Pueraria lobata was purchased from QianCao Chinese Herbal Pieces Co. Ltd., Beijing, China. White wine (38% strength) was sourced from Shunxin Agriculture Co. Ltd., Beijing, China. White vinegar was purchased from Xianglian Sauce and Food Co. Ltd., Changsha, China. The total polysaccharide content was measured using a Total Polysaccharide Determination Kit (Keming Biotechnology Co., Ltd., Suzhou, China). Decoction pots were purchased from Joyoung Co., Ltd., Jinan, China. A digital electric heater was purchased from LiChen Instrument Technology Co., Ltd., Shanghai, China.

Traditional process: (1) Distilled water was used as the solvent for decoction. (2) 50 g of P. lobata slices were purified to remove impurities and soaked in distilled water for 30 min. (3) 50 g of P. lobata slices that had been soaked in Step (2) were placed in 500 mL of the decoction prepared in Step (1) and boiled for 50 min. The mixture was boiled at high heat in a decoction pot for 30 min, then simmering at low heat for an additional 20 min. The decoction was separated and stored. The residual herbal material from the initial decoction was reintroduced into the decoction obtained in step (2) and boiled for 40 min. The mixture was heated for 25 min, then stewed at low heat for 15 min in a decoction pot. The decoction was then strained. Finally, the two batches of decoction were combined to obtain a P. lobata decoction (13, 14).

Processing method 1 (PM1): (1) Distilled water and edible white wine were prepared in a volume ratio of 10:1 to form a decoction solution. Steps (2) and (3) were performed according to the traditional process.

Processing method 2 (PM2): (1) Distilled water and white vinegar were prepared at a volume ratio of 10:1; Steps (2) and (3) were performed according to the traditional process.

Processing method 3 (PM3): (1) Distilled water was used as the solvent for decoction. (2) 50 g of P. lobata slices were soaked in distilled water for 30 min. (3) After soaking as described in step (2), 50 g of P. lobata slices were boiled in the 500 mL decoction solution obtained from step (1) for 50 min. The medicinal liquid was removed to obtain a decoction from P. lobata slices. The boiling temperature was maintained at 100°C, and an electric heater was utilized for the boiling process.

Processing method 4 (PM4): (1) Distilled water and edible white wine were prepared according to the volume ratio of 10:1; Steps (2) and (3) mirror those outlined in treatment process 3.

Processing method 5 (PM5): (1) Distilled water and edible white vinegar were prepared at a volume ratio of 10:1. The procedures outlined in steps (2) and (3) were identical to those detailed in treatment Process 3.

The total content of polysaccharides in P. lobata slices was extracted according to a previously reported method (15). In this process, 1 mL of P. lobata decoction was used, and 1 mL of water was added to dissolve the polysaccharides. The extract was heated in a metal bath at 100°C for 2 h, then centrifuged at 10000 g for 10 min after cooling, and the supernatant was collected. 200 μL of supernatant was taken and mixed thoroughly with 800 μL of ethanol. After standing overnight at 4°C, 100 μL of Reagent 1 and 0.5 mL of concentrated sulfuric acid were added and incubated in a water bath at 90°C, then cooled with tap water. The absorbance (A) was measured at 490 nm, and the polysaccharide content was determined according to the manufacturer’s instructions (12).

A boiling tube containing 1 mL of decoction and 10 mL of acid mixture (GR grade HNO₃: GR grade HClO₄ = 4:1) was kept at 25°C for 16 h, then heated to 95°C and kept at this temperature in a water bath for 2 h until the digested sample was transparent. The cooling volume was fixed at 25 mL with pure water before filtration. The digested samples were analyzed using Inductively Coupled Plasma Mass Spectrometry. The reagent blanks were treated the same way as the samples (12).

For sample preparation, P. lobata was powdered using a high-speed tissue disintegrator and then homogenized with a standard sieve having a pore size of 0.83 mm. The P. lobata sample (1.0 g) was weighed in a beaker, and 50 mL of 70% ethanol solution was added. After 25 min of ultrasonic extraction, the mixture was filtered to collect the filtrate for further processing. The isoflavone content of P. lobata and its presence in the decoction were determined using an ultraviolet spectrophotometer (16). The puerarin stock solution was accurately pipetted and diluted with 70% ethanol to concentrations of 0.005, 0.01, 0.025, 0.05, and 0.1 mg/mL to serve as puerarin standard solutions. A 70% ethanol solution was used as a blank control group. The absorbance was measured at a wavelength of 250 nm.

The process of stir-fried begins with preheating the bran in decoction pot, then adding P. lobata slices and stirring constantly until the surface of P. lobata turns golden brown color. Finally, the burnt bran was filtered out, and the filtrate was allowed to cool. Orthogonal experiments were designed based on heating heat (weak medium, medium, high), preheating time (10 s, 15 s, 20 s), stir-fried with bran time (1 min, 2 min, 3 min), and the amount of wheat bran (20, 30, 40%), the design scheme were shown in Tables 1, 2. The detection method for heavy metals was based on the previously mentioned text.

Raw data were processed using Microsoft Excel (Microsoft Corporation, Redmond, WA, United States). Statistical analyses and charting were performed using OriginPro 2023 software (OriginLab Corporation, Northampton, MA, United States). Pairwise comparisons were analyzed using Student’s t-test. Differences were considered statistically significant at p < 0.05. All reported results are the averages of three identical experimental runs.

