Visual indicator film for freshness of fresh-cut fruits, preparation method, use and detection method

Abstract

The present invention provides a visual indicator film for the freshness of fresh-cut fruits, a preparation method, the use and a detection method. The preparation method comprises the following steps: preparing a Co-based MOF; washing and activating the Co-MOF; preparing a CMC-Na-based thin film; and preparing a CMC-Na-based thin film, and thereby obtaining an indicator film for phenol red (PR) or an indicator film for bromothymol blue (BTB). In the present invention, an irregular spherical Co-based metal organic framework (Co-MOF) loaded with PR or BTB is prepared, and the nanocomposites Co-MOF / PR and Co-MOF / BTB are obtained after dyes and the MOF are adsorbed by means of the hydrogen bond interaction. Then, the nanocomposites are embedded into a sodium carboxymethyl cellulose (CMC-Na) base material to prepare CO2-responsive packaging materials, namely, a CMC-Na / PR / Co-MOF and a CMC-Na / BTB / Co-MOF, which are used for monitoring the freshness of fruits in real time and in situ. The Co-MOF-based composite thin film has good water resistance, time-temperature stability and mechanical properties, and also exhibits enhanced ultraviolet-visible light barrier properties, dye migration resistance and thermal stability. The visual indicator film for the freshness of fresh-cut fruits can effectively reflect the concentration of CO2, and is suitable for real-time and in-situ monitoring of the freshness of fruits.

Description

Visual indicator film for freshness of fresh-cut fruits, preparation method, application and detection method TECHNICAL FIELD

[0001] The present application belongs to the technical field of intelligent food packaging, and particularly relates to a visual indicator film for freshness of fresh-cut fruits, a preparation method, an application and a detection method, especially for freshness detection of fresh-cut fruits. BACKGROUND

[0002] At present, fresh-cut fruits have become a key and rapidly expanding industry due to their convenience and the satisfaction of consumer demand. The treatment of the surface of fruits not only simplifies the preparation and carrying of products, but also maintains the nutritional ingredients, flavor and freshness of the products, thereby promoting the widespread popularity of fresh-cut fruits. However, fresh-cut fruits are prone to bacterial contamination and mechanical damage, which can cause a sharp decline in the quality (including nutrition and freshness) of the fruits, manifesting as flesh browning, increased respiration rate and ethylene production, and ultimately accelerating the loss of freshness. Despite this, consumers may not be able to accurately identify these quality changes, especially in the early stages. Therefore, it is particularly important to develop a cost-effective and non-destructive visual monitoring method to evaluate the freshness of fruits.

[0003] The original intention of intelligent packaging is to monitor the state of food throughout its life cycle, identify or alert any changes in food quality, and provide convenience for consumers. For example, in an intelligent packaging system, a pH-responsive indicator film exhibits visible color changes corresponding to the freshness and shelf life of food by interacting with volatile gases produced by microorganisms and enzymes during food degradation. In view of the negative impact of petroleum-based materials on the environment, renewable, biodegradable and environmentally friendly biopolymers have received increasing attention in the development of intelligent packaging materials.

[0004] Sodium carboxymethylcellulose (CMC-Na) is a non-toxic, renewable and biocompatible polymer extracted from cellulose. It protects fresh-cut products from external contamination by forming a viscous pseudoplastic film, and is widely used in the sustainable food packaging industry due to its unique film-forming and oxygen barrier properties. However, the widespread use of pure CMC-Na films in food packaging is limited by their low mechanical properties, poor water resistance and high light transmittance. In order to improve the performance of packaging films, numerous studies have modified the films using nanomaterials, ultimately constructing flexible polymer / rigid nanoparticle composites. Therefore, the development of additive materials that can enhance the physical properties and activity of current composite films is crucial for promoting the application of CMC-Na-based films in fresh-cut fruit packaging.

[0005] CO2-sensitive pigments are another important responsive factor in food packaging, and acid-base indicators have been used to monitor the freshness of fruits. In recent years, synthetic dyes and natural colorants are often used as sensing materials in polymer matrices of intelligent packaging to detect changes in food freshness. However, due to external influences (temperature, ambient light, etc.) and safety issues, the practical application of natural colorants and synthetic dyes is limited. In addition, dye migration is a long-standing problem that leads to inaccurate visual evaluation.

[0006] SUMMARY

[0007] In view of the above technical problems, the present application provides a visual indication film for fresh-cut fruit freshness, a preparation method, an application and a detection method.

[0008] In the present application, metal organic frameworks (MOF) are composed of coordination polymers that self-assemble into ordered structures through coordination bonds, with metal ions or clusters as nodes and organic ligands as links. The synthesis process is simple, has long-term stability, and adjustable functionality. In addition, the unique inorganic-organic hybrid characteristics of MOF materials help to improve their compatibility with the polymer matrix, making them an ideal choice for manufacturing nanoscale composites with improved mechanical properties, moisture resistance and salt selectivity. Therefore, the present application uses MOF as a nanofiller for enhancing the matrix and carrier system to develop intelligent packaging films suitable for monitoring the freshness of fresh-cut fruits.

[0009] In the present application, phenol red (PR) and bromothymol blue (BTB) have pH-sensitive properties. The incorporation of PR or BTB into Co-MOF can form a colored nanohybrid, and further combination with CMC-Na can produce a CMC-Na-based packaging material with anti-dye migration and improved physical properties. The visual indication film for fresh-cut fruit freshness of the present application can effectively reflect the CO2 concentration and is suitable for real-time in-situ monitoring of fruit freshness.

[0010] In the present application, PR and BTB two pigments are prepared by hydrogen bond interaction using the adsorption mechanism of MOF, and the prepared Co-MOF / PR and Co-MOF / BTB composites are embedded into the CMC-Na film, so that the CO2-responsive pigments are not easy to fall off. In addition, the present application focuses on solving the problems of mechanical properties, water resistance, barrier properties and other related properties of natural polymers. The structure of the prepared Co-MOF to CMC-Na film and the overall performance of the encapsulation have been significantly improved. The composite film of the present application has good pH response characteristics and time-temperature stability under different acid-base conditions and CO2 systems. Therefore, the indication film of the present application is a CO2-responsive composite film, which combines the advantages of natural biopolymers and multifunctional nanofillers, and is suitable for visual monitoring of the packaging process of fresh-cut fruits.

[0011] The present application prepares irregular spherical cobalt (Co) based metal organic framework (Co-MOF) loaded with phenol red (PR) or bromothymol blue (BTB), and obtains nanocomposites Co-MOF / PR and Co-MOF / BTB after the dye is adsorbed based on hydrogen bond interaction with the MOF. Then, the nanocomposites are embedded into a sodium carboxymethyl cellulose (CMC-Na) substrate to prepare CO2 responsive packaging materials (CMC-Na / PR / Co-MOF and CMC-Na / BTB / Co-MOF) for real-time and in-situ monitoring of fruit freshness. The Co-MOF based composite film has excellent water resistance, time-temperature stability and mechanical properties, while showing enhanced ultraviolet-visible light barrier, dye anti-migration performance and thermal stability. The visual indicator film for freshness of fresh-cut fruits can effectively reflect the CO2 concentration and is suitable for real-time and in-situ monitoring of fruit freshness.

