A reducing transparent nanofiber membrane and its preparation method and application

Abstract

The present invention relates to a reducing transparent nanofiber membrane, its preparation method, and application. The preparation method uses bacterial cellulose nanofiber membrane as a substrate, polycarboxylic acids / anhydrides, and biomass thiol compounds as raw materials, and chemically modifies the fiber membrane in the presence of a catalyst and a carboxyl activator to produce a transparent nanofiber membrane with reactive anti- / deoxidation properties for oils and fats. Compared to existing technologies, the present invention utilizes natural, low-toxic raw materials, mild reaction conditions, and simple steps, making it suitable for large-scale industrial production. The resulting transparent nanofiber membrane exhibits strong reducing properties, high stability, and a high specific surface area, without concerns about miscibility of oxidation inhibitors with edible oils or potential threats to human ingestion. Therefore, the membrane has broad application value in the field of food safety.

Description

Technical Field

[0001] The present invention relates to the field of polymer materials, and in particular to a reducing transparent nanofiber membrane and its preparation and application. Background Art

[0002] Because edible oils, especially plant-based oils (such as rapeseed oil, corn oil, and peanut oil), contain large amounts of unsaturated fatty acids, they are susceptible to oxidation and deterioration under the combined effects of light, oxygen, and high temperatures during storage and repeated or prolonged cooking. Peroxides have been found to be primary oxidation products in aged edible oils, which can further oxidize to form carcinogenic and genotoxic secondary oxidation products such as aldehydes and ketones. In food safety, the quality and safety of oils are evaluated by their peroxide value (PV). Excessively high PV values ​​in aged edible oils, i.e., excessive peroxide concentrations, can lead to excessive consumption of glutathione and related enzymes in animal cells, causing oxidative stress in the living system and thus endangering health. Studies have also shown that oxidized corn oil has a significantly higher PV value than unoxidized corn oil and can cause significant pathological differences in mice with DSS-induced enteritis.

[0003] To effectively inhibit the oxidation of edible oils during storage, researchers have explored the effects of various antioxidants, such as vitamin E, vitamin C, tea polyphenols, and curcumin, on the oil oxidation process. These antioxidants primarily act as reactive oxygen free radical scavengers to prevent the production of lipid peroxides, thereby effectively avoiding the formation of secondary oxidation products. However, antioxidants are often added directly to edible oils in the form of small molecules. This limitation, coupled with their miscibility with edible oils and potential toxicity upon human ingestion, limits their dosage and, consequently, their application. In response to this, researchers have developed a novel food packaging film material with sustained-release antioxidants. This material effectively regulates the release rate of antioxidants, achieving long-lasting antioxidant effects. However, this approach, which relies solely on antioxidants, also has drawbacks. Once food has undergone oxidative aging, these antioxidants lack the ability to deoxidize the aged oils, leading to significant waste in food logistics and storage, significantly reducing the economic benefits of food companies.

[0004] In order to make up for the limitations of antioxidant packaging materials, the development of new film materials with reactive anti- / deoxidation functions is of practical significance in ensuring food safety and food economic benefits. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a method for preparing a transparent nanofiber membrane with all-natural ingredients, easy preparation, and reactive anti- / de-oxidation properties for oily foods (such as edible oils).

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A first aspect of the present invention provides a method for preparing a reducing transparent nanofiber membrane, comprising the following steps:

[0008] Using bacterial cellulose nanofiber membrane as the substrate and polycarboxylic acids / anhydrides and biomass thiol compounds as raw materials, the fiber membrane is chemically modified under the action of catalysts and carboxyl activators to obtain a transparent nanofiber membrane with reactive anti- / deoxidation function for oils and fats.

[0009] Furthermore, the polycarboxylic acid / acid anhydride is selected from one of citric acid, 1,2,3,4-butanetetracarboxylic acid, malic acid, itaconic acid, 3,3',4,4'-benzophenonetetracarboxylic acid, 3,3',4,4'-benzophenonetetracarboxylic acid diglycoside, 1,2,4-benzenetricarboxylic acid, and 1,2,4-benzenetricarboxylic acid-1,2-anhydride ethylene ester;

[0010] The biomass thiol compound is selected from one of glutathione, cysteine, cysteamine and homocysteine.

[0011] Furthermore, the polycarboxylic acid / acid anhydride is selected from one of the compounds corresponding to the following structural formulas:

[0012]

[0013] Furthermore, the biomass thiol compound is selected from one of the compounds corresponding to the following structural formulas:

[0014]

[0015] Further, the following steps are included:

[0016] S1: using ethanol as a solvent, preparing an ethanol solution of a polycarboxylic acid / anhydride compound at a concentration of 25 to 120 g / L, adding a catalyst at a molar ratio of 1:0.1 to 1 to the polycarboxylic acid / anhydride compound to obtain a reaction solution A, and adjusting the pH of the reaction solution A to 1.5 to 3.5 using a sodium hydroxide aqueous solution;

