AFFAFFAdvanced Functional Foods3106-9843LUMOSCIENCE PUBLISHING LIMITEDA17 2/F HING YIP CENTRE, 31 HING YIP STREET KWUN TONG KOWLOON HONG KONG10.64187/aff.2026.v2.i1.006ORIGINAL RESEARCH ARTICLEImproving the Quality of Ready-to-Eat Jujubes by Alkaline Electrolyzed Water Inactivation Coupled with Sequential Hot Air and Microwave DryingJiangDifeiAdilMuhammadXiaoXinglong*fexxl@scut.edu.cnYuYigang*yuyigang@scut.edu.cnCollege of Food Science and Engineering, South China University of Technology, Guangdong, Guangzhou Province 510640, China*Email: fexxl@scut.edu.cn;Email: yuyigang@scut.edu.cn;3032026219812312202630320262632026
In China, the consumer market for dried jujubes is enormous, accounting for the vast majority of jujube-based food consumption. However, the conventional hot air drying offers poor microbial inactivation efficacy to the treated jujubes, and causes considerable damage to their nutritional components and sensory properties. To solve these problems, offering ready-to-eat (RTE) jujubes, a preparation method combining alkaline electrolyzed water (AEW) inactivation and sequential hot air-microwave drying was established and confirmed. The experimental results confirmed that the combination of AEW inactivation and sequential hot air and microwave drying could produce high-quality RTE jujubes efficiently. Under the optimal treatment conditions, the retention rates of vitamins (B₁, B₂, and C) and bioactive substances (total polyphenols, total flavonoids, total triterpenic acids) in the prepared RTE jujubes were all higher than those in the samples processed via the conventional hot air drying. Specifically, the treated jujubes showed 36.88% and 13.67% higher contents of vitamin C and TPs, respectively, than those of the conventional samples. Meanwhile, the combined treatment negligibly changed the surface color and texture of the products, greatly enriched the aroma, and thus improved the edible quality of the prepared jujubes. In addition, all microbial indicators of the prepared jujubes fully met the requirements of the National Standard of China (GB14884-2016), indicating their good food safety. In summary, the combination of AEW inactivation and sequential hot air and microwave drying is an effective processing method to improve the comprehensive quality of RTE jujubes, which is promising to promote the development of the jujube industry.
Ready-to-eat jujubesAlkaline electrolyzed water inactivationSequential hot air and microwave dryingComprehensive qualityheader-authors-shortJiang, D.F., Adil, M., Xiao, X.L., et al.1. Introduction
Jujube (Ziziphus jujuba Mill.), a species of the genus Ziziphus (family Rhamnaceae), is indigenous to the middle and lower reaches of the Yellow River in China and has a cultivation history of millennia.1 Benefiting from its strong tolerance and adaptability to arid, saline-alkaline, and barren soils, along with its rich nutritional value2,3 and sustainable economic value4, jujube cultivation has expanded rapidly in northwest China, with annual output value exceeding 100 billion RMB. Jujube fruits contain various bioactive compounds, including polyphenols, flavonoids, triterpenoids, polysaccharides, vitamin C (VC), and cyclic nucleotides. The disclosed biological effects of these compounds include antioxidant, anti-inflammatory, antibacterial, hepatoprotective, gastroprotective, hypoglycemic, and anticancer activities, thereby underscoring the health potential of the fruit.5–7
Postharvest handling poses persistent challenges for jujubes in both the quality and safety. Mechanical damage occurred during harvesting, and physiological senescence happened at the storage period facilitates microbial invasion. Which in turn, promotes the colonization and proliferation of the spoilage organisms and pathogens on the fruit surface.8,9 Black spot disease, caused by Alternaria alternata, leads to dark brown lesions on jujube, accelerating its spoilage.10 Pathogenic microorganisms such as Escherichia coli and Listeria monocytogenes also tend to adhere to the jujube surface, potentially running the serious risk of compromising food safety.11 Routine washing with tap water removes surface debris only, providing limited microbial inactivation. Chlorine-based disinfectants (e.g., sodium hypochlorite) have been confirmed to inactivate a broad range of microorganisms effectively for a long time. However, the concerns about chemical residues and potential health risks have led to the reduced use of these disinfectants in food processing operations. The other postharvest handling treatments, such as ozone11 and ultraviolet (UV) radiation12, have also been applied in surface disinfection. Though these methods are milder and more environmentally friendly, they still do some harm to the fruit’s appearance and sensory properties. Therefore, an effective postharvest strategy for jujube products, especially ready-to-eat (RTE) ones with strict safety requirements, must be able to control the safety microbial level while preserving high levels of sensory and nutritional quality of the products to those of the original fruit.
In recent years, alkaline electrolyzed water (AEW) has emerged as a promising technology for cleaning, disinfection, and preservation in many food systems.13–15 AEW is generated via the electrolysis of saturated salt solutions, yielding a product characterized by the advantages of high pH, elevated reduction potential, and strong penetration capacity.16 Unlike traditional disinfectants, AEW acts as a mild, non-toxic, and residue-free disinfection, which provides effective microbial inactivation while preserving the good food quality of the related products.17–21 Moreover, it has been reported to effectively preserve the nutritional quality in key nutrients, freshness of many kinds of food commodities, and lengthen their storage and shelf lives without compromising product safety. These investigations span diverse product categories, including fresh fruits and vegetables, raw meat, and aquatic products, with consistent positive results about AEW.17,18,20,21
Due to the high moisture content and poor storability of fresh jujubes, over 90% of the global jujube crop is subjected to drying and ultimately consumed as dried products.7 A variety of industrial drying technologies have been investigated to enhance processing efficiency and improve end-product quality, including freeze drying, vacuum drying, hot air drying, microwave drying, and hybrid processes like microwave-vacuum-freeze drying.22,23 Among these disclosed methods, hot air drying dominates industrial treatments of the commercial dried food products, owing to its advantages of simple equipment configuration, well-established processing protocols, and low production costs. However, this technique is associated with several drawbacks, including prolonged drying durations, uneven internal moisture distribution, significant nutrient degradation, undesirable flavor alterations, and moderate microbial inactivation.24–28 Moreover, microwave drying is another widely used technique for the dehydration of common fruits and vegetables to afford dry products. It has the positive characteristics of fast drying speed, high drying quality, and significant sterilization effect.22,23,29 However, microwave drying is not universally applicable to all kinds of fruits and vegetables. When applied to the samples with non-uniform moisture distributions, this method makes it difficult to achieve consistent final moisture levels and runs the high risk of over-drying and sample scorching. Several related studies have also reported that microwave drying may potentially affect the flavor and texture of the treated samples.30–32 Furthermore, given the relatively low market value of the conventional dried jujubes, standalone microwave drying, an energy-intensive and costly process, is not economically viable for the industrial production of RTE jujubes.
