Preparation method of visual sensor based on mulberry anthocyanins and its application in aquatic product monitoring

Abstract

This invention provides a method for preparing a visual sensor based on mulberry anthocyanins and its application in aquatic product monitoring. Using mulberry anthocyanins as an indicator, combined with citric acid, chitosan, and polyvinyl alcohol, a novel freshness sensor is constructed on filter paper via an impregnation method. The anthocyanins (MAs) exhibit a significant color change within the pH range of 3–12, and their main components are various cyanidin and pelargonidin glycosides. Testing showed that the sensor prepared by the membrane method exhibited the best stability, remaining viable for over 15 days at 4°C, and demonstrating high sensitivity to volatile ammonia. Specifically, the sensor with 1.25% MAs added showed a sensitivity of 11.79%–18.35% to 0.025%–0.1% NH3 within 30 minutes. When used for monitoring the freshness of Litopenaeus vannamei, the sensor's color difference value (ΔE) was highly correlated with storage time and TVB-N value, with the color gradually changing from purplish-red to grayish-blue, achieving visual and qualitative detection of aquatic product freshness.

Description

Technical Field

[0001] This invention relates to the field of food freshness detection technology, and more specifically, to a method for preparing a visual sensor based on mulberry anthocyanins and its application in aquatic product monitoring. Background Technology

[0002] Aquatic products are rich in high-quality protein, unsaturated fatty acids, and other nutrients, making them popular with consumers. However, due to their high water content and fragile structure, they are prone to spoilage during storage and transportation, producing volatile nitrogenous compounds (such as ammonia and trimethylamine). This not only affects the taste and nutritional value but may also harm human health. Therefore, establishing a rapid, accurate, and visualized method for monitoring the freshness of aquatic products is of great significance for ensuring food safety and reducing resource waste.

[0003] Currently, the main methods for detecting the freshness of aquatic products include sensory evaluation, physicochemical testing, and microbiological testing. Sensory evaluation is greatly affected by subjective factors and has limited accuracy; physicochemical testing (such as the determination of volatile basic nitrogen (TVB-N)) is accurate and reliable, but it is complex to operate, time-consuming, requires professional instruments and technicians, and is difficult to achieve rapid on-site testing; microbiological testing has problems such as long testing cycles and cumbersome procedures, and cannot meet the needs of real-time monitoring.

[0004] Anthocyanins (ACNs) are natural water-soluble pigments widely found in plant tissues. Their molecular structure contains phenolic hydroxyl groups, making them highly sensitive to changes in environmental pH. They can exhibit different color changes through structural transformations, making them ideal natural pH indicators. Mulberries, a common fruit, are rich in anthocyanins (MAs), and their wide availability, low cost, and non-toxicity make them suitable for preparing natural visual sensors. However, natural anthocyanins are easily affected by environmental factors such as light, temperature, and oxygen, exhibiting poor stability. When directly applied to sensor fabrication, they are prone to color fading and decreased response performance, limiting their practical applications.