[0017] S2: immersing the bacterial cellulose nanofiber membrane in the reaction solution A for 1 to 10 minutes, then drying, baking, and sequentially washing with distilled water and ethanol, and drying to obtain a fiber membrane A;

[0018] S3: Using PBS buffered aqueous solution as solvent, prepare a carboxyl activator solution with a concentration of 10-100 g / L to obtain reaction solution B;

[0019] S4: immersing the fiber membrane A in the reaction solution B, and performing carboxyl activation at room temperature for 10 to 60 minutes to obtain a fiber membrane B;

[0020] S5: using PBS buffered aqueous solution as solvent, preparing a biomass thiol solution with a concentration of 10-100 g / L to obtain reaction solution C;

[0021] S6: Immersing the fiber membrane B in the reaction solution C to perform a biomass thiol condensation reaction at a temperature of 4 to 25° C. for a reaction time of 0.5 to 3 hours to obtain a fiber membrane C;

[0022] S7: The fiber membrane C is washed with PBS buffer and ethanol in sequence and dried to obtain a reducing transparent nanofiber membrane.

[0023] Furthermore, in S1, the catalyst is selected from one of polyphosphoric acid, sodium hypophosphite, sodium dihydrogen phosphate, sodium pyrophosphate, trisodium phosphate, and disodium hydrogen phosphate.

[0024] Furthermore, in S1, the carboxyl activator is 1-ethyl-(3-dimethylaminopropyl)carbodiimide.

[0025] The second aspect of the present invention provides a reducing transparent nanofiber membrane obtained by the above preparation method.

[0026] The third aspect of the present invention provides an application of the above-mentioned reducing transparent nanofiber membrane in materials for inhibiting oil oxidation. The thiol groups on the transparent nanofiber membrane act as scavengers of active oxygen free radicals, inhibiting the redox reaction between active oxygen free radicals and unsaturated double bonds in oils and fats, causing oxidation and deterioration of the oils and fats.

[0027] The fourth invention of the present invention provides an application of the above-mentioned reducing transparent nanofiber membrane in the deoxidation material of aged oil, which removes lipid peroxides produced during the storage of edible oil through redox reaction, reduces the peroxide concentration in the oil, and avoids the generation of secondary oxidation products of the oil.

[0028] Compared with the prior art, the present invention has the following technical advantages:

[0029] The method provided by the present invention utilizes natural, low-toxic raw materials, mild reaction conditions, and simple procedures, making it suitable for large-scale industrial production. The transparent nanofiber membranes prepared using this method exhibit strong reducing properties, high stability, and a high specific surface area. These membranes eliminate concerns about the miscibility of oxidation inhibitors with edible oils and potential risks of human ingestion, thus possessing broad application value in the food safety field. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 1 is a schematic flow chart of the preparation process of Example 1;

[0031] Figure 2 is the reaction equation corresponding to Examples 1 to 4;

[0032] Figure 3 The effect of citric acid concentration on the ester bond and carboxylate concentrations on the modified bacterial cellulose nanofiber membrane during the preparation process of Example 1;

[0033] Figure 4 This is a curve showing the effect of baking time and temperature on ester bond formation in the citric acid esterification reaction during the preparation process of Example 1;

[0034] Figure 5 This is a curve showing the effect of baking time and temperature on carboxylate concentration in the citric acid esterification reaction during the preparation process of Example 1;

[0035] Figure 6 The effect of cysteamine concentration during the preparation process of Example 1 on the surface thiol concentration of the final material;

[0036] Figure 7 is an electron microscope image of the transparent nanofiber membrane prepared in Example 1;

[0037] Figure 8 is an infrared spectrum of the transparent nanofiber membrane prepared in Example 1;

[0038] Figure 9 The electron microscope-energy dispersive X-ray spectroscopy results of the transparent nanofiber membrane prepared under the preferred conditions of Example 1;

[0039] Figure 10 is the tensile stress-strain curve of different nanofiber membrane materials prepared in Example 1;

[0040] Figure 11 is the Young's modulus of different nanofiber membrane materials prepared in Example 1;

[0041] Figure 12 This is a diagram showing the mechanism of the transparent nanofiber membrane prepared in Example 1 for reactive deoxidation of oils and fats;

[0042] Figure 13 This is a time-dependent curve of the detoxification of cumene hydroperoxide, a representative lipid peroxide, by the transparent nanofiber membrane prepared in Example 1;

[0043] Figure 14 is a time-dependent histogram of the deoxidation performance of the transparent nanofiber membrane prepared in Example 1;

[0044] Figure 15 This is a time-dependent result diagram of the transparent nanofiber membrane prepared in Example 1's ability to inhibit oil oxidation. DETAILED DESCRIPTION

[0045] The method provided by the present invention uses bacterial cellulose nanofiber membrane as a substrate, polycarboxylic acid / acid anhydride and biomass thiol compound as raw materials, and sequentially carries out esterification and condensation reactions under the action of a catalyst and a carboxyl activator to obtain a transparent nanofiber membrane with reactive anti- / deoxidation function. Specifically, the polycarboxylic acid / acid anhydride compound described in the present invention can be citric acid, 1,2,3,4-butanetetracarboxylic acid, malic acid, itaconic acid, 3,3',4,4'-benzophenonetetracarboxylic acid, 3,3',4,4'-benzophenonetetracarboxylic acid diglycoside, 1,2,4-benzenetricarboxylic acid-1,2-anhydride ethylene ester, preferably citric acid; the thiol compound described in the present invention can be glutathione, cysteine, cysteamine, homocysteine, preferably cysteamine.

