A composite filter aid

Abstract

This invention belongs to the technical field of filtration aids for food-grade enzyme preparation production, specifically relating to a composite filter aid comprising food-grade diatomaceous earth, food-grade expanded perlite, microcrystalline cellulose, food-grade silica powder, food-grade calcium carbonate, food-grade magnesium carbonate, hydroxypropyl methylcellulose, a diatomaceous earth porous framework compound modified with a low-adsorption sodium alginate pectin silica hydration layer for glucose oxidase, a perlite microcrystalline cellulose chitosan tannic acid-bridged gradient pore modification compound, and food-grade silica. Its preparation includes preparation of a basic mixture, preparation of a composite mixture, drying and cooling, and sieving. This invention forms a loose and stable filter cake, reduces filtration resistance, and improves the filtration speed, filtrate clarity, and enzyme activity retention rate of the glucose oxidase production solution.

Description

Technical Field

[0001] This invention belongs to the technical field of filtration aids for food-grade enzyme preparation production, and specifically relates to a composite filtration aid. Background Technology

[0002] Glucose oxidase is an important industrial enzyme preparation widely used in food preservation, baking quality improvement, juice clarification, beer clarification, feed processing, pharmaceutical diagnostics, and biocatalysis. Its production typically involves microbial fermentation. The enzyme solution after fermentation often contains mycelium, cell debris, unused culture medium components, colloidal polysaccharides, soluble proteins, pigments, inorganic salts, and a large number of fine suspended particles. Direct subsequent concentration, purification, or formulation processing can easily lead to problems such as filter cloth clogging, filter bed compaction, increased filtration resistance, decreased filtration speed, and turbidity of the filtrate. Furthermore, some impurities may affect the stability of glucose oxidase and the appearance of the product. Therefore, in the production of glucose oxidase, how to improve filtration efficiency, increase filtrate clarity, and maintain a high enzyme activity recovery rate through appropriate filter aids is a key technical problem that needs to be solved in the industrial production of enzyme preparations.

[0003] Commonly used filter aids in enzyme production include diatomaceous earth, perlite, cellulose, and their blends. Diatomaceous earth, with its porous structure, low density, large specific surface area, good chemical stability, and strong filter cake support, is a commonly used filter aid. Perlite, with its low density, porosity, and good liquid permeability, can improve the pore structure of the filter cake. Cellulose-based materials possess flexibility and bridging capabilities, which are beneficial for improving the integrity of the filter cake. However, traditional filter aids typically rely on single inorganic channels or simple physical mixing, making it difficult to simultaneously achieve optimal filtration rate, filtrate clarity, and glucose oxidase activity retention. Especially in fermentation broths with high viscosity, high solids content, or fluctuating mycelial debris content, ordinary diatomaceous earth filter cakes are prone to localized compaction or pore blockage. While coarse-particle filter aids can increase liquid permeability, their clarification effect is insufficient. Fine-particle filter aids, while beneficial for trapping fine impurities, can easily increase filtration resistance, leading to insufficient production stability.

[0004] Furthermore, glucose oxidase is a protein-based active substance. The hydroxyl groups, charged groups, and microporous surfaces of ordinary silica materials may non-specifically adsorb onto the enzyme protein, leading to enzyme activity loss. For food-grade enzyme production, filter aids not only need excellent filtration performance but also require attention to raw material sources, residue risks, heavy metal control, leachate control, and subsequent filter cake removal. Most existing filter aids only emphasize filtration speed or cost advantages, failing to adequately consider reducing enzyme protein adsorption, improving the gradient pore structure of the filter cake, enhancing the filter cake's resistance to compaction, and meeting the comprehensive requirements of food-grade glucose oxidase production. Therefore, it is necessary to develop a composite filter aid based on food-grade porous inorganic and cellulose materials, incorporating low-enzyme-adsorption hydrophilic modified structures and bridging gradient pore structures, to achieve good clarity while improving filtration speed and minimizing glucose oxidase activity loss. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a composite filter aid comprising the following components in parts by weight: 35-55 parts food-grade diatomaceous earth, 10-25 parts food-grade expanded perlite, 5-15 parts microcrystalline cellulose, 3-10 parts food-grade silica powder, 2-8 parts food-grade calcium carbonate, 1-5 parts food-grade magnesium carbonate, 0.3-1.5 parts hydroxypropyl methylcellulose, 8-18 parts of a glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound, 6-16 parts of a perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modification compound, and 0.5-2 parts food-grade silica.

[0006] According to a preferred embodiment of the present invention, the preparation steps of the composite filter aid include:

[0007] S1. By weight, mix 35-55 parts of food-grade diatomaceous earth, 10-25 parts of food-grade expanded perlite, 3-10 parts of food-grade silica micropowder, 2-8 parts of food-grade calcium carbonate, and 1-5 parts of food-grade magnesium carbonate to obtain a basic mixture; add 5-15 parts of microcrystalline cellulose, 0.3-1.5 parts of hydroxypropyl methylcellulose, 8-18 parts of glucose oxidase low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound, and 6-16 parts of perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modification compound to the basic mixture, mix, and obtain a composite mixture;

[0008] S2. Add 0.5-2 parts of food-grade silica to the composite mixture, mix, and obtain a mixed powder; dry the mixed powder at 55-65℃, cool, and sieve.

