Color superposition type starch-based nanofiber capable of monitoring food freshness and preparation method of color superposition type starch-based nanofiber

Abstract

The invention discloses a color superposition type starch-based nanofiber for food freshness monitoring and a preparation method of the color superposition type starch-based nanofiber, and relates to the field of intelligent food freshness monitoring and food packaging. The preparation method comprises the following steps: (1) constructing a continuous phase consisting of starch octenyl succinate, polyvinyl alcohol and a water-soluble pH color response indicator; (2) constructing a dispersion phase containing plant essential oil and a water-insoluble pH color response indicator; (3) uniformly mixing the continuous phase and the dispersed phase, and homogenizing and emulsifying to construct an oil-in-water emulsion system with electrostatic spinning performance; and (4) preparing the starch-based nanofiber membrane with the core-shell structure through an emulsion electrostatic spinning technology. The shell indicator and the core indicator of the nanofiber can form synergistic color response when the pH of the environment changes, the response visual effect and the multi-dimensional response sensitivity to food spoilage can be remarkably enhanced through the internal and external color superposition effect of the core-shell structure fiber, and the problem that the color response of a single pH indicator is not remarkable at the present stage is solved. Meanwhile, the starch-based emulsion electrostatic spinning nanofiber membrane has excellent biodegradability and water-insoluble active component loading capacity, effectively solves the technical bottlenecks of poor environmental tolerance, single function and the like in the existing food packaging, can be widely applied to the fields of food intelligent active packaging and drug controlled release, and has wide application prospects. And an innovative solution is provided for promoting innovation of a green packaging technology and upgrading of a food safety monitoring system.

Description

Technical Field

[0001] The present invention relates to the field of intelligent monitoring of food freshness and food packaging, and in particular to a color-superimposed starch-based nanofiber capable of monitoring food freshness and a preparation method thereof. Background Art

[0002] Changing consumer lifestyles have increased the demand for freshness and safety in the food market, making ensuring food quality and safety a top priority. However, traditional petroleum-based food packaging is often unable to effectively detect and maintain the freshness of food during storage and transportation, resulting in approximately one-third of food being lost due to spoilage each year. The lack of intelligence and single functionality have become the main limitations of traditional food packaging. In addition, solutions to problems such as microplastic pollution, environmental degradation, and the depletion of non-renewable resources caused by petroleum-based packaging also need to be solved urgently. Therefore, the development of green and sustainable smart active food packaging has become the key to breaking the deadlock: through the strategic integration of biodegradable matrices, visual pH sensors and active ingredients, a biodegradable smart active packaging that is both biodegradable, visually displays freshness, and has an extended shelf life is constructed, providing a forward-looking solution to the above challenges.

[0003] Meat spoilage stems from microbial contamination, protein oxidation, and the degradation of nitrogenous compounds triggered by endogenous enzymatic reactions, a process that is accompanied by a rapid increase in the ambient pH. This dynamic change enables instrument-free, real-time monitoring of meat quality by triggering the color response of smart pH indicators, the core working mechanism of visual pH sensors. Based on this principle, integrating natural anthocyanins, which exhibit excellent pH color response, into natural polymer matrices has become an effective strategy for constructing biodegradable smart packaging. This strategy not only enables bio-based films to rapidly detect food freshness, but also avoids the potential health risks of chemically synthesized indicators and petroleum-based plastics. Among natural polymers, starch is one of the most attractive candidates for constructing biodegradable matrices due to its abundant availability, strong biodegradability, low cost, and excellent barrier, film-forming, and safety properties. It demonstrates great potential in the global "plastic reduction" effort.

[0004] Integrating functional active ingredients into film matrices to impart antimicrobial and antioxidant properties, rather than directly adding them to food, has become a key strategy for developing active food packaging. Natural active ingredients such as curcumin, carotenoids, and chlorophyll have become promising alternatives to chemical preservatives due to their potent antioxidant and antimicrobial properties. Furthermore, these ingredients exhibit pH-color responsiveness, enabling real-time freshness monitoring while extending the shelf life of food. However, their poor water solubility, low bioavailability, and instability often require organic solvents for loading, potentially posing health and environmental risks. Furthermore, the low color change response of single visual pH indicators can lead to a lack of precision in indicating food freshness at varying pH values. Therefore, the development of biodegradable smart active packaging capable of simultaneously loading multiple visual pH indicators has become a key breakthrough. By integrating multiple pH-responsive indicator systems, this approach can broaden the color response range and synergistically extend the shelf life of food, significantly enhancing the intelligent advancement of environmentally friendly smart active food packaging.

[0005] Emulsion electrospinning, based on nano-driven engineering, is a simple, cost-effective, and gentle electrohydrodynamic technique that can transform emulsions into ultrafine core-shell fibers at room temperature. This effectively circumvents the harsh conditions (such as high temperature and mechanical shear) associated with traditional film fabrication methods (such as casting, extrusion, and blow molding) that can deactivate heat-sensitive substances. Furthermore, compared to the closed surfaces of traditional films, the resulting nanofibrous membranes possess a high surface area and porosity, providing more responsive sites and significantly improving monitoring sensitivity. Compared to traditional electrospinning techniques (co-blending, secondary carrier, and coaxial spinning), emulsion electrospinning enables simultaneous encapsulation of both water-insoluble and water-soluble substances using a single-axis nozzle, without the need for organic solvents. This strategy not only overcomes the inactivation of surfactants caused by incomplete encapsulation in co-blending electrospinning but also cleverly circumvents the complex operations and stringent process variables associated with coaxial electrospinning, which holds important practical significance for promoting "green electrospinning." Notably, the alkaline environment created by meat spoilage can simultaneously activate the dual pH-responsive system in the core-shell structure, broadening the colorimetric recognition range through a color superposition effect. Currently, there are no reports on the construction of biodegradable smart active packaging by co-loading water-insoluble and water-soluble pH indicators into starch-based core-shell nanofibers using emulsion electrospinning technology. Summary of the Invention

[0006] In order to overcome the main limitations of traditional food packaging, namely the lack of intelligence and single functionality, the primary purpose of the present invention is to provide a biodegradable intelligent active packaging that combines biodegradability, freshness visualization and extended shelf life by strategically integrating a biodegradable matrix, a visual pH sensor and active ingredients.