[0046] The structure of the polycarboxylic acid / anhydride compound of the present invention is as follows:

[0047]

[0048] The structure of the thiol compound preferably described in the present invention is as follows:

[0049]

[0050] The catalyst for the esterification reaction process of the present invention is polyphosphoric acid, sodium hypophosphite, sodium dihydrogen phosphate, sodium pyrophosphate, trisodium phosphate, and disodium hydrogen phosphate, preferably sodium hypophosphite.

[0051] The carboxyl group activating agent for the condensation reaction of the thiol compound of the present invention is preferably 1-ethyl-(3-dimethylaminopropyl)carbodiimide.

[0052] As a preferred embodiment of the present invention, the polycarboxylic acid / anhydride compound is citric acid, and the biomass thiol is cysteamine; the transparent nanofiber membrane with reactive anti- / deoxidation function is prepared by a method comprising the following steps:

[0053] (1) using ethanol as a solvent, preparing a polycarboxylic acid / acid anhydride compound ethanol solution with a concentration of 25 to 120 g / L, adding a catalyst with a molar ratio of 1:0.1 to 1 to the polycarboxylic acid / acid anhydride compound to obtain a reaction solution A, and adjusting the pH of the reaction solution to 1.5 to 3.5 using a sodium hydroxide aqueous solution;

[0054] (2) Bacterial cellulose nanofiber membrane (Guilin Qihong Technology) (3×3cm 2 ) Immerse in reaction solution A for 1 to 10 minutes;

[0055] (3) drying the fiber membrane in a fume hood;

[0056] (4) baking the dried fiber membrane at a temperature of 100 to 180° C. for 5 to 30 minutes;

[0057] (5) Wash the fiber membrane with distilled water and ethanol in sequence and dry it in a fume hood;

[0058] (6) Using PBS buffer solution (pH = 7.4) as solvent, prepare a carboxyl activator solution with a concentration of 10-100 g / L to obtain reaction solution B;

[0059] (7) Immersing the fiber membrane obtained in step (5) in reaction solution B, and performing carboxyl activation at room temperature for 10–60 minutes;

[0060] (8) Using PBS buffer solution (pH = 7.4) as a solvent, a biomass thiol solution with a concentration of 10 to 100 g / L was prepared to obtain reaction solution C;

[0061] (9) immersing the fiber membrane obtained in step (7) in the reaction solution C to carry out a biomass thiol condensation reaction at a reaction temperature of 4 to 25° C. and a reaction time of 0.5 to 3 hours;

[0062] (10) The final fiber membrane was washed with PBS buffer solution and ethanol in sequence and dried.

[0063] (11) The prepared fiber membrane was placed in a sealed vacuum bag and stored at low temperature (–4°C) until use.

[0064] The nanofiber membrane prepared by the present invention is translucent in dry state and becomes transparent in water or edible oil system; the nanofiber membrane has stable properties and is insoluble in solvents such as water, edible oil, ethanol, hexane, etc.

[0065] The transparent nanofiber membrane prepared by the present invention has a high thiol loading amount of 1400-5000 μmol / g, is stable in nature, and has good reducibility.

[0066] The present invention protects the use of the transparent nanofiber membrane in inhibiting grease oxidation and deoxidizing aged grease.

[0067] Specifically, the transparent nanofiber membrane prepared by the present invention uses the thiol groups on its surface as active oxygen free radical scavengers to achieve the purpose of inhibiting oil oxidation; the thiol groups on the surface of the transparent nanofiber membrane also act as reducing agents to remove lipid peroxides generated during the storage of oil through redox reactions.

[0068] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Any features such as preparation methods, materials, structures or composition ratios not explicitly described in this technical solution are considered to be common technical features disclosed in the prior art.

[0069] The abbreviations in the following examples have the following meanings: SHP stands for sodium hypophosphite; EDC stands for 1-ethyl-(3-dimethylaminopropyl)carbodiimide; BC stands for bacterial cellulose nanofiber membrane; BC-CA stands for nanofiber membrane modified with citric acid; BC-SH stands for nanofiber membrane modified with citric acid and cysteamine in sequence.

[0070] The infrared spectral data of the material obtained in Example 1 were measured using an infrared spectrometer (Nicolet 8700, ThermoScientific, USA); the electron microscopic spectrum of the material was measured using an electron microscope (Quattro ESEM, ThermoFisher Scientific, USA); and the thiol content of the material was measured using an UV-visible spectrophotometer (Evolution 600, Thermo Scientific, USA) using Ellman's reagent as a colorimetric probe.