Given the relatively low market prices of RTE jujube products, their processors require mature, reliable, and economical technologies. Considering the poor moisture uniformity of freshly harvested jujubes, hot air drying can effectively homogenize their moisture distribution. Combined sequential hot-air and microwave drying is proposed here as an energy cost-effective approach for RTE jujube products. Which overcomes the drawbacks of the above-listed single methods, thereby enhancing the nutritional quality, sensory properties, and microbial safety of the dried jujubes while maintaining industrial feasibility. Further integrating AEW pretreatment with sequential hot-air and microwave drying may improve the microbial safety and overall quality of the prepared RTE jujube products. Accordingly, this study optimizes the processing conditions and evaluates the products in terms of nutritional quality, sensory attributes, and food safety, establishing an RTE jujube processing strategy. This integrated strategy hopefully improves processing efficiency while avoiding chemical disinfectant residues, and part costs of microbial control level and food quality.
2. Materials and Methods2.1 Materials
Jujube samples used in this study were collected from Maigaiti County, Kashgar Prefecture, Xinjiang Uygur Autonomous Region, China. The cultivar used in this study was Huizao (gray jujube), which ripens in late August and is harvested in late November after natural drying on-tree. Sodium chloride, disodium hydrogen phosphate, potassium dihydrogen phosphate, glacial acetic acid, sodium carbonate, sodium nitrite, sodium hydroxide, and 95% ethanol were purchased from Guangzhou Reagent Chemical Factory (Guangzhou, Guangdong, China). Plate Count Agar (PCA), Violet Red Bile Agar (VRBA), Brilliant Green Lactose Bile Broth (BGLB), and Sabouraud Dextrose Agar (SDA) were obtained from Guangdong Huankai Microbial Technology Co., Ltd. (Guangzhou, Guangdong, China). Acetonitrile, Folin-Ciocalteu phenol reagent, 2,6-dichloroindophenol, gallic acid, rutin, oleanolic acid, vanillin, and perchloric acid were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Thiamine hydrochloride, riboflavin, and ascorbic acid were purchased from Sigma-Aldrich (Germany). All reagents were of analytical grade except for the chromatographic-grade reagents used for high-performance liquid chromatography (HPLC).
2.2 Jujube Collection and Preparation
After being uniformly harvested by local fruit growers, jujube samples were randomly selected and transported to the laboratory at South China University of Technology (Guangdong, China) within 48 h after the collection. After sorting to remove defective fruits, jujubes with no visible defects, no disease symptoms, and uniform sizes were randomly selected for subsequent experiments.
2.3 Microbial Enumeration
Microbial contents of jujube samples, including total plate count (TPC), coliforms, and molds, were determined in accordance with the microbial limit requirements specified in the National Standard of China (GB 14884-2016). Sample preparation and analysis were performed according to the methods specified in the National Standard of China (GB 4789.2-2022, GB 4789.3-2016, and GB 4789.15-2016). The spread plate method was used to enumerate TPC, coliforms, and molds. Typical or suspected coliform colonies were subjected to gas production verification. All experiments were performed in triplicate to ensure data reliability and representativeness, and the average values were calculated. Microbial counts were expressed as colony-forming units per gram (CFU/g). Notably, the GB 4789 series of National Standard of China utilized in the present work shows excellent equivalence with the corresponding ISO international standards, ensuring the comparability and global recognition of the study’s findings.
2.4 AEW Inactivation Experiment
Single-factor experiments were designed to investigate the effects of AEW parameters, including electrolyte (NaCl) concentration, treatment duration, and solid-liquid ratio (m:V), on microbial inactivation efficiency. The TPC was determined, and the inactivation rate was calculated. The electrolyte (NaCl) concentration gradients for AEW were set at 0 mg·L⁻¹, 10 mg·L⁻¹, 50 mg·L⁻¹, 100 mg·L⁻¹, 500 mg·L⁻¹, and 1,000 mg·L⁻¹. The treatment duration gradients were 1 min, 2 min, 5 min, 10 min, and 15 min. The solid-liquid ratio gradients were 1:10, 1:20, 1:50, 1:100, and 1:200 (m:V, g:mL). When investigating one factor, the other two factors were fixed at 100 mg·L⁻¹ (electrolyte (NaCl) concentration), 10 min (treatment duration), and 1:200 (solid-liquid ratio (m:V, g:mL)). Based on the inactivation rates obtained from the single-factor experiments, a 3 × 3 orthogonal experiment was designed to explore and verify the optimal processing conditions for AEW as a pre-inactivation pretreatment for jujubes. The results were expressed as inactivation rates.
2.5 Determination of Moisture Content
Moisture content was determined using the direct drying method in accordance with the National Standard of China (GB 5009.3-2016). The test sample was cut into particles with a diameter of less than 3 mm. A 10 g aliquot of the sample (accurate to 0.0001 g) was weighed and transferred into a pre-weighed weighing bottle. After capping and precise weighing, the bottle was placed in an oven at 105 ℃. After drying for 4 h, the bottle was removed from the oven, cooled in a desiccator for 0.5 h, and then weighed. Repeat the above operation until the mass difference between two consecutive weighings does not exceed 2 mg, at which point a constant mass is reached.
2.6 VC Retention Rate and Color Difference
VC content was determined using the 2,6-dichloroindophenol titration method.33 A 2 g aliquot of the sample was mixed with 40 mL of glacial acetic acid solution (20 g/L) and homogenized. The supernatant was collected and titrated with a standard 2,6-dichloroindophenol solution until a persistent pink color was observed (end point of titration). L-ascorbic acid was used as the standard reference, and the results were expressed as mg/100 mg on a dry weight basis. The VC retention rate was calculated by comparing the VC content of the experimental group with that of the control group.
Color parameters were measured using a colorimeter (Ci60 integrating sphere colorimeter X-Rite Inc., Grand Rapids, MI, USA). The L* (lightness), a* (redness), and b* (yellowness) values were measured at 3 random points on the sample surface, and the total color difference (∆E) between the experimental group and the control group was calculated.33 The formula for ∆E is as follows (Equation 1):
∆E=(L*-L)2+(a*-a)2+(b*-b)2
where L, a, and b represent the color values of the control group; L*, a*, and b* represent the color values of the experimental group.
2.7 Hot Air and Microwave Drying Experiment2.7.1 Effects of Hot Air Drying and Microwave Drying on VC Content and Color of Jujubes
Existing hot air drying processes for jujubes typically involve subjecting the raw materials to a temperature range of 70–80 ℃ for several hours, an operation that severely impairs the overall quality of jujubes. To mitigate the quality deterioration induced by hot air drying and optimize the corresponding process parameters, hot air drying was conducted at temperature gradients of 55 ℃, 60 ℃, 65 ℃, 70 ℃, and 75 ℃, with treatment duration gradients set at 60 min, 90 min, 120 min, 150 min, and 180 min. When investigating one factor, the others were fixed at 65 ℃ (hot air drying temperature) and 120 min (hot air drying duration).
Microwave drying is characterized by high efficiency and rapid processing, taking only tens of seconds to several minutes when used alone for jujube drying. Given the objective of optimizing parameters for sequential hot air and microwave drying, and considering that pre-drying with hot air effectively reduces initial jujube moisture content, the microwave power was fixed at a constant 800 W, while treatment durations were set at five gradients: 20 s, 30 s, 40 s, 50 s, and 60 s.