[0007] Another object of the present invention is to provide a nanofiber membrane loaded with a dual pH response system in a core-shell structure, which is used to expand the color response range of a single pH indicator through the superposition effect of internal and external colors, thereby improving the response accuracy of smart active food packaging to changes in environmental pH values.

[0008] Another object of the present invention is to provide an emulsion electrospinning process for preparing core-shell structured nanofibers of a dual-load system, so as to replace the coaxial electrospinning process with low efficiency, complex operation and high process variable requirements, as well as the blending electrospinning process with incomplete encapsulation.

[0009] Another object of the present invention is to provide a strategy for preparing green starch-based food packaging, which effectively avoids environmental degradation and waste of resources.

[0010] To achieve the above object, the present invention relates to the following technical solutions: A method for preparing color-superimposed starch-based nanofibers capable of monitoring food freshness comprises the following steps: (1) Octenyl succinate starch and polyvinyl alcohol are dissolved in deionized water at high temperature, mixed evenly, and then allowed to stand and cool to room temperature to obtain an octenyl succinate starch / polyvinyl alcohol mixed solution.

[0011] (2) A water-soluble pH color response indicator is added to the octenyl succinate starch / polyvinyl alcohol mixed solution, and the mixture is stirred thoroughly at room temperature to finally obtain a continuous phase.

[0012] (3) Add a water-insoluble pH color response indicator to the plant essential oil and heat and stir to obtain a dispersed phase after mixing evenly.

[0013] (4) The water-soluble pH color response indicator / octenyl succinate starch / polyvinyl alcohol continuous phase is mixed with the water-insoluble pH color response indicator / plant essential oil dispersed phase, and homogenized and emulsified to obtain an oil-in-water emulsion.

[0014] (5) The oil-in-water emulsion was subjected to emulsion electrospinning to prepare starch-based nanofibers with color-stacked core-shell structures for monitoring food freshness.

[0015] Preferably, in the octenyl succinate starch / polyvinyl alcohol mixed solution described in step (1), the mass volume ratio of octenyl succinate starch is 4% to 8%, more preferably 4% to 6%, and most preferably 4%.

[0016] Preferably, in the octenyl succinate starch / polyvinyl alcohol mixed solution described in step (1), the mass volume ratio of polyvinyl alcohol is 3% to 7%, more preferably 5% to 7%, and most preferably 7%.

[0017] Preferably, in the octenyl succinate starch / polyvinyl alcohol mixed solution described in step (1), the octenyl succinate starch / polyvinyl alcohol is stirred in a water bath at 80-95° C. for 1-3 hours.

[0018] Preferably, in the continuous phase described in step (2), the water-soluble pH color response indicator includes one or more of roselle anthocyanidin, purple sweet potato anthocyanidin, mulberry anthocyanidin, perilla leaf anthocyanidin, blueberry anthocyanidin, purple cabbage anthocyanidin, black wolfberry anthocyanidin and betaine.

[0019] Preferably, in the continuous phase of step (2), the amount of the water-soluble pH color response indicator added to water is 0.02-0.24 g / mL, more preferably 0.02-0.20 g / mL, and most preferably 0.15 g / mL.

[0020] Preferably, in the continuous phase of step (2), the mixed solution of the water-soluble pH color response indicator / octenyl succinate starch / polyvinyl alcohol is uniformly stirred in a water bath at 30-40°C for 4-5 hours, more preferably at 40°C for 5 hours. The entire process is carried out in a dark and closed environment.

[0021] Preferably, in the dispersed phase of step (3), the water-insoluble pH color response indicator comprises one or more of curcumin, carotenoids and chlorophyll.

[0022] Preferably, in the dispersed phase of step (3), the plant essential oil comprises one or more of tea tree essential oil, ginger essential oil, clove essential oil, geranium essential oil, rose essential oil, thyme essential oil and lemon essential oil.

[0023] Preferably, in the dispersed phase of step (3), the amount of the water-insoluble pH color response indicator added to the plant essential oil is 0.01-0.2 g / mL, more preferably 0.02-0.18 g / mL, and most preferably 0.025 g / mL.

[0024] Preferably, in the dispersed phase of step (3), a water-insoluble pH color response indicator is added to the plant essential oil and stirred uniformly at 30-50°C for 30-120 minutes, more preferably at 45°C for 100 minutes. The entire process is carried out in a dark and closed environment.

[0025] Preferably, in the emulsion system described in step (4), the volume ratio of the water-insoluble pH color response indicator / plant essential oil dispersed phase to the water-soluble pH color response indicator / octenyl succinate starch / polyvinyl alcohol continuous phase is 1:9 to 3:7, more preferably 1:9.

[0026] Preferably, in the emulsion system described in step (4), the homogenization and emulsification method includes one or more of high-pressure homogenization, rotor-stator homogenization and ultrasonic homogenization.

[0027] Preferably, in the emulsion system described in step (4), the homogenization rate is 8000-14000 rpm, more preferably 10000-13000 rpm, most preferably 11000 rpm, and the homogenization time is preferably 1-2 min.

[0028] Preferably, in the emulsion system described in step (4), the conductivity of the water-in-oil emulsion used for emulsion electrospinning is 350-900 μs / cm, the emulsion particle size is 80-300 nm, and the Zeta potential range is ±20-±40 mV.

