A solid preparation of phytochemicals: improvement of the solubility and bioavailability of astragaloside IV based on β-cyclodextrin microencapsulation

Astragaloside IV/β-cyclodextrin inclusion complex (AsIV/β-CD IC) was successfully prepared by low-temperature sedimentation technology. Under the preparation conditions inclusion temperature 60 °C, inclusion time 3 h, core–wall material molar ratio 1:1, and inclusion rate 750 rpm, the optimal encapsulation efficiency reached 81.63%. It was characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray diffractometer (XRD) and thermogravimetric (TGA). Physicochemical characterization of the AsIV/β-CD IC indicated that the complexation of AsIV/β-CD was successful, and the thermal stability of AsIV was improved clearly. Through granulation technology, astragalus saponin granules (ASG) with uniform size from AsIV/β-CD IC were obtained. ASG possessed significant water solubility and storage stability beside fine taste. In addition, ASG demonstrated beneficial bioactivity in antioxidant and antibacterial functions. Antioxidative stress detection showed that ASG could inhibit the increase of malondialdehyde (MDA) content and the decrease of superoxide dismutase (SOD) content in cells caused by lipopolysaccharides (LPS). In addition, in vitro antibacterial experiments of ASG showed that ASG has obvious antibacterial activity against 6 strains, especially the minimum inhibitory concentration (MIC) value of Escherichia coli and Bacillus subtilis reached 12.5 mg/mL.


Introduction
Astragalus membranaceus (Fisch.)Bunge.(A. membranaceus) is a common healthy and medicinal plant widely distributed in temperate regions of North America, Europe and Asia (Salehi et al. 2021).Astragaloside IV (AsIV), an active ingredient derived from A. membranaceus, is known as a signature compound for its various excellent biological activities (Zhou et al. 2012), including antiinflammatory, antioxidant, neuroprotective and anticancer effects (Luo et al. 2021;Shahrajabian et al. 2019).Studies have shown that AsIV is not only an excellent immunomodulator but in some aspects even surpasses ginsenosides (Zheng et al. 2020) and is of great value in the treatment of cardiovascular and cerebrovascular diseases, respiratory diseases, diabetes and related complications (Facioni et al. 2018;Kwon et al. 2021).AsIV, despite its natural origin and excellent effects, has poor solubility and stability, resulting in low value adding products and limiting its direct application in solid formulations (Chen et al. 2022;Huang et al. 2006;Qing et al. 2017).Therefore, the combination with embedding technology can make critical problem overcome and resolved.
Embedding technology (microencapsulation) refers to the use of natural or synthetic polymers to encapsulate solid, liquid or gaseous materials to form a semipermeable material (Kang et al. 2019;Muhoza et al. 2019).Microencapsulation is widely used in the fields of pharmacy, food, fragrance and agrochemicals (Ban et al. 2020;Assadpour and Jafari 2019).The fine particles or powders formed by microencapsulation have good dispersion and flowability and can be homogeneously mixed with other ingredients for easy dosing and transport (Sun et al. 2020).There are a variety of wall materials used for embedding in domestic and international studies, including starch, maltodextrin and gum Arabic.β-cyclodextrin has become a good choice of wall material for its special structure with a hydrophobic center and hydrophilic surface, which allows for uniform dispersion during use and protection of the guest molecular material (Samborska et al. 2021).The non-polar guest molecule is embedded in a cavity with β-cyclodextrin as the host, forming a complex that can be in solution or crystalline form (Sharif et al. 2018).Covalent bonds are not formed or broken during the inclusion process, but are driven by an enthalpy change (ΔH).The enthalpy-rich water molecules are released from the cyclodextrin cavity, where the host-guest molecules form a dynamic equilibrium (Cetin Babaoglu et al. 2017).The formation of inclusion complexes offers significant advantages for improving the solubility of insoluble molecules (Jiang et al. 2016), enhancing light and thermal stability (Molina et al. 2019), masking the odor of specific substances and modifying the release of drugs (Cai et al. 2019).
Sugar alcohols, a class of polyols formed by the hydrogenation of the reducing carbon group of sugar, are used as fillers and flavoring agents in granules.The polyhydroxy groups they contain stimulate the taste buds and produce a sweet taste, which serves to mask the bitterness and bad odor of the medicine and reduce irritation to the gastrointestinal tract.Sugar alcohols absorb heat when dissolved or diluted and have a distinct cooling sensation when dissolved in the mouth.The higher heat resistance prevents Maillard reaction at high temperature (Ribeiro and Veloso 2021).Studies have shown that it also has functional properties such as low caloric value, non-stimulation of insulin secretion and alleviation of diabetes.Among them, the low hygroscopicity of mannitol can play a certain role in the preparation to prevent moisture and isolate the air.The excellent water solubility of xylitol can increase the solubility of solid formulations and proliferate Bifidobacterium in the intestinal tract, playing a role in regulating intestinal flora and improving intestinal function (Li et al. 2021).The combination of sugar alcohol and cyclodextrin can compensate each other for the deficiencies of these two carriers.While increasing the solubility of the drug, it acts as a flavor correction and effectively prolongs the residence time of the drug in the body.
Therefore, in view of the poor water solubility and stability of AsIV, this study aimed to prepare an AsIV/β-CD inclusion complex (IC) by low-temperature precipitation technology and optimize the process parameters.Solid granulation technology formulates IC into Astragalus saponin granules (ASG) to further improve the taste and stability, aiming to obtain a new type of active substance for solid preparation.Recent years, a solid lipid nanoparticle-enriched hydrogel for local delivery of AsIV is being investigated (Chen et al. 2013); Peng et al. used sodium alginate-gelatin as a carrier for the encapsulation and slow release of AsIV (Peng et al. 2012), supercritical antisolvent (SAS) method for the preparation of AsIV nanoparticles for cancer treatment (Chen et al. 2022).In this work, the first combined application of AsIV, β-CD and sugar alcohols, ASG may have application in promoting the bioavailability of low water-soluble drugs.
ASG was used to significantly improve the water solubility, thermal stability, biocompatibility and bioavailability of AsIV, and the added value of the product was enhanced, which will provide some directions for the development and utilization of AsIV in food and pharmaceutical applications.

