slices, and the results showed that intermittent treatment had varying effects on drying characteristics, energy consumption, and quality. In comparison to infrared-combined hot air drying (IRHAD), the effective drying time of red jujube slices was reduced by 40 to 100 min, the energy consumption decreased by 11.91% to 34.34%, and there were also varying degrees of improvement in the quality indicators. It was discovered that excessively long or short active drying and tempering periods had a negative impact on the drying process. Therefore, these factors should be further broken down and improved in the future. This research holds great importance for the future advancement and widespread use of IIRHAD in fruit and vegetable materials.

1. Introduction

Chinese jujube (Zizyphus jujuba Mill.), belonging to the plant family Rhamnaceae, is one of China’s traditional fruit specialties. The jujube fruit contains a variety of nutrients, including polysaccharides, proteins, ascorbic acid, cyclic AMP, and phenolics [1,2]. Regretfully, fresh jujube fruits ripen quickly after being harvested and cannot be kept in storage for longer than 10 days in natural environments [3]. Therefore, it is crucial to investigate jujube preservation techniques in order to increase its shelf life.

Hot air drying is a popular traditional drying method that is inexpensive, simple to use, and appropriate for large-scale manufacturing. However, it has a number of disadvantages, including low drying efficiency, high energy consumption, subpar product quality, and others [4]. As compared to hot air drying, IRHAD uses the special heat transfer properties of infrared radiation to irradiate the material’s surface. The radiant energy that enters the material’s interior is absorbed by the molecules, causing them to vibrate violently, raising the material’s temperature and producing the drying effect. As a result, the material’s drying speed and quality are strengthened. It has been applied to the drying of many fruits and vegetables [5,6,7]. However, drying is a very energy-intensive process, accounting for about 15% of the total industrial energy consumption [8]. Under the current global trend of energy saving and emission reduction, all countries are looking for more energy-saving and efficient drying technology [9]. When drying fruits and vegetables, the drying quality is also crucial. For example, using an excessively high temperature can speed up the drying process but also cause color browning, nutritional loss, texture degradation, and other issues [10,11,12]. Therefore, finding a balance between drying efficiency, energy consumption, and quality is the key research direction of drying technology nowadays.

Intermittent drying involves resting periods or tempering between predefined phases of the drying process. As an emerging drying technology, it can realize the control of the drying process by changing the input of drying energy [13]. Intermittent drying is composed of a series of cycles, each of which includes an active drying time and a tempering period in which the material exits the drying environment. It has been proven that throughout the tempering period, moisture diffuses from the inside of the material to the surface, driven by a humidity gradient, resulting in moisture redistribution [14]. The moisture redistribution phenomenon causes the material’s internal moisture to naturally migrate to the surface, improving the drying rate during the following active drying time. The surface of the material has less moisture than its interior, making it easier to remove. [15]. Depending on the humidity difference between the material and the environment at the time of retarding, a small amount of water evaporation or reabsorption phenomenon will also accompany the tempering period [16]. The results of previous studies have shown that the energy consumption in the drying process depends largely on the material processed, parameter settings, drying method, retardation time, and environmental conditions [17,18,19].

Intermittent drying can effectively reduce the drying energy consumption and improve the drying quality, which is a promising drying technology; however, there is a lack of research on the infrared hot air intermittent drying of fruit and vegetable materials. Therefore, this study adopts different intermittent rates (IR) for intermittent drying of red jujube slices and compares them with conventional IRHAD to investigate the effects of IIRHAD on the drying characteristics, energy consumption, and quality. This will provide references for the further optimization and promotion of the intermittent drying process for fruits and vegetables, with the goal of further improving the drying rate, reducing the drying energy consumption, and improving the drying quality based on IRHAD.

2. Material and Methods

2.1. Materials

Jujube with an undamaged appearance and consistent color, shape, and size were purchased from the local farmers’ market in Shihezi City, Xinjiang, China, and stored in a refrigerator at (4 ± 1) °C. The material was taken out at the start of the drying test, cleaned, dried with absorbent paper, and pitted before the jujubes were cut into thin slices with a thickness of 0.3 cm and a diameter of 2.2 cm with a slicer. The average dry basis moisture content of the samples was determined to be 0.267 g/g [20].

