An industrial preparation method for sucralose

Abstract

This invention discloses an industrial preparation method for stevia in different specifications. The method utilizes a temperature-variable linear desorption process using a supersaturated calcium hydroxide-alcohol aqueous solution. The desorption process requires a low volume fraction of alcohol-water solution, 44-55%, avoiding the need for high-volume-fraction alcohol-water solutions in conventional methods for high-content stevia products and reducing the amount of alcohol used. This invention only requires adding a jacket to existing industrial chromatography columns, eliminating the need for traditional crystallization and decolorization processes, making it easily scalable for industrial production. It can simultaneously obtain two different stevia products with varying content specifications. This process has low equipment requirements, is simple, environmentally friendly, widely applicable, and suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of sweet tea extract preparation technology, and in particular to an industrial preparation method for sweet tea glycosides. Background Technology

[0002] Sweet tea, a perennial deciduous shrub belonging to the genus Rubus in the Rosaceae family, is a newly discovered species in recent years, mainly growing in the subtropical regions of South Asia. Its leaves have a sweet taste and are often used to make tea, hence the name "sweet tea." In the 1990s, Professors Liu Fuzhi and Wang Riwei analyzed the macro-components and inorganic elements of sweet tea according to national standards for tea and other food products, and determined the content of sweeteners (sweet tea leaves contain various chemical components: mainly 5-10% sweet tea glycosides, 8-18% sweet tea polyphenols, and 2.9-4.2% flavonoids. It also contains 18 kinds of amino acids). Toxicological tests were also conducted, showing that the content of tea polyphenols, amino acids, and caffeine in sweet tea is lower than that in ordinary tea, but the content of flavonoids is higher, and it also contains up to 10% sweeteners, which are not found in ordinary green tea. All toxicological tests on sweet tea were negative, with no mutations or aberrations, making it safe for consumption. It is a valuable wild plant beverage and sweetener resource. The main sweet component of sweet tea is steviol glycoside, also known as stevia glycoside or raspberry glycoside, a tetracyclic diterpenoid glycoside formed by the combination of stevia and glucose, and chemically similar to stevia glycoside. Its sweetness is close to that of sucrose, with a relative sweetness 150 times that of a 2% sucrose solution. As a high-sweetness, low-calorie, and pure-tasting non-sucrose natural sweetener, steviol glycoside is receiving increasing attention and is the most studied active ingredient in sweet tea, with significant progress made in its extraction and purification.

[0003] CN105166198 discloses a method for preparing high-purity sucralose, which involves raw material crushing, water extraction, membrane filtration and concentration, drying, double crystallization with alcohol, and drying. This process utilizes two crystallization steps, resulting in a large amount of mother liquor and cumbersome subsequent processing.

[0004] CN102702284 discloses a production process for high-purity sucralose, which involves extraction, concentration, flocculation, centrifugation, macroporous resin chromatography, ion exchange resin chromatography, desalting resin chromatography, composite decolorization, concentration, and drying. This process is extremely complex, requiring targeted decolorization and desalting, and involves three chromatographic processes, resulting in very high production costs.

[0005] CN114053315 describes a method and its application for the comprehensive extraction of catechins, total polyphenols, and total flavonoids from sweet tea. The method involves water extraction, macroporous adsorption resin chromatography, gradient ethanol-water elution, high-ethanol eluent, anion exchange resin pass-through, concentration, and crystallization to obtain catechins. For total polyphenols, the water washing and low-ethanol eluent are combined, concentrated, flocculated, precipitated, pH adjusted, macroporous adsorption resin chromatography, ethanol-water elution, ethyl acetate extraction, concentration, and drying. For total flavonoids, the supernatant undergoes macroporous resin adsorption, ethanol-water desorption, polyamide resin adsorption and elution, concentration, and ethyl acetate concentration and drying. This process is extremely complex, requiring three chromatographic steps to obtain catechins and polyphenols, making large-scale production relatively difficult.

[0006] CN201811102849.4 discloses a method for extracting catechins from sweet tea leaves, comprising the following steps: (1) crushing the sweet tea leaves, sieving, adding water, continuously countercurrent extraction, and filtering; (2) adding an enzyme preparation for enzymatic hydrolysis, inactivation, cooling, flocculation, filtering, and washing the filter residue with water; (3) ultrafiltration and nanofiltration; (4) adsorption onto a macroporous adsorption resin chromatography column, washing with water, discarding the washing liquid, eluting with an organic solvent gradient, and concentrating under reduced pressure; (5) adsorption onto an alumina chromatography column, washing with water, and concentrating under reduced pressure; (6) adding activated carbon, stirring, filtering, concentrating under reduced pressure, and spray drying; (7) dissolving in an organic solvent, filtering, crystallizing, filtration, vacuum drying, and crushing to obtain the catechin product. The crystallization process also generates a significant amount of mother liquor.