[0005] Chitosan (CS) possesses excellent biocompatibility, film-forming properties, and antibacterial activity, while polyvinyl alcohol (PVA) exhibits superior flexibility and water solubility. The composite membrane formed by the cross-linking of these two components effectively improves the stability of anthocyanins. Furthermore, citric acid (CA), as a co-pigment, can form a complex with anthocyanins, further enhancing their color stability and pH response sensitivity. Therefore, combining mulberry anthocyanins with a chitosan-PVA composite membrane to prepare a highly stable and responsive visual sensor holds promise for rapid visual monitoring of aquatic product freshness, addressing the shortcomings of existing detection methods. Thus, this paper proposes a method for preparing a visual sensor based on mulberry anthocyanins and its application in aquatic product monitoring. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing methods for detecting the freshness of aquatic products, such as complex operation, inability to provide real-time visual monitoring, and poor stability and insufficient response performance of natural anthocyanin sensors. This invention provides a method for preparing a visual sensor based on mulberry anthocyanins. The sensor has a simple preparation process, low cost, good stability, and sensitive response, and can realize real-time visual monitoring of the freshness of aquatic products. It also provides the application of this sensor in monitoring the freshness of aquatic products.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: a method for preparing a visual sensor based on mulberry anthocyanins, comprising the following steps: Step 1, preparation of mulberry powder and extraction of mulberry anthocyanins (MAs): fresh mulberries are placed in an oven and dried to constant weight at 60°C, then pulverized with a pulverizer and passed through a 100-mesh sieve to remove coarse particles, thus preparing mulberry powder; the mulberry powder is then weighed, added to a 95% ethanol solution, and intermittently stirred and extracted at room temperature of 25°C. The extract is filtered and then evaporated to dryness. The MAs powder is collected and stored in the dark and refrigerated. Step 2, Preparation of composite membrane solution: Prepare CS solution and PVA solution separately, mix them at a volume ratio of 1:1 to obtain CS@PVA composite membrane solution, add MAs and citric acid (CA) to it, stir evenly to obtain MAs@CS@PVA membrane solution; Step 3, Sensor preparation: Immerse filter paper in MAs@CS@PVA membrane solution, and dry at room temperature after immersion to obtain a visual sensor.

[0008] As a preferred embodiment of the present invention, in step 1, the ratio of mulberry powder to 95% ethanol solution is 1:5 (g / mL), and the extraction time is not less than 6 hours. The extract is filtered and evaporated to dryness at 35°C and 100 mb.

[0009] As a preferred technical solution of the present invention, in step 2, the concentration of the CS solution is 1.5%, and the preparation method is to dissolve 3g of CS in 200mL of 1.0% acetic acid aqueous solution and stir at room temperature until completely dissolved; the concentration of the PVA solution is 1.5%, and the preparation method is to dissolve 3g of PVA in 200mL of pure water and stir in a 60℃ water bath for 4h until completely dissolved.

[0010] As a preferred technical solution of the present invention, in step 2, the concentration of CA in the MAs@CS@PVA membrane solution is 0.5mM, and the mass fraction of MAs is 0.5%, 0.75%, 1.0%, 1.25%, or 1.5%.

[0011] As a preferred technical solution of the present invention, in step 3, the filter paper has a specification of 20mm×20mm, the soaking time is 10min, and the drying method is natural drying at room temperature.

[0012] As a preferred technical solution of the present invention, the sensor is prepared by any one of the methods described in claims 1-5 based on the visualization sensor of mulberry anthocyanins. The sensor uses qualitative filter paper as a substrate and is loaded with mulberry anthocyanins, chitosan-polyvinyl alcohol composite membrane and citric acid. It can exhibit significant color changes in the pH range of 3-11.

[0013] As a preferred embodiment of the present invention, the mass fraction of MAs in the sensor is 1.25%, and the sensor is stored at 4°C in the dark for 15 days. With an E value ≤ 0.99 ± 0.21, it exhibits high sensitivity to volatile ammonia at concentrations of 0.025%–0.1%, with a response sensitivity of 11.79%–18.35% within 30 minutes.

[0014] As a preferred technical solution of the present invention, the sensor is fixed to the top of a sealed container and placed in a sealed environment together with the aquatic products. The sensor detects color changes and... Changes in the E-value are used to monitor the freshness of aquatic products in real time.

[0015] As a preferred technical solution of the present invention, the aquatic product is Litopenaeus vannamei, and the monitoring conditions are 25℃ or 4℃, light-proof and sealed storage. The color change of the sensor is recorded by taking pictures with a smartphone, and the L, a, b, R, G, B parameters measured by a colorimeter are used to calculate the color. E value, and simultaneously detect TVB-N value of aquatic products, to establish The relationship between E-value and the freshness of aquatic products.

[0016] As a preferred embodiment of the present invention, when the TVB-N value of the whiteleg shrimp exceeds 20 mg / 100g, the sensor color changes from purplish-red to grayish-purple or grayish-blue. If the E value is ≥14.61, the aquatic product is considered to be spoiled and inedible.