2.2. Drying Equipment

In this study, an infrared hot air combined drying test platform (made by Shihezi University) was used to carry out the drying test. A carbon fiber heating plate was used as the infrared heat source, the heating plate was 11 cm away from the material, the wind speed was 1.3 m/s during drying, and the temperature of the infrared heating plate was controlled by using the console. Figure 1 shows the equipment used in this test.

2.3. Procedure

In this study, a 3 × 3 factorial drying scheme was used [21] with three active drying periods (20, 40, and 60 min) and three intermittent times (20, 40, and 60 min) comprising nine IRs. There was also a continuous drying group with 0 intermittent time. To facilitate illustration, the experimental groups were assigned numbers, as indicated in

Table 1. Intermittent drying time and energy consumption reduction rate of jujube slice.

Group Number	Drying Methods	Effective Drying Time (min)	Total Drying Time (min)	Reduction Rate of Energy Consumption
1	IRHAD	220	220	0
2	IR = 20:20	180	340	12.74%
3	IR = 20:40	160	440	21.88%
4	IR = 20:60	180	660	11.91%
5	IR = 40:20	160	220	25.76%
6	IR = 40:40	120	200	34.34%
7	IR = 40:60	160	340	27.42%
8	IR = 60:20	180	220	19.39%
9	IR = 60:40	180	260	21.32%
10	IR = 60:60	180	300	18.83%

, and the experiment was repeated three times for each group. It should be noted that previous studies have found that the drying quality of jujube is better when the hot air temperature is 50~60 °C [22,23]. Considering the heat dissipation of the infrared plate, the hot air temperature should not be very high, so the hot air temperature in this experiment is fixed at 50 °C, and the temperature of the infrared heating plate is fixed at 70 °C. Figure 2 shows the principle of moisture redistribution in infrared hot air intermittent drying from a visual perspective.

Before the experiment, the dryer was preheated to the target temperature and operated for 15 min to obtain a stable state. Next, a stainless-steel metal mesh tray containing 100 g of material was equally spread out and placed inside the drying chamber. The tray was then taken out during the slow recovery period, and the dryer was left to stand at 25 °C with a 60% relative humidity and no wind. The material should be withdrawn during tempering, and each time it is taken out of the drying chamber and returned, its current weight should be measured. The materials were weighed using a high-precision electronic scale (model SAM30, KIMO, Paris, France). When the moisture content of the dry base of the material was less than 0.05 g/g, the drying was stopped, and the material was sealed and packed after natural cooling.

2.4. Drying Characteristics Analysis

The drying characteristics curve of the jujube flakes used the image of dry basis moisture content with drying time (M_t). The dry basis moisture content of jujube slices at moment t was calculated as follows [3]:

$${\mathrm{M}}_{\mathrm{t}}=\frac{{\mathrm{W}}_{\mathrm{t}}-{\mathrm{W}}_0}{{\mathrm{W}}_0}$$

The drying rate (DR) is calculated as follows:

$$\mathrm{DR}=\frac{{\mathrm{W}}_{\mathrm{t}2}-{\mathrm{W}}_{\mathrm{t}1}}{{\mathrm{t}}_2-{\mathrm{t}}_1}$$

where M_t is the dry base water content, g/g, W_t is the t moment of the total mass of the material, g, t is the drying time, min, and W₀ is the dry material mass, g.

2.5. Energy Consumption Analysis

Energy consumption is measured using a high-precision metering socket (model DL333502, Deli Group, Ningbo, China). The power reading is zeroed using the zero function before measurement and placed on the socket, after which the drying test bench’s power plug is inserted into it, and the current power consumption (KW·h) is displayed in real-time after start-up.

2.6. Color Evaluation

The color was measured using a colorimeter (model SMY-2000SF, Beijing Mingyang Science and Technology Development Co., Ltd., Beijing, China). The ideal L*, a*, and b* values of fresh pieces were considered as references in the following array: L* represents lightness, b* represents yellowness and blueness, and a* represents redness and greenness. Before the measurements, calibration was done three times using a standard whiteboard. The colors of the fresh samples were measured to obtain three ideal parameters of L₀, a₀, and b₀ values, and the color difference was described as the color changes according to Liu et al. [5]. The specific formula is as follows:

$$\Delta \mathrm{E}=\sqrt{{\left({\mathrm{L}}^{*}-{\mathrm{L}}_0\right)}^2+{\left({\mathrm{a}}^{*}-{\mathrm{a}}_0\right)}^2+{\left({\mathrm{b}}^{*}-{\mathrm{b}}_0\right)}^2}$$