[0071] Example 1

[0072] Modification of bacterial cellulose nanofiber membrane with citric acid and cysteamine:

[0073] The specific steps are:

[0074] (1) Using ethanol as a solvent, a 50 g / L citric acid ethanol solution was prepared, SHP was added at a molar ratio of 1:0.5 to citric acid, and the pH was adjusted to 2.5 by dropwise addition of sodium hydroxide solution to obtain reaction solution A;

[0075] (2) Bacterial cellulose nanofiber membrane (Guilin Qihong Technology) (3×3cm 2 ) was immersed in the reaction solution for 5 minutes;

[0076] (3) drying the fiber membrane in a fume hood;

[0077] (4) baking the dried fiber membrane at a temperature of 150° C. for 5 minutes;

[0078] (5) Wash the fiber membrane with distilled water and ethanol in sequence and dry it in a fume hood;

[0079] (6) Using PBS buffer solution (pH = 7.4) as solvent, prepare an 80 g / L EDC solution to obtain reaction solution B;

[0080] (7) Immersing the fiber membrane obtained in step (5) in reaction solution B, and performing carboxyl activation at room temperature for 30 minutes;

[0081] (8) Using PBS buffer solution (pH = 7.4) as solvent, prepare a cysteamine solution with a concentration of 80 g / L to obtain reaction solution C;

[0082] (9) Immerse the fiber membrane obtained in step (7) in reaction solution C to carry out cysteamine condensation reaction at a reaction temperature of 25° C. and a reaction time of 1 hour.

[0083] (10) The final fiber membrane is washed with PBS buffer and ethanol in sequence and dried.

[0084] (11) The prepared fiber membrane was placed in a sealed vacuum bag and stored at low temperature (–4°C) until use.

[0085] Example 2

[0086] Modification of bacterial cellulose nanofiber membrane with citric acid and glutathione:

[0087] The specific steps are:

[0088] (1) Using ethanol as a solvent, a 50 g / L citric acid ethanol solution was prepared, SHP was added at a molar ratio of 1:0.5 to citric acid, and the pH was adjusted to 2.5 by dropwise addition of sodium hydroxide solution to obtain reaction solution A;

[0089] (2) Bacterial cellulose nanofiber membrane (Guilin Qihong Technology) (3×3cm 2 ) was immersed in the reaction solution for 5 minutes;

[0090] (3) drying the fiber membrane in a fume hood;

[0091] (4) baking the dried fiber membrane at a temperature of 150° C. for 5 minutes;

[0092] (5) The fiber membrane was washed with distilled water and ethanol in sequence and dried in a fume hood;

[0093] (6) Using PBS buffer solution (pH = 7.4) as solvent, prepare an 80 g / L EDC solution to obtain reaction solution B;

[0094] (7) Immersing the fiber membrane obtained in step (5) in reaction solution B, and performing a carboxyl activation reaction at room temperature for 30 minutes;

[0095] (8) Using PBS buffer solution (pH = 7.4) as solvent, a glutathione solution with a concentration of 100 g / L was prepared to obtain reaction solution C;

[0096] (9) The fiber membrane obtained in step (7) is immersed in reaction solution C to carry out condensation reaction of the thiol compound. The reaction temperature is 25° C. and the reaction time is 1 hour.

[0097] (10) The final fiber membrane is washed with PBS buffer and ethanol in sequence and dried.

[0098] (11) The prepared fiber membrane was placed in a sealed vacuum bag and stored at low temperature (–4°C) until use.

[0099] Example 3

[0100] Modification of bacterial cellulose nanofiber membrane with citric acid and cysteine:

[0101] The specific steps are:

[0102] (1) Using ethanol as a solvent, a 50 g / L citric acid ethanol solution was prepared, SHP was added at a molar ratio of 1:0.5 to citric acid, and dissolved in ethanol. The pH was adjusted to 2.5 by dropwise addition of sodium hydroxide solution to obtain reaction solution A;

[0103] (2) Bacterial cellulose nanofiber membrane (Guilin Qihong Technology) (3×3cm 2 ) was immersed in the reaction solution for 5 minutes;

[0104] (3) drying the fiber membrane in a fume hood;

[0105] (4) baking the dried fiber membrane at a temperature of 150° C. for 5 minutes;

[0106] (5) Wash the fiber membrane with distilled water and ethanol in sequence and dry it in a fume hood;

[0107] (6) Using PBS buffer solution (pH = 7.4) as solvent, prepare an 80 g / L EDC solution to obtain reaction solution B;

[0108] (7) Immersing the fiber membrane obtained in step (5) in reaction solution B, and performing a carboxyl activation reaction at room temperature for 30 minutes;

[0109] (8) Using PBS buffer solution (pH = 7.4) as solvent, prepare cysteine ​​at a concentration of 85 g / L to obtain reaction solution C;

[0110] (9) The fiber membrane obtained in step (7) is immersed in reaction solution C to carry out condensation reaction of the thiol compound. The reaction temperature is 4° C. and the reaction time is 1 hour.

