A curcumin emulsion and its application in the preparation of gelatin active films

Abstract

This invention discloses a curcumin emulsion and its application in the preparation of gelatin-based active films. A protein-polysaccharide covalent complex is prepared via a hydrothermal reaction, and this complex is used as an emulsifier to construct a highly stable curcumin emulsion. This emulsion is then introduced into a gelatin system to prepare an active composite film. The emulsion exhibits high encapsulation efficiency and good thermal stability, ionic stability, and storage stability. The resulting composite film has a uniform structure and possesses good barrier properties and sustained-release characteristics. Application results show that this composite film can significantly reduce weight loss during fruit and vegetable storage, delay browning, and has antioxidant and antibacterial effects. This invention achieves efficient delivery and stabilization of curcumin while enhancing the functional properties of gelatin-based films, demonstrating promising application prospects.

Description

Technical Field

[0001] This invention belongs to the field of food science and functional materials technology, specifically relating to a β-lactoglobulin / β-glucan glycosylation complex-stabilized curcumin emulsion and its application in the preparation of gelatin active films. Background Technology

[0002] Curcumin, a lipophilic polyphenolic compound derived from the rhizome of turmeric, possesses various biological activities such as antioxidant, anti-inflammatory, and antibacterial properties, and is commonly used as a natural colorant and functional ingredient in food processing. However, curcumin suffers from poor water solubility, sensitivity to light and heat, and low bioavailability, severely limiting its practical application. To improve its stability and utilization efficiency, constructing an efficient delivery system has become a key technological approach. Currently, emulsions, as a typical hydrophobic active substance delivery system, are widely used due to their simple preparation process and good biocompatibility. Encapsulation and protection can be achieved by dissolving curcumin in the oil phase to form an oil-in-water emulsion. However, emulsions are thermodynamically unstable systems, prone to flocculation, aggregation, and phase separation during storage and processing, thus requiring high-performance emulsifiers to maintain their stability. Proteins, due to their amphiphilic structure, can adsorb at the oil-water interface to form an interfacial film and are widely used for emulsion stabilization; however, single proteins are prone to denaturation or desorption under high temperatures or extreme pH conditions, leading to insufficient emulsion stability. To improve the emulsifying properties of proteins, glycosylation modification has gradually become a research hotspot. Through the covalent bonding of proteins and polysaccharides, complex structures with steric hindrance effects can be formed at the interface, thereby enhancing the stability of the emulsion. However, existing research mostly focuses on monosaccharide or oligosaccharide systems, with relatively little research on glycosylation systems involving polysaccharides with larger molecular structures and greater bioactivity, such as β-glucan, and their application in the delivery of active substances still needs further development. On the other hand, gelatin, as a natural protein-based material, has good film-forming properties and biodegradability, and has broad application prospects in the food packaging field. However, pure gelatin films suffer from poor mechanical properties, insufficient water resistance, and limited functionality, especially lacking antioxidant and antibacterial activities, making it difficult to meet the needs of food preservation. Introducing active substances such as curcumin into the membrane system can endow it with functionality, but due to the strong hydrophobicity of curcumin, direct addition easily leads to aggregation and precipitation, affecting its dispersibility and stability.

[0003] Based on the emulsion encapsulation strategy, curcumin can be pre-encapsulated before being introduced into the gelatin system, thereby improving its dispersibility and achieving sustained release. However, current research on using protein-polysaccharide covalent complexes to stabilize emulsions and further construct active composite films is still relatively limited, and the structural regulation and functional synergistic mechanisms of related systems remain unclear. Therefore, it is necessary to develop an emulsion delivery system based on protein-polysaccharide covalent complexes and apply it to active composite films to improve the stability of curcumin and its application effect in food packaging. Summary of the Invention

[0004] The purpose of this invention is to provide a curcumin emulsion and its application in the preparation of gelatin active films.

[0005] A method for preparing curcumin emulsion, comprising the following steps: (1) Dissolve β-lactoglobulin and β-glucan in water at a ratio of 1:(0.5-6) ​​by mass to obtain a mixed solution with a protein concentration of 5-10 mg / mL, which is used as the aqueous phase; (2) The mixed solution described in step (1) is reacted at 50-90℃ for 0.5-4 h to carry out a wet heat glycosylation reaction. After the reaction is completed, it is rapidly cooled and freeze-dried to obtain a covalent complex. (3) Dissolve curcumin in the oil phase, use the covalent complex as an emulsifier, mix it with the aqueous phase at a volume ratio of 1:(2-4), and then homogenize and sonicate to obtain curcumin emulsion.