2.7. Non-Enzymatic Browning Index

Non-enzymatic browning index analysis was performed according to a method by Deng et al. [24]. A UV spectrophotometer (model UV-1900i, Shimadzu, Japan) was utilized. For the measurement, 2.0 g of the sample was firstly ground well using 20 mL of distilled water, and the mixture was centrifuged at 10,000 r/min for 30 min at 4 °C, after which the supernatant was loaded into a spectrophotometer dish for the measurement, and the absorbance of the supernatant at 420 nm was taken as the degree of browning.

2.8. Rehydration Ratio

Rehydration ratio (R) analysis was performed according to a method by Shi et al. [4]. A glass containing deionized water was placed into a water bath at 40 °C. Under the condition of constant water temperature, 5 g of dried jujube slices were added to 50 mL of water, and after 30 min, the jujube slices were removed, and the surface was dried with absorbent paper. The rehydration rate was calculated as follows:

$$\mathrm{R}=\frac{{\mathrm{m}}_2}{{\mathrm{m}}_1}$$

where m₂ is the mass of the jujube slices after rehydration, g, and m₁ is the mass of the jujube slices before rehydration, g.

2.9. Ascorbic Acid

Ascorbic acid (A) analysis was performed according to a method by Deng et al. [24]. A total of 1.0 g of jujube powder was weighed into a mortar, 20 mL of 20 g/L oxalic acid solution was added, and the mixture was fixed in an ice bath with 20 g/L oxalic acid solution and shaken. The sample was centrifuged at 8000 r/min for 10 min, and the ascorbic acid content of the sample was calculated by 2,6-dichlorophenol indophenol to reverse the titration method, expressed on a dry basis (mg/100 g DW), which was calculated as follows:

$$\mathrm{A}=\frac{\mathrm{c}·{\mathrm{V}}_1·{\mathrm{V}}_2}{{\mathrm{V}}_3·\mathrm{W}}$$

where A is the content of ascorbic acid, mg/g; c is the concentration of the ascorbic acid standard solution, mg/mL; V₁ is the volume of the ascorbic acid standard solution consumed for the titration of 5 mL of 2,6-dichloroindophenol sodium salt, mL; V₂ is the total volume of the sample solution, mL; V₃ is the volume of the sample solution consumed for the titration of 5 mL of 2,6-dichloroindophenol sodium salt, mL; and W is the dry weight of the taken sample, g.

2.10. Total Phenols

The total phenol content (TPC) was determined according to the method by Ai et al. [25]. A weighed amount of 10 mg of the gallic acid standard was dissolved in distilled water to obtain a solution with a concentration of 100 mg/L, and the configured solution was stored in a light-insulated refrigerator at 4 °C. The absorbance of 1 mL of the extract was measured at 765 nm using a UV spectrophotometer. The total phenol content was calculated as follows.

$$\mathrm{TPC}=\frac{{\mathrm{x}}_{\mathrm{p}}·{\mathrm{V}}_{\mathrm{p}}·{\mathrm{n}}_1}{{\mathrm{m}}_{\mathrm{p}}}$$

where x_p is the experimentally measured mass concentration of the total phenols, mg/mL; V_p is the volume of the extract, mL; n₁ is the dilution factor; and m_p is the dry basis mass of the sample, g.

2.11. Total Flavonoids

The total flavonoid content (TFC) was determined according to the method by Zhang et al. [26]. The absorbance was used as the vertical coordinate, and the mass concentration of the standard solution was used as the horizontal coordinate to draw the standard curve. The absorbance was used to calculate the mass concentration to obtain the regression equation. For the determination, 1.00 g of the material was weighed, and 20 mL of 95% ethanol solution was added in a ratio of 1:20. The mixture was sonicated at 50 °C, 150 W for 40 min, and the clear liquid was extracted with a 0.45 µm organic filter membrane, and the absorbance of 1 mL of the extract was measured by ultraviolet spectrophotometer at 510 nm. The total flavonoid content was expressed as rutin equivalents per gram (mg RE/g DW) and was calculated as follows.

$$\mathrm{TFC}=\frac{{\mathrm{x}}_{\mathrm{f}}·{\mathrm{V}}_{\mathrm{f}}·{\mathrm{n}}_2}{{\mathrm{m}}_{\mathrm{f}}}$$

where x_f is the experimentally measured mass concentration of the total flavonoids, mg/mL; V_f is the volume of the extract, mL; n₂ is the dilution factor; and m_f is the dry basis mass of the sample, g.

