A nanoporous filtration separation medium and its preparation method and application

Abstract

The present invention discloses a nanoporous filtration separation medium and its preparation method and application. A polymer-based nanoporous medium with precise pore size distribution is constructed through template-induced self-assembly technology, and combined with a surface functionalization modification strategy, efficient retention of milk fat components and selective permeation of whey proteins are achieved. Based on the synergistic mechanism of size screening and electrostatic repulsion, milk fat enrichment and milk protein purification are completed simultaneously under mild operating conditions, significantly reducing energy consumption and component losses in traditional processes. Long-term operating stability is maintained by regulating the nanopore structural parameters and surface chemical properties. This technology provides an innovative solution for the development of high-purity buffalo milk fat products and skimmed buffalo milk products.

Description

Technical Field

[0001] The invention belongs to the technical field of lipid separation, and specifically relates to a nanopore filtration separation medium and a preparation method and application thereof. Background Art

[0002] Buffalo milk has become the core raw material for the development of high-end dairy products because its milk fat content is significantly higher than that of ordinary milk and it is rich in a variety of high-value-added lipid components. However, the efficient separation of milk fat faces multiple technical bottlenecks. Traditional non-filtration technologies such as centrifugation rely on density difference separation. Although they can handle high-solid content systems, they consume extremely high energy during the centrifugation process, and the high-speed shearing during the process can easily destroy heat-sensitive lipids, and the separation accuracy is low. Although the solvent extraction method improves efficiency through organic solvents such as n-hexane, the residual solvent threatens food safety and destroys the natural emulsification structure of milk fat. The enzymatic method uses lipase to hydrolyze the milk fat membrane, but the free fatty acids in the product are prone to cause rancidity. The process also requires additional enzyme inactivation and deacidification steps, and the process is complicated.

[0003] Although existing filtration technology can avoid the thermal damage and chemical pollution of non-filtration methods, it still has significant limitations: the pore size distribution of traditional membrane materials is wide, resulting in a contradiction between milk fat retention and whey protein permeability; the interaction between the chemical properties of the membrane surface and the milk components can easily cause pollution and blockage, especially casein micelles form a dense filter cake layer on the membrane surface, causing rapid flux decay, requiring frequent shutdowns for cleaning, and accelerating the aging of membrane materials. The preparation of some inorganic membranes relies on highly toxic etchants such as hydrofluoric acid, which poses a risk of ion residues and does not meet the safety standards for dairy processing; the homogeneous pore size design cannot match the complex component characteristics of buffalo milk, resulting in difficulty in achieving both separation efficiency and flux. Further design at the material level is still needed.

[0004] Therefore, in the current technical system, non-filtration methods sacrifice quality and safety, and filtration methods are limited by material properties, making it difficult to balance separation accuracy, flux stability, and food safety. In particular, for the high-protein and high-lipid characteristics of buffalo milk, existing technologies cannot meet the core requirements of lipid separation. Therefore, developing a nanoporous filtration medium that combines high separation accuracy, long-term stability, and food safety has become the key to breaking through the bottleneck of the industry. Summary of the invention

[0005] Purpose of the invention: The technical problem to be solved by the present invention is to provide a nanoporous filtration separation medium to address the deficiencies of the prior art, achieve efficient separation of milk fat and other milk components, and improve permeation flux and diversified regulation performance.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a nanoporous filtration separation medium, characterized in that it comprises the following steps:

[0008] S1: Preparation of monodisperse alumina nanospheres:

[0009] Aluminum isopropoxide and acetylacetone are dissolved in cyclohexane in a molar ratio of 1:0.8-1.2, 0.5-1.5 wt% nitric acid is added as a hydrolysis catalyst, and the mixture is refluxed at 80-90°C for 3-7 hours. After centrifugation and washing, monodispersed γ-Al with a particle size of 300-350 nm is obtained. 2 O 3 Nanospheres, particle size variation coefficient (CV) <5%.

[0010] Specifically, in step S1, during the centrifugal washing, the centrifugal speed is 10,000-15,000 rpm, and the centrifugal time is 10-15 minutes.

