Original paper
Simultaneous dewatering and wax extraction of Chinese winter jujube (Ziziphus jujuba Mill. cv. Dongzao) fruit by subcritical dimethyl ether
2021 年 27 巻 5 号 p. 711-723
詳細
2021 年 27 巻 5 号 p. 711-723
Due to its highly perishable nature, fresh Chinese winter jujube fruit is preserved, commonly by drying. In this study, dewatering of fresh Chinese winter jujube was carried out by subcritical dimethyl ether (DME). Additionally, wax was simultaneously extracted from the fruit during the dewatering process. The maximum dehydration efficiency was 93.33 ± 0.34% when the dewatering experiment was conducted at 55 °C with 8 cycles. The content of total soluble solids, total sugars, and total flavonoids of dried jujube fruit (JD) by subcritical DME dewatering was somewhat lower than those in the freeze-dried jujube fruit (JF). However, the content of total phenols and VC in JD was close to those in JF, and the protein content in JD was higher than that in JF. The extracted wax primarily comprised acids, the most abundant of which was hexadecanoic acid. It had melting points of 53.71 °C–54.62 °C. The study demonstrates that subcritical DME can be used to dry Chinese winter jujube fruit. There is a small loss of nutritional content, but the speed, economy, and environmental friendliness of the process are advantages compared to other methods. Results further suggest that jujube wax has potential for use in the cosmetic and food industries.
Chinese winter jujube (Ziziphus jujuba Mill. cv. Dongzao) is an important economic crop in northern China (Wang et al., 2019a). The fruits are rich in many valuable nutrients such as vitamin C (VC), cyclic adenosine monophosphate (cAMP), polysaccharides, amino acids, phenolic compounds, and minerals (Kou et al., 2019; Wang et al., 2019b; Sun et al., 2019). Though it has high quality and abundant nutrition, unfortunately, fresh jujube fruits have a short shelf life (less than ten days) at ambient temperatures, due to rapid ripening after harvest (Hui et al., 2015). As a result, most jujubes are preserved, by drying or processing, soon after harvest.
Drying is efficient and widely used to preserve jujubes. The common drying methods are sun-, oven-, and microwave-drying. These drying methods belong to thermal drying and can result in significant declines in nutrient content and quality. To avoid this degradation, non-thermal drying methods such as freeze-drying and osmotic dehydration have been studied during the last decade. Recently, subcritical dimethyl ether (DME) as a new solvent has received wide attention. DME in subcritical state has distinctive properties, namely the affinity to both lipophilic and hydrophilic substances. Subcritical DME is used for drying raw materials due to its unique capacity to merge with the water of the sample (Kanda et al., 2011; Yano et al., 1978; Kanda et al., 2010; Qin et al., 2019). Some researchers have reported the dewatering of foods with subcritical DME, but they used it primarily to extract other components (Hoshino et al., 2014; Yu et al., 2012). There have been no studies specifically about the drying of fruits and vegetables using subcritical DME.
Jujube fruit is rich in wax (Li et al., 2014). Chemically, natural wax is a complex mixture of long-chain aliphatic compounds (e.g., fatty acids, alkanes, aldehydes, ketones, and esters) and cyclic compounds (e.g., triterpenoids and steroids). It has functions in protecting the plant from stress in the environment and attack by pathogens (Yin et al., 2011). Natural wax is hydrophobic; thus, it can be applied in biodegradable plastic/packaging materials, making it both economically and ecologically beneficial (Canizares et al., 2020). Natural wax can be extracted from jujube fruit with subcritical DME.
In the present study, subcritical DME was used to dewater the fresh fruit of Chinese winter jujube. Wax was also extracted from the fruit during the dewatering process. The changes in nutritional composition, physical and chemical properties and microstructures in the fruit as a result of subcritical DME dewatering were investigated. In addition, the composition of the wax extracted from the jujube fruit was characterized. The study is the first to explore the application of subcritical DME in the drying of fruit. It is also the first to assess the composition of jujube fruit wax, revealing it has qualities that make it potentially useful in industry.
Materials Fresh Chinese winter jujube fruits were purchased from a market in Zhengzhou, China. The fruit was stored in the refrigerator (4 °C) before the experiment.
DME dewatering The fresh Chinese winter jujube fruit was washed and wiped with absorbent paper, then pitted, and sliced into 2–3 mm thickness. The prepared fresh jujube slices were used for the followed subcritical DME dewatering and freeze-drying experiments. The subcritical dewatering experiment was performed using subcritical fluid experimental equipment (Henan Subcritical Biological Technology Co., Ltd, China). The schematic of the subcritical fluid experimental equipment is shown in Fig. 1.
Schematic of the subcritical fluid experimental equipment: 1. solvent tank; 2. meter regulator; 3. process tank; 4. vaporizing tank; 5. buffer tank; 6. compressor; 7. condenser; 8. water pump and hot water tank; 9. vacuum pump.
Unlike other solvents, subcritical DME has good solubility with respect to both water and lipophilic substances (Fang et al., 2018). The process of dehydrating the jujube fruit by subcritical DME dewatering is shown in Fig. 2. When subcritical DME dewatering is finished, the dried jujube fruit can be directly collected from the process tank. The mixture containing subcritical DME and the extracts is transferred into a vaporizing tank. With the assistance of a compressor, subcritical DME can be recovered and reused. The byproducts remaining are jujube juice and wax. Jujube juice is mainly water with a small amount of hydrophilic substances. Due to the hydrophobicity of wax, the jujube wax and juice can be easily separated.
