Analysis of the changes in quality and characteristics of hot air drying of Xinjiang jujube (Zizyphus jujuba Mill. cv. Junzao) following a delayed harvest

Dry processing is ineffective in preserving fresh jujubes (Zizyphus jujuba Mill.), contributing largely to the delayed jujube harvest in Xinjiang. However, no studies have evaluated the impact of delayed harvest periods on processing quality. Therefore, the present study investigated the effects of different delayed harvest periods on the characteristics of the quality of jujubes in Xinjiang after hot air drying. Six batches (S1–S6) were sampled over a 7-d period. Various indicators of jujubes changed significantly during the extended harvest period (P < 0.05). The water content of the fruit decreased progressively. While the percentages of soluble solids, total sugars, and reducing sugars increased continuously, the total weight of these parameters in a single jujube fruit decreased continuously. The proportion of ascorbic acid, total weight, and drying time decreased steadily. The fruit had the highest ascorbic acid content at the S4 stage after hot air drying (87.14 mg 100 g−1). Fewer color differences were recorded in hot air-dried fruits as compared with fresh jujubes; the cracking rate decreased after hot air drying, but the fruit could be rehydrated more effectively. A comprehensive evaluation revealed that jujubes harvested in the S4 stage were better suited for dry processing.


Test materials
The jujube used in the experiment were harvested from farm No. 8, 10th regiment, Alar City, Xinjiang Uygur Autonomous Region, China (81°12′15.53″E,40°38′16.21″N),1,023 m a.s.l.Branches with the same orientation and fruit maturity were selected from 30 jujube trees that were 10 years old and had grown uniformly.Jujubes on the branches were marked (as designated by the Xinjiang Special Forestry and Fruit Industry Export Marketing Board 22 .The first picking was performed on September 16, 2022, with a 7-d interval for each sample.Six batches of samples, designated S1-S6, were collected.The samples were cleaned with alcohol, sealed, and stored at 4 °C to prevent the loss of moisture and nutrients.Batches of experiments were conducted the next day to minimize errors that occur when sampling the fruit over a longer period.Table 1 depicts the surface conditions of jujubes at different picking stages.Figure 1 depicts the photographs of jujubes at different picking stages in this experiment.
The experimental site was selected from the key Laboratory of Modern Agricultural Engineering, Tarim University, Alar, Xinjiang Uygur Autonomous Region, and the experiment ran from September 16, 2022 to October 30, 2022.All experiments followed the relevant national, international, or institutional guidelines and regulations.

Determination of moisture content and average weight
The weight and moisture content were determined as described by Pu 23 .

Determination of total sugars and reducing sugars
The TS content was determined as described in GB/T 10782-2021, "General rules for the quality of preserves" 24 .The RS content was determined using the direct titration method in GB5009.7-2016"Determination of reducing sugars in foodstuffs" 25 .The TA was determined from the collected filtrate by neutralization titration with sodium hydroxide 26 .Malic acid was the primary acid in the jujube 27 .

Determination of the soluble solids (SSC)
Ten jujubes were selected, and 5 g of samples were randomly weighed from various parts of different jujubes and pulverized.The filtrate was collected and deposited in the test area of a hand-held refractometer (LH-Y28, Lu Heng China) for measurement and reading at 22 °C.The results were expressed as a percentage of the total mass fraction.

Determination of ascorbic acid (VC) content
Ascorbic acid content was determined using the 2,6-dichloroindophenol titration method described in GB 5009.86-2016 28.

Determination of color
The color variation between picked and dried jujubes from S1 to S6 was determined using a spectrophotometer (YS6003, Sanenshi China) and the CIELAB color chart system.The brightness L*, green-red value a*, and blueyellow value b* were determined.The color difference in values between the colors at the different picking stages and the jujubes dried in hot air were analyzed and calculated as follows 29 : where ∆E denotes the value between different colors; ∆L*, ∆a*, and ∆b* denote the difference in color parameters between the fruit picked at stages S2-S6 and S1, respectively.