[0017] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention uses mulberry anthocyanins as a natural indicator, which is widely available, inexpensive, non-toxic, and harmless, meeting food-grade testing requirements and avoiding the potential harm to the environment and human body posed by artificially synthesized indicators. Simultaneously, through chitosan-polyvinyl alcohol composite film encapsulation and citric acid co-pigment modification, the stability of mulberry anthocyanins is significantly improved, solving the problems of easy degradation and color fading of natural anthocyanins, allowing the sensor to be stably stored at 4°C for more than 15 days. The E value is only 0.99±0.21.

[0018] The sensor prepared by this invention is simple in process and easy to operate, requiring no complex instruments or equipment, and is suitable for large-scale production. The sensor is highly sensitive to volatile ammonia, and its response sensitivity reaches 11.79%, 12.96% and 18.35% within 30 minutes at ammonia concentrations of 0.025%, 0.05% and 0.1%, respectively. Moreover, the color change is significant and can be directly identified by the naked eye, realizing the visual real-time monitoring of the freshness of aquatic products.

[0019] The sensor of this invention has a good correlation with the freshness of aquatic products. The E value was significantly positively correlated with the storage time and TVB-N value of aquatic products, and the linear fitting coefficient R was [missing value]. 2 With a maximum value of 0.9967, it can accurately reflect the degree of spoilage of aquatic products, providing a fast, accurate, and low-cost new method for detecting the freshness of aquatic products, and has broad prospects for practical application. Attached Figure Description

[0020] Figure 1 The following are provided for this invention: (a) UV-Vis absorption spectra of MAs (50 mg / L) at different pH values ​​(3-12), with the inset showing the color changes at different pH values ​​under sunlight; (b) Schematic diagram of the color response mechanism of ACNs. Figure 2 Ultra-high performance liquid chromatogram (a) and high-resolution mass spectrum (b) of MAs (50 mg / L) provided for this invention; Figure 3 (ae) Scanning electron microscope images of the sensor substrate and sensors prepared by different methods provided for this invention; (f) Infrared spectra of MAs, sensor substrate and sensors prepared by different methods; Figure 4 XRD patterns of (a) MAs, sensor substrate, and sensors prepared by different methods provided for this invention; (b) water contact angles of sensor substrate and sensors prepared by different methods (A: FP; B: MAs-FP; C: CS@PVA-FP; D: MAs-CS@PVA-FP; E: MAs@CS@PVA-FP). Figure 5 Color stability of sensors prepared by different methods provided by the present invention at different temperatures: (a) 25°C; (b) 4°C; (c) Color response of sensors prepared by different methods under different pH conditions. Figure 6 (a) Photostress stability of MAs@CS@PVA-FP at 25°C; (b) Color response diagrams of MAs@CS@PVA-FP with different MAs contents under different pH conditions provided for this invention; Figure 7 The sensitivity of MAs@CS@PVA-FP with different MAs contents to different concentrations of volatile ammonia provided by this invention is shown in the figures: (a) 0.025% NH3; (b) 0.1% NH3; (c) 0.05% NH3. The color response of MAs@CS@PVA-FP with different MAs contents to different concentrations of volatile ammonia is also shown in the figures: (d) 0.025% NH3; (e) 0.05% NH3; (f) 0.1% NH3. Figure 8 Stability graphs of MAs@CS@PVA-FP prepared in different batches for this invention; Figure 9 The following images are provided for this invention: (ab) Correlation between color change of MAs@CS@PVA-FP and shrimp freshness under 25℃ storage conditions; (c) Photographs of shrimp freshness monitored in real time by MAs@CS@PVA-FP; (de) Correlation between color change of MAs@CS@PVA-FP and shrimp freshness under 4℃ storage conditions; (f) Graph of shrimp freshness monitored in real time by MAs@CS@PVA-FP. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0022] Therefore, the following detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely illustrates some embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. It should be noted that, in the absence of conflict, the embodiments and features and technical solutions in the embodiments of the present invention can be combined with each other. It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0023] Example 1: A method for preparing a visual sensor based on mulberry anthocyanins, comprising the following steps: Step 1, preparation of mulberry powder and extraction of mulberry anthocyanins (MAs): Fresh mulberries are placed in an oven and dried to constant weight at 60°C, then pulverized with a pulverizer and passed through a 100-mesh sieve to remove coarse particles, thus preparing mulberry powder; the mulberry powder is then weighed, and 95% ethanol solution is added, and the mixture is intermittently stirred and extracted at room temperature of 25°C. The extract is filtered and then evaporated to dryness. The MAs powder is collected and stored in the dark and refrigerated. Step 2, Preparation of composite membrane solution: Prepare CS solution and PVA solution separately, mix them at a volume ratio of 1:1 to obtain CS@PVA composite membrane solution, add MAs and citric acid (CA) to it, stir evenly to obtain MAs@CS@PVA membrane solution; Step 3, Sensor preparation: Immerse filter paper in MAs@CS@PVA membrane solution, and dry at room temperature after immersion to obtain a visual sensor.