[0029] Preferably, in the emulsion electrospinning process described in step (5), the emulsion flow rate is 0.2~0.5mL / h, the internal environmental humidity of the electrospinning machine is 30~35%, the internal environmental temperature of the electrospinning machine is 30~35℃, the distance between the electrospinning machine needle and the receiver is 10~15cm, the electrospinning machine voltage is 16~20kV, the speed of the electrospinning machine drum receiver is 85~200rpm, and the working time is 6~8h.

[0030] The above technical solution provides a color-superimposed starch-based nanofiber capable of monitoring the freshness of food and a preparation method thereof. Compared with the prior art, the present invention has the following advantages and beneficial effects: (1) The present invention utilizes emulsion electrospinning technology to prepare a biodegradable intelligent active food packaging that integrates a biodegradable matrix, a visual pH sensor, and active ingredients. The packaging has the functions of biodegradability, freshness visualization, and shelf life extension, overcoming the main limitations of existing traditional food packaging, such as the lack of intelligence and single functionality.

[0031] (2) The present invention adopts an emulsion electrospinning process to prepare a core-shell structured nanofiber with dual loading of water-insoluble substances and water-soluble substances, so as to replace the coaxial electrospinning process with low efficiency, complex operation and high process variable requirements, as well as the blended electrospinning process with incomplete encapsulation.

[0032] (3) The present invention adopts the emulsion electrospinning process to prepare a nanofiber membrane loaded with a dual pH response system in a core-shell structure. Through the superposition effect of the internal and external colors of the fiber core-shell structure, the color response range of a single pH indicator is expanded, thereby improving the response accuracy of smart active food packaging to changes in environmental pH values.

[0033] (4) The present invention adopts the emulsion electrospinning process to prepare a strategy for preparing starch-based biodegradable food packaging, which effectively avoids environmental degradation and resource waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is an optical microscope image of a double-loaded starch-based emulsion loaded with curcumin, a water-insoluble pH color-responsive indicator, and anthocyanins, a water-soluble pH color-responsive indicator, prepared in Example 1; Figure 2 Optical microscope images of starch-based emulsions with no loading, single loading, and dual loading of pH color response indicators prepared in Comparative Example 1; Figure 3 X-ray diffraction spectra of starch-based nanofibers with no loading, single loading, and dual loading of pH color response indicator prepared in Comparative Example 2; Figure 4 Scanning electron microscope images and transmission electron microscope images of starch-based nanofibers with no loading, single loading, and dual loading of pH color response indicators prepared in Comparative Example 2; Figure 5 This is the natural degradation process of the starch-based nanofiber membrane prepared in Comparative Example 2; Figure 6 The diameters of the inhibition zones of starch-based nanofibers with no loading, single loading, and dual loading of pH color response indicators prepared in Comparative Example 2; Figure 7 DPPH radical scavenging activity of starch-based nanofibers with no loading, single loading, and dual loading of pH color response indicator prepared in Comparative Example 2; Figure 8 Color response behavior of starch-based nanofibers with no loading, single loading, and dual loading of pH color response indicator prepared in Comparative Example 2 at different pH values; Figure 9 The dual-loaded starch-based nanofibers loaded with curcumin, a water-insoluble pH color response indicator, and anthocyanins, a water-soluble pH color response indicator, prepared in Comparative Example 2, were used for the preservation and freshness indication of shrimp at 4°C. DETAILED DESCRIPTION

[0035] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process. However, the protection scope of the present invention is not limited to the following embodiment.

[0036] Example 1 Octenylsuccinate starch (4%, w / v) and polyvinyl alcohol (7%, w / v) were dissolved in distilled water and magnetically stirred at 95°C for 2.5 hours to obtain an octenylsuccinate starch / polyvinyl alcohol (SP) mixed solution. After the SP solution cooled, roselle anthocyanidins (3%, w / v) were added and stirred at 40°C for 5 hours to obtain an octenylsuccinate starch / polyvinyl alcohol / roselle anthocyanidin (SPR) mixed solution. Subsequently, curcumin was added to clove essential oil (the mass volume ratio of curcumin to clove essential oil was 2.5%) and stirred in a 45°C water bath for 100 minutes to fully dissolve the curcumin and obtain a curcumin / clove essential oil (OC) dispersion. The curcumin / clove essential oil dispersed phase and the SPR continuous phase were mixed in a volume ratio of 3:7 and homogenized. The mixture was homogenized at a speed of 11,000 rpm using a high-speed shearing machine for 1 min to obtain an octenylsuccinate starch / polyvinyl alcohol / roselle anthocyanidin-curcumin / clove essential oil (SPR-OC) emulsion system.

[0037] Optical microscope image of the double-loaded emulsion system.

[0038] After removing bubbles using an ultrasonicator, 3 μL of fresh SPR-OC emulsion was dropped onto a microscope slide and covered with a coverslip. The microstructure of each emulsion was recorded using a BX53F optical microscope under a 100x oil immersion lens. The particle size of each emulsion was measured at 25°C using a particle size analyzer to obtain the particle size distribution of the SPR-OC dual-loaded emulsion system.

[0039] like Figure 1 The particle size of the SPR-OC dual-loaded emulsion is 88.19±8.33nm, which is at a relatively low particle size level (nanoscale). This indicates that the octenylsuccinate starch / polyvinyl alcohol polymer effectively integrates and maintains the emulsifying properties of octenylsuccinate starch and the ability of polyvinyl alcohol to capture the oil phase. It has good emulsifying properties and can be quickly adsorbed to the oil / water interface to form a barrier to prevent the aggregation of oil droplets, successfully achieving the effective loading of the dual pH-responsive system (Roselle anthocyanins and curcumin).

