A preparation method of quercetin

Abstract

The invention relates to a method for preparing quercetin, and particularly to a method for preparing high-quality quercetin from sophora japonica seeds. In order to overcome the problems of a large amount of sewage, a long production cycle, a complex operation process, difficulty in large-scale production, a low recovery rate, and difficulty in meeting the USP standard for the quality standard of quercetin in an existing process, a method for preparing quercetin is provided. The quercetin is obtained by extraction, extraction and concentration, wherein the extraction step includes multiple extraction processes, and the acidic organic solvent selected for each extraction has a different volume ratio of the acidic solvent to the organic solvent. Hydrolysis and extraction are organically combined and completed in one step, thereby shortening the production cycle.

Description

Technical Field

[0001] The present invention relates to a method for preparing quercetin, in particular to a method for preparing high-quality quercetin from sophora japonica seeds. Background Art

[0002] Quercetin is a flavonol compound with multiple biological activities. It is widely distributed in dicotyledonous plants in the form of glycosides, such as rutin, quercetin, hyperoside, etc. It is the most important bioflavonoid in the human diet.

[0003] At present, quercetin is mainly prepared by the method of breaking rutinosyl groups. For example, Chinese patent CN101220013A discloses a method for extracting and isolating quercetin from the whole herb of Herba Siegesbeckiae. The method is as follows: ① Take a portion of the whole herb of Herba Siegesbeckiae, add water, boil and extract, add lime water to the pH of 8-9 under stirring, then add 3% boric acid and boil under this pH condition for 20-30 minutes; ② Filter while hot, repeatedly filter the residue under reduced pressure, combine the filtrates, and purify them with concentrated hydrochloric acid at 60-70 degrees Celsius. The combined solution was adjusted to pH 5, the acid solution was allowed to stand for 24 hours, and filtered; ③ the precipitate was washed with water until neutral, dried at 60 degrees to obtain crude rutin, recrystallized with boiling water, and dried at 70-80 degrees Celsius to obtain pure rutin; ④ refined rutin was added with 2% H2SO4 at a ratio of 1:80, heated for 2 hours, cooled and filtered; ⑤ the residual impurities were washed with water until neutral, then recrystallized with 95% ethanol, separated by dextran gel column chromatography (Sphadex LH20), and methanol was used as eluent to obtain yellow-green pure quercetin.

[0004] Chinese patent CN107915702A discloses a method for producing quercetin, comprising the following steps: (1) crushing Sophora japonica seeds, soaking them in saturated lime water with a certain amount of borax, sodium sulfite and a buffer, then heating and filtering, acidifying the filtrate with hydrochloric acid to a pH of 4, filtering and drying to obtain crude rutin; (2) dissolving the crushed crude rutin with methanol; (3) adding reaction liquid ammonia water, EDTA and sodium bisulfite to the dissolved crude rutin solution in batches, heating and stirring to completely dissolve it, and reflux and stir to react; (4) adding activated carbon to the reactor, reflux and stir to decolorize, and filter; (5) recovering methanol under vacuum decompression until a large amount of crystals appear, placing it in a crystallization pool for cooling and crystallization for 4 hours, filtering, washing with pure water, drying and crushing to obtain rutin; (7) adding rutin, purified water and concentrated sulfuric acid under normal pressure, heating and pressurizing the reaction, separating and drying to obtain quercetin.

[0005] Chinese patent CN105541776A discloses a method for extracting rutin from Sophora japonica rice and preparing quercetin. By utilizing the different solubility of rutin in hot and cold water, rutin is separated and extracted from the Sophora japonica rice. Rutin is then purified by reflux in ethanol to obtain highly pure refined rutin. The rutin is then hydrolyzed under acidic conditions to obtain quercetin, which is then recrystallized from ethanol to obtain yellow needle-shaped crystals, a high-quality quercetin product.

[0006] The aforementioned patent documents employ various techniques to cleave the rutinosyl groups to yield crude quercetin. This is then refined through a purification process to obtain refined quercetin. Currently, quercetin purification techniques primarily focus on macroporous resins, extraction, polyamide chromatography, gel chromatography, preparative chromatography, and recrystallization.