[0009] In this invention, the preparation process of the composite filter aid mainly involves the physical compounding of porous inorganic particles, cellulose-based materials, hydrophilically modified diatomaceous earth materials, and bridging gradient pore materials. Food-grade diatomaceous earth, food-grade expanded perlite, food-grade silica powder, food-grade calcium carbonate, and food-grade magnesium carbonate are first mixed to form a basic mixture. The food-grade diatomaceous earth provides a porous siliceous framework and an interface for impurity trapping; the food-grade expanded perlite provides low-density coarse-pore support; the food-grade silica powder is used to adjust the fine particle size distribution; and the food-grade calcium carbonate and food-grade magnesium carbonate further enhance the support of the filter cake and maintain the mild and stable nature of the system. Subsequently, microcrystalline cellulose, hydroxypropyl methylcellulose, a glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound, and a perlite-microcrystalline cellulose-chitosan-tannic acid bridging gradient pore modification compound were added to the base mixture. The chain structures of microcrystalline cellulose and hydroxypropyl methylcellulose act as bridges and dispersion aids between particles. The glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound introduces a hydrophilic hydration layer with low enzyme protein adsorption into the system. The perlite-microcrystalline cellulose-chitosan-tannic acid bridging gradient pore modification compound introduces a gradient pore framework that resists compaction. Further addition of food-grade silica to the composite mixture allows it to adhere to the surface of the composite powder, further improving the powder's flowability and the uniformity of fine particle distribution. This results in a stable filter aid particle system formed after drying, cooling, and sieving. During use, this composite filter aid involves the interpenetration of food-grade diatomaceous earth, food-grade expanded perlite, microcrystalline cellulose, food-grade silica powder, food-grade silica, sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compounds with low adsorption of glucose oxidase, and perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compounds. These compounds, together with mycelial fragments, colloidal impurities, and fine suspended matter in the material to be filtered, form a loose and stable filter cake. The coarse pores in the loose filter cake ensure the flow capacity, the hydrophilic hydration layer reduces the non-specific adsorption of glucose oxidase on the surface of silica particles, and the bridged gradient pore structure effectively reduces the compaction of the filter cake under pressure difference, thereby synergistically improving the filtration speed, filtrate clarity, and glucose oxidase activity retention rate.

[0010] According to a preferred embodiment of the present invention, in step S1, the mixing time of food-grade diatomaceous earth, food-grade expanded perlite, food-grade silica powder, food-grade calcium carbonate and food-grade magnesium carbonate is 10-20 minutes.

[0011] According to a preferred embodiment of the present invention, in step S2, the product is dried at 55-65°C until the moisture content is ≤3%.

[0012] According to a preferred embodiment of the present invention, the preparation method of the glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomite porous framework compound includes:

[0013] A1. By weight, add 100 parts of food-grade diatomaceous earth to 180-260 parts of deionized water, stir, filter, wash, dry at 100-110℃, and sieve to obtain pretreated food-grade diatomaceous earth; add 2-6 parts of food-grade sodium alginate, 1-4 parts of food-grade low-methoxyl pectin, and 0.1-0.3 parts of food-grade sodium citrate to 800-1200 parts of deionized water, stir at 35-45℃ to obtain a composite solution; add the pretreated food-grade diatomaceous earth to the composite solution, stir at 30-40℃, add 5-15 parts of food-grade silica sol, and continue stirring at 30-40℃ to obtain a diatomaceous earth-polysaccharide-silica composite dispersion;

[0014] A2. Dissolve 0.5-2 parts of food-grade calcium chloride in 20-60 parts of deionized water to obtain a calcium chloride aqueous solution; add the calcium chloride aqueous solution dropwise to the diatomaceous earth-polysaccharide-silica composite dispersion, control the pH to 5.0-6.0, stir, filter, wash, dry with hot air at 45-55℃, then vacuum dry at 55-65℃, pulverize, and sieve.

[0015] In this invention, during the preparation of the glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound, after stirring, filtering, washing, drying, and sieving with deionized water, the soluble impurities and loose fine powder adhering to the particle surface of the food-grade diatomaceous earth are effectively removed, allowing the porous framework and surface silanol sites of the diatomaceous earth to be fully exposed, facilitating interfacial adsorption with subsequent hydrophilic components. After adding food-grade sodium alginate, food-grade low-methoxy pectin, and food-grade sodium citrate to deionized water, the food-grade sodium alginate and food-grade low-methoxy pectin each unfold into anionic polymer chains in the aqueous phase, while the food-grade sodium citrate has a mild chelating effect on the calcium ions released by the subsequently added food-grade calcium chloride, reducing the risk of local aggregation caused by the instantaneous strong cross-linking of calcium ions with polysaccharide segments, thereby obtaining a uniform composite solution. After pretreated food-grade diatomaceous earth is added to the composite solution, the carboxyl and hydroxyl groups on the chains of food-grade sodium alginate and food-grade low-methoxyl pectin combine with the silanol groups on the surface of food-grade diatomaceous earth through hydrogen bonding and electrostatic adsorption, forming a polysaccharide adsorption layer on the surface and at the pore entrance of the food-grade diatomaceous earth. After the addition of food-grade silica sol, the silanol groups on the surface of the sol particles associate with the food-grade sodium alginate, food-grade low-methoxyl pectin, and the surface of food-grade diatomaceous earth through hydrogen bonding, becoming embedded around the siliceous framework. When food-grade calcium chloride aqueous solution is added dropwise to the diatomaceous earth-polysaccharide-silica composite dispersion, calcium ions undergo ion bridging with the carboxyl groups of the guluronic acid segment in food-grade sodium alginate and the carboxyl groups of galacturonic acid in food-grade low-methoxyl pectin, gradually fixing the polysaccharide layer originally adsorbed on the surface of the food-grade diatomaceous earth. Under weakly acidic conditions, food-grade low-methoxyl pectin forms continuous cross-linked regions with calcium ions, while the guluronic acid segments of food-grade sodium alginate form an egg-box structure with calcium ions. Simultaneously, food-grade silica sol particles moderately aggregate due to the compression of the electrical double layer and are confined to the surface of food-grade diatomaceous earth by the polysaccharide-calcium cross-linked layer, forming a composite hydration layer composed of polysaccharides, silica, and food-grade diatomaceous earth. After filtration, washing, drying, pulverizing, and sieving, the compound retains the porous framework of food-grade diatomaceous earth while forming a composite layer rich in hydrophilic groups on the surface. This reduces the direct contact between glucose oxidase and the exposed silica surface, thereby minimizing the non-specific adsorption of enzyme proteins.

[0016] According to a preferred embodiment of the present invention, in step A1, the drying time at 100-110°C is 3-5 hours; the stirring time at 35-45°C is 50-70 minutes.

[0017] According to a preferred embodiment of the present invention, in step A2, the hot air drying time at 45-55°C is 6-8 hours; the vacuum drying time at 55-65°C is 1.5-2.5 hours.

