Sensor for indicating freshness and preparation method and application thereof

Abstract

The invention discloses a sensor for indicating freshness and a preparation method and application thereof, and belongs to the technical field of food quality monitoring. According to the composite natural pigment colorimetric sensor, hydroxypropyl methyl cellulose and sodium alginate are used as polymer substrates, a film substrate is formed by combining the polymer substrates with a film-forming aid glycerol, and alkannin and curcumin are loaded on the film-forming substrate. The sensor can visually judge or compare the freshness degree of salmon by observing the color change of the intelligent sensor, the freshness change is clear at a glance, the sensor can also be used for detecting the freshness of penaeus vannamei, the application prospect is wide, and a new thought is provided for development and application of a novel aquatic product freshness intelligent response sensor.

Description

Technical Field

[0001] The present invention relates to the technical field of food quality monitoring, and particularly to a sensor for indicating freshness, a preparation method thereof, and an application thereof. Background Art

[0002] The real-time monitoring of the freshness of aquatic products is crucial for food quality and consumer safety. The spoilage of aquatic products not only causes huge economic losses and resource waste but also poses a serious threat to health. Since aquatic products generally contain a large amount of protein and fatty acids, they are extremely prone to deterioration during transportation and storage, reducing their nutritional value and producing harmful substances to the human body. Therefore, it is imperative to conduct real-time visual monitoring of the freshness of aquatic products.

[0003] In the past, various methods for evaluating freshness have been reported, but some of them are time-consuming in operation, some are destructive to samples, some require personnel with professional training to participate, and some are not suitable for on-site or large-scale operations. Therefore, it is crucial to develop a method for portable, non-destructive, and real-time monitoring of the freshness of aquatic products to ensure the quality and safety of aquatic products and protect the health of consumers. In this regard, using pH-sensitive pigments to prepare freshness indicator films is a feasible direction.

[0004] Currently, in the prior art, Patent CN 114805875 A discloses a low-temperature forming visual intelligent indicator film for the freshness of pork, a preparation method thereof, and an application thereof; in this method, a sodium alginate solution and a gelatin solution are homogenized, then glycerol is added, and finally a shikonin solution is added to obtain a film-forming solution; the film-forming solution is ultrasonically homogenized and defoamed to obtain a film-forming liquid; the film-forming liquid is cast and spread on a film embryo, and then formed at a low temperature, and the film is peeled off to obtain an intelligent indicator film in which shikonin is dispersed in a molecular free state. However, shikonin is purplish-red, and the color change of the prepared freshness indicator film is not obvious and is not easily observable by the naked eye. Summary of the Invention

[0005] Technical Problem

[0006] Using natural pigments to prepare freshness indicator films is a simple, portable, and real-time method for monitoring the freshness of aquatic products. However, some of the indicator films in the current prior art still have the problem that the color change is not obvious and cannot be distinguished by the naked eye.

[0007] Technical Content

[0008] To achieve the above object and other related objects, the technical solution provided by the present invention is: a sensor for colorimetric comparison of composite natural pigments, using hydroxypropyl methylcellulose and sodium alginate as polymer matrices, combining with a film-forming aid glycerol to form a film matrix, and the film matrix is loaded with composite natural pigments shikonin and curcumin.

[0009] The present invention provides a sensor for the colorimetry of a composite natural pigment. The sensor uses hydroxypropyl methylcellulose and sodium alginate as a polymer matrix, combines with a film-forming aid glycerol to form a film matrix, and a composite natural pigment is loaded on the film matrix.

[0010] Furthermore, the composite pigment is shikonin and curcumin.

[0011] Furthermore, the mass ratio of shikonin to curcumin in the composite pigment is 0.8 - 1.2:1.

[0012] Preferably, the mass ratio of shikonin to curcumin in the composite pigment is 1:1.

[0013] Furthermore, the loading amount of the composite natural pigment is 1 - 2% of the mass of the film matrix.

[0014] Preferably, the loading amount of the composite natural pigment is 1.667% of the mass of the film matrix.

[0015] The present invention also provides a preparation method of a sensor for the colorimetry of a composite natural pigment. The preparation method includes the following steps:

[0016] Mix sodium alginate and hydroxypropyl methylcellulose with water by stirring, add the composite pigment and glycerol and continue stirring to obtain a film-forming solution; pour the film-forming solution into a mold and dry it to obtain the sensor for the colorimetry of the composite natural pigment.

