Method for concentrating osmanthus absolute

Abstract

The application belongs to the technical field of biology and particularly relates to a method for concentrating osmanthus pure distillate. The method comprises step-by-step freezing of the osmanthus pure distillate, filtration to remove ice crystals after the freezing is completed, and concentration of the osmanthus pure distillate through a nano-composite polyamide composite membrane. The application can produce more ice crystals in the freezing process and the particle size of the ice crystals is larger, so that the ice crystals are removed more easily, thereby improving the efficiency of the freezing concentration, reducing the thermal loss of effective components, improving the concentration efficiency of the osmanthus pure distillate, and finally obtaining osmanthus pure distillate with higher concentration.

Description

Technical Field

[0001] This invention belongs to the field of biotechnology, specifically relating to a method for concentrating osmanthus hydrosol. Background Technology

[0002] Osmanthus hydrosol is a byproduct of the steam distillation process of osmanthus essential oil. It possesses a unique osmanthus aroma and certain health benefits, such as antioxidant and anti-inflammatory properties. As people's understanding of the value of osmanthus hydrosol deepens and the demand for high-quality osmanthus hydrosol increases, developing more efficient and gentle concentration technologies to maximize the preservation of the aroma components and bioactive substances in osmanthus hydrosol has become an important direction in current research and production.

[0003] Traditional osmanthus hydrosol extraction mainly employs steam distillation, which involves heating a mixture of osmanthus flowers and water, followed by condensation and separation. However, this method has some drawbacks, such as a long distillation time and the potential loss of some aromatic components due to thermal decomposition.

[0004] To improve the quality and concentration of osmanthus hydrosol, the application of concentration technology has become particularly important. Currently, common concentration methods include vacuum concentration and membrane concentration. While vacuum concentration is low-cost and highly efficient, some ester compounds and volatile aroma components in osmanthus are easily decomposed by heat, leading to significant loss of aromatic substances and even unpleasant odors. Membrane concentration, especially reverse osmosis (RO), can be carried out at room temperature, effectively avoiding the damage to osmanthus aroma components caused by high temperatures and better preserving the original freshness, purity, and flavor of the extract. However, during membrane concentration, impurities such as organic matter and microorganisms in the osmanthus hydrosol can easily form a fouling layer on the membrane surface, leading to a decrease in membrane flux. For example, polysaccharides in osmanthus hydrosol may adsorb onto the membrane surface, clogging the pores and reducing the membrane's separation performance. This not only affects concentration efficiency but also increases the cost of cleaning and replacing the membrane. Summary of the Invention

[0005] The purpose of this invention is to provide a method for concentrating osmanthus hydrosol, which reduces production costs, improves concentration efficiency, and increases the concentration of active ingredients in osmanthus hydrosol.

[0006] This invention provides a method for concentrating osmanthus hydrosol, the method comprising the following steps:

[0007] Osmanthus hydrosol was gradually frozen to -10℃ to -5℃. When the hydrosol cooled to -2℃ to 0℃, dry ice was added to promote ice crystal formation. After freezing, the ice crystals were removed by filtration, and the filtrate was collected. The filtrate was heated to 2℃ to 4℃ and then gradually frozen to -10℃ to -5℃ again. When the hydrosol cooled to -2℃ to 0℃, dry ice was added to promote ice crystal formation. After freezing, the ice crystals were removed by filtration to obtain pretreated osmanthus hydrosol. The pretreated osmanthus hydrosol was then passed through a reverse osmosis membrane to obtain concentrated osmanthus hydrosol.

[0008] Preferably, the osmanthus hydrosol is frozen stepwise to -8℃ to -5℃.

[0009] Preferably, the step-by-step freezing includes primary freezing, secondary freezing, and tertiary freezing; primary freezing involves lowering the temperature of the osmanthus hydrosol to -1°C and adding dry ice; secondary freezing involves lowering the temperature of the osmanthus hydrosol from -1°C to -4°C and maintaining it at -4°C for 30-60 minutes; tertiary freezing involves lowering the temperature of the osmanthus hydrosol from -4°C to -8°C to -5°C and maintaining it at -8°C to -5°C for 30-60 minutes.