[0007] CN201710465808.0 discloses a method for preparing sucralose, characterized by the following steps: raw material crushing, flash extraction, centrifugation, macroporous resin purification, desorption, enzymatic hydrolysis, organic membrane fractionation and decolorization, concentration, crystallization, and drying to obtain the finished product. (Note: The text also mentions a Chinese invention patent application (CN108752231A) which discloses a method for extracting theanine and simultaneously extracting sucralose and tea polyphenols from sweet tea. This method involves the following steps: using sweet tea leaves as raw material, crushing, cold water permeation, adsorption using macroporous resin, polyamide resin, and ion exchange resin respectively, and crystallization to obtain high-content theanine, sucralose, and tea polyphenols. This method can achieve the simultaneous preparation of three high-content components through a simple process, but industrialization is difficult.)

[0008] Chinese invention patent application CN108752231A discloses a method for extracting theanine from sweet tea and simultaneously extracting catechins and tea polyphenols. The method is achieved through the following steps: using sweet tea leaves as raw material, crushing and permeating with cold water, adsorbing with macroporous resin, polyamide resin and ion exchange resin respectively, and obtaining high contents of theanine, sweet tea polyphenols and catechins through crystallization. The process is simple to achieve the simultaneous preparation of three high contents of components, but it is difficult to industrialize.

[0009] The extraction and purification methods for stevia in the aforementioned patents generally involve extracting stevia raw materials with water and / or alcohol, followed by solid-liquid separation, membrane filtration, adsorption and desorption using macroporous resin, decolorization with ion exchange resin or activated carbon, concentration, and drying to obtain a high-content product. Resin chromatography is a crucial step in increasing the product content. Typically, after each macroporous adsorption resin chromatography, two or even three desorption processes using alcohol-water solutions of different concentrations are performed, requiring the use of high-volume-fraction alcohol-water solutions. If only a single-gradient alcohol-water solution is used for desorption, the stevia content of the obtained product is generally low. Furthermore, some technologies employ extremely cumbersome process steps to increase content, making industrial application difficult. Most of the published patents for stevia obtain it through a process involving high-ethanol desorption and crystallization steps. High-ethanol desorption is risky and costly, and the crystallization mother liquor may contain some stevia due to insufficient crystallization. Summary of the Invention

[0010] This invention discloses an industrial preparation method for stevia glycosides. The method utilizes a temperature-variable linear desorption process using a supersaturated calcium hydroxide-alcohol aqueous solution. The desorption process requires a low volume fraction of alcohol-water solution, typically 44-55%, avoiding the need for high-volume-fraction alcohol-water solutions in conventional methods for high-content stevia glycoside products and reducing the amount of alcohol used. This invention only requires adding a jacket to existing industrial chromatography columns, eliminating the need for traditional crystallization and decolorization, making it easily scalable for industrial production. It can simultaneously yield two different stevia glycoside products with varying content. This process has low equipment requirements, is simple, environmentally friendly, widely applicable, and suitable for large-scale production.

[0011] The present invention achieves the above objectives through the following technical solutions:

[0012] An industrial preparation method for betaine of different specifications includes the following steps:

[0013] (1) Extraction: The dried sweet tea leaves are extracted, separated into solid and liquid, flocculated, passed through an ultrafiltration membrane, and the permeate is collected.

[0014] (2) Column loading: The permeate is loaded onto the macroporous adsorption resin;

[0015] (3) Column washing and desorption: After washing the column with alkaline water, linear temperature-variable desorption is performed at 0-10℃ to 60-80℃ using supersaturated calcium hydroxide alcohol aqueous solution. The desorption solution is collected in segments to obtain desorption solution I and desorption solution II. When the content of catechin is ≥96% by dry weight, the collection of desorption solution I is stopped and the collection of desorption solution II begins.

[0016] (4) Extraction I: After concentrating the desorption solution I, add a mixture of alcohol and water and mix well. Add a low-polarity organic solvent and mix well. Let stand for extraction and take the aqueous phase.