[0017] Furthermore, the composite pigment is shikonin and curcumin.

[0018] Furthermore, the mass ratio of shikonin to curcumin in the composite pigment is 0.8 - 1.2:1.

[0019] Preferably, the mass ratio of shikonin to curcumin in the composite pigment is 1:1.

[0020] Furthermore, the content of sodium alginate in the film-forming solution is 0.01 - 0.02 g / mL.

[0021] Furthermore, the content of hydroxypropyl methylcellulose in the film-forming solution is 0.01 - 0.02 g / mL.

[0022] Furthermore, the content of the composite pigment in the film-forming solution is 1 - 2 mg / mL.

[0023] Preferably, the content of the composite pigment in the film-forming solution is 1 mg / mL.

[0024] Furthermore, the content of glycerol in the film-forming solution is 0.01 - 0.02 g / mL.

[0025] Furthermore, the stirring is carried out at 40 - 70 °C for 0.5 - 6 hours.

[0026] Further, the drying is carried out in an environment of 20 - 25°C and 40 - 50% RH for 12 - 48 hours.

[0027] The present invention also provides a method for detecting freshness using the above-mentioned composite natural pigment colorimetric sensor, and the method includes:

[0028] Placing the composite natural pigment colorimetric sensor on the product to be tested and sealing it for a period of time;

[0029] When the sample to be tested is salmon, when the color difference of the composite natural pigment colorimetric sensor becomes 30, the product to be tested reaches a spoiled state and is inedible;

[0030] When the sample to be tested is shrimp, when the color difference of the composite natural pigment colorimetric sensor becomes 20, the product to be tested reaches a spoiled state and is inedible.

[0031] The beneficial effects of the present invention compared with the prior art are:

[0032] 1. The color change of the composite natural pigment colorimetric sensor of the present invention is obvious at a glance, and can intuitively judge or compare the freshness of salmon, and the detection effect is applicable to South American white shrimp. It is a sensor with a wide range of applications and can visually detect the freshness of aquatic products.

[0033] 2. The sensor prepared by the present invention is sensitive in reaction and can detect trimethylamine at the μM level. It can also achieve real-time detection in the freshness detection of aquatic products, and reflect the degree of salmon spoilage through color difference changes, providing new ideas for the development and application of new aquatic product freshness sensors. Description of the Drawings

[0034] Figure 1 Are the apparent color and ultraviolet absorption spectra of shikonin, curcumin and their mixed solutions at different pH values; among them, (a) - (g) correspond to Comparative Example 1, Examples 1 - 5 and Comparative Example 2 respectively.

[0035] Figure 2 (a) is the FTIR spectrum of shikonin, curcumin, sodium alginate, and hydroxypropyl methylcellulose powder, Figure 2 (b) is the FTIR spectrum of Comparative Example 1, Comparative Example 2, and Examples 1, 2, 3, 4, 5.

[0036] Figure 3 Are the X-ray diffraction patterns of Comparative Example 1, Comparative Example 2, and Examples 1, 2, 3, 4, 5.

[0037] Figure 4Thermal field scanning electron microscope images of Comparative Example 1, Comparative Example 2, and Example 1, Example 2, Example 3, Example 4, and Example 5.

[0038] Figure 5 Relationship between the a value and trimethylamine vapor concentration for Comparative Example 1, Comparative Example 2, and Example 1, Example 2, Example 3, Example 4, and Example 5 exposed to different concentrations of trimethylamine vapor.

[0039] Figure 6 Changes and color parameters of salmon and Comparative Example 1, Comparative Example 2, and Example 1, Example 2, Example 3, Example 4, and Example 5 stored at 4°C for 12 days; where (a) shows the storage changes of salmon, and (b) - (e) show the changes in color difference values.

[0040] Figure 7 Change in the content of volatile basic nitrogen in salmon at different storage days at 4°C.

[0041] Figure 8 Changes and color parameters of white shrimp from South America and Comparative Example 1, Comparative Example 2, and Example 1, Example 2, Example 3, Example 4, and Example 5 stored at 4°C for 12 days; where (a) shows the storage changes of white shrimp from South America, and (b) - (e) show the changes in color difference values.