[0024] In step 1, the ratio of mulberry powder to 95% ethanol solution is 1:5 (g / mL), and the extraction time is no less than 6 hours. The extract is filtered and evaporated to dryness at 35°C and 100 mb.

[0025] In step 2, the concentration of the CS solution is 1.5%, and the preparation method is to dissolve 3g of CS in 200mL of 1.0% acetic acid aqueous solution and stir at room temperature until completely dissolved; the concentration of the PVA solution is 1.5%, and the preparation method is to dissolve 3g of PVA in 200mL of pure water and stir in a 60℃ water bath for 4h until completely dissolved.

[0026] In step 2, the concentration of CA in the MAs@CS@PVA membrane solution is 0.5 mM, and the mass fraction of MAs is 0.5%, 0.75%, 1.0%, 1.25%, or 1.5%.

[0027] In step 3, the filter paper is 20mm×20mm in size, the soaking time is 10 minutes, and the drying method is natural drying at room temperature.

[0028] The sensor uses qualitative filter paper as a substrate and is loaded with mulberry anthocyanins, chitosan-polyvinyl alcohol composite membrane and citric acid, and can exhibit significant color changes in the pH range of 3-11.

[0029] The mass fraction of MAs in the sensor is 1.25%, and the sensor is stored at 4°C in the dark for 15 days. With an E value ≤ 0.99 ± 0.21, it exhibits high sensitivity to volatile ammonia at concentrations of 0.025%–0.1%, with a response sensitivity of 11.79%–18.35% within 30 minutes.

[0030] The sensor was fixed to the top of a sealed container and placed in a sealed environment along with the aquatic products. The sensor's color change and... Changes in the E-value are used to monitor the freshness of aquatic products in real time.

[0031] The aquatic product in question is Litopenaeus vannamei (whiteleg shrimp). Monitoring conditions include storage at 25℃ or 4℃ in a light-proof, sealed environment. Color changes are recorded by taking photos with a smartphone and then combined with L, a, b, R, G, and B parameters measured by a colorimeter to calculate... E value, and simultaneously detect TVB-N value of aquatic products, to establish The relationship between E-value and the freshness of aquatic products.

[0032] When the TVB-N value of the whiteleg shrimp exceeds 20 mg / 100g, the sensor color changes from purplish-red to grayish-purple or grayish-blue. If the E value is ≥14.61, the aquatic product is considered to be spoiled and inedible.

[0033] Example 2: A method for preparing a visual sensor based on mulberry anthocyanins, comprising the following steps: Preparation of mulberry powder and extraction of mulberry anthocyanins (MAs): Fresh mulberries were placed in an oven and dried to constant weight at 60°C. Then, they were pulverized with a pulverizer and passed through a 100-mesh sieve to remove coarse particles, thus preparing mulberry powder. The mulberry powder was then weighed and added to a 95% ethanol solution. The mixture was intermittently stirred and extracted at room temperature (25°C). The extract was filtered and then evaporated to dryness. The MAs powder was collected and stored in the dark under cold conditions.