[0017] (5) Extraction II: Add butanol to the aqueous layer from step (4), stir thoroughly, let stand for extraction, and take the butanol phase;

[0018] (6) Concentration to remove alcohol: The collected butanol phase and desorption solution II are concentrated to remove alcohol, and concentrated solution I and concentrated solution II are obtained respectively.

[0019] (7) Membrane filtration: After diluting concentrate I and concentrate II with pure water, they were passed through a nanofiltration membrane and retentate I and retentate II were collected.

[0020] (8) Concentration and drying: After concentrating and drying the retentate I and retentate II from step (7), two different specifications of sweet tea glycoside products can be obtained.

[0021] Preferably, in step (1), the content of sweet tea glycosides in the sweet tea raw material is not less than 3%; the extraction is carried out by using water as the extraction solvent and by tank extraction or continuous countercurrent extraction; the mass ratio of the extracted sweet tea raw material to water is 1:6 to 10.

[0022] Preferably, in step (1), the extraction temperature is 80-100°C and the extraction time is 1-3 hours.

[0023] In step (1), the solid-liquid separation aims to separate the raw material residue from the extract, including any one or a combination of horizontal screw centrifugation and disc centrifugation; the flocculation involves adding 1-5% flocculant to the clear liquid after solid-liquid separation, wherein the flocculant is any one of the organic polymer chitosan or polyacrylamide; after stirring and flocculating evenly, solid-liquid separation is performed again to separate the sweet tea raw material residue from the extract, including any one or a combination of horizontal screw centrifugation and disc centrifugation.

[0024] Preferably, in step (1), the ultrafiltration membrane has a molecular weight cutoff of 10,000 to 40,000 Da, more preferably 20,000 to 30,000 Da.

[0025] Preferably, in step (2), the diameter-to-height ratio of the macroporous adsorption resin is 1:5 to 10, and the flow rate on the column is 1 to 2 BV / h; the macroporous adsorption resin is any one of D101, HPD-100, D1400, D4020, AB-8, and DM130.

[0026] In step (2), degassing is required before loading the column; the degassing method is vacuum treatment or ultrasonic treatment. The vacuum treatment is to maintain a vacuum degree of -0.1MPa to -0.05MPa for 10 to 20 minutes; the ultrasonic treatment is to use 40 to 60KHZ and 150 to 400W for 5 to 10 minutes.

[0027] In step (3), the alkaline solution is a 0.5-3 wt% aqueous solution of sodium hydroxide and / or potassium hydroxide. The flow rate of the alkaline solution washing column is 2-5 BV / h, and the volume of the alkaline solution washing column is 2-4 BV. While the macroporous adsorption resin adsorbs betaine, it also adsorbs a large number of impurities. When the alkaline solution washes the column, it can change the polarity of some impurities, thereby causing them to detach from the surface of the macroporous adsorption resin groups and be washed off with the alkaline solution washing column.

[0028] Preferably, in step (3), the concentration of calcium hydroxide in the supersaturated calcium hydroxide alcohol aqueous solution is 0.15-0.3 wt%. A small amount of undissolved calcium hydroxide is normal, as the system is supersaturated. The supersaturated calcium hydroxide alcohol aqueous solution is passed through a ceramic membrane before being loaded onto the column. The insoluble calcium hydroxide in the supersaturated calcium hydroxide alcohol aqueous solution is filtered to prevent it from flowing into the chromatography column and causing blockage.

[0029] The supersaturated calcium hydroxide aqueous solution is prepared in an alcohol-water solvent system with an alcohol volume fraction of 44-55%, preferably ethanol.

[0030] Preferably, in step (3), the linear desorption method is to linearly increase the column temperature and the temperature of the desorption liquid from 0-10℃ to 60-80℃, with a heating rate of 7-12℃ / BV and a flow rate of 1-2BV / h.

[0031] Preferably, in step (3), the collection is carried out in stages. The low-temperature stage collects the desorption solution I with a low content of catechins. As the temperature increases, the content of catechins in the desorption solution gradually increases and the content of impurities decreases, and the desorption solution II is collected.

