A chromogenic hydrogel for detecting the freshness of vegetable oil, and its preparation method and application

Abstract

The present invention discloses a chromogenic hydrogel for detecting the freshness of vegetable oils, and its preparation method and application, belonging to the technical field of food freshness detection. The preparation method of the chromogenic hydrogel for detecting the freshness of vegetable oils of the present invention comprises the following steps: dispersing nanocellulose obtained by TEMPO oxidation treatment with acrylamide, potassium persulfate, and tetramethylethylenediamine in an aqueous solution, and allowing to stand for cross-linking to obtain a nanocellulose-acrylamide composite hydrogel; mixing ascorbic acid, ferrous sulfate, and sulfosalicylic acid to form a peroxide indicator; allowing the nanocellulose-acrylamide composite hydrogel and the peroxide indicator to stand for adsorption to obtain a chromogenic hydrogel; the chromogenic hydrogel, as a vegetable oil freshness indicator, has a larger specific surface area and a higher water content than traditional film-type indicators, and has the advantages of fast response speed, more sensitive color development, and stronger stability; and has the advantages of low price, no need for complex instruments, and simple and convenient operation.

Description

Technical Field

[0001] The invention relates to a chromogenic hydrogel for detecting the freshness of vegetable oil, a preparation method and application thereof, and belongs to the technical field of food freshness detection. Background Art

[0002] Cooking oil is an essential food in our daily lives, but it is susceptible to oxidation and deterioration due to the effects of light, heat, and oxygen, producing a series of toxic byproducts, including peroxides, aldehydes, ketones, and acids. These oxidation products not only produce an unpleasant odor but also pose a serious threat to human health. Studies have shown that long-term consumption of oxidized cooking oil may cause damage to organs such as the lungs, liver, kidneys, and heart.

[0003] Current methods for detecting the degree of oil oxidation include Fourier transform infrared spectroscopy, near-infrared spectroscopy, nuclear magnetic resonance, Raman spectroscopy, electron spin resonance, and high-performance liquid chromatography-mass spectrometry. While these methods can provide highly accurate and effective results, the requirement for chemical reagents, expensive equipment, and complex procedures limit their practical application. This makes them unable to meet the needs of ordinary consumers who simply determine the freshness and edibility of cooking oils.

[0004] During the oxidative deterioration of oils and fats, unsaturated fatty acid chains break, forming peroxides. The peroxide value (PV) increases during the initial oxidation process, indicating the onset of oil fission. The PV is an important criterion for assessing oil freshness and quality, reflecting the degree of oxidative rancidity. Currently, most food freshness indicators are pH-sensitive, based on carbon dioxide and volatile alkaline nitrogen. Peroxide-based indicators are not yet available.

[0005] Therefore, it is of great significance to develop a simple method suitable for ordinary consumers to conveniently and visually evaluate the oxidation degree of edible oils. Summary of the Invention

[0006] [Technical Issues]

[0007] In the prior art, the determination of the degree of oil oxidation usually involves expensive equipment, chemical reagents, and complicated operations, which in turn limits its application in daily life.

[0008] [Technical solution]

[0009] In response to the above-mentioned technical problems, the present invention provides a method for detecting the freshness of vegetable oil based on the response of a colorimetric hydrogel. The method uses bamboo as raw material to extract cellulose, uses oxidized nanocellulose as a hydrogel substrate, uses acrylamide as a monomer, potassium persulfate as an initiator, and tetramethylethylenediamine as a cross-linking agent to prepare a composite hydrogel. Nanocellulose and acrylamide construct a double-cross-linked hydrogel, and the rigid network and the flexible network are highly cross-linked, thereby greatly improving the physical and chemical properties of the hydrogel and solving the defects of single-component hydrogels such as poor mechanical properties and single function. A mixed solution of ascorbic acid, ferrous sulfate, and sulfosalicylic acid is used as a peroxide indicator. The principle of the color development of the indicator is that peroxide oxidizes iron ions from divalent to trivalent, and the trivalent iron ions can produce a red complex with sulfosalicylic acid, so that the peroxide content can be judged by the depth of color and the freshness of the edible oil can be indirectly evaluated. The composite hydrogel is combined with the peroxide indicator to prepare a colorimetric hydrogel that is sensitive to edible oil peroxides. This color-developing hydrogel can sensitively indicate the peroxide content in edible oil, instantly detect whether the peroxide content in the edible oil exceeds the standard, and monitor its freshness; compared with traditional hydrophilic membrane freshness indicators, it can increase the reaction speed and enhance the reaction sensitivity.