[0012] The present application achieves the above technical purpose through the following technical means.

[0013] A preparation method of a visual indicator film for freshness of fresh-cut fruits, comprising the following steps:

[0014] Step S1. Preparation of Co-based MOF:

[0015] Step S1.1 Preparation of crude Co-MOF

[0016] Dissolve cobalt acetate tetrahydrate and trimesic acid in N,N-dimethylformamide, and after ultrasonic treatment to mix the reactants uniformly, transfer the mixture to an autoclave, heat and cool to room temperature;

[0017] Step S1.2 Co-MOF washing and activation

[0018] Centrifuge the precipitate produced in step S1.1, wash with solvent, and vacuum dry and activate to obtain purple Co-MOF powder;

[0019] Step S2. Preparation of indicator film:

[0020] Step S2.1 Preparation of CMC-Na based film

[0021] First, dissolve the Co-MOF prepared in step S1 and glycerol, CMC-Na powder in distilled water under magnetic stirring to form a homogeneous system, and add phenol red PR-ethanol dye and sodium hydroxide solution or bromothymol blue BTB-ethanol dye and sodium hydroxide solution to the homogeneous system under continuous magnetic stirring to obtain a uniform solution;

[0022] Step S2.2 Stable standing of CMC-Na based film

[0023] The uniform solution obtained in step S2.1 is cast on a mold, and transferred to a constant temperature and humidity incubator. After the film is demolded from the mold, a phenol red PR indicator film or a bromothymol blue BTB indicator film is obtained.

[0024] In the above scheme, in the step S1.1,

[0025] The amounts of the cobalt acetate tetrahydrate, trimesic acid and N,N-dimethylformamide are 3 mmol: 2 mmol: 100 ml respectively; the ultrasonic time is 30 min; the heating temperature is 160℃; and the heating time is 12 h. Preferably, the autoclave is a steel autoclave with a polytetrafluoroethylene lining.

[0026] In the above scheme, in the step S1.2,

[0027] The centrifugation condition is 8000 r / min for 5 min; the washing solvent is ethanol and N,N-dimethylformamide; the washing mode is alternating washing with ethanol and N,N-dimethylformamide; and the vacuum drying temperature is 60℃, and the heating time is 8 h.

[0028] In the above scheme, in the step S2.1,

[0029] The Co-MOF is 1-4.0 wt%, and the amounts of glycerol, CMC-Na and distilled water are 3 g: 10 g: 400 mL;

[0030] The magnetic stirring condition is 90℃, 1200 rpm, and stirring for 0.5 h.

[0031] In the above scheme, in the step S2.1,

[0032] The amounts of the phenol red PR-ethanol dye or bromothymol blue BTB-ethanol dye and sodium hydroxide are 1:1;

[0033] The stirring condition after adding the dye and sodium hydroxide is 30℃, 1200 rpm, and stirring for 0.5 h.

[0034] In the above scheme, in the step S2.2,

[0035] The constant temperature and humidity incubator condition is 24 h, 25℃ and 50% relative humidity. Preferably, the mold is a square polytetrafluoroethylene mold, and the parameters of the polytetrafluoroethylene mold are 100.0 mm in length and 10.0 mm in depth.

[0036] A visual indicator film for the freshness of fresh-cut fruits is prepared according to the preparation method of the visual indicator film for the freshness of fresh-cut fruits.

[0037] A visual indicator film of fresh-cut fruit freshness prepared according to the preparation method of the visual indicator film of fresh-cut fruit freshness, or the application of the visual indicator film of fresh-cut fruit freshness in monitoring fruit freshness.

[0038] A detection method of a visual indicator film of fresh-cut fruit freshness, the visual indicator film of fresh-cut fruit freshness being a visual indicator film of fresh-cut fruit freshness prepared according to the preparation method of the visual indicator film of fresh-cut fruit freshness, or the visual indicator film of fresh-cut fruit freshness, the detection method comprising the following steps:

[0039] (1) bonding the air-permeable filter paper layer with the visual indicator film of fresh-cut fruit freshness to the polyethylene preservative film to form a packaging material capable of covering the top opening of the packaging box;

[0040] (2) soaking the visual indicator film of fresh-cut fruit freshness sample in a buffer solution with pH 2-10 for 10 min, and capturing the color change and the three-color value in the sample using a camera and a colorimeter, respectively;

[0041] (3) using CO2 slow-release tablets to generate a CO2 concentration of 0-5.0%, and recording the color response of the visual indicator film of fresh-cut fruit freshness to CO2;

[0042] (4) storing the visual indicator film of fresh-cut fruit freshness sample in a sealed environment at 4℃, 25℃ and 37℃ for 28 days, and measuring the ΔE value using a colorimeter for evaluating the color stability of the visual indicator film of fresh-cut fruit freshness;

[0043] (5) establishing a standard curve by recording the relationship between the daily recorded fruit firmness FH and total soluble solid TSS content of the fruit, respectively, and the color difference value of the visual indicator film of fresh-cut fruit freshness;

[0044] (6) testing the FH and TSS of the fresh-cut fruit sample: using the instrument to puncture the cut surface of the peeled fresh-cut fruit, and recording the fruit firmness FH of three surfaces; slicing the fruit, selecting fruit slices with substantially uniform thickness, wrapping them with gauze, and then squeezing out the juice, which is dropped into the sample port of the sugar meter for detection, and recording the total soluble solid TSS of three surfaces of the fruit;

[0045] (7) collecting the daily response color difference value of the visual indicator film of fresh-cut fruit freshness, fitting the FH and TSS data obtained according to the method of step (6), combining with the standard curve established in step (5), fitting to obtain the optimal Co-MOF loading amount, and preparing the optimal visual indicator film of fresh-cut fruit freshness according to the optimal Co-MOF loading amount, the visual indicator film of fresh-cut fruit freshness being a phenol red PR indicator film or a bromothymol blue BTB indicator film;

[0046] (8) For the detection of fresh-cut fruits: the packaging material containing the optimal visualized indicator film of fresh-cut fruits freshness obtained in step (7) is covered on the packaging box containing the fresh-cut fruits to be detected, and the color change of the indicator film is observed to detect the freshness of the fresh-cut fruits.

[0047] Further, the optimal phenolphthalein PR loading amount is 4.0wt%, the optimal phenolphthalein PR indicator film is CMC-Na / PR / Co-MOF 4%, the standard curve of ΔE value and FH is y=0.336x-9.813 (R 2 =0.9887), and the standard curve of ΔE value and TSS is y=0.226x+1.484 (R 2 =0.9781);

[0048] The optimal bromothymol blue BTB loading amount is 4.0wt% by fitting the ΔE value and FH, the optimal bromothymol blue BTB indicator film is CMC-Na / BTB / Co-MOF 4%, the standard curve is y=0.512x-14.912 (R 2 =0.9923); the optimal bromothymol blue BTB loading amount is 2.0wt% by fitting the ΔE value and TSS, the optimal bromothymol blue BTB indicator film is CMC-Na / BTB / Co-MOF 2%, and the standard curve is y=0.267x+0.747 (R 2 =0.9769).