[0042] Figure 9 Changes and color parameters of salmon and Comparative Example 3 stored at 4°C for 12 days; where (a) shows the storage changes of salmon, and (b) - (e) show the changes in color difference values. Detailed implementation mode

[0043] The following specific embodiments illustrate the implementation mode of the present invention. Those familiar with this technology can easily understand other advantages and effects of the present invention from the content disclosed in this embodiment.

[0044] It should be noted that the structures, ratios, sizes, etc. shown in the drawings of this specification are only used to cooperate with the content disclosed in the specification for those familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention. Therefore, they do not have technical substantive significance. Any modification of the structure, change in the proportional relationship, or adjustment of the size. The following embodiments are provided to better understand the present invention, rather than to limit the present invention. The experimental methods in the following embodiments are all conventional methods unless otherwise specified. The experimental materials used in the following embodiments are all obtained from regular biochemical reagent stores unless otherwise specified.

[0045] In the following examples, hydroxypropyl methyl cellulose (HPMC) was provided by Shanghai Aladdin Biochemical Technology Co., Ltd.; shikonin (SHK) was provided by Shenzhen Ketong Technology Co., Ltd., Shanghai Energy Biochemical Co., Ltd.; curcumin (CUR) was provided by Macklin Biochemical Technology Co., Ltd. (Shanghai, China); trimethylamine solution (TMA, 35%) was provided by Macklin Biochemical Technology Co., Ltd. (Shanghai, China); sodium alginate (SA), glycerol, and ammonia (25%) were provided by Tianjin Damao Chemical Reagent Factory; all experimental reagents were of analytical grade. Frozen Atlantic salmon was provided by Run Kong Food Supply Chain (Guangdong) Co., Ltd.

[0046] Comparative Example 1

[0047] Step 1: Add 0.010 g of shikonin powder to 10 mL of ethanol, stir for 2 - 4 h under magnetic stirring conditions, and sonicate for 15 min to prepare a shikonin alcohol solution for storage.

[0048] Step 2: Mix 0.2 g of sodium alginate and 0.4 g of hydroxypropyl methyl cellulose with 15 mL of deionized water. After continuously stirring in a 60°C water bath for 30 min, add the shikonin alcohol solution (10 mL, 1 mg / mL) and 0.2 g of glycerol. Continue stirring for 2 - 3 h to ensure uniform mixing and eliminate hidden air bubbles to obtain a film-forming solution. Pour the film-forming solution onto a disposable plastic petri dish, with a flat thickness of 0.9 cm, and dry it in a constant temperature and humidity chamber at 25°C and 50% RH for 24 h to obtain a film named SHK / HA film, which is a sensor for natural pigment colorimetry.

[0049] Example 1

[0050] Step 1: Add 0.008 g of shikonin powder and 0.002 g of curcumin powder to 10 mL of ethanol, stir for 2 - 4 h under magnetic stirring conditions, and sonicate for 15 min to prepare a composite pigment alcohol solution for storage.

[0051] Step 2: Mix 0.2 g of sodium alginate and 0.4 g of hydroxypropyl methyl cellulose with 15 mL of deionized water. After continuously stirring in a 60°C water bath for 30 min, add the composite pigment alcohol solution (10 mL, 1 mg / mL) and 0.2 g of glycerol. Continue stirring for 2 - 3 h to ensure uniform mixing and eliminate hidden air bubbles to obtain a film-forming solution. Pour the film-forming solution onto a disposable plastic petri dish, with a flat thickness of 0.9 cm, and dry it in a constant temperature and humidity chamber at 25°C and 50% RH for 24 h to obtain a film named S8C2 / HA film, which is a sensor for composite natural pigment colorimetry.

[0052] Example 2

[0053] Step 1: Add 0.006 g of shikonin powder and 0.004 g of curcumin into 10 mL of ethanol, stir for 2 - 4 h under magnetic stirring conditions, and ultrasonicate for 15 min to obtain a composite pigment ethanol solution, which is stored for later use.