[0040] Comparative Example 1 Octenylsuccinate starch (4%, w / v) and polyvinyl alcohol (7%, w / v) were dissolved in distilled water and magnetically stirred at 95°C for 2.5 h to obtain a SP mixed solution. After the SP solution cooled, roselle anthocyanidins (3%, w / v) were added and stirred at 40°C for 5 h to obtain a SPR mixed solution. A clove essential oil (10%, v / v) dispersed phase was mixed with the SP and SPR continuous phases at a volume ratio of 3:7, respectively, and homogenized to obtain octenylsuccinate starch / polyvinyl alcohol-clove essential oil (SP-O) and octenylsuccinate starch / polyvinyl alcohol / roselle anthocyanidins-clove essential oil (SPR-O) oil-in-water emulsions. Subsequently, curcumin was added to the clove essential oil (the mass volume ratio of curcumin to clove essential oil was 2.5%) and stirred in a 45°C water bath for 100 min to fully dissolve the curcumin / clove essential oil (OC) dispersed phase. The OC dispersed phase was mixed with the SP and SPR continuous phases at a volume ratio of 3:7 and homogenized to produce starch octenylsuccinate / polyvinyl alcohol-curcumin / clove essential oil (SP-OC) and SPR-OC emulsion systems. Four emulsion systems were prepared: SP-O, SPR-O, SP-OC, and SPR-OC. All emulsion systems were homogenized at 11,000 rpm for 1 minute using a high-speed shear.

[0041] Optical microscope images of each emulsion system.

[0042] After removing bubbles using an ultrasonicator, 3 μL of fresh emulsions (SP-O, SP-OC, SPR-O, and SPR-OC) were dropped onto a microscope slide and covered with a coverslip. The microstructure of each emulsion was recorded using a BX53F optical microscope. All images were acquired under a 100x oil immersion lens. The particle size of each emulsion was measured at 25°C using a particle size analyzer, yielding the particle size distribution of the four composite emulsion systems.

[0043] like Figure 2The particle sizes of the four emulsion systems have a smaller particle size range (nanoscale), which indicates that the octenyl succinate starch / polyvinyl alcohol polymer retains the excellent emulsifying properties of both and has good emulsifying properties. Compared with the SP-O and SP-OC emulsion systems, the particle sizes of the SPR-O and SPR-OC emulsions loaded with Roselle anthocyanins are slightly reduced, indicating that the addition of hydrophilic anthocyanins changes the interfacial activity of the octenyl succinate starch / polyvinyl alcohol complex. In addition, by observing the microstructure and particle size distribution of the SP-O and SPR-O stable emulsions, it was found that the SP-OC and SPR-OC emulsion systems loaded with curcumin also showed a trend of decreasing particle size, which indicates that the addition of a small amount of curcumin will reduce the interfacial tension at the oil-water interface. It is worth noting that compared with the SPR-O (pink) and SP-OC (light yellow) emulsion systems loaded with a single pH color response indicator, the SPR-OC dual pH color response indicator loaded emulsion system exhibits a more significant orange-yellow color, indicating that there is indeed a color superposition effect between the outer layer of roselle anthocyanin and the inner layer of curcumin in the water-in-oil emulsion, providing an ideal precursor for the subsequent preparation of core-shell structured nanofibers that can achieve a color superposition effect.

[0044] Example 2 Octenylsuccinate starch (4%, w / v) and polyvinyl alcohol (7%, w / v) were dissolved in distilled water and magnetically stirred at 95°C for 2.5 h to obtain a SP mixed solution. After the SP solution cooled, blueberry anthocyanidins (3%, w / v) were added and stirred at 40°C for 5 h to obtain a starch octenylsuccinate / polyvinyl alcohol / blueberry anthocyanidin mixed solution. Subsequently, chlorophyll was added to peppermint essential oil (the mass volume ratio of chlorophyll to peppermint essential oil was 2.5%) and stirred in a 45°C water bath for 100 min to fully dissolve the chlorophyll / peppermint essential oil dispersion. The chlorophyll / peppermint essential oil dispersion was then mixed with the starch octenylsuccinate / polyvinyl alcohol / blueberry anthocyanidin continuous phase at a volume ratio of 3:7 and homogenized using a high-speed shear at 11,000 rpm for 1 min to obtain a starch octenylsuccinate / polyvinyl alcohol / blueberry anthocyanidin-chlorophyll / peppermint essential oil emulsion system. The emulsion electrospinning process was then carried out at a spinning voltage of 20 kV, a dope flow rate of 0.4 mL / h, an internal humidity of 32%, a temperature of 30°C, a spinning distance of 15 cm, a drum speed of 120 rpm, and a spinning time of 6.5 hours. The resulting starch-based biodegradable green nanofibers consisted of a shell composed of starch octenylsuccinate / polyvinyl alcohol / blueberry anthocyanins and a core composed of chlorophyll / peppermint essential oil. These nanofibers can reflect the freshness of meat products in real time through changes in ambient pH, extending their shelf life.

[0045] Example 3 Octenylsuccinate starch (4%, w / v) and polyvinyl alcohol (7%, w / v) were dissolved in distilled water and magnetically stirred at 95°C for 2.5 hours to obtain a SP mixed solution. After the SP solution cooled, perilla leaf anthocyanins (3%, w / v) were added and stirred at 40°C for 5 hours to obtain an octenylsuccinate starch / polyvinyl alcohol / perilla leaf anthocyanin mixed solution. Subsequently, carotenoids were added to thyme essential oil (the mass volume ratio of carotenoids to thyme essential oil was 2.5%) and stirred in a 45°C water bath for 100 minutes to fully dissolve the carotenoids and obtain a carotenoid / thyme essential oil dispersion. A carotenoid / thyme oil dispersed phase was mixed with a starch octenylsuccinate / polyvinyl alcohol / blueberry anthocyanidin continuous phase in a volume ratio of 3:7 and homogenized. The mixture was then homogenized using a high-speed shear at 11,000 rpm for 1 minute to obtain an emulsion system consisting of starch octenylsuccinate / polyvinyl alcohol / perilla leaf anthocyanidin-carotenoid / thyme oil. This emulsion was then electrospun at a voltage of 20 kV, a dope flow rate of 0.4 mL / h, an internal humidity of 32%, a temperature of 30°C, a spinning distance of 15 cm, a drum speed of 120 rpm, and a spinning time of 6.5 hours. The resulting starch-based biodegradable green nanofibers consisted of a starch octenylsuccinate / polyvinyl alcohol / perilla leaf anthocyanidin shell and a carotenoid / thyme oil core. These nanofibers can reflect the freshness of meat products in real time by monitoring changes in ambient pH, thereby extending their shelf life.