[0010] The present invention is achieved through the following technical solutions:

[0011] The first object of the present invention is to provide a method for preparing a chromogenic hydrogel for detecting the freshness of vegetable oil, the method comprising the following steps:

[0012] (1) Preparation of TEMPO-oxidized nanocellulose

[0013] Cellulose is dispersed in water, and NaBr, 2,2,6,6-tetramethylpiperidinium-nitrogen-oxide free radical (TEMPO), and NaClO aqueous solution are added. The reaction is stirred continuously, and the pH value of the reaction system is adjusted to 9-10. Anhydrous ethanol is then added to terminate the reaction, and the mixture is dialyzed and freeze-dried to obtain TEMPO-oxidized nanocellulose.

[0014] (2) Preparation of nanocellulose-acrylamide composite hydrogel

[0015] Dispersing the TEMPO-oxidized nanocellulose prepared in step (1) in water, then adding acrylamide, potassium persulfate, and tetramethylethylenediamine, stirring evenly to obtain a mixed system, and allowing it to stand at room temperature for spontaneous crosslinking to obtain a nanocellulose-acrylamide composite hydrogel;

[0016] (3) Preparation of color-developing hydrogel

[0017] Ascorbic acid, ferrous sulfate and sulfosalicylic acid are mixed and ultrasonically treated to obtain a peroxide indicator, which is then added to the nanocellulose-acrylamide composite hydrogel prepared in step (2) and allowed to stand for adsorption to obtain a color-developing hydrogel.

[0018] In one embodiment, the cellulose in step (1) is bamboo cellulose, which is extracted from bamboo.

[0019] In one embodiment, the mass ratio of the cellulose, NaBr, 2,2,6,6-tetramethylpiperidine-nitrogen-oxyl free radical and NaClO in step (1) is 5-10:0.5:0.1:35.

[0020] In one embodiment, the pH value of the reaction system is adjusted in step (1) by using a 1-2 mol / L NaOH aqueous solution.

[0021] In one embodiment, the mass concentration of potassium persulfate in the mixed system of step (2) is 0.1 to 0.9 mg / mL; preferably 0.5 to 0.7 mg / mL; the mass concentration of tetramethylethylenediamine is 0.1 to 0.9 mg / mL; preferably 0.7 to 0.9 mg / mL; the mass concentration of TEMPO-oxidized nanocellulose is 5 mg / mL; and the mass concentration of acrylamide is 1 mg / mL.

[0022] In one embodiment, each 10 mL of the mixed system in step (2) contains 50 mg of TEMPO-oxidized nanocellulose, 10 mg of acrylamide, 5 mg of potassium persulfate and 7 μg of tetramethylethylenediamine.

[0023] In one embodiment, the molar ratio of ferrous sulfate to ascorbic acid in the peroxide indicator in step (3) is 1:1-20, preferably 1:10-20.

[0024] In one embodiment, the molar ratio of ferrous sulfate to sulfosalicylic acid in the peroxide indicator of step (3) is 1-3:1-3; preferably 1:2.

[0025] In one embodiment, the molar ratio of ascorbic acid, sulfosalicylic acid and ferrous sulfate in step (3) is 1:10 to 30:10.

[0026] In one embodiment, the volume ratio of the peroxide indicator to the nanocellulose-acrylamide composite hydrogel in step (3) is 5-10:2-5, ml:ml.

[0027] In one embodiment, the adsorption time in step (3) is 12 to 24 hours.

[0028] The second object of the present invention is to provide a chromogenic hydrogel prepared by the above-mentioned method for preparing a chromogenic hydrogel for detecting the freshness of vegetable oil.

[0029] The third object of the present invention is to provide a use of the above-mentioned chromogenic hydrogel in detecting the freshness of vegetable oil.