[0034] Preparation of composite membrane solutions: ① Preparation of 1.5% chitosan (CS) solution: Dissolve 3 g CS in 200 mL of 1.0% acetic acid aqueous solution and stir at room temperature until completely dissolved; ② Preparation of 1.5% polyvinyl alcohol (PVA) solution: Dissolve 3 g PVA in 200 mL of pure water and stir in a 60℃ water bath for 4 h until completely dissolved; ③ Preparation of CS@PVA composite membrane solution: Mix the CS solution cooled to room temperature with the PVA solution at a volume ratio of 1:1 until homogeneous; ④ Preparation of MAs@CS@PVA membrane solution: Add mulberry anthocyanins and 4.8 mg citric acid (CA) to 50 mL of CS@PVA composite membrane solution and stir until homogeneous to obtain MAs@CS@PVA membrane solutions with a mass fraction of 0.5%, 0.75%, 1.0%, 1.25%, or 1.5% and a CA concentration of 0.5 mM.

[0035] Fabrication of the visualization sensor: A 20 mm × 20 mm qualitative filter paper was immersed in the MAs@CS@PVA membrane solution for 10 min, and then dried at room temperature to obtain a visualization sensor based on mulberry anthocyanins (X-MAs@CS@PVA-FP, where X is the mass fraction of MAs).

[0036] Preferably, in step 3, the mass fraction of the MAs is 1.25%, and the sensor prepared under this condition has the best color stability and volatile ammonia response sensitivity.

[0037] The specific steps for applying the above-mentioned visualization sensor based on mulberry anthocyanins in monitoring the freshness of aquatic products are as follows: The sensor was fixed to the top of the sealed container, and the aquatic product samples were placed in the sealed container and stored at 25°C or 4°C in the dark. During storage, the color changes of the sensor were periodically recorded by taking photos with a smartphone, and the color parameters (L, a, b, R, G, B) of the sensor were measured using a colorimeter to calculate the color change values. E; Based on the sensor's color changes and The E value, combined with the TVB-N value of aquatic products, determines the freshness of the aquatic products; when the sensor color changes from purplish-red to purplish-pink, the aquatic products are close to spoilage; when the color changes to grayish-blue, the aquatic products are completely spoiled.

[0038] The aquatic product in question is Litopenaeus vannamei. According to the national food safety standard (GB 2733-2015), when the TVB-N value of Litopenaeus vannamei exceeds 20 mg / 100 g, it is deemed inedible. At this point, the sensor... The E value is greater than 15.

[0039] Example 3: Extraction of Mulberry Anthocyanins (MAs) Accurately weigh 100 g of mulberry powder and place it in an Erlenmeyer flask. Add 500 mL of 95% ethanol solution, mix thoroughly, and extract intermittently with stirring at 25°C for 6 h. Filter the extract with qualitative filter paper to remove residue. Place the filtered extract in a rotary evaporator and evaporate to dryness at 35°C and 100 mbar. Collect the obtained mulberry anthocyanin powder, place it in a brown sealed bottle, and store it in the dark at 4°C for later use.

[0040] Example 4: Preparation of composite membrane solution and sensor 1. Preparation of 1.5% CS solution: Weigh 3 g of chitosan, add 200 mL of 1.0% acetic acid aqueous solution, and stir continuously at room temperature until the chitosan is completely dissolved to obtain a transparent CS solution.

[0041] 2. Preparation of 1.5% PVA solution: Weigh 3 g of polyvinyl alcohol, add 200 mL of pure water, place in a 60℃ water bath, stir for 4 h until the polyvinyl alcohol is completely dissolved, cool to room temperature, and obtain a transparent PVA solution.

[0042] 3. Preparation of CS@PVA composite membrane solution: Mix CS solution and PVA solution at a volume ratio of 1:1, stir at room temperature for 30 min to make them evenly mixed, and obtain CS@PVA composite membrane solution.