[0046] Example 4 Octenylsuccinate starch (4%, w / v) and polyvinyl alcohol (7%, w / v) were dissolved in distilled water and magnetically stirred at 95°C for 2.5 h to obtain a SP mixed solution. After the SP solution cooled, purple sweet potato anthocyanidins (3%, w / v) were added and stirred at 40°C for 5 h to obtain a starch octenylsuccinate / polyvinyl alcohol / purple sweet potato anthocyanidin mixed solution. Subsequently, chlorophyll was added to rose essential oil (the mass volume ratio of chlorophyll to rose essential oil was 2.5%) and stirred in a 45°C water bath for 100 min to fully dissolve the chlorophyll / rose essential oil dispersion. The chlorophyll / rose essential oil dispersion was then mixed with the starch octenylsuccinate / polyvinyl alcohol / purple sweet potato anthocyanidin continuous phase at a volume ratio of 3:7 and homogenized using a high-speed shear at 11,000 rpm for 1 min to obtain a starch octenylsuccinate / polyvinyl alcohol / purple sweet potato anthocyanidin-chlorophyll / rose essential oil emulsion system. The emulsion electrospinning process was then carried out at a spinning voltage of 20 kV, a dope flow rate of 0.4 mL / h, an internal humidity of 32%, a temperature of 30°C, a spinning distance of 15 cm, a drum speed of 120 rpm, and a spinning time of 6.5 hours. The resulting starch-based biodegradable green nanofibers consisted of a shell composed of starch octenylsuccinate / polyvinyl alcohol / purple potato anthocyanins and a core composed of chlorophyll / rose essential oil. These nanofibers can reflect the freshness of meat products in real time through changes in ambient pH, extending their shelf life.

[0047] Comparative Example 2 Octenylsuccinate starch (4%, w / v) and polyvinyl alcohol (7%, w / v) were dissolved in distilled water and stirred at 95°C for 2.5 h to obtain an octenylsuccinate starch / polyvinyl alcohol (SP) mixed solution. After the SP solution cooled, roselle anthocyanidins (3%, w / v) were added and stirred at 40°C for 5 h to obtain an octenylsuccinate starch / polyvinyl alcohol / roselle anthocyanidin (SPR) mixed solution. Clove essential oil (10%, v / v) was then dispersed with the SP and SPR continuous phases at a volume ratio of 3:7 and homogenized to obtain octenylsuccinate starch / polyvinyl alcohol-clove essential oil (SP-O) and octenylsuccinate starch / polyvinyl alcohol / roselle anthocyanidin-clove essential oil (SPR-O) oil-in-water emulsions. Subsequently, curcumin was added to clove essential oil (the mass volume ratio of curcumin to clove essential oil was 2.5%) and stirred in a 45°C water bath for 100 minutes to fully dissolve the curcumin / clove essential oil (OC) dispersion. The OC dispersion was then mixed with the SP and SPR continuous phases at a volume ratio of 3:7 and homogenized to produce octenylsuccinate starch / polyvinyl alcohol-curcumin / clove essential oil (SP-OC) and octenylsuccinate starch / polyvinyl alcohol / roselle anthocyanidin-curcumin / clove essential oil (SPR-OC) emulsion systems. Four emulsion systems, SP-O, SPR-O, SP-OC, and SPR-OC, were ultimately obtained. All emulsions were homogenized using a high-speed shear at 11,000 rpm for 1 minute. Then the emulsion electrospinning process was carried out, the spinning machine voltage was 20kV, the spinning liquid flow rate was 0.4mL / h, the internal environmental humidity of the electrospinning machine was 32%, the internal environmental temperature of the electrospinning machine was 30℃, the spinning distance was 15cm, the spinning machine drum speed was 120rpm, and the spinning time was 6.5h. After completion, six different electrospun nanofibers were obtained.

[0048] X-ray diffraction pattern.

[0049] The crystal structures of starch octenylsuccinate, polyvinyl alcohol, roselle anthocyanins, curcumin, and electrospun nanofiber samples (SP, SPR, SP-O, SPR-O, SP-OC, SPR-O, and SPR-OC) were characterized using XRD-3D X-ray diffractometer (XRD). The operating voltage and current were set at 40 kV and 40 mA, respectively.

[0050] like Figure 3The XRD diffraction patterns of SP-OC and SPR-OC nanofibers revealed that the characteristic diffraction peak corresponding to pristine curcumin disappeared, replaced by an amorphous pattern with diffuse characteristic peaks. This is because curcumin transformed from a highly crystalline to an amorphous state during the emulsion electrospinning process, successfully embedding into the core layer of the nanofibers. Furthermore, the sharp characteristic peak of polyvinyl alcohol at 2θ = 19.9° was replaced by a diffuse amorphous peak in all nanofiber samples, indicating that starch octenylsuccinate and polyvinyl alcohol did not simply form a mixture, but rather formed a complex through hydrogen bonding interactions. Compared to SP and SPR prepared by mixed electrospinning, the diffraction peak at 2θ = 19.9° in the SP-O, SPR-O, and SP-OC nanofibers prepared by emulsion electrospinning was weakened, indicating that the loading of clove essential oil and curcumin led to the formation of more hydrogen bonds, improving the compatibility between the components.