[0010] Preferably, the cooling rate of the first-stage freezing is 2~5℃ / min; the cooling rate of the second-stage freezing is 0.5~1℃ / min; and the cooling rate of the third-stage freezing is 0.5~1℃ / min.

[0011] Preferably, the freezing process also includes stirring at 100-200 rpm.

[0012] Preferably, the amount of dry ice used is 1% to 5% of the volume of osmanthus hydrosol.

[0013] Preferably, the heating rate is 1~2℃ / min.

[0014] Preferably, the nanocomposite polyamide composite film is prepared by doping nanoparticles into a polyamide precursor solution.

[0015] Preferably, the nanoparticles include nano-alumina or nano-zinc oxide.

[0016] The beneficial effects of this invention are:

[0017] Freeze concentration is a concentration method that utilizes the solid-liquid equilibrium principle between ice and aqueous solution to remove water from the solution in a solid state. This invention, by adjusting the freezing time and cooling rate, can generate more ice crystals during the freezing process, and the ice crystals have a larger particle size, making them easier to remove. This improves the efficiency of freeze concentration, reduces the heat loss of effective ingredients, and also improves the concentration efficiency of osmanthus hydrosol, ultimately resulting in a higher concentration of osmanthus hydrosol.

[0018] This invention extends the freezing time of hydrosol at -4℃, which is beneficial for the formation of more ice crystals and increases the size of the ice crystals. This reduces the difficulty of subsequent ice crystal removal and allows for the removal of more free water, thereby achieving efficient concentration of osmanthus hydrosol.

[0019] This invention utilizes a nanocomposite polyamide membrane for reverse osmosis treatment, which not only improves concentration efficiency but also further preserves the small-molecule active ingredients in osmanthus hydrosol. The nanoparticles enhance the membrane's hydrophilicity and antifouling properties, thereby improving its performance. Detailed Implementation

[0020] To further illustrate the present invention, the solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0021] In the embodiments of the present invention, the osmanthus hydrosol used in the examples and comparative examples were all from the same batch; the polyamide composite membrane used in comparative example 1 was purchased from Guangzhou Yuanteng Environmental Protection Technology Co., Ltd., model RE8040-FEN34, item number RE8040-FEN34.

[0022] Example 1

[0023] Nano-zinc oxide was uniformly dispersed in an aqueous monomer solution (m-phenylenediamine), and then the base membrane (polyacrylonitrile) was cleaned with deionized water. The aqueous monomer solution containing nanoparticles was then uniformly coated onto the surface of the cleaned base membrane. The base membrane coated with the aqueous solution was then immersed in an oil solution, allowing the aqueous and oil monomers to undergo an interfacial polymerization reaction on the base membrane surface. After the reaction was completed, the membrane surface was cleaned with deionized water to remove unreacted monomers and impurities. The cleaned membrane was then placed in a drying oven to dry and remove moisture, resulting in a nano-zinc oxide composite polyamide composite membrane for later use.

[0024] 1 L of osmanthus hydrosol was placed in a freeze-concentration apparatus and stirred at 150 rpm. The temperature was lowered from room temperature to -1°C at a rate of 5°C / min. 10 mL of dry ice was added, and the cooling rate was adjusted to 1°C / min, lowering the temperature to -4°C and holding at -4°C for 45 min. The temperature was then lowered to -6°C at a rate of 1°C / min and held at -6°C for 45 min. The mixture was filtered to remove ice crystals. The temperature was then increased to 4°C at a rate of 2°C / min, and the gradient freezing process was repeated: the temperature was lowered from room temperature to -1°C at a rate of 5°C / min. 10 mL of dry ice was added, and the cooling rate was adjusted to 1°C / min, lowering the temperature to -4°C and holding at -4°C for 45 min. The temperature was then lowered to -6°C at a rate of 1°C / min and held at -6°C for 45 min. The mixture was filtered to remove ice crystals, and the temperature was then increased to 4°C at a rate of 2°C / min to obtain pretreated osmanthus hydrosol. The pretreated osmanthus hydrosol was then passed through a nano-zinc oxide composite polyamide membrane to obtain concentrated osmanthus hydrosol.