2.12. Statistical Analysis

Data were analyzed using SPSS 23.0 (SPSS Inc, Chicago, IL, USA), and all data were expressed as the mean and standard deviation of at least three replicate determinations. Analysis of variance (ANOVA) was performed at the p < 0.05 level to determine the significance between data.

3. Results and Discussion

3.1. Drying Characteristics

Figure 3 and Figure 4 and Table 1 demonstrate the dry basis moisture content of jujube slices with respect to the complete drying time and effective drying time at different drying interval ratios during IIRHAD, respectively. When compared to IRHAD, intermittent drying saved a variable amount of effective drying time while increasing the overall drying time. The drying characteristics will be discussed below.

When the active drying period is 20 min, the effective drying time required for a single tempering of 20 min (Group 2) is the same as that required for a tempering of 60 min (Group 4), as seen in Table 1, and consumes 20 min more than a single tempering of 40 min (Group 3). From Figure 3a, it can be observed that during the initial tempering period, the water content of all the material groups decreased to some extent, and such a phenomenon was also found in the intermittent drying of olives by Jumah et al. [27]. This is because the material’s moisture content is still high at this phase, allowing the sensible heat stored during the initial active drying period to evaporate some of the moisture during tempering [28]. During the subsequent tempering period, as the material’s moisture content decreases, the stored sensible heat’s influence on the remaining moisture diminishes. Additionally, due to the difference in the moisture concentration between the material and the environment, the material will undergo a certain degree of rehydration, leading to an increase in moisture content at the end of the tempering process. This, to some extent, slows down the drying rate [29,30]. However, when the moisture content declines, it becomes more difficult to extract the residual moisture from the jujube slices. A tempering period of 60 min permits moisture to penetrate more fully to the surface, resulting in a faster drying rate in the later stage. This can be seen in Figure 3a and Figure 4a. Group 2 has a consistently lower moisture content than Group 4 throughout the third to seventh active drying sessions. However, as the drying process progresses and the drying rate of Group 2 gradually drops, Group 4 is able to sustain a higher drying rate, allowing it to equalize Group 2’s moisture content in the final two active drying periods. Group 3 had the relatively shortest effective drying time, which was analyzed as having a moderate tempering time, thus sufficient time for moisture redistribution during tempering and relatively weak rehydration.

When the active drying period is 40 min, compared with the active drying period of 20 min, the effective drying time required for tempering for 20 min (Group 5) and 60 min (Group 7) is the same but 20 min shorter. This may be attributed to the longer active drying period being able to remove most of the water in the material faster in the middle of the drying period, reducing the number of tempering cycles and subsequently minimizing the effect of the rehydration phenomenon on the drying efficiency. Tempering for 40 min (Group 6) required the shortest time and was the only batch drying group with a total drying time shorter than continuous drying.

However, during the 60 min active drying period, it was observed that the effective drying time remained consistent across the tempering period levels, while the total drying time increased with longer tempering periods. After 60 min of continuous drying, it was observed that the surface layer of the material had dried, and there was a significant difference between the internal and external moisture concentration. This allows the moisture inside the material to diffuse to the surface rapidly during the tempering period. Since the material’s dry base moisture content at the end of the initial active drying period is only 0.0835, extending the tempering time can facilitate the diffusion of more moisture to the material’s surface layer. However, because of the low remaining moisture content, the subsequent active drying period will quickly remove the moisture diffused to the surface layer, resulting in a small difference in the drying rate at different tempering times. Figure 3c and Figure 4c indicate that intermittent drying occurs during the second and third active drying periods of the moisture content decline curve, which essentially overlap; increasing the tempering phase will just increase the total drying time in vain. It can be shown that for the intermittent infrared-coupled hot air drying of jujube slices, the active drying period is too long and does not adequately reflect the benefits of intermittent drying.

