3.3.2. Fruit Firmness

The firmness of strawberry fruits significantly reduced after all treatments during storage of 4, 8, 12, and 16 days in the two experiments, as shown in Figure 3. The coating treatments showed a higher firmness value than the control strawberry fruits. The firmness of strawberries treated with *A. vera* gel at 20% and 40% was found to be lower than that of other coated samples with *A. vera* gel combined with EO during different days of storage, showing that the treated and control strawberry fruit become less firm due to reaping (Figure 3). Compared to that of the control, the highest firmness value was that of strawberry fruits treated with *A. vera* gel 40%, followed by the fruits treated with *A. vera* gel 20% and those treated with *A. vera* gel with EO 1%. *A. vera* gel coatings retarded the postharvest ripening process and reduced the firmness of table grape and sour cherry [67,68]. Lower water vapor from fruits subjected to *A. vera* gel coating results in maintained turgor pressure of the cell wall [69,70]. The treated fruits with *A. vera* with EO showed slightly higher firmness, that could be due to the higher hydrophobic properties in this treatment. In agreement with this work, the coated strawberry fruits with *A. vera* gel alone and combined with basil EO had a lower softening rate compared to the control treatment [71].

**Figure 3.** Firmness (N) (mean ± S.E.) of strawberry fruits stored at 5 ◦C as affected by coating treatments when stored for different lengths of time in both experiments. The mean ± S.E. of treatments in the figures with the same letters shows a nonsignificant difference according to Duncan multiple range test for *p* ≤ 0.05. AV: *A. vera* gel. (**a**) First experiment; (**b**) second experiment.

### *3.4. Color Value*

The data in Tables 4 and 5 show that the color value of strawberry fruits was influenced by the different coating treatments. The L\* of the strawberry fruits increased in lightness in both coating treatments during the shelf-life study. In other words, a coating of *A. vera* gel or *A. vera* gel with lemongrass EO helped to maintain the lightness of the fruit compared to the control samples (Tables 4 and 5). The chromaticity coordinate a\* value, representing the red-green color, was slightly affected by the coatings and remained stable in all treatments during the shelf life. The chromaticity coordinate b\* value of strawberry fruits slightly reduced over storage time and coating treatment by *A. vera* gel only. However, *A. vera* gel with lemongrass EO increased this parameter by day 16 in the two experiments. Coated and control fruits showed a significant decrease in hue during the first 8 days of storage and the hue increased as the storage period was increased to 16 days (Tables 4 and 5).

**Table 4.** Color values of strawberry fruits as affected by coating treatments after 0, 8, and 16 days of storage at 5 ◦C in Experiment 1.

The mean values with the same superscript letter/s in the same column show a nonsignificant difference according to Duncan multiple range test for *p* ≤ 0.05.

**Table 5.** Color values of strawberry fruits as affected by coating treatments after 0, 8, and 16 days of storage at 5 ◦C in Experiment 2.

The mean values with the same superscript letter/s in the same column show a nonsignificant difference according to Duncan multiple range test for *p* ≤ 0.05.

The color of strawberry fruits is an important property for product reception by the consumer; although the coating did not change the fruit initially color [72], and with the increased storage time, the fruit became redder and darker. This increase was probably due to a reduction in both the respiration rate and some enzymatic processes, maintaining the quality of the fruit and preventing its browning [73]. At the end of storage time, control fruits and those treated with gluten plus CaCl2 had a low L\* value (darker color) [74].


*3.5. X-ray Diffraction (XRD) Analysis of Coal Samples* 3.5.1. XRD Peak Fitting

In order to analyze the microstructure of outburst coal, the XRD patterns of coal samples were fitted by peak separation, as shown in Figure 11. The original spectrum was divided into 002 peaks, *γ* peaks and 100 peaks by Guassian [41] formula. The fitting formula is:

$$y = y\_0 + \frac{A}{w \cdot \sqrt{\frac{\pi}{2}}} \cdot e^{-2\left(\frac{x - x\_0}{w}\right)^2} \tag{10}$$

where *y*0 is the baseline position, *w* is the diffraction peak width, *A* is the diffraction pattern area and *xc* is the 2*θ* angle of the diffraction center.