Dehydration of the jujube fruit by subcritical DME dewatering.
For each experiment, jujube fruit slices (500 g) were placed into the process tank. After vacuum pumping, dimethyl ether was injected into the process tank. During the process, the ratio of the sample to solvent was 1:20 (sample: solvent, w/v) to ensure maximum dehydration efficiency, and the process temperature (25 °C, 40 °C, and 55 °C) was controlled at the set temperature using recycled hot or cool water. The jujube was processed 8 cycles, with for 20 min for each cycle. When the dewatering process was finished, the extracts were transferred into the vaporizing tank. The dimethyl ether in the extracts was separated by vacuum distillation and condensed for recycling. The remaining liquid (jujube juice) was collected from the bottom of the vaporizing tank; the solid (jujube wax) attached to the inner wall of the vaporizing tank was dissolved in ethanol and then collected after the ethanol volatilization. The dried jujube fruit (JD) treated by subcritical DME process at temperatures 25 °C, 40 °C, and 55 °C were named as JD25, JD40, and JD55, respectively. Correspondingly, the jujube juices obtained at process temperatures 25 °C, 40 °C, and 55 °C were named as JJ25, JJ40, and JJ55, respectively, and the collected jujube waxes were named as JW25, JW40, and JW55, respectively.
To explore the changes in nutritional composition of dried jujube fruits during the subcritical DME treatment process, cycle number single-factor experiments were carried out. In the single-factor experiments, the process temperature and the ratio of the sample to solvent was fixed at 40 °C and 1:20 (sample: solvent, w/v), respectively. The JD treated by subcritical DME dewatering process with 1, 2, 3, 4, 5, 6, 7, and 8 cycles were collected as JD40-1, JD40-2, JD40-3, JD40-4, JD40-5, JD40-6, JD40-7, and JD40-8, respectively. Correspondingly, the jujube juices (JJ) obtained from subcritical DME dewatering process at the 1st, 2nd, 3rd, 4th, 5th, and 6th cycles were collected as JJ40-1, JJ40-2, JJ40-3, JJ40-4, JJ40-5, and JJ40-6, respectively; the jujube waxes (JW) obtained from subcritical DME dewatering process at the 1st, 2nd, 3rd, 4th, 5th, 6th, and 7th cycles were collected as JW40-1, JW40-2, JW40-3, JW40-4, JW40-5, and JW40-6, and JW40-7, respectively. It is worth noting that, due to the very low yields, the jujube juices obtained from subcritical DME dewatering process at the 7th and 8th cycles, and the jujube wax obtained from subcritical DME dewatering process at the 8th cycle, were not collected.
Freeze-drying Freeze-drying was used as a control. The prepared fresh jujube slices were frozen at −20 °C for 24 h before the drying. Then the frozen jujube fruits were quickly transferred into the freeze dryer (LGJ-10D, Sihuan Scientific Instrument Co., Ltd., China). During the drying process, the cold trap was kept at a temperature of approximately −55 °C, and pressure was maintained at 10–20 Pa. The freeze-drying process lasted 24 h. The freeze-dried jujube fruits were collected as JF.
Dehydration efficiency calculations he freeze-dried and subcritical DME dried jujube samples were sealed in polyethylene bags and stored in a desiccator before analysis. For the dried jujube samples, the dehydration efficiency was determined using the following equation:
 |
The moisture content (dry basis) of fresh and dried jujube fruit was determined by drying in an oven at 105 °C until a constant weight.
Quality determination The quality of the dried jujube fruits was determined by their browning degree, and total sugar, soluble solids, protein, phenol, flavonoid, and VC content. For analysis, 10 g of fresh jujube fruits were pulped, or 10 g of dried jujube fruits were ground into powder. Each sample was then suspended in 100 mL of 80% methanol with a sonic power of 100 W at 30 °C for 30 min. The extract was separated and concentrated, then diluted to 25 mL with distilled water. Before the measurement, each extract was diluted tenfold with distilled water. These diluted extracts were used to determine quality and antioxidant activities of the fresh and dried jujube fruits.
Browning degree was expressed as the absorbance of the sample measured at 420 nm. Briefly, 5 mL of each diluted methanol extract was mixed with 10 mL alcohol, and then centrifuged at 4 000 ×g for 10 min. Absorbance at 420 nm of the supernatant was determined in a UV-1100 spectrophotometer (Shanghai MAPADA Instruments Co., Ltd. China). The total sugar content was measured by the phenol sulfuric acid method. Total soluble solids (TSS) were measured by an Abbe refractometer (2W, Shanghai Yuguang Instrument Co. Ltd., China). Protein content was determined with a micro-Kjeldahl apparatus (Kjeltec™ 8400, Foss, Hillerød, Denmark). The total phenolic content of the fresh jujube pulp and subcritical DME pulp was determined according to the Folin-Ciocalteu method described in the literature (Rahman et al., 2018), and the result was expressed as garlic acid equivalent (mg GA/100 g). Total flavonoid content was measured according to the method described in the literature (Choi et al., 2012). The VC content was measured by 2, 6-dichlorophenolindophenol (DCPIP) titration method as described previously by Zhang et al (Zhang et al., 2014).