[0054] Step 2: Mix 0.2 g of sodium alginate and 0.4 g of hydroxypropyl methylcellulose with 15 mL of deionized water. After continuously stirring in a 60 °C water bath for 30 min, add the composite pigment ethanol solution (10 mL, 1 mg / mL) and 0.2 g of glycerol. Continue stirring for 2 - 3 h to ensure uniform mixing and eliminate hidden air bubbles to obtain a film-forming solution. Pour the film-forming solution onto a disposable plastic petri dish by casting, with a flat thickness of 0.9 cm, and dry it in a constant temperature and humidity chamber at 25 °C and 50% RH for 24 h to obtain a film, named S6C4 / HA film, which is a sensor for colorimetric detection of composite natural pigments.

[0055] Example 3

[0056] Step 1: Add 0.005 g of shikonin powder and 0.005 g of curcumin into 10 mL of ethanol, stir for 2 - 4 h under magnetic stirring conditions, and ultrasonicate for 15 min to obtain a composite pigment ethanol solution, which is stored for later use.

[0057] Step 2: Mix 0.2 g of sodium alginate and 0.4 g of hydroxypropyl methylcellulose with 15 mL of deionized water. After continuously stirring in a 60 °C water bath for 30 min, add the composite pigment ethanol solution (10 mL, 1 mg / mL) and 0.2 g of glycerol. Continue stirring for 2 - 3 h to ensure uniform mixing and eliminate hidden air bubbles to obtain a film-forming solution. Pour the film-forming solution onto a disposable plastic petri dish by casting, with a flat thickness of 0.9 cm, and dry it in a constant temperature and humidity chamber at 25 °C and 50% RH for 24 h to obtain a film, named S5C5 / HA film, which is a sensor for colorimetric detection of composite natural pigments.

[0058] Example 4

[0059] Step 1: Add 0.004 g of shikonin powder and 0.006 g of curcumin into 10 mL of ethanol, stir for 2 - 4 h under magnetic stirring conditions, and ultrasonicate for 15 min to obtain a composite pigment ethanol solution, which is stored for later use.

[0060] Step 2: Mix 0.2 g of sodium alginate and 0.4 g of hydroxypropyl methylcellulose with 15 mL of deionized water. After continuously stirring for 30 min in a 60 °C water bath, add the composite pigment alcohol solution (10 mL, 1 mg / mL) and 0.2 g of glycerol. Continue stirring for 2 - 3 h to ensure uniform mixing and eliminate hidden bubbles, obtaining a film-forming solution. Pour the film-forming solution by casting into a disposable plastic petri dish with a flat thickness of 0.9 cm, and dry it in a constant temperature and humidity chamber at 25 °C and 50% RH for 24 h to obtain a film, named S4C6 / HA film, which is a sensor for colorimetric analysis of composite natural pigments.

[0061] Example 5

[0062] Step 1: Add 0.002 g of shikonin powder and 0.008 g of curcumin to 10 mL of ethanol, stir for 2 - 4 h under magnetic stirring conditions, and ultrasonicate for 15 min to obtain a composite pigment alcohol solution, which is stored for later use.

[0063] Step 2: Mix 0.2 g of sodium alginate and 0.4 g of hydroxypropyl methylcellulose with 15 mL of deionized water. After continuously stirring for 30 min in a 60 °C water bath, add the composite pigment alcohol solution (10 mL, 1 mg / mL) and 0.2 g of glycerol. Continue stirring for 2 - 3 h to ensure uniform mixing and eliminate hidden bubbles, obtaining a film-forming solution. Pour the film-forming solution by casting into a disposable plastic petri dish with a flat thickness of 0.9 cm, and dry it in a constant temperature and humidity chamber at 25 °C and 50% RH for 24 h to obtain a film, named S2C8 / HA film, which is a sensor for colorimetric analysis of composite natural pigments.

[0064] Comparative Example 2

[0065] Step 1: Add 0.010 g of curcumin to 10 mL of ethanol, stir for 2 - 4 h under magnetic stirring conditions, and ultrasonicate for 15 min to obtain a curcumin alcohol solution, which is stored for later use.