[0049] Compared with the prior art, the present application has the following beneficial effects:

[0050] (1) The present application synthesizes Co-BTC MOF by adopting the solvothermal method and the anion exchange principle, which is universal and simple in operation, and avoids operations such as centrifugation to maintain the stability and uniformity of MOF; at the same time, the MOF nano filler used has excellent moisture and heat resistance. The present application fully utilizes the pH response property of the dye and the hydrogen bond interaction between PR and BTB dyes and MOF, ensures that the adsorption process does not require energy, vacuum and other conditions, and is simple and efficient. Co-MOF has stable chemical properties, so that it is embedded in the three-dimensional network structure of the gel without changing the film-forming property and applicability of the matrix, which promotes the attachment of nanoparticles and is conducive to the establishment of a stable enhanced indicator film monitoring system.

[0051] (2) The application applies a biological polymer CMC-Na, which has the characteristics of non-toxic, renewable and biocompatible, and in addition, protects fresh-cut fruits from external pollution by forming a viscous pseudoplastic film, and has unique film-forming and oxygen barrier properties. The embedding of Co-MOF fillers improves the moisture resistance, mechanical properties, barrier properties, thermal stability and dye migration properties of the original CMC-Na film. In addition, the indicator film is a composite film, which has a significant pH response behavior under different acid-base conditions and CO2 concentration systems, and the time-temperature stability shows satisfactory results (ΔE less than 5), indicating that the composite film is suitable as an indicator packaging material.

[0052] (3) The Co-MOF prepared in the application has excellent adsorption capacity for PR and BTB, realizes the improvement of dye visual grading, and the incorporation of Co-MOF enhances the overall physicochemical stability of the original matrix. The addition of Co-MOF leads to more obvious visual differences of the color developing film, and the film containing MOF has good time-temperature stability. In addition, the linear correlation between ΔE and FH (R 2 = 0.9923) and TSS (R 2 = 0.9769) is established, which provides a practical method for real-time monitoring of the freshness of fresh-cut fruits. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 is a general scheme of the application, wherein Figure 1(a) is a synthesis scheme of Co-MOF, Figure 1(b) is the interaction of Co-MOF with two dyes, Figure 1(c) is the preparation process of CMC-Na-based composite film, Figure 1(d) is the enhancement effect of CMC-Na-based composite film, Figure 1(e) is the CO2 response of the composite film, and Figure 1(f) is the packaging test of fresh-cut apples.

[0054] Figure 2 is the related characterization of nano-Co-MOF and the color change diagram after the dye is mixed with MOF, wherein Figure 2(a) and (b) are FTIR and XRD spectra of Co-MOF, Co-MOF / PR and Co-MOF / BTB samples, respectively, Figure 2(c-e) are SEM mapping images of Co-MOF, Co-MOF / PR and Co-MOF / BTB samples, Figure 2(f) is an EDS image of Co-MOF, and Figure 2(g-j) are various colors of dyes and dye solutions based on MOF under pH 2.0-10.0, and the corresponding UV-Vis spectra.

[0055] Figure 3 is a diagram of film related characterization and performance test, wherein Figure 3(a-d) are FTIR spectra, DSC analysis and UV-Vis spectra of pure CMC-Na, CMC-Na / PR and CMC-Na / BTB films loaded with different Co-MOF concentrations, respectively, and Figure 3(e-h) are the pigment migration of CMC-Na / PR, CMC-Na / BTB, CMC-Na / PR / Co-MOF and CMC-Na / BTB / Co-MOF films in different simulated food fluids, respectively.

[0056] Figure 4 is SEM micrographs of the surface morphology of CMC-Na (a), CMC-Na / PR (b), CMC-Na / BTB (c) and films containing different Co-MOF concentrations (d-i), respectively.

[0057] Figure 5(a) is the film color of CMC-Na / PR and CMC-Na / BTB with different Co-MOF contents under 0.0-5.0% CO2 concentration, and Figure 5(b) is the appearance, FH and TSS changes of fresh-cut apples during 4℃ storage.

[0058] Figure 6 is the color change of CMC-Na / PR and CMC-Na / BTB films with different Co-MOF contents after 28 days at 4℃, 25℃ and 37℃.

[0059] Figure 7 is the color change of CMC-Na / PR and CMC-Na / BTB films loaded with different Co-MOF concentrations, and the correlation of its ΔE with the FH and TSS of fresh-cut apples, wherein Figure 7(a) is CMC-Na / PR, Figure 7(b) is CMC-Na / PR / Co-MOF 1%, Figure 7(c) is CMC-Na / PR / Co-MOF 2%, Figure 7(d) is CMC-Na / PR / Co-MOF 4%, Figure 7(e) is CMC-Na / BTB, Figure 7(f) is CMC-Na / BTB / Co-MOF 1%, Figure 7(g) is CMC-Na / BTB / Co-MOF 2%, and Figure 7(h) is CMC-Na / BTB / Co-MOF 4%. DETAILED DESCRIPTION

[0060] In order for those skilled in the art to better understand the technical solutions of the present application, the following specific embodiments will be described in detail, but the following embodiments do not limit the protection scope of the present application. In the embodiments of the present application, what is not described in detail is completed by using conventional experimental methods, and what is not described in detail in the processes involved in the embodiments is understood and easily realized by those skilled in the art according to the product instructions or the basic knowledge in the art, and therefore will not be described in detail.

[0061] The application provides a fresh-cut fruit freshness visual indication film, integrates a nanocomposite material into a CMC-Na base material, and prepares a CO2 response packaging material for real-time and in-situ monitoring of fruit freshness.

[0062] The application also provides a preparation method of the fresh-cut fruit freshness visual indication film, as shown in FIG. 1, including the following steps:

[0063] Preparation of the packaging nanofiller Co-MOF

[0064] The application synthesizes Co-BTC MOF by adopting a solvothermal method and an anion exchange principle, is universal and simple to operate, and avoids operations such as centrifugation to maintain the stability and uniformity of the MOF. The synthesis of Co-MOF and the interaction relationship between the MOF and the dyes are indicated by using a series of characterization such as FT-IR, XRD, EDS and SEM. Then, the Co-MOF / PR and Co-MOF / BTB complexes are subjected to ultraviolet-visible spectroscopy characterization at different pH values. The synthesis scheme of the Co-MOF and the interaction with the two dyes are as shown in FIG. 1.

