3.1 Morphological observation of capsaicin emulsion
The morphology of capsaicin emulsion before spray drying the capsaicin emulsion is shown in Fig. 1. Light micrographs captured the structure information of capsaicin emulsion (Fig. 1A1-G1). When sodium caseinate and wheat starch acetate were 7: 3, 5: 5, and 3: 7, the droplets of capsaicin emulsion were tiny and uniform distribution, indicating that the emulsifiers of these three complex samples had an excellent emulsification effect, effectively prevented the fusion of droplets, and could stabilize the emulsion well (Fig. 1C1-E1). However, when sodium caseinate and acetate wheat starch esters were 10: 0 and 9: 1, the emulsion showed the phenomenon of small oil droplets and tiny oil droplets sticking to each other in clusters, and the tiny oil droplets were long on the surface of the big oil droplets, indicating that the emulsification effect was poor (Fig. 1A1, F1). In addition, large droplets and aggregates were observed in the emulsion stabilized by a single acetate wheat starch ester. Droplet instability occurs due to coalescence (Alvarez-Cerimedo et al., 2010), indicating that single acetate wheat starch ester had a poor emulsifying effect and could not stabilize the emulsion well (Fig. 1G1).
The results of the AFM analysis are shown in Fig. 1(A2-G2). It could be seen from the figure that, except for the emulsions with sodium caseinate and acetate wheat starch ester of 1:9 and 0:10, the phenomenon of oil droplets aggregation is more prominent, tiny droplets join into clusters or gather into groups, and large droplets appear. The oil droplets in the other systems were tiny and evenly distributed. The results showed that the emulsifying effect of sodium caseinate with a small amount of wheat starch acetate as an emulsifier was better. Still, the emulsifying effect of single wheat starch acetate was poor. Ma & Chatterton (2021) showed that molecules of Nacas-polysaccharide complexes interact with each other through electrostatic attraction or covalent bonds to stabilize the emulsion according to various environmental factors.
3.2 Rheological properties of capsaicin emulsion
Shear rheological properties of capsaicin emulsion prepared by sodium caseinate combined with wheat starch acetate as an emulsifier at 25°C and 50°C are shown in Fig. 2. As shown in Fig. 2, when the ratio of sodium caseinate to wheat starch ester acetate was 0:10 and 1:9, the apparent viscosity of the emulsion system increased temporarily at a low shear rate. However, the apparent viscosity of other sample emulsions decreased with the increase in shear rate, and shear thinning appeared, which is typical of non-Newtonian fluids (Torres et al., 2018). Moreover, the shear stress-shear rate rheological curves were convex to varying degrees in the shear stress axis, indicating that the sample emulsions have the characteristics of pseudoplastic fluids.
The relevant results are recorded in Table 1. R2 of all curves was between 0.9989 and 0.9999, indicating that the simulation had an excellent fitting degree. The fluid index N values of the emulsion systems were all less than 1, except for the samples with 1:9 sodium caseinate and wheat starch acetate esters at 25°C and the samples with 10:0, 9:1, and 3:7 sodium caseinate and wheat starch acetate esters at 50°C, indicating that the system was a typical pseudoplastic fluid. The K value of the consistency coefficient in the system increased with the decrease of the content of sodium caseinate in the emulsifier, indicating that a strong and dense three-dimensional network structure was formed in the system and the thickening effect was enhanced. Moreover, the consistency coefficient K of the capsaicin emulsion system at 25°C was higher than that at 50°C, while the fluid index n value at 25°C was lower than that at 50°C. This suggested that temperature can influence the consistency of the emulsion and the liquid. That is to say, at a relatively low temperature, a strong bond was formed between sodium caseinate and modified starch to keep the emulsion system relatively stable. Therefore, the tensile deformation degree was low under the action of external stress, and the system showed a higher consistency coefficient K value and a lower fluid index n value at low temperature.