[0066] Step 2: Mix 0.2 g of sodium alginate and 0.4 g of hydroxypropyl methylcellulose with 15 mL of deionized water. After continuously stirring for 30 min in a 60 °C water bath, add the curcumin alcohol solution (10 mL, 1 mg / mL) and 0.2 g of glycerol. Continue stirring for 2 - 3 h to ensure uniform mixing and eliminate hidden bubbles, obtaining a film-forming solution. Pour the film-forming solution by casting into a disposable plastic petri dish with a flat thickness of 0.9 cm, and dry it in a constant temperature and humidity chamber at 25 °C and 50% RH for 24 h to obtain a film, named CUR / HA film, which is a sensor for colorimetric analysis of natural pigments.

[0067] Example 6

[0068] 1. Ultraviolet-visible absorption spectrum: The ultraviolet spectra of shikonin and curcumin solutions and the composite pigment solution at different pH values were obtained using an ultraviolet-visible spectrophotometer (SP-2500, UV-Vis, Shanghai Spectrum Instruments Co., Ltd.). The specific operation is as follows: Weigh 0.010 g of the powder sample, dissolve it in 10 mL of ethanol, take 100 μL and place it in a 10 mL sample bottle, and finally use 10 mL of solution prepared with BR buffer solution of different pH values for testing to obtain the ultraviolet-visible absorption spectrum.

[0069] Figure 1 It is the ultraviolet spectrum diagram of shikonin and curcumin solutions and the composite pigment solution at different pH values. It can be seen from the figure that for shikonin, the maximum light absorption peak appears in the range of 490 - 560 nm. Due to the structural transformation, a peak shift is observed in the alkaline state. While for curcumin, as the pH increases, the β-diketone structure changes to the keto-enol structure, and the maximum light absorption peak undergoes a red shift. The composite pigment also shows a corresponding shift, indicating that these two pigments can have an obvious indication effect on pH changes.

[0070] 2. Infrared spectrum: Using a Fourier transform infrared spectrometer produced by PerkinElmer Co., Ltd., the potassium bromide tablet method and attenuated total reflection (ATR reflection) were adopted. The specific operation is as follows: Weigh spectroscopic pure potassium bromide powder and shikonin powder, curcumin powder, sodium alginate powder, and hydroxypropyl methylcellulose powder at a ratio of 150:1 respectively, place them in a mortar and grind them until the powder size is less than 2.5 μm. After grinding, take out the sample and put it into a tablet press to make a transparent or semi-transparent and evenly thick and thin round tablet. Take out the sample and conduct the test. For Comparative Examples 1 and 2 and Examples 1 - 5, the attenuated total reflection accessory was used for measurement, the number of scans was 32, and the resolution was 4 cm -1 , and the measurement range was 4000 - 500 cm -1 , to obtain the infrared spectrum.

[0071] Figure 2 (a) is shikonin powder, curcumin powder, sodium alginate powder, and hydroxypropyl methylcellulose powder, Figure 2 (b) is the FTIR spectrum diagrams of Comparative Examples 1 and 2 and Examples 1 - 5. In curcumin, the characteristic peak observed at 3503 cm -1 is due to the stretching vibration of phenol, and the peaks observed at 1628 and 1511 cm -1 are due to carbonyl and vinyl respectively. The peak found at 1283 cm -1 refers to the C=O stretching frequency of the ether group, and the peak found at 1029 cm -1 is due to the C bending vibration of the C-H olefin group. The absorption peak at 3400 cm -1 is the stretching vibration absorption of the hydroxyl group in shikonin, and 1623 cm-1 The absorption band belongs to the symmetric stretching vibration of sodium alginate - COOH; 2933 cm -1 The stretching vibration of the unsaturated carbon - hydrogen bond - CH=, 3466 cm -1 There is an absorption peak belonging to the hydroxyl group of hydroxypropyl methylcellulose at; 1356 - 1650 cm -1 At is the stretching vibration absorption of the carboxyl C - O - in sodium alginate and the bending vibration absorption of C - H in - CH2 and - CH on hydroxypropyl methylcellulose, 1535 - 1686 cm -1 The absorption peak at is the stretching vibration absorption of the benzene ring and C=C on the C ring in shikonin; the stretching vibration absorption of C=O in the carboxyl group of sodium alginate; 966 - 1115 cm -1 At is the bending vibration absorption of the hydrogen atom on the double - bond carbon in shikonin and the stretching vibration absorption of the ether bond in sodium alginate and hydroxypropyl methylcellulose. The peaks of most of the comparative examples and examples based on hydroxypropyl methylcellulose are similar, only with slight changes in peak intensity. The change in peak intensity is likely due to the physical interaction (van der Waals interaction and hydrogen bonding) between the phenolic and naphthoquinone groups in the biopolymer and the colorant. The results of the infrared spectroscopy experiment prove the successful preparation of the composite film.