Materials
Astragalus was harvested from Heilongjiang Province, China, and preserved in the Key Laboratory of Forest Plant Ecology, Ministry of Education, Northeast Forestry University (Heilongjiang, China).Astragaloside IV was purchased from the National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China).β-cyclodextrin, xylitol and mannitol were purchased from Shanghai Macklin Biochemical Co., Ltd (Shanghai, China).

Extraction and refinement of AsIV
The green extraction of AsIV refers to the method of Sui-xin Wang (Wang et al. 2019) with slight changes.Ultrasoundassisted enzymatic assay was used to induce increments of AsIV (250 W, 50 min).The surfactant water system consisted of 2% (w/v) TX-114 and 0.03% (w/v) Gemini 16-5-16.The HPLC chromatographic profile of the purified AsIV is shown in Fig. S1, with a purity of 99.1%.

Preparation of AsIV/β-CD IC
The complex solution was obtained by slow dropwise addition of AsIV-ethanol solution (1:3, w/w) to a saturated aqueous solution of β-cyclodextrin.The filter cake was washed 1 ~ 2 times after filtering under reduced pressure.Loose AsIV/β-CD IC was obtained after freeze-drying.

One-factor optimization
Four influencing factors were controlled: molar ratio of AsIV to β-CD (3:1, 2:1, 1:1, 1:2 and 1:3), inclusion temperature (50, 55, 60, 65 and 70 °C), inclusion time (1, 2, 3, 4 and 5 h) and inclusion rate (400, 500, 600, 700 and 800 r/ min).The encapsulation efficiency (EE) calculated by different optimization conditions is: The actual quality of AsIV contained in the inclusion compound was quantitatively determined by external standard method.Three groups of parallel experiments were carried out for each group of treatments according to the specifications, and the optimal preparation conditions were selected.