The reduction in the effective drying time suggests that the material requires less time in the heating environment, directly impacting the energy consumption for drying, as will be analyzed below.

3.2. Energy Consumption

In this study, the intermittent method of periodic switching of heat sources is adopted to reduce the drying energy consumption by saving the effective drying time. Combined with Figure 5 and Table 1, it can be seen that the energy consumption under the same effective drying time is basically the same, and the energy consumption decreases with a reduction in the effective drying time, i.e., the effective drying time is positively correlated with the drying energy consumption.

From Table 1, it can be found that relative to continuous drying, the drying energy consumption after adding intermittent treatment has a reduction of 11.91% to 34.34%, which proves the effectiveness of energy saving with intermittent drying. Furthermore, it can be observed that an appropriate increase in the intermittent time or tempering at a lower moisture content generally helps to reduce energy consumption, such as an active drying period of 40 min, where an intermittent time from 20 min to 40 min can be beneficial. Similarly, if the intermittent time is fixed at 20 min, the active drying period from 20 min to 40 min can also lead to reduced energy consumption. These findings align with the results of Filippin and Gan et al. [31,32]. It should be noted that a prolonged tempering time and high moisture content can lead to reduced energy consumption, as explained in the preceding subsection.

In this study, the material leaves the drying environment during the intermittent period, but the drying device still maintains operation. To accomplish the goal of lowering the overall drying energy consumption, this time might be used to dry other materials during the actual production process.

While the effective drying time determines the energy savings, the total drying time also significantly impacts the economics of the dried product. Taking the Group 2 and 3 experiments as an example, although increasing the interval time enhances the energy-saving effect, the total drying time also increases significantly. Excessive drying time may not be suitable for some materials, as it can lead to more serious rehydration and quality degradation phenomena [33]. Therefore, to achieve optimal drying results, the IR should be carefully selected. In addition, intermittent drying allows for the full utilization of heat input to remove the moisture diffused to the material’s surface rather than overheating the product, as in continuous drying. This not only efficiently utilizes the heat energy but also reduces quality deterioration. The next section will discuss the effect of intermittent drying on drying quality.

3.3. Color

Color is an important index to evaluate the drying effect of materials [34]. Degradation of chlorophyll, non-enzymatic browning, and the Maillard reaction are the main reasons for the color change [35]. The browning value (B) represents the degree of browning of jujube slices, and the color difference value (∆E) indicates the overall color changes of the product.

Table 2. Color indexes of jujube slices under different intermittent drying methods.

Group Number	Color Indexes
	L*	a*	b*	∆E	B
Fresh	45.75 ± 0.04 g	5.79 ± 0.02 a	23.62 ± 0.03 a	-	-
1	68.75 ± 0.84 a	3.18 ± 0.12 f	16.88 ± 0.13 g	24.42 ± 0.13 a	0.95 ± 0.08 a
2	62.96 ± 0.71 e	5.08 ± 0.06 c	20.04 ± 0.06 bc	17.89 ± 0.06 f	0.62 ± 0.09 f
3	64.14 ± 0.86 cd	4.74 ± 0.12 d	19.89 ± 0.15 c	19.06 ± 0.03 ef	0.76 ± 0.07 de
4	66.98 ± 0.77 b	4.12 ± 0.06 ef	19.46 ± 0.12 e	22.01 ± 0.09 b	0.89 ± 0.03 b
5	62.08 ± 0.58 ef	5.19 ± 0.02 c	19.67 ± 0.16 de	17.11 ± 0.15 g	0.58 ± 0.05 g
6	61.32 ± 0.45 f	5.32 ± 0.07 b	20.15 ± 0.09 b	16.26 ± 0.04 h	0.55 ± 0.02 h
7	64.08 ± 0.72 d	4.79 ± 0.03 d	19.67 ± 0.08 d	19.08 ± 0.07 ef	0.77 ± 0.03 d
8	64.42 ± 0.61 cd	4.82 ± 0.05 d	19.96 ± 0.14 bc	19.34 ± 0.11 e	0.72 ± 0.04 e
9	65.08 ± 0.58 c	4.22 ± 0.11 e	19.63 ± 0.13 de	20.10 ± 0.06 d	0.88 ± 0.11 bc
10	66.12 ± 0.71 b	4.15 ± 0.16 ef	18.61 ± 0.07 f	21.35 ± 0.08 c	0.84 ± 0.07 c

Note: Different lowercase letters in the same column indicate significant differences (p < 0.05).

summarizes the color parameters of fresh samples, IRHAD, and IIRHAD materials at each IR.