[0025] Example 2

[0026] Nano-zinc oxide was uniformly dispersed in an aqueous monomer solution (m-phenylenediamine), and then the base membrane (polyacrylonitrile) was cleaned with deionized water. The aqueous monomer solution containing nanoparticles was then uniformly coated onto the surface of the cleaned base membrane. The base membrane coated with the aqueous solution was then immersed in an oil solution, allowing the aqueous and oil monomers to undergo an interfacial polymerization reaction on the base membrane surface. After the reaction was completed, the membrane surface was cleaned with deionized water to remove unreacted monomers and impurities. The cleaned membrane was then placed in a drying oven to dry and remove moisture, resulting in a nano-zinc oxide composite polyamide composite membrane for later use.

[0027] Place 1L of osmanthus hydrosol into a freeze-concentrating apparatus and turn on the stirrer at a speed of 150rpm. Cool it from room temperature to -1℃ at a cooling rate of 5℃ / min, add 10mL of dry ice, adjust the cooling rate to 0.5℃ / min, cool it to -4℃, and maintain it at -4℃ for 45min. Continue cooling at a rate of 0.5℃ / min to cool it to -5℃, and maintain it at -5℃ for 45min. The ice crystals were removed by filtration, and then the temperature was increased to 4°C at a rate of 2°C / min. The gradient freezing process was repeated: the temperature was lowered from room temperature to -1°C at a rate of 5°C / min, 10 mL of dry ice was added, the cooling rate was adjusted to 0.5°C / min, and the temperature was lowered to -4°C and held at -4°C for 45 min; the temperature was then lowered to -5°C at a rate of 0.5°C / min and held at -5°C for 45 min. The ice crystals were removed by filtration, and then the temperature was increased to 4°C at a rate of 2°C / min to obtain the pretreated osmanthus hydrosol; the pretreated osmanthus hydrosol was passed through a nano-zinc oxide composite polyamide composite membrane to obtain the concentrated osmanthus hydrosol.

[0028] Example 3

[0029] Nano-alumina was uniformly dispersed in an aqueous monomer solution (m-phenylenediamine), and then the base film (polyacrylonitrile) was cleaned with deionized water. The aqueous monomer solution containing nanoparticles was then uniformly coated onto the surface of the cleaned base film. The base film coated with the aqueous solution was then immersed in an oil solution, allowing the aqueous and oil monomers to undergo an interfacial polymerization reaction on the surface of the base film. After the reaction was completed, the film surface was cleaned with deionized water to remove unreacted monomers and impurities. The cleaned film was then placed in a drying oven to dry and remove moisture, resulting in a nano-alumina composite polyamide composite film for later use.

[0030] Place 1L of osmanthus hydrosol into a freeze-concentrating apparatus and turn on the stirrer at a speed of 150rpm. Cool it from room temperature to -1℃ at a cooling rate of 5℃ / min, add 10mL of dry ice, adjust the cooling rate to 1℃ / min, cool it to -4℃, and hold it at -4℃ for 45min. Continue to adjust the cooling rate to 1℃ / min, cool it to -5℃, and hold it at -5℃ for 45min. The ice crystals were removed by filtration, and then the temperature was increased to 4°C at a rate of 2°C / min. The gradient freezing process was repeated: the temperature was lowered from room temperature to -1°C at a rate of 5°C / min, 10 mL of dry ice was added, the cooling rate was adjusted to 1°C / min, and the temperature was lowered to -4°C and held at -4°C for 45 min; the cooling rate was further adjusted to 1°C / min, and the temperature was lowered to -5°C and held at -5°C for 45 min. The ice crystals were removed by filtration, and then the temperature was increased to 4°C at a rate of 2°C / min to obtain the pretreated osmanthus hydrosol; the pretreated osmanthus hydrosol was passed through a nano-alumina composite polyamide composite membrane to obtain the concentrated osmanthus hydrosol.