Drying equipment
Twenty jujubes of roughly the same size, shape, and hardness were selected from each batch, removed from 4 °C storage, brought to room temperature, and wiped dry.The starting weight was the whole weight of the fruit.The drying conditions were set at 65 °C, 1.5 m s −1 wind speed, and 40% relative humidity using a hot air thin layer drying test apparatus (homemade by the Key Laboratory of Modern Agricultural Engineering, Tarim University) 30 .
The dryer was switched on, and the heating and air supply device was initiated during the trial.After 15 min of operation, the material bin was preheated to set parameters and attain a constant value.The jujubes were then evenly distributed in the drying chamber trays.The weight sensor within the material bin was automatically weighed and recorded every hour.Tray quality was evaluated to determine their real-time moisture content until the dry base of the jujubes reached < 33.3%.The drying test was completed at this point.The fruit was left to cool, and the surface cracks were examined using a stereo microscope.The jujubes were bagged, sealed under a vacuum, and stored at room temperature before testing.

Drying parameters (1) Moisture content of the dry base
The moisture content of jujubes varies as they dry.M t denotes the dry basis moisture content of jujubes at time t, and it is expressed as follows: where G t denotes the total weight of all jujubes in the material tray at time t (g).G 0 denotes the starting weight of all jujubes in the material tray (g), and M 0 denotes the initial wet base moisture content of the jujubes (%). (

2) Drying rate
The drying rate of any stage (Drying rate) is expressed as a function of the moisture content of the dry base at the start and end stages versus time as follows 31 : where M t 1 denotes the dry basis moisture content at the start of the stage.M t 2 denotes the dry basis moisture content at the end of the stage.t 1 denotes the start of the phase, and t 2 represents the end of the stage.
(3) Effective diffusion coefficient of moisture The effective diffusion moisture coefficient during hot air drying of jujubes was calculated according to Fick's second law equation 32 : Where MR represents moisture content ratio; D eff denotes effective moisture diffusion coefficient (m 2 s −1 ); r e denotes volume equivalent radius (m), and t denotes drying time (s).
Simultaneously taking the logarithm of both sides of the equation yields the following: (1) The slope method was used in calculating the effective diffusion coefficient of moisture.The slope expression from Eq. ( 5) is as follows:

Observation of the superficial surface cracks
The high heat and mass transfer rate during jujube hot air drying accelerates heat expansion, water loss, and shrinking, which readily cause cracks.The presence of cracks results in nutrient loss, an enrichment of the microbial population and their entry into the fruit, and other conditions that influence drying quality.Each batch included 20 jujubes without initial cracks selected for drying.The surface of the jujubes was examined using a stereomicroscope (TD-2KH, Sanmang Teda China), and the rates of jujube cracking at different picking stages after hot air drying were calculated using the statistical method that is employed to calculate the cracks that occur in hazelnut (Corylus spp.) after drying 33 .
where P denotes the rate of cracked fruits; N d denotes the number of stepped jujubes with cracks after hot air drying, and N w denotes the total number of dried jujubes.

Determination of the rehydration ratio
Rehydration is a crucial indicator of dried jujube quality.Five dried jujube were randomly selected, weighed, and placed at 50 °C for 40 min, dried on absorbent paper, and left to dry for 20 min and re-weighed.The rate of rehydration of jujubes was calculated as follows 34 : where R denotes the rehydration ratio; M d denotes the initial weight, and M w denotes the weight after rehydration.

Integrated methods of evaluation
The Analytical Hierarchy Process (AHP) was used on a scale of 1-9.A pairwise comparison matrix of the factors was constructed, followed by a consistency test.The indicators used to evaluate the jujubes after hot air drying required normalization.The positive indicators included ascorbic acid content and rehydration ratio after hot air drying, whereas the negative indicators included drying time, cracking rate, and color difference.The normalization formula y i is illustrated in Eqs. ( 9) and ( 10): where y i denotes the normalized values of the indicators; x i denotes the actual values of the indicators, and the x max and x min denotes the maximum and minimum values of the indicators, respectively.
The combined score for the drying quality of jujubes at different picking stages y was determined as follows: where y 1 , y 2 , y 3 , y 4 and y 5 are the normalized values for ascorbic acid content, rehydration ratio, drying time, cracking rate, and color difference, respectively, and l 1 , l 2 , l 3 , l 4 and l 5 represent the corresponding weights.