The composition and content of free amino acids in fresh and dried samples were analyzed by automatic amino acid analyzer (S-433D, Sykam, Germany) with a PEEK column (4.6 i.d.×150 mm, particle size 7 µm, 10% crosslink). Firstly, 500 mg sample was weighed in a glass tube, followed by the addition of 6 mol/L hydrochloric acid (10 mL) and phenol (3–4 drops). Then the sample was hydrolyzed at 110 °C ± 1 °C for 22 h. Deionized water was added to the hydrolysate to make up to 50 mL, and then 1 mL of this solution was transferred into a 50 mL centrifuge tube to evaporate in a vacuum evaporator at 40–50 °C. The residue was dissolved in 1 mL of water (pH 2.2) with ultrasound for 1 min, and then filtered through a 0.22 µm microfiltration membrane. Finally, 50 mL filtrate was injected into the instrument. The gradient elution was programmed according to the previous study (Zhou et al., 2019). The proline was detected at 440 nm, and other amino acids were detected at 570 nm.
Scanning electron microscopy (SEM) observations The flesh pieces (5 × 5 × 1 mm) were cut from the subcritical DME dried jujube fruits. The specimens were fixed on the SEM stub with double sided adhesive tape and coated with 20 nm of gold using an ion sputter coater (EICO IB-5). The samples were finally observed on a scanning electron microscope (Quanta 250 FEG, FEI, Czech) under low vacuum conditions with an accelerating voltage of 3 kV.
Antioxidant activities of jujube fruits The FRAP assay was carried out according to a previous study with some modifications (Salami et al., 2020). 1 mL of jujube extracts was added to 2.0 mL of a working FRAP solution and then left at 37 °C for 30 min in the dark. A standard curve was plotted by using ferrous sulfide. Results were expressed as mg of ferrous sulfide equivalents (FSE) /g DW.
The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activities of the samples were measured according to the method reported in our previous study with some modifications (Qin et al., 2018). 1 mL of jujube extracts diluted with 80% methanol was allowed to react with 4 mL of DPPH working solution, and the reaction solution was kept at 25 °C for 20 min in the dark. A standard curve was plotted by using VC. Results were expressed as mg of VC equivalents/g DW.
ABTS radical scavenging activities of the samples were assessed according to a previously reported protocol with some modifications (Yap et al., 2020). Briefly, 2 mmol/L ABTS solution was prepared with pH 7.4 phosphate buffer saline (PBS) buffer. Next, 500 mL of potassium persulfate (70 mmol/L) was added to 50 mL of ABTS solution. Then the mixture was incubated at room temperature in the dark for 15 h to generate radical cation ABTS•+. Before using, the ABTS•+ solution was diluted with PBS buffer (pH 7.4) so that the measured OD734 nm was 0.7 ± 0.03. Finally, ABTS•+ solution (3.6 mL) was mixed with 0.4 mL jujube extract and kept at room temperature for 15 min. The absorbance of the mixture was measured at 734 nm. VC was used to prepare the standard curve, and the results were expressed as mg of VC equivalents/g DW.
Chemical analysis of jujube wax Fourier transform infrared (FT-IR) spectra of the jujube wax samples were obtained on a Nicolet iS10 FT-IR spectrometer. The transmittance of the sample was measured in the 4 000–400 cm−1 wavelength range. 32 scans were used for each sample.
The thermal characteristics of the jujube wax samples were determined by a differential scanning calorimetry (DSC) 822 instrument (Mettler-Toledo, Switzerland). The jujube wax samples were weighed (2 mg) into a closed aluminum pan and loaded along with a similar empty reference pan. The temperature program was as follows: the first stage, heating from 20 °C to 105 °C at a rate of 10 °C/min and maintaining at 105 °C for 1 min; the second stage, cooling from 105 °C to 20 °C and maintaining at 20 °C for 1 min; the third stage, heating from 20 °C to 105 °C at a rate of 10 °C/min and maintaining at 105 °C for 1 min. The weights during the third stage were recorded to determine the melting point range.
The acid value of jujube wax was determined according to GB 5009.229-2016 (National Standards of China, 2016). Briefly, the jujube wax was dissolved in 50 mL of ethanol/ether (V:V = 1:2) solution, and 3–4 drops of phenolphthalein indicator was added to the mixture. Then the mixture was titrated against 0.1 moL/L KOH until a faint pink color persisted for 15 s. Blank test was done without jujube wax. The calculation was carried out by the following equation:
 |
where, V1 was the volume of KOH solution used for sample determination, V0 was the volume of KOH solution used for blank determination, c was the concentration of KOH solution, 56.1 was the molar mass of KOH, m was the mass of sample.
Prior to GC-MS analyses, the jujube wax was submitted to a silylation reaction (Chu et al., 2018). The reaction was done as follows: 10 mg of extracted wax was dissolved in 100 µL pyridine, and then 100 µL bis-N, N-(trimethylsilyl) trifluoroacetamide (BSTFA, Sigma-Aldrich, USA) was mixed with the pyridine. This mixture was incubated at 70 °C for 40 min. Then, pyridine and excessive BSTFA were evaporated under a stream of nitrogen. The dried sample was re-dissolved in chloroform for GC-MS analysis. The extracted waxes were analyzed using GC (Agilent 7890A) equipped with an HP-5MS capillary column (30 m × 0.25 mm × 0.25 µm) and coupled to a mass-selective detector (Agilent 5975c). The GC column temperature was held at 50 °C for 2 min, then increased to 230 °C at 20 °C/min, then increased to 270 °C at 10 °C/min and held for 15 min at 270 ºC. For GC-MS identification, an electron ionization system (ionization energy of 74 eV, ionization source temperature 230 °C) was used.