[0006] The mass ratio of β-lactoglobulin to β-glucan is 1:2.

[0007] The high-speed homogenization conditions are 8000-12000 r / min for 5-10 min, and the ultrasonic treatment time is 5-15 min.

[0008] The oil phase is edible oil, and the mass ratio of curcumin to edible oil is (1-5):100.

[0009] The edible oil is one or more of soybean oil, corn oil, peanut oil, rapeseed oil, and hemp seed oil.

[0010] The method for preparing the curcumin emulsion is characterized in that the amount of the covalent complex in step (3) is (1-4)% of the mass of the aqueous phase.

[0011] The application of the curcumin emulsion in the preparation of gelatin active film.

[0012] The application of the curcumin emulsion in the preparation of gelatin active films, and the method for preparing the gelatin active films are as follows: (1) Dissolve gelatin in water to prepare a gelatin solution with a mass concentration of 3-5%, and add 10-30% glycerin by mass of gelatin; (2) Add the curcumin emulsion of claim 1 to the gelatin solution, the amount of addition is 0.5-3%, and defoam after homogenization; (3) Pour the film-forming solution into the mold and dry it at 50-60℃ to obtain a gelatin active film.

[0013] The beneficial effects of this invention are as follows: Addressing the problems of poor water solubility, photothermal sensitivity, low bioavailability, and insufficient stability and easy aggregation / stratification in traditional emulsion systems, as well as the shortcomings of gelatin-based films such as weak mechanical properties, poor water resistance, and insufficient functional activity, this invention provides a β-lactoglobulin / β-glucan glycosylation complex-stabilized curcumin emulsion and its application in gelatin active films, achieving the goals of efficient curcumin delivery, improved emulsion stability, and functionalization of packaging materials. This invention achieves high stability and high encapsulation efficiency of the curcumin emulsion by constructing a β-lactoglobulin / β-glucan glycosylation complex, and further endows the gelatin active film with excellent mechanical properties, antioxidant properties, and controlled-release characteristics. The process of this invention is simple and the parameters are controllable, making it suitable for industrial applications in food functional packaging and active delivery systems. Attached Figure Description

[0014] Figure 1 The static state of covalently stable emulsions under different oil-water ratios and emulsifier concentrations over 7 days.

[0015] Figure 2 Curcumin standard curves (a) and encapsulation efficiency (b) of different interface emulsions; different letters on the bar chart indicate significant differences between groups (P<0.05).

[0016] Figure 3 Effect of different ion concentrations on the stability of three interfacial emulsions; (a) represents curcumin retention rate; (b) represents emulsification index; (c) represents the appearance of the emulsion after standing; different letters on the bar chart indicate significant differences between groups (P<0.05).

[0017] Figure 4 The effect of temperature on the stability of emulsions at different interfaces; (a) shows the curcumin retention rate of the three emulsions at 50℃, 70℃ and 90℃; (b) shows the emulsification index of the three emulsions; (c) shows the static appearance of the three emulsions after treatment at different temperatures; different letters in the bar chart indicate significant differences between the groups (P<0.05).

[0018] Figure 5 Storage stability of emulsions with different interfaces; retention of curcumin after 15 days of storage at 25℃ (a) and 4℃ (b).

[0019] Figure 6 The effects of emulsion coating treatment on fresh fruit; Figures a and b show the appearance and weight loss of citrus fruits during storage, respectively; Figures c, d, and e show the changes in appearance, weight loss, and color difference of sliced ​​apples, respectively. Different lowercase letters indicate significant differences between groups within the same storage period (P < 0.05).

[0020] Figure 7Scanning electron microscope images of the composite thin film; a represents the surface morphology of the film (3000×); b represents the cross-sectional morphology of the film (500×).

[0021] Figure 8 Appearance changes (a) and browning index (b) of sliced ​​apples with different film packaging during storage; different lowercase letters indicate significant differences between groups within the same storage period (P<0.05). Figure 9 Weight loss rate of sliced ​​apples packaged in different films during storage; different lowercase letters indicate significant differences between groups within the same storage period (P<0.05). Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Other implementation methods obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the protection scope of the present invention.