Box-Behnken design optimization
Design Expert 11 software was used to carry out the Box-Behnken design based on the single factor experiment.
The model was used to analyze the relationship between four factors and three levels (Table S1) to determine the optimal preparation conditions for IC.

SEM investigations
The microscopic morphologies of AsIV, β-cd and IC under different magnifications were assessed using scanning electron microscopy (SEM).Samples were vacuum coated, observed and photographed under a cold field emission scanning electron microscope (JSM-7500F, JEOL, Japan) with an accelerating voltage of 5 kV and an electric current of 10 mA.

X-ray diffraction (XRD) analysis
Phase analysis of the samples was performed using an X-ray diffractometer (D-max-2200, Japan) at a voltage of 40 kV and a current of 30 mV.The diffraction angle range is set from 5° to 80° with a scanning step size 0.02° and a scanning speed 5°/min (Wang et al. 2021).
(1) EE% = actual content of AsIV in the IC (g) total AsIV content (g) × 100% Fourier transform infrared (FTIR) spectroscopy The formation of IC was verified by a Fourier-transform infrared spectrometer (IRAffinity-1S, Shimadzu, Japan).Samples were mixed with dried anhydrous KBr (1:100, w/w) and ground until a smooth fine powder was formed.The FTIR spectrum of the sample was acquired in a spectrum range of 4000-500 cm −1 (Jiang et al. 2021).

Thermogravimetric (TGA) analysis
The thermal stability was determined using a synchronous thermal analyzer (STA449F5, Netzsch, Germany).Samples were scanned from 30 to 600 °C at a rate of 10℃/min under nitrogen purge flow at 50 mL/min (Cai et al. 2019.

Preparation and quality inspection of ASGs
The best formulation of astragalus saponin granules (ASG) was obtained by using wet granulation technology and optimizing the different excipient ratios of granules according to five sensory evaluation indexes.In a nutshell, soft material was made under the blending of wetting agent by mixing IC powder with mannitol and xylitol in the ratio (6:2:2, W/W).ASGs were obtained after drying wet granules with several times of precise sieving.Table S2 demonstrates that ASGs are white round particles with uniform dimensions.
The sensory evaluation criteria are shown in Table S3.The evaluation team consisted of 20 members, and the samples were randomly numbered.

Antioxidant activity detection
The antioxidant activity of ASG was detected by using AsIV as a control experiment under lipopolysaccharide (LPS) modeling conditions (Mizobuchi and Soma 2021).The cell 1 3 samples were processed by ultrasound-assisted fragmentation.For the determination of MDA content, the mixture was cooled after a 100 °C water bath for 60 min and centrifuged, and the absorbance of the supernatant at 532 nm and 600 nm was measured.As for the SOD enzyme activity assay, the absorbance value A was measured at 560 nm after a 37 °C water bath for 30 min.
wherein V t is the total volume of the reaction system; ε: the MDA molar absorption coefficient; V s : the volume of sample added; d: the optical path of 96-well plate; 10 9 : the unit conversion factor; Cpr: the sample protein concentration; IR: the inhibition rate; and F: the sample dilution factor.

Antibacterial activity detection
Six strains were inoculated in sterile nutrient agar (NA) Petri dishes, and they concentration reached 0.5, as measured by a McFarland turbidimeter (Delogu et al. 2015).ASG was prepared into a mother liquor with a mass concentration of 100 g/L with sterile water.They were evenly diluted with sterile water to 9 concentration gradients and prepared into liquids containing ASG with mass concentrations of 100, 50, 25, 12.5, 6.25, 3.13, 1.56 and 0.78 mg/mL.They were prepared into agar plates containing various concentrations of drug solution for testing the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) (Wang et al. 2021).The Kirby-Bauer test (K-B) was used to determine the bacteriostatic effect of ASG.Blank control was set at the same time, and the diameter of the inhibition zone was measured after culturing at 37 °C for 18 h.