Statistical analysis All the above experiments were carried out in triplicate; the collected data are expressed as the mean ± standard deviation. The statistical analysis was performed using SPSS software version 21.0 (SPSS Inc., Chicago, USA). Differences among groups were analyzed by one-way analysis of variance (ANOVA) and Duncan's multiple range tests (p < 0.05).
Dehydration by subcritical DME dewatering The temperatures and number of cycles during the subcritical DME dewatering process affect the dehydration efficiency. As can be seen in Fig. 3A, dehydration efficiency increased as subcritical DME process temperature increased, over a range of 78.86–93.33%. Correspondingly, the moisture of jujube fruit decreased with the increasing temperature. The dehydration efficiency reached the maximum (JD55, 93.33 ± 0.34%) at 55 °C, which was in close proximity to that by freeze-drying (JF, 94.84 ± 0.32%). However, the microstructure of dried jujube fruit obtained at 55 °C was partially destroyed (see Fig. 4). Therefore, the optimal process temperature was 40 °C. At subcritical DME process temperature 40 °C, JD40 has the dehydration efficiency value of 89.31 ± 0.54%, and its moisture content was 8.50 ± 0.33%.
Dehydration efficiency and moisture content of dried jujube fruits dehydrated at different temperatures (A) and cycles (B). A, JD25, JD40, JD55, represent dried jujube fruits (JD) treated by subcritical DME process at temperatures 25 °C, 40 °C, and 55 °C, respectively; JF, freeze-dried jujube fruit. B, JD40-1, JD40-2, JD40-3, JD40-4, JD40-5, JD40-6, JD40-7, and JD40-8 represent dried jujube fruits (JD) treated by subcritical DME dewatering process at 40 °C with 1, 2, 3, 4, 5, 6, 7, 8 cycles, respectively.
SEM images of dried jujube fruit. A, JD25, JD40, JD55, represent dried jujube fruits (JD) treated by subcritical DME process at temperatures 25 °C, 40 °C, and 55 °C, respectively; JF, freeze-dried jujube fruit. JD40-1, JD40-3, and JD40-6 represent dried jujube fruits (JD) treated by subcritical DME dewatering process at 40 °C with 1, 3, and 6 cycles, respectively.
As can be seen in Fig. 3B, the more subcritical DME process cycles, the higher the dehydration efficiency and the lower the moisture content. For 1–5 cycles, the dehydration efficiency of subcritical DME process increased with increasing number of the cycles. For 6–8 cycles, further increase in the number of the cycles did not increase dehydration efficiency.
Properties and microstructure of fresh and dried jujube fruits The results of analyzing the physical and chemical properties of fresh and dried jujube fruits are listed in Table 1. In comparison with fresh jujube, dried jujube had a lower content of nutritional components. The content of TSS, total sugars, and total flavonoids in JD samples (JD25, JD40, JD55) was lower than those in JF, presumably due to the transfer of components into the juice during the subcritical DME dewatering process. However, the content of total phenols and VC in JD40 and JD55 was close to those in JF, and the protein content in JD samples (JD25, JD40, JD55) was higher than that in JF. The higher protein content in JD samples suggested that subcritical DME process produced minimal damage to protein. Browning degree is an important parameter of jujube quality (Fang et al., 2011). JD samples had a lower browning degree (0.015 ± 0.001–0.034 ± 0.001) than that of JF (0.205 ± 0.009). These data implied that more pigments in jujube fruits were leached during DME dewatering process.
| Browning degree (Abs) | TSS# (%) | Total phenols (mg GA/100g DW#) | Total sugars (%) | Protein (%) | Total flavonoids (mg/g) | VC (mg/100g DW) | |
|---|---|---|---|---|---|---|---|
| FF# | 0.132±0.006b* | 12.304±0.523a | 998.380±39.918def | 23.256±1.092a | 6.791±0.339ab | 2.416±0.021a | 1836.902±71.845c |
| JF | 0.205±0.009a | 10.502±0.481b | 1030.802±41.507cde | 22.468±1.013ab | 5.074±0.225d | 1.850±0.062c | 1368.228±58.411de |
| JD25 | 0.030±0.002cd | 8.467±0.323c | 1072.484±23.624bc | 13.671±0.684efg | 6.593±0.3295ab | 1.143±0.003gh | 1090.98±54.549f |
| JD40 | 0.025±0.001def | 8.484±0.364c | 987.775±19.389ef | 12.852±0.643g | 6.236±0.311bc | 1.170±0.010gh | 1239.72±61.986e |
| JD55 | 0.023±0.001ef | 8.502±0.38c | 961.744±18.087f | 12.690±0.635g | 5.910±0.295c | 1.283±0.006f | 1428.25±71.412d |
| JD40-1 | 0.015±0.001g | 8.439±0.347c | 1179.255±28.963a | 21.960±1.098ab | 6.797±0.134a | 1.962±0.009b | 2124.33±106.216a |
| JD40-2 | 0.029±0.001cde | 8.444±0.325c | 1105.175±25.258b | 21.300±1.065b | 6.771±0.195ab | 1.629±0.015d | 1761.36±88.067b |
| JD40-3 | 0.034±0.002c | 8.507±0.421c | 1039.372±21.968cd | 19.232±0.962c | 6.724±0.181ab | 1.331±0.018e | 2069.80±103.489b |
| JD40-4 | 0.033±0.002c | 8.488±0.336c | 1015.671±20.783de | 15.670±0.783d | 6.712±0.391ab | 1.307±0.005ef | 2594.54±129.726c |
| JD40-5 | 0.029±0.001cde | 8.502±0.425c | 998.463±19.923def | 14.892±0.745de | 6.694±0.338ab | 1.184±0.022g | 1763.37±88.168c |
| JD40-6 | 0.022±0.001f | 8.451±0.312c | 994.436±18.721def | 14.523±0.726def | 6.688±0.299ab | 1.146±0.002gh | 1318.05±65.902c |
| JD40-7 | 0.028±0.001cdef | 8.465±0.323c | 990.51±19.525ef | 13.267±0.663fg | 6.505±0.325ab | 1.137±0.032h | 1459.98±72.998d |
| JD40-8 | 0.022±0.001f | 8.482±0.334c | 985.221±19.261ef | 12.803±0.640g | 6.337±0.217abc | 1.143±0.003gh | 1738.64±86.932de |
Process temperatures and number of cycles affects the composition of the dried jujube fruit. The total phenol, total sugar, and protein content decreased with the rising temperature, while the total flavonoid and VC content increased with the rising temperature. The browning degree, TSS and protein content were minimally influenced by the cycles, while total phenol, total sugar, and total flavonoid content decreased with increasing the number of cycles. In other words, a certain amount of components such as sugars, phenolics and flavonoids are lost during the subcritical DME process.