[0007] The above process uses alkaline water to extract and purify the whole herb of Herba Epimedii or Sophorae japonicae, and then hydrolyzes it to produce quercetin. The whole process produces a large amount of wastewater and a long cycle. In addition, the subsequent refining process is complicated to operate, making it difficult to scale up production. The recovery rate is low, and the quality standard of quercetin is difficult to meet the United States Pharmacopoeia (USP) standard. Summary of the Invention

[0008] The present invention aims to provide a method for preparing quercetin, which overcomes the problems of the existing process, such as large amount of wastewater, long production cycle, complicated operation process, difficulty in large-scale production, low recovery rate, and difficulty in meeting the USP standard for quercetin quality.

[0009] The technical solution of the present invention is:

[0010] A method for preparing quercetin is characterized in that it comprises the following steps:

[0011] Step 1, extraction;

[0012] Fresh Sophora japonica seeds are added into an acidic organic solvent and extracted by a stepwise concentration extraction method;

[0013] The acidic organic solvent is a mixed solvent of an acidic solvent and an organic solvent;

[0014] The step-by-step concentration extraction method is a multiple extraction method, wherein the acidic organic solvent selected for each extraction has a different volume ratio of the acidic solvent to the organic solvent;

[0015] Step 2, extraction;

[0016] The extracts from multiple extractions in step 1 are combined and placed in an extraction device. Strong alkaline water is first added for extraction. When the pH of the aqueous layer is 7±0.5, it is separated and collected to obtain a first aqueous layer, which is used as a raw material for extracting the effective components therein or for other purposes; weak alkaline water is added to the separated organic layer for extraction, and when the pH of the aqueous layer is 7-9, it is separated to obtain a second aqueous layer, which contains kaempferol and other impurity flavonoids. The second aqueous layer is used as a raw material for extracting the effective components therein or for other purposes; the organic layer proceeds to step 3; the strong alkaline water is an aqueous solution of a strong base, and the weak alkaline water is an aqueous solution of a weak base;

[0017] Step 3, concentration;

[0018] The organic layer of step 2 is concentrated, crystallized, centrifuged, and precipitated, dried, and crystallized to obtain quercetin.

[0019] Furthermore, step 1 is specifically as follows: placing fresh Sophora japonica seeds in a Soxhlet extraction device, and sequentially adding a stepwise acidic organic solvent thereto for N extractions; N≥3;

[0020] The step-wise acidic organic solvents are a plurality of acidic organic solvents whose acidity decreases from strong to weak;

[0021] The extraction temperature from the first to the N-1th extraction is 90-100° C., the extraction time is 1-2 hours, and the ratio of the acidic solvent to the organic solvent in the acidic organic solvent is 5-1:1;

[0022] The Nth extraction temperature is 50-80° C., the extraction time is 30-60 min, and the ratio of the acidic solvent to the organic solvent in the acidic organic solvent is 1-5:1.

[0023] The quercetin 3-O-rutinoside, also known as rutin, contained in the Sophora japonica seed raw material requires an acidic reaction to produce quercetin. The traditional process involves extracting the rutin first and then hydrolyzing it to produce quercetin. However, the present invention combines the extraction and hydrolysis processes into one, performing hydrolysis reactions simultaneously during multiple extractions. The first N-1 main reactions are the hydrolysis of rutin, while the Nth main reaction is the extraction of quercetin, resulting in different corresponding reaction conditions.

[0024] Rutin hydrolysis requires acid, but as the hydrolysis reaction progresses from the beginning to the end, too long a strong acid dose will damage the nucleus of rutin, so the amount of acid is reduced successively. The nucleus of rutin is 2-phenyl-3-hydroxychromone.

[0025] Furthermore, in step 1, N=3, and the total mass of the acidic organic solvent used in the three extractions is 5-7 times the mass of the fresh Sophora japonica seeds.

[0026] Furthermore, the acidic solvent in step 1 includes at least one of methanesulfonic acid, trichloroacetic acid, glacial acetic acid and oxalic acid; and the organic solvent includes one or two of ethyl acetate and dichloromethane mixed in proportion.

[0027] Acidic solvents are all strong acids among organic acids. Compared with inorganic acids, the reaction using organic acids is milder and causes less damage to the parent nucleus during hydrolysis.

[0028] Quercetin has low polarity. According to the principle of like dissolves like, the selected organic solvent has low polarity. If a solvent with high polarity is selected, more impurities may be introduced, affecting the extraction effect.

[0029] Furthermore, the strong alkaline water in step 2 includes sodium hydroxide solution or potassium hydroxide solution; the weak alkaline water includes ammonia water, saturated lime water or sodium carbonate solution.