Statistical analysis
One-way ANOVA of the data was performed using SPSS 12.0 software.Significant differences among means were evaluated at P ≤ 0.05.The resulting data were presented as Mean ± SE.All experiments were processed in parallel three times and performed simultaneously.

Response surface optimization of AsIV/β-CD IC preparation
The saturated aqueous solution method was chosen for the preparation of ICs, mainly because in both ensuring high encapsulation efficiency and shortening the drying time to obtain the package (Xi et al. 2015).It is more conducive to reduce energy consumption and thermal degradation in the drying-process.Response surface optimization experiments were carried out on the basis of the single factor experimental results (Fig. S2).The response values of four factors tested by Design-Expert software are shown in Table S4.Simulation equation of the respective variables of AsIV/ β-CD IC is: Table 1 shows that the influence of each factor on the EE of IC is: X 3 > X 1 > X 2 > X 4 according to the size of the F value.In addition, the coefficient of determination (R 2 = 0.981) indicates that the model fits well and the regression equation is representative.It can also be seen from the variance analysis results of the regression equations that the residual (P = 0.8709) is not significant, indicating that the equation has little experimental error.As shown in Fig. 1c, e, f, the 3D projection contour lines of X 1 X 4 , X 2 X 4 , X 3 X 4 are elliptical, indicating that the interaction between the two is significant.The optimal conditions for IC preparation predicted by the regression model were as follows: inclusion temperature 65℃, inclusion time 3 h, core-wall molar ratio 1:1 and inclusion rate 784 r/min.The predicted value of EE is 79.45%.The actual plan of the experiment was adjusted according to the above optimal process, and the operation was performed in parallel 3 times.The results showed that the average EE was 80.26% and the model was realistic and reliable. (5)

Scanning electron microscope (SEM) observation
The formation of inclusion complexes can change the appearance of β-CD, and SEM can distinguish IC from unwrapped AsIV and β-CD (Srinivasan et al. 2014).
AsIV shows irregular strip-like crystals with a clusterlike structure as a whole (Fig. 2a).β-CD presents a massive irregular structure with strong porosity (Fig. 2b).
The AsIV/β-CD IC exhibits a more regular layered cubic crystal structure with a smooth surface (Fig. 2c).This is because the molecules of AsIV enter the large-capacity cavity of β-CD, which makes the morphology of β-CD change greatly and forms a stable "host-guest" supramolecular structure (Yin et al. 2021).In addition, the normal Ltd., UK) as shown in Fig. S5, and the average particle size was 179.3 ± 2.6 nm.

X-ray diffraction (XRD) detection
Powder ray diffraction is another useful method to assess whether complexes are formed (Xi et al. 2015).The XRD patterns of AsIV, β-CD and AsIV/β-CD ICD are shown in Fig. 3a.We can observe that AsIV has a good crystalline structure, with four characteristic diffraction peaks in the 2θ range of 7.31°-18.86°.β-CD is one of the nanocrystals, and its diffraction peak will have a smaller half-peak width and diffraction intensity, accompanied by lattice expansion.In addition, the smaller the grain size is, the greater the broadening of the X-ray diffraction band.The spectrogram of IC is highly similar to that of β-CD, and the strong diffraction peaks of β-CD appear at 2θ = 14.68°, 21.36° and 28.93°.The strong diffraction peaks of IC characteristic peaks are slightly lower than those of β-CD, appearing at 2θ = 13.89,20.54° and 27.06°.This is due to the fact that AsIV is encapsulated in the cyclodextrin cavity, resulting in a slight decrease in its diffraction peak.The spectra of the inclusion complex are obviously different from those of AsIV.Most of the characteristic peaks of AsIV disappear, weaken or shift, indicating that the crystal form has changed obviously.