[0065] The specific preparation steps are as follows:

[0066] S1. Preparation of Co-MOF

[0067] Firstly, 1.5 mmol of cobalt acetate tetrahydrate (Co(Ac)2·4H2O) and 1.0 mmol of trimesic acid (H3BTC) are dissolved in 50 mL of N,N-dimethylformamide (DMF), and ultrasonic treatment is performed at 25℃ for 30 min to uniformly mix the reactants; then, the mixture is transferred into a 100 mL polytetrafluoroethylene-lined steel autoclave, kept at 160℃ for 12 h, and cooled to room temperature. Then, the generated precipitate is centrifuged (8000 r / min, 5 min), washed with ethanol and DMF alternately for 3 times, and finally dried at 60℃ under vacuum for 8 h to obtain a purple Co-MOF powder.

[0068] S2. Co-MOF and interaction with two dyes

[0069] (1) FT-IR is used to indicate the successful preparation of the MOF and the formation of hydrogen bonds between the pigment and the Co-MOF

[0070] Firstly, as shown in FIG. 2(a), no characteristic vibration band related to the non-ionized carboxyl group in the BTC 3- is observed in the Co-BTC MOF, for example, 3010 cm -1 (O-H stretching vibration), 1683 cm -1 (C=O stretching vibration) and 610 cm -1(C=O bending vibration), which confirmed the successful synthesis of Co-MOF. In addition, the stretching peaks of C=O and Co-O of Co-MOF slightly shifted to low wave number after adsorbing PR and BTB, which was due to the formation of hydrogen bonds between the dyes and Co-MOF.

[0071] (2) XRD was used to show the successful preparation of MOF and the adsorption mechanism

[0072] As shown in Figure 2(b), the prepared Co-MOF had typical sharp diffraction peaks, and after adsorbing dyes, the typical diffraction peaks of Co-MOF did not shift obviously, which indicated that the morphology of MOF did not change obviously, but experienced physical adsorption of dye molecules around it.

[0073] (3) SEM and EDS were used to show the microstructure of MOF, the successful preparation and the existing mode with dyes

[0074] As shown in Figure 2(c), the synthesized Co-MOF was irregular spherical, and the average diameter was about 1.0 μm. In addition, Co-MOF / PR and Co-MOF / BTB composites showed certain aggregation phenomenon (Figures 2(d-e)), which was mainly because Co-MOF adsorbed dyes. EDS mode showed that C (red), O (green) and Co (blue) elements existed in the synthesized Co-MOF (Figure 2(f)), which further confirmed the successful synthesis of MOF.

[0075] (4) UV-Vis spectrum was used to test the color grading of dyes and dye derivatives

[0076] As shown in Fig. 2(g), the color of PR changed significantly from yellow (pH 2-6) to orange (pH 7) and finally to purple (pH 8-10). Correspondingly, the maximum absorption peak of BCG shifted from 433 nm to 558 nm with the increase of pH. After the introduction of Co-MOF, the PR / Co-MOF composite showed orange at pH 6, followed by a transition from red to purple, presenting a multi-level color change. As shown in Fig. 2(h), under alkaline conditions, the absorption peak at 558 nm showed significant differences and a wider wavelength range, highlighting the obvious visual distinction. In addition, BTB changed from initial bright yellow (pH 2-5) to light green at pH 6. After that, the color gradually transitioned from light green to turquoise (pH 7) and finally to dark blue (pH 8-10). As can be observed from Fig. 2(i), the maximum absorption peak of BTB solution shifted from 433 nm to 618 nm. Although there was no significant visual difference between the BTB group and the BTB / Co-MOF group, in the UV spectrum, the absorption intensity at pH 7-8 was significantly enhanced after the addition of Co-MOF, and the visual appearance changed slightly. In summary, Co-MOF effectively improved the color grading ability of PR and BTB in different pH buffer solutions due to its unique color change and adsorption properties. In addition, the range of color change is consistent with the change of pH during the deterioration process of fresh-cut fruits. These results indicate that PR / Co-MOF and BTB / Co-MOF are ideal choices for the development of colorimetric films.

[0077] Preparation method of CMC-Na-based intelligent colorimetric film

[0078] The present application applies the biopolymer CMC-Na, which has the characteristics of non-toxic, renewable and biocompatible. In addition, it protects fresh-cut products from external contamination by forming a viscous pseudoplastic film, with unique film-forming and oxygen barrier properties. A series of characterization methods such as FT-IR, DSC and SEM are used to study the interaction between CMC-Na, MOF and dye. The moisture resistance, mechanical properties, barrier properties, thermal stability and dye migration properties of the original CMC-Na film are evaluated by embedding Co-MOF fillers.

[0079] The specific preparation steps are as follows:

[0080] S1. Preparation of CMC-Na-based intelligent colorimetric film

[0081] Firstly, the desired amount of Co-MOF, 0.6 g of glycerol and 2.0 g of CMC-Na were stirred and dissolved in 80 mL of distilled water to form a homogeneous system, which was dissolved at 90 °C for 0.5 h at 1200 rpm. Then, 1.0 mL of PR and 1.0 mL of NaOH (0.1 mol / L) were added to the above homogeneous system, and the mixture was stirred at 30 °C for 0.5 h. Then, the resulting uniform solution was cast onto a square polytetrafluoroethylene mold (100.0 mm in length, 10.0 mm in depth), transferred to a constant temperature and humidity incubator for 24 h at 25 °C and 50% relative humidity. After the film was demolded from the mold, it was stored in a sealed container overnight. The Co-MOF loadings of 0, 1.0, 2.0 and 4.0 wt% (based on CMC-Na) were named CMC-Na / PR, CMC-Na / PR / Co-MOF1%, CMC-Na / PR / Co-MOF2% and CMC-Na / PR / Co-MOF4%, respectively. In addition, the preparation process of the BTB dye series film was also the same as above, which was named CMC-Na / BTB, CMC-Na / BTB / Co-MOF1%, CMC-Na / BTB / Co-MOF2% and CMC-Na / BTB / Co-MOF4%, respectively.

[0082] S2. Microscopic mechanism of CMC-Na-based smart colorimetric films

[0083] (1) FT-IR was used to show the interaction between CMC-Na-based films and pigments and MOFs

[0084] As shown in Figure 3(a), the FTIR spectrum of the initial CMC-Na exhibited typical functional group characteristic peaks at 3254 cm -1 , 2922 cm -1 and 2876 cm -1 , 1592 cm -1 and 1412 cm -1 , 1022 cm -1 , corresponding to the broad -OH stretching band, the asymmetric stretching of -CH3 and -CH2- groups, the asymmetric and symmetric stretching of COO- groups and the stretching vibration of C-O-C, respectively. Due to the interaction between the hydroxyl groups in PR and BTB and the hydrophilic groups in CMC-Na, the -OH stretching band on the CMC-Na film was shifted. After the addition of Co-MOF, the Co element in Co-MOF formed a coordination interaction with CMC-Na, resulting in the asymmetric stretching (from 1592 cm -1 to 1589 cm -1 ) and symmetric stretching (from 1412 cm -1 to 1415 cm -1) showed a significant change in the position of the absorption peak. It is worth noting that the -OH stretching peak of the CMC-Na / PR / Co-MOF group and the CMC-Na / BTB / Co-MOF group moved from 3250 cm -1 and 3247 cm -1 to 3244 cm -1 and 3242 cm -1 respectively, due to the hydrogen bonding interaction between COO- in Co-MOF and -OH in CMC-Na chains. Finally, the increase in MOF content did not result in a significant wavelength shift.