[0051] Scanning electron microscopy and transmission electron microscopy images of nanofibers.

[0052] The microscopic morphologies of electrospun nanofibers at different voltages were observed using a Regulus 8220 scanning electron microscope, and single fibers on a copper grid were observed using a JEM 1400 Flash transmission electron microscope at an accelerating voltage of 120 kV to observe their core-shell structure.

[0053] like Figure 4 The emulsion electrospun nanofibers were further characterized by transmission electron microscopy to verify their core-shell structure and the distribution of the oil phase. In the transmission electron microscopy images of the SP and SPR nanofibers, only the dark inner region was observed, with no clear boundaries, indicating that both were solid fibers without a core-shell structure. The complex formed by octenyl succinate starch, polyvinyl alcohol, and roselle anthocyanins was evenly distributed on the surface and interior of the fibers. Unlike the SP and SPR fibers, the images of the SP-O, SPR-O, SP-OC, SPR-O, and SPR-OC nanofibers clearly showed a gray outer layer and a black inner layer. This phenomenon can be attributed to the difference in electron density, which leads to different electron transport capabilities of the inner and outer materials, thereby affecting the transmission rate of the electron beam, thus revealing the core-shell structure. Therefore, the SPR-OC nanofibers do have a core-shell structure and have successfully achieved the simultaneous loading of two pH color-responsive indicators: water-soluble roselle anthocyanins and water-insoluble curcumin. It is worth noting that compared with SPR-O (light pink) loaded with Roselle anthocyanins alone and SP-OC (light yellow) loaded with curcumin alone, the SPR-OC nanofiber membrane (orange-yellow) co-loaded with Roselle anthocyanins and curcumin has a more significant visual effect in color, indicating that there is a color superposition effect between the two, which will play a positive role in the subsequent visualization process of food freshness.

[0054] Biodegradability testing of nanofiber membranes.

[0055] The dried nanofiber membranes were cut into rectangular shapes and placed in a 500 mL beaker containing natural soil. The membranes were then placed in an incubator for observation. To simulate natural conditions, the temperature and humidity were set at 30°C and 45%, respectively, and distilled water was used for regular irrigation. A polyethylene (PE) film served as a control. The membranes' status was recorded daily using a digital camera.

[0056] like Figure 5 The biodegradability of each nanofiber membrane was evaluated in soil for a week under simulated natural environmental conditions. As can be seen in the figure, with the exception of the polyethylene (PE) membrane in the control group, all other starch-based nanofiber membranes completed natural degradation after 5 days. Throughout the degradation process, the continuous penetration of water molecules breaks the hydrogen bonds within each fiber membrane, leading to curling, wrinkling, and cracking, until the nanofiber membrane loses its original shape. Mechanical fragmentation of the fiber membrane, triggered by enzymatic degradation by soil microorganisms, is the primary cause of its natural degradation. This process provides carbon and nitrogen sources for microbial growth and metabolism. In contrast, PE membrane undergoes virtually no degradation in soil, which could pose a threat to the ecological environment. In summary, compared to petroleum-based food packaging, this series of starch-based nanofiber membranes demonstrates superior biofriendliness. They can be disposed of in the natural environment without causing plastic pollution, making them a viable alternative to traditional petroleum-based food packaging when needed.

[0057] Determination of antibacterial and antioxidant properties of nanofiber membranes.

[0058] 36 mg of nanofiber membrane was dissolved in 0.9 mL of phosphate buffered saline to prepare a sample solution. Subsequently, Escherichia coli ( E. coli ) and Staphylococcus aureus ( S. aureus ) were cultured in sterile nutrient broth for 24 h. 6A bacterial suspension (100 μL) containing 100 colony-forming units (CFU) was mixed with agar (10 mL) and then poured into a Petri dish containing an Oxford cup (8.5 mm diameter). The cup was removed after the solid medium solidified. 100 μL of sample solution was added to the wells of the solid medium and then incubated at 37°C for 24 h. The diameter of the inhibition zone was measured using a caliper, and the inhibition zone was recorded using a digital camera. Subsequently, 0.2 g of the fibers were soaked in 100 mL of deionized water and magnetically stirred for 1 h to obtain a sample aqueous solution. The sample aqueous solution was mixed with a 75 μM DPPH ethanol solution at a volume ratio of 1:4 and reacted in the dark for 24 h. The absorbance of the solution at 517 nm was measured using a UV spectrophotometer to assess DPPH radical scavenging activity. During the 28-day storage period of each nanofiber mat, DPPH radical scavenging activity was repeatedly tested to evaluate antioxidant stability.

[0059] like Figure 6 , SP and SPR nanofibers did not show obvious inhibition zone, indicating that the octenyl succinate starch / polyvinyl alcohol complex and low content of Roselle anthocyanins had no significant inhibitory effect on antibacterial activity. With the loading of clove essential oil, SP-O nanofiber membrane began to E. coli and S. aureus The results showed significant inhibitory effects, with inhibition zone diameters of 10.37±0.32 mm and 10.91±0.35 mm, respectively. It is worth noting that SP-OC and SPR-OC showed the greatest inhibitory effects on these two target bacteria. E. coli The inhibition zone diameters of 13.42±0.36mm and 13.77±0.41mm were respectively S. aureus The inhibition zone diameters were shown to be 14.49 ± 0.32 mm and 14.61 ± 0.38 mm, respectively. This was attributed to the slow release of curcumin during bacterial culture, which promoted the increased permeability of the bacterial cell wall or cell membrane, leading to the destruction and dissolution of the bacterial cell structure and ultimately the death of the bacteria.