[0032] The inventor's previous patent CN202410954867.4 discloses an industrial preparation method for mogroside V, which uses a supersaturated calcium hydroxide alcohol-water solution for linear temperature-switching desorption. Since the solubility of calcium hydroxide decreases with increasing temperature, its pH value also decreases with increasing temperature, thus affecting the polarity of the aqueous solution; water temperature and polarity are also correlated, with decreasing polarity as temperature increases. By selecting a supersaturated calcium hydroxide alcohol-water solution as the eluent, the pH value and polarity of the supersaturated calcium hydroxide alcohol-water solution gradually decrease during the linear temperature increase process. That is, in the early stage of elution during the linear temperature rise process, the eluent has a higher pH value and greater polarity, and both pH and polarity begin to decrease as the temperature rises. This linear temperature-switching adsorption can reduce the amount and concentration of ethanol used, and achieves high mogroside yield and high purity. The inventors have found that this linear temperature-switching adsorption method is also applicable to the extraction of mogroside V. In the early stage of linear temperature-dependent adsorption-desorption, the eluent has a relatively high pH and polarity, exhibiting a lower elution capacity for steatosides but a higher elution capacity for impurities. This allows for effective retention of steatosides and removal of impurities under the initial lower temperature conditions. As the temperature gradually increases in the later stages, the steatosides on the chromatography column are effectively and completely eluted, resulting in low impurity content.

[0033] Preferably, in step (4), the alcohol-water mixture is a mixture of C1-3 alcohol and water, wherein the C1-3 alcohol is selected from at least one of methanol, ethanol, and propanol; the volume ratio of concentrate: alcohol: water is 1:5 to 10:5 to 10; the low-polarity organic solvent is selected from at least one of carbon tetrachloride, hexane, cyclohexane, and petroleum ether, and the volume of the low-polarity organic solvent is 3 to 6 times the volume of the alcohol.

[0034] Preferably, in step (4), the standing and layering extraction involves thoroughly stirring the mixed solution until it is fully settled, allowing it to stand until two uniform phases appear, and then collecting the aqueous phase through an extraction tower.

[0035] Adding a low-polarity organic solvent to a mixture of desorption concentrate and alcohol-water solution allows the alcohol to carry impurities and pigments into the low-polarity organic solvent, while the stevia glycosides remain in the aqueous phase, thus achieving the purpose of impurity removal.

[0036] Preferably, in step (5), the butanol is n-butanol or isobutanol; the volume ratio of butanol to water is 1:3-6. The inventors have found that using butanol for the second extraction in step (5) can simultaneously ensure both the purity and yield of catechins. Other solvents, such as n-heptane or isopropanol, cannot achieve this effect.

[0037] Water and butanol are immiscible. Further extraction can remove water-soluble impurities and pigments, while stevia glycosides are retained in the butanol phase, thus achieving further purification.

[0038] Two extractions can achieve a good decolorization effect and also serve as a means of further purification.

[0039] Preferably, in step (6), the concentration aims to remove some water and alcohol, and the removal of alcohol odor is sufficient when no alcohol odor can be detected. The concentration method can be any one of rotary evaporation concentration, falling film concentration, single-effect evaporation concentration, and multi-effect evaporation concentration.

[0040] Preferably, in step (7), the pure water is diluted to a soluble solids content of 2-5%, and the membrane filtration is performed to retain a molecular weight cutoff of 200-500 Da.

[0041] Preferably, in step (8), the concentration is aimed at removing some water and alcohol, concentrating to 40-60% soluble solids. The concentration method is any one of rotary evaporation concentration, falling film concentration, single-effect evaporation concentration, and multi-effect evaporation concentration; the drying is aimed at removing moisture, including but not limited to one of spray drying, vacuum drying, microwave drying, belt drying, and forced-air drying; the drying process also includes pulverization and sieving, passing through 100-200 mesh.

[0042] The principle of this invention is as follows:

[0043] Sweet tea raw materials were extracted with water as a solvent and initially separated into solid and liquid components. Impurities were removed using an organic macromolecular flocculant, followed by another solid-liquid separation and ultrafiltration to obtain a pre-purified column buffer. The buffer was then degassed and loaded onto a macroporous adsorption resin chromatography column with a heat-insulating layer for adsorption. The column was then washed with alkaline water, which has strong impurity removal capabilities, followed by linear temperature-dependent fractional desorption using a supersaturated calcium hydroxide alcohol solution. (The solubility of calcium hydroxide decreases with increasing temperature, leading to a decrease in its pH value and affecting the polarity of its aqueous solution. Water polarity is also correlated with temperature; increasing temperature decreases water polarity. Using a supersaturated calcium hydroxide alcohol solution as the eluent, as the temperature linearly increased from 0℃ to 70℃, the calcium hydroxide concentration decreased, the pH value of the eluent gradually decreased, and the increasing temperature also affected the polarity of the eluent, which gradually decreased. That is, in the early stage of elution, the eluent had a higher pH value and higher adsorption capacity.) The system effectively removes impurities, but the initial eluent is highly polar and has limited elution capacity for stevia glycosides. Therefore, a linear elution method is employed. This allows for effective removal of impurities while retaining stevia glycosides at lower initial temperatures, followed by thorough elution of the stevia glycosides from the column at higher temperatures. Simultaneously, the polarity of water decreases at higher temperatures, thus reducing the overall polarity of the eluent system. This allows for obtaining a lower polarity eluent without requiring an excessively high ethanol volume fraction, thereby reducing the amount and concentration of ethanol used.

[0044] The fractionally collected eluent still contained impurities and pigments. After concentrating the eluent, propanol and water were added and mixed thoroughly. Then, carbon tetrachloride was added. Some impurities and pigments were absorbed into the carbon tetrachloride along with the propanol, while the betaine remained in the aqueous phase, thus achieving purification. Subsequently, butanol was added to the aqueous phase and mixed thoroughly. Some impurities and pigments were again partitioned and separated between the aqueous and butanol phases, while the betaine remained in the butanol, achieving a second purification. Through two extractions, the concentrate was decolorized and further purified.

[0045] Next, the two extracts were concentrated, diluted, filtered through a nanofiltration membrane, concentrated again, and dried to obtain two different contents of sweet tea glycoside products.

[0046] The beneficial effects of the method of the present invention are as follows:

[0047] I. This invention provides an industrial preparation method for stevia glycosides, obtaining two stevia glycoside products with different content specifications. Product I: Stevia glycoside content ≥80%, pure white; Product II: Stevia glycoside content ≥99%, pure white. The total yield of Products I and II is ≥93%.

[0048] Second, this invention utilizes a temperature-variable linear desorption process using a supersaturated calcium hydroxide alcohol-water solution. The volume fraction of the alcohol-water solution used in the desorption process is low, requiring only 44-55%. This eliminates the need for the high volume fraction of alcohol-water solution required for obtaining high-content catechin products, and also reduces the amount of alcohol used. Furthermore, this process eliminates the need for crystallization and resin decolorization.

[0049] Third, this invention only requires adding a sandwich layer to existing industrial chromatography columns, eliminating the need for traditional crystallization and decolorization, making it easy to industrialize and simultaneously obtaining two different concentrations of betaine products. This process has low equipment requirements, is simple, environmentally friendly, widely applicable, and suitable for large-scale production. Detailed Implementation

[0050] The technical solution of the present invention will be explained in detail below by way of embodiments.

[0051] The present invention will be further described below with reference to the embodiments.

[0052] The sweet tea raw material used in this invention was provided by Hunan Huacheng Bioresources Co., Ltd., with a sweet tea glycoside content of 3.65%. Unless otherwise specified, all raw materials or chemical reagents used in the embodiments of this invention were obtained through conventional commercial channels.

[0053] The testing methods used in this invention are described below:

[0054] The content of catechins was determined by high performance liquid chromatography (HPLC) (refer to the local standard: DBS45 Local Food Safety Standard for the Determination of Catechin Content in Sweet Tea).

[0055] Unless otherwise specified, the "%" mentioned in the embodiments of this invention refers to the percentage by mass.

[0056] Example 1

[0057] (1) Preparation of column loading liquid: 200 kg of sweet tea raw material was extracted using a continuous countercurrent extraction unit with a material-to-water ratio of 1:8, an extraction temperature of 85℃, and an extraction time of 2 h. After disc centrifugation, 3% chitosan was added for flocculation, followed by disc centrifugation and passing through a 15000 Da ultrafiltration membrane. The permeate was collected as the column loading liquid.

[0058] (2) Column loading: The degassed loading solution is loaded onto the prepared chromatography column. The macroporous adsorption resin in the chromatography column is HPD-100 with a diameter-to-height ratio of 1:7 and the loading flow rate is 1 BV / h.