The flowchart of the boiling process of P. lobata slices used in this study is shown in Figure 1. The effect of different boiling processes on total polysaccharide retention was assessed by determining the total polysaccharide content in P. lobata. As shown in Figure 2, the polysaccharide content in the decoction prepared by the traditional process in P. lobata was 15.4 mg/g, accounting for 10.8% of the total polysaccharide content in P. lobata (142.5 mg/g). The highest total polysaccharide content, 17.9 mg/g, was found in the decoction of PM3, while the lowest was 11.4 mg/g in PM1. The total polysaccharide contents for PM2, PM4, and PM5 were 11.5 mg/g, 13.3 mg/g, and 12.5 mg/g, respectively. High temperature can lead to the degradation of polysaccharides, which is consistent with previously reported studies (17, 18). We adopted PM3 to optimise the preparation process of P. lobata decoction and obtained P. lobata decoction with high polysaccharide content.

Polysaccharides are one of the main active ingredients in traditional Chinese medicine (19). Polysaccharides have different pharmacological effects such as immune modulating, antitumour, antioxidant, and antiviral (20). Previously, Yao et al. showed that the content of polysaccharide components in shiitake mushrooms could be reduced by steaming, boiling, and air-frying for 5–20 min. In addition, the loss of crude polysaccharides and adenosine monophosphate was minimized by roasting for 5–15 min (21). In this study, we analyzed the total polysaccharide content of P. lobata and its decoction under different steaming and boiling conditions. The results showed that the total polysaccharide content loss in P. lobata was the lowest in processing method 3. One potential explanation for these results is that when water is used as a solvent, polysaccharides dissolve more easily in water than in alcohol or vinegar. This is due to the polarity of water and its ability to form hydrogen bonds, which reduces the loss of total polysaccharide content during the decoction process (22). In addition, under the same solvent conditions, the total polysaccharide content of the decoction steamed using the electric heating pads may be higher than that of the traditional Chinese medicine soup pot method.

Heavy metal pollution not only affects the quality of traditional Chinese medicinal materials but also degrades the environment and threatens human life and health through the food chain (23, 24). Lead (Pb), Cadmium (Cd), Chromium (Cr), Copper (Cu), and Arsenic (As) are widely recognized as harmful heavy metals to the human body (25). The contamination of Chinese medicinal materials by heavy metals is a serious issue that must be addressed (26). In this study, five heavy metals (As, Pb, Cd, Cr, and Cu) in P. lobata were analyzed.

To evaluate the effects of different processing methods on isoflavones, the isoflavone content of P. lobata decoctions prepared using different processing methods was determined. As shown in Figure 4, the isoflavone content in P. lobata was 3581.5 mg/g. In contrast, the isoflavone content in the decoction prepared by the traditional process was 250.8 mg/g, accounting for 7% of the isoflavone content in P. lobata. Among the processing methods, PM1 (283.9 mg/g), PM4 (269.5 mg/g), and PM5 (289.6 mg/g) exhibited higher isoflavone contents compared to that of the traditional process. There was a significant increase of 13.4% in isoflavone content by PM5. In contrast, the other processing methods (PM2 with 203.6 mg/g and PM3 with 213.8 mg/g) exhibited a slight decrease in isoflavone content compared to that of the traditional process, which could be attributed to the high temperature heating that caused the hydrolysis of isoflavones, thus resulting in no significant increase in the isoflavone content (37).

Isoflavones, as the main bioactive compounds in P. lobata, have various pharmacological effects (33). Wang et al. found that ambient temperature and moisture content could affect the stability of daidzin in Astragalus membranaceus slices (38, 39). In this work, PM5 only retained 7% of the isoflavone content in P. lobata, suggesting that heating may lead to the degradation of isoflavones in P. lobata.

In this study, although the decoction treated by PM3 had the highest total polysaccharide content, PM3 was not as effective as PM4 in reducing the heavy metal content and retaining the isoflavone content. The optimal processing method, PM4, significantly improved the isoflavone content in P. lobata (p < 0.001) and reduced the levels of the five heavy metals (p < 0.05), resulting in the production of 13.2 mg/g polysaccharides, 0.21 μg/g of the five heavy metals and 269.5 mg/g of isoflavones in the decoction. The total heavy metal content of the decoction of P. lobata treated by PM4 was reduced by 96.5%, and its isoflavone content was increased to 7.5% compared to the traditional process. Enhancing the content of isoflavones in the decoction can enhance the pharmacological activity of the decoction and promote the application of P. lobata decoction in the food industry (40).

The “Fu Chao” method, a traditional processing technique in Chinese herbal medicine, involves mixing and stir-fried medicinal herbs with wheat bran (41). This process enhances the therapeutic effects of the herbs and tones the spleen (41). In this study, we investigated the effects of different stir-fried with bran processes on the heavy metal content in P. lobata. A flowchart of the process for stir-fried with bran of P. lobata slices used in this study was shown in Figure 5. As shown in Figure 6, the content of all five heavy metals (Pb, Cd, Cr, Cu, As) in the stir-fried with bran of P. lobata was reduced compared with the untreated P. lobata. The heavy metal content of P. lobata slices processed by methods 6 and 8 was significantly reduced by 58.8 and 52.5%, respectively. In particular, the lowest overall heavy metal content in P. lobata was observed in method 6 by heating over medium heat for 20 s and roasting with 30% bran for 1 min.

Stir-fried with bran has many clinical applications in traditional Chinese medicine. For example, Peng et al. determined the optimal processing parameters for stir-fried with bran by conducting an orthogonal variance analysis and principal component analysis on Fructus Aurantii (42). The optimal method for stir-fried with bran in this study was method 6, with medium heat, preheating time of 20 s, and 30% bran frying for 1 min. The total heavy metal content of P. lobata treated by this method was reduced by 58.8% compared to the control (7.424 μg/g vs. 18.032 μg/g).