[0111] (10) The final fiber membrane is washed with PBS buffer and ethanol in sequence and dried.

[0112] (11) The prepared fiber membrane was placed in a sealed vacuum bag and stored at low temperature (–4°C) until use.

[0113] Example 4

[0114] Modification of bacterial cellulose nanofiber membrane with citric acid and homocysteine:

[0115] The specific steps are:

[0116] (1) Using ethanol as a solvent, a 50 g / L citric acid ethanol solution was prepared, SHP was added at a molar ratio of 1:0.5 to citric acid, and dissolved in ethanol. The pH was adjusted to 2.5 by dropwise addition of sodium hydroxide to obtain reaction solution A;

[0117] (2) Bacterial cellulose nanofiber membrane (Guilin Qihong Technology) (3×3cm 2 ) was immersed in the reaction solution for 5 minutes;

[0118] (3) drying the fiber membrane in a fume hood;

[0119] (4) baking the dried fiber membrane at a temperature of 150° C. for 5 minutes;

[0120] (5) Wash the fiber membrane with distilled water and ethanol in sequence and dry it in a fume hood;

[0121] (6) Using PBS buffer solution (pH = 7.4) as solvent, prepare an 80 g / L EDC solution to obtain reaction solution B;

[0122] (7) Immersing the fiber membrane obtained in step (5) in reaction solution B, and performing a carboxyl activation reaction at room temperature for 30 minutes;

[0123] (8) Using PBS buffer solution (pH = 7.4) as solvent, a homocysteine ​​solution with a concentration of 95 g / L was prepared to obtain reaction solution C;

[0124] (9) The fiber membrane obtained in step (7) is immersed in reaction solution C to carry out condensation reaction of the thiol compound. The reaction temperature is 4° C. and the reaction time is 1 hour.

[0125] (10) The final fiber membrane is washed with PBS buffer and ethanol in sequence and dried.

[0126] (11) The prepared fiber membrane was placed in a sealed vacuum bag and stored at low temperature (–4°C) until use.

[0127] Characterization and verification results

[0128] Figure 1Schematic diagram of the preparation process of the transparent nanofiber membrane using the above-mentioned Example 1 as an example; citric acid from natural sources is used as a hydroxyl activator; the modification of cellulose hydroxyl groups by polycarboxylic acids increases the sites on the unit active hydroxyl groups that can be subsequently grafted with thiol compounds, which helps to optimize the final functionality of the nanofiber membrane.

[0129] Figure 2 The chemical reaction diagram of the preparation process of the above Examples 1 to 4 is shown. Citric acid is used as a natural, edible hydroxyl activator to achieve the grafting modification of cysteamine, glutathione, cysteine ​​and homocysteine ​​on the surface of bacterial cellulose nanofibers.

[0130] In order to explore the optimal conditions for the preparation of biomass thiol-modified bacterial cellulose nanofiber membrane (BC-SH) in Example 1, the applicant screened the citric acid concentration, citric acid baking temperature and time, and cysteamine concentration. Figure 3 As shown in the figure, the baking temperature of the citric acid modification step was controlled at 120℃ and the baking time was 20min, and the ester bonds (1731cm -1 / 2900cm -1 ) and sodium carboxylate (1585cm -1 / 2900cm -1 ) relative absorption intensity. As citric acid concentration increased, both ester bond and sodium carboxylate concentrations increased; however, the increase in sodium carboxylate was more pronounced than in ester bond concentration. This indicates that under these baking conditions, citric acid prefers single-end grafting to the cellulose surface rather than double-end crosslinking, which facilitates the provision of more reactive sites (i.e., free carboxyl groups) for subsequent cysteamine grafting. Ultimately, balancing the single-end grafting efficiency of citric acid and compound utilization, 80 g / L was selected as the optimal citric acid concentration.

[0131] Figure 4 and Figure 5 The relative infrared absorption intensities of the ester bond and sodium carboxylate at different baking temperatures and times at a citric acid concentration of 80 g / L were demonstrated, allowing for further optimization of the citric acid modification reaction. Taking into account factors such as single-end citric acid grafting efficiency, free carboxyl group concentration, and energy consumption, the optimal baking temperature and time were 150°C and 5 minutes, respectively.

[0132] Further screening of cysteamine concentrations was performed to optimize the preparation of the final functional nanofiber membrane. First, a condensation reaction was performed on citric acid-modified bacterial cellulose nanofiber membrane (BC-CA) using a cysteamine concentration of 10 g / L, 20 g / L, 50 g / L, 80 g / L, and 100 g / L, controlling the molar ratio of the carboxyl activator EDC to cysteamine. The thiol concentrations on the resulting BC-SH nanofiber membrane were quantitatively characterized using Ellman's reagent.

[0133] The Ellman reagent test steps are:

[0134] (1) Prepare Ellman's reagent concentrate: Dissolve Ellman's reagent powder in 0.1 M sodium phosphate, 1 mM EDTA buffer solution to a concentration of 4 mg / mL. Store the prepared concentrate at 4°C for no more than 7 days.