As can be seen from Table 2, the L* of the jujube slices dried by infrared hot air under different interval ratios were significantly increased compared with the fresh samples, and the values of a* and b* were decreased, which were similar to the results obtained by Qian et al. [36]. Hou et al. [37] studied that the changes in the L* and a* values may be due to non-enzymatic Maillard browning during the drying of jujube slices. Compared to continuous drying, the sixth group had the best color, with a relative reduction of 33.41% for ∆E and 40.21% for B. This may be attributed to the fact that this group had the shortest effective drying time, and therefore, the high-temperature drying environment had the least effect on the color of the material. However, the effective drying time is not the only factor that affects the color change, and it can be found that the effective drying time is the same for the second and the tenth groups; however, the color difference value differs greatly. Liu et al. [38] demonstrated that prolonged heating led to a more noticeable color deterioration in the material. The tenth group experienced a 60 min active drying period, subjecting the material to a longer period of high temperature, while the second group had only a 20 min active drying period. During the tempering process, the temperature of the material in the second group was promptly reduced, preventing overheating and preserving the color more effectively.

In addition, the total drying time will also have an effect on the material’s color. For example, consider the fourth and ninth groups. The effective drying time for these two groups is the same, but the active drying period of the fourth group is smaller than that of the ninth group. However, the color difference value is significantly higher for the fourth group than for the ninth group. This difference may be attributed to the fact that the total drying time for the fourth group is 660 min, which is two and a half times longer than that of the ninth group. The prolonged air contact with the external environment leads to a higher degree of oxidation reaction in the material.

3.4. Rehydration Ratio

The rehydration rate mainly depends on the integrity of the internal structure of the material, such as the degree of drying on the destruction of cell walls, cell membranes, and organelles [39]. The rehydration rates of jujube slices under various drying methods are depicted in Figure 6. It can be observed that the rehydration rates during intermittent drying are reduced by 3.44% to 19.12% compared to continuous drying. Additionally, the rehydration rates between each intermittent group slightly increase with the prolongation of the active drying period. This finding is consistent with the results of Chaima [29] and Tepe [16], who conducted experiments on intermittent drying with pecan fruits and apples, respectively. This is because the temperature has a significant impact on the cellular organization of the material. Prolonged continuous drying keeps the material in a high-temperature state, making the cell walls, cell membranes, and organelles more susceptible to damage [40]. Due to the destruction of the cell structure, the number of intracellular and intercellular voids increases, which reduces the difficulty of water flow in the rehydration process and increases the water absorption capacity of the material [41]. During intermittent drying, the temperature of jujube flakes is reduced during tempering, which decreases the degree of destruction of the internal structure compared to continuous drying. As a result, the rehydration capacity is also reduced.

It should be noted that the effect of intermittent drying on the rehydration properties of different materials may be different. For example, Saleh et al. [42] found that whether or not using intermittent treatment did not affect the rehydration properties of the material in their intermittent drying experiments on carrots. Pan et al. [43] found that the intermittent treatment enhanced the rehydration capacity of the material after intermittent drying at variable temperatures on pumpkin. This may be due to the unique physical and chemical properties of each material in the drying process, which produce different effects. Therefore, the impact of intermittent drying on the rehydration rate of the specific material should also be determined through rehydration testing.

3.5. Ascorbic Acid

As shown in

Table 3. Nutrient content of jujube slices after drying.

Group Number	Total Phenolic/(mg·g−1)	Total Flavonoids/(mg·g−1)	Ascorbic ACID/(mg·g−1)
Fresh	27.38 ± 0.44 a	38.17 ± 0.16 a	0.29 ± 0.07 a
1	19.82 ± 0.74 f	26.35 ± 0.33 f	0.15 ± 0.04 i
2	24.24 ± 0.87 b	33.81 ± 0.45 ab	0.23 ± 0.03 c
3	25.21 ± 0.71 ab	32.29 ± 0.59 c	0.24 ± 0.03 b
4	23.22 ± 0.97 bc	30.78 ± 0.67 cd	0.21 ± 0.01 d
5	22.81 ± 0.46 cd	28.47 ± 0.34 de	0.20 ± 0.01 e
6	26.14 ± 0.75 a	34.44 ± 0.21 b	0.25 ± 0.02 b
7	23.31 ± 0.34 c	31.02 ± 0.13 c	0.19 ± 0.03 f
8	21.16 ± 0.61 de	28.01 ± 0.76 e	0.18 ± 0.01 g
9	21.47 ± 0.81 d	28.85 ± 0.64 d	0.17 ± 0.02 h
10	20.83 ± 0.94 e	26.82 ± 0.48 ef	0.17 ± 0.04 h