[0011] Preferably, in step S1, the amount of nitric acid added is 0.8%-1.2% of the mass of aluminum isopropoxide; the reflux reaction temperature is 85-88° C., the reaction time is 5-5.5 hours; the centrifugal speed is 12,000 rpm, and the centrifugal time is 12 minutes.

[0012] S2: Self-assembly of nanosphere arrays:

[0013] The alumina nanospheres obtained in step S1 are dispersed in an ethanol / water mixture (volume ratio 4:1) to form a 5-8 wt% colloidal solution, and self-assembled on a polyethersulfone substrate using the Langmuir-Blodgett method to obtain a hexagonal close-packed array; the number of deposited layers is controlled by the number of pull-ups (2-6 times), and the single layer thickness is 150-180 nm.

[0014] Specifically, in step S2, the Langmuir-Blodgett method conditions are: surface pressure 20-25 mN / m, pulling speed 1-3 mm / min; drying temperature 60-80° C., and drying time 2-4 hours.

[0015] Preferably, in step S2, the solid content of the colloidal solution is 6 wt%, the number of deposition layers is 3, the pulling speed is 2 mm / min, and the drying temperature is 70°C / 3 h.

[0016] S3: Polyimide precursor filling:

[0017] The polyamic acid / N-methylpyrrolidone (NMP) solution (solid content 12-18 wt%) was vacuum infused into the gaps between the nanospheres at a pressure of -0.05~-0.08 MPa for 20-30 minutes, followed by gradient temperature imidization: heating rate 5 ℃ / min, 80℃ / 1 h→150℃ / 1 h→280℃ / 2 h to form an alumina-polyimide composite membrane.

[0018] Preferably, in step S3, the viscosity of the polyamic acid solution is 1200±100 cP (25°C), the vacuum pressure is -0.07 MPa, and the nitrogen flow rate at 280°C is 0.5 L / min.

[0019] S4: Selective etching of alumina template:

[0020] Alumina nanospheres were removed by etching with a phosphoric acid solution of 10-15% mass concentration at 60-70°C to obtain a through-hole polyimide nanoporous medium with a pore size of 300-350 nm; the etching time was 40-60 minutes, the pore size uniformity deviation was ≤±8%, and the porosity was ≥85%.

[0021] Preferably, in step S4, the phosphoric acid concentration is 12%, the etching temperature is 65° C., the ultrasonic power is 120 W / 50 min, and 0.1 M NaOH is used for neutralization and rinsing after etching.

[0022] S5: Surface hydrophilization treatment:

[0023] The nanoporous medium was immersed in a 0.5-1.0 M ammonium persulfate aqueous solution and treated under ultraviolet light (254 nm, 30-50mW / cm²) for 10-15 minutes to carboxylate the membrane surface and reduce the contact angle from 110° to below 35°. This process can enhance hydrophilicity, better filter lipids, and better prevent casein adsorption through electrostatic repulsion.

[0024] Preferably, in step S5, the ammonium persulfate concentration is 0.8 M, the UV intensity is 40 mW / cm², the treatment time is 12 minutes, and the flushing water flow rate is 20 mL / min.

[0025] Furthermore, the nanoporous filtration separation medium prepared by the above preparation method is also within the protection scope of the present invention.

[0026] Furthermore, the present invention also claims protection for the use of the above-mentioned nanoporous filtration separation medium in buffalo milk lipid separation, specifically: preheating the buffalo milk to 45°C, filtering the nanoporous medium at a transmembrane pressure of 0.8 bar and a cross-flow rate of 3 m / s to obtain low-fat buffalo milk; the milk fat content in the low-fat buffalo milk is ≤0.8%, the whey protein recovery rate is ≥80%, and the flux attenuation rate after continuous operation for 12 hours is ≤20%.