VC contents of the blank control group and each treatment group were determined, and the VC retention rate was calculated and expressed as a percentage. The surface color of the blank control group and each treatment group was measured, and the color difference was calculated.
2.7.2 Effects of Hot Air Drying and Microwave Drying on Moisture Content and Microbial Count of Jujubes
The hot air drying temperature was set at 65 ℃, and the changes in jujube moisture content and microbial count with drying duration were determined at this temperature. The microwave drying power was fixed at 800 W, and the changes in jujube moisture content and microbial count with drying duration were measured at this power. Moisture content was expressed as a percentage, and the microbial count was expressed as CFU/g.
2.8 Vitamins (B₁, B₂, and C) Content
Vitamin contents in jujubes at various treatment stages were quantified using high-performance liquid chromatography (HPLC, Agilent), with vitamin B₁ (VB₁, thiamine hydrochloride), vitamin B₂ (VB₂, riboflavin), and VC (ascorbic acid) as external standards.34 The mobile phase consisted of potassium dihydrogen phosphate solution (50 mM, pH 3.0) and acetonitrile. The chromatographic column was a TC-C₁₈ column (Agilent, USA), with a column temperature of 30 ℃ and a run time of 25 min. VB₁ content was expressed as thiamine (μg/g DW), VB₂ content as riboflavin (μg/g DW), and VC content as total ascorbic acid (μg/g DW). Retention rates of the three vitamins were calculated individually and expressed as percentages.
2.9 Total Polyphenol Content
Total polyphenol content was determined using the Folin‑Ciocalteu colorimetric method.35 A 0.5 mL aliquot of gallic acid standard solution (0.04–0.20 mg/mL) or polyphenol extract was transferred into a 10 mL colorimetric tube, followed by the addition of 0.5 mL of Folin-Ciocalteu reagent (0.5 N). The mixture was mixed thoroughly. After standing for 5 min, 2.5 mL of 10% (w/v) Na₂CO₃ solution was added, and deionized water was added to adjust the total volume to 10 mL. The solution was vortexed thoroughly and then incubated in the dark for 1 h. Absorbance was measured at 760 nm. Total polyphenol content was expressed as gallic acid equivalents (mg/g DW).
2.10 Total Flavonoids Content
Total flavonoid content was determined by the aluminum nitrate-sodium nitrite colorimetric method.36 A 1.0 mL aliquot of rutin standard solution (0.10–0.50 mg/mL) or sample extract was pipetted into a 10 mL colorimetric tube. Then, 0.3 mL of 5% NaNO₂ solution was added, followed by the sequential addition of 0.3 mL of 10% Al(NO₃)₃ solution. The mixture was mixed thoroughly and allowed to stand for 5 min for precipitation. Subsequently, 4.0 mL of 4% NaOH solution was added to the mixture, and the total volume was made up to the mark with 60% ethanol solution. The solution was vortexed thoroughly and incubated for 15 min. Absorbance was measured at 510 nm. Total flavonoid content was expressed as rutin equivalents (mg/g DW).
2.11 Total Triterpenic Acids Content
Total triterpenic acid content was determined using the vanillin-perchloric acid colorimetric method.37 A 0.1 mL aliquot of oleanolic acid standard solution (0.2–1.2 mg/mL) or sample extract was added to a test tube and evaporated to dryness in a water bath (T ≥ 85 ℃). Subsequently, 0.2 mL of vanillin-glacial acetic acid solution (5:95, w/v) and 0.8 mL of perchloric acid (HClO₄) were added, and the mixture was mixed thoroughly. The mixture was then incubated in a water bath (T = 60 ℃) for 15 min, followed by cooling to room temperature. Next, 5.0 mL of glacial acetic acid was added and mixed thoroughly. Absorbance was measured at 547 nm. Total triterpenic acid content was expressed as oleanolic acid equivalents (mg/g DW).
2.12 Texture Profile Analysis
Texture parameters of jujube samples were determined using a rheometer (HR-1/TA), with minor modifications to the method described by Kong et al.38 A P36R probe was used, and the pre-test descending speed (V₁), test penetration speed (V₂), and post-test return speed (V₃) were all set at 2 mm/s. The measurement conditions were as follows: 25% compression ratio relative to the initial height of jujubes, 5 s holding time, 5 g trigger force, and 2 compression cycles. Hardness, springiness, cohesiveness, gumminess, chewiness, and resilience of the samples were automatically calculated by the instrument software. Six parallel tests were performed for each sample, and the values of each texture parameter were analyzed and calculated by the instrument software.
2.13 Electronic Nose Analysis
The experiment was performed according to the method reported by Liu et al.39 A 2.0 g aliquot of the jujube sample was weighed and transferred into a 20 mL headspace vial. After equilibration at 25 ℃ for 2 h, the sample was tested using an iNose electronic nose (Ruifen International Trade Co., Ltd., Shanghai, China). The test parameters were set as follows: 10 s preparation time, 1.0 L/min filtered air flow rate, and 2 min detection time. Before each measurement, the sensor was purged with clean air at 1.0 L/min for 2 min to remove any residual volatiles and reset the signal to baseline.
2.14 Gas Chromatography-Mass Spectrometry Analysis
The experiment was performed according to the method described by Cheng et al.40, with appropriate modifications. An accurately weighed 2.000 g aliquot of the sample was transferred into a 15 mL extraction vial, which was then sealed immediately. Volatile components were extracted by headspace solid-phase microextraction (HS-SPME), and analyzed using a gas chromatography-mass spectrometer (Shimadzu GC-2030AM). A DVB/CAR/PDMS extraction fiber was used. The sample was equilibrated at 50 ℃ for 40 min and desorbed at 250 ℃ for 5 min.
Chromatographic conditions: An SH-Polar WAX column (60 m × 0.25 mm, 0.25 μm) was used. The initial column temperature was held at 40 ℃ for 3 min, then raised to 120 ℃ at 5 ℃/min, followed by a further increase to 200 ℃ at 10 ℃/min and held for 5 min. The injector temperature was set at 250 ℃, and the transfer line temperature at 240 ℃. Helium (He) was used as the carrier gas at a flow rate of 1.0 mL/min. Splitless injection mode was used.
Mass spectrometric conditions: An electron ionization (EI) source was used with an electron energy of 70 eV. The ion source temperature was set at 230 ℃, and the quadrupole temperature at 150 ℃. Full scan mode was used, with a mass range of 35–550 u.
2.15 Statistical Analysis
Data collected in this study were subjected to statistical analysis. All treatments were performed in three biological replicates and three technical replicates. Analysis of variance (ANOVA) was performed using IBM SPSS Statistics 26 to evaluate significant differences among treatments. Data were plotted using Origin 2024, and comparisons were performed among different treatment groups. Multiple comparisons were performed using the least significant difference (LSD) test at a significance level of p < 0.05. Results are expressed as mean ± standard error (SE).