[0030] A fourth object of the present invention is to provide a method for detecting the freshness of vegetable oil based on the response of a chromogenic hydrogel, the method comprising the following steps:

[0031] (1) Construction of quantitative relationship model

[0032] Vegetable oil was oxidized using the Schall oven method. The oil samples were placed in glass bottles and accelerated in an oven. Samples were taken at different time points and the peroxide value (PV) of the samples was measured.

[0033] The samples collected at different time points were mixed with an equal volume of deionized water to form an emulsion. 100 μL of the emulsion was reacted with the chromogenic hydrogel prepared above, and the absorbance response value (ΔA) of the reaction system was measured using an ultraviolet spectrophotometer. A quantitative relationship model for vegetable oil was constructed with the measured peroxide value (PV) as the horizontal axis and the absorbance response value (ΔA) as the vertical axis.

[0034] (2) Detection of vegetable oil freshness

[0035] The vegetable oil to be tested is ultrasonically mixed with an equal volume of deionized water to form an emulsion. The emulsion is reacted with the color-developing hydrogel prepared above. The absorbance response value is measured using an ultraviolet spectrophotometer. The peroxide value of the vegetable oil is calculated based on the quantitative relationship model constructed in step (1), and the freshness of the vegetable oil is judged based on the peroxide value.

[0036] A fifth object of the present invention is to provide a method for detecting the freshness of vegetable oil based on hydrogel response, the method comprising the following steps:

[0037] (1) Construction of quantitative relationship model

[0038] Vegetable oil was oxidized using the Schall oven method. The oil samples were placed in glass bottles and accelerated in an oven. Samples were taken at different time points and the peroxide value (PV) of the samples was measured.

[0039] The samples collected at different time points were mixed evenly with an equal volume of deionized water to form an emulsion. 100 μL of the emulsion was then reacted with the chromogenic hydrogel prepared above, and the absorbance response value (ΔA) of the reaction system was measured using an ultraviolet spectrophotometer. Meanwhile, photos were taken with a smartphone, and the R, G, and B values of the images were analyzed using Image J software. The chromaticity change value (ΔR) was calculated, and a color change function model was constructed with the measured peroxide value (PV) as the horizontal axis and the chromaticity change value (ΔR) as the vertical axis.

[0040] (2) Detection of vegetable oil freshness:

[0041] The vegetable oil to be tested was ultrasonically mixed with an equal volume of deionized water to form an emulsion. The emulsion was reacted with the color-developing hydrogel prepared above. A smartphone was used to take a photo and Image J software was used to analyze the R, G, and B values and the color change value (ΔR) of the image. The peroxide value of the vegetable oil was calculated based on the quantitative relationship model constructed in step (1). The freshness of the vegetable oil was judged based on the peroxide value.

[0042] A sixth object of the present invention is to provide a method for preparing a colorimetric card for detecting the freshness of vegetable oil, the method comprising:

[0043] Vegetable oil was oxidized using the Schall oven method. The oil samples were placed in glass bottles and accelerated in an oven. Samples were taken at different time points and the peroxide value (PV) of the samples was measured.

[0044] In addition, the samples taken at different time points were mixed evenly with an equal volume of deionized water to form an emulsion. 100 μL of the emulsion was reacted with the chromogenic hydrogel prepared above. A smartphone was used to take photos, and the R, G, and B values of the images were analyzed using Image J software to calculate the chromaticity change value (ΔR). A standard colorimetric card was constructed based on the measured peroxide value (PV) and chromaticity change value (ΔR).

[0045] The seventh object of the present invention is to provide a standard colorimetric card prepared by the above-mentioned method for preparing a colorimetric card for detecting the freshness of vegetable oil.

[0046] An eighth object of the present invention is to provide a method for detecting the freshness of vegetable oil, wherein the vegetable oil to be tested is ultrasonically mixed with an equal volume of deionized water to form an emulsion, the emulsion is reacted with the chromogenic hydrogel prepared above, a smartphone is used to take a photo and Image J software is used to obtain the color change value (ΔR) of the reaction system, and the freshness of the vegetable oil is judged according to a standard colorimetric card.