[0018] According to a preferred embodiment of the present invention, the preparation method of the perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compound includes:

[0019] B1. By weight, add 100 parts of food-grade expanded perlite to 180-260 parts of deionized water, stir, filter, wash, dry at 95-105℃, and sieve to obtain pretreated food-grade expanded perlite; add 20-40 parts of microcrystalline cellulose to 240-720 parts of deionized water, stir at 20-30℃ to obtain a microcrystalline cellulose dispersion; add 3-8 parts of food-grade chitosan to an aqueous solution of 180-720 parts of acetic acid, and stir at 30℃. Stirring at -35℃ yields a chitosan solution; 2-6 parts tannic acid are added to 50-210 parts deionized water and stirred at 20-30℃ to obtain a tannic acid solution; pretreated food-grade expanded perlite is added to the microcrystalline cellulose dispersion and stirred at 25-35℃, then the chitosan solution is added, followed by a food-grade sodium bicarbonate aqueous solution. The pH is adjusted to 4.5-5.5, and the mixture is stirred at 30-40℃ to obtain a perlite-microcrystalline cellulose-chitosan composite dispersion.

[0020] B2. Add the tannic acid solution dropwise to the perlite-microcrystalline cellulose-chitosan composite dispersion and stir at 25-35℃; control the pH to 4.5-5.5, allow to stand for aging, filter, wash, dry at 40-50℃, then dry again at 65-75℃, pulverize, and sieve.

[0021] In this invention, during the preparation of the perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compound, after food-grade expanded perlite is stirred, filtered, washed, dried, and sieved with deionized water, the floating powder and soluble impurities on the particle surface are reduced, and the porous structure and surface hydroxyl sites are exposed, which is beneficial to the adhesion of subsequent components. Microcrystalline cellulose, after being added to deionized water, forms a microcrystalline cellulose dispersion, and the microcrystalline cellulose particles form a flexible dispersion network through the interaction of hydroxyl groups between molecular chains and mechanical bonding. After food-grade chitosan is added to an aqueous solution of acetic acid, the amino groups on the food-grade chitosan molecular chains are protonated to form positively charged ammonium groups, thereby maintaining sufficient dispersion in a weakly acidic aqueous phase, resulting in a chitosan solution. Tannic acid, after being added to deionized water, forms a tannic acid solution; the phenolic hydroxyl groups on the tannic acid molecules have the ability to interact with the surfaces of food-grade chitosan, microcrystalline cellulose, and food-grade expanded perlite at multiple points. After the pretreated food-grade expanded perlite is added to the microcrystalline cellulose dispersion, the microcrystalline cellulose adheres to the surface of the food-grade expanded perlite and forms bonding between the particles. After adding chitosan solution, the protonated amino groups on the food-grade chitosan chains bind to the negatively charged sites on the surface of food-grade expanded perlite through electrostatic adsorption, and then bind to the hydroxyl groups on the microcrystalline cellulose chains through hydrogen bonding, thus depositing on the surface of the food-grade expanded perlite and microcrystalline cellulose. Adding food-grade sodium bicarbonate aqueous solution and adjusting the pH to a weakly acidic range partially neutralizes the free acetic acid in the solution, reducing the acidity of the system while maintaining the food-grade chitosan in a dispersed and adsorbed state, preventing excessive precipitation due to the loss of protonated amino groups in excessively high pH conditions. After adding tannic acid solution dropwise to the perlite-microcrystalline cellulose-chitosan composite dispersion, the phenolic hydroxyl groups in the tannic acid undergo hydrogen bonding and electrostatic interactions with the amino groups on the food-grade chitosan chains, and simultaneously form hydrogen bonds with the hydroxyl groups on the surfaces of microcrystalline cellulose and food-grade expanded perlite. This multi-point bridging assembles the previously loosely adsorbed food-grade expanded perlite, microcrystalline cellulose, and food-grade chitosan into a more stable composite structure. Maintaining the system's pH within a slightly acidic range prevents rapid precipitation of food-grade chitosan due to its near-neutral pH, while also facilitating the formation of a more uniform composite deposition of tannic acid and chitosan. During static aging, the bridging structure further stabilizes, and a multi-scale porous framework composed of food-grade expanded perlite, microcrystalline cellulose, food-grade chitosan, and tannic acid gradually takes shape. After filtration, washing, drying, pulverizing, and sieving, the resulting modified compound possesses both a rigid framework and a bridging structure, exhibiting a gradient pore distribution and good resistance to filter cake compaction.

[0022] According to a preferred embodiment of the present invention, in step B1, the drying time at 95-105°C is 2.5-3.5 h; and the stirring time at 30-35°C is 1.5-2.5 h.

[0023] According to a preferred embodiment of the present invention, in step B2, the stirring time at 25-35°C is 80-100 min; the standing aging time is 1.5-2.5 h; the drying time at 40-50°C is 5-7 h; and the drying time at 65-75°C is 1.5-2.5 h.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The composite filter aid of the present invention comprises a basic porous support system consisting of food-grade diatomaceous earth, food-grade expanded perlite, food-grade silica powder, food-grade calcium carbonate, and food-grade magnesium carbonate, which can form a filter cake with liquid flow capacity and support strength during filtration. Food-grade diatomaceous earth provides a porous siliceous skeleton, which is beneficial for trapping mycelial fragments, colloidal impurities, and fine suspended matter in glucose oxidase production liquid; food-grade expanded perlite reduces the bulk density of the filter cake and improves the flow path of the filtrate; food-grade silica powder improves the fine particle gradation and enhances the ability to capture fine impurities; food-grade calcium carbonate and food-grade magnesium carbonate improve the support of the filter cake and help maintain a mild filtration environment. The combination of the above components can reduce filtration resistance, reduce local compaction of the filter cake, and improve filtration speed and filtration stability.

[0026] (2) This invention introduces a porous diatomaceous earth framework compound modified with a sodium alginate-pectin-silica hydration layer to reduce glucose oxidase adsorption, enabling the composite filter aid to possess both porous retention capacity and low enzyme protein adsorption performance. In the sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound, food-grade sodium alginate, food-grade low-methoxyl pectin, food-grade sodium citrate, food-grade silica sol, and food-grade calcium chloride form a hydrophilic hydration layer on the surface of food-grade diatomaceous earth. This reduces the direct contact between glucose oxidase and the exposed silica surface, thereby reducing the non-specific adsorption of glucose oxidase on the filter aid surface. Compared with ordinary food-grade diatomaceous earth, this invention improves clarity while maintaining glucose oxidase activity and reducing the effective enzyme loss caused by filter aid adsorption.