[0135] (2) Accurately weigh a certain mass of nanofiber membrane (~2 mg) and place it in 20 mL of 0.1 M sodium phosphate, 1 mM EDTA buffer solution, and then add 400 μL of Ellman's concentrate;

[0136] (3) After the sample was placed in the dark for 15 minutes, the absorption intensity of the solution at 412 nm was tested by UV-visible spectrophotometer. 412 =1.6959×C SH +0.0576,R 2 =0.9996) to obtain the thiol concentration (mM) in the buffer solution;

[0137] (4) Calculate the thiol content (μmol / g) on ​​the fiber membrane based on the buffer volume and fiber membrane concentration.

[0138] like Figure 6 As shown in the results, as the cysteamine concentration varied within the range of 20–50 g / L, the thiol content on the fiber membrane increased significantly, from 1500 μmol / g to 3943 μmol / g. The thiol content peaked at 5039 μmol / g at 80 g / L. Based on this, the optimal cysteamine concentration was 80 g / L.

[0139] Figure 7 The UV-visible light transmission spectra and optical images of the nanofiber membrane prepared under the preferred conditions of Example 1, in the dry state, soaked in water, and soaked in corn oil, respectively. The images show that after modification with citric acid and cysteamine, the nanofiber membrane becomes translucent in the dry state; however, after soaking in water or oil, the nanofiber membrane becomes transparent, making it suitable for transparent food packaging and edible oil packaging.

[0140] The microscopic morphology of the nanofiber membrane prepared under the preferred conditions of Example 1 is as follows Figure 8 Electron micrographs of the modified bacterial cellulose nanofiber membranes (BC membranes) show no significant change in size compared to unmodified bacterial cellulose nanofiber membranes (BC membranes), with fiber diameters ranging from 80 to 100 nm. The fiber surface is also rougher than that of the unmodified membranes. Furthermore, the modified membranes exhibit intact microstructures, with no apparent fiber defects or breakage, demonstrating that the mild modification reaction conditions do not significantly affect the membrane's strength.

[0141] Figure 9 The following are infrared spectra of the nanofiber membranes at various stages of the preparation process under the preferred conditions of Example 1. Specifically, the BC membrane was directly tested for infrared spectroscopy; the BC-CA membrane was washed with 0.01M NaOH solution at room temperature for 5 minutes to convert the carboxylic acids on the membrane surface to carboxylates, separating their characteristic peaks from those of ester bonds. The membrane was then dried in a 60°C vacuum oven before being tested for infrared spectroscopy; the cysteamine-modified BC-SH membrane was dried under nitrogen and then directly tested for infrared spectroscopy.

[0142] Figure 9 As shown, BC-CA is higher than BC at 1731 cm -1 and 1585cm -1 New peaks appeared at , representing ester bonds and sodium carboxylates in the nanofiber membrane, respectively. This demonstrates that citric acid was covalently grafted onto the nanofiber membrane via ester bonds, retaining a limited number of free carboxyl groups, providing reactive sites for the subsequent thiol modification. The infrared spectrum of the BC-SH membrane remained unchanged after cysteamine modification. This is due to the low grafting amount of cysteamine, which obscures its characteristic infrared peaks from the bacterial cellulose backbone signal. Therefore, it is crucial to verify the successful grafting of thiol compounds through other means.

[0143] Figure 10 SEM-EDS analysis was used to characterize the changes in the concentration and types of elements on the surface of the fiber membranes before and after modification. In the BC membrane sample, the concentrations of carbon and oxygen were 86.57 wt% and 13.43 wt%, respectively, while no sulfur was detected. After modification with citric acid and then cysteamine, the surface contents of the BC-SH membrane were 69.99 wt%, 24.38 wt%, 3.77 wt%, and 1.85 wt%, respectively. The presence of nitrogen and sulfur confirmed the successful modification of the fiber membrane surface with cysteamine.

[0144] The tensile strength tests of the nanofiber membranes prepared at each stage in Example 1, including the BC membrane, BC-CA membrane, BC-SH membrane, and edible oil-soaked BC-SH membrane, were conducted according to the following parameters:

[0145] (1) The fiber membrane sample is in the shape of a dumbbell, with a clamping distance of 40 mm, a clamping width of 16 mm, a parallel width of 8 mm, and a parallel length of 10 mm;

[0146] (2) The stretching speed is 5 mm / min;

[0147] (3) The test endpoint is the tensile fracture of the fiber membrane.

[0148] The stress-strain curves of various fiber membranes prepared under the preferred conditions of Example 1 are as follows: Figure 11 All fiber membranes experienced an elastic phase, a yield phase, a strengthening phase, and finally a fracture phase. Compared to BC and BC-CA membranes, the thiol-modified BC-SH membrane exhibited a certain decrease in strength both in the dry state and under edible oil immersion conditions, with its fracture stress at ~23,000 kPa, a decrease of approximately 20%. This may be because the carboxyl activator EDC used in the thiol grafting process activates the carboxyl groups while also causing further crosslinking of the hydroxyl groups and free carboxyl groups on the citric acid.