**Figure 11.** Peak separation fitting results of coal samples.

It can be found from Figure 11 and Table 4 that the 2*θ* angle range of 002 diffraction peak is 24.92◦–25.08◦, with high intensity and good symmetry. Moreover, the 2*θ* angle range of 100 diffraction peaks is 42.73◦–42.85◦ and the diffraction peaks are wide and low, indicating that the degree of aromatic ring condensation in coal is not high. For the *γ* band, it can be clearly seen from the graph that its area is large, reflecting the rich branched chain structure of aliphatic hydrocarbons and aliphatic hydrocarbons in coal.

**Table 4.** Diffraction peak fitting of the main 2*θ* parameters.

3.5.2. Structure Analysis of Aromatic Microcrystals

The layer spacing of *d*002 and *d*100, the average stacking height of *Lc* and the ductility of *La* of the aromatic layer can be calculated from the diffraction angle and the half-peak width [49].

$$d\_{002/100} = \frac{\lambda}{2\sin\theta\_{002/100}}\tag{11}$$

$$L\_c = \frac{K\_1 \lambda}{\beta\_{002} \cos \theta\_{002}} \tag{12}$$

$$L\_{\mathfrak{a}} = \frac{K\_2 \lambda}{\beta\_{100} \cos \theta\_{100}} \tag{13}$$

$$M\_c = \frac{L\_c}{d\_{002}}\tag{14}$$

where *λ* is the wavelength of X-ray, and 0.15405 nm is used for the experiment with copper target irradiation; *θ*002/100 is the Bragg angle of 002 diffraction peak and 100 diffraction peak, respectively; *β*002 and *β*100 are the half-peak widths of 002 diffraction peak and 100 diffraction peak, respectively; *K*1 and *K*2 are Debye-Scherrer constants, with *K*1 as 0.89, *K*2 as 1.84 and *Mc* as the number of effective stacked aromatic slices.

In general, the interlayer spacing of the coal aromatic layer is between cellulose (*d*002 = 3.975 × 10−1 nm) and graphite (*d*002 = 3.354 × 10−1 nm). Therefore, the coalification degree *P* is used to determine the percentage of condensed aromatic ring, and the relative content of aromatic layer and aliphatic layer structure is obtained. The calculation formula is as follows:

$$P = \frac{3.975 - d\_{002}}{3.975 - 3.354} \times 100\% \tag{15}$$

The calculation results are shown in Table 5. It can be found from the table that *d*002 of primary coal is 3.548, indicating that its coalification degree is high. The number of effective stacking aromatic slices of primary coal is about 4, and the coalification degree is 68.83%, indicating that the coal sample has a high degree of coalification, with fewer side chains and functional groups, and the internal arrangement of molecules is orderly and stable, and the condensation degree of aromatic nucleus is high. Combined with the data of *d*002 and *d*100, the number of aromatic flakes in outburst coal is lower than primary coal, which suggests that the prominent heating effect increases the degree of aromatization, increases the d100 and *d*002 values and, finally, changes the dense ring structure in coal. Through the coal degree index *P*, it can be seen that the primary coal is slightly higher than the outburst coal. Combined with the analysis of *d*002 and *d*100 as basically unchanged, this shows that the content of side chain in the primary coal is lower than that of the outburst coal, resulting in the increase of the relative number of aromatic rings, which is finally reflected in the increase of *P*.

**Table 5.** XRD microstructure parameters of coal samples.

#### **4. Conclusions**

increases the *d*100 and *d*002 values and, finally, changes the dense ring structure in coal. Moreover, from the coal degree index P, it can be seen that the primary coal has a smaller increase than the outburst coal.

**Author Contributions:** Conceptualization: A.J., S.T.; Methodology: H.L.; Formal analysis and investigation: A.J., H.L.; Writing-original draft preparation: A.J.; Writing-review and editing: S.T.; Funding acquisition: S.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China under Grant number 52104079 and the Science & Technology Foundation of Guizhou Province under Grant number [2020]4Y050.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** All data, models, and code generated or used during the study appear in the submitted article.

**Acknowledgments:** The authors appreciate the support from above funders.

**Conflicts of Interest:** The authors declare that they have no competing financial interests.

#### **References**