[0059] (3) Column washing and desorption: The column was washed with a 2% sodium hydroxide aqueous solution at a flow rate of 3 BV / h and a volume of 3 BV. At 0℃, a supersaturated calcium hydroxide alcohol aqueous solution with a calcium hydroxide addition of 0.15 wt% and an ethanol volume fraction of 48% was prepared for temperature-variable linear desorption. The heating rate was 7℃ / BV to 75℃, and the flow rate was 1.5 BV / h. The content of catechin in the desorbate was monitored. If the content of catechin was less than 97% based on the dry weight of the product, desorbate I was collected. When the content of catechin was ≥97% based on the dry weight, desorbate II was collected. The supersaturated calcium hydroxide alcohol aqueous solution was passed through a ceramic membrane with a molecular weight cutoff of 10000 Da before being loaded onto the column to remove insoluble matter.

[0060] (4) Extraction I: After concentrating the desorbed liquid I into alcohol, add n-propanol and water in a volume ratio of concentrate: n-propanol: water of 1:7:8. After stirring thoroughly, add carbon tetrachloride in a volume of 5 times that of n-propanol and stir evenly. Let stand for extraction and take the aqueous phase.

[0061] (5) Extraction II: Add n-butanol to the aqueous phase from step (4) at a volume ratio of 1:5 of n-butanol to aqueous phase, stir thoroughly, let stand for extraction, and take the n-butanol phase;

[0062] (6) Concentration to remove alcohol: The collected n-butanol phase and desorption solution II are concentrated to remove alcohol, and concentrated solution I and concentrated solution II are obtained respectively.

[0063] (7) Membrane filtration: Concentrate I and concentrate II were diluted with pure water and then passed through a nanofiltration membrane with a molecular weight cutoff of 300 Da to collect retentate I and retentate II.

[0064] (9) Concentration and drying: After concentrating and drying the retentate I and retentate II from step (7), two different specifications of sweet tea glycoside products can be obtained.

[0065] After weighing and testing, two different grades of sucralose products with varying contents were obtained: Product I: 3.02 kg, sucralose content 84.71%, pure white; Product II: 4.49 kg, sucralose content 98.53%, pure white. The total yield of sucralose from both products was 95.65%.

[0066] Example 2

[0067] Other conditions were the same as in Example 1, except that in step (3), the temperature range for linear desorption of the supersaturated calcium hydroxide aqueous solution with temperature variation was changed from 0 to 75°C to 10-60°C. Final product I: 2.86 kg, catechin content 85.15%; Product II: 4.47 kg, catechin content 98.42%, pure white, total catechin yield of products I and II: 93.63%.

[0068] Example 3

[0069] Other conditions were the same as in Example 1, except that in step (3), the heating rate was changed from 7°C / BV to 12°C / BV. Final product I: 3.07 kg, 82.50% catechin content; Product II: 4.41 kg, 97.58% catechin content, pure white, total catechin yield of products I and II: 93.64%.

[0070] Comparative Example 1

[0071] The other conditions were the same as in Example 1, except that in step (3), desorption was carried out at 20°C throughout, i.e., temperature-dependent desorption from 0 to 75°C was not performed. The polarity of the eluent did not change, making it impossible to collect the product with high catechin content in segments, and the experiment failed.

[0072] Comparative Example 2

[0073] Other conditions were the same as in Example 1, except that n-butanol was replaced with an equal volume of n-heptane. Final product I: 0.21 kg, stevia content 76.31%; Product II: 0.24 kg, stevia content 97.36%, off-white in color. The total yield of stevia from products I and II was 5.37%.

[0074] Comparative Example 3

[0075] All other conditions were the same as in Example 1, except that n-butanol was replaced with an equal volume of isopropanol. Phase separation was unsuccessful, and the experiment failed.

Claims

1. An industrial preparation method for betaine of different specifications, characterized in that, Includes the following steps: (1) Extraction: The dried leaves of sweet tea are extracted, separated into solid and liquid, flocculated, passed through an ultrafiltration membrane, and the permeate is collected; (2) Column loading: The permeate is loaded onto the macroporous adsorption resin; (3) Column washing and desorption: After washing the column with alkaline water, linear temperature variable desorption is performed at 0-10℃ to 60-80℃ using supersaturated calcium hydroxide alcohol aqueous solution. The desorbent is collected in segments to obtain desorbent I and desorbent II. When the content of catechin is ≥96% by dry weight, the collection of desorbent I is stopped and the collection of desorbent II is started. The linear temperature variable desorption method is to linearly increase the column temperature and the temperature of the desorbent from 0-10℃ to 60-80℃, with a heating rate of 7~12℃ / BV and a flow rate of 1~2BV / h. (4) Extraction I: After concentrating the desorption solution I, add a mixture of alcohol and water and mix well. Add a low-polarity organic solvent and mix well. Let stand for extraction and take the aqueous phase. (5) Extraction II: Add butanol to the aqueous layer from step (4), stir thoroughly, let stand for extraction, and take the butanol phase; (6) Concentration to remove alcohol: The collected butanol phase and desorption solution II are concentrated to remove alcohol, and concentrated solution I and concentrated solution II are obtained respectively. (7) Membrane filtration: After diluting concentrate I and concentrate II with pure water, they were passed through a nanofiltration membrane and the retentate I and retentate II were collected. (8) Concentration and drying: After concentrating and drying the retentate I and retentate II from step (7), two different specifications of sweet tea glycoside products can be obtained.