[0027] (3) This invention further introduces a perlite-microcrystalline cellulose-chitosan-tannic acid bridging gradient pore modification compound, and combines it with microcrystalline cellulose, hydroxypropyl methylcellulose, and food-grade silica to form a loose filter cake with synergistic coarse and fine pores during the filtration process. The perlite-microcrystalline cellulose-chitosan-tannic acid bridging gradient pore modification compound can enhance the bridging and pore connectivity between particles, while microcrystalline cellulose and hydroxypropyl methylcellulose are beneficial for improving the integrity of the filter cake, and food-grade silica is beneficial for improving the flowability and dispersion uniformity of the powder. Therefore, this invention can balance filtration speed, filtrate clarity, and glucose oxidase activity retention rate, and is suitable for the filtration treatment of glucose oxidase production liquid and related food-grade enzyme preparation intermediate liquid. Detailed Implementation

[0028] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0029] Example 1

[0030] This embodiment provides a method for preparing a composite filter aid, the steps of which include:

[0031] S1. Add 45g of food-grade diatomaceous earth, 17.5g of food-grade expanded perlite, 6.5g of food-grade silica powder, 5g of food-grade calcium carbonate, and 3g of food-grade magnesium carbonate to a three-dimensional mixer and mix for 15 minutes to obtain a basic mixture. Add 10g of microcrystalline cellulose, 0.9g of hydroxypropyl methylcellulose, 13g of glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound, and 11g of perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modification compound to the basic mixture and continue mixing for 25 minutes to obtain a composite mixture.

[0032] S2. Add 1.25g of food-grade silica to the composite mixture and continue mixing for 8 minutes to obtain a mixed powder. Spread the mixed powder evenly on a stainless steel tray with a thickness of 10mm, place it in a forced-air drying oven, and dry it at 60℃ until the moisture content is ≤3%. After drying, cool it to 25℃ and sieve it through a 60-mesh sieve to obtain the composite filter aid.

[0033] Preparation of porous diatomaceous earth framework compounds modified with sodium alginate-pectin-silica hydration layer and low adsorption of glucose oxidase:

[0034] A1. Add 100g of food-grade diatomaceous earth to 220g of deionized water and place it in a glass reaction vessel equipped with a mechanical stirrer. Stir at 300r / min for 30min at 25℃. After stirring, filter through a filter cloth and collect the filter cake. Wash the filter cake with 200g of deionized water twice, filtering through a filter cloth after each wash to obtain a washed filter cake. Spread the washed filter cake evenly on a stainless steel tray with a thickness of 10mm and place it in a forced-air drying oven. Dry at 105℃ for 4h. After drying, cool to 25℃ and sieve through an 80-mesh sieve to obtain pretreated food-grade diatomaceous earth. Add 4g of food-grade sodium alginate and 2.5g of food-grade... Low-methoxyl pectin and 0.2g of food-grade sodium citrate were added to 1000g of deionized water and placed in a glass reaction vessel equipped with a mechanical stirrer. The mixture was stirred at 400r / min for 60min at 40℃ to fully disperse the food-grade sodium alginate and low-methoxyl pectin, resulting in a composite solution. All of the pretreated food-grade diatomaceous earth was added to the composite solution, and the mixture was stirred at 350r / min for 45min at 35℃ to fully wet the pretreated food-grade diatomaceous earth. Subsequently, 10g of food-grade silica sol was added, and the mixture was stirred at 350r / min for another 60min at 35℃ to obtain a diatomaceous earth-polysaccharide-silica composite dispersion.

[0035] A2. Add 1.25g of food-grade calcium chloride to 40g of deionized water and stir at 300r / min for 10min at 25℃ until the calcium chloride is completely dissolved, obtaining a calcium chloride aqueous solution. Add the calcium chloride aqueous solution dropwise to the diatomaceous earth-polysaccharide-silica composite dispersion over 25min, stirring at 350r / min during the addition and controlling the pH of the system to 5.5. After the addition is complete, continue stirring at 350r / min at 35℃ for 60min, then filter using a filter cloth and collect the filter cake. Use 30... The filter cake was washed three times with 0g of deionized water, and filtered through a filter cloth after each wash to obtain the washed filter cake. The washed filter cake was spread evenly on a stainless steel tray with a thickness of 8mm and placed in a forced-air drying oven at 50℃ for 7h. Then it was transferred to a vacuum drying oven and vacuum dried at 60℃ for 2h at a vacuum degree of -0.08MPa. After drying, it was cooled to 25℃ and pulverized using a pulverizer. The pulverized powder was then sieved through a 60-mesh sieve to obtain a porous diatomaceous earth framework compound modified with a low adsorption capacity of glucose oxidase by sodium alginate-pectin-silica hydration layer.

[0036] Preparation of perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compounds:

[0037] B1. Add 100g of food-grade expanded perlite to 220g of deionized water and place it in a glass reaction vessel equipped with a mechanical stirrer. Stir at 250r / min for 30min at 25℃. After stirring, filter through a filter cloth and collect the filter cake. Wash the filter cake with 200g of deionized water twice, filtering through a filter cloth after each wash to obtain a washed filter cake. Spread the washed filter cake evenly on a stainless steel tray with a thickness of 10mm and place it in a forced-air drying oven to dry at 100℃ for 3h. After drying, cool to 25℃ and sieve through a 60-mesh sieve to obtain pretreated food-grade expanded perlite. Add 30g of microcrystalline cellulose to 480g of deionized water and place it in a glass reaction vessel equipped with a mechanical stirrer. Stir at 400r / min for 30min at 25℃ to obtain a microcrystalline cellulose dispersion. Add 5.5g of food-grade chitosan to 450g of acetic acid. In an aqueous solution, the mixture was stirred at 400 r / min for 2 h at 32.5 °C to obtain a chitosan solution. The aqueous solution of acetic acid was prepared by mixing 4.5 g of acetic acid and 445.5 g of deionized water. 4 g of tannic acid was added to 130 g of deionized water and stirred at 300 r / min for 15 min at 25 °C to obtain a tannic acid solution. Pretreated food-grade expanded perlite was added to a microcrystalline cellulose dispersion and stirred at 350 r / min for 40 min at 30 °C. The chitosan solution was then added and stirred at 350 r / min for another 20 min at 30 °C. Subsequently, a food-grade sodium bicarbonate aqueous solution was added to adjust the pH to 5.0, and the mixture was stirred at 350 r / min for 60 min at 35 °C to obtain a perlite-microcrystalline cellulose-chitosan composite dispersion. The food-grade sodium bicarbonate aqueous solution was prepared by mixing 1 g of food-grade sodium bicarbonate and 99 g of deionized water.