[0030] Different alkaline reagents have different purposes and different effects on removing impurities. Strong alkaline water is used to neutralize acid and separate the target products quercetin and rutin, while weak alkaline water is used to remove impurities such as kaempferol. Other strong or weak base aqueous solutions also have the same effect.

[0031] Furthermore, in step 2, the mass concentrations of the sodium hydroxide solution, potassium hydroxide solution, ammonia water, and sodium carbonate solution are all 5%.

[0032] Furthermore, in step 1, the acidic solvent is oxalic acid, and the organic solvent is ethyl acetate;

[0033] The mass ratio of oxalic acid to ethyl acetate in the first extraction was 2:1, the extraction time was 60 minutes, and the extraction temperature was 100°C;

[0034] The mass ratio of oxalic acid to ethyl acetate in the second extraction was 1:1, the extraction time was 60 minutes, and the extraction temperature was 100°C;

[0035] The third extraction was performed with a mass ratio of oxalic acid to ethyl acetate of 1:5, an extraction time of 30 minutes, and an extraction temperature of 80°C.

[0036] In step 2, the strong alkaline water is sodium hydroxide solution;

[0037] The weakly alkaline water added is saturated lime water, and the weakly alkaline water is added to make the pH of the water layer reach 9 for layer separation.

[0038] Furthermore, the method further comprises step A, preparing rhamnose: separating rhamnose and glucose from the first water layer obtained in step 2 to obtain a rhamnose product.

[0039] The beneficial effects of the present invention are:

[0040] 1. The present invention adds fresh sophora japonica seeds to an acidic organic solvent and selects a stepwise concentration extraction method for multiple extractions, organically combines hydrolysis and extraction, completes the extraction in one step, and shortens the production cycle.

[0041] 2. The present invention utilizes Soxhlet extraction, which requires less solvent, facilitates subsequent extraction, and reduces pollution.

[0042] 3. In the process of extracting the extract, the present invention uses different alkaline water extractions. First, strong alkaline water is used to neutralize the residual acid in the organic layer, and the rutin produced by hydrolysis is dissolved in the water layer and separated from the target product. Then, rhamnose and glucose are separated to obtain a high-value-added rhamnose product. Secondly, weak alkaline water is used to continue extraction and impurity removal. Quercetin has one more hydroxyl group at the 3' position than kaempferol, so it is more acidic. Kaempferol and other impurity flavonoids are soluble in the weak alkaline water layer, and quercetin remains in the organic phase. Finally, 98% quercetin is obtained only by concentrating the organic phase, and the impurities meet the USP standard. The recovery rate of the effective ingredient reaches more than 90%. The process cycle is short, easy to operate, easy to scale production, and the product quality is high, which meets the needs of the high-end market. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a high performance liquid chromatogram of quercetin obtained in Example 1 of the present invention;

[0044] Figure 2 This is a high performance liquid chromatogram of quercetin obtained in Example 2 of the present invention;

[0045] Figure 3 This is a high performance liquid chromatogram of quercetin obtained in Example 3 of the present invention;

[0046] Figure 4 This is a high performance liquid chromatogram of quercetin obtained in Example 4 of the present invention;

[0047] Figure 5 This is a high performance liquid chromatogram of quercetin obtained in Example 5 of the present invention. DETAILED DESCRIPTION

[0048] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0049] Example 1

[0050] In this embodiment, quercetin is prepared by the following steps:

[0051] 1. Extraction

[0052] Fresh Sophora japonica seeds are placed in a Soxhlet extraction device, a stepwise acidic organic solvent is added, and the extraction is carried out by heating and stirring. Specifically:

[0053] 100 kg of fresh Sophora japonica seed raw material is placed in a Soxhlet extraction device, 300 kg of acidic organic solvent (including 200 kg of oxalic acid and 100 kg of ethyl acetate) is added, heated and stirred for extraction for 1 hour, and the extraction temperature is 100° C.; then 100 kg of acidic organic solvent (including 50 kg of oxalic acid and 50 kg of ethyl acetate) is added, heated and stirred for extraction for 1 hour, and the extraction temperature is 100° C.; then 300 kg of acidic organic solvent (including 50 kg of oxalic acid and 250 kg of ethyl acetate) is added, heated and stirred for extraction for 30 minutes to obtain an extract, and the extraction temperature is 80° C.

