All figures and tables were presented in the supplementary material 2.
Morphology and microstructure of G. elata
Figure S1 shows that the surface of G. elata tablets was relatively flat without any significant shrinkage prior to steam treatment. In all steamed groups, except the PGB-CFD treatment group, the appearance of G. elata was exhibited slightly yellow due to the dissolution of polysaccharides attached to its surface during the steaming process. The surface polysaccharide content was reduced. Research by Wu et al. [8] and Ai Ziping et al. [9] also supported the conclusion. The microstructure was most compact in the PGB-CFD treatment group. PGB-VMD treatment group had a porous and loose spongy structure with protrusions and pores. Yue et al. [10] showed that Codonopsis pilosula was comparable after microwave vacuum drying with rotation. The surface tissue morphology and microstructure of G. elata did not differ significantly between the PGB-NAD and PGB-HAD treatment groups, with both showing a slightly transparent appearance and clearly visible veins.
In the unprocessed groups, the surface morphology of G. elata in the NGB-NAD and NGB-HAD treatment groups showed different degrees of shrinkage and bending, and the degree of shrinkage and bending of G. elata after NAD was greater than that after HAD. However, the results of turmeric drying study [11] differed from our study, which could be due to the different experimental equipment and parameters. With the exception of the NGB-NAD treatment group, the appearance of the other groups was all white in the NGB treatment method, as the unprocessed treatment reduced the loss of G. elata polysaccharides compared to the steam processed treatment. The polysaccharides of G. elata was potentially declined over time due to a slow drying process and not-enzyme killing treatment in NGB-NAD treatment group. Additionally, the color of ginger was adversely affected after natural air drying [12], which is consistent with the results of this study.
The hardness and moisture contents
As shown in Fig. S2, the moisture content of G. elata was highest after CFD and significantly different from HAD and VMD. The moisture content of G. elata after drying did not differ significantly between the CFD and VMD treatment groups, even when the raw material was processed. However, there was a significant difference between the NAD and HAD treatment groups. During the steaming of G. elata tablets, a large number of air pockets were created, which, when combined with the G. elata polysaccharides, formed a solid structure that facilitated the transfer of internal water to the surface of the G. elata, thereby accelerating the drying process.
For hardness, there was a clear variation between the different drying methods within the unprocessed treatment group, with hardness decreasing in the following order: NAD > HAD > VMD > CFD. Zeng et al. [13] found that the hardness of Agaricus bisporus after hot air drying was 5.4 times that of freeze drying, which is consistent with our conclusions. The order of hardness from highest to lowest in the steam treated group was: NAD and HAD > CFD and VMD. The hardness of the unprocessed group after NAD and HAD was significantly greater than that of the steam processed group, because the case hardening was weakened after steam processing. Regardless of whether the G. elata tablets had been processed or not, there was no significant difference in their hardness after VMD. This was due to the fact that the heat was evenly distributed and, under the same vacuum conditions, the processing of the G. elata had no significant effect on its hardness after drying.
The contents of eight functional components
The chromatogram in Fig. S3 showed a resolution of over 1.2 for the peaks of gastrodin, p-hydroxybenzyl alcohol, vanillin, vanillyl alcohol, parishin A, parishin B, parishin C, and parishin E, which was considered to be good resolution. The results of the regression equations, R2 values, precision, reproducibility, stability, mean recoveries of standard addition and recovery rate are shown in Table S1.
As shown in Fig. S4, the total content of the eight functional components in G. elata tablets was the highest in the PGB-CFD treatment group, with the highest amount of parishin A. This was due to the fact that the CFD had a low drying temperature, which minimized the deterioration of G. elata caused by high temperature and reduced the biochemical reaction rate within the G. elata material. In addition, steam processing significantly reduced the activity of microorganisms and enzymes [14]. Next was the NGB-VMD group, where the VMD was highly efficient, and prevented internal deterioration and microbial activity of the material. The NGB-NAD treatment group had the lowest total content. The main reason for this was the longer drying time required for NAD and the fact that G. elata had not been processed. The active ingredient of the samples was degraded by the action of microorganisms and enzymes, resulting in a decrease in functional ingredients and a reduction in the quality of the dried products.
Compared with the unprocessed group, the total content of eight functional components in the steam processed group was significantly higher when the NAD, HAD and CFD methods were used. The results of a study conducted by Xie et al. [15] suggested that steam decreased the β-glucosidase activity, which suppressed the enzymatic hydrolysis reaction and consequently amplified the content of gastrodin, which is consistent with this study. The VMD group showed an opposite result. On the one hand, the loss of water-soluble functional components such as gastrodin and p-hydroxybenzyl alcohol was minimised without any processing. On the other hand, VMD could accelerate the process of reducing the moisture content of G. elata tablets and inhibit enzymatic and microbial activity. Thus, it may also serve as a protective functional component.