[0038] B2. Tannic acid solution was added dropwise to perlite-microcrystalline cellulose-chitosan composite dispersion over 30 minutes, with stirring at 350 rpm at 30°C during the addition. After addition, stirring was continued at 350 rpm at 30°C for 90 minutes. The pH of the system was controlled at 5.0, followed by static aging at 25°C for 2 hours. After aging, the mixture was filtered through a filter cloth, and the filter cake was collected. The filter cake was washed three times with 300 g of deionized water, and filtered through a filter cloth after each wash to obtain a washed filter cake. The washed filter cake was spread evenly on a stainless steel tray with a thickness of 8 mm and placed in a forced-air drying oven at 45°C for 6 hours, followed by drying at 70°C for 2 hours. After drying, the mixture was cooled to 25°C and pulverized using a pulverizer. The pulverized mixture was then sieved through a 60-mesh sieve to obtain the perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compound.

[0039] Example 2

[0040] The difference between this embodiment and Embodiment 1 is that this embodiment provides a method for preparing a composite filter aid, the steps of which include:

[0041] S1. Add 35g of food-grade diatomaceous earth, 10g of food-grade expanded perlite, 3g of food-grade silica powder, 2g of food-grade calcium carbonate, and 1g of food-grade magnesium carbonate to a mixing device and mix for 10 minutes to obtain a basic mixture. Add 5g of microcrystalline cellulose, 0.3g of hydroxypropyl methylcellulose, 8g of glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound, and 6g of perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modification compound to the basic mixture and continue mixing for 20 minutes to obtain a composite mixture.

[0042] S2. Add 0.5g of food-grade silica to the composite mixture and mix for 5 minutes to obtain a mixed powder. Place the mixed powder in a forced-air drying oven and dry it at 55℃ until the moisture content is ≤3%. Cool it to 25℃ and sieve it to obtain the composite filter aid.

[0043] Preparation of porous diatomaceous earth framework compounds modified with sodium alginate-pectin-silica hydration layer and low adsorption of glucose oxidase:

[0044] A1. Add 100g of food-grade diatomaceous earth to 180g of deionized water, stir at 25℃ for 20min, filter, wash twice with deionized water, place the washed filter cake in a forced-air drying oven, dry at 100℃ for 3h, cool and sieve to obtain pretreated food-grade diatomaceous earth; add 2g of food-grade sodium alginate, 1g of food-grade low-methoxyl pectin and 0.1g of food-grade sodium citrate to 800g of deionized water, stir at 35℃ for 50min to fully disperse the food-grade sodium alginate and food-grade low-methoxyl pectin and form a uniform solution to obtain a composite solution; add the pretreated food-grade diatomaceous earth to the composite solution, stir at 30℃ for 40min to fully wet the pretreated food-grade diatomaceous earth and enter its pores and surface structure; then add 5g of food-grade silica sol, continue stirring at 30℃ for 50min to obtain a diatomaceous earth-polysaccharide-silica composite dispersion.

[0045] A2. Dissolve 0.5g of food-grade calcium chloride in 20g of deionized water to obtain a calcium chloride aqueous solution. Add the calcium chloride aqueous solution dropwise to a diatomaceous earth-polysaccharide-silica composite dispersion, controlling the pH at 5.0 during the dropwise addition. After the dropwise addition is complete, continue stirring for 50min, filter, and wash with deionized water to obtain a washed filter cake. Place the washed filter cake in a forced-air drying oven and dry at 45℃ for 6h, then transfer it to a vacuum drying oven and vacuum dry at 55℃ for 1.5h. After cooling, pulverize and sieve to obtain a glucose oxidase low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound.

[0046] Preparation of perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compounds:

[0047] B1. Add 100g of food-grade expanded perlite to 180g of deionized water, stir at 25℃ for 20min, filter, wash twice with deionized water, place the washed filter cake in a forced-air drying oven, dry at 95℃ for 2.5h, cool and sieve to obtain pretreated food-grade expanded perlite; add 20g of microcrystalline cellulose to 240g of deionized water, stir at 20℃ for 25min to obtain microcrystalline cellulose dispersion; add 3g of food-grade chitosan to 180g of acetic acid aqueous solution. Chitosan solution was obtained by stirring at 30℃ for 1.5 h; 2 g of tannic acid was added to 50 g of deionized water and stirred at 20℃ for 10 min to obtain tannic acid solution; pretreated food-grade expanded perlite was added to microcrystalline cellulose dispersion and stirred at 25℃ for 35 min, then chitosan solution was added and stirring continued until the system was homogeneous; then food-grade sodium bicarbonate aqueous solution was added to adjust the pH to 4.5, and stirred at 30℃ for 50 min to obtain perlite-microcrystalline cellulose-chitosan composite dispersion.

[0048] B2. Add tannic acid solution dropwise to perlite-microcrystalline cellulose-chitosan composite dispersion and stir at 25℃ for 80 min; control pH at 4.5, let stand for aging for 1.5 h, filter, wash with deionized water to obtain washed filter cake; place the washed filter cake in a forced-air drying oven and dry at 40℃ for 5 h, then dry at 65℃ for 1.5 h, cool, pulverize, and sieve to obtain perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compound.

[0049] Example 3

[0050] The difference between this embodiment and Embodiment 1 is that this embodiment provides a method for preparing a composite filter aid, the steps of which include:

[0051] S1. Add 55g of food-grade diatomaceous earth, 25g of food-grade expanded perlite, 10g of food-grade silica powder, 8g of food-grade calcium carbonate, and 5g of food-grade magnesium carbonate to a mixing device and mix for 20 minutes to obtain a basic mixture. Add 15g of microcrystalline cellulose, 1.5g of hydroxypropyl methylcellulose, 18g of glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound, and 16g of perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modification compound to the basic mixture and continue mixing for 30 minutes to obtain a composite mixture.

[0052] S2. Add 2g of food-grade silica to the composite mixture and mix for 10min to obtain a mixed powder. Place the mixed powder in a forced-air drying oven and dry it at 65℃ until the moisture content is ≤3%. Cool it to 25℃ and sieve it to obtain the composite filter aid.