Statistical analysis
The data are expressed as mean ± SD.The data were subjected to a one-way analysis of variance (ANOVA) using SPSS 25.0 (IBM, Inc., Armonk, NY, USA) to examine significant differences in the median fruit index at different harvest periods (P < 0.05).The correlation of fruit quality indices was analyzed using Pearson's method.Furthermore, the correlation Plot plugin in Origin 2018 (OriginLab, Northampton, MA, USA) was employed to generate a correlation heat map of quality indicators.The consistency of the matrix was assessed using MATLAB.

Moisture content and weight change analysis
The moisture content and weight are the key parameters used to evaluate the growth state of jujubes 35 .As shown in Fig. 2, the dry basis moisture content and mean weight of jujubes picked at different stages of the delayed harvest period ranged from 208 to 56% and 13.32-29.38g, respectively.The jujubes were dehydrated during the (5) lnMR = ln 6 www.nature.com/scientificreports/delayed picking period with significant differences (P < 0.05) between the moisture content and mean weight of those picked at S1-S4.The most significant reduction in water content was observed in stages S2-S3 and S3-S4.The fruit in this ripening period incurred the primary amount of dehydration during the delayed harvest period, compared with those harvested at stages S1-S2.There was less variation in moisture content, and the jujubes weighed less during the S1-S2 stages.These changes potentially occurred because the branches continually supply water and other materials to the fruit 36 .The range of change in moisture content in the S4-S6 stage decreased significantly (P < 0.05).The surface of the jujube lost water rapidly during this period, and natural environmental factors were insufficient for the internal water of the jujube to rapidly migrate to the surface and diffuse into the air 37 .

Changes in the contents of nutrients
The total weight of material within a single jujube fruit was used to reveal the changes in the indicators during the delayed harvest periods as the moisture content and average weight of the batches of jujubes continued to decrease.SS represents solid substances that are water soluble in fruits, including sugars, acids, and trace elements.Sugar is the most essential nutrient and flavoring element in jujube 38 .As depicted in Table 2 and Fig. 3, the TS and RS ranges of variation were 28.13-56.22%and 16.73-38.02%,respectively, while the SS range was 32.41-63.89%.The percentage content of both the TS and RS increased as the harvest time was extended, possibly due to the evaporation of water in the fruit, resulting in a further concentration of dry matter.Furthermore, the total content within a single fruit increased and decreased, peaking at the S2 stage.The percentage TS content and total content of a jujube within a single fruit showed the highest increase from S1 to S2. Jujube is considered to be in the "reddening stage" and starch is metabolized to SS 39 .Simultaneously, the total content of compounds in a single jujube decreased at the S2-S6 stages, indicating that the jujubes absorbed internal nutrients during respiration.Fruit metabolism is reduced following a drop in its moisture content, and the loss rate of total nutritional content at each stage similarly decreases with the steady decrease in respiration 40 .
Ascorbic acid (Vitamin C) is an essential vitamin that the human body cannot synthesize 41 .The percentage of ascorbic acid varied from 377.25 mg 100 g −1 to 111.61 mg 100 g −1 during delayed harvest, following the same trend as the total content, and demonstrated a distinct declining trend during the S1-S6 stages, as previously described 42 .A hypothesis exists that the continuous loss of ascorbic acid during ripening and drying is related to its high susceptibility to oxidation.
TA can significantly change the taste of fruit in general 43 .The level of total TA in a single jujube fruit increased slightly from S1 to S2 throughout the delayed harvest period.The total content of jujube gradually decreased