[0072] 3. X - ray diffraction pattern: The X - ray diffraction (XRD) patterns of Comparative Examples 1 and 2 and Examples 1 - 5 were collected using a Shimadzu XRD - 7000S diffractometer (Shimadzu, Japan) in the range of 2θ from 10° to 70°.

[0073] As Figure 3 shown, Comparative Examples 1 and 2 and Examples 1 - 5 have a broad diffraction peak at 2θ = 20°, indicating that the prepared composite natural pigment colorimetric sensor exhibits semi - crystalline characteristics. After adding curcumin, the positions of the diffraction peaks of the films of Examples 1 - 5 do not shift, also showing semi - crystalline characteristics; for Examples 1 - 5 with added curcumin, the positions of the diffraction peaks do not shift, and the intensity increases slightly, indicating that the crystallinity of Examples 1 - 5 after adding curcumin is better.

[0074] 4. Thermal field emission scanning electron microscopy: The surface morphologies of Comparative Examples 1 and 2 and Examples 1 - 5 were observed using a thermal field emission scanning electron microscope (FSEM, JSM - 7800F, JEOL, Japan) with an acceleration voltage of 5.0 kV. The samples were coated with gold by vacuum sputtering for 120 s before testing.

[0075] Figure 4The specimens of Comparative Examples 1 and 2 and Examples 1-5 were all intact, without voids or cracks. The two colorants were evenly dispersed on the polymer matrix without forming aggregates. There was slight delamination, granularity, obvious unevenness, and obvious wrinkles in Comparative Examples 1 and 2 and Examples 1-5, which increased the specific surface area in contact with volatile gases and effectively improved the gas-sensing performance of Comparative Examples 1 and 2 and Examples 1-5.

[0076] 5. Detection limit for TMA: TMA is a volatile gas produced by typical spoiled meat products. At room temperature, the colorimetric sensor was placed in the headspace of the test chamber. A TMA solution was injected into the test chamber. After 20 min, the reaction reached equilibrium, and the color of the colorimetric sensor was measured using a benchtop colorimeter (YS6060, Shenzhen 3nh Technology Co., Ltd.).

[0077] The color changes of Comparative Examples 1 and 2 and Examples 1-5 were described using the a* value. As Figure 5 shown, when the TMA concentration reached 260 μM, the a* value changed with the change in TMA concentration. There was a linear relationship between a* and the TMA concentration in the range of 20-160 μM of the TMA concentration, and the corresponding calibration curve was expressed as an equation. Therefore, the detection limit (LOD) of Comparative Examples 1 and 2 and Examples 1-5 for TMA was determined by using the equation.

[0078] According to the relationship between the a value and the trimethylamine vapor concentration in each comparative example and example, the detection limit (Table 1) and the linear range of detection of trimethylamine for each example and comparative example were calculated. As can be seen from Table 1, the detection limits of Examples 1, 2, 3, and 5 were lower than those of Comparative Examples 1 and 2, demonstrating that the composite natural pigment could enhance the sensitivity of the colorimetric sensor to trimethylamine, further proving that the examples had good application potential in food freshness indication.

[0079] Table 1 Detection limit and linear range of the colorimetric sensor for trimethylamine

[0080] Comparative Example 1 Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 2 Detection Limit (μM) 29.58 22.50 17.77 19.70 37.85 15.04 26.72 Linear Range (μM) 40～160 160～260 80～180 80～200 120～240 80～200 40～160

[0081] 6. Salmon storage experiment: 30 g of thawed Atlantic salmon was placed in a disposable plastic petri dish with a diameter of 90 mm. The membranes (i.e., Comparative Examples 1 and 2 and Examples 1-5) were cut into rectangles of 3.5 cm × 4 cm, fixed to the lid of the petri dish with glue, placed in the headspace of the petri dish without contacting the salmon, and the petri dish was stored in a refrigerator at 4 °C for 12 days. During storage, the color of the colorimetric sensor was measured using a benchtop colorimeter, and the changes in the salmon meat were photographed and recorded.