[0047] Compared with the prior art, the present invention has the following advantages:

[0048] 1) The present invention extracts and prepares nanocellulose from moso bamboo. The surface of bamboo fiber is rich in active functional groups, and it has the advantages of being renewable, biodegradable, non-toxic, and low-cost. Bamboo nanofiber not only inherits all the advantages of biomass sources, but also has nanoscale effects such as small size, high specific surface area, and high surface reactivity. In addition, its rigid structure shows the function of enhancing various aspects of polymer materials similar to inorganic nanoparticles.

[0049] 2) The present invention uses TEMPO-oxidized nanocellulose as the main body of the gel and uses acrylamide to construct a double-crosslinked network. It has the characteristics of large specific surface area, strong adsorption capacity, high mechanical strength, and good biocompatibility. It can solve the shortcomings of traditional cellulose hydrogels such as poor flexibility and single function, and greatly improve the physical and chemical properties of the hydrogel.

[0050] 3) The present invention uses nanocellulose-acrylamide composite hydrogel as a carrier of the indicator and cleverly combines the nanocellulose-acrylamide composite hydrogel with peroxide detection by utilizing the principle of redox. This can be used to predict the freshness of edible oil and ensure the safety of consumption for ordinary consumers.

[0051] 4) The present invention uses hydrogel as a freshness indicator. Compared with traditional film indicators, it has a larger specific surface area and higher water content, and has the advantages of faster response, more sensitive color development, and greater stability. At the same time, compared with laboratory detection methods for oil peroxides, the present invention has the advantages of being inexpensive, requiring no complex instruments, and being simple and convenient to operate.

[0052] 5) The present invention is the first to develop a peroxide-sensitive colorimetric hydrogel and successfully apply it to the non-destructive detection of oil samples, which has the advantages of real-time, rapid and simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 1 is a graph showing the changes in peroxide value (PV) and absorbance ΔA value of different oil samples in Example 1 of the present invention;

[0054] Figure 2 1 is a linear relationship diagram of the peroxide value (PV) and absorbance ΔA value of different oil samples in Example 1 of the present invention;

[0055] Figure 3 This is a function fitting data diagram of the peroxide value (PV) and color change value (ΔR) of different oil samples in Example 2 of the present invention;

[0056] Figure 4 A colorimetric response and peroxide value (PV) standard colorimetric chart formulated for Example 2 of the present invention;

[0057] Figure 5 This is a data diagram showing the effect of different addition amounts of potassium persulfate on absorbance ΔA values in Example 3 of the present invention;

[0058] Figure 6 This is a data graph showing the effect of different addition amounts of tetramethylethylenediamine on absorbance ΔA values in Example 4 of the present invention;

[0059] Figure 7 This is a data graph showing the effect of different molar ratios of ascorbic acid and ferrous sulfate on absorbance ΔA value in Example 5 of the present invention;

[0060] Figure 8 This is a data graph showing the effect of different molar ratios of ascorbic acid and sulfosalicylic acid on absorbance ΔA value in Example 6 of the present invention. DETAILED DESCRIPTION

[0061] The present invention will be further described below with reference to the examples, but the present invention is not limited by the following examples.

[0062] 1. Determination of peroxide value:

[0063] The measurement was carried out according to the national standard GB5009.227-2016.

[0064] 2. Colorimetric value calculation method

[0065]

[0066] Where R, G, and B are the red, green, and blue values of the colorimetric hydrogel, respectively, and R0, G0, and B0 are the color parameter values of the blank hydrogel, respectively.

[0067] Example 1

[0068] A method for detecting the freshness of vegetable oil based on chromogenic hydrogel response, the method comprising the following steps:

[0069] (1) Extraction of cellulose

[0070] The bamboo was crushed into 100 mesh, immersed in a 1 mol / L NaOH aqueous solution with mechanical stirring for 24 h, taken out and rinsed with deionized water, and then steamed and bleached in a 1% NaClO2 aqueous solution at 70°C, with the pH value of the system controlled at 5. The steaming and bleaching was repeated several times until the bamboo fiber turned white, and then rinsed with deionized water and stored at 4°C.