[0031] Example 4

[0032] Nano-alumina was uniformly dispersed in an aqueous monomer solution (m-phenylenediamine), and then the base film (polyacrylonitrile) was cleaned with deionized water. The aqueous monomer solution containing nanoparticles was then uniformly coated onto the surface of the cleaned base film. The base film coated with the aqueous solution was then immersed in an oil solution, allowing the aqueous and oil monomers to undergo an interfacial polymerization reaction on the surface of the base film. After the reaction was completed, the film surface was cleaned with deionized water to remove unreacted monomers and impurities. The cleaned film was then placed in a drying oven to dry and remove moisture, resulting in a nano-alumina composite polyamide composite film for later use.

[0033] Place 1L of osmanthus hydrosol into a freeze-concentrating apparatus and turn on the stirrer at a speed of 150rpm. Cool it from room temperature to -1℃ at a cooling rate of 5℃ / min, add 10mL of dry ice, adjust the cooling rate to 0.5℃ / min, cool it to -4℃, and hold it at -4℃ for 45min. Adjust the cooling rate to 1℃ / min and continue cooling to -8℃, and hold it at -8℃ for 45min. The ice crystals were removed by filtration, and then the temperature was increased to 4°C at a rate of 2°C / min. The gradient freezing process was repeated: the temperature was lowered from room temperature to -1°C at a rate of 5°C / min, 10 mL of dry ice was added, the cooling rate was adjusted to 0.5°C / min, and the temperature was lowered to -4°C and held at -4°C for 45 min; the cooling rate was adjusted to 1°C / min, and the temperature was lowered to -8°C and held at -8°C for 45 min. The ice crystals were removed by filtration, and then the temperature was increased to 4°C at a rate of 2°C / min to obtain pretreated osmanthus hydrosol; the pretreated osmanthus hydrosol was passed through a nano-alumina composite polyamide composite membrane to obtain concentrated osmanthus hydrosol.

[0034] Comparative Example 1

[0035] The difference from Example 1 is that the osmanthus hydrosol, after being frozen and concentrated, is passed through a conventional polyamide composite membrane, as follows:

[0036] 1 L of osmanthus hydrosol was placed in a freeze-concentration apparatus and stirred at 150 rpm. The mixture was cooled from room temperature to -1°C at a rate of 5°C / min. 10 mL of dry ice was added, and the cooling rate was adjusted to 1°C / min, lowering the temperature to -4°C and holding at -4°C for 45 min. The temperature was then further lowered to -6°C at a rate of 1°C / min and held at -6°C for 45 min. The mixture was filtered to remove ice crystals. The temperature was then increased to 4°C at a rate of 2°C / min, and the gradient freezing process was repeated: the mixture was cooled from room temperature to -1°C at a rate of 5°C / min. 10 mL of dry ice was added, and the cooling rate was adjusted to 1°C / min, lowering the temperature to -4°C and holding at -4°C for 45 min. The temperature was then further increased to 1°C / min, lowering the temperature to -6°C and holding at -6°C for 45 min. The mixture was filtered to remove ice crystals, and the temperature was then increased to 4°C at a rate of 2°C / min to obtain pretreated osmanthus hydrosol. The pretreated osmanthus hydrosol was then passed through a polyamide composite membrane to obtain concentrated osmanthus hydrosol.