2. The preparation method according to claim 1, characterized in that, In step (1), the content of sweet tea glycosides in the sweet tea raw material is not less than 3%; the extraction is carried out by using water as the extraction solvent and by tank extraction or continuous countercurrent extraction; the mass ratio of the extracted sweet tea raw material to water is 1:6~10.

3. The preparation method according to claim 2, characterized in that, In step (1), the extraction temperature is 80~100℃; the extraction time is 1~3h.

4. The preparation method according to claim 2, characterized in that, In step (1), the molecular weight cutoff of the ultrafiltration membrane is 10,000 to 40,000 Da.

5. The preparation method according to claim 4, characterized in that, The ultrafiltration membrane has a molecular weight cutoff of 20,000 to 30,000 Da.

6. The preparation method according to claim 1, characterized in that, In step (2), the diameter-to-height ratio of the macroporous adsorption resin is 1:5~10, and the flow rate on the column is 1~2 BV / h; the macroporous adsorption resin is any one of D101, HPD-100, D1400, D4020, AB-8, and DM130.

7. The preparation method according to claim 6, characterized in that, In step (2), degassing is required before loading the column; the degassing method is vacuum treatment or ultrasonic treatment; the vacuum treatment is to maintain a vacuum degree of -0.1MPa to -0.05MPa for 10~20min; the ultrasonic treatment is 40~60KHZ, 150~400W, for 5~10min.

8. The preparation method according to claim 1, characterized in that, In step (3), the alkaline water is a 0.5-3wt% aqueous solution of sodium hydroxide and / or potassium hydroxide, the flow rate of the alkaline water washing column is 2-5 BV / h, and the volume of the alkaline water washing column is 2-4 BV.

9. The preparation method according to claim 1, characterized in that, In step (3), the concentration of calcium hydroxide in the supersaturated calcium hydroxide alcohol aqueous solution is 0.15-0.3 wt%; the supersaturated calcium hydroxide alcohol aqueous solution is prepared in an alcohol-water solvent system with an alcohol volume fraction of 44-55%.

10. The preparation method according to claim 9, characterized in that, The alcohol is ethanol.

11. The preparation method according to claim 1, characterized in that, In step (4), the alcohol-water mixture is a mixture of C1-3 alcohol and water, wherein the C1-3 alcohol is selected from at least one of methanol, ethanol, and propanol; the volume ratio of concentrate: alcohol: water is 1:5~10:5~10; the low polarity organic solvent is selected from at least one of carbon tetrachloride, hexane, cyclohexane, and petroleum ether, and the volume of the low polarity organic solvent is 3~6 times the volume of the alcohol.

12. The preparation method according to claim 1, characterized in that, In step (5), the butanol is n-butanol or isobutanol; the volume ratio of butanol to water phase is 1:3~6.

13. The preparation method according to claim 1, characterized in that, In step (6), the concentration method is any one of rotary evaporation concentration, falling film concentration, single-effect evaporation concentration, and multi-effect evaporation concentration; and / or In step (7), the pure water is diluted to a soluble solids content of 2-5%, and the membrane filtration is performed to retain a molecular weight cutoff of 200-500 Da.

14. The preparation method according to claim 1, characterized in that, In step (8), the soluble solids are concentrated to 40-60%; the concentration method is any one of rotary evaporation concentration, falling film concentration, single-effect evaporation concentration, and multi-effect evaporation concentration; the drying is for the purpose of removing moisture, and is spray drying, vacuum drying, microwave drying, belt drying, or forced air drying; after drying, the product is pulverized and sieved through 100-200 mesh.