[0085] (2) SEM was used to show the microstructure of each type of film

[0086] As shown in Figure 4(a), the SEM images show that the CMC-Na film has a normal, regular and uniform structure. In addition, the inclusion of dyes in the polymer film affected the structure, resulting in a surface that is uneven, irregular and slightly protruding, due to the interaction forces between the dye and the substrate. With the continuous addition of Co-MOF (Figure 3(d-i)), some particles appeared on the surface of the film, and the number and size of the particles increased. In addition, the addition of 4% Co-MOF resulted in an increase in disturbance in the film structure, resulting in more and wider spots and a higher non-homogeneous surface (Figures 3(f and i)). The aggregation of these particles in the film contributes to its stiffness, which directly leads to an increase in the mechanical and barrier properties of the film.

[0087] (3) DSC was used to show the thermal stability of MOFs-based films

[0088] As shown in Figure 3(b), the CMC-Na film without pigments showed a clear endothermic peak at 271.1 °C, indicating the melting temperature. After adding PR and BTB, the CMC-Na / PR and CMC-Na / BTB films showed a slight peak shift at low frequency, with endothermic peaks at 272.7 °C and 272.3 °C, respectively. Compared with the pure CMC-Na matrix, the addition of dyes increased the melting temperature of the film, thereby enhancing the thermal stability of the film. In addition, with the increase in Co-MOF content, the peaks tended to slightly move to a relatively higher temperature. It is worth noting that all CMC-Na with Co-MOF loading showed endothermic peaks at higher temperatures than pure CMC-Na. This is due to the structure of the nanocomposite, which requires more energy to break the stability. Therefore, the peaks observed at higher temperatures in the Co-MOF-filled films confirm that the encapsulation system has better thermal stability in different environments.

[0089] S3. Performance testing of CMC-Na-based smart colorimetric films

[0090] The application also provides a test scheme for (1) waterproof performance, (2) mechanical property, (3) barrier property, and (4) dye migration performance of the packaging film, comprising:

[0091] (1) The initial weight of the sample is recorded as M1, and then dried in an oven at 110°C for 1.5h to obtain the dried weight M2. Then, the dried sample is immersed in distilled water and gently stirred for 1.5h, and then dried again at 110°C for 1.5h, at which time the weight is recorded as M3. In addition, the initial film sample is soaked in distilled water at room temperature for 1.0h, and then the surface moisture is removed using water absorption paper, and the obtained weight is recorded as M4. The calculation method of the characteristic parameters is as follows: Water content rate (%) = (M2-M1) / M1x100 Water solubility (%) = (M2-M3) / M2x100 Water absorption rate (%) = (M4-M1) / M1x100

[0092] The water contact angle test of the film is to fix the film horizontally on a metal stage, drop water on the surface, and then capture the image using a horizontal microscope.

[0093] (2) First, the thickness of the film sample at different positions is measured using a digital micrometer. According to the GB 13022-1991 standard, the tensile strength and elongation at break of the film are determined using a universal texture meter. The sample size is set to 20mmx60mm, the initial clamping distance is 40mm, the pulling speed is set to 0.06mm / s, and a 150kg load cell is used. Tensile strength (MPa) = F / A Elongation at break (%) = (ΔL / L0)x100

[0094] F: the critical force for film rupture, unit: Newton (N),

[0095] A: the cross-sectional area of the film, unit: square millimeter (mm 2 ),

[0096] ΔL: the increment of the rupture length of the film, unit: millimeter (mm),

[0097] L0: the initial length of the film sample, unit: millimeter (mm).

[0098] (3) The water vapor permeability (WVP) test of the film is to cover the film on the end of a centrifuge tube containing deionized water, and place the centrifuge tube in a desiccator. The weight is recorded every 24 hours for 5 days. In addition, the transmission spectrum of the film in the ultraviolet-visible region (300-800nm) is recorded using a UV-Vis spectrophotometer to evaluate the ultraviolet-visible light barrier property of different films.

[0099] (4) The migration of PR and BTB in CMC-Na matrix was tested by UV-Vis spectrophotometer. The films were cut into 10 mm x 10 mm squares and immersed in four kinds of simulated food fluids for 7 days. The simulated fluids were deionized water representing neutral food, 10% ethanol solution representing alcoholic food, 95% ethanol solution representing fatty food, and 3% acetic acid solution representing acidic food

[0100] The above performance test method specifically includes the following steps:

[0101] (1) Water resistance

[0102] As shown in Table 1, the water content of pure CMC-Na film was 19.29 ± 0.22%, and the addition of dyes reduced the water content, which was due to the benzene ring structure in the dye molecule that enhanced the water resistance of the film. In addition, as confirmed by FTIR analysis, the addition of Co-MOF further reduced the water content, as it could form strong intermolecular interactions with the polymer matrix, including hydrogen bonds and electrostatic interactions.

[0103] The hydrophobicity of the dyes caused the water solubility of CMC-Na / PR and CMC-Na / BTB films to decrease from 15.18 ± 0.40% to 13.83 ± 0.77% and 13.03 ± 1.45%, respectively. At the same time, FTIR analysis showed that the hydrogen bonds in CMC-Na / PR / Co-MOF and CMC-Na / BTB / Co-MOF films reduced the free hydroxyl groups in the composite films, limiting their interaction with water molecules through hydroxyl groups, ultimately leading to a decrease in the water solubility value of the films.

[0104] The water absorption rate of the original CMC-Na film was relatively high, at 563.69 ± 6.89%, which was due to the hydrophilic nature of the CMC polymer film, which easily absorbed water and caused swelling. The swelling of the colorimetric film occurred when water molecules were effectively trapped within the three-dimensional network structure of the colorimetric film. This phenomenon enhanced the release of pH-sensitive dyes, ultimately enhancing the sensitivity of the intelligent packaging film. In addition, due to the hydrophobicity of the dyes and Co-MOF, the water absorption rates of different films were reduced to varying degrees.

[0105] The water contact angle value of CMC-Na film was 43.47 ± 1.55°, indicating that the pure CMC-Na film had hydrophilicity, which was caused by a large number of hydrophilic groups in the CMC molecule, indicating that the surface wettability of the film was related to the surface chemical properties and surface microstructure. In addition, Co-MOF particles were filled into the CMC-Na film to obtain a certain roughness, which jointly determined the adhesion level between the liquid droplets and the surface. The water contact angle values of CMC-Na / PR / Co-MOF 1%, CMC-Na / PR / Co-MOF 2%, CMC-Na / PR / Co-MOF 4% and CMC-Na / BTB / Co-MOF 4% were all greater than 90°, indicating that the surfaces of these films had hydrophobicity, which had a positive effect on the indicating function of Co-MOF-based films. Therefore, the prepared CMC-Na / Co-MOF series of films had excellent water resistance.