Color changes
The appearance and color of the fruit is the primary determinant of consumer purchase 45 .Table 3 outlines the change in color at the delayed harvest stage.
The brightness L* of jujubes decreased significantly (P < 0.05) throughout the delayed harvest process in the S1-S4 stage.The S1-S2 stage decreased dramatically from 51.26 to 39.82 due to the green coloration on the epidermis of the S1 stage fruit.The contrast between green and red increased.The brightness in the S2-S4 stage dropped from 39.82 to 31.48 because of the rapid water loss between these stages.The light reflection decreased parallel with water loss from the epidermal cells, and the fruit turned from a luminous maroon to a dull dark red.However, there was no statistically significant change in brightness between the S4 and S6 stages (P < 0.05).
The surface condition of jujubes progressively stabilized after the S4 stage.The red-green value a* was significantly lower in the S1 stage than in the S2-S6 stages.The red-green value a* did not change significantly after the S2 stage, which was consistent with the S1 stage.The "red-change" stage is considered the most critical for changes in the compounds responsible for fruit coloring 46 .The blue-yellow value b* decreased significantly between S1 and S4 and did not change significantly between S4 and S6, which is consistent with the trend of brightness L*.The chromatic aberration value of the S2-S4 stages increased and stabilized during the S4-S6 stage such that the chromatic aberration value of each stage of S1-S4 was significant.However, there was no significant difference in the S4-S6 stage and these results may be because water content influences the material coloration; water on the surface of the jujube is completely lost after the S4 stage, and the color change remains insignificant.Thus, while differences in fruit coloration might indicate jujube maturity in the growth phase 47 , they cannot be used as a criterion for the delayed harvest procedure.

Quality correlation analysis
The correlation coefficient between the jujube quality indices at the different stages of delayed harvest is depicted in Fig. 4, and the SS content significantly positively correlated with the TS, RS, TA, red and green values (P < 0.05), and ascorbic acid and blue-yellow values (P < 0.05).Ascorbic acid positively correlated with the TS, RS, red-green values, and TA (P < 0.05), and negatively with the blue-yellow values (P < 0.05).The TS and RS, TA, and redgreen values all had a significant positive correlation, whereas the blue-yellow values had a significant negative correlation (P < 0.05).The red-green and blue-yellow values had a significant negative correlation (P < 0.05).In  a nutshell, changes in various indicators are co-related rather than independent, exhibiting positive or negative correlations during the delayed harvesting process.The SS comprised soluble sugars, acids, minerals, and other water-soluble compounds, and the SS accounted for the largest proportion.As the harvesting process was delayed, the moisture content of jujubes decreased, while the percentage content of SS, TS, RS, and TA increased.
However, the unstable nature of VC increased the rate of loss on the trees throughout the drying process.As a result, VC was a negative correlation with the other nutritional indicators.Brightness and indicators were weakly correlated.There was a high correlation between the blue-yellow and red-green values for the other indicators.Thus, the blue-yellow and red-green values indicate the delayed harvest stages.

Drying characteristics
Influence of the picking stage on the kinetics of hot air drying of jujubes Figure 5 depicts the hot air drying rate curves for the different picking stages.The drying time required for each batch of jujubes from S1 to S6 was 22 h, 19 h, 14 h, 10 h, 7 h, and 6 h, respectively.Thus, the overall time for hot air drying decreased as the jujubes from the delayed harvest matured naturally, and the rate of drying of the samples from S1-S4 exhibited an increasing trend and then decreased.The jujubes were in the preheating and initial heating stage at the start of the drying period, and the drying rate increased first.The internal regulation of moisture diffusion decreased at a later stage 48 .The drying curves of the samples in S5-S6 were more comparable to those of uniform drying, indicating that lowering the initial moisture content hastened the preheating and  heating process of the jujubes.It was easier to disperse moisture from the surface of the jujube during hot air drying than it was to move moisture from the interior of the jujube to the surface.However, at the end of drying, the drying rate in the S5-S6 stage was higher than at the S1-S4 stage.
Along with the characteristics of shrinkage of the porous media that holds moisture during drying, the high initial moisture content of jujube is regarded to be more destructive to the tissue structure during hot air drying due to the long drying period and quick rate of mass transfer.Furthermore, the drying rate inside the early drying stage is substantially lower than that on the surface of the jujube.Uneven water migration produces biological stress, causing the jujube to shrink, deform, and eventually destroy the water migration channel.Shrinkage of jujubes with a low initial moisture content is not readily visible due to slower heat and mass transfer, better preservation of the pore structure of the flesh, and increased efficiency of water passage during delayed harvest periods.