The content of polysaccharides, total phenols, and total flavonoids in G. elata
As shown in Fig. S5a, the order of total phenol content from highest to lowest was NGB-CFD, NGB-VMD, NGB-NAD, PGB-CFD, NGB-HAD, PGB-HAD, PGB-NAD, and PGB-VMD. This study confirmed the changes in total phenolic content of Bletilla striata scented tea following different drying treatments [16]. However, Ncube Buyile and colleagues [17] showed that the total phenolic content of Corchorus olitorius leaves was increased by cooking. The difference may be related to the phenolic species. In G. elata, a variety of phenols were found, such as gastrodin, epicatechin, cinnamic acid and so on. The concentration of free phenolics in G. elata from different sources was usually higher than that of bound phenolics [18]. During steaming, the free phenols present in G. elata tended to be lost, leading to a decrease in the total phenolic content.
As shown in Fig. S5b, the flavonoid content of NGB-CFD was the highest, followed by NGB-HAD, PGB-VMD, PGB-CFD, PGB-NAD, NGB-NAD and finally PGB-HAD and NGB-VMD. G. elata is known to contain flavonoids such as rutin, epicatechin, glycyrrhizin, heteroflavonoid C, and glicladone. The NGB-CFD group exhibited the highest levels of total flavonoids and total phenolics due to the unprocessed treatment, which prevented the loss of flavonoids during steaming and the CFD, which ensured the maximum retention of active ingredients. Guo et al. [19] found that freeze drying of Sophora japonica flowers resulted in higher total flavonoid content than hot air and vacuum drying. Other studies had shown that heating fresh turmeric resulted in a decrease in total phenolics and total flavonoids [11], reinforcing the findings.
As depicted in Fig. S5c, the polysaccharide content of G. elata raw material was relatively high, and the content range under different drying methods was 6%-20%, slightly higher than Fang Wei’s research results of 12%-17% [20]. NGB-CFD had the highest polysaccharide content, followed by PGB-CFD, NGB-VMD, PGB-NAD, NGB-HAD, PGB-HAD, PGB-VMD, and NGB-NAD with the lowest. CFD was advantageous for preserving polysaccharide content in both the unprocessed and steam processed methods. This was followed by the NGB-VMD group. Compared to the PGB-VMD treatment group, the NGB-VMD treatment group was able to prevent the loss of polysaccharides during the steaming process. Although it could not inactivate the microorganisms and enzymes on the G. elata tablets, VMD was able to rapidly reduce the water activity of the material, thereby significantly reducing the action time of the microorganisms and enzymes and consequently the loss of polysaccharides. The NGB-NAD treatment group had the lowest polysaccharide content. Due to the lack of steam treatment, the polysaccharides of G. elata had been subjected to decomposition by microorganisms and enzymes such as β-glucosyl enzyme during a prolonged drying process [20], resulting in the lowest content.
Evaluation of the antioxidant capacity
Figure S6 shows that the best total antioxidant capacity was the NGB-CFD group (68.54%), followed by the NGB-NAD group (44.83%) and the PGB-HAD group was the worst, with only 29.38%. The best DPPH free radical scavenger was the NGB-CFD group (50.86%), followed by the NGB-VMD group (33.47%), and the worst was the PGB-HAD group (28.87%). The NGB-CFD group had the best ABTS free radical scavenging ability (41.05%). This was followed by the NGB-HAD group (46.43%) and the weakest was the PGB-NAD group (17.06%). Freeze drying of both coffee fruits [21] and green beans [22] showed a more effective scavenging capacity for DPPH free radicals compared to HAD and NAD. Freeze-dried blackberry extract had the greatest DPPH, ABTS free radical scavenging capacity and total reducing power [23], which supports this conclusion. The unprocessed group of G. elata had a higher antioxidant capacity than the steam-processed group, as shown in Fig. S5, which showed higher levels of total phenolics and flavonoids in the unprocessed group. This was in line with the conclusion of the study.
The capacity of reducing blood pressure levels
As can be seen in Fig. S7, the PGB-CFD, PGB-VMD, and NGB-CFD groups showed the highest rate of ACE enzyme inhibition, while the PGB-NAD group showed the lowest rate. The highest rate of ACE enzyme inhibition had been observed in lyophilized mushrooms [24], supporting this conclusion. The PGB-CFD and NGB-CFD groups had a relatively high polysaccharide content, and G. elata polysaccharides were found to have an inhibitory effect on ACE enzyme activity, with an IC50 of 0.66 mg/mL[25]. The PGB-VMD group had a high concentration of gastrodin, p-hydroxybenzyl alcohol, and parishins, which can effectively reduce blood pressure by suppressing vascular inflammatory factors. The inhibition rate of ACE enzyme was the lowest in the PGB-NAD group because it contained less polysaccharide, gastrodin, p-hydroxybenzyl alcohol and parishins. The inhibiton rate of ACE enzyme can be an indicator of a substance’s potential to lower blood pressure, suggesting that the PGB-CFD, PGB-VMD, and NGB-CFD treatment groups have a promising ability to lower blood pressure.
The quality evaluation score
The content of each substance was scored from high to low and the acceptability of the appearance form, with a full score of 8 points. The results are shown in Table S2.
According to Table S2, the NGB-CFD treatment group, in which G. elata was directly sliced and vacuum freeze dried without steam treatment, had the highest overall evaluation score of 69 points. The NGB-VMD treatment group, in which G. elata was directly sliced without steam treatment and then vacuum microwave dried, had the second highest score of 56 points.