Fourier transform infrared (FTIR) analysis
The presence of functional groups and the formation of intramolecular hydrogen bonds between the cyclodextrin and the guest moiety can be confirmed by Fourier-transform infrared spectroscopy (Cai et al. 2019).Figure 3b shows the FT-IR spectrogram of AsIV, β-CD and AsIV/β-CD IC, and the substance structures with FTIR functional group comparison are located in Table S5.AsIV belongs to tetraline triterpenoid saponins, and the peaks in curve a are characteristic of the molecule.The contraction vibration peak at 2900 cm −1 is the characteristic peak of C-H, and the strong absorption band appearing around 3380 cm −1 is classified as the expansion vibration of O-H (Tavares et al. 2021).From 3120 to 3581 cm −1 , β-CD exhibits a wide O-H telescopic vibration belt; the CH 2 telescopic vibration at 2926 cm −1 and the C=O bending vibrations at 1645 cm −1 , 937 cm −1 and 756 cm −1 exhibited characteristic peaks of β-glycosid bonds in the cyclic structure (curve b) (Vyas et al. 2008).
The FTIR spectra of IC and β-CD have resemblance, which indicates that AsIV and β-CD form an IC, rather than forming a new compound (Timilsena et al. 2017).It was observed that due to the shielding of β-CD, the feature absorption peak of AsIV disappeared.We speculate that AsIV was successfully embedded in the hydrophobic chamber of β-CD.The peak change in comparison curves b and c, 1022 cm −1 and 1159 cm −1 is caused by expanding the C-O and C-O-C of the β-cyclodextrin cavity due to the embedding process.The spectrum of AsIV/β-CD IC showed a spectral band at 943 cm −1 , which was caused by the rolling vibration of the end of the AsIV pyran ring.Therefore, the formation of IC can be explained by spectral changes.In addition, the structure of the "host-guest" in IC can be determined by the displacement of hydrogen elements in the NMR pattern (Fig. S4).The characteristic peak of AsIV/ β-CD IC is very close to that of β-CD, which can prove that AsIV has successfully entered the cavity structure of β-CD, and the interaction between the host-guest molecules forms IC, and this result also verifies the FTIR detection results.

Thermogravimetric (TGA) analysis
Thermal stability before and after IC formation was evaluated using TGA in the temperature range of 30-600 °C to analyze thermal degradation and weight loss processes (Rakmai et al. 2017).Figure 3c shows the TGA and derivative thermogravimetry (DTG) curves of AsIV, β-CD and AsIV/β-CD IC.AsIV has only one weight loss zone at 207-310 °C, the highest weight loss rate is 79.43%, and most of the AsIV (84.25%) volatilizes and decomposes.The thermal weight loss of β-CD is divided into two stages: The first stage occurs when the temperature is lower than 100 °C, which is mainly related to the evaporation of water and the vitrification of β-CD; the weight of β-CD decreases sharply at 320 °C, the maximum weight loss rate occurs at approximately 338 °C, and the weight loss rate is approximately 65.38%.This is mainly due to the degradation of β-cyclodextrin molecules, the breakage of cross-linking bonds, the partial breakage of the main chain and the decomposition of β-cyclodextrin glucose units.The weightlessness states of β-CD and AsIV/β-CD IC were very similar.The weightless peak of AsIV/β-CD IC shifted to a higher temperature relative to AsIV (319 °C vs 235 °C), indicating that the interaction between AsIV and β-CD greatly affected the thermal stability of IC.Apparently, the thermal stability of AsIV was significantly enhanced in IC due to the inclusion of β-CD.

Property analysis and sensory evaluation of the of ASG
The sensory evaluation radar diagram of nine samples prepared according to different proportion of excipients is shown in Fig. S5.It can be seen that when the mass ratio of IC to xylitol and mannitol is 6:2:2, the sensory score reaches the highest (93.01 points).ASG is an easy to accept product with off-white color, cool, sweet and non-sticky taste.The water-soluble modification effect of ASG is shown in Table S2.It can be observed that the AsIV solution (a) after stirring at a constant speed for 24 h is still turbid; in contrast, the ASG solution (b) is clear and transparent.This also verifies our speculation very well.The ASG obtained by the combination of inclusion technology and granulation technology significantly improved the water solubility of AsIV.