3. Results3.1 Optimization of AEW Inactivation3.1.1 Single-Factor Experiments
The microbial inactivation results of the single-factor experiment by AEW (Figure 1) indicated that the inactivation efficacy increased with the increasing NaCl concentration. The inactivation rate reached a maximum with good stability when the NaCl concentration was increased to 100 mg·L⁻¹. No further improvement was observed with the continued increase in NaCl concentration after that value. In addition, the inactivation rate increased gradually with increasing treatment duration, reaching 100% at the treatment durations of 10 min and longer. As for the solid-liquid ratios (m:V), the inactivation rate increased steadily with increasing solid-liquid ratio, then leveled off. A satisfactory inactivation effect was achieved when the solid-liquid ratio was set at 1:100, and no further improvement in the inactivation rate was observed with a continued increase in the solid-liquid ratio after that value.
Microbial inactivation results of Single-factor experiments by AEW. (A) Effect of NaCl concentration on microbial inactivation rate. (B) Effect of treatment duration on microbial inactivation rate. (C) Effect of solid-liquid ratio on microbial inactivation rate. Different lowercase letters indicate significant differences (p < 0.05) among treatments.
3.1.2 Orthogonal Experiments and Stability Verification
Based on the above results of single-factor experiments, the factors and levels of the orthogonal experiments were set in Table 1. The conditions and results of the orthogonal experiments are presented in Table 2.
Factor and level settings of the orthogonal experiments.
No.
A
B
C
Electrolyte Concentration (mg/L)
Treatment Duration (min)
Solid-Liquid Ratio (m:V, g:mL)
1
50
5
1:50
2
100
10
1:100
3
150
15
1:200
Microbial inactivation results of orthogonal experiments
No.
A
B
C
D (Blank Column)
Inactivation rate (%)
1
1
1
1
1
81.33 ± 5.24
2
1
2
3
2
86.93 ± 6.76
3
1
3
2
3
85.30 ± 5.72
4
2
1
3
3
92.07 ± 3.06
5
2
2
2
1
99.91 ± 0.11
6
2
3
1
2
97.45 ± 1.04
7
3
1
2
2
94.82 ± 2.67
8
3
2
1
3
95.41 ± 3.02
9
3
3
3
1
99.18 ± 0.33
k1
0.8452
0.8941
0.9140
-
-
k2
0.9648
0.9408
0.9334
-
-
k3
0.9647
0.9398
0.9273
-
-
R
0.1196
0.0468
0.0195
-
-
Note: The calculations of the k values are based on the mean values of the parallel experiments.
Range (R) analysis was performed on the orthogonal experiment results (Table 2). The R value of electrolyte concentration was 0.1196, that of treatment duration was 0.0468, and that of solid-liquid ratio (m:V) was 0.0195. The order of influence of the three factors on microbial inactivation rate is as follows: electrolyte concentration > treatment duration > solid-liquid ratio (m:V). The optimal experimental conditions were determined as follows: electrolyte concentration of 100 mg/L, treatment duration of 10 min, and solid-liquid ratio (m:V) of 1:100. In addition, the analysis of variance (ANOVA) was performed on the orthogonal experiment results, indicating that electrolyte concentration had a significant effect on the microbial inactivation rate (Table 3).
Analysis of Variance (ANOVA) for the orthogonal experiment results.
Factor
Type Ⅲ Sum of Squares
Degree of Freedom (df)
Mean Square
F-Value
p-Value (Significance)
Electrolyte Concentration
0.029
2
0.014
25.445
0.038
Treatment duration
0.004
2
0.002
3.808
0.208
Solid-Liquid Ratio (m:V)
0.001
2
0
0.529
0.654
Error
0.001
2
0.001
-
-
R2 = 0.968 (Adjusted R2 = 0.870)
Triplicate experiments were performed under the optimal experimental conditions, and the TPC, coliforms, and molds of the prepared samples were determined separately. The experimental results are shown in Table 4. The inactivation effect of the parallel replicate experiments was relatively stable, with average microbial inactivation rates of 99.29%, indicating significant inactivation efficacy.
Results of parallel replicate experiments.
No.
Before AEW Inactivation
After AEW Inactivation
Inactivation Rate*
TPC
Coliforms
Molds
TPC
Coliforms
Molds
(CFU/g)
(CFU/g)
(CFU/g)
(CFU/g)
(CFU/g)
(CFU/g)
(%)
1
3890 ± 163
1410 ± 44
390 ± 46
27 ± 3
7 ± 1
4 ± 0
99.31 ± 0.26
2
4210 ± 191
1290 ± 37
420 ± 17
30 ± 4
9 ± 1
3 ± 0
99.29 ± 0.21
3
4030 ± 188
1350 ± 48
370 ± 26
26 ± 2
6 ± 0
4 ± 1
99.35 ± 0.18
Note:*The microbial inactivation rate was calculated based on the change in TPC.
3.2 Optimization of Sequential Hot Air and Microwave Drying Conditions3.2.1 Effects of Hot Air Drying and Microwave Drying on VC Content and Color of Jujubes
Hot air drying temperature exerted significant effects on VC retention rate and surface color of the obtained jujube samples. When the drying temperatures were set at 65 ℃ and below, the VC retention rates and color difference variations were well controlled. In contrast, drying temperatures of 70 ℃ and above induced substantial VC losses and pronounced changes in the surface color of jujubes (Figures 2 (A1, A2)).
Effects of hot air drying and microwave drying on VC content and surface color of jujubes. (A1) Effect of hot air drying temperature on VC retention rate. (A2) Effect of hot air drying temperature on surface color of jujubes. Figures. (B1) Effect of hot air drying duration on VC retention rate. (B2) Effect of hot air drying duration on surface color of jujubes. (C1) Effect of microwave drying duration on VC retention rate. (C2) Effect of microwave drying duration on surface color of jujubes. Different lowercase letters indicate significant differences (p < 0.05) among the treatments.
Hot air drying duration exerted relatively minor effects on the VC retention rate and surface color of the obtained jujube samples. When the drying temperatures were maintained at 65 ℃, the VC retention rates of jujubes exhibited a steady declining trend with prolonged drying duration; simultaneously, no abrupt changes were observed in the surface color of the jujubes. When drying durations were no longer than 120 min, the color difference varied uniformly with prolonged time. When the drying durations exceeded 120 min, the surface color of jujubes tended to stabilize and showed no significant changing trend (Figures 2 (B1, B2)).
Microwave drying duration had no significant influence on the VC retention rate of jujubes, with the average retention rates of all treatments exceeding 90% (Figure 2 (C1)). In contrast, microwave drying duration exerted a significant effect on the surface color of jujubes. When the treatment durations were extended to 40 s and above, the surface color of jujubes in the experimental groups differed significantly from that of the control group (Figures 2 (C1, C2)).