[0082] Figure 6(a) It can be seen that as the storage time of salmon increases, the color of Comparative Example 1 changes from purple to purplish gray, and Examples 1-5 mainly show a change from yellowish green to reddish brown, and the color of Comparative Example 2 changes from yellow to reddish brown. However, from Figure 6 (b), it can be seen that there are differences in the total color difference values of Examples 1-5. In particular, the color difference value of Example 3 is the highest (36.77), and the color difference values of Example 1 (33.25), Example 2 (30.59), and Example 5 (34.37) are all higher than those of Comparative Example 1 (13.66) and Comparative Example 2 (25.54). This shows that the compound pigment of shikonin and curcumin is superior to the indicating effect of single pigments, and the colorimetric sensor prepared from the compound pigment can indicate the freshness of salmon and has potential application ability in the freshness monitoring of animal aquatic products.

[0083] 7. Determination of total volatile basic nitrogen (TVB-N): Frozen Atlantic salmon from Walmart Supermarket (Dalian, China) was cut into square blocks, and each salmon sample weighed about 10 ± 0.5 g. To be closer to the actual storage conditions, each piece of chilled salmon meat was sealed and packaged in a self-sealing bag. These self-sealing bags containing salmon meat were placed in a refrigerator and stored at 4.0 ± 0.5 °C for 12 d. At the same time, the change in the TVB-N value in the salmon meat during storage was determined using the national standard GB 5009.228—2016 "Determination of Total Volatile Basic Nitrogen in Foods". GB 2733—2015 "Fresh and Frozen Animal Aquatic Products" stipulates that the TVB-N content of seawater fish should not be higher than 30 mg / 100 g for fresh meat.

[0084] It can be seen from Figure 7 that the TVB-N content was 26.43 mg / 100 g on the 9th day of salmon storage and reached 32.66 mg / 100 g on the 10th day, exceeding the TVB-N content limit in GB 2733-2015, indicating that the salmon reached a spoiled state and was inedible on the 10th day of storage.

[0085] 8. Storage experiment of white shrimp Litopenaeus vannamei: 30 g of slow-frozen white shrimp Litopenaeus vannamei was placed in a disposable plastic petri dish with a diameter of 90 mm. The membranes (i.e., Comparative Examples 1 and 2 and Examples 1-5) were cut into rectangles of 3.5 cm × 4 cm and fixed on the lid of the petri dish with glue, placed in the headspace of the petri dish without contacting the white shrimp Litopenaeus vannamei. The petri dish was placed in a refrigerator at 4 °C for 10 days. During storage, a desktop colorimeter was used to test the color of the colorimetric sensor and the change of the white shrimp Litopenaeus vannamei was photographed and recorded.

[0086] Figure 8 (a) It can be seen that as the storage time of salmon increases, the color of Comparative Example 1 changes from purple to purplish gray, and Examples 1-5 mainly show a change from yellowish green to reddish brown, and the color of Comparative Example 2 changes from yellow to reddish brown. However, from Figure 8It can be seen from (b) that there are differences in the total color difference values of Examples 1-5. In particular, the color difference value of Example 3 is the highest (37.04), and that of Example 2 (31.59) is higher than those of Comparative Example 1 (19.19) and Comparative Example 2 (29.64). This shows that the compound pigment of shikonin and curcumin is superior to the indicating effect of single pigments. The colorimetric sensor prepared from the compound pigment can indicate the freshness of whiteleg shrimp and has potential application ability in the freshness monitoring of animal aquatic products.

[0087] Comparative Example 3

[0088] Step 1: Shikonin powder (SHK) and sodium copper chlorophyllin powder (CHL) were mixed and added to 10 mL of ethanol according to the mass ratios of 1:0, 7:3, 5:5, 3:7, and 0:1 (total mass is 0.010 g) respectively. Under the condition of magnetic stirring, it was stirred for 2-4 h and ultrasonicated for 15 min to prepare a compound pigment alcohol solution, which was stored for later use.