[0054] 2. Extraction

[0055] The extract is placed in an extraction device, and 200 kg of 5% sodium hydroxide solution is first added for extraction. When the pH value of the aqueous solution is measured to be equal to 7±0.5, the aqueous layer is separated and collected, and rhamnose and glucose are separated to obtain a high-value-added rhamnose product; 200 kg of saturated lime water is then added to the organic layer after extraction and separation, and the lower aqueous layer is separated when the pH value reaches 9. The aqueous layer obtained contains kaempferol and other impurity flavonoids, and the organic layer proceeds to step 3.

[0056] 3. Concentration

[0057] The organic layer was concentrated and recovered, and allowed to stand for more than 12 hours for crystallization, centrifuged, and the precipitate was dried to obtain 7.8 kg of quercetin with a content of 98.84%. The quercetin was tested by high performance liquid chromatography. The results were as follows: Figure 1 All indicators are in accordance with USP regulations.

[0058] The quercetin content of fresh Sophora japonica seed raw material hydrolyzed was 8.2%, and the recovery rate of the active ingredient was (7.8*0.9884) / (100*0.082)*100%=94%.

[0059] The following table shows the comparison of the quercetin recovery rate, quercetin content, and the final amount of waste liquid obtained by the traditional process and this example. It can be seen that the method of the present invention produces less waste liquid and a higher recovery rate of active ingredients than the traditional process for preparing quercetin.

[0060]

[0061] Example 2

[0062] In this embodiment, quercetin is prepared by the following steps:

[0063] 1. Extraction

[0064] Fresh Sophora japonica seeds are placed in a Soxhlet extraction device, a stepwise acidic organic solvent is added, and the extraction is carried out by heating and stirring. Specifically:

[0065] 100 kg of fresh Sophora japonica flower raw material is placed in a Soxhlet extraction device, 300 kg of an acidic organic solvent (including 250 kg of trichloroacetic acid and 50 kg of dichloromethane) is added, and the mixture is heated and stirred for extraction for 2 hours at an extraction temperature of 90° C. Then 150 kg of an acidic organic solvent (including 100 kg of trichloroacetic acid and 50 kg of dichloromethane) is added, and the mixture is heated and stirred for extraction for 1.5 hours at an extraction temperature of 95° C. Then 150 kg of an acidic organic solvent (including 50 kg of trichloroacetic acid and 100 kg of dichloromethane) is added, and the mixture is heated and stirred for extraction for 60 minutes at an extraction temperature of 75° C. to obtain an extract.

[0066] 2. Extraction

[0067] The extract is placed in an extraction device, and 150 kg of 5% potassium hydroxide solution is first added for extraction. When the pH value of the aqueous solution is measured to be equal to 7±0.5, the aqueous layer is separated and collected, and rhamnose and glucose are separated to obtain a high-value-added rhamnose product; then 200 kg of 5% ammonia water is added to the organic layer after extraction and separation, and the lower aqueous layer is separated when the pH value reaches 8. The aqueous layer obtained contains kaempferol and other impurity flavonoids, and the organic layer proceeds to step 3.

[0068] 3. Concentration

[0069] After the organic layer was concentrated and crystallized, centrifuged, and the precipitate was dried to obtain 7.5 kg of quercetin with a content of 98.56%. The results of HPLC were as follows: Figure 2 All indicators are in accordance with USP regulations.

[0070] The quercetin content of fresh Sophora japonica seed raw material was detected by hydrolysis and was 8.2%, and the recovery rate of the active ingredient was (7.5*0.9856) / (100*0.082)*100%=90.14%.

[0071] Example 3

[0072] In this embodiment, quercetin is prepared by the following steps:

[0073] 1. Extraction

[0074] Fresh Sophora japonica seeds are placed in a Soxhlet extraction device, a stepwise acidic organic solvent is added, and the extraction is carried out by heating and stirring. Specifically:

[0075] 100 kg of fresh Sophora japonica seed raw material is placed in a Soxhlet extraction equipment, 200 kg of acidic organic solvent (including 150 kg of trichloroacetic acid and 50 kg of dichloromethane) is added, and the mixture is heated and stirred for extraction for 1.5 hours at an extraction temperature of 95° C.; then 200 kg of acidic organic solvent (including 100 kg of trichloroacetic acid and 100 kg of dichloromethane) is added, and the mixture is heated and stirred for extraction for 1 hour at an extraction temperature of 100° C.; then 100 kg of acidic organic solvent (including 50 kg of trichloroacetic acid and 50 kg of dichloromethane) is added, and the mixture is heated and stirred for extraction for 40 minutes at an extraction temperature of 70° C.