[0071] (2) Preparation of TEMPO-oxidized nanocellulose

[0072] 5 g of the bamboo cellulose prepared in step (1) was stirred and dispersed in 480 mL of water, and 0.5 g of NaBr, 0.1 g of 2,2,6,6-tetramethylpiperidinium-nitrogen-oxyl free radical (TEMPO), and 35 g of a 12 wt % NaClO aqueous solution were added while stirring continuously. A 1 mol / L NaOH aqueous solution was added dropwise to maintain the pH value of the reaction system at 10. When the pH value of the system no longer decreased, 2 mL of anhydrous ethanol was added to terminate the reaction. After dialysis to remove unreacted small molecular impurities in the reaction system, the mixture was freeze-dried at -20°C for 48 h to obtain TEMPO-oxidized nanocellulose.

[0073] (3) Preparation of nanocellulose-acrylamide composite hydrogel

[0074] 50 mg of the TEMPO-oxidized nanocellulose prepared in step (2) was ultrasonically crushed and uniformly dispersed in 10 mL of deionized water, and 10 mg of acrylamide, 5 mg of potassium persulfate, and 7 μg of tetramethylethylenediamine were added and stirred to obtain a mixed system. The system was allowed to stand at room temperature for spontaneous cross-linking to obtain a nanocellulose-acrylamide composite hydrogel.

[0075] (4) Preparation of color-developing hydrogel

[0076] Prepare 10 mL of a ferrous sulfate-ascorbic acid mixed solution (ascorbic acid concentration is 0.001 mol / L, ferrous sulfate concentration is 0.01 mol / L); prepare 10 mL of 0.02 mol / L sulfosalicylic acid, mix the two, and ultrasonicate for 5 minutes to evenly disperse the solution to obtain a peroxide indicator. Take 200 μL of the above peroxide indicator and add it to 250 μL of the nanocellulose-acrylamide composite hydrogel prepared in step (3). Let it stand for adsorption for 12 hours. Rinse three times with ultrapure water to remove the residual indicator on the gel surface to obtain a color-developing hydrogel.

[0077] (5) Construction of quantitative relationship model

[0078] Camellia oil, soybean oil, peanut oil, olive oil, linseed oil, rapeseed oil, and corn oil were oxidized using the Schall oven method. The oil samples were placed in glass bottles and accelerated oxidation was carried out in an oven at 62±1°C. Camellia oil, soybean oil, peanut oil, olive oil, rapeseed oil, and corn oil were sampled at 0, 1, 2, 3, 4, 6, 8, 10, 12, and 15 days, respectively, and linseed oil was sampled at 0, 1, 2, 3, 12, 24, 36, 48, and 72 hours, respectively. The peroxide value (PV) of the oil samples after different oxidation times was determined using the national standard GB5009.227–2016.

[0079] In addition, after the oil samples with different oxidation times were mixed evenly with an equal volume of deionized water to form an emulsion, 100 μL of the emulsion was reacted with 250 μL of the color-developed hydrogel prepared in step (4), and the absorbance value of the reaction system was measured at 510 nm using an ultraviolet spectrophotometer. A quantitative relationship model for different oils was constructed with the peroxide value (PV) as the horizontal axis and the absorbance response value ΔA (minus the blank control value without adding vegetable oil) as the vertical axis. The model had a good linear relationship, R 2 All are greater than 0.9; see Table 1 and Figures 1-2 As shown:

[0080] (6) Detection of vegetable oil freshness

[0081] After sampling the vegetable oil to be tested, it was ultrasonically mixed with an equal volume of deionized water to form an emulsion. 100 μL of the emulsion was reacted with 250 μL of the chromogenic hydrogel prepared in step (4) of Example 1. The absorbance was measured by ultraviolet light, and the peroxide value of the vegetable oil was calculated based on the quantitative relationship model. The freshness of the vegetable oil was judged based on the peroxide value. If it was lower than 9.82 mmol / kg (GB2716-2018), the freshness met the national standard; if it was higher than this value, the freshness of the vegetable oil did not meet the standard.