Table 1
Fitting parameters of Herschel-Bulkley equation for sodium caseinate-acetylated wheat starch-capsaicin emulsion. a, b
Temperatures | sodium caseinate: acetylated wheat starch | τ0 (Pa) | K (Pa·s− 1) | n | R2 |
25°C | 10: 0 | 0.0035 ± 0.0001bc | 0.0111 ± 0.0000d | 0.9897 ± 0.0001b | 0.9997 |
9: 1 | 0.0045 ± 0.0005ab | 0.0139 ± 0.0001c | 0.9799 ± 0.0013bc | 0.9999 |
7: 3 | 0.0054 ± 0.0004a | 0.0080 ± 0.0000e | 0.9705 ± 0.0007c | 0.9996 |
5: 5 | 0.0032 ± 0.0004bc | 0.0070 ± 0.0001e | 0.9522 ± 0.0026d | 0.9994 |
3: 7 | -0.0019 ± 0.0008d | 0.0153 ± 0.0002b | 0.9081 ± 0.0028e | 0.9996 |
1: 9 | 0.0027 ± 0.0001c | 0.0136 ± 0.0001c | 1.0037 ± 0.0017a | 0.9991 |
0: 10 | -0.0011 ± 0.0000d | 0.0256 ± 0.0007a | 0.9456 ± 0.0059d | 0.9995 |
50°C | 10: 0 | 0.0056 ± 0.0002a | 0.0041 ± 0.0000e | 1.0163 ± 0.0016a | 0.9996 |
9: 1 | 0.0047 ± 0.0005ab | 0.0041 ± 0.0000e | 1.0025 ± 0.0022b | 0.9993 |
7: 3 | 0.0040 ± 0.0005b | 0.0044 ± 0.0001e | 0.9874 ± 0.0056c | 0.9989 |
5: 5 | 0.0037 ± 0.0000b | 0.0078 ± 0.0000c | 0.9202 ± 0.0010e | 0.9996 |
3: 7 | 0.0040 ± 0.0004b | 0.0048 ± 0.0001d | 1.0012 ± 0.0028b | 0.9994 |
1: 9 | -0.0027 ± 0.0000d | 0.0112 ± 0.0001b | 0.9832 ± 0.0014c | 0.9991 |
0: 10 | 0.0012 ± 0.0000c | 0.0134 ± 0.0002a | 0.9697 ± 0.0037d | 0.9996 |
a τ0 is the yield stress. K is the consistency coefficient. n is the flow behavior index. R2 is the degree of fit. |
b All values are means of triplicate determinations ± SD. Different letters within the same column indicate a significantly different (p < 0.05). |
3.3 Morphology observation of capsaicin microcapsules
Many characteristics of microcapsules are related to their structures. The appearance of six microcapsules constructed by sodium caseinate and wheat starch acetate and the morphology observed by scanning electron microscopy (SEM) are shown in Fig. 1. Capsaicin microcapsule powder had a fine and fluffy texture, no agglomeration, uniform color, and dispersion, indicating that the powder product formed after rapid evaporation of water by spray drying is of good quality (Fig. 1H). Figure 1(A3ཞF3) shows that most powders are granular and not bonded together. The shape of each microcapsule particle is almost spherical, and the shell structure of the spherical surface is dense and continuous, but the particle size is not uniform. In addition, some microcapsule particles appear depression around. It was a common phenomenon in spray drying technology that the wall material shrunken due to rapid dehydration of emulsion (Rosenberg & Young 1993).
The surface of most microcapsule particles was smooth and intact, without obvious cracks or pores, indicating that microencapsulation had an excellent protective effect on capsaicin, and only a few microcapsule particles had holes and damage on the surface. This was consistent with the encapsulation efficiency results in the following paper. Although there were significant differences in the encapsulation efficiency among microcapsule products, they were all higher than 73.20%, which was a high level (Table 2). In addition, compared with other microcapsules, the uniformity of capsaicin microcapsules was poor when the sodium caseinate: acetate wheat starch ester was 1:9, and the phenomenon of particle adhesion existed. This might be related to higher moisture content (Table 2).