The appearances of JF and JD (see Fig. S1) are different. The flesh of JF was yellow-green, while the flesh of the JD became yellow-white after subcritical DME dewatering process. This was probably caused by a substantial decrease in the wax and pigment contents of these samples, as these substances can be dissolved in subcritical DME in the dewatering process.
After subcritical DME dewatering process, JD had a by-product, jujube juice. The juice contained small amounts of TSS, protein, phenols, flavonoids, and VC, and could be used to develop jujube beverages. This is due to the ability of subcritical DME to dissolve hydrophilic substances in jujube fruits, such as water-soluble protein and phenolic compounds (Furukawa et al., 2016; Sato et al., 2003).
The jujube juices obtained from subcritical DME process at different temperatures (JJ25, JJ40, JJ55) differed (see Table 2). Protein was not detected in any of the three samples. This may be due to the extremely low content in the jujube juice. In this experiment, the protein content in the juice was detected by Kjeldahl method. The Bradford method may be a better choice for the determination of protein in the juice, as it has higher sensitivity for proteins than the Kjeldahl method (Kamizake et al., 2003). The browning degree and VC content of the jujube juice had no significant changes with the increase of subcritical DME process temperature, while the total sugar, total phenol and total flavonoid contents decreased with the rising subcritical DME temperatures. This was because polysaccharides, phenolics, and flavonoids are sensitive to heat and can degrade at high temperatures.
| Protein (%) | Total sugars (%) | Browning degree (Abs) | Total phenols (mg GA/100 g) | Total flavonoids (mg/g) | VC (mg/100 g) | |
|---|---|---|---|---|---|---|
| JJ25# | / | 11.42±0.371a* | 0.009±0.001c | 3.970±0.141b | 3.890±0.105b | 0.118±0.003c |
| JJ40 | / | 9.720±0.246c | 0.018±0.001a | 2.900±0.095c | 2.096±0.051d | 0.159±0.005a |
| JJ55 | / | 9.450±0.233c | 0.013±0.001b | 2.570±0.079d | 2.181±0.257d | 0.106±0.002d |
| JJ40-1 | / | 5.550±0.188f | 0.013±0.001b | 4.250±0.143a | 4.460±0.202a | 0.109±0.002d |
| JJ40-2 | / | 5.900±0.205f | 0.015±0.002b | 2.870±0.084c | 2.635±0.154c | 0.084±0.002e |
| JJ40-3 | / | 6.870±0.224e | 0.010±0.001c | 2.120±0.056e | 1.172±0.033e | 0.069±0.001f |
| JJ40-4 | / | 8.940±0.257d | 0.005±0.001e | 1.060±0.033f | 0.548±0.009f | 0.065±0.001f |
| JJ40-5 | / | 10.600±0.330b | 0.003±0.001f | 0.660±0.026g | 0.285±0.010g | 0.124±0.004b |
| JJ40-6 | 0.025±0.001 | 9.780±0.299c | 0.007±0.001d | 0.570±0.018g | 0.205±0.005g | 0.085±0.002e |
For a clearer understanding of the changes in composition of the juice during the whole subcritical DME process, juice after each cycle was analyzed. The protein was only detected in JJ40-6 among the samples obtained from different subcritical DME process cycles. The total phenol and total flavonoid contents in juice obtained from the 1st, 2nd, and 3rd cycles were higher than those from the 4th, 5th, and 6th cycles. Although the total sugar percentages in the juices obtained from the 4th, 5th, and 6th cycles were higher than those from the 1st, 2nd, and 3rd cycles, their volumes were lower. So, the sugars in the juices from the first three cycles were higher than those from the latter cycles. These data indicate that some phenolics, flavonoids and sugars were mainly dissolved in dimethyl ether solvent and lost during the subcritical DME process.