[0082] Table 1. Quantitative relationship models for different vegetable oils

[0083] vegetable oil Linear equations <![CDATA[Linear relationship R 2 > Linear range mmoL / kg Camellia oil y=1.3622+0.0867*x 0.9647 4.6～19.7 soybean oil y=1.1765+0.0863*x 0.9629 0.7～14.7 linseed oil y=0.9182+0.0867*x 0.9639 3.0～16.1 peanut oil y=1.3622+0.0580*x 0.9560 3.5～24.9 olive oil y=0.4110+0.1085*x 0.9786 1.6～13.0 corn oil y=1.5623+0.0700*x 0.9600 0.6～19.7 rapeseed oil y=0.6661+0.0740*x 0.9726 1.2～11.8

[0084] Example 2

[0085] Camellia oil, soybean oil, peanut oil, olive oil, linseed oil, rapeseed oil, and corn oil were oxidized using the Schall oven method. The oil samples were placed in glass bottles and accelerated oxidation was carried out in an oven at 62±1°C. Camellia oil, soybean oil, peanut oil, olive oil, rapeseed oil, and corn oil were sampled at 0, 1, 2, 3, 4, 6, 8, 10, 12, and 15 days, respectively, and linseed oil was sampled at 0, 1, 2, 3, 12, 24, 36, 48, and 72 hours, respectively. The peroxide value (PV) of the oil samples after different oxidation times was determined using the national standard GB5009.227–2016.

[0086] In addition, after the oil samples with different oxidation times were mixed with an equal volume of deionized water to form an emulsion, 100 μL of the emulsion was reacted with 250 μL of the color-developing hydrogel prepared in step (4) of Example 1, and the R, G, and B values of the image were analyzed using Image J software on a smartphone; a color change function model was constructed with the measured peroxide value (PV) as the horizontal coordinate and the chromaticity change value (ΔR) as the vertical coordinate, as shown in Tables 2 and 3. Figure 3 As shown: Or construct a standard color chart as Figure 4 As shown;

[0087] After sampling the vegetable oil to be tested, it was ultrasonically mixed with an equal volume of deionized water to form an emulsion. 100 μL of the emulsion was reacted with 250 μL of the color-developing hydrogel prepared in step (4) of Example 1. A smartphone was used to take a photo, and the colorimetric data chromaticity value ΔR was obtained using Image J software. The color was quantified according to the functional relationship model, and the peroxide value of the vegetable oil was calculated to determine the freshness of the vegetable oil; alternatively, the freshness of the vegetable oil was determined according to a standard colorimetric card.

[0088] Table 2. Functional relationship models of different vegetable oils

[0089]

[0090]

[0091] Example 3

[0092] In the process of preparing the nanocellulose-acrylamide composite hydrogel in step (3) of Example 1, the volume of the mixed system remains unchanged, and the content of the initiator potassium persulfate (KPS) is adjusted, as shown in Table 3, to prepare different nanocellulose-acrylamide composite hydrogels; then, different color-developing hydrogels are prepared in the same manner as in step (4) of Example 1; camellia oil is ultrasonically mixed with an equal volume of deionized water to form an emulsion, 100 μL of the emulsion is reacted with 250 μL of the color-developing hydrogel, and the absorbance response value ΔA of the reaction system is measured by an ultraviolet spectrophotometer. The results are as follows: Figure 5 As shown:

[0093] Table 3. Different potassium persulfate addition amounts

[0094]

[0095] from Figure 1 It can be seen that the composite hydrogel prepared with different added amounts of initiator will affect the absorbance response value during the vegetable oil freshness detection process. As the added amount increases, the absorbance response value also increases. When it increases to 0.5 mg / mL, it reaches the highest. When the amount of potassium persulfate is increased, the absorbance response value decreases. This shows that when the initiator concentration reaches 0.5 mg / mL, the detection sensitivity of the color hydrogel prepared is better.

[0096] Example 4

[0097] In the process of preparing the nanocellulose-acrylamide composite hydrogel in step (3) of Example 1, the volume of the mixed system remains unchanged, and the content of the crosslinking agent tetramethylethylenediamine (TEMED) is adjusted, as shown in Table 4, to prepare different nanocellulose-acrylamide composite hydrogels; and then, different chromogenic hydrogels are prepared in the same manner as in step (4) of Example 1, and camellia oil is ultrasonically mixed with an equal volume of deionized water to form an emulsion. 100 μL of the emulsion is reacted with 250 μL of the chromogenic hydrogel, and the absorbance response value ΔA of the reaction system is measured by an ultraviolet spectrophotometer. The results are as follows: Figure 6 As shown:

[0098] Table 4. Different addition amounts of tetramethylethylenediamine

[0099]

[0100] from Figure 2 It can be seen that the amount of cross-linker added will affect the absorbance response value during the vegetable oil freshness detection process, and there is no regularity. Only when the cross-linker concentration reaches 0.7μg / mL, the absorbance response value reaches the highest, indicating that when the cross-linker concentration reaches 0.7μg / mL, the detection sensitivity of the color hydrogel prepared is better.