In vitro release evaluation
In vitro release profiles of ASG, AsIV/β-CD IC and AsIV in three dissolution media are shown in Fig. 4. It can be seen that the cumulative dissolution amount of ASG in any dissolution medium is better than that of AsIV/β-CD IC and unencapsulated AsIV, and has a sustained release effect.The final cumulative dissolution rates of ASG, AsIV/β-CD IC and AsIV in deionized water (Fig. 4a) were 49.79%, 30.82% and 19.63%, respectively.The cumulative dissolution of ASG over 48 h was approximately 1.61 times that of AsIV/β-CD IC and 2.53 times that of unencapsulated AsIV.The final cumulative dissolution rates of ASG, AsIV/β-CD IC and AsIV in SGF (Fig. 4b) were 55.45%, 35.78% and 23.59%, respectively, which were about 0.55 and 1.35 times greater than that of AsIV/β-CD IC and AsIV.The final cumulative dissolution rates of ASG, AsIV/β-CD IC and AsIV in SIF (Fig. 4c) were 33.53%, 19.56% and 12.29%, respectively.The final dissolution of ASG within 48 h was approximately 1.71 and 2.73 times higher than that of AsIV/β-CD IC and AsIV.Since the cyclodextrin structure in the ASG formula is generated by the action of glucosyltransferase, it is easy to be hydrolyzed in an acidic environment.Therefore, the solubility of ASG in SGF at low pH was significantly increased compared to deionized water as a control.On the contrary, the dissolution effect of ASG is slightly weaker in SIF under alkaline condition.

Antioxidant activity detection of ASG
Oxygen free radicals can generate lipid peroxides by applying to lipid unsaturated fatty acids, it can be decomposed into MDA and other complex compounds.Lipid oxidation levels can be detected by measuring MDA content (Hu et al. 2020;Wang et al. 2018).Figure 5a shows that LPS-induced HepG2 cells significantly upregulated the level of MDA after 24 h (P < 0.0001), indicating that the model was successfully established in the LPS group.However, the administration group pretreated with different concentrations of AsIV or ASG showed a significantly lower MDA level than the model group, and with the increase in the added AsIV concentration, the MDA level gradually decreased in a dosedependent manner.Figure 5b shows that ASG significantly inhibited the level of MDA and significantly down-regulated the level of lipid peroxidation in HepG2 cells.There was no significant difference in the down-regulation of MDA expression between the AsIV group and the ASG group, indicating that both could significantly inhibit the LPSinduced increase of MDA levels in HepG2 cells.
SOD has dual identities of superoxide anion scavenging enzyme and H 2 O 2 generating enzyme, which lays its important role in biological antioxidant activity (Zhang et al. 2020).Figure 5c shows that SOD level of drug group 4 h after pretreatment was obviously higher than model group, and SOD level gradually increased with increasing AsIV or ASG concentration.The ASG group had significantly increased SOD levels and increased superoxide dismutase activity in HepG2 cells (Fig. 5d).The up-regulation of SOD expression in the AsIV group and ASG group was very close, and both AsIV monomer and ASG could significantly increase the SOD level.In conclusion, ASG not only has similar antioxidant levels to monomer, but also exhibits better effects than AsIV at certain doses.The experimental results verified our speculation that the encapsulation process and granulation technology did not affect the biological activity of AsIV, and even improved the antioxidant activity of the monomer.While improving the water solubility of AsIV, the bioavailability of the monomer is improved, which provides a theoretical basis for the further development and utilization of Astragalus saponins in the fields of food and medicine.