[0043] 4. Preparation of MAs@CS@PVA membrane solution: Take 50 mL of CS@PVA composite membrane solution, add 0.625 g of mulberry anthocyanin powder (MAs mass fraction 1.25%) and 4.8 mg of citric acid, stir at room temperature for 1 h to completely dissolve and mix evenly to obtain 1.25%-MAs@CS@PVA membrane solution (CA concentration 0.5 mM).

[0044] 5. Sensor preparation: Immerse a 20 mm × 20 mm qualitative filter paper in the above 1.25%-MAs@CS@PVA membrane solution for 10 min, then remove it and place it in a fume hood to air dry at room temperature for 24 h to obtain a 1.25%-MAs@CS@PVA-FP visualization sensor. Seal and protect from light for later use.

[0045] Example 5: Sensor Characterization and Performance Testing Sensor characterization: The prepared 1.25%-MAs@CS@PVA-FP sensor, as well as blank CS@PVA-FP, MAs-FP, and MAs-CS@PVA-FP, were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and water contact angle (WCA) measurement.

[0046] SEM results showed that the 1.25%-MAs@CS@PVA-FP sensor surface exhibited a uniform rough fiber structure, with mulberry anthocyanins uniformly distributed in the chitosan-polyvinyl alcohol composite membrane, ensuring both the sensor's air permeability and improving the loading stability of anthocyanins. FT-IR spectroscopy indicated hydrogen bonding interactions between mulberry anthocyanins and chitosan and polyvinyl alcohol, further verifying the composite membrane's encapsulation effect on anthocyanins. XRD results showed a decrease in the crystallinity of the composite membrane, improving the sensor's flexibility. WCA testing indicated that the sensor surface possessed suitable hydrophobicity, preventing performance degradation caused by direct contact with aquatic product juices.

[0047] Color stability test: The 1.25%-MAs@CS@PVA-FP sensor was stored at 25℃ and 4℃ in the dark for 15 days respectively. The color parameters (L, a, b values) of the sensor were measured periodically using a colorimeter. The results were then calculated according to the formula... E = √[(Li - L0)] 2 +(ai-a0) 2 +(bi-b0) 2 ]calculate E value (where L0, a0, b0 are the initial color parameters, and Li, ai, bi are the color parameters for the i-th day).

[0048] The results showed that under storage conditions of 4℃, the sensor remained functional after 15 days. The E value was only 0.99±0.21, and no obvious color change was perceptible to the naked eye; under storage conditions of 25℃, after 15 days... The E value is 4.23±0.85, maintaining good color stability. Furthermore, comparing storage of the sensor in a bright and dark environment at 25℃, it was found that light accelerates color changes, and the sensor stored in the dark exhibits significantly better stability than that stored in the light. Therefore, the sensor must be stored away from light.

[0049] Volatile ammonia response test: The 1.25%-MAs@CS@PVA-FP sensor was exposed to 10 mL of ammonia water atmosphere with different concentrations (0.025%, 0.05%, 0.1%). Color changes were recorded by taking pictures every 5 minutes over 30 minutes using a smartphone. At the same time, the color parameters (R, G, B) of the sensor were measured using a colorimeter. The response sensitivity of the sensor was calculated according to the formula sensitivity (%) = [(R0-Ri)+(G0-Gi)+(B0-Bi)] / (R0+G0+B0).

[0050] The results showed that as the ammonia concentration increased and the exposure time prolonged, the sensor's response sensitivity gradually increased, and the color gradually changed from purplish-red to grayish-blue. At 30 min, the sensor's response sensitivity was 11.79%, 12.96%, and 18.35% at ammonia concentrations of 0.025%, 0.05%, and 0.1%, respectively. The color change was obvious and could be directly distinguished by the naked eye, indicating that the sensor has excellent response performance to volatile ammonia.

[0051] Preparation stability test: Following the method in Example 2, six batches of 1.25%-MAs@CS@PVA-FP sensors were prepared. The initial color parameters (L0, a0, b0) of each batch of sensors were measured using a colorimeter, and the results were calculated. The E-value was analyzed using IBM SPSS statistical software.