[0053] Preparation of porous diatomaceous earth framework compounds modified with sodium alginate-pectin-silica hydration layer and low adsorption of glucose oxidase:

[0054] A1. Add 100g of food-grade diatomaceous earth to 260g of deionized water, stir at 25℃ for 40min, filter, wash twice with deionized water, place the washed filter cake in a forced-air drying oven, dry at 110℃ for 5h, cool and sieve to obtain pretreated food-grade diatomaceous earth; add 6g of food-grade sodium alginate, 4g of food-grade low-methoxyl pectin and 0.3g of food-grade sodium citrate to 1200g of deionized water, stir at 45℃ for 70min to fully disperse the food-grade sodium alginate and food-grade low-methoxyl pectin and form a homogeneous solution to obtain a composite solution; add the pretreated food-grade diatomaceous earth to the composite solution, stir at 40℃ for 50min to fully wet the pretreated food-grade diatomaceous earth and enter its pores and surface structure; then add 15g of food-grade silica sol, continue stirring at 40℃ for 70min to obtain a diatomaceous earth-polysaccharide-silica composite dispersion.

[0055] A2. Dissolve 2g of food-grade calcium chloride in 60g of deionized water to obtain a calcium chloride aqueous solution. Add the calcium chloride aqueous solution dropwise to a diatomaceous earth-polysaccharide-silica composite dispersion, controlling the pH at 6.0 during the dropwise addition. After the dropwise addition is complete, continue stirring for 70min, filter, and wash with deionized water to obtain a washed filter cake. Place the washed filter cake in a forced-air drying oven and dry at 55℃ for 8h, then transfer it to a vacuum drying oven and vacuum dry at 65℃ for 2.5h. After cooling, pulverize and sieve to obtain a glucose oxidase low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound.

[0056] Preparation of perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compounds:

[0057] B1. Add 100g of food-grade expanded perlite to 260g of deionized water, stir at 25℃ for 40min, filter, wash twice with deionized water, place the washed filter cake in a forced-air drying oven, dry at 105℃ for 3.5h, cool and sieve to obtain pretreated food-grade expanded perlite; add 40g of microcrystalline cellulose to 720g of deionized water, stir at 30℃ for 35min to obtain microcrystalline cellulose dispersion; add 8g of food-grade chitosan to 720g of acetic acid aqueous solution. Chitosan solution was obtained by stirring at 35℃ for 2.5 h; 6 g of tannic acid was added to 210 g of deionized water and stirred at 30℃ for 20 min to obtain tannic acid solution; pretreated food-grade expanded perlite was added to microcrystalline cellulose dispersion and stirred at 35℃ for 45 min, then chitosan solution was added and stirring continued until the system was homogeneous; then food-grade sodium bicarbonate aqueous solution was added to adjust the pH to 5.5, and stirred at 40℃ for 70 min to obtain perlite-microcrystalline cellulose-chitosan composite dispersion.

[0058] B2. Add tannic acid solution dropwise to perlite-microcrystalline cellulose-chitosan composite dispersion and stir at 35℃ for 100 min; control pH at 5.5, let stand for 2.5 h, filter, wash with deionized water to obtain the washed filter cake; place the washed filter cake in a forced-air drying oven and dry at 50℃ for 7 h, then dry at 75℃ for 2.5 h, cool, pulverize, and sieve to obtain perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compound.

[0059] Comparative Example 1

[0060] The difference between this comparative example and Example 1 is that the glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomite porous framework compound is not added, and 13g of food-grade diatomite is used to replace 13g of glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomite porous framework compound. The rest is the same as in Example 1.

[0061] Comparative Example 2

[0062] The difference between this comparative example and Example 1 is that the perlite-microcrystalline cellulose-chitosan-tannic acid bridging gradient pore modification compound is not added, and 11g of food-grade expanded perlite is used instead of 11g of perlite-microcrystalline cellulose-chitosan-tannic acid bridging gradient pore modification compound. The rest is the same as in Example 1.

[0063] Comparative Example 3

[0064] The difference between this comparative example and Example 1 is that the glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomite porous framework compound and the perlite-microcrystalline cellulose-chitosan-tannic acid bridging gradient pore modification compound are not added. In addition, 13g of food-grade diatomite is used to replace 13g of glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomite porous framework compound, and 11g of food-grade expanded perlite is used to replace 11g of perlite-microcrystalline cellulose-chitosan-tannic acid bridging gradient pore modification compound. The rest is the same as in Example 1.

[0065] The performance of the composite filter aids provided in the above embodiments and comparative examples was tested using the following methods:

[0066] The same batch of glucose oxidase production liquid was used as the filtrate. Before the test, the filtrate was placed in a constant temperature environment of 25°C for 30 minutes to equilibrate. After being slowly stirred evenly with a clean glass rod, it was divided into portions, with 2000g of the filtrate taken from each group. The composite filter aids prepared in Examples 1, 2, 3, Comparative Examples 1, 2, and 3 were used as test samples. Before the test, each test sample was placed in a sealed desiccator for 2 hours to allow its temperature to equilibrate to 25°C.

[0067] Weigh 40g of the test sample (2% of the mass of the filtrate) and add it to 2000g of the filtrate. Stir with a paddle stirrer at 100r / min for 10min, maintaining the temperature at 25℃ during stirring to obtain the filtrate mixture. Use an experimental plate and frame filter with the same filter cloth, filtration area, and assembly method for constant pressure filtration. The effective filtration area is 0.02m², the filtration pressure is 0.20MPa, and the filtration temperature is 25℃. Before filtration, wet the filter cloth with 200g of deionized water and purge the air from the device. Then, add the filtrate mixture to the filter device all at once, start timing, and collect the filtrate. Record the volume of filtrate obtained within 10min after the start of filtration. Calculate the filtration flux per unit filtration area using the formula: "filtrate volume within 10min ÷ effective filtration area ÷ filtration time", in L / (m²·h).

[0068] Ten minutes after the start of filtration, 10 mL of the stable filtrate was taken and placed in a cuvette. Using deionized water as a blank, the transmittance of the filtrate was measured at 550 nm. Each sample was measured three times and the average value was taken. The unit is .

[0069] Ten minutes after filtration begins, take 20 mL of the stable filtrate and place it in the turbidity meter sample bottle, avoiding air bubbles adhering to the bottle wall. After standing for 1 minute, measure the turbidity of the filtrate. Measure each sample three times and take the average value. The unit is NTU.