Antibacterial activity detection of ASG
It can be seen that ASG has broad-spectrum antibacterial activity after observing and measuring the average diameter of the inhibition zone under different strains (Fig. S6).ASG inhibited six strains to different degrees, especially Escherichia coli (Fig. S6a) and Bacillus subtilis (Fig. S6e) with relatively strong inhibitory, and the average inhibition zone diameters, respectively, were 10.4 mm and 10.1 mm.The MIC of ASG against E. coli and B. subtilis was 12.5 mg/ mL; the MIC against Pseudomonas aeruginosa was 25 mg/ mL; and the MIC against S. aureus, C. albicans and P. acnes was 50 mg/mL.The MBC of ASG was 50 mg/mL for E. coli and B. subtilis, and the MBC for the other four strains was 100 mg/mL (Table 2).The dissolution of ASG releases the encapsulated AsIV, which confirms that ASG also has broad-spectrum antibacterial activity.The addition of AsIV increases the content of nucleic acid substances in the bacterial suspension, which indicates that AsIV damages the integrity of the cell membrane of the bacteria and causes macromolecular substance overflow to achieve bacteriostatic effects.This result offers some thoughts to the development and production of novel antibiotics using encapsulation techniques under the severe situation that human beings are facing increasing bacterial resistance or adverse reactions to antibiotics.

Storage stability
The saponin contents of ASG, AsIV/β-CD IC and unencapsulated AsIV during storage at 4 °C, 25 °C and 37 °C for 60 days are shown in Fig. 6.It can be seen that the stability of saponins in all samples stored at 4 °C (Fig. 6a) was significantly better than that in other two storage conditions (Fig. 6b, c) (P < 0.05).This reflects that the higher the storage temperature, the lower the retention of saponins.ASG was 86.48% and 72.43% of the initial saponin concentration after 30 and 60 days at 4 °C, respectively, while at 4 °C, the saponin retention rates were 78.83%-67.29%and 59.97%-49.35% in AsIV/β-CD IC and unencapsulated AsIV (Fig. 6a).Similar trends were observed at 25 °C near room temperature and 37 °C near human body temperature (Fig. 6b, c), and the saponin retention percentages of ASG were approximately 76.23 and 51.77%, respectively, after one month storage.However, the saponins in AsIV are only 40.82% and 38.04%.These results indicate that ASG prepared from a combination of sugar alcohol and cyclodextrin has better stability at all three temperatures during storage compared to IC prepared from cyclodextrin alone and unencapsulated saponins.The interaction between the components of the wall material forms a protective film around the saponins, which slows down the thermal damage and degradation of the saponins, and makes the ASG structure more compact, thus maintaining the stability of astragalus saponins during storage.

Conclusions
In conclusion, AsIV was successfully encapsulated in the β-cyclodextrin cavity by embedding technology to form AsIV/β-CD IC.The best encapsulation efficiency obtained by optimization was 81.63%.The formation of ICs was confirmed by morphology visualization, XRD, FT-IR physicochemical characterization and TGA analysis, which verified the remarkably improvement in the thermostability of embedded IC.The resulting IC was reprocessed into ASG using granulation technology.The formation of ASG significantly improves the taste and water solubility of AsIV, which has good in vitro release effect and storage stability, and also confirms its excellent antioxidant and antibacterial effects.Therefore, the combination of embedding technology and granulation technology greatly improved the bioavailability of AsIV.This research provides some ideas for the application of water-soluble preparations of new medicinal active ingredients for food and medicine.

Fig. 1
Fig.13D response surface models of various interaction terms for the optimization of AsIV/β-CD IC

Fig. 4 a
Fig. 4 a Cumulative dissolution of AsIV, AsIV/β-CD IC and ASG in deionized water; b cumulative dissolution of AsIV, AsIV/β-CD IC and ASG in SGF; and c cumulative dissolution of AsIV, AsIV/β-CD IC and ASG in SIF

Fig. 5
Fig. 5 Inhibition of LPS-induced oxidative stress in hepatocytes by AsIV and ASG. a effects of AsIV on MDA content in LPS-induced hepatocytes; b effects of ASG on MDA content in LPS-induced hepatocytes; c effects of AsIV on SOD content in LPS-induced hepatocytes; and d effects of ASG on SOD content in LPS-induced

Table 2
MIC and MBC detection results of ASG for six strains "−" means the bacterial growth in the test tube is inhibited; "+" means bacterial growth and the culture solution is turbid; "△" means the number of colonies < 5 after transfection in a Petri dish, and "▲" means the number of colonies after transfection in a Petri dish > 5