Note: Different lowercase letters in the same column indicate significant differences (p < 0.05).

, the ascorbic acid content of red jujube slices increased by 13.33% to 60.00% after the intermittent drying treatment compared to conventional infrared hot air drying. Ascorbic acid is not very stable and is easily affected by the temperature, making it likely to break down during the drying process. The brief high-temperature phase during intermittent drying reduces the duration for which the jujube slices are exposed to high temperatures, thereby minimizing the degradation of ascorbic acid. It can also be found that the drying time also affects the ascorbic acid content. For instance, in groups 3 and 4, as the effective drying time and total drying time increase, the ascorbic acid content decreases accordingly. This is consistent with the findings of Fang et al. [44], who observed that the drying time significantly affects the ascorbic acid content due to prolonged drying causing a more severe oxidation process, leading to the degradation of ascorbic acid. The drying study conducted by Geng et al. [9] on sea buckthorn also found that the retention of ascorbic acid in the material was higher after vacuum pulsation drying compared to hot air drying, which was attributed to the fact that the vacuum drying environment reduced the oxidization phenomenon of the material, which in turn reduced the degradation of ascorbic acid.

3.6. TPC and TFC

TPC and TFC are both antioxidant substances. Table 3 shows the TPC and TFC contents of red jujube slices under different drying methods. It can be found that compared with continuous drying, there is a significant difference in the contents of TPC and TFC of intermittent drying jujube slices, which have an increase of 5.09~31.89% and 1.78~26.88%, respectively. Since the antioxidant substances in jujube are vulnerable to thermal oxidation or degradation during drying, it was found that high temperatures damaged the plant cell structure, leading to the release and loss of TPC and TFC from the cell matrix [9]. Intermittent drying reduced the sustained high temperature of the material, resulting in a shorter duration of exposure to high temperatures and thus reducing the thermal degradation of TPC and TFC. In the second, fifth, and eighth drying groups, the extended active drying period led to a longer single heating time for the jujube slices. This resulted in a decrease in the TPC from 24.24 ± 0.87 to 21.16 ± 0.61 and for the TFC, from 33.81 ± 0.45 to 28.01 ± 0.76.

It can also be observed that temperature is not the only factor affecting the TPC content, as shown in drying group 6, which has the shortest effective drying period and the highest TPC content. Because bound phenols are linked to oligosaccharides or polysaccharides by ester bonds, higher temperatures may cause ester bond breakage, resulting in an increased TPC content [45]. The study by Gong et al. [46] also showed that a proper drying treatment could improve the TPC to a certain extent.

4. Conclusions

In order to further reduce the energy consumption during drying and improve the quality of the drying process based on IRHAD, this study focused on sliced jujube as the material and utilized nine different combinations of IR with varying active drying periods and intermittent periods to conduct drying tests. The results show that compared with IRHAD, the addition of intermittent treatment shortens the effective drying time and reduces energy consumption, in which the effective drying time is reduced by 40~100 min, the complete drying time of other drying groups, except the sixth group, is increased, and the drying energy consumption is reduced by 11.91%~34.34%. In terms of quality, the color difference and browning degree of the intermittent drying of red jujube slices were smaller (p < 0.05), and the nutrients, such as TPC, TFC, and ascorbic acid, were retained to a higher degree (p < 0.05). In summary, IIRHAD is a superior drying method.

Furthermore, it has been found that the process of infrared hot air intermittent drying is influenced by the active drying period, tempering period, effective drying time, and complete drying time. As a result, the interactions between these variables should also be considered to establish an efficient drying control process. In the future, we should consider integrating a variable intermittent ratio or temperature control technology to further explore the potential of intermittent drying and expand the applicability of the drying technology to a wider range of materials.