[0101] Example 5

[0102] During the preparation of the chromogenic hydrogel in step (4) of Example 1, the added volume of each substance in the peroxide indicator remained unchanged, ensuring that the concentrations of sulfosalicylic acid and ferrous sulfate were 0.02 mol / L and 0.01 mol / L, respectively. The molar ratio of ferrous sulfate and ascorbic acid was adjusted, as shown in Table 5, to prepare different chromogenic hydrogels; camellia oil was ultrasonically mixed with an equal volume of deionized water to form an emulsion, 100 μL of the emulsion was reacted with 250 μL of the chromogenic hydrogel, and the absorbance response value ΔA of the reaction system was measured by ultraviolet spectrophotometer. The results are as follows: Figure 7 As shown:

[0103] Table 5 Different molar ratios of iron ion and ascorbic acid

[0104]

[0105] Example 6

[0106] In the preparation process of the chromogenic hydrogel in step (4) of Example 1, the added volume of each substance in the peroxide indicator remains unchanged, ensuring that the ascorbic acid concentration and the ferrous sulfate concentration are 0.001 mol / L and 0.01 mol / L, respectively. The molar ratio of ferrous sulfate and sulfosalicylic acid is adjusted as shown in Table 6 to prepare different chromogenic hydrogels; camellia oil and an equal volume of deionized water are ultrasonically mixed to form an emulsion, 100 μL of the emulsion is reacted with 250 μL of the chromogenic hydrogel, and the absorbance response value ΔA of the reaction system is measured by ultraviolet spectrophotometer. The results are as shown in FIG. Figure 8 As shown:

[0107] Table 6 Different molar ratios of iron ion and sulfosalicylic acid

[0108]

[0109] The peroxide-indicating composite hydrogel prepared in this example has good indication sensitivity to edible oil peroxide, a characteristic oxidation product, and can effectively detect the freshness of edible oil in real time.

[0110] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present disclosure. It should be understood that the above are only specific embodiments of the present disclosure and are not intended to limit the present disclosure. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present disclosure should be included in the scope of protection of the present disclosure.

Claims

1. A method for preparing a chromogenic hydrogel for detecting the freshness of vegetable oil, characterized in that: The method comprises the following steps: (1) Preparation of TEMPO-oxidized nanocellulose Cellulose is dispersed in water, and NaBr, 2,2,6,6-tetramethylpiperidinium-nitrogen-oxide free radical (TEMPO), and NaClO aqueous solution are added. The reaction is stirred continuously, and the pH value of the reaction system is adjusted to 9-10. Anhydrous ethanol is then added to terminate the reaction, and the mixture is dialyzed and freeze-dried to obtain TEMPO-oxidized nanocellulose. (2) Preparation of nanocellulose-acrylamide composite hydrogel Take the TEMPO-oxidized nanocellulose dispersed in water prepared in step (1), then add acrylamide, potassium persulfate, and tetramethylethylenediamine, stir evenly to obtain a mixed system, and let it stand at room temperature for spontaneous cross-linking to obtain a nanocellulose-acrylamide composite hydrogel; the mass concentration of potassium persulfate in the mixed system is 0.5-0.7 mg / mL; the mass concentration of tetramethylethylenediamine is 0.7-0.9 mg / mL; the mass concentration of TEMPO-oxidized nanocellulose is 5 mg / mL; the mass concentration of acrylamide is 1 mg / mL; (3) Preparation of color-developing hydrogel Ascorbic acid, ferrous sulfate, and sulfosalicylic acid are mixed and ultrasonically treated to obtain a peroxide indicator, which is then added to the nanocellulose-acrylamide composite hydrogel prepared in step (2) and allowed to stand for adsorption to obtain a color-developing hydrogel; the molar ratio of ascorbic acid, sulfosalicylic acid, and ferrous sulfate is 1:10 to 30:10; The color-developing hydrogel is used for indicating the content of peroxide in vegetable oil.