[0070] Take the filtrate before filtration and the stable filtrate 10 minutes after filtration begins, respectively, and measure the enzyme activity using the same glucose oxidase activity assay method. Calculate the glucose oxidase activity retention rate as "glucose oxidase activity in the filtrate after filtration ÷ glucose oxidase activity in the filtrate before filtration × 100%", with units of 1.

[0071] During the filtration process, the filtrate volume was recorded for 0-2 minutes after the start of filtration and 2 minutes before the end of filtration. The average flux for the corresponding stage was calculated, and the flux retention rate at the end of filtration was calculated as "average flux for 2 minutes before the end of filtration ÷ average flux for 0-2 minutes after the start of filtration × 100%", with the unit being 1.

[0072] Take 5g of each test sample and place it in a pre-weighed weighing dish. Dry it at 105℃ until the difference between two consecutive weighings is ≤0.002g. Calculate the moisture content according to the formula: "mass before drying minus mass after drying ÷ mass before drying × 100%".

[0073] Take 100g of each test sample and place it in a 60-mesh standard sieve. Use a vibrating sieve machine to sieve for 10 minutes. Weigh the mass of powder that passes through the sieve. Calculate the 60-mesh sieve passing rate as "mass of powder passing through the 60-mesh sieve ÷ mass of sample before sieving × 100%".

[0074] After filtration, open the filter device, remove the filter cake completely, and observe whether the filter cake is intact, whether there are cracks, whether there is local compaction and signs of decreased liquid permeability. Record the state of the filter cake as "loose and intact, not obviously compacted", "loose and intact, slightly compacted", "relatively intact, locally compacted", "relatively intact, obviously compacted" or "obviously compacted, poor liquid permeability". Each of the above tests was performed in parallel 3 times, and the results in the table are the average of the 3 tests.

[0075] The performance test data above are shown in Table 1.

[0076] Table 1: Performance Test Results

[0077] Test Project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 <![CDATA[Filtration flux per unit filtration area (L / (m 2 ·h))]]> 286 268 301 251 214 196 550nm transmittance (%) 94.6 93.8 95.1 90.7 92.3 88.9 Filtrate turbidity (NTU) 3.8 4.5 3.4 8.6 6.9 12.4 Glucose oxidase activity retention rate (%) 96.8 95.9 97.2 91.6 95.1 90.3 Flux retention rate at the end of filtration (%) 88.5 86.9 89.2 80.4 72.6 68.3 Moisture (%) 2.4 2.7 2.2 2.5 2.6 2.5 Passing rate of 60-mesh sieve (%) 96.8 96.1 97.0 96.5 96.3 96.4 Filter cake state Loose and intact, not obviously compacted Loose and intact, slightly compacted Loose and intact, not obviously compacted Relatively complete, locally compacted Relatively intact, with obvious compaction Significant compaction and poor fluid permeability

[0078] As can be seen from the above, Examples 1-3 improved the filtration speed, filtrate clarity, glucose oxidase activity retention rate, and filter cake stability compared to Comparative Examples 1-3. This indicates that the present invention solves the problems of insufficient filtration speed, filtrate turbidity, enzyme activity loss, and filter cake compaction that are easily encountered in the filtration of glucose oxidase production liquid by existing ordinary filter aids.

[0079] Specifically, the filtration flux per unit filtration area in Examples 1-3 is 286 L / (m²). 2 ·h), 268L / (m 2 ·h) and 301L / (m 2The concentrations of ·h) were significantly higher than those of Comparative Example 1 (251 L / (m³)). 2 ·h), 214L / (m²) of Comparative Example 2 2 ·h) and Comparative Example 3 196L / (m 2 In Comparative Example 2, without the addition of the perlite-microcrystalline cellulose-chitosan-tannic acid bridging gradient pore modification compound, the filtration flux increased from 286 L / (m²) in Example 1. 2 The volume of water (·h) decreased to 214 L / (m³). 2 The result (·h) indicates that the modified compound can enhance the pore connectivity and compaction resistance of the filter cake.

[0080] Comparative Example 3, without the addition of either of the two modifying compounds (a diatomaceous earth porous framework compound modified with low adsorption of glucose oxidase using sodium alginate-pectin-silica hydration layer and a perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modifying compound), had the lowest filtration flux, only 196 L / (m³). 2 The result (·h) indicates that it is difficult to form a stable high-throughput filter cake by relying solely on conventional components such as food-grade diatomaceous earth, food-grade expanded perlite, and microcrystalline cellulose.

[0081] The 550nm transmittance of Examples 1-3 were 94.6%, 93.8%, and 95.1%, respectively, and the filtrate turbidity was 3.8 NTU, 4.5 NTU, and 3.4 NTU, respectively. These values ​​were all superior to those of Comparative Example 1 (90.7% and 8.6 NTU), Comparative Example 2 (92.3% and 6.9 NTU), and Comparative Example 3 (88.9% and 12.4 NTU). This demonstrates that the present invention, through the synergistic effect of the glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomite porous framework compound and the perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modification compound, not only improves the retention capacity of fine suspended impurities and colloidal impurities but also avoids excessive filter cake density leading to a decrease in flux.

[0082] The glucose oxidase activity retention rates of Examples 1-3 were 96.8%, 95.9%, and 97.2%, respectively, which were significantly higher than those of Comparative Example 1 (91.6%) and Comparative Example 3 (90.3%). This indicates that when the porous framework compound of diatomaceous earth modified with low-adsorption sodium alginate-pectin-silica hydration layer is lacking, glucose oxidase is more likely to undergo non-specific adsorption on ordinary silica surfaces, leading to enzyme activity loss. The glucose oxidase activity retention rate of Comparative Example 2 was 95.1%, which is close to that of the Examples, but its filtration flux and the flux retention rate at the end of filtration were significantly reduced. This indicates that the low-adsorption modified diatomaceous earth structure alone is insufficient to solve the problems of filter cake compaction and filtration rate decay.

[0083] The flux retention rates at the end of filtration in Examples 1-3 were 88.5%, 86.9%, and 89.2%, respectively, all significantly higher than those in Comparative Example 1 (80.4%), Comparative Example 2 (72.6%), and Comparative Example 3 (68.3%). Furthermore, the filter cakes in Examples 1-3 were loose and intact or slightly compacted, while Comparative Examples 2 and 3 showed significant compaction and poor flowability, respectively. This further demonstrates that the perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compound plays a crucial role in maintaining the loose structure and filtration stability of the filter cake.