[0106] (2) Mechanical properties

[0107] As shown in Table 1, after adding PR and BTB in the CMC-Na matrix, the tensile strength decreased from 36.70 ± 1.85 MPa to 35.34 ± 1.12 and 34.46 ± 1.07 MPa (not significant), respectively. The decrease in strength can be attributed to the replacement of stronger polymer-polymer interactions (mainly hydrogen bonds provided by hydroxyl groups) with weaker polymer-dye interactions in the film network, hindering polymer chain-chain interactions and forming flexible structural regions in the film. It is worth noting that with the increase of Co-MOF content from 1% to 4%, the tensile strength of CMC-Na / PR / Co-MOF increased significantly from 38.83 ± 1.05 MPa to 52.12 ± 1.36 MPa, and the tensile strength of CMC-Na / BTB / Co-MOF increased from 40.13 ± 0.29 MPa to 48.66 ± 1.60 MPa, indicating that the inhibition effect of Co-MOF filler improved the rigidity of CMC-Na film.

[0108] Following the same trend, the elongation at break of neat CMC-Na film was about 2.49 ± 0.37%, and after adding a small amount (1%) of Co-MOF, the EB of CMC-Na / PR / Co-MOF and CMC-Na / BTB / Co-MOF films increased to 4.01 ± 0.36% and 6.07 ± 0.73%, respectively. In addition, the elongation at break of the films also improved significantly with the increase of MOF content. Generally, the mechanical reinforcement effect of fillers mainly depends on their dispersibility and interaction with the polymer matrix. Corresponding to the FTIR and SEM results, the improvement of mechanical properties can be attributed to the strong hydrogen bonding interaction between well-dispersed MOFs and the polymer matrix, which prevents phase separation and promotes effective stress transfer at the interface. In summary, the mechanical properties of CMC-Na / Co-MOF composite films further improved with the increase of Co-MOF content.

[0109] (3) Barrier properties

[0110] As shown in Table 1, the water vapor permeability of the films decreased with the mixing of dyes compared to the neat CMC-Na film, which can be attributed to the hydrophobicity of PR and BTB. In addition, the increase of Co-MOF fillers led to the decrease of water vapor permeability of CMC-Na / PR / Co-MOF and CMC-Na / BTB / Co-MOF films. It can be seen that the dependence of water vapor permeability on film thickness is obvious, and with the increase of thickness, the migration of water through the layer is inhibited. In addition, due to the incorporation of MOF, the original structure of the film is changed, and the hydrophobicity of MOF further hinders the diffusion of water vapor in the film, eventually leading to the decrease of water vapor permeability of the film.

[0111] As shown in Figure 3(c-d). It was observed that the CMC-Na film exhibited relatively high UV transmittance (40-50%) in the wavelength range of 300-400 nm, indicating that a large amount of UV light passed through the CMC-Na film. This result indicates that the UV barrier performance of the pure CMC-Na film is poor. In addition, the behavior in the visible and infrared regions is similar to that of UV light. After adding the pigments, the absorption peaks of PR and BTB groups appeared at 570 nm and 621 nm, respectively, similar to the above UV-visible spectrum analysis. With the increase of Co-MOF loading, the light transmittance of CMC-Na / PR / Co-MOF and CMC-Na / BTB / Co-MOF composite films gradually decreased, which was due to the increase of film thickness and the rich coloring of MOF. Therefore, the addition of Co-MOF can enhance the water vapor and UV-visible light barrier performance of CMC-Na film.

[0112] (4) Dye migration

[0113] Figure 3 (e-f) shows the leakage of PR in CMC-Na films, which produces pink, orange and yellow in solution. The maximum absorption peak of PR in deionized water is 3% acetic acid and 10% ethanol, which is observed at 433 nm, and PR has another absorption peak at 558 nm in deionized water and 10% ethanol. After filling Co-MOF into the film, the absorbance of all groups decreased significantly, which is attributed to the adsorption of pigments by MOF and the formation of more compact agglomerated structures within the film. In addition, the color of CMC-Na / BTB group changed to blue or yellow, and the UV-Vis results showed that the absorbance peak appeared obviously at around 433 nm in deionized water and 3% acetic acid, while another absorbance peak appeared at 618 nm in 10% ethanol. With the addition of MOF, the overall color concentration and absorbance value decreased significantly. According to the discussion of the water resistance of the film, the incorporation of hydrophobic materials can effectively enhance the water resistance of the film.

[0114] Table 1. Physical properties of CMC-Na, CMC-Na / PR and CMC-Na / BTB films loaded with different concentrations of Co-MOF

[0115] These values are expressed as mean ± standard deviation. Values in the same column followed by different letters represent a significant difference (P < 0.05) between the values of the two.

[0116] Color response and stability of CMC-Na-based smart colorimetric films

[0117] As can be seen from Table 2, the color change of the film is similar to that of the dye solution. In the acidic to neutral pH range, all the color films are mainly light yellow and sienna. With the pH value changing from neutral to alkaline, the CMC-Na / PR and CMC-Na / BTB films start to show red-violet and blue, respectively. Due to the color and adsorption properties of Co-MOF, the color of the pH-sensitive film gradually changes after adding Co-MOF, which corresponds to the calculated AE value.

[0118] Table 2. Color and AE values of CMC-Na / PR and CMC-Na / BTB films loaded with different concentrations of Co-MOF in pH 2-10 buffer solution.

[0119] After fresh-cut processing, respiration and metabolism of the fruit and external microorganisms produce CO2, leading to a decrease in fruit freshness. During the early stages of storage, subtle changes in the appearance and nutritional quality of fresh-cut fruits are difficult to observe. Therefore, CO2 is usually targeted as the gas of interest to monitor the quality changes of fruits in packages in real-time. At different concentrations of CO2 (0.0-5.0%), the color of CMC-Na / PR and CMC-Na / BTB films changed from purple-red to yellow and dark blue to yellow, respectively, and the transparency of the films gradually increased during this process, as shown in Figure 5(a). The visual color was similar to the color reported in Table 2. However, the addition of Co-MOFs resulted in a more obvious visual distinction within the same pH range.

[0120] Generally, if the difference in ΔE values before and after storage is less than 5, the naked eye cannot observe the color change of the film, indicating that the film has good stability. Table 3 and Figure 6 show the results of 8 types of films after 4 weeks at 4°C, 25°C and 37°C, as well as their corresponding real pictures. The time-temperature stability of CMC-Na-based films without Co-MOFs was insufficient (ΔE value difference greater than 5), except for CMC-Na / PR at 25°C. As more Co-MOFs were filled into the film, the difference in ΔE decreased. In addition, when the Co-MOF dosage reached 4%, the film had the best time-temperature stability. Filling the internal voids of the film with MOF materials can hinder the penetration of water and gas, thereby improving the indication efficiency. On the other hand, Co-MOFs, as a hydrophobic material, prevent moisture from being absorbed into the color-developing film. The above results show that Co-MOF-based films are the best choice for practical applications.