Table 2
The yield, encapsulation efficiency, water, solubility and wetness of sodium caseinate-acetylated wheat starch-capsaicin microcapsules. a
sodium caseinate: acetylated wheat starch | Yield (%) | Encapsulation efficiency (%) | Moisture (%) | Solubility (%) | Wetness (s) |
10: 0 | 77.02 ± 1.08a | 73.20 ± 0.36d | 0.87 ± 0.19b | 97.64 ± 1.25a | 357.73 ± 2.57a |
9: 1 | 69.37 ± 0.08b | 74.28 ± 0.40cd | 1.59 ± 0.14a | 96.31 ± 0.04a | 349.57 ± 1.52a |
7: 3 | 65.12 ± 0.79c | 79.02 ± 0.96b | 1.00 ± 0.19ab | 92.38 ± 0.69b | 293.48 ± 3.19b |
5: 5 | 51.45 ± 0.82d | 83.31 ± 0.58a | 1.22 ± 0.19ab | 88.40 ± 0.63c | 295.41 ± 3.39b |
3: 7 | 43.56 ± 1.23e | 84.05 ± 1.35a | 1.21 ± 0.02ab | 81.99 ± 0.40d | 277.47 ± 3.46c |
1: 9 | 29.74 ± 0.51f | 75.96 ± 0.07c | 1.42 ± 0.14ab | 77.88 ± 1.22e | 200.87 ± 1.56d |
a All values are means of triplicate determinations ± SD. Different letters within the same column indicate a significantly different (p < 0.05). |
3.4 FT-IR spectroscopy analysis
Whether capsaicin microcapsules introduce new functional groups or form new chemical bonds can be detected by Fourier Transform infrared spectrometer (FTIR), and then the molecular structure can be characterized. The infrared spectrum obtained is shown in Fig. 3. In the present study, the infrared spectrogram shapes of natural wheat starch and acetic acid-modified starch were similar. In the 3200–3600 cm− 1, the broad and strong -OH characteristic stretching vibration peak appeared in native wheat starch. The absorption peak occurred in the stretching vibration of -CH at 2930 cm− 1, and the strong absorption peak was at 1650 cm− 1. This was probably caused by water bonded to starch (De Cássia Sousa Mendes et al., 2021).
After being modified by acetic acid, the stretching vibration characteristic peak of -OH was weakened in the range of 3200-3600cm− 1, while a new absorption peak appeared at 1730cm− 1, which was the stretching vibration distinct peak of -C = O. It was proved that new groups were introduced into the starch treated by esterification, and the esterification reaction between original starch and acetic acid took place under alkaline conditions. Sodium caseinate is an amphiphilic protein molecule presenting strong amide I and amide II absorptions at 1650 and 1530 cm-1 (C-N stretching). It also showed C-H stretching at 2960 cm− 1, 2930 cm− 1 and 2850 cm− 1 indicating its high hydrophobicity. That corresponds to the results reported by Wang et al. (2016). Two prominent characteristic peaks were generated at 1650 cm− 1 and 1530 cm− 1. In addition, the distinct absorption peaks of pure capsaicin were 2930 cm− 1, 2850 cm− 1, 1750 cm− 1 and 1153 cm− 1 (Da Silva Anthero et al., 2022).
It was worthy of being noticed that in addition to the wall material characteristic peak of capsaicin microcapsules with sodium caseinate and starch modified by acetic acid as wall material, there was also a more substantial absorption peak near 1750 cm− 1 than that of pure capsaicin, with higher intensity and more prolonged and narrower peak type. This was caused by C = O stretching vibration and the inclusion of capsaicin, which proved that capsaicin had a robust binding effect with the wall material system and could form a stable microcapsule system. Moreover, the absorption peak of the microcapsule powder was sharper and stronger than the characteristic peak of sodium caseinate at 2960 cm− 1, 2930 cm− 1 and 2850 cm− 1. Also, a new distinct peak was formed at 1153 cm− 1, which was also related to the capsaicin encapsulated in the microcapsule.
3.5 Yield, encapsulation efficiency, moisture content, solubility and wettability analysis
Yield refers to the proportion of solid mass before and after spray drying, which reflects the efficiency of spray drying and is closely related to industrial processing costs. As shown in Table 2, the yield of the six microcapsules ranges from 29.74–77.02%. The yield of capsaicin microcapsules with sodium caseinate as a single wall material could reach 77.02%. The increase of acetylated wheat starch content in the embedding carrier led to a significant decrease in the yield of the microcapsule system (P < 0.05).
Encapsulation efficiency indicates the extent to which the wall material encapsulates capsaicin. Gómez-Aldapa et al. (2019) mentioned that microencapsulation efficiencies of 24.4%-81.03% have been reported at different concentrations and wall materials (in various proportions). In this study, the encapsulation efficiency of the capsaicin microcapsule system was 73.20–84.05% (Table 2). The higher encapsulation efficiency was observed at 83.31% and 84.05% in the ratio of sodium caseinate to wheat starch acetate was 5:5 and 3:7. However, when the ratio of sodium caseinate to wheat starch acetate was 7:3, it showed a lower encapsulation efficiency (79.02%).