[0037] Comparative Example 2

[0038] The difference from Example 1 is that the osmanthus hydrosol, after being frozen and concentrated, does not pass through a reverse osmosis membrane, as detailed below:

[0039] Place 1L of osmanthus hydrosol into a freeze-concentrating apparatus and start stirring at 150rpm. Cool the hydrosol from room temperature to -1℃ at a rate of 5℃ / min, add 10mL of dry ice, adjust the cooling rate to 1℃ / min, and cool to -4℃, holding at -4℃ for 45min. Continue cooling at 1℃ / min until the hydrosol reaches -6℃, holding at -6℃ for 45min. Filter to remove ice crystals, then heat to 4℃ at a rate of 2℃ / min and repeat the gradient freezing process: cool the hydrosol from room temperature to -1℃ at a rate of 5℃ / min, add 10mL of dry ice, adjust the cooling rate to 1℃ / min, and cool to -4℃, holding at -4℃ for 45min. Continue cooling at 1℃ / min until the hydrosol reaches -6℃, holding at -6℃ for 45min, filter to remove ice crystals, and then heat to 4℃ at a rate of 2℃ / min to obtain concentrated osmanthus hydrosol.

[0040] Comparative Example 3

[0041] The difference from Example 1 is that the osmanthus hydrosol is not concentrated by freezing, but is directly passed through a reverse osmosis membrane, as follows:

[0042] Nano-zinc oxide was uniformly dispersed in an aqueous monomer solution (m-phenylenediamine), and then the base membrane (polyacrylonitrile) was cleaned with deionized water. The aqueous monomer solution containing nanoparticles was then uniformly coated onto the surface of the cleaned base membrane. The base membrane coated with the aqueous solution was then immersed in an oil solution, allowing the aqueous and oil monomers to undergo an interfacial polymerization reaction on the base membrane surface. After the reaction was completed, the membrane surface was cleaned with deionized water to remove unreacted monomers and impurities. The cleaned membrane was then placed in a drying oven to dry and remove moisture, resulting in a nano-zinc oxide composite polyamide composite membrane for later use.

[0043] 1L of osmanthus hydrosol was passed through a nano zinc oxide composite polyamide composite membrane to obtain concentrated osmanthus hydrosol.

[0044] Comparative Example 4

[0045] The difference from Example 1 is that the cooling rate of the osmanthus hydrosol was maintained at 2℃ / min throughout, as detailed below:

[0046] Nano-zinc oxide was uniformly dispersed in an aqueous monomer solution (m-phenylenediamine), and then the base membrane (polyacrylonitrile) was cleaned with deionized water. The aqueous monomer solution containing nanoparticles was then uniformly coated onto the surface of the cleaned base membrane. The base membrane coated with the aqueous solution was then immersed in an oil solution, allowing the aqueous and oil monomers to undergo an interfacial polymerization reaction on the base membrane surface. After the reaction was completed, the membrane surface was cleaned with deionized water to remove unreacted monomers and impurities. The cleaned membrane was then placed in a drying oven to dry and remove moisture, resulting in a nano-zinc oxide composite polyamide composite membrane for later use.

[0047] 1 L of osmanthus hydrosol was placed in a freeze-concentration apparatus and stirred at 150 rpm. The temperature was lowered from room temperature to -1°C at a rate of 2°C / min, 10 mL of dry ice was added, and the temperature was further lowered to -4°C and held at -4°C for 45 min. The temperature was then lowered to -6°C at a rate of 2°C / min and held at -6°C for 45 min. The ice crystals were removed by filtration. The temperature was then increased to 4°C at a rate of 2°C / min, and the gradient freezing process was repeated: the temperature was lowered from room temperature to -1°C at a rate of 2°C / min, 10 mL of dry ice was added, and the temperature was then lowered to -4°C and held at -4°C for 45 min. The temperature was then lowered to -6°C at a rate of 2°C / min and held at -6°C for 45 min. The ice crystals were removed by filtration, and the temperature was then increased to 4°C at a rate of 2°C / min to obtain pretreated osmanthus hydrosol. The pretreated osmanthus hydrosol was then passed through a nano-zinc oxide composite polyamide membrane to obtain concentrated osmanthus hydrosol.