[0121] Table 3. Color parameters of CMC-Na / PR and CMC-Na / BTB films with different Co-MOF contents after 28 days at 4°C, 25°C and 37°C

[0122] Packaging applications of CMC-Na-based smart colorimetric films

[0123] The present application also provides a method for real-time and in-situ freshness CO2 response packaging detection of fresh-cut fruit freshness visualized indicator films, comprising the following steps:

[0124] (1) Using hot melt adhesive technology, the air-permeable filter paper layer with fresh-cut fruit freshness visualized indicator film is bonded to the polyethylene preservative film to form a packaging material that can cover the top opening of the packaging box;

[0125] (2) Soak the fresh-cut fruit freshness visualized indicator film sample in a buffer solution with a pH of 2-10 for 10 minutes, and use a camera and a colorimeter to capture the color change and tristimulus values in the sample, respectively.

[0126] (3) Using CO2 slow-release tablets to generate a CO2 concentration of 0-5.0%, recording the color response of the visual indicator film of fresh-cut fruit freshness to CO2.

[0127] (4) The visual indicator film sample of fresh-cut fruit freshness was stored in a sealed environment at 4°C, 25°C and 37°C for 28 days, and the ΔE value was measured using a colorimeter to evaluate the color stability of the film.

[0128] (5) Establish a standard curve between the daily recorded fruit firmness (FH), total soluble solids (TSS) content and the color difference value of the film, respectively;

[0129] (6) FH and TSS test of fresh-cut fruit sample: use the instrument to puncture the cut surface of peeled fresh-cut fruit, record the fruit firmness of three surfaces; cut the fruit into slices, select fruit slices with basically consistent thickness, wrap them with gauze, squeeze out the juice, drop it on the sample port of the sugar meter, measure the sugar content of the three surfaces of the fruit, and record the total soluble solids TSS of the three surfaces of the fruit;

[0130] (7) Collect the daily response color difference value of the visual indicator film of fresh-cut fruit freshness, and fit it with the FH and TSS data obtained according to the method of step (6), combined with the standard curve established in step (5), the optimal Co-MOF loading is obtained by fitting, the optimal phenol red PR loading is 4.0wt%, the optimal phenol red PR indicator film is CMC-Na / PR / Co-MOF4%, the standard curve of ΔE value and FH is y=0.336x-9.813(R 2 =0.9887), the standard curve of ΔE value and TSS is y=0.226x+1.484(R 2 =0.9781); the optimal bromothymol blue BTB loading is 4.0wt% by fitting the ΔE value and FH, the optimal bromothymol blue BTB indicator film is CMC-Na / BTB / Co-MOF4%, the standard curve is y=0.512x-14.912(R 2 =0.9923); the optimal bromothymol blue BTB loading is 2.0wt% by fitting the ΔE value and TSS, the optimal bromothymol blue BTB indicator film is CMC-Na / BTB / Co-MOF2%, the standard curve is y=0.267x+0.747(R 2 =0.9769);

[0131] According to the optimal Co-MOF loading, the optimal visual indicator film of fresh-cut fruit freshness is prepared, and the visual indicator film of fresh-cut fruit freshness is phenol red PR indicator film or bromothymol blue BTB indicator film;

[0132] (8) For the detection of fresh-cut fruits: The packaging material containing the optimal visual indicator film of fresh-cut fruits freshness obtained from step (7) is covered onto the packaging box containing the fresh-cut fruits to be tested, and the color change of the indicator film is observed to detect the freshness of the fresh-cut fruits.

[0133] Due to the interaction of polyphenol oxidase with polyphenol and oxygen, fresh-cut apples rapidly browning within 1 day, while no significant increase in browning was observed within 1-4 days. Subsequently, browning was most pronounced at day 5, with a thin layer of patches formed on the surface of the apples. During storage, the firmness and TSS of fresh-cut apples were also analyzed as physicochemical indicators. The results in Fig. 5(b) show that the firmness of fresh-cut apples decreased significantly due to the continuous loss of water, as plant cells with higher water content tend to have greater elasticity and firmness. On the other hand, the TSS of apples decreased rapidly within the first 24 h of storage, followed by a gradual slowdown in the rate of decomposition. This effect is attributed to the initial high oxygen, low carbon dioxide environment, which accelerated the metabolism of microorganisms and apples. As the CO2 concentration increased, respiration was inhibited, and the decline in TSS levels was mitigated. In addition, the firmness and TSS of fresh-cut apples decreased below the threshold value at 5 days (FH < 7 and TSS < 13), indicating that the apples were no longer suitable for consumption.

[0134] Fig. 7(a-h) shows the color of fresh-cut apples based on CMC-Na monitoring of 8 composite films, and the fitting effect of ΔE with FH and TSS. As the number of days of storage increased, the ΔE value gradually decreased, and the film color changed significantly, which was highly similar to the observation in the film under different CO2 concentrations, indicating that PR and BTB are suitable for preparing films to monitor the freshness of CO2-producing fruits. In addition, compared with other PR groups, the ΔE value of the CMC-Na / PR / Co-MOF 4% film had a stronger correlation with FH and TSS, with R 2 values of 0.9887 and 0.9781, respectively, and a higher correlation means a better fitting degree. Similarly, the optimal correlation of ΔE with FH and TSS corresponded to the CMC-Na / BTB / Co-MOF 4% film (R 2 = 0.9923) and the CMC-Na / BTB / Co-MOF 2% film (R 2= 0.9769). It can be seen that the film with high fitting degree has higher Co-MOF concentration, which is due to the alignment of the color of Co-MOF with the original dyes PR and BTB. In addition, the respiration of fruits and external microorganisms leads to the increase of humidity in the packaging environment, thereby interfering with the indication of the film, while Co-MOF as a multifunctional nano-filler of CMC-Na improves the physical properties and color stability of the film. In summary, the color difference parameters of Co-MOF-based film have high fitting degree with the physicochemical indexes of fresh-cut apples, and the film shows excellent stability during the test, indicating that it is very suitable for intelligent packaging applications.

[0135] The above examples are only used to illustrate the technical solutions described in the present application and not to limit the present application; therefore, although the present application has been described in detail with reference to the above examples, those of ordinary skill in the art should understand that the present application can still be modified or replaced equivalently; and all technical solutions and improvements that do not deviate from the spirit and scope of the present application should be covered within the scope of the claims of the present application.