Fig. 4 shows representative SEM images of dried jujube fruit obtained by subcritical DME processing at different treatment temperatures. There were significant differences in the flesh microstructure of JD samples treated at different treatment temperatures or cycles. Micrographs of the JD sample treated at 55 °C (JD55) has a heterogeneous porous structure because the cell walls have shrunken significantly. After the jujube sample was dewatered at 55 °C, the dimethyl ether remaining in the sample expanded rapidly, disrupting the cell wall (Lee et al., 2011). The cell wall structure of the sample dewatered at 25 °C (JD25) was disrupted less; however, some collapse can be seen. This phenomenon may be related to cell lysis due to incomplete dehydration. A porous but nonshrunken structure is observed in JD40. This indicates that 40 °C was a suitable temperature to maintain the original shape of cell walls. The number of subcritical DME dewatering process cycles also affect the microstructure of dried jujube samples, and this is evident in the differences in SEM images JD40-1, JD40-3, and JD40-6. Due to incomplete dehydration, the collapse with the cell structure intact is observed in JD40-1. While for JD40-6, the residual DME solvent apparently rapidly expanded, disrupting its fluffy structure. Comparing to JD40-1 and JD40-6, JD40-3 had a uniform porous structure.
There were significant differences between the flesh microstructure of JF and JD samples (see Fig. S2 and Fig. 4). Micrographs of JF sample presented a heterogeneous porous structure while JD samples presented relatively homogeneous porous structures. These results predicted that subcritical DME dewatering process could be used to produce the dried fruit with the uniform microstructure.
Amino acid analysis Amino acid (AA) composition and content influence the quality of jujube fruit to some extent. Table 3 reveals the free amino acid (FAA) composition of jujube fruits. The amino acids analyzed here are in two categories: essential amino acids (EAAs), and non-essential amino acids (NEAAs). Proline (Pro) and aspartic acid (Asp) are the most abundant free amino acids in the fruits. The most prevalent amino acids in both fresh and freeze-dried jujubes were Pro and Asp, but in subcritical DME dewatering jujube sample, the most prevalent was Pro alone. In comparison with fresh and freeze-dried jujube fruits, the subcritical DME dehydrated jujube sample had a higher content of total free amino acids. Amino acids exist in two states in jujube fruit: free, and protein-bound (Hildebrandt et al., 2015). Higher free amino acid content after DME processing indicates the protein-bound amino acids may have been transformed into free ones during the subcritical DME dewatering process. The amino acid compositions in fresh and freeze-dried jujube fruit were the most alike. The fresh, freeze-dried, and subcritical DME dewatered jujube fruit possessed similar patterns in the NEAA and EAA compositions.
| Amino acids | FF# | JD40 | JF | FF | JD40 | JF |
|---|---|---|---|---|---|---|
| (mg/100 g DW)# | (mg/100 g DW) | (mg/100 g DW) | (% of total AA) | (% of total AA) | (% of total AA) | |
| Asp# | 432.36±16.61b* | 276.66±11.83c | 530.82±22.50a | 23.75±0.98a | 6.38±0.21c | 20.77±0.83b |
| Thr | 31.61±1.08b | 359.82±14.99a | 43.77±2.01b | 1.74±0.02b | 8.30±0.35a | 1.71±0.06b |
| Ser | 54.61±1.73c | 87.88±3.39a | 71.60±2.57b | 3.00±0.09a | 2.03±0.05c | 2.80±0.09b |
| Glu | 144.58±6.22b | 248.98±9.44a | 242.25±11.02a | 7.94±0.19b | 5.74±0.17c | 9.48±0.36a |
| Gly | 72.33±2.61b | 109.60±4.48a | 102.27±4.11a | 3.97±0.06a | 2.53±0.04b | 4.00±0.11a |
| Ala | 80.48±3.02c | 112.52±4.62a | 98.57±3.93b | 4.42±0.15a | 2.60±0.09c | 3.86±0.04b |
| Cys | 12.53±0.52b | 15.77±0.45a | 12.63±0.43b | 0.69±0.01a | 0.36±0.01c | 0.49±0.01b |
| Val | 75.28±2.76b | 94.53±3.72a | 91.51±3.57a | 4.13±0.14a | 2.18±0.07c | 3.58±0.11b |
| Met | 4.93±0.15c | 9.59±0.38b | 14.59±0.60a | 0.27±0.01b | 0.22±0.01c | 0.57±0.01a |
| Ile | 68.80±2.44c | 94.09±3.71a | 84.57±3.22b | 3.78±0.15a | 2.17±0.07c | 3.31±0.14b |
| Leu | 105.76±2.28c | 146.56±4.32a | 130.24±4.51b | 5.81±0.20a | 3.38±0.08c | 5.10±0.17b |
| Tyr | 47.16±1.35c | 55.65±2.28b | 64.71±2.93a | 2.59±0.10a | 1.28±0.04b | 2.53±0.11a |
| Phe | 56.74±1.83c | 64.65±2.23b | 74.08±2.70a | 3.12±0.08a | 1.49±0.02c | 2.90±0.07b |
| His | 77.58±2.87c | 111.84±4.59a | 96.65±3.83b | 4.26±0.11a | 2.58±0.02c | 3.78±0.09b |
| Lys | 94.16±3.70c | 110.73±4.53b | 123.88±5.09a | 5.17±0.13a | 2.55±0.07c | 4.85±0.11b |
| Arg | 85.97±3.29b | 118.71±4.59a | 125.10±5.25a | 4.72±0.11a | 2.74±0.06b | 4.89±0.14a |
| Pro | 375.79±15.78c | 2317.33±94.86a | 648.85±22.44b | 20.64±0.85c | 53.46±2.37a | 25.38±1.01b |
| Total | 1820.66±50.03c | 4334.91±88.74a | 2556.07±57.80b | 100.00 | 100.00 | 100.00 |