3.2.2 Effects of Hot Air Drying and Microwave Drying on Moisture Content and Microbial Load of Jujubes
After jujubes were subjected to AEW inactivation treatments, their moisture content increased slightly. Under the hot air drying condition of 65 ℃, the moisture content of jujubes decreased to approximately 21.5 wt% after 150 min of drying, and then stabilized (Figure 3A). Further prolonging drying duration did not cause significant differences in jujubes’ moisture content. Under the microwave drying condition of 800 W, the drying rate within 20–40 s was the fastest compared with those at other drying durations. The drying rate stabilized when the moisture content of jujubes decreased to around 22 wt%, and the moisture content could be reduced to approximately 20 wt% within 40–80 s (Figure 3B).
Effects of hot air drying at 65 ℃ and microwave drying at 800 W on the moisture content of Jujubes. (A) Effect of hot air drying on moisture content. (B) Effect of microwave drying on moisture content. Different lowercase letters indicate significant differences (p < 0.05) among treatments.
The microbial inactivation effect of hot air drying at 65 ℃ on jujubes was negligible (Figure 4A), with microbial reduction rates of only approximately 8.5% after 180 min of drying. In contrast, the microbial inactivation effect of microwave drying at 800 W was extremely significant; microbial reduction rates of approximately 99% were achieved by treating jujubes at 800 W for not less than 50 s (Figure 4B).
Effects of hot air drying at 65 ℃ and microwave drying at 800 W on microbial inactivation efficacy of jujubes. (A) Effects of hot air drying at 65 ℃ on microbial inactivation efficacy. (B) The effects of microwave drying at 800 W on microbial inactivation efficacy. Different lowercase letters indicate significant differences (p < 0.05) among the treatments.
3.2.3 Parameter Optimization for Sequential Hot Air and Microwave Drying of Jujubes
Based on the single-factor experiment results of hot air drying, temperatures of 70 ℃ and above exerted a significant destructive effect on VC. In contrast, drying duration exerted a minimal impact on the VC retention rate during drying treatments at 65 ℃. Meanwhile, hot air drying temperatures of 70 ℃ and above also induced significant changes in the surface color of jujubes (Figures 2(A1, A2)). To maximize VC retention, minimize color difference, and shorten the overall drying duration, a hot air drying temperature of 65 ℃ was selected as the optimal condition.
Based on the single-factor experiment results of microwave drying, the treatment duration had no significant effect on the VC retention rate. However, when the treatment durations exceeded 40 s, the surface color of jujubes changed considerably. To avoid severe discoloration of the jujube peel, the microwave drying duration should be controlled within 40 s (Figures 2 (C1, C2)). Considering that microwave drying has a slight destructive effect on VC (Figure 2 C1) and provides high drying efficiency (Figure 3 B), the microwave drying duration should be extended as much as possible within the permitted duration range to shorten the total drying duration. Therefore, 40 s was determined as the optimal microwave drying duration.
Given that the hot air drying durations at 65 ℃ exerted minimal impacts on the VC retention rate and surface color of jujubes, with uniform variations observed for both indicators (Figures 2 (B1, B2)), the hot air drying duration needs to be ultimately determined based on the requirement for the moisture content of jujubes at the end of the hot air drying stages. The drying endpoint of RTE jujubes was set at a moisture content of 20.0 wt%. Given that the microwave drying power and duration had already been set up, the moisture content of jujubes prior to microwave drying needed to be controlled at approximately 22.5 wt% to ensure the target moisture content of 20.0 wt% in the final samples after microwave drying treatments. Meanwhile, the hot air drying temperature had also been fixed at 65 ℃, and it took 120 min to reduce the jujube moisture content to around 22.5 wt% under those conditions (Figure 3A). Therefore, a hot air drying duration of 120 min was finalized as the optimal parameter.
In summary, the raw jujubes with an initial moisture content of 24.5 ± 1.1 wt% experienced a slight increase in moisture content to 28.1 ± 0.6 wt% after AEW inactivation treatment. Subsequently, the moisture content of these jujubes was reduced to approximately 22.5 ± 0.1 wt% after hot air drying at 65 ℃ for 120 min, and further decreased to 20.0 ± 0.1 wt% following microwave drying at 800 W for 40 s.
During this sequential drying process, hot air drying provided a microbial reduction rate of approximately 5%, whereas microwave drying gave a microbial reduction rate of about 93%, resulting in a total microbial reduction rate of 98%, which declared a significant microbial inactivation efficacy.
3.3 Comprehensive Quality Evaluations of RTE Jujubes
Integrating the advantages of the three aforementioned treatment methods by the developed sequential combination in this work, RTE jujubes were prepared under the optimized conditions. The resultant samples were evaluated in terms of nutritional value, sensory quality, and food safety. The sample designations and their corresponding treatment protocols are summarized in Table 5.
Sample Information.
Sample ID
Mode of Treatment
Kgj-r
Harvested from Maigaiti County, Kashgar Prefecture, Xinjiang Uygur Autonomous Region, and simply rinsed with tap water.
Kgj-r+AEW
Kashgar grey jujube treated with AEW for inactivation.
Kgj-r+AEW+Hd
Kashgar grey jujube treated with AEW for inactivation, followed by hot air drying at 65 ℃ for 120 min.
Kgj-r+AEW+Hd+Md(RTE jujubes)
Kashgar grey jujube treated with AEW for inactivation, followed by hot air drying at 65 ℃ for 120 min and microwave drying at 800 W for 40 s.
Kgj-r+Hd
Kashgar grey jujube simply rinsed with tap water, followed by hot air drying at 65 ℃ for 360 min.
Kgj-p
RTE Jujubes of the same variety and origin were purchased from the official online store of a certain brand.
3.3.1 Contents of Vitamins (B₁, B₂ and C) in RTE Jujubes
The contents of vitamins (B₁, B₂, and C) in jujubes obtained by various processing modes were determined and shown in Figure 5. Following the sequential applications of AEW inactivation, hot air drying, and microwave drying, the contents of the three vitamins exhibited overall significant decreasing trends. Taking the vitamin contents in the raw Kashgar grey jujubes (Kgj-r) as benchmarks, the changes in vitamins of other jujube samples were assessed. The average retention rates of VB₁, VB₂, and VC in the RTE jujubes reached 72.55%, 83.28%, and 64.91%, respectively. As to Kgj-r+Hd by the hot air drying conditions at 65 ℃ for 360 min, the average retention rates of vitamins B₁ and B₂ were 67.27% and 80.41%, respectively, whereas that of VC was only 47.42%. These results indicated that sequential hot air and microwave drying significantly improved the VC retention rate, with an increase of 17.49%. Meanwhile, it should be noted that the contents of VB₁ and VB₂ were less affected by various processing; although their contents also exhibited decreasing trends, the changes were relatively insignificant compared with those of VC. Compared with those in the Kgj-p group, the commercially available RTE products of the same variety and origin, the contents of the vitamins in the RTE jujubes (Kgj-r+AEW+Hd+Md) were higher. These results fully demonstrated that the RTE jujubes prepared by the proposed process retained higher vitamin contents than those of the control groups.
Effects of various processing on vitamin (B₁, B₂, and C) contents in jujubes. Different lowercase letters indicate significant differences (p < 0.05) in the same indicator among different samples.