[0060] like Figure 7Compared with SP nanofibers (2.2±0.7%), the free radical scavenging activity of SPR nanofibers (15.1±1.6%) was slightly increased, indicating that the Roselle anthocyanins attached to the fiber shell possessed some antioxidant activity. Because clove essential oil contains a large number of unsaturated double bonds, the loading of clove essential oil imparted robust free radical scavenging capabilities to the nanofibers. Specifically, loading clove essential oil onto SP (2.1±0.7%) and SPR (15.1±1.6%) nanofibers significantly enhanced the antioxidant properties of SP-O (30.4±3.1%) and SPR-O (40.3±2.7%) nanofibers. In contrast, curcumin loading significantly improved the free radical scavenging properties of the fibers, with SPR-OC nanofibers achieving the highest antioxidant activity, reaching 80.2±3.4%.

[0061] pH color response test of nanofiber membrane.

[0062] The SPR-OC dual-loaded core-shell structured nanofiber membrane was placed in buffer solutions with different pH values ​​to test its color response behavior to different pH values, and the color changes were recorded using a camera.

[0063] like Figure 8, demonstrating the color response behavior of each sample under different pH conditions. The SP, SP-O, and SPR-O nanofiber membranes showed no significant changes in color within the pH range of 2–12, indicating a lack of pH color response. Notably, the SPR, SP-OC, and SPR-OC samples exhibited pH-dependent color response, demonstrating that the pH-responsive color change properties of Roselle anthocyanins and curcumin were maintained after loading into the core-shell structure of the fibers. In contrast, the SPR-OC nanofibers exhibited multi-stage and significantly different pH-dependent color response behaviors. SPR-OC exhibited a bright yellow color at pH 2–5, gradually transitioning to orange-yellow (pH = 6) and reddish-brown (pH = 7), and finally to purple-red (pH 8–11) and dark brown (pH ≥ 11). The emergence of this new color response (purple-red) is attributed to the unique core-shell structure of the emulsion electrospun nanofibers. The pH range of 2–6 corresponds to the bright yellow color of curcumin in the core layer and the pink color of roselle anthocyanins in the shell layer. The combined effect of these two colors results in the orange-yellow color of SPR-OC within the pH range of 2–6. Subsequently, at pH 7, this process induces the simultaneous color changes of curcumin in the core layer and roselle anthocyanins in the shell layer, turning orange and purple, respectively. The orange core layer and purple shell layer are responsible for the reddish-brown color of SPR-OC nanofibers at pH 7. As the ambient pH increases (8–11), the roselle anthocyanins in the shell layer turn purple, and the curcumin in the shell layer turns orange-red. This superposition of the inner and outer colors of the core-shell structure of the SPR-OC fibers results in a novel indicator color, purple-red. This significantly enhances the warning effect of biodegradable smart active packaging while addressing the undesirable limitation of the low color change of pure curcumin films. Ultimately, the core curcumin and the shell roselle anthocyanins turn red and yellow, respectively, resulting in a dark brown pH response endpoint for SPR-OC at pH > 11.

[0064] Application of dual-loaded nanofiber membrane for shrimp preservation and freshness indication.

[0065] SP and SPR-OC films were cut into rectangular films and placed in Petri dishes. Shrimp were then encapsulated in the films. Fresh shrimp not encapsulated in the films served as a control. The Petri dishes were placed in a constant-temperature incubator at 25°C and 75% humidity. Color changes in the fiber films of each group were observed and recorded with a camera at different time periods. Total volatile basic nitrogen (TVB-N), pH, and viable bacterial count (TVC) of pork and shrimp were measured using a K9860 Kjeldahl nitrogen analyzer and a pH meter at the aforementioned time periods.

[0066] like Figure 9At 4°C, fresh shrimp samples initially exhibited a turquoise color, corresponding to lower TVB-N, TVC, and pH values. This stage corresponds to the initial bright yellow color of SPR-OC, indicating the shrimp are "fresh" (TVB-N < 30 mg / 100 g). As storage time increased to 60 hours, shrimp in the control and SP groups began to develop localized redness, attributed to the inherent red exposure of free astaxanthin released from shrimp protein degradation. However, shrimp in the SPR-OC group retained their turquoise color, representative of freshness, for 60 hours, and SPR-OC remained bright yellow. At 90 h, the shrimps in the control group (TVB-N=33.07±1.61 mg / 100 g, TVC=5.42±0.13 log CFU / g, pH=7.65±0.16) and the SP group (TVB-N=31.45±1.22 mg / 100 g, TVC=5.13±0.14 log CFU / g, pH=7.42±0.22) deteriorated significantly and emitted a foul odor. However, the shrimp encapsulated with SPR-OC (TVB-N = 25.82 ± 2.12 mg / 100 g, TVC = 4.75 ± 0.12 log CFU / g, pH = 7.02 ± 0.18) showed redness only in some areas. At this stage, the shrimp still met the TVB-N standard for fresh seafood (TVB-N ≤ 30 mg / 100 g) as specified in the Chinese standard GB 2733-2015, the generally accepted TVC standard for fresh seafood (TVC ≤ 5.0 log CFU / g), and the acceptable pH limit for fresh seafood (pH ≤ 7.8). This stage is considered "sub-fresh," corresponding to the orange coloration of the SPR-OC. The significantly fresher appearance of the shrimp in the SPR-OC group over the same period suggests that the SPR-OC sample effectively slowed the deterioration rate of fresh shrimp. As deterioration progressed, shrimp in the SPR-OC group turned completely red after 120 h, and the color of SPR-OC changed from orange-red at 120 h (TVB-N = 39.52 ± 2.33 mg / 100 g, TVC = 5.44 ± 0.12 log CFU / g, pH = 7.57 ± 0.15) to purple-red at 150 h (TVB-N = 58.86 ± 1.68 mg / 100 g, TVC = 6.51 ± 0.24 log CFU / g, pH = 8.15 ± 0.19), corresponding to the “deterioration” and “complete corruption” stages, respectively.