[0149] according to Figure 11 The slope of the elastic phase of the stress-strain curve of the fiber membrane is fitted to obtain the Young's modulus of different fiber membranes, which further characterizes the mechanical properties of the material. Figure 12 As shown in Figure 3, the Young's modulus values ​​of the BC, BC-CA, and BC-SH films range from 4.9 to 6.1 MPa. After being soaked in oil, the Young's modulus of the BC-SH film decreases to 4.4 MPa, a decrease of approximately 28%.

[0150] Through a variety of experimental characterization methods, it was proved that the thiol group was successfully modified on the surface of the BC membrane. Thanks to the strong reducing property of the thiol group, it can be used to inhibit the oxidation of edible oil or the reduction of lipid peroxides in aged edible oil. The specific application mechanism is as follows Figure 13 As shown in Figure 2, two moles of thiol groups on the BC-SH membrane reduce one mole of lipid peroxide to alcohol through a redox reaction, and the thiol is oxidized to form a disulfide bond.

[0151] First, the reactivity of the BC-SH membrane with peroxides was tested by adding known concentrations of cumene hydroperoxide (C-OOH) to edible oils (such as peanut oil) as a lipid peroxide model, verifying its feasibility in deoxidizing aged edible oils. Specifically, ~20 mg of BC-SH membrane was immersed in 2 mL of fresh peanut oil containing 70 mmol / L C-OOH. After different immersion times, the C-OOH concentration in the peanut oil system was measured to assess the BC-SH membrane's ability to reduce peroxides.

[0152] The concentration of peroxides in solvents or edible oils is determined by the following colorimetric test methods:

[0153] (1) Process formula of the indicator: the "solvent" is 2:1 v / v chloroform / methanol; solution D is 3.94 mol / L ammonium thiocyanate aqueous solution; solution E is a mixture of 0.144 mol / L ferrous sulfate aqueous solution and 0.132 mol / L barium chloride dissolved in 0.4 mol / L HCl solution in a ratio of 1:1 v / v; indicator D and indicator E are mixed in a ratio of 1:1 v / v to obtain a peroxide indicator.

[0154] (2) Peroxide detection: Take 100 μL of edible oil sample and dilute it in 5 mL of "solvent" to make sample S1; take 200 μL of sample S1 and dilute it in 3.14 mL of "solvent" to make sample S2; add 34.5 μL of peroxide indicator to sample S2 and let it stand at room temperature in a dark environment for 20 minutes.

[0155] (3) The absorption intensity of sample S2 at 510 nm was measured by UV-visible spectrophotometer. The standard curve (A) obtained by C-OOH standard solution was used. 510 =0.0237×C OOH +0.156,R 2 =0.9985) to calculate the peroxide concentration in the sample.

[0156] like Figure 14 As shown, the BC-SH membrane's reduction reaction of C-OOH in an edible oil environment reached equilibrium within 2 hours, with an equilibrium detoxification capacity of 348 mg / g. The rapid reduction of C-OOH in edible oil by the BC-SH membrane prepared under the preferred conditions of Example 1 demonstrates its feasibility for deoxidation of aged edible oils.

[0157] Based on this, the BC-SH membrane was further used to test its deoxidation performance on four commonly used edible oils.

[0158] The four types of edible oils must first be artificially aged. The specific steps are as follows:

[0159] (1) Olive oil, peanut oil, sunflower oil, and rapeseed oil freshly purchased from the supermarket were sealed in 50 mL centrifuge tubes, aerated with nitrogen, and stored at –80°C as blank samples (before oxidation) until use;

[0160] (2) Transfer four edible oils into 50 mL centrifuge tubes and store them in an oven at 37°C for 10 days.

[0161] (3) Nitrogen was introduced into the oxidized edible oil and the oil was sealed and stored at –80°C until use.

[0162] Approximately 20 mg of BC-SH membrane was immersed in 2 mL of four edible oils, either before or after oxidation. After immersion for varying times (4 or 20 hours), the peroxide concentrations in the oils were measured. The peroxide indicator method was used to quantify the changes in lipid peroxide concentrations in the edible oils before and after BC-SH membrane treatment, thus evaluating the performance of BC-SH membranes in deoxidation applications of aged oils.

[0163] like Figure 15 As shown, the three edible oils except olive oil contain less lipid peroxides (i.e., lower PV values) when freshly opened. The lipid peroxide concentrations before oxidation are 13.27 mg / L for olive oil, 2.99 mg / L for peanut oil, 0.274 mg / L for sunflower oil, and 0.184 mg / L for rapeseed oil; after artificial oxidation at 37°C for 7 days, the lipid peroxide concentrations increase to 22.62 mg / L for olive oil, 14.14 mg / L for peanut oil, 9.92 mg / L for sunflower oil, and 10.93 mg / L for rapeseed oil, verifying that the lipid peroxide concentration in edible oils increases under high temperature and aerobic conditions.