Moreover, the encapsulation efficiency of capsaicin microcapsules with sodium caseinate as a single carrier was the lowest (73.20%), establishing that the microcapsules formed by acetylated wheat starch and sodium caseinate as composite wall material could significantly improve the encapsulation efficiency of the whole system (P < 0.05). Low encapsulation efficiency indicates that the capsaicin content on the surface of microcapsules is high, which is easy to be oxidized by the environment and its stability is reduced. In addition, it can be seen from Table 2 that the moisture content of these six microcapsules is all lower than 2.00%, an relatively low level. This made microcapsules not easy to mildew degradation, moisture absorption agglomeration and agglomeration and reduced the flow and dispersion of active ingredients.
Moreover, microbial growth and lipid oxidation were minimized, improving the storage stability of microcapsules. Except that the ratio of sodium caseinate to acetylated wheat starch was 9:1, the moisture content of the other five microcapsules had no significant difference (P < 0.05), but the moisture content tended to increase with the increase of modified starch (Table 2). That tendency was also verified by Frascareli et al. (2012) in a study with coffee oil encapsulation. The high moisture content may be due to the rapid drying process of modified starch and the rapid formation of the shell structure. At this point, the internal water has not spread to the surface in time to cause water evaporation (Tonon et al., 2011)
Solubility is the last step of particle dissolution and is an essential factor in determining the quality of raw material powder in the food industry (Fernandes et al., 2014). Table 2 shows that the solubility of capsaicin microcapsules is high (77.88%-97.64%), among which the solubility of the microcapsules prepared by single sodium caseinate as wall material is the highest at 97.64%. With the increase in the content of acetylated wheat starch, the solubility of capsaicin powder decreased significantly (P < 0.05). Pure capsaicin is insoluble in water and a fat-soluble pigment, challenging to use in the food industry. Microencapsulating capsaicin can substantially improve the solubility so that the product can release the core material when it meets water and plays a role.
Wettability is the capability of a bulk powder to absorb, defined as the capability of a bulk powder to absorb a liquid under the effect of capillary forces. The wettability of dried powders is one of the most crucial handling characteristics associated with reconstituting powders (Korma et al., 2019). From Table 2, the wettability of microcapsules with sodium caseinate as single wall material is poor, and the time of total immersion in water is 357.73 s. With the increase in the content of acetylated wheat starch in the carrier, the wettability of microcapsules had an upward trend. Specifically, when the ratio of sodium caseinate and acetylated wheat starch was 1:9, the wettability of microcapsules was better, and the time was 200.87 s. The results showed that acetylated wheat starch had the shortest immersion time and sodium caseinate had the longest immersion time. This may be due to sodium caseinate being insoluble in water at room temperature. Simultaneously, casein molecules appeared as a mixture of monomers and complexes, which tended to aggregate in an aqueous sodium caseinate dispersion. These monomers could not adequately remove hydrophobic surfaces when interacting with water (Morris et al. 2004). Thus, the dissolution of capsaicin microcapsules with sodium caseinate as wall material was limited, and the time of powder wholly dissolved in water was prolonged.
3.6 Chromaticity value
The color determination results of microcapsule powder are shown in Table 3. L*, a*, b* values of the product were recorded, and the color difference ΔE values were obtained by calculation. L* represents lightness (0 is black, 100 is white), a* expresses the red-green value (positive value is red, a negative value is green), and b * is the yellow-blue value (positive value represents yellow, a negative value is blue).