[0048] Comparative Example 5

[0049] The difference from Example 1 is that the osmanthus hydrosol was directly cooled to -6°C and maintained at -6°C for 90 minutes, as detailed below:

[0050] Nano-zinc oxide was uniformly dispersed in an aqueous monomer solution (m-phenylenediamine), and then the base membrane (polyacrylonitrile) was cleaned with deionized water. The aqueous monomer solution containing nanoparticles was then uniformly coated onto the surface of the cleaned base membrane. The base membrane coated with the aqueous solution was then immersed in an oil solution, allowing the aqueous and oil monomers to undergo an interfacial polymerization reaction on the base membrane surface. After the reaction was completed, the membrane surface was cleaned with deionized water to remove unreacted monomers and impurities. The cleaned membrane was then placed in a drying oven to dry and remove moisture, resulting in a nano-zinc oxide composite polyamide composite membrane for later use.

[0051] Place 1L of osmanthus hydrosol into a freeze-concentrating apparatus and start stirring at 150rpm. Cool it from room temperature to -1℃ at a rate of 5℃ / min, add 10mL of dry ice, adjust the cooling rate to 1℃ / min, cool it to -6℃, and hold it at -6℃ for 90min. Then, raise the temperature to 4℃ at a rate of 2℃ / min and repeat the freezing process: cool it from room temperature to -1℃ at a rate of 5℃ / min, add 10mL of dry ice, adjust the cooling rate to 1℃ / min, cool it to -6℃, and hold it at -6℃ for 90min. Filter to remove ice crystals, and then raise the temperature to 4℃ at a rate of 2℃ / min to obtain pretreated osmanthus hydrosol. Pass the pretreated osmanthus hydrosol through a nano-zinc oxide composite polyamide composite membrane to obtain concentrated osmanthus hydrosol.

[0052] Comparative Example 6

[0053] The difference from Example 1 is that the concentration is carried out by vacuum concentration, as detailed below:

[0054] After filtering the osmanthus hydrosol, pour it into a rotary evaporator flask. Attach the rotary evaporator flask to the rotary device of the vacuum concentrator, turn on the rotation function, and set the rotation speed to 80-100 r / min. Start the heating system and slowly raise the temperature to 55℃ and maintain it stable. Start the vacuum pump and adjust the vacuum level to 0.05 MPa and maintain it stable. During the concentration process, monitor the temperature and vacuum level in real time, ensuring that the temperature remains stable at 55℃ and the vacuum level remains stable at 0.05 MPa. Observe the solution; when the solution becomes viscous and the color changes from pale yellow to brownish-yellow, turn off the heating system and the rotary device. Slowly open the vacuum regulating valve to restore the system to atmospheric pressure, remove the rotary evaporator flask, and collect the concentrated osmanthus hydrosol.

[0055] Test Example 1

[0056] The concentrated osmanthus hydrosols prepared in Example 1 and Comparative Examples 1 to 6 were used as samples. The concentration factor and active ingredient content of each group of samples were statistically analyzed, and the results are shown in Tables 1 and 2.

[0057] Table 1. Concentration effect test results

[0058]

[0059] Table 2 Results of Active Ingredient Content Detection

[0060]

[0061] As shown in Tables 1 and 2, the method of the present invention is beneficial for removing more free water and improving concentration efficiency. Combined with Example 1 and Comparative Example 6, it can be seen that compared with the traditional vacuum concentration method, the method of the present invention is more beneficial for retaining the active ingredients in osmanthus hydrosol. After concentration, the content of active ingredients in the osmanthus hydrosol of Example 1 is significantly higher than that of Comparative Example 2 and Comparative Example 3, indicating that the present invention can further amplify the concentration efficiency by combining freeze concentration and reverse osmosis membrane filtration.