| EAAs# | 439.58±11.97c | 897.28±34.86a | 567.78±18.38b | 24.14±1.00a | 20.70±0.52b | 22.21±0.99b |
| NEAAs# | 1381.08±39.05c | 3437.63±81.88a | 1988.30±59.41b | 75.86±2.79a | 79.30±2.96a | 77.79±2.88a |
| Aromatic# | 103.91±2.19c | 120.30±4.01b | 138.79±5.94a | 5.71±0.12a | 2.78±0.11b | 5.43±0.22a |
| Sulfur# | 17.45±0.40c | 25.36±0.99b | 27.22±1.06a | 0.96±0.02b | 0.59±0.01c | 1.06±0.04a |
Antioxidant activity of jujube fruits Jujube fruit is a potential source of natural antioxidants as it may have various efficient oxygen radical scavengers, such as phenolic compounds, flavonoids, VC, and polysaccharides (Zhou et al., 2014). To investigate the influence of drying methods on the antioxidant activity of the jujube fruit samples, ABTS, DPPH and FRAP assays were carried out, and the corresponding results are shown in Table 4. Freeze-dried jujube fruits exhibited the highest antioxidant activities in all three assays. Surprisingly, the antioxidant activities of fresh jujube extracts were lower than that of the freeze-dried jujube extracts. As can be seen in Table 1, the content of flavonoids and VC in fresh jujube fruit is higher than in freeze-dried, while the content of total phenols in the fresh was similar to that in the freeze-dried. However, the antioxidant activities of freeze-dried fruit are higher than fresh. This may be because of the different extraction efficiency of antioxidant substances in the fresh and freeze-dried fruit extracts. The subcritical DME dewatered jujube fruit had the lowest antioxidant activities in all three assays. This can be explained by the loss of components such as phenolics, flavonoids and VC, which make a major contribution to antioxidant activity, during the dewatering process. All jujube extracts had lower antioxidant activities in the DPPH assay than in the ABTS assay. The DPPH assay is used in hydrophobic systems; the ABTS assay is used in hydrophilic and lipophilic system. Similar results were found in the comparison of antioxidant activities of jujube fruits dried by freeze-drying, sun-drying and microwave drying (Wang et al., 2016).
| ABTS (mg VC/g DW#) | DPPH (mg VC/g DW) | FRAP (mg FeSO4/g DW) | |
|---|---|---|---|
| FF# | 32.04 ± 0.75b* | 19.02 ± 0.32b | 17.95 ± 0.45b |
| JF | 47.18 ± 0.26a | 29.12 ± 0.24a | 33.57 ± 0.66a |
| JD40 | 25.34 ± 0.27c | 11.29 ± 0.30c | 9.36± 0.81c |
FT-IR analysis of jujube wax The FT-IR spectra of jujube waxes are displayed in Fig. 5. The peak at 3 446 cm−1 is the characteristic absorption peak of OH groups. The peaks at 2 927 cm−1 and 2 850 cm−1 correspond to saturated hydrocarbon C-H stretching vibration. The peaks at 1 730 cm−1 and 1 702 cm−1 show the strong absorption of C=O. It is speculated that the sample may contain carboxyl groups, which implies the wax samples contained organic acids or esters. The peak at 1 463 cm−1 relates to CH2 scissoring originating from alkanes. The peak at 1 379 cm−1 derives from CH3 symmetric deformation or OH deformation of carboxyl monomers (Farber et al., 2019); this proves the presence of alkanes or carboxylic acids. The peak at 1 168 cm−1 is related to stretching vibration of a C-O ester group. The peaks at 1 051 cm−1 and 999 cm−1 are assigned to R-O stretching. The peak at 742 cm−1 is assigned to CH2 in-plane rocking vibration, which shows that the sample contained alkanes. Overall, JW25, JW40, and JW55 have similar spectral features, but their peak intensities differ. This means they have similar compositions but the percentages of the compounds in them are different, a conclusion which is in accordance with the GC-MS data.
FT-IR spectra of jujube wax. JW25, JW40, JW55 represent wax extracted from jujube treated by subcritical DME process at temperatures 25 °C, 40 °C, and 55 °C, respectively.
DSC analysis of jujube wax The thermal properties of a wax partially determine its application value in areas such as cosmetics and food. DSC is frequently used to determine the thermal properties of the wax. DSC analysis provides information about phase transitions and melting points of wax. Fig. 6 presents the DSC curves of jujube waxes. As can be seen in Fig. 6, all the DSC curves reflect a negative heat flow, which indicates the phase transition of melting is endothermic. JW25, JW40, and JW55 have similar thermal properties, and their melting points ranged between 53.71 °C and 54.62 °C. For use in the cosmetic industry, waxes should have melting points in the range of 48–56 °C (Li et al., 2015). The jujube waxes in this study were all within the required standard values and thus can potentially be used in cosmetics.
DSC curves of jujube wax. JW25, JW40, JW55 represent wax extracted from jujube treated by subcritical DME process at temperatures 25 °C, 40 °C, and 55 °C, respectively.