3.3.2 Contents of Bioactive Substances (Total Polyphenols, Total Flavonoids and Total Triterpenic Acids) in RTE Jujubes
The variation trends of total polyphenols, total flavonoids, and total triterpenic acids in jujubes prepared at various processing modes (Figure 6) were similar to those of vitamins (Figure 5). As to the RTE jujubes, the average retention rates of total polyphenols, total flavonoids, and total triterpenic acids reached 81.06%, 76.07%, and 90.91%, respectively (Figure 6). As for Kgj-r+Hd jujubes under the hot air drying conditions at 65 ℃ for 360 min, their average retention rate of total polyphenols was 71.31%, representing a 9.75% reduction compared with that of the RTE jujubes. Meanwhile, the average retention rates of total flavonoids and total triterpenic acids were 73.81% and 85.81%, with respective decreases of 2.26% and 5.10%. These differences were statistically significant (p < 0.05). Compared with those of the Kgj-p group, the contents of total polyphenols, total flavonoids, and total triterpenic acids in the RTE jujubes were significantly higher. These results fully confirmed that the RTE jujubes prepared by the proposed method retained higher contents of bioactive substances.
Effects of various processing modes on bioactive substances (total polyphenols, total flavonoids, and total triterpenic acids) contents in jujubes. Different lowercase letters indicate significant differences (p < 0.05) in the same indicator among different samples.
3.3.3 Surface Color and Whole Fruit Texture of RTE Jujubes
The surface color and texture properties of jujubes prepared at various processing modes were determined and shown in Figures 7–9. The surface color of the RTE jujubes was close to that of the raw material, with no significant differences in L* (lightness), a* (redness), and b* (yellowness) values (Figure 7). No obvious changes in the surface color appearances were observed in the optical photographs compared with those of the other samples (Figure 8), indicating that the proposed process had no significant effect on the surface color of the RTE jujubes. However, combined AEW inactivation and hot air drying led to a decrease in the L* value, along with increases in the a* and b* values of jujube peel. Hot air drying maintained the L* value while causing decreases in the a* and b* values of jujube peel. Microwave drying led to an increase in the L* value as well as decreases in the a* and b* values of jujube peel. All the aforementioned differences were statistically significant (p < 0.05). Compared with that of the Kgj-r+Hd (single hot air drying) group, sequential hot air-microwave drying reduced the changes in jujube surface color relatively. Compared with that of the Kgj-r+p group, the RTE jujubes prepared by the proposed process exhibited a similar surface color to the commercially available products, with no significant difference in visual appearance.
Significant changes were observed in the texture parameters of the RTE jujubes. Among the six measured parameters (hardness, springiness, cohesiveness, gumminess, chewiness, and resilience), hardness, gumminess, and chewiness increased significantly, whereas springiness, cohesiveness, and resilience decreased significantly (Figure 9). Such changes are common during various drying processes, especially in the later stages, mainly due to the gradual water loss of jujubes and the resulting altered internal water distribution. It is worth noting that compared with those of the Kgj-r+Hd and Kgj-r+p group, the RTE jujubes prepared by the proposed process showed significant differences (p < 0.05) in three parameters, namely hardness, cohesiveness, and resilience. Specifically, hardness was relatively higher, whereas cohesiveness and resilience were relatively lower. These findings declared that the incorporation of microwave drying had a significant impact on the texture properties of jujubes.
Effects of various processing modes on the surface color of jujubes. Different lowercase letters indicate significant differences (p < 0.05) in the same indicator among different samples.
Effects of various processing modes on the surface color of jujubes. Letters A through F denote optical photographs of the six jujube sample groups. (A) Kgj-r. (B) Kgj-r+AEW. (C) Kgj-r+AEW+Hd. (D) Kgj-r+AEW+Hd+Md (RTE jujubes). (E) Kgj-r+Hd. (F) Kgj-p.
Effects of various processing modes on texture properties of jujubes. (A) Effects of the processing procedures on the hardness, gumminess, and chewiness of jujubes. (B) Effects of the processing procedures on the springiness, cohesiveness, and resilience of jujubes. Different lowercase letters indicate significant differences (p < 0.05) in the same indicator among different samples.
3.3.4 Volatile Compounds of RTE Jujubes
The detection and evaluation of jujube aroma compounds were performed using an electronic nose and GC-MS. The results were shown in Figures 10 and 11.
The electronic nose measurement results showed that the volatile flavor compounds in jujubes were dominated by amines S1 and volatile organic compounds S8 (Figure 10 C). The cumulative variance contribution rate of the first two principal components derived from principal component analysis (PCA) reached 85.3% (Figure 10 A), which could explain most of the flavor characteristics of jujubes. As indicated by the response scores of sensors S1 and S8 in the radar chart, hot air drying significantly enhanced jujube aroma, with the treated jujubes exhibiting a richer flavor profile (Figure 10 C). Based on the sample group distribution in the PCA score plot (Figure 10 B), the flavor characteristics of the RTE jujubes were relatively similar to those of the raw jujubes. These findings indicated that the RTE jujubes retained desirable aroma characteristics, which would likely improve consumer acceptability.
Electronic nose analysis of the effects of various processing modes on volatile flavor compounds in various jujubes. (A) PCA score plot. (B) PCA loading plot. (C) Radar chart of electronic nose sensor responses.
GC-MS identified a total of 90 volatile flavor compounds (Figure 11), including 21 organic acids, 28 ketones, 11 esters, 6 alcohols, 10 aldehydes, 3 alkenes, 4 alkanes, and 7 aromatic compounds. Among the six sample groups, acetic acid, hexanoic acid, and 3-hydroxy-2-butanone were the three most abundant compounds, which tightly dominated the aroma characteristics of the RTE jujubes.
Based on the quantification results of volatile flavor compounds in each group, AEW inactivation significantly reduced the proportions of acetic acid and hexanoic acid in jujube volatile flavor compounds, while exerting minimal effect on the contribution of 3-hydroxy-2-butanone. In contrast, hot air drying significantly increased the proportions of acetic acid and hexanoic acid and decreased the contribution of 3-hydroxy-2-butanone in jujube volatile flavor compounds. Compared with those of the two control groups, the RTE jujubes prepared by the proposed process had similar proportions of acetic acid and hexanoic acid, along with a higher contribution of 3-hydroxy-2-butanone. This phenomenon might be attributed to the short-term high-temperature environment induced by microwave drying, where small-molecule products were generated via the Maillard reaction during carbohydrate conversion, thereby providing a richer creamy aroma to the jujubes. In addition, the proportions of butyric acid, heptanoic acid, capric acid, and valeric acid in the RTE jujubes all exceeded 2%, which disclosed that these components also acted as important contributors to the jujube aroma.
Under the optimal process combination established in this study, RTE jujubes were prepared. The TPC, coliforms, and mold count of the products were determined and compared with those of the raw jujube materials (Table 6). With reference to the microbial limit requirements specified in the National Standard of China (GB 14884-2016), the safety quality of the RTE jujubes processed by alkaline AEW combined with sequential hot air and microwave drying fully met the standard requirements.