Table 3
The color of sodium caseinate-acetylated wheat starch-capsaicin microcapsules. a
sodium caseinate: acetylated wheat starch | L* (%) | a* (%) | b* (%) | ΔE |
10: 0 | 91.61 ± 0.06ab | 6.09 ± 0.04c | 18.78 ± 0.18e | 0.21 ± 0.12e |
9: 1 | 91.26 ± 0.24bc | 6.76 ± 0.13b | 19.62 ± 0.25de | 1.30 ± 0.32d |
7: 3 | 92.17 ± 0.22a | 4.74 ± 0.11e | 19.80 ± 0.34d | 1.81 ± 0.10d |
5: 5 | 91.64 ± 0.18ab | 5.42 ± 0.03d | 21.58 ± 0.13c | 2.99 ± 0.12c |
3: 7 | 90.85 ± 0.31c | 6.35 ± 0.22bc | 23.45 ± 0.53b | 4.89 ± 0.59b |
1: 9 | 83.55 ± 0.27d | 8.34 ± 0.37a | 28.41 ± 0.41a | 10.22 ± 0.52a |
a All values are means of triplicate determinations ± SD. Different letters within the same column indicate a significantly different (p < 0.05). |
The appearance of the six capsaicin microcapsules is orange and delicate. When the ratio of sodium caseinate to acetylated wheat starch was 7:3, the brightness of microcapsules was significantly higher than the others, and the L* value was 92.17, which was closer to white. While the L* was the lowest value of 83.55 when the ratio of sodium caseinate to acetylated wheat starch was 1:9, and the color was darker than other kinds of powder. The samples with larger whiteness showed that capsaicin was less exposed on the surface of microcapsules, with a better embedding effect and higher encapsulation efficiency. Specifically, the lightness was higher (91.64), and the loading rate was also higher (83.31%) than other microcapsules when the ratio of sodium caseinate to acetylated wheat starch was 5:5 (Table 3).
In terms of a* and b* values, both values increased significantly with the increase of modified starch content (P < 0.05). When the sodium caseinate to the starch ester of acetate was 1:9, a* was the highest value of 8.34, indicating a reddish color. The b* value was also the maximum (28.41) under this ratio, illustrating that the product's color is more inclined to be yellow (Table 3). In terms of color difference ΔE value, the color difference of microcapsules was significant (P < 0.05), and the value increased gradually (Table 3). This suggested that with the decrease of sodium caseinate content in microcapsule wall material, the color of microcapsule gradually approaches deep orange-yellow. However, except that the ratio of sodium caseinate to acetylated wheat starch was 1:9, the color difference of other microcapsules powder could not be distinguished by human eyes (ΔE < 5).
3.7 Particle size distribution
The mean particle size and particle size distribution of capsaicin microcapsule powder formed by spray drying were recorded in Table 4. The mean particle size indicates the thickness of microcapsule particles, and the particle size distribution reflects the concentration and uniformity of particles. As shown in Table 4, the spray-dried microcapsule powder in this study is micron. During the preparation of capsaicin emulsion, the interaction between protein and starch was increased by high-speed shear and ultrasonic waves. As a result, the interface between the core material and the wall material was broken, which promoted the combination of the three components in the system. Therefore, microcapsule powder formed by emulsion with higher modified starch content in the wall material has a larger particle size.
Table 4
The mean particle size and size distribution of sodium caseinate-acetylated wheat starch-capsaicin microcapsules. a
sodium caseinate: acetylated wheat starch | d (4,3) (µm) | d (3, 2) (µm) | Particle size distributions (µm) |
D (10) | D (50) | D (90) |
10: 0 | 1.01 ± 0.01d | 0.76 ± 0.01d | 0.44 ± 0.00d | 0.81 ± 0.01d | 1.93 ± 0.03b | |
9: 1 | 1.05 ± 0.01d | 0.78 ± 0.01d | 0.44 ± 0.01d | 0.88 ± 0.02d | 1.95 ± 0.01b | |
7: 3 | 1.15 ± 0.03d | 0.83 ± 0.01d | 0.46 ± 0.01d | 0.97 ± 0.07d | 2.08 ± 0.04b | |
5: 5 | 30.40 ± 0.57c | 6.13 ± 0.29c | 6.22 ± 0.08c | 30.05 ± 0.49c | 56.10 ± 1.27a | |
3: 7 | 32.27 ± 0.15b | 7.66 ± 0.32b | 6.86 ± 0.05b | 31.77 ± 0.23b | 59.27 ± 0.31a | |
1: 9 | 33.90 ± 1.27a | 9.08 ± 0.01a | 7.50 ± 0.12a | 34.40 ± 0.85a | 60.00 ± 3.54a | |
a All values are means of triplicate determinations ± SD. Different letters within the same column indicate a significantly different (p < 0.05). |
From Table 4, the d (4, 3) and d (3, 2) of the microcapsules were 33.90 µm and 9.08 µm when the ratio of sodium caseinate to acetylated wheat starch was 1:9, respectively. Moreover, the mean particle size of the microcapsules decreased significantly with the increase of sodium caseinate content in the wall material (P < 0.05). Nevertheless, the particle size distribution of microcapsules was similar to that reported by Álvarez-Henao et al. (2018). This may be because the emulsion with high viscosity can form larger droplets in atomization, and the microcapsule powder constructed by spray drying has a larger particle size.