2. The chromogenic hydrogel prepared by the method for preparing the chromogenic hydrogel for detecting the freshness of vegetable oil according to claim 1.

3. Use of the chromogenic hydrogel according to claim 2 in detecting the freshness of vegetable oils.

4. A method for detecting the freshness of vegetable oil based on chromogenic hydrogel response, characterized in that: The method comprises the following steps: (1) Construction of quantitative relationship model The vegetable oil sample is placed in a glass bottle and accelerated oxidized in an oven. Samples are taken at different time points, and the peroxide value (PV) of the sampled samples is measured. The sampled samples taken at different time points are mixed uniformly with an equal volume of deionized water to form an emulsion. The emulsion is reacted with the chromogenic hydrogel according to claim 2, and the absorbance response value ΔA of the reaction system is measured by ultraviolet spectrophotometry. The quantitative relationship model of vegetable oil was constructed with the measured peroxide value PV as the horizontal axis and the absorbance response value ΔA as the vertical axis; (2) Detection of vegetable oil freshness The vegetable oil to be tested is ultrasonically mixed with an equal volume of deionized water to form an emulsion, the emulsion is reacted with the color-developing hydrogel according to claim 2, the absorbance response value is measured using an ultraviolet spectrophotometer, and the peroxide value of the vegetable oil is calculated based on the quantitative relationship model constructed in step (1), and the freshness of the vegetable oil is judged based on the peroxide value.

5. A method for detecting the freshness of vegetable oil based on chromogenic hydrogel response, characterized in that: The method comprises the following steps: (1) Construction of quantitative relationship model The vegetable oil sample is placed in a glass bottle and accelerated oxidized in an oven. Samples are taken at different time points, and the peroxide value (PV) of the sampled samples is measured. The sampled samples taken at different time points are mixed evenly with an equal volume of deionized water to form an emulsion. The emulsion is reacted with the color-developing hydrogel according to claim 2. A smartphone is used to take a photo. The R, G, and B values of the photo are analyzed using Image J software, and the chromaticity change value (ΔR) is calculated. A color change function model is constructed with the measured peroxide value (PV) as the horizontal coordinate and the chromaticity change value (ΔR) as the vertical coordinate. (2) Detection of vegetable oil freshness: The vegetable oil to be tested is ultrasonically mixed with an equal volume of deionized water to form an emulsion, and the emulsion is reacted with the color-developing hydrogel according to claim 2. A smartphone is used to take a photo and Image J software is used to analyze the R, G, and B values of the image and the color change value ΔR. The peroxide value of the vegetable oil is calculated based on the quantitative relationship model constructed in step (1), and the freshness of the vegetable oil is judged based on the peroxide value.

6. A method for preparing a colorimetric card for detecting the freshness of vegetable oil, characterized in that: The method comprises: The vegetable oil samples were placed in glass bottles and accelerated oxidized in an oven. Samples were taken at different time points and the peroxide value (PV) of the samples was measured. In addition, the samples taken at different time points were mixed evenly with an equal volume of deionized water to form an emulsion. The emulsion was reacted with the chromogenic hydrogel according to claim 2. A smartphone was used to take a photo. The R, G, and B values of the image were analyzed using Image J software, and the chromaticity change value ΔR was calculated. A standard colorimetric card was constructed using the measured peroxide value PV and chromaticity change value ΔR.

7. A standard colorimetric card prepared by the method for preparing a colorimetric card for detecting the freshness of vegetable oil according to claim 6.

8. A method for detecting the freshness of vegetable oil, characterized in that: The method comprises ultrasonically mixing the vegetable oil to be tested with an equal volume of deionized water to form an emulsion, reacting the emulsion with the color-developing hydrogel according to claim 2, taking a photo with a smartphone and using Image J software to obtain the color change value ΔR of the reaction system, and judging the freshness of the vegetable oil using the standard colorimetric card according to claim 7.

Images