Components of extracted jujube wax The results for the wax analysis are shown in Fig. 7. Acids, esters, amides, alkanes, alkenes, and aldehydes were found in the wax extracted by subcritical DME. Acids represented 89.85%– 96.87% of the total wax extracted by subcritical DME. The high percentages of acids in jujube wax samples were confirmed by their high acid values. Acid value can be used to indicate the content of acids present in the wax (Hwang et al., 2002). The acid values of the jujube wax samples were 128.81 ± 3.44–150.47 ± 4.12 mg KOH/g, which were close to the acid value of sugarcane peel wax (Inarkar et al., 2012). The high acid values implied a large amount of acid existed in the jujube wax samples. In jujube wax samples, the predominant acids were hexadecanoic, trans-9-octadecenoic and octadecanoic. In addition to acid compounds, relatively large quantities of alkanes were detected. Tetracosane was the main alkane. The proportions of other compounds in the wax were low. As can be seen in Fig. 7A, alkenes were lost in JW25, and alkenes and amides were lost in JW55, but the major compounds (acids and alkanes) in both were found in percentages similar to JW40.
Composition of wax extracted by subcritical DME from jujube fruit. A, JW25, JW40, JW55 represent wax extracted from jujube treated by subcritical DME process at temperatures 25 °C, 40 °C, and 55 °C, respectively. B, JW40-1, JW40-2, JW40-3, JW40-4, JW40-5, JW40-6, JW40-7 represent wax extracted from jujube treated by subcritical DME process at the 1st, 2nd, 3rd, 4th, 5th, 6th, and 7th cycles, respectively.
The yields of jujube wax extracted after different numbers of subcritical DME process cycles were significantly different. Due to the extremely low yield of JW40-8, its data is not shown in Fig. 7B. The amount of jujube wax extracted from the 1st cycle amounted to 90% of the total amount of jujube wax extracted with 8 cycles. In other words, wax is mainly extracted from the jujube fruit in the early stage of subcritical DME dewatering.
The composition of jujube wax changed in response to variations in the subcritical DME dehydration process (see Fig. 7B). The proportions of acids extracted in the eight cycles were 60.14% (cycle 1), 72.87% (cycle 2), 72.02% (cycle 3), 53.02% (cycle 4), 59.80% (cycle 5), 60.77% (cycle 6), 78.69% (cycle 7). This indicates that acids are extracted throughout the dewatering process. The proportions of other compounds extracted from the jujube wax showed large variations over the process cycles. The esters were only present in JW40-7. The alkene proportions in JW40-4, JW40-5, JW40-6 and JW40-7 were higher than those of JW40-1, JW40-2 and JW40-3. The amide proportion in JW40-4, JW40-5, JW40-6 were higher than those of JW40-1, JW40-2, and JW40-3. These data indicate that alkenes, alkenes, and amides were predominantly extracted during the late stages of the subcritical DME process (the 4th, 5th, 6th, and 7th cycles). The aldehyde proportions in JW40-2 and JW40-3 were higher than those of other samples. The alkane proportions in JW40-1 were much higher than those of the other six jujube wax samples. Furthermore, some long-chain alkanes, including tetracosane (C24), hexacosane (C26), heptacosane (C27), hentriacontane (C31), and dotriacontane (C32), were only present in JW40-1. These data show that aldehydes and alkanes in jujube waxes were predominantly extracted during the early stages of the DME process (the 1st, 2nd, and 3rd cycles). The wax yield from the subcritical DME dewatering process (425 mg/100 g fresh fruit) was higher than that from the traditional chloroform extraction method (120 mg/100 g fresh fruit). Subcritical DME is a more efficient and environmentally friendly solvent than chloroform for the extraction of wax. The extracted jujube wax could be used as edible film to improve the shelf life of fresh fruits (Goslinska et al., 2019).
Fresh jujube fruits were dewatered using subcritical dimethyl ether (DME). The maximum dewatering efficiency was 89.31 ± 0.54% under the operation conditions used (temperature, 40 °C; cycle number, 8). Subcritical DME dewatering caused less damage to the microstructure of the dried jujube fruit than freeze-drying. When the jujube fruit was dewatered, some components were lost in the juice, reducing the content of total phenolic compounds, flavonoids, proteins, and vitamin C in the dried fruit. Thus, the nutritional content of jujube fruit that had been processed by subcritical DME dewatering was somewhat lower than that of freeze-dried fruit. The study demonstrates that subcritical DME can be used to dry Chinese winter jujube fruit. There is a small loss of nutritional content, but the speed, economy, and environmental friendliness of the process are advantages compared to freeze-drying.
During the dewatering process, wax was extracted. The extracted jujube wax was composed of acids, alkanes, esters, amides, alkenes, and aldehydes, of which the most abundant component was hexadecanoic acid. The extracted wax had melting points ranging between 53.71 °C and 54.62 °C, which were all within the required standard values of commercial wax. These results suggest that jujube wax could be used in the cosmetic and food industries.
Appendix A. Supplementary data E-supplementary data of this work can be found in the online version of the paper.
Acknowledgements We sincerely acknowledge the financial support by the earmarked fund for Technological Innovation Talents of Colleges and Universities (19HASTIT012), the earmarked fund for Modern Agro-industry Technology Research System (CARS14-1-29), and Postdoctoral Research Grant in Henan Province (201902039).
Conflict of interest There are no conflicts of interest to declare.