[0084] The moisture content of Examples 1-3 was 2.4%, 2.7% and 2.2% respectively, all of which met the requirement of moisture content ≤3%. The passing rates through the 60-mesh sieve were 96.8%, 96.1% and 97.0% respectively, which also met the requirement of 95% full passing through the 60-mesh sieve. This shows that the present invention maintains good product quality stability and powder particle size consistency while improving filtration performance.

[0085] In summary, Examples 1-3, compared with Comparative Examples 1-3, demonstrate that the present invention, through the synergistic effect of two modified compounds (a glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound and a perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compound) with food-grade diatomaceous earth, food-grade expanded perlite, microcrystalline cellulose, food-grade silica powder, and other components, solves the technical problem that existing composite filter aids cannot simultaneously achieve filtration speed, filtrate clarity, glucose oxidase activity retention rate, and filter cake compaction stability.

Claims

1. A composite filter aid, characterized in that, The product comprises the following components in parts by weight: 35-55 parts food-grade diatomaceous earth, 10-25 parts food-grade expanded perlite, 5-15 parts microcrystalline cellulose, 3-10 parts food-grade silica powder, 2-8 parts food-grade calcium carbonate, 1-5 parts food-grade magnesium carbonate, 0.3-1.5 parts hydroxypropyl methylcellulose, 8-18 parts of a glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound, 6-16 parts perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modification compound, and 0.5-2 parts food-grade silica.

2. The composite filter aid of claim 1, wherein, The preparation steps of the composite filter aid include: S1. By weight, mix 35-55 parts of food-grade diatomaceous earth, 10-25 parts of food-grade expanded perlite, 3-10 parts of food-grade silica micropowder, 2-8 parts of food-grade calcium carbonate, and 1-5 parts of food-grade magnesium carbonate to obtain a basic mixture; add 5-15 parts of microcrystalline cellulose, 0.3-1.5 parts of hydroxypropyl methylcellulose, 8-18 parts of glucose oxidase low-adsorption sodium alginate-pectin-silica hydration layer modified diatomaceous earth porous framework compound, and 6-16 parts of perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modification compound to the basic mixture, mix, and obtain a composite mixture; S2. Add 0.5-2 parts of food-grade silica to the composite mixture, mix, and obtain a mixed powder; dry the mixed powder at 55-65℃, cool, and sieve.

3. The composite filter aid of claim 2, wherein, In step S1, the food-grade diatomaceous earth, food-grade expanded perlite, food-grade silica powder, food-grade calcium carbonate, and food-grade magnesium carbonate are mixed for 10-20 minutes.

4. The composite filter aid of claim 2, wherein, In step S2, the product is dried at 55-65℃ until the moisture content is ≤3%.

5. The composite filter aid according to claim 1, characterized in that, The preparation method of the glucose oxidase-low-adsorption sodium alginate-pectin-silica hydration layer modified diatomite porous framework compound includes: A1. By weight, add 100 parts of food-grade diatomaceous earth to 180-260 parts of deionized water, stir, filter, wash, dry at 100-110℃, and sieve to obtain pretreated food-grade diatomaceous earth; add 2-6 parts of food-grade sodium alginate, 1-4 parts of food-grade low-methoxyl pectin, and 0.1-0.3 parts of food-grade sodium citrate to 800-1200 parts of deionized water, stir at 35-45℃ to obtain a composite solution; add the pretreated food-grade diatomaceous earth to the composite solution, stir at 30-40℃, add 5-15 parts of food-grade silica sol, and continue stirring at 30-40℃ to obtain a diatomaceous earth-polysaccharide-silica composite dispersion; A2. Dissolve 0.5-2 parts of food-grade calcium chloride in 20-60 parts of deionized water to obtain a calcium chloride aqueous solution; add the calcium chloride aqueous solution dropwise to the diatomaceous earth-polysaccharide-silica composite dispersion, control the pH to 5.0-6.0, stir, filter, wash, dry with hot air at 45-55℃, then vacuum dry at 55-65℃, pulverize, and sieve.

6. The composite filter aid according to claim 5, characterized in that, In step A1, the drying time at 100-110℃ is 3-5 hours; the stirring time at 35-45℃ is 50-70 minutes.

7. The composite filter aid according to claim 5, characterized in that, In step A2, the hot air drying time at 45-55℃ is 6-8 hours; the vacuum drying time at 55-65℃ is 1.5-2.5 hours.

8. The composite filter aid according to claim 1, characterized in that, The preparation method of the perlite-microcrystalline cellulose-chitosan-tannic acid bridged gradient pore modified compound includes: B1. By weight, add 100 parts of food-grade expanded perlite to 180-260 parts of deionized water, stir, filter, wash, dry at 95-105℃, and sieve to obtain pretreated food-grade expanded perlite; add 20-40 parts of microcrystalline cellulose to 240-720 parts of deionized water, stir at 20-30℃ to obtain a microcrystalline cellulose dispersion; add 3-8 parts of food-grade chitosan to an aqueous solution of 180-720 parts of acetic acid, and stir at 30℃. Stirring at -35℃ yields a chitosan solution; 2-6 parts tannic acid are added to 50-210 parts deionized water and stirred at 20-30℃ to obtain a tannic acid solution; pretreated food-grade expanded perlite is added to the microcrystalline cellulose dispersion and stirred at 25-35℃, then the chitosan solution is added, followed by the addition of food-grade sodium bicarbonate aqueous solution. The pH is adjusted to 4.5-5.5, and the mixture is stirred at 30-40℃ to obtain a perlite-microcrystalline cellulose-chitosan composite dispersion. B2. Add the tannic acid solution dropwise to the perlite-microcrystalline cellulose-chitosan composite dispersion and stir at 25-35℃; control the pH to 4.5-5.5, allow to stand for aging, filter, wash, dry at 40-50℃, then dry again at 65-75℃, pulverize, and sieve.

9. The composite filter aid according to claim 8, characterized in that, In step B1, the drying time at 95-105℃ is 2.5-3.5h; the stirring time at 30-35℃ is 1.5-2.5h.

10. The composite filter aid according to claim 8, characterized in that, In step B2, the stirring time at 25-35℃ is 80-100 min; the standing aging time is 1.5-2.5 h; the drying time at 40-50℃ is 5-7 h; and the drying time at 65-75℃ is 1.5-2.5 h.