[0023] Unless otherwise specified, all instruments and equipment used in this invention can be obtained through conventional commercial channels; unless otherwise specified, all methods used are conventional methods in the art; unless otherwise specified, all raw materials used can be obtained from publicly available commercial sources. Process conditions not explicitly stated in the embodiments can be performed in accordance with conventional techniques in the art or product manuals. Example 1

[0024] A method for preparing a gelatin active film containing curcumin emulsion, comprising the following steps: (1) Preparation of glycosylated complex: Weigh β-lactoglobulin solution (2% by mass) and β-glucan (at a mass ratio of 1:2), adjust the pH of the mixed solution to 7.0, stir and dissolve thoroughly, and then place it in a water bath at 70℃ for 3 h to carry out Maillard reaction to obtain β-lactoglobulin / β-glucan glycosylated complex solution.

[0025] (2) Preparation of curcumin emulsion: Curcumin was dissolved in corn oil to form an oil phase (3% curcumin). The covalent complex obtained in step (1) was dissolved in the aqueous phase as an emulsifier and mixed at an oil-water volume ratio of 3:7. The covalent complex was added as an emulsifier at a ratio of 3%. The above glycosylated complex solution was used as the aqueous phase and pre-emulsified for 8 min under the conditions of a high-speed tissue disperser (12000 r / min). Then, a second homogenization was performed using an ultrasonic cell disruptor with an amplitude of 60%, a working / intermittent period of 5 s, and a total duration of 10 min to obtain a stable curcumin emulsion.

[0026] (3) Preparation of gelatin active membrane: 1.6 g of gelatin particles were added to 40 mL of ultrapure water and stirred at 60 °C until completely dissolved to obtain a 4% gelatin solution. 0.4 mL of glycerol was added as a plasticizer and stirred at room temperature for 20 min to obtain a film-forming solution. Freshly prepared curcumin emulsion was then added to the film-forming solution at a ratio of 1.25% (v / v). After initial mixing at room temperature for 5 min, the solution was homogenized at 12000 r / min for 3 min using a high-speed tissue disperser and sonicated for 5 min to remove air bubbles. Pure gelatin membrane without curcumin emulsion was used as a control. Finally, the film-forming solution was poured into a square petri dish (13 cm × 13 cm) and dried in an oven at 55 °C for 16 h to obtain a composite membrane. The dried composite membrane was placed in an environment with a temperature of 35 °C and a relative humidity of 75% for 24 h.

[0027] Example 2: Effects of oil-water volume ratio and emulsifier concentration on emulsion stability According to Specific Example 1, the remaining steps remain unchanged. Step (1) "Preparation of curcumin emulsion" is processed as follows: curcumin is dissolved in corn oil to form an oil phase. The covalent complex obtained in step (1) is dissolved in the aqueous phase as an emulsifier and mixed at an oil-water volume ratio of 1:(0.1-9). The covalent complex obtained in step one is added as an emulsifier at a ratio of 1-5%. The above glycosylated complex solution is used as the aqueous phase and pre-emulsified for 8 min under the conditions of a high-speed tissue disperser (12000 r / min). Then, a secondary homogenization is performed using an ultrasonic cell disruptor with an amplitude of 60%, a working / intermittent period of 5 s, and a total duration of 10 min to obtain a stable curcumin emulsion.

[0028] like Figure 1 As shown, when the oil-to-water ratio was 1:9 and 3:7, the emulsion remained stable within 7 days without stratification; however, when the oil-to-water ratio increased to 5:5 and 7:3, stratification began to appear from day 5. This is because a high oil volume fraction reduces the droplet spacing, increases the rate of collision and aggregation, and increases the compressive stress on the interfacial film, leading to demulsification and stratification. Furthermore, an excessively high oil-to-water ratio (9:1) results in a relative deficiency of emulsifier, failing to completely cover the oil-water interface and thus disrupting the emulsion's homogeneity. When the emulsifier concentration was 3%, 4%, and 5%, the emulsion system was stable; however, significant stratification occurred at concentrations of 1% and 2%. The emulsifier concentration directly affects the integrity and density of the interfacial film: insufficient concentration results in low droplet surface coverage, weak interfacial film strength, and easy droplet merging and stratification; a suitable concentration can form a stable interfacial barrier, effectively inhibiting Ostwald ripening and flocculation. Therefore, subsequent experiments selected an emulsifier concentration of 3% and an oil-to-water ratio of 3:7 to prepare a covalent interfacial emulsion.