Microbial counts of RTE jujubes.
No.
Kgj-r
Kgj-r+AEW+Hd+Md(RTE jujubes)
Inactivation Rate*
TPC
Coliforms
Molds
TPC
Coliforms
Molds
（CFU/g）
（CFU/g）
（CFU/g）
（CFU/g）
（CFU/g）
（CFU/g）
（%）
1
3890 ± 163
1410 ± 44
390 ± 46
4 ± 1
1 ± 0
0 ± 0
99.90 ± 0.14
2
4210 ± 191
1290 ± 37
420 ± 17
0 ± 0
0 ± 0
0 ± 0
100.00 ± 0.00
3
4030 ± 188
1350 ± 48
370 ± 26
3 ± 1
0 ± 0
0 ± 0
99.93 ± 0.07
Note:*The microbial inactivation rate was calculated based on the change in TPC.
4. Discussion
In this study, jujubes were selected as the research subject to optimize the optimal treatment conditions for AEW inactivation. Under the optimal conditions, an averaged total microbial inactivation rate of 99.32% was confirmed in triplicate inactivation experiments, with remarkable inactivation efficacy on coliforms and molds. Notably, AEW also exhibited favorable inactivation performance against a broad spectrum of microorganisms. Khalid et al.41 investigated the inactivation effect of AEW on E. coli adhering to stainless steel surfaces with meat residues, reporting an inactivation rate of approximately 81.35% when the initial E. coli concentration was 10⁵ CFU/cm². Huang et al.42 applied sequential treatment with AEW and acidic electrolyzed water, achieving an inactivation rate of 99.99% against Listeria monocytogenes at an initial bacterial concentration of 10⁶–10⁸ CFU/g in the control group. Liu et al.43 employed the same sequential electrolyzed water treatment and achieved a 94.4% inactivation rate against Pseudomonas aeruginosa biofilms at an initial bacterial concentration of 10⁷–10⁸ CFU/g in the control group. In this study, the initial microbial concentrations of jujube samples were at the levels of approximately 10³ CFU/g, indicating relatively low challenge in inactivation. This enabled the AEW inactivation to give stable and remarkable effects.
Meanwhile, as a mild inactivation method, AEW treatment exerted minimal impact on the quality attributes of jujubes. In this study, the contents of VB₁, VB₂, VC, total polyphenols, total flavonoids, total triterpenic acids, as well as surface color, whole-fruit texture properties, and volatile flavor compounds in jujube samples before and after AEW treatment were determined and analyzed. The experimental results confirmed that AEW treatment only exerted slight effects on the vitamin contents, surface color, and volatile flavor compounds of jujubes. Although these indices exhibited statistical significance, the magnitudes of the actual differences were negligible. Specifically, the changes in the contents of the three vitamins after AEW treatment were all less than 10%, the proportions of major volatile flavor compounds showed no significant alterations, and no visually distinguishable differences were observed in the surface color of the tested samples. These results suggest that the effects of AEW treatment on jujube quality attributes can only be accurately identified via precision analytical instruments and are imperceptible to consumers. Moreover, some published studies have also confirmed that AEW treatment causes minimal damage to the nutritional components and quality attributes of the treated fruits and vegetables, and even improves certain quality-related indices. For instance, Belay et al.44 found that nectarines treated with AEW retained good freshness during the designed storage, particularly in terms of texture and color attributes. Nyamende et al.17 reported that AEW treatment significantly reduced the microbial count on apple surfaces by 3 log units, while simultaneously improving the attributes in hardness, color, texture, contents of polyphenols and flavonoids, as well as antioxidant activity.
Drying is a critical processing step in the production of RTE jujubes, exerting a pronounced influence on the comprehensive quality of the final products. This study investigated the effects of sequential hot air-microwave drying on the key quality attributes of jujubes and quantified the critical physicochemical indices of the jujubes prepared under the optimized treatment conditions. Among these indices, vitamins are key quality markers for fruit products. Jujubes are naturally rich in various vitamins.7 As far as optimizing drying parameters is concerned, the findings of this study agree well with those of Niu et al.24 the loss of VC increased with the increased drying temperature and the extension of drying duration. This phenomenon can be attributed to the combined effects of oxidation and thermal degradation. Elevating the temperature accelerates the degradation of VC, while prolonging drying duration extends the exposure time of jujubes to hot air, which actually increases oxidative damage. Based on the variation in VC retention rate, it can be concluded that prolonging drying duration has a significantly weaker destructive effect on VC in jujubes than increasing drying temperature. Polyphenols, flavonoids, and triterpenic acids act as representative bioactive compounds in fruits. They have also been reported to be abundant in jujubes.2 In this study, the average retention rate of total polyphenols in jujubes Kgj-r+Hd subjected to hot air drying at 65 ℃ for 360 min was 71.31%. In contrast, Lu et al.2 reported a total polyphenol retention rate of 69.53% when jujubes were dried at 70 ℃. These results confirm that drying at higher temperatures tends to induce more severe degradation of polyphenols in dried jujubes
Appearance and texture of the commercial jujubes are important factors influencing consumers’ product preferences, especially the surface color, which exerts a more significant impact on the buying decisions.27 Consistent with the findings of Fang et al.27, the results of drying whole jujubes under comparable experimental conditions demonstrated that higher hot air drying temperatures induced more pronounced alterations in the surface color. Drying temperatures of 70 ℃ and above exerted particularly significant influences on surface color, whereas the effects of various treatment durations on color difference were relatively minor. Generally speaking, owing to the inherent characteristics of microwave drying, namely its high heating efficiency and volumetric heating mode, this technique enables rapid dehydration of the treated samples. Consequently, the parenchyma cells of the treated jujubes lose turgor pressure due to water loss, undergo rapid shrinkage and collapse, and form densely porous network structures. Meanwhile, it exerts considerable damage to the cohesions among the cells.45,46 Macroscopically, these microstructural changes are manifested as an increase in jujube hardness, along with reductions in cohesiveness and springiness. Therefore, when applying microwave drying to fruits and vegetables, attention should be paid to the drying power and duration. Excessively high power and prolonged treatment times can exert significant impacts on the textural properties of the prepared samples, potentially affecting consumer acceptability.
The characteristic aroma of jujubes is composed of a complex mixture of volatile flavor compounds, and the relative proportions of these compounds constitute the aroma profiles.23 Aroma is also one of the key sensory factors influencing consumers’ buying preferences. It should be pointed out that the relative proportions of 3-hydroxy-2-butanone of all six sample groups, including the raw jujube group (Kgj-r) in this work, exceed 15%, which is much higher than the reported value of 3-hydroxy-2-butanone in other kinds of fresh jujubes.47–49 The explanations are as follows. The natural origins of 3-hydroxy-2-butanone are dominated by microbial metabolic synthesis, supplemented by minor contributions from plant endogenous biosynthesis and non-enzymatic transformation pathways.50 This compound is widely distributed in ripe fruits (e.g., apples, strawberries, grapes, cantaloupes), vegetables (e.g., asparagus, broccoli, tomatoes), and grains (e.g., wheat, corn)50, yet it is present only in trace amounts in the other kinds of fresh jujubes. As to this work, the elevated relative contents may be associated with the delayed harvest in the producing regions of the kind of jujube. The mature jujubes hang on the trees for up to three months before the harvest, during which time yeasts colonizing the fruit surfaces may produce 3-hydroxy-2-butanone via fermentation.51 Samples of hot air drying exhibit relatively low aldehyde contents, which is consistent with the experimental findings reported by Wan et al.26 This phenomenon may be attributed to the oxidation of fatty acids and the metabolism of amino acids under elevated temperature conditions during drying treatments. Spadafora et al.52 pointed out that the multiple factors, including fruit maturity, storage temperature, and processing technology, all exert significant effects on the compositions of volatile flavor compounds in fruits.