Effect of the picking stage on the effective diffusion coefficient of water in jujubes
The effective moisture diffusion coefficient reflects how rapidly moisture inside the material migrates to the surface during the drying process, which is influenced by the changes in material structure, moisture content, and drying temperature.The results are shown in Table 4.The effective moisture diffusion coefficient ranged from 9.3621 × 10 −10 m 2 s −1 to 10.7400 × 10 −10 m 2 s −1 from S1 to S6, exhibiting an increasing trend followed by a decreasing trend 43 .These data suggest that jujubes picked in the early stages have a high initial moisture content and rapid water diffusion during the initial drying stage; on the other hand, they produce severely wrinkled and crusted fruit, which distorts and damages the internal water transfer channels, reducing water diffusion.In contrast, jujubes picked too late exhibited a comparably slow water diffusion mechanism owing to their low initial moisture content and water primarily bonded internally.Jujubes e picked at S4 during the delayed harvest process had the highest effective water diffusion coefficients due to variations in moisture content and cracking.

Drying quality of jujube at different picking stages
Cracking rate of jujube after hot air drying at different stages The formation of drying cracks is related to the drying method, moisture content, mechanical properties, and tissue structure of the material 49 .The cracking rate of jujubes during hot air drying continuously decreased with the prolonged picking stage.The early stage of hot air drying is the primary stage in which cracks form, and a higher initial moisture content increases the likelihood of crack formation in the early stage.The cracking rate of jujubes at the end of drying was 16.7%, 13.3%, 11.6%, 5.0%, 0%, and 0% at initial dry base moisture contents of 208%, 158%, 111%, 78%, 63% and 56%, respectively (Table 5).It is believed that hot air drying of jujubes with a high moisture content (S1-S3) heats the internal water in the fruit, which rapidly vaporizes.Figure 6a depicts the condition of fresh jujube before drying.The fruit could not completely discharge the water in time, causing internal expansion and cracking of the peel under stress (Fig. 6b).This phenomenon generally occurs during the accelerated drying phase.Owing to the strong water ability of the crack site to secure the water molecules, the mass transfer rate of the cracked area will be significantly higher than that of the rest of the area (Fig. 6c), resulting in crack diffusion, juice outflow, linear deep shrinkage, and other conditions that impact drying quality (Fig. 6d).Therefore, the drying process parameters, including low temperatures and increased humidity, should www.nature.com/scientificreports/be regulated when jujubes have high moisture to limit the heat mass transfer rate of the surface during the early drying phase.