[0027] The present invention is based on nanopore design and polymer material regulation strategy to achieve efficient retention of milk fat and high-precision screening of other nutrients such as whey protein. Specifically, the nanopores of the functional layer retain 200-500nm milk fat globules through the size exclusion effect, while allowing whey protein to pass freely; the surface carboxylation treatment gives the membrane negative electrical properties, reducing casein adsorption through electrostatic repulsion. Milk fat enrichment and whey protein recovery are completed simultaneously through a single cross-flow filtration, breaking through the technical bottleneck of traditional technology that separation accuracy and flux cannot be achieved at the same time, and completely retaining the heat-sensitive nutrients of buffalo milk. The present invention can meet the needs of high-end dairy product development for the coordinated acquisition of high-purity milk fat and functional milk protein, and promote the upgrading of dairy processing towards precise separation and full resource utilization.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) This invention solves the problem of balancing efficiency and flux stability in buffalo milk lipid separation by exploring a controllable construction method of nanopore structure. Compared with traditional centrifugation and filtration processes, this technology provides a more promising means for the selective retention of milk fat components, and shows certain advantages in maintaining the permeability of whey protein, which may provide technical support for the development of high value-added dairy products.

[0030] (2) In terms of process design, by optimizing the surface properties and pore structure of the material, a more precise and controllable structure is achieved than other filtration processes. This feature may help to extend the operating cycle, reduce the frequency of cleaning and maintenance, and provide a reference direction for the optimization of the continuous production process of dairy products.

[0031] (3) The preparation process focuses on food processing safety requirements, and uses environmentally friendly processes to replace the use of traditional toxic reagents. The selection of relevant materials and the setting of process parameters follow the food contact material specifications as much as possible, laying the foundation for the compliance of technical applications.

[0032] (4) The technical solution retains a certain amount of parameter adjustment space. By adjusting key variables such as the template concentration, it can adapt to the component fluctuations of different batches of buffalo milk. This flexibility may help improve the adaptability of the technology in actual production and provide more options for process optimization.

[0033] (5) At the level of industrial implementation, the existing technology route is compatible with conventional dairy processing equipment to a certain extent, and the difficulty of technological transformation is reduced through modular design. Its mild operating conditions may provide a transitional solution for upgrading traditional processes.

[0034] (6) The universality of the technical principle suggests its potential application value. In addition to the currently verified buffalo milk system, it may inspire the improvement and exploration of other milk source separation technologies and accumulate experience for continuous innovation in the dairy processing field. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become more clear.

[0036] Figure 1 It is the technical route and working principle diagram of the present invention.

[0037] Figure 2 The graph is a graph of the peak particle size of the monodisperse alumina nanospheres of the present invention and the synthesis time, which reflects the size adjustability thereof.

[0038] Figure 3 This is a scanning electron microscope (SEM) image of the ordered aluminum oxide nanosphere array film of the present invention, showing a hexagonal close-packed structure.

[0039] Figure 4 This is a scanning electron microscope (SEM) image of the polyimide nanoporous medium of the present invention, showing the morphology of the through-hole channels and the uniformity of the pore size.

[0040] Figure 5 The Zeta potential statistics of the polyimide nanoporous medium of the present invention before and after surface charge modification, wherein the Zeta potential of the unmodified membrane at pH 6.8 is -10.5 mV, which is increased to -34.8 mV after UV / ammonium persulfate carboxylation treatment, confirming the significant enhancement of the surface negative charge density, providing a theoretical basis for electrostatic repulsion for inhibiting the adsorption of casein micelles.

[0041] Figure 6 The flux attenuation trends of the medium of the present invention and the traditional organic membrane during the continuous treatment of buffalo milk for 12 h are demonstrated. The flux retention rate of the medium of the present invention is ≥80%, while the traditional membrane decays to about 60% of the initial value.

[0042] Figure 7 Display of pore size-retention rate relationship: The bar graph shows the corresponding relationship between the retention rate of media with different pore sizes (280 / 320 / 360 nm) for milk fat (200-500 nm), verifying the decisive role of precise pore size control on separation selectivity.

[0043] Figure 8 Display of pore size-permeability relationship: The bar graph shows the correspondence between the permeability of whey protein (<50 nm) with media of different pore sizes (280 / 320 / 360 nm), verifying the decisive role of precise control of pore size on separation selectivity. DETAILED DESCRIPTION

[0044] The present invention can be better understood with reference to the following examples.