Claims

1. A method for preparing a visual indicator film for freshness of fresh-cut fruit, characterized by, The method comprises the following steps: Step S1. Preparation of Co-based MOF: Step S1.1 Preparation of crude Co-MOF Dissolve cobalt acetate tetrahydrate and trimesic acid in N,N-dimethylformamide, and after ultrasonic treatment to mix the reactants uniformly, transfer the mixture to an autoclave, and after heating, cool to room temperature; Step S1.2 Co-MOF washing and activation Centrifuge the precipitate produced in step S1.1, wash with solvent, and vacuum dry and activate to obtain purple Co-MOF powder; Step S2. Preparation of indicator film: Step S2.1 Preparation of CMC-Na-based film First, dissolve the Co-MOF prepared in step S1 and glycerol, CMC-Na powder in distilled water under magnetic stirring to form a homogeneous system, and add phenol red PR-ethanol dye and sodium hydroxide solution or bromothymol blue BTB-ethanol dye and sodium hydroxide solution to the homogeneous system under continuous magnetic stirring to obtain a uniform solution; Step S2.2 Stable standing of CMC-Na-based film Pour the uniform solution obtained in step S2.1 onto a mold, transfer it to a constant temperature and humidity incubator, and after the film is demolded from the mold, obtain a phenol red PR indicator film or a bromothymol blue BTB indicator film.

2. The method of preparing a visual freshness indicator film for fresh-cut fruit according to claim 1, characterized in that, In the step S1.1, The amounts of cobalt acetate tetrahydrate, trimesic acid and N,N-dimethylformamide are 3 mmol: 2 mmol: 100 ml respectively; the ultrasonic treatment time is 30 min; the heating temperature is 160℃; and the heating time is 12 h.

3. The method of preparing a visual freshness indicator film for fresh-cut fruit according to claim 1, characterized in that, In the step S1.2, The centrifugation conditions are 8000 r / min for 5 min; the washing solvent is ethanol and N,N-dimethylformamide; the washing method is alternating washing with ethanol and N,N-dimethylformamide; and the vacuum drying temperature is 60℃ for 8 h.

4. The method of preparing a visual freshness indicator film for fresh-cut fruit according to claim 1, characterized in that, In the step S2.1, The amount of Co-MOF is 1-4.0 wt%, and the amounts of glycerol, CMC-Na and distilled water are 3 g: 10 g: 400 mL; The magnetic stirring conditions are 90℃, 1200 rpm, and stirring for 0.5 h.

5. The method of preparing a visual freshness indicator film for fresh-cut fruit according to claim 1, characterized in that, In the step S2.1, The amounts of phenol red PR-ethanol dye or bromothymol blue BTB-ethanol dye and sodium hydroxide are 1:1; The stirring conditions after adding the dye and sodium hydroxide are 30℃, 1200 rpm, and stirring for 0.5 h.

6. The method of preparing a visual freshness indicator film for fresh-cut fruit according to claim 1, characterized in that, In the step S2.2, The constant temperature and humidity incubator conditions are 24 h, 25℃ and 50% relative humidity.

7. A visual freshness indicating film for fresh cut fruit, characterized in that, The fresh-cut fruit freshness visualized indicator film prepared according to the preparation method of the fresh-cut fruit freshness visualized indicator film according to any one of claims 1-6.

8. A fresh-cut fruit freshness visualized indicator film prepared according to the preparation method of the fresh-cut fruit freshness visualized indicator film according to any one of claims 1-6, or the application of the fresh-cut fruit freshness visualized indicator film according to claim 7 in monitoring fruit freshness.

9. A method of detecting a visual freshness indicator film for fresh-cut fruit, characterized in that, The fresh-cut fruit freshness visualized indicator film is a fresh-cut fruit freshness visualized indicator film prepared by the preparation method of the fresh-cut fruit freshness visualized indicator film according to any one of claims 1-6, or the fresh-cut fruit freshness visualized indicator film according to claim 7, and the detection method comprises the following steps: (1) The air-permeable filter paper layer with the fresh-cut fruit freshness visualized indicator film is adhered to the polyethylene preservative film to form a packaging material capable of covering the top opening of the packaging box; (2) The fresh-cut fruit freshness visualized indicator film sample is soaked in a buffer solution with a pH of 2-10 for 10 min, and a camera and a colorimeter are used to capture the color change and the three-color value in the sample, respectively; (3) The CO2 slow-release tablets are used to generate a CO2 concentration of 0-5.0%, and the color response of the fresh-cut fruit freshness visualized indicator film to CO2 is recorded; (4) The fresh-cut fruit freshness visualized indicator film sample is stored in a sealed environment at 4℃, 25℃ and 37℃ for 28 days, and the ΔE value is measured by using a colorimeter to evaluate the color stability of the fresh-cut fruit freshness visualized indicator film; (5) The standard curve is established by recording the relationship between the daily fruit firmness FH and the total soluble solid TSS content of the fresh-cut fruit, respectively, and the color difference value of the fresh-cut fruit freshness visualized indicator film; (6) FH and TSS test of fresh-cut fruit sample: the peeled fresh-cut fruit is instrumentally punctured with a cutting surface, and the fruit firmness FH of three surfaces is recorded; the fruit is cut into slices, and the fruit slices with substantially uniform thickness are wrapped with gauze and squeezed to extract juice, which is dropped into the sample port of a sugar meter for detection, and the total soluble solid TSS of three surfaces of the fruit is recorded; (7) The color difference value of the fresh-cut fruit freshness visualized indicator film is collected daily, and is fitted with the FH and TSS data obtained according to the method of step (6), combined with the standard curve established in step (5), to obtain the optimal Co-MOF loading amount, and the optimal fresh-cut fruit freshness visualized indicator film is prepared according to the optimal Co-MOF loading amount, and the fresh-cut fruit freshness visualized indicator film is a phenol red PR indicator film or a bromothymol blue BTB indicator film; (8) For detection of fresh-cut fruit: the packaging material containing the optimal fresh-cut fruit freshness visualized indicator film obtained in step (7) is covered on the packaging box containing the fresh-cut fruit to be detected, and the color change of the indicator film is observed to detect the freshness of the fresh-cut fruit.

10. The method of detecting the freshness of fresh-cut fruit using the visual indicator film for fresh-cut fruit according to claim 9, characterized by, The optimal phenol red PR loading is 4.0wt%, and the optimal phenol red PR indicator film is CMC-Na / PR / Co-MOF 4%, the standard curve of ΔE value and FH is y=0.336x-9.813 (R 2 =0.9887), and the standard curve of ΔE value and TSS is y=0.226x+1.484 (R 2 =0.9781); The ΔE value fitted with FH obtained the optimal bromothymol blue BTB loading of 4.0 wt%, the optimal bromothymol blue BTB indicating film was CMC-Na / BTB / Co-MOF 4%, and the standard curve was y=0.512x-14.912 (R 2 =0.9923); the ΔE value fitted with TSS obtained the optimal bromothymol blue BTB loading of 2.0 wt%, the optimal bromothymol blue BTB indicating film was CMC-Na / BTB / Co-MOF 2%, and the standard curve was y=0.267x+0.747 (R 2 =0.9769).

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