[0164] After 2 or 20 hours of treatment with the BC-SH membrane, the lipid peroxide concentrations in the four edible oils were significantly reduced. Compared to the lipid peroxide concentrations in the edible oils before oxidation, the BC-SH membrane achieved deoxidation efficiencies of 77.75% for olive oil, 94.26% for peanut oil, 72.26% for sunflower oil, and 59.93% for rapeseed oil after 20 hours of standing at room temperature.

[0165] The deoxidation efficiency of BC-SH membrane is calculated by formula (1):

[0166]

[0167] The strong reducing properties of BC-SH membranes also suggest potential applications in inhibiting edible oil oxidation. Therefore, ~20 mg of BC-SH membrane was placed in 2 mL of pre-oxidized peanut oil and stored open at 37°C for 10 days. Another 2 mL of pre-oxidized peanut oil was stored in the same environment for 10 days as a control group. A peroxide indicator was used to quantify changes in lipid peroxide concentrations in the different systems.

[0168] The BC-SH membrane can significantly inhibit the oxidation of peanut oil and reduce the concentration of lipid peroxides in the system. The oil oxidation inhibition rate of the BC-SH membrane is calculated by formula (2) to be 72.92%. The experimental results show that the BC-SH membrane has a significant inhibitory effect on edible oil oxidation.

[0169]

[0170] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for preparing a reducing transparent nanofiber membrane, characterized in that: The following steps are involved: Using bacterial cellulose nanofiber membrane as the substrate, polycarboxylic acids / anhydrides and biomass thiol compounds as raw materials, the fiber membrane is chemically modified under the action of catalysts and carboxyl activators to obtain a transparent nanofiber membrane with reactive anti- / deoxidation function for oils and fats; The specific steps include: S1: Using ethanol as a solvent, a polycarboxylic acid / anhydride compound ethanol solution with a concentration of 25-120 g / L is prepared, and a catalyst is added at a molar ratio of 1:0.1-1 to the polycarboxylic acid / anhydride compound to obtain a reaction solution A. The pH of the reaction solution A is adjusted to 1.5-3.5 using a sodium hydroxide aqueous solution; S2: immersing the bacterial cellulose nanofiber membrane in the reaction solution A for 1 to 10 minutes, then drying, baking, and sequentially washing with distilled water and ethanol, and drying to obtain a fiber membrane A; S3: Using PBS buffered water as solvent, prepare a carboxyl activator solution with a concentration of 10-100 g / L to obtain reaction solution B; S4: immersing the fiber membrane A in the reaction solution B, and performing carboxyl activation at room temperature for 10 to 60 minutes to obtain a fiber membrane B; S5: Using PBS buffered water as solvent, prepare a biomass thiol solution with a concentration of 10-100 g / L to obtain reaction solution C; S6: immersing the fiber membrane B in the reaction solution C to perform a biomass thiol condensation reaction at a temperature of 4 to 25°C for a reaction time of 0.5 to 3 hours to obtain a fiber membrane C; S7: The fiber membrane C is washed with PBS buffer and ethanol in sequence and dried to obtain a reducing transparent nanofiber membrane.

2. The method for preparing a reducing transparent nanofiber membrane according to claim 1, characterized in that: The polycarboxylic acid / acid anhydride is selected from one of citric acid, 1,2,3,4-butanetetracarboxylic acid, malic acid, itaconic acid, 3,3',4,4'-benzophenonetetracarboxylic acid, 3,3',4,4'-benzophenonetetracarboxylic acid diglycoside, 1,2,4-benzenetricarboxylic acid, and 1,2,4-benzenetricarboxylic acid-1,2-anhydride ethylene ester; The biomass thiol compound is selected from one of glutathione, cysteine, cysteamine and homocysteine.

3. The method for preparing a reducing transparent nanofiber membrane according to claim 1, characterized in that: In S1, the catalyst is selected from one of polyphosphoric acid, sodium hypophosphite, sodium dihydrogen phosphate, sodium pyrophosphate, trisodium phosphate, and disodium hydrogen phosphate.

4. The method for preparing a reducing transparent nanofiber membrane according to claim 1, characterized in that: In S1, the carboxyl activating agent is 1-ethyl-(3-dimethylaminopropyl)carbodiimide.

5. A reducing transparent nanofiber membrane obtained by the preparation method according to any one of claims 1 to 4.

6. A use of the reducing transparent nanofiber membrane as claimed in claim 5 in a material for inhibiting oil oxidation, characterized in that: The thiol groups on the transparent nanofiber membrane act as scavengers of active oxygen free radicals, inhibiting the redox reaction between active oxygen free radicals and unsaturated double bonds in oils and fats, which causes oxidation and deterioration of oils and fats.

7. An application of the reducing transparent nanofiber membrane as claimed in claim 5 in deoxidation materials for aged oils and fats, characterized in that: The thiol groups on the surface of the transparent nanofiber membrane act as reducing agents, removing lipid peroxides produced during the storage of edible oil through redox reactions, reducing the peroxide concentration in the oil and avoiding the formation of secondary oxidation products of the oil.

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