[0045] Combination Figure 1The preparation process of the nanoporous filtration separation medium of the present invention is as follows:

[0046] S1: Preparation of monodispersed alumina nanospheres;

[0047] S2: self-assembling the monodispersed alumina nanospheres in step S1 on a substrate to form an ordered nanosphere array film;

[0048] S3: filling the polyimide precursor solution into the gaps between the nanospheres of the ordered nanosphere array film in step S2, and forming an aluminum oxide-polyimide composite film after curing;

[0049] S4: dissolving and removing the alumina nanospheres in step S3 to obtain a polymer nanoporous medium with through pores;

[0050] S5: performing surface hydrophilization treatment on the polymer nanoporous medium of step S4 to obtain a carboxyl-functionalized nanoporous filtration medium.

[0051] Example 1: Preparation of 280 nm pore size nanoporous medium

[0052] S1. Preparation of monodisperse alumina nanospheres for constructing nanoporous filter media for buffalo milk lipid separation:

[0053] Aluminum isopropoxide and acetylacetone were dissolved in cyclohexane at a molar ratio of 1:1, 1.0 wt% nitric acid was added as a catalyst, and the reaction was refluxed at 85 °C for about 3 hours. The reaction time can determine the size of the nanospheres ( Figure 2 ), and the reaction was stopped after the desired particle diameter was reached through dynamic monitoring. After the reaction was completed, the particles were centrifuged at 12,000 rpm for 12 minutes and washed with ethanol three times to obtain monodispersed γ-Al with a particle size of 280 ± 10 nm. 2 O 3 Nanospheres are used as representatives for subsequent operations.

[0054] S2. Self-assembly of nanosphere arrays for subsequent construction of nanoporous filtration media for buffalo milk lipid separation:

[0055] The obtained nanospheres were dispersed in a mixture of ethanol / water (4:1 v / v) to prepare a 6 wt% colloidal solution. Three layers were self-assembled on a polyethersulfone substrate using the Langmuir-Blodgett method, with the surface pressure controlled at 23 mN / m, the pulling speed at 2 mm / min, and dried at 70°C for 3 hours to form an ordered nanosphere array film. Figure 3 The structure of the nanosphere array membrane is shown.

[0056] S3, polyimide precursor filling and curing, template etching and pore formation, construction of alumina-polyimide composite film:

[0057] A polyamic acid / NMP solution with a solid content of 15% was perfused under a vacuum of -0.07 MPa for 30 minutes, followed by gradient temperature imidization (80℃ / 1 h→150℃ / 1 h→280℃ / 2 h) to obtain an alumina-polyimide composite membrane. A 12% phosphoric acid solution was used for etching at 65℃ under ultrasonic assistance (120 W) for 50 minutes to completely remove the alumina template and obtain a polyimide nanoporous medium with a pore size of 320±25 nm. Figure 4 Demonstrated its nanostructure.

[0058] S4. Surface carboxylation modification of polyimide nanoporous media:

[0059] The membrane was immersed in 0.8 M ammonium persulfate solution, treated under 40 mW / cm² 254 nm UV light for 12 minutes, and rinsed with deionized water. The Zeta potential changed before and after surface charge modification. The Zeta potential of the unmodified membrane at pH 6.8 was -10.5 mV, which increased to -34.8 mV after UV / ammonium persulfate carboxylation treatment, confirming the significant enhancement of the surface negative charge density, providing a theoretical basis for electrostatic repulsion to inhibit casein micelle adsorption ( Figure 5 ).

[0060] Example 2: Preparation of 320 nm pore size nanoporous medium

[0061] S1. Preparation of monodisperse alumina nanospheres for constructing nanoporous filter media for buffalo milk lipid separation:

[0062] Aluminum isopropoxide and acetylacetone were dissolved in cyclohexane at a molar ratio of 1:1, 1.0 wt% nitric acid was added as a catalyst, and the reaction was refluxed at 85°C for about 4.5 hours. The reaction time can determine the size of the nanospheres ( Figure 2 ), and the reaction was stopped after the desired particle diameter was reached through dynamic monitoring. After the reaction was completed, the particles were centrifuged at 12,000 rpm for 12 minutes and washed with ethanol three times to obtain monodispersed γ-Al with a particle size of 320±10 nm. 2 O 3 Nanospheres are used as representatives for subsequent operations.