[0029] Example 3: Determination of curcumin encapsulation efficiency in emulsions with different interfaces Curcumin standard curve construction and encapsulation efficiency determination of different emulsions: First, 50 mg of curcumin was weighed and dissolved in anhydrous ethanol. The maximum absorption wavelength of curcumin was determined to be 425 nm using a UV spectrophotometer in the range of 200-800 nm. Then, curcumin-ethanol solutions with concentrations of 0.2, 0.5, 1, 2, 5, 10, and 25 μg / mL were prepared, using anhydrous ethanol as a blank control. The absorbance values ​​were measured at the maximum absorption wavelength. Each concentration was measured in triplicate, and the average value was taken to obtain the relationship between absorbance and curcumin concentration, and a curcumin standard curve was plotted.

[0030] To determine the curcumin content in the emulsion, 1 mL of fresh emulsion was added to 9 mL of anhydrous ethanol, and the mixture was shaken at 750 rpm for 40 min to completely break the emulsion. The emulsion was then centrifuged at 8000 rpm for 10 min. The supernatant was filtered through a 0.22 μm filter membrane, and the absorbance at 425 nm was measured. The total curcumin content was calculated based on the standard curve. Separately, 1 mL of the emulsion was centrifuged at 8000 rpm for 20 min, and the precipitate was collected. The precipitate was reconstituted with anhydrous ethanol and centrifuged again. The resulting supernatant was filtered through a filter membrane, and the absorbance was measured at the same wavelength to determine the free curcumin content. The encapsulation efficiency (EE) was calculated using the following formula: EE(%) = ((Total curcumin content - Free curcumin content) / Total curcumin content) × 100 Figure 2 (a) shows that curcumin solution exhibits good linearity across different concentration ranges at 425 nm. Its standard curve equation is: "y = 0.1436x + 0.0251", R0 2 =0.9991. Figure 2 (b) The encapsulation capacity of different interfacial emulsions for hydrophobic curcumin was evaluated. It was observed that when curcumin emulsions were prepared using β-lactoglobulin, a mixture of β-lactoglobulin / β-glucan, and covalent complexes as emulsifiers, the covalent group achieved the highest encapsulation efficiency, reaching 91.57%. The single β-lactoglobulin emulsion exhibited the lowest encapsulation efficiency, at 65.01%. This is because the interfacial layer formed by β-lactoglobulin alone at the oil-water interface is relatively weak, resulting in low encapsulation efficiency. When β-lactoglobulin and β-glucan are used in combination, the polysaccharide promotes protein adsorption at the interface through non-covalent interactions, jointly enhancing emulsion stability and interfacial layer integrity, thereby improving the encapsulation capacity of curcumin. The covalent complex formed through glycosylation can construct a denser and more stable interfacial layer at the oil-water interface. This compact structure effectively prevents the uncontrolled release of curcumin, thus exhibiting the highest encapsulation efficiency. These results indicate that constructing an emulsion system using the interaction between β-lactoglobulin and β-glucan can significantly improve the encapsulation efficiency of hydrophobic functional factors on proteins, with covalent binding showing the most significant effect.

[0031] Example 4: Physical stability analysis of emulsions with different interfaces (1) Ionic stability: To investigate the stability of the three emulsions under environmental stress, the effect of different ionic strengths on the three emulsions was studied. Freshly prepared emulsions were taken and mixed with equal volumes of sodium chloride solutions of different concentrations to achieve final NaCl concentrations of 50, 100, 150, 200, and 250 mmol / L, respectively. After thorough mixing, the samples were left to stand at room temperature for 24 hours, and the appearance and curcumin content of each emulsion were observed and recorded.

[0032] like Figure 3 As shown in (a), with increasing ion concentration, the retention rate of curcumin in all three emulsions decreased after standing for 1 day. The covalent group interfacial emulsion showed the smallest decrease in retention rate, while the β-lactoglobulin group showed the largest decrease. This is because the addition of salt ions shields the surface charge carried by the protein molecules adsorbed at the interface by the β-lactoglobulin group, weakening the electrostatic repulsion between droplets and enhancing van der Waals attraction, thereby inducing droplet aggregation and triggering phase separation. In physically mixed systems, the presence of salt ions disrupts the weak interaction between β-lactoglobulin and β-glucan, causing polysaccharide molecules to be adsorbed onto the surface of multiple droplets simultaneously, triggering bridging flocculation and ultimately leading to emulsion instability. When β-glucan chains are firmly bound to β-lactoglobulin molecules through covalent bonds, a thicker and denser adsorption layer is formed at the oil-water interface. This interfacial layer provides a stronger steric hindrance effect, effectively preventing droplet approach and aggregation even in high-salt environments, thus maintaining the stability of the system. Figure 3 (bc) shows the changes in the appearance and