Comprehensive evaluation of the delayed harvest stages
As illustrated in Table 6, the average quality of hot air-dried jujube at each picking stage ranged between 11.29 and 13.25 g.Considering the changes in the nutrient content of fresh fruits at different stages and the actual drying conditions, it is believed that jujubes at the picking stage have a high initial moisture content, an extended hot air drying time, and a rapid heat and mass transfer rate, resulting in the thermal decomposition and sugar precipitation 11 .In contrast, low moisture content impacts the quality of jujube after drying due to nutrient depletion by respiration during the delayed harvest process.Unlike heat-tolerant substances such as sugars, ascorbic acid is highly vulnerable to oxidation and thermal degradation during high-temperature drying; determining ascorbic acid concentrations is better suited to evaluating the quality of dried fruit 50 .In the present investigation, because the weight after drying differed significantly (P < 0.05), we determined the mass of ascorbate in a single dried fruit by multiplying the percentage of ascorbate measured by the mass after drying.The initial content of ascorbate decreased with the extension of the picking stage; however, the rate of ascorbate retention increased owing to the reduction in hot air drying time required.Surface water evaporation and chemical reactions such as enzymatic browning, non-enzymatic browning, and caramelization reactions can cause differences in the color of jujubes during high-temperature drying 51 .The color of jujube was compared before and after drying.The results revealed that the brightness L* of jujube in the S1-S3 stage decreased by 34%, 32%, and 21.3% during the different stages, respectively, whereas the brightness L* of jujube in the S4-S6 stage did not change significantly.These data demonstrated that hot air drying potentially influenced the brightness L*, and a higher moisture content resulted in a longer drying time and a higher degree of influence.However, the L* of the S1 stage remained the highest.The red and green value a* improved significantly during the S1 stage, i.e., the green faded and the red colors brightened compared with before drying, and there was no significant change in the other stages.These observations indicated that hot air drying primarily influenced the contents of green compounds on the jujube surface 34 .The blue-yellow value b* of the samples at each picking stage after drying was significantly the highest in S1 and the lowest in S6 (P < 0.05), comparable to before drying.Furthermore, there was no significant variation in color at each stage after drying (P < 0.05).Rehydration reflects the structural damage of the jujubes after drying.The jujube harvested at the S4-S6 stage rehydrated the fastest after drying, while that picked at the S1-S2 stage rehydrated the slowest.It is considered that the rapid thermal expansion and water-loss shrinkage during the hot air drying of jujube with a high initial moisture content caused more irreversible damage to the tissue structure of fruit and a poor intercellular water storage capacity 52 .The five indicators described above were combined with the drying time and the crack rate to assess jujube quality after hot air drying.These five indices, combined with the drying time and crack rate, defined the quality of jujube after drying.A consistency test was performed after constructing a pairwise comparison matrix of factors using the 1-9 ratio scale approach.The weights of ascorbic acid content, rehydration ratio, drying time, crack rate, and color difference were set at 0.3, 0.15, 0.3, 0.15, and 0.1, respectively 53 .The comprehensive scores of hot-air drying quality at (11) y = y 1 l 1 + y 2 l 2 + y 3 l 3 + y 4 l 4 + y 5 l 5 Vol.:(0123456789) Scientific Reports | (2023) 13:16732 | https://doi.org/10.1038/s41598-023-43594-w

Figure 2 .
Figure 2. Graph of moisture content and weight change.

Figure 3 .
Figure 3. Variation in the total content of individual jujube fruits at each stage of delayed harvest.

Figure 4 .
Figure 4. Correlation coefficient diagram showing the quality of jujubes at different stages of delayed harvest.Based on Pearson's correlation analysis, red represents positive correlation, blue represents negative correlation, darker colors represent higher correlations, * indicates 0.05 level, and the numbers in the graph indicate correlation coefficients between indicators.

Figure 5 .
Figure 5. Hot air drying rate curves for different stages of delayed harvest.

Figure 6 .
Figure 6.Dynamics of hot air drying cracks in jujubes.

Table 1 .
Sensory characteristics of jujubes at different picking stages.Determination of the titratable acidsTen jujubes were selected, and 15 g of flesh was randomly weighed from different parts of the fruit and crushed.

Table 2 .
Variation in the content of jujube components at various stages of delayed harvest.
Vol:.(1234567890) Scientific Reports | (2023) 13:16732 | https://doi.org/10.1038/s41598-023-43594-wwww.nature.com/scientificreports/during the S2-S6 stage.Minyan 44 et al. also discovered that the titrable acid content of jujube decreased with maturity during the jujube growing phase.As a result, jujube was sweeter and less sour after delayed harvesting, which explains why we selected it as the main raw material for Xinjiang dried fruit processing.

Table 3 .
Color changes in jujubes at different stages of delayed harvest.

Table 4 .
Variation in the effective diffusion coefficient of jujube moisture at different stages of delayed harvest.

Table 5 .
Changes in the formation of cracks during hot air drying of jujubes at various stages of delayed harvest.

Table 6 .
Statistics on the quality of jujubes after drying at various stages of delayed harvest.

Table 7 .
Results of the comprehensive evaluation of the jujube picking stage.