[0063] Steps S2-S5 are the same as those in Example 1.

[0064] Example 3: Preparation of 360 nm pore size nanoporous medium

[0065] S1. Preparation of monodisperse alumina nanospheres for constructing nanoporous filter media for buffalo milk lipid separation:

[0066] Aluminum isopropoxide and acetylacetone were dissolved in cyclohexane at a molar ratio of 1:1, 1.0 wt% nitric acid was added as a catalyst, and the reaction was refluxed at 85 °C for about 6.5 hours. The reaction time can determine the size of the nanospheres ( Figure 2 ), and the reaction was stopped after the desired particle diameter was reached through dynamic monitoring. After the reaction was completed, the particles were centrifuged at 12,000 rpm for 12 minutes and washed with ethanol three times to obtain monodispersed γ-Al with a particle size of 320±10 nm. 2 O 3 Nanospheres are used as representatives for subsequent operations.

[0067] Steps S2-S5 are the same as those in Example 1.

[0068] Example 4: Comparison of flux stability between nanoporous filtration separation media and traditional polyethersulfone membrane

[0069] The carboxylated polyimide nanoporous medium prepared in Example 2 was used to compare the flux stability with a commercially available polyethersulfone (PES) organic filter membrane (brand: Merck Millipore, model: SLGPR33RB, nominal pore size 0.22 μm). The buffalo milk parameters tested were as follows: milk fat content 4.5±0.3%, whey protein content 4.0±0.2%, pH 6.8±0.1. The operating parameters were set to a transmembrane pressure of 0.8 bar and a cross-flow rate of 3 m / s, and the system was run continuously for 12 hours in a laboratory-grade flat membrane filtration system (effective membrane area 0.01 m²).

[0070] like Figure 6 As shown in the figure, the initial flux of the medium of the present invention is 120 L / (m²·h), and the flux retention rate is more than 80% after 12 hours of operation; while the initial flux of the traditional PES membrane is also 120 L / (m²·h), and the flux decays to about 60% after 12 hours.

[0071] Example 5

[0072] Tests of milk fat retention and protein permeability at different pore sizes:

[0073] Parallel experiments were conducted using the nanoporous media prepared in Example 1 (280 nm pore size), Example 2 (320 nm pore size), and Example 3 (360 nm pore size). The parameters of buffalo milk tested were as follows: milk fat content 4.5±0.3%, whey protein content 4.0±0.2%, and pH value 6.8±0.1. The operating parameters were set to a transmembrane pressure of 0.8 bar and a cross-flow rate of 3 m / s, and the system was operated continuously in a laboratory-grade flat membrane filtration system (effective membrane area 0.01 m²). Each batch was run for 12 hours, and the average value was taken after 3 repetitions.

[0074] The milk fat retention rate was determined by centrifugation: 10 mL of the permeate was centrifuged at 12,000 rpm for 15 minutes, the upper milk fat layer was separated and weighed, and the retention rate was calculated (retention rate = 1-permeate milk fat mass / stock solution milk fat mass). The whey protein transmittance was determined by ultraviolet spectrophotometry: the permeate was diluted 10 times, the absorbance was measured at a wavelength of 280 nm, and the protein concentration was calculated by comparing with the standard curve (transmittance = permeate protein concentration / stock solution protein concentration).

[0075] like Figure 7 and Figure 8 As shown in the figure, the 280 nm medium has the highest retention rate for milk fat, but the whey protein permeability is relatively low; the 360 ​​nm medium has a lower milk fat retention rate, while the whey protein permeability is increased. The 320 nm medium strikes a balance between the two, as shown in the figure. Figure 7 , Figure 8 The middle bar graph shows.