In addition, the viscosity of emulsion also affects the particle size distribution of microcapsules (Esquivel-Chávez et al., 2021). Generally speaking, the particle size of microcapsule powder is not uniform, and the distribution range is extensive. From Table 4, capsaicin microcapsules are widely distributed in the field of 1–60 µm, and these values are lower than those reported in previous studies by Günel et al. (2021). The d (4, 3) and D (3, 2) of the microcapsules were close when the ratio of sodium caseinate to acetylated wheat starch was 10:0, 9:1 and 7:3, indicating that the particle size of the microcapsules was relatively uniform. Moreover, the particle size uniformity of other products was poor and the distribution range was wide.
3.8 Thermal properties analysis
TA software was used to analyze and compare DSC curves to study the thermal characteristics of capsaicin microcapsules, which is significant for evaluating product quality, safety, and stability. DSC curves and glass-transition temperature of the six capsaicin microcapsules are shown in Fig. 4. As shown, the natural capsaicin dissolved at 210.02°C. And the first endothermic peak of the six capsaicin microcapsules formed after spray drying appeared near 120°C, which was the glass-transition temperature (Tg).
This result indicated that the stable vitrification state could be maintained at room temperature and that the capsaicin microcapsules could still maintain a relatively complete structure after heat treatment. This may be related to the stable structure of starch - protein - capsaicin complex. However, according to Zhang et al. (2021), the microcapsule powders' glass transition temperature depended on the Tg of the wall material and related to the interactions among individual ingredients. Furthermore, the Tg of microcapsules was the lowest when the ratio of sodium caseinate to acetate wheat starch ester was 9:1, indicating that the emulsifier with this ratio had a poor embedding effect on capsaicin and weak resistance to heat.
3.9 Storage stability analysis
Under different storage conditions, the retention rates of three kinds of sodium caseinate - acetylated wheat starch - capsaicin microcapsules with encapsulation efficiency are shown in Fig. 5. It can be found that temperature has a significant effect on capsaicin microcapsules. After 15 days of storage at 50°C, the sodium caseinate: acetylated wheat starch was 7:3, 5:5, and 3:7, and the retention rates of capsaicin microcapsules were 77.81%, 83.22% and 70.23%, respectively (Fig. 5a). At the same time, the retention rates of capsaicin microcapsules were 86.98%, 87.47% and 83.25% after 15 days of storage at 25°C (Fig. 5b). It suggested that capsaicin microcapsules had specific heat resistance, and the system formed with 5N-5AW as the sample wall material had a more substantial heat protection effect on capsaicin.
After 15 days of UV-light storage, the retention rates of the microcapsules with the ratio of sodium caseinate to acetylated wheat starch of 7:3, 5:5 and 3:7 were 76.27%, 80.53% and 64.91%, respectively. After 15 days of sunlight storage, the retention rates of the microcapsules were 83.36%, 84.37% and 78.43%, respectively. It could be noticed that the retention rate was more than 60.00% after 15 days of storage under different light, illustrating that microcapsules had a photoprotective effect on capsaicin. Interestingly, when the ratio of sodium caseinate to acetylated wheat starch was 5:5, the retention rate of the microcapsules was the highest after 15 days of storage under the light. This result showed that the microcapsules formed by the compound samples could effectively improve the photostability of capsaicin.
3.10 Principal component analysis
Principal component analysis (PCA) was used to compare the differences among components of microcapsules. As shown in Fig. 6, no variables are directly related to the principal component. The contribution value of the first principal component (PC1) is 80.29%, which mainly affects yield, encapsulation efficiency, moisture content, solubility, wettability, chromaticity value, particle size distribution, Tg, shear rheological properties and storage stability (except the retention rate at 50°C). On the other hand, the contribution value of the second principal component (PC2) was 19.71%, which mainly affected R (50°C), illustrating that the physicochemical properties and storage stability of microcapsules were mutually influenced and interacted, which jointly determined the properties of microcapsules (Fig. 6).