[0067] The synchronized pH-color superposition of core curcumin and shell roselle anthocyanins in SPR-OC fibers effectively overcomes the limitations of single indicators due to their low color change under varying pH conditions. Furthermore, SPR-OC can dynamically indicate the freshness stages of meat through color changes: fresh (bright yellow), sub-fresh (orange), spoiled (red), and corrupted (purple-red). Strong correlations between color changes and spoilage indicators (TVB-N, TVC, and pH) confirm its reliability as a freshness indicator. Furthermore, thanks to the sustained release of clove essential oil and curcumin from the fiber, SPR-OC successfully extends the shelf life of fresh shrimp under both cold chain and ambient temperature storage conditions.

[0068] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing color-superimposed starch-based nanofibers capable of monitoring food freshness, characterized in that: The starch-based material has a core-shell structure, wherein the shell layer is formed by electrospinning an octenylsuccinate starch / polyvinyl alcohol solution loaded with a water-soluble pH color response indicator, and the core layer is formed by electrospinning a plant essential oil / water-insoluble pH color response indicator. The preparation method comprises the following steps: (1) Octenyl succinate starch and polyvinyl alcohol are used as emulsifiers to prepare a mixed solution with a fiber-forming matrix; (2) The continuous phase is obtained by adding a water-soluble pH color-responsive indicator to an octenylsuccinate starch / polyvinyl alcohol mixed solution; (3) The dispersed phase is obtained by adding a water-insoluble pH color-responsive indicator to the plant essential oil; (4) fully mixing the continuous phase obtained in step (2) and the dispersed phase obtained in step (3) and homogenizing and emulsifying them to obtain an oil-in-water emulsion with electrospinning properties; (5) The oil-in-water emulsion obtained in step (4) is subjected to emulsion electrospinning to obtain a core-shell structured degradable starch-based nanofiber membrane co-loaded with a dual pH color response indicator.

2. The method for preparing color-superimposed starch-based nanofibers capable of monitoring food freshness according to claim 1, characterized in that: In the step (1), in the octenyl succinate starch / polyvinyl alcohol mixed solution, the mass volume ratio of octenyl succinate starch is 4% to 8%, and the mass volume ratio of polyvinyl alcohol is 3% to 7%; preferably, the octenyl succinate starch / polyvinyl alcohol mixed solution is stirred in a water bath at 80 to 95° C. for 1 to 3 hours, and a uniform solution is formed and allowed to stand and cool to room temperature.

3. The method for preparing color-superimposed starch-based nanofibers capable of monitoring food freshness according to claim 1, characterized in that: In the step (2), the water-soluble pH color response indicator includes one or more of roselle anthocyanidin, purple sweet potato anthocyanidin, mulberry anthocyanidin, perilla leaf anthocyanidin, blueberry anthocyanidin, purple cabbage anthocyanidin, black wolfberry anthocyanidin and betaine; preferably, the added amount of the water-soluble pH color response indicator is 0.02~0.24g / mL.

4. The method for preparing color-superimposed starch-based nanofibers capable of monitoring food freshness according to claim 1, characterized in that: In the step (2), a water-soluble pH color response indicator is added to the cooled octenyl succinate starch / polyvinyl alcohol mixed solution, and the mixture is stirred uniformly at 30-40° C. for 4-5 hours. The entire process is carried out in a light-proof and closed environment to obtain a water-soluble pH color response indicator / octenyl succinate starch / polyvinyl alcohol continuous phase.

5. The method for preparing color-superimposed starch-based nanofibers capable of monitoring food freshness according to claim 1, characterized in that: In the step (3), the water-insoluble pH color response indicator includes one or more of curcumin, carotenoids and chlorophyll; and the plant essential oil includes one or more of tea tree essential oil, ginger essential oil, clove essential oil, geranium essential oil, rose essential oil, thyme essential oil and lemon essential oil.

6. The method for preparing color-superimposed starch-based nanofibers capable of monitoring food freshness according to claim 1, characterized in that: In the step (3), the amount of the water-insoluble pH color response indicator added to the plant essential oil is 0.01-0.2 g / mL; preferably, the water-insoluble pH color response indicator is added to the plant essential oil and uniformly stirred at 30-50° C. for 30-120 min. The entire process is carried out in a dark and closed environment to obtain a water-insoluble pH color response indicator / plant essential oil dispersed phase.

7. The method for preparing color-superimposed starch-based nanofibers capable of monitoring food freshness according to claim 1, characterized in that: In the step (4), the volume ratio of the water-insoluble pH color response indicator / plant essential oil dispersed phase to the water-soluble pH color response indicator / octenyl succinate starch / polyvinyl alcohol continuous phase is 1:9 to 3:

7.

8. The method for preparing color-superimposed starch-based nanofibers capable of monitoring food freshness according to claim 1, characterized in that: In the step (4), the homogenization and emulsification method includes one or more of high-pressure homogenization, rotor-stator homogenization and ultrasonic homogenization; the homogenization rate is 8000~14000rpm; the conductivity of the water-in-oil emulsion used for emulsion electrospinning is 350~900μs / cm, the emulsion particle size is 80~300nm, and the Zeta potential range is ±20~±40mV.

9. The method for preparing color-superimposed starch-based nanofibers capable of monitoring food freshness according to claim 1, characterized in that: In the step (5), the parameters of the emulsion electrospinning process are as follows: the emulsion flow rate is 0.2-0.5 mL / h, the internal environmental humidity of the electrospinning machine is 30-35%, the internal environmental temperature of the electrospinning machine is 30-35°C, the distance between the electrospinning machine needle and the receiver is 10-15 cm, the electrospinning machine voltage is 16-20 kV, the speed of the electrospinning machine drum receiver is 85-200 rpm, and the working time is 6-8 h.

10. Application of a color-superimposed core-shell starch-based nanofiber capable of monitoring food freshness, prepared by the method according to any one of claims 1 to 9, in the field of intelligent active food packaging and drug controlled release.

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