[0052] The results showed that the six batches of sensors The E-values ​​varied to some extent, but were all within acceptable ranges, with the optimal batch being the best. The E-value is 0.87, indicating the worst batch. The E value of 3.92 indicates that the fabrication process of this sensor has good stability and repeatability, which can meet the needs of large-scale production and practical application.

[0053] Example 6: Application of sensors in monitoring the freshness of Litopenaeus vannamei. Sample preparation: Select whiteleg shrimp of uniform appearance and size (20±0.5 g / s), remove the shrimp whiskers and legs, place them in a flat glass weighing bottle with a diameter of 60 mm, fix the 1.25%-MAs@CS@PVA-FP sensor on the top of the bottle cap, seal and store them at 25℃ and 4℃ in the dark.

[0054] Index Testing: During storage, periodically (every 4 hours at 25℃, every 12 hours at 4℃) use a smartphone to record the color changes of the sensor, and use a colorimeter to measure the sensor's color parameters (L, a, b, R, G, B), and calculate... E value; at the same time, in accordance with the national food safety standard (GB 2733-2015), the TVB-N value of shrimp samples was determined using an intelligent nitrogen analyzer. The specific method is as follows: take 20g ± 0.1g of edible shrimp, stir evenly, add 100 mL of pure water, soak for 30 min, filter, titrate with 0.01 M standard hydrochloric acid titrant, and indicate the endpoint with methyl red-bromocresol green mixed indicator solution. Calculate the TVB-N value according to the formula X=[(V1-V2)×14c] / (10m)×100 (where X is the TVB-N content, V1 is the volume of hydrochloric acid consumed by the sample, V2 is the volume of hydrochloric acid consumed by the blank, c is the hydrochloric acid concentration, and m is the sample mass).

[0055] Results analysis: Under storage conditions at 25℃, the initial TVB-N value was 5.07 mg / 100 g, and the sensor color was purple-red. The E value was 0; after 20 hours of storage, the TVB-N value rose to 24.56 mg / 100 g (exceeding the edible threshold of 20 mg / 100 g), and the sensor color turned grayish-purple. The E value was 15.70; after 24 hours of storage, the TVB-N value reached 32.52 mg / 100 g, and the sensor color turned grayish-blue. The E value is 19.66. Under storage conditions of 4℃, the initial TVB-N value is 5.52 mg / 100 g, and the sensor color is purple-red. The E value was 0.35; after 24 hours of storage, the TVB-N value was 17.6 mg / 100 g (close to spoilage), and the sensor color turned purplish-pink. The E value was 10.22; after 48 hours of storage, the TVB-N value rose to 35.52 mg / 100 g (completely decomposed), and the sensor color turned grayish-blue. The E value is 14.61.

[0056] Correlation analysis: [This refers to the analysis of sensors.] Linear fitting was performed on the E value with the shrimp sample storage time and TVB-N value. The results showed that at 25℃, the storage time and TVB-N value were correlated. The fitted equation for the E value is Y = 0.78611X + 0.7785(R²). 2 =0.9553), TVB-N value and The fitted equation for the E value is Y = 0.77447X - 5.35062 (R²). 2 =0.9967); at 4℃, storage time and The fitted equation for the E value is Y = 0.26671X + 2.22636 (R²). 2 =0.9446), TVB-N value and The fitted equation for the E value is Y = 0.50401X - 0.70663 (R²). 2 =0.8752), indicating that the sensor The E-value is highly correlated with the freshness index of shrimp samples, and this can be detected by changes in sensor color and... The E value accurately determines the freshness of shrimp.

[0057] The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described herein. Although the present invention has been described in detail with reference to the above embodiments, the present invention is not limited to the specific embodiments described above. Therefore, any modifications or equivalent substitutions to the present invention, as well as all technical solutions and improvements that do not depart from the spirit and scope of the invention, are covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a visual sensor based on mulberry anthocyanins, characterized in that, Includes the following steps: Step 1: Preparation of mulberry powder and extraction of mulberry anthocyanins (MAs): Fresh mulberries were placed in an oven and dried to constant weight at 60°C. Then, they were pulverized with a pulverizer and passed through a 100-mesh sieve to remove coarse particles, thus preparing mulberry powder. The mulberry powder was then weighed and added to a 95% ethanol solution. The mixture was intermittently stirred and extracted at room temperature (25°C). The extract was filtered and evaporated to dryness. The MAs powder was collected and stored in the dark and refrigerated. Step 2, Preparation of composite membrane solution: Prepare CS solution and PVA solution separately, mix them at a volume ratio of 1:1 to obtain CS@PVA composite membrane solution, add MAs and citric acid (CA) to it, stir evenly to obtain MAs@CS@PVA membrane solution; Step 3, Sensor preparation: Immerse filter paper in MAs@CS@PVA membrane solution, and dry at room temperature after immersion to obtain a visual sensor.

2. The method for preparing a visual sensor based on mulberry anthocyanins according to claim 1, characterized in that, In step 1, the ratio of mulberry powder to 95% ethanol solution is 1:5 (g / mL), and the extraction time is no less than 6 hours. The extract is filtered and evaporated to dryness at 35°C and 100 mb.

3. The method for preparing a visual sensor based on mulberry anthocyanins according to claim 1, characterized in that, In step 2, the concentration of the CS solution is 1.5%, and the preparation method is to dissolve 3g of CS in 200mL of 1.0% acetic acid aqueous solution and stir at room temperature until completely dissolved; the concentration of the PVA solution is 1.5%, and the preparation method is to dissolve 3g of PVA in 200mL of pure water and stir in a 60℃ water bath for 4h until completely dissolved.

4. The method for preparing a visual sensor based on mulberry anthocyanins according to claim 1, characterized in that, In step 2, the concentration of CA in the MAs@CS@PVA membrane solution is 0.5 mM, and the mass fraction of MAs is 0.5%, 0.75%, 1.0%, 1.25%, or 1.5%.

5. The method for preparing a visual sensor based on mulberry anthocyanins according to claim 1, characterized in that, In step 3, the filter paper is 20mm×20mm in size, the soaking time is 10 minutes, and the drying method is natural drying at room temperature.

6. A visual sensor based on mulberry anthocyanins, characterized in that, The sensor is prepared using the method described in any one of claims 1-5 based on mulberry anthocyanin visualization. The sensor uses qualitative filter paper as a substrate and is loaded with mulberry anthocyanin, chitosan-polyvinyl alcohol composite membrane and citric acid. It exhibits significant color changes within the pH range of 3-11.

7. The visual sensor based on mulberry anthocyanins according to claim 6, characterized in that, The mass fraction of MAs in the sensor is 1.25%, and the sensor is stored at 4°C in the dark for 15 days. With an E value ≤ 0.99 ± 0.21, it exhibits high sensitivity to volatile ammonia at concentrations of 0.025%–0.1%, with a response sensitivity of 11.79%–18.35% within 30 minutes.

8. The application of the visual sensor according to claim 7 in monitoring the freshness of aquatic products, characterized in that, The sensor was fixed to the top of a sealed container and placed in a sealed environment along with the aquatic products. The sensor's color change and... Changes in the E value are used to monitor the freshness of aquatic products.

9. The application of the visual sensor according to claim 8 in monitoring the freshness of aquatic products, characterized in that, The aquatic product in question is Litopenaeus vannamei (whiteleg shrimp). Monitoring conditions include storage at 25℃ or 4℃ in a light-proof, sealed environment. Color changes are recorded by taking photos with a smartphone and then combined with L, a, b, R, G, and B parameters measured by a colorimeter to calculate... E value, and simultaneously detect TVB-N value of aquatic products, to establish The relationship between E-value and the freshness of aquatic products.

10. The application of the visual sensor according to claim 9 in monitoring the freshness of aquatic products, characterized in that, When the TVB-N value of the whiteleg shrimp exceeds 20 mg / 100g, the sensor color changes from purplish-red to grayish-purple or grayish-blue. If the E value is ≥14.61, the aquatic product is considered to be spoiled and inedible.

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