[0076] 2. Extraction

[0077] The extract is placed in an extraction device, and 250 kg of 5% sodium hydroxide solution is first added for extraction. When the pH value of the aqueous solution is measured to be equal to 7±0.5, the aqueous layer is separated and collected, and rhamnose and glucose are separated to obtain a high-value-added rhamnose product; then 250 kg of 5% sodium carbonate is added to the organic layer after extraction and separation, and the lower aqueous layer is separated when the pH value reaches 7. The aqueous layer obtained contains kaempferol and other impurity flavonoids, and the organic layer proceeds to step 3.

[0078] 3. Concentration

[0079] After the organic layer was concentrated and crystallized, it was centrifuged, precipitated and dried to obtain 7.7 kg of quercetin with a content of 98.71%. The results of HPLC were as follows: Figure 3 All indicators are in accordance with USP regulations.

[0080] The quercetin content of fresh Sophora japonica seed raw material hydrolyzed was 8.2%, and the recovery rate of the active ingredient was (7.7*0.9871) / (100*0.082)*100%=92.69%.

[0081] Example 4

[0082] In this embodiment, quercetin is prepared by the following steps:

[0083] 1. Extraction

[0084] Fresh Sophora japonica seeds are placed in a Soxhlet extraction device, a stepwise acidic organic solvent is added, and the extraction is carried out by heating and stirring. Specifically:

[0085] 100 kg of fresh Sophora japonica flower raw material is placed in a Soxhlet extraction equipment, 200 kg of acidic organic solvent (including 150 kg of trichloroacetic acid and 50 kg of dichloromethane) is added, and the mixture is heated and stirred for extraction for 1.7 hours at an extraction temperature of 98° C.; then 100 kg of acidic organic solvent (including 50 kg of trichloroacetic acid and 50 kg of dichloromethane) is added, and the mixture is heated and stirred for extraction for 1.2 hours at an extraction temperature of 90° C.; then 200 kg of acidic organic solvent (including 50 kg of trichloroacetic acid and 150 kg of dichloromethane) is added, and the mixture is heated and stirred for extraction for 55 minutes to obtain an extract at an extraction temperature of 50° C.

[0086] 2. Extraction

[0087] The extract is placed in an extraction device, and 150 kg of 5% sodium hydroxide solution is first added for extraction. When the pH value of the aqueous solution is measured to be equal to 7±0.5, the aqueous layer is separated and collected, and rhamnose and glucose are separated to obtain a high-value-added rhamnose product; then 200 kg of 5% saturated lime water is added to the organic layer after extraction and separation, and the lower aqueous layer is separated when the pH value reaches 8. The aqueous layer obtained contains kaempferol and other impurity flavonoids, and the organic layer proceeds to step 3.

[0088] 3. Concentration

[0089] After the organic layer was concentrated and crystallized, centrifuged, and the precipitate was dried to obtain 7.5 kg of quercetin with a content of 98.66%. The results of HPLC were as follows: Figure 4 All indicators are in accordance with USP regulations.

[0090] The quercetin content of fresh Sophora japonica seed raw material hydrolyzed was 8.2%, and the recovery rate of the active ingredient was (7.5*0.9866) / (100*0.082)*100%=90.23%.

[0091] Example 5

[0092] In this embodiment, quercetin is prepared by the following steps:

[0093] 1. Extraction

[0094] Fresh Sophora japonica seeds are placed in a Soxhlet extraction device, a stepwise acidic organic solvent is added, and the extraction is carried out by heating and stirring. Specifically:

[0095] 100 kg of fresh Sophora japonica flower raw material is placed in a Soxhlet extraction equipment, 250 kg of acidic organic solvent (including 200 kg of trichloroacetic acid and 50 kg of dichloromethane) is added, and the mixture is heated and stirred for 1.4 hours at an extraction temperature of 100° C.; then 200 kg of acidic organic solvent (including 150 kg of trichloroacetic acid and 50 kg of dichloromethane) is added, and the mixture is heated and stirred for 2 hours at an extraction temperature of 98° C.; then 200 kg of acidic organic solvent (including 50 kg of trichloroacetic acid and 150 kg of dichloromethane) is added, and the mixture is heated and stirred for 35 minutes to obtain an extract at an extraction temperature of 60° C.