[0076] The present invention provides a nanoporous filtration separation medium and its preparation method and application ideas and methods. There are many methods and ways to implement the technical solution. The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the protection scope of the present invention. All components not specified in this embodiment can be implemented by existing technologies.

Claims

1. A method for preparing a nanoporous filtration separation medium, characterized in that: The steps include: S1: Preparation of monodispersed alumina nanospheres; S2: self-assembling the monodispersed alumina nanospheres in step S1 on a substrate to form an ordered nanosphere array film; S3: filling the polyimide precursor solution into the gaps between the nanospheres of the ordered nanosphere array film in step S2, and forming an aluminum oxide-polyimide composite film after curing; S4: dissolving and removing the alumina nanospheres in step S3 to obtain a polymer nanoporous medium with through pores; S5: performing surface hydrophilization treatment on the polymer nanoporous medium of step S4 to obtain a carboxyl-functionalized nanoporous filtration medium.

2. The method for preparing a nanoporous filtration separation medium according to claim 1, characterized in that: In step S1, the monodisperse alumina nanospheres are prepared by the following method: Aluminum isopropoxide and acetylacetone are dissolved in cyclohexane at a molar ratio of 1:0.8-1.2, 0.5-1.5 wt% nitric acid is added as a hydrolysis catalyst, and the mixture is refluxed at 80-90°C for 3-7 hours. After centrifugation and washing, monodispersed γ-Al2O3 nanospheres with a particle size of 300-350 nm are obtained, and the particle size variation coefficient is <5%.

3. The method for preparing a nanoporous filtration separation medium according to claim 2, characterized in that: The amount of nitric acid added is 0.8%-1.2% of the mass of aluminum isopropoxide; the temperature of the reflux reaction is 85-88°C, and the reaction time is 5-5.5 hours; the speed of the centrifugal washing is 10,000-15,000 rpm, and the centrifugal time is 10-15 minutes.

4. The method for preparing a nanoporous filtration separation medium according to claim 1, characterized in that: In step S2, the alumina nanospheres obtained in step S1 are dispersed in a mixture of ethanol and water to form a 5-8 wt% colloidal solution, and self-assembled on a polyethersulfone substrate using the Langmuir-Blodgett method to obtain a hexagonal close-packed array; the number of deposited layers is controlled by 2-6 pulling times, and the single layer thickness is 150-180 nm.

5. The method for preparing a nanoporous filtration separation medium according to claim 4, characterized in that: The Langmuir-Blodgett method conditions are: surface pressure of 20-25 mN / m, pulling speed of 1-3 mm / min; drying temperature of 60-80° C., and drying time of 2-4 hours.

6. The method for preparing a nanoporous filtration separation medium according to claim 1, characterized in that: In step S3, a polyamic acid / N-methylpyrrolidone solution with a solid content of 12-18 wt% is vacuum-infused into the gaps between the nanospheres at an infusion pressure of -0.05~-0.08 MPa for 20-30 minutes, followed by gradient temperature increase for imidization to form an alumina-polyimide composite film; wherein the viscosity of the polyamic acid solution at 25°C is 1200±100 cP; the gradient temperature increase conditions are: heating from room temperature to 80°C at a heating rate of 5°C / min for 1 h, then heating to 150°C for 1 h, and finally heating to 280°C for 2 h.

7. The method for preparing a nanoporous filtration separation medium according to claim 1, characterized in that: In step S4, a phosphoric acid solution with a mass concentration of 10-15% is used to etch and remove the alumina nanospheres at 60-70° C. to obtain a through-hole polyimide nanoporous medium with a pore size of 300-350 nm; the etching time is 40-60 minutes, the pore size uniformity deviation is ≤±8%, and the porosity is ≥85%.

8. The method for preparing a nanoporous filtration separation medium according to claim 1, characterized in that: In step S5, the nanoporous medium is immersed in a 0.5-1.0 M ammonium persulfate aqueous solution and treated under ultraviolet light for 10-15 minutes to carboxylate the membrane surface and reduce the contact angle to below 35°.

9. The nanoporous filtration separation medium prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the nanoporous filtration separation medium according to claim 9 in the separation of buffalo milk lipids.

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