It is worth noting that, given the concentrated cultivation history and primary consumption of jujubes in China, there are currently no specific microbial limit standards specifically formulated for RTE jujubes (dried fruit products) at the international level. Instead, only relevant general food standards and recommended industry practices are available for reference. Neither the U.S. Food and Drug Administration (FDA) nor the European Union (EU) has established mandatory limits for the TPC of RTE dried fruits. However, a limit of ≤ 10⁴ CFU/g has been widely adopted as an internal hygiene qualification threshold, which is highly consistent with the TPC requirements specified in the National Standard of China (GB 14884-2016). As to coliform limit requirements, GB 14884-2016 is fully consistent with the relevant specifications outlined in the EU’s food microbiological criteria (EC No 2073/2005), whereas the FDA has not established mandatory limits for this indicator. For mold limits, GB 14884-2016 (≤ 50 CFU/g) is more stringent than the European industrial internal control threshold (≤ 100 CFU/g). Additionally, the FDA’s Compliance Policy Guide (CPG 7110.09) explicitly specifies that mold testing for dried fruits shall utilize the Howard Mold Count (HMC) method (AOAC Official Method 970.75), with sample compliance determined by a positive field rate of < 20% rather than the conventional colony counting method (CFU/g). Notably, jujube samples with the mold limit specification (≤ 50 CFU/g) stipulated in the National Standard of China (GB 14884-2016), typically exhibit a Howard Mold Count (HMC) positive field rate of ≤ 10%, thereby concurrently satisfying the relevant compliance requirements of the FDA.
The AEW inactivation process, coupled with sequential hot air and microwave drying, enables the efficient production of high-quality RTE jujubes. However, scaling this technology for large-scale industrial applications still requires overcoming some technical and manufacturing bottlenecks. First, the subtle textural alterations in the jujubes induced by microwave drying may exert some measurable impacts on consumer sensory experiences. It is imperative to clarify how such changes influence market uptake by scaling up experimental sample sizes and conducting multi-regional consumer preference surveys. In addition, the process’s sophisticated configuration demands systematic optimization of all operational parameters during the preliminary industrialization. Quality inconsistencies stemming from fluctuations in raw material attributes and dynamic shifts in production environments represent core barriers to consumer acceptance and product market competitiveness. Meanwhile, although sequential hot air and microwave drying boosts drying efficiency relatively to conventional single hot air drying, it also leads to substantial increases in energy costs, equipment investment, and operational and maintenance expenses. Compounding these challenges with the technical hurdles of integrating disparate drying units and coordinating robust multi-parameter control systems, it further raises the barrier level of industrial scalability. Notably, this technology is promising to extend to diverse fruit categories, developing tailored drying protocols that balance efficiency and quality for different fruit varieties, while accounting for their unique physicochemical properties and quality requirements remains a challenge.
5. Conclusion
This study investigated the microbial inactivation efficacy of AEW treatment on jujubes, and identified the optimal treatment conditions as follows: electrolyte (NaCl) concentration of 100 mg·L⁻¹, treatment duration of 10 min, and solid-liquid ratio (m:V, g:mL) of 1:100. In addition, the drying effects of sequential hot air and microwave drying on jujubes were explored. And the optimal drying parameters were confirmed as follows: hot air drying temperature of 65 ℃, hot air drying duration of 120 min, microwave drying power of 800 W, and microwave drying duration of 40 s. The characterization results demonstrated that the integration of AEW inactivation and sequential hot air and microwave drying can produce high-quality RTE jujubes, with the advantages of high nutrient retention, superior edible quality, high food safety, and significantly enhanced production efficiency. Under the optimal conditions, the retention rates of vitamins (B1, B2, C) and bioactive compounds (total polyphenols, total flavonoids, and total triterpenic acids) in the prepared RTE jujubes were all higher than those of the jujubes processed via the conventional hot air drying method. Specifically, compared with those of the control samples via conventional hot air drying, the contents of VC and total polyphenols of the prepared RTE jujubes were increased by 36.88% and 13.67%, respectively. Meanwhile, this combined processing method minimized alterations in surface color and texture properties, enriched the characteristic flavor profile, and consequently improved the edible quality of the final products. Furthermore, all microbial indices of the final jujube products fully complied with the requirements specified in the National Standard of China (GB 14884-2016), indicating high food safety compliance. In conclusion, considering the three core aspects of nutrient retention, edible quality, and food safety, the RTE jujubes prepared via the integrated processing technology exhibit favorable comprehensive qualities. Therefore, this technology is promising for the industrial production of high-quality RTE jujubes.
AI Use Declaration
During the preparation and revision of this manuscript, the authors used the following artificial intelligence–assisted tool(s): [DeepSeek], solely for the purpose of improving the clarity, readability, and linguistic quality of the text. All scientific content, including the study design, data acquisition, data analysis, interpretation of results, discussion, and conclusions, was conceived, generated, and critically verified by the authors. The authors take full responsibility for the accuracy, originality, and integrity of the published work.
Acknowledgments
Not applicable.
Author Contributions
Difei Jiang: Conceptualization, Data curation, Formal analysis, Methodology, Validation, Visualization, Writing - original draft. Muhammad Adil: Formal analysis, Writing - review & editing. Xinglong Xiao: Formal analysis, Writing - review & editing. Yigang Yu: Conceptualization, Project administration, Resources, Supervision, Writing - review & editing. All authors have reviewed and agreed to the final version of this manuscript for publication.
Conflicts of Interest
The authors declare they have no competing interests. The commercial products, instruments, and software mentioned in this study were used solely for experimental purposes. Their mention does not imply endorsement or promotion by the authors or the journal.
Data Availability Statement
Not applicable.
Funding
This research is supported by the Food Processing Aid Team from South China University of Technology for Xinjiang, the Guangzhou Science and Technology Plan Project (2024B03J1177), and the Fundamental Research Funds for the Central Universities (2025ZYGXZR087).
Cite this Article
Jiang, D.F., Adil, M., Xiao, X.L., et al. Improving the Quality of Ready-to-eat Jujubes by Alkaline Electrolyzed Water Inactivation Coupled with Sequential Hot Air and Microwave Drying. Adv Funct Foods. 2026;2(1):98–123. https://doi.org/10.64187/aff.2026.v2.i1.006
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