[0096] 2. Extraction

[0097] The extract is placed in an extraction device, and 200 kg of 5% sodium hydroxide solution is first added for extraction. When the pH value of the aqueous solution is measured to be equal to 7±0.5, the aqueous layer is separated and collected, and rhamnose and glucose are separated to obtain a high-value-added rhamnose product; then 200 kg of 5% saturated lime water is added to the organic layer after extraction and separation. When the pH value of the lower aqueous layer reaches 8, it is separated to obtain an aqueous layer containing kaempferol and other impurity flavonoids, and the organic layer proceeds to step 3.

[0098] 3. Concentration

[0099] After the organic layer was concentrated and crystallized, it was centrifuged, precipitated and dried to obtain 7.6 kg of quercetin with a content of 98.47%. The results of HPLC were as follows: Figure 5 All indicators are in accordance with USP regulations.

[0100] The quercetin content of fresh Sophora japonica seed raw material hydrolyzed was 8.2%, and the recovery rate of the active ingredient was (7.6*0.9847) / (100*0.082)*100%=91.26%.

Claims

1. A method for preparing quercetin, characterized in that: The following steps are involved: Step 1, extraction; Fresh Sophora japonica seeds were added to an acidic organic solvent and extracted using a stepwise concentration extraction method; specifically: Place fresh Sophora japonica seeds in a Soxhlet extraction apparatus, and sequentially add a stepwise acidic organic solvent thereto for N extractions; N ≥ 3; The step-wise acidic organic solvents are a plurality of acidic organic solvents with acidity ranging from strong to weak; the acidic organic solvents selected for each extraction have different volume ratios of the acidic solvent to the organic solvent; The acidic organic solvent is a mixed solvent of an acidic solvent and an organic solvent; the acidic solvent is selected from at least one of methanesulfonic acid, trichloroacetic acid, glacial acetic acid and oxalic acid; the organic solvent is selected from one or two of ethyl acetate and dichloromethane in proportion; Step 2, extraction; The extracts from multiple extractions in step 1 are combined and placed in an extraction device. Strong alkaline water is first added for extraction. When the pH of the aqueous layer is 7±0.5, it is separated and collected to obtain a first aqueous layer, which is used as a raw material for extracting the effective components therein; weak alkaline water is added to the separated organic layer for extraction, and when the pH of the aqueous layer is 7-9, it is separated to obtain a second aqueous layer, which contains kaempferol and other impurity flavonoids, and is used as a raw material for extracting the effective components therein; the organic layer proceeds to step 3; the strong alkaline water is an aqueous solution of a strong base, and the weak alkaline water is an aqueous solution of a weak base; The strong alkaline water is selected from sodium hydroxide solution or potassium hydroxide solution with a mass concentration of 5%; the weak alkaline water is selected from ammonia water, saturated lime water or sodium carbonate solution with a mass concentration of 5%; Step 3, concentration; The organic layer of step 2 is concentrated, crystallized, centrifuged, and precipitated, dried, and crystallized to obtain quercetin.

2. The method for preparing quercetin according to claim 1, wherein: In step 1, the extraction temperature from the first to the N-1th extraction is 90-100° C., the extraction time is 1-2 hours, and the ratio of the acidic solvent to the organic solvent in the acidic organic solvent is 1-5:1; The Nth extraction temperature is 50-80° C., the extraction time is 30-60 min, and the ratio of the acidic solvent to the organic solvent in the acidic organic solvent is 1:

5.

3. The method for preparing quercetin according to claim 1 or 2, wherein: In step 1, N=3, and the total mass of the acidic organic solvent used for the three extractions is 5-7 times the mass of the fresh Sophora japonica seeds.

4. The method for preparing quercetin according to claim 3, wherein: In step 1, the acidic solvent is oxalic acid and the organic solvent is ethyl acetate; The mass ratio of oxalic acid to ethyl acetate in the first extraction was 2:1, the extraction time was 60 minutes, and the extraction temperature was 100°C; The mass ratio of oxalic acid to ethyl acetate in the second extraction was 1:1, the extraction time was 60 minutes, and the extraction temperature was 100°C; The third extraction was performed with a mass ratio of oxalic acid to ethyl acetate of 1:5, an extraction time of 30 minutes, and an extraction temperature of 80°C. In step 2, the strong alkaline water is sodium hydroxide solution; The weakly alkaline water added is saturated lime water, and the weakly alkaline water is added to make the pH of the water layer reach 9 for layer separation.

5. The method for preparing quercetin according to claim 4, wherein: The method further comprises step A, preparing rhamnose: separating rhamnose and glucose from the first water layer obtained in step 2 to obtain a rhamnose product.

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