[0062] Test Example 2

[0063] Antioxidant activity test

[0064] DPPH free radical scavenging assay: Each group of samples was mixed with 0.16 mM DPPH solution in a 96-well plate and incubated at 30°C in the dark for 30 minutes. The absorbance at 517 nm was then measured using a microplate reader. Using α-tocopherol as a positive control and anhydrous ethanol as a blank control, the scavenging rate was calculated using the following formula:

[0065] DPPH free radical scavenging rate % = (1 - A1 / A0) × 100%

[0066] Where A0 represents the absorbance of anhydrous ethanol and DPPH solution, and A1 represents the absorbance of sample and DPPH solution.

[0067] ABTS free radical scavenging assay: ABTS and potassium persulfate were prepared into a working solution, which was then mixed with different concentrations of osmanthus extract in a 96-well plate. After incubation at 30°C in the dark for 6 minutes, the absorbance at 734 nm was measured using a microplate reader. Using α-tocopherol as a positive control and anhydrous ethanol as a blank control, the scavenging rate was calculated according to the following formula:

[0068] ABTS radical scavenging rate % = (1 - As / Ae) × 100%

[0069] Where Ae represents the absorbance of anhydrous ethanol and ABTS solution, and As represents the absorbance of sample and ABTS solution.

[0070] The results are shown in Table 3.

[0071] Table 3. Results of antioxidant activity detection for each sample

[0072]

[0073] As shown in Table 3, the method provided in Example 1 resulted in the highest concentration and strongest antioxidant activity of osmanthus hydrosol, indicating that the method of the present invention is more conducive to preserving the active ingredients in osmanthus hydrosol.

[0074] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for concentrating osmanthus hydrosol, characterized in that, The method includes the following steps: Osmanthus hydrosol was gradually frozen to -10℃ to -5℃. When the osmanthus hydrosol cooled to -2℃ to 0℃, dry ice was added to promote ice crystal formation. After freezing, the ice crystals were removed by filtration, and the filtrate was collected. The filtrate was heated to 2℃ to 4℃ and then gradually frozen to -10℃ to -5℃ again. When the osmanthus hydrosol cooled to -2℃ to 0℃, dry ice was added to promote ice crystal formation. After freezing, the ice crystals were removed by filtration to obtain pretreated osmanthus hydrosol. The pretreated osmanthus hydrosol was passed through a nano-composite polyamide membrane to obtain concentrated osmanthus hydrosol. The progressive freezing process includes primary freezing, secondary freezing, and tertiary freezing. Primary freezing involves lowering the temperature of the osmanthus hydrosol to -1°C and adding dry ice. Secondary freezing involves lowering the temperature of the osmanthus hydrosol from -1°C to -4°C and maintaining it at -4°C for 30-60 minutes. Tertiary freezing involves lowering the temperature of the osmanthus hydrosol from -4°C to -8°C to -5°C and maintaining it at -8°C to -5°C for 30-60 minutes. The cooling rate of the first-stage freezing is 2~5℃ / min; the cooling rate of the second-stage freezing is 0.5~1℃ / min; and the cooling rate of the third-stage freezing is 0.5~1℃ / min.

2. The method according to claim 1, characterized in that, The osmanthus hydrosol was gradually frozen to -8℃ to -5℃.

3. The method according to claim 1, characterized in that, The freezing process also involves stirring at 100-200 rpm.

4. The method according to claim 1, characterized in that, The amount of dry ice used is 1% to 5% of the volume of osmanthus hydrosol.

5. The method according to claim 1, characterized in that, The heating rate is 1~2℃ / min.

6. The method according to claim 1, characterized in that, The nanocomposite polyamide composite film is prepared by doping nanoparticles into a polyamide precursor solution.

7. The method according to claim 6, characterized in that, The nanoparticles include nano-alumina or nano-zinc oxide.

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