[0089] Step 2: 0.2 g of sodium alginate and 0.4 g of hydroxypropyl methylcellulose were mixed with 15 mL of deionized water. After continuously stirring in a 60 °C water bath for 30 min, the compound pigment alcohol solution (10 mL, 1 mg / mL) and 0.2 g of glycerol were added. Stirring was continued for 2-3 h to ensure uniform mixing and eliminate hidden bubbles to obtain a film-forming solution. The film-forming solution was cast into a disposable plastic petri dish, and the flattened thickness was 0.9 cm. It was dried in a constant temperature and humidity box at 25 °C and 50% RH for 24 h to obtain a film, that is, a colorimetric sensor of the compound natural pigment.

[0090] The colorimetric sensor of the compound natural pigment prepared above was used for the storage experiment of salmon: 30 g of slow-frozen Atlantic salmon was placed in a disposable plastic petri dish with a diameter of 90 mm. The film was cut into a rectangle of 3.5 cm × 4 cm and fixed on the petri dish cover with glue, placed in the headspace of the petri dish without contacting the salmon. The petri dish was placed in a 4 °C refrigerator for storage for 12 days. During the storage period, a desktop colorimeter was used to test the color of the colorimetric sensor and take pictures to record the changes of the salmon meat.

[0091] Figure 9 It can be seen that for the mixed pigment of shikonin and sodium copper chlorophyllin, as the storage time increases, the colorimetric sensor mainly reflects the role of shikonin, and the color difference values of the compound pigments 23.98 (7:3), 14.07 (5:5), and 18.48 (3:7) are all lower than the color difference value of single shikonin (26.31), indicating that the compound effect of shikonin and sodium copper chlorophyllin is not as good as that of shikonin and curcumin.

[0092] The embodiments provided above are not intended to limit the scope covered by the present invention, nor are the described steps intended to limit the order of their execution. Obvious improvements made by those skilled in the art in combination with the existing well-known general knowledge also fall within the protection scope defined by the claims of the present invention.

Claims

1. A sensor for colorimetric determination of a composite natural pigment, characterized in that, The sensor uses hydroxypropyl methylcellulose and sodium alginate as polymer matrices, combines with the film-forming aid glycerol to form a film matrix, and a composite natural pigment is loaded on the film matrix; the composite pigment is shikonin and curcumin; the mass ratio of shikonin to curcumin in the composite pigment is 0.8-1.2:1; The loading amount of the composite natural pigment is 1-2% of the mass of the film matrix.

2. The sensor according to claim 1, wherein The mass ratio of shikonin to curcumin in the composite pigment is 1:

1.

3. The sensor according to claim 1, wherein The loading amount of the composite natural pigment is 1.667% of the mass of the film matrix.

4. A method for preparing the composite natural pigment colorimetric sensor according to any one of claims 1 to 3, characterized in that, The preparation method includes the following steps: Stir and mix sodium alginate and hydroxypropyl methylcellulose with water, add the composite pigment and glycerol and continue to stir to obtain a film-forming solution; pour the film-forming solution into a mold and dry it to obtain the sensor for colorimetric analysis of the composite natural pigment; The composite pigment is shikonin and curcumin; the mass ratio of shikonin to curcumin in the composite pigment is 0.8-1.2:

1.

5. According to the preparation method described in claim 4, characterized in that, The content of sodium alginate in the film-forming solution is 0.01-0.02 g / mL.

6. According to the preparation method described in claim 4, characterized in that, The content of hydroxypropyl methylcellulose in the film-forming solution is 0.01-0.02 g / mL.

7. According to the preparation method described in claim 4, wherein The content of the composite pigment in the film-forming solution is 1-2 mg / mL.

8. According to the preparation method described in claim 4, characterized in that, The content of glycerol in the film-forming solution is 0.01-0.02 g / mL.

9. A method for detecting freshness using the composite natural pigment colorimetric sensor according to any one of claims 1-3, the method comprising: Place the composite natural pigment colorimetric sensor on the product to be tested and seal it for a period of time; When the product to be tested is salmon, when the color difference of the composite natural pigment colorimetric sensor becomes 30, the product to be tested reaches a spoiled state and is inedible; when the product to be tested is shrimp, when the color difference of the composite natural pigment colorimetric sensor becomes 20, the product to be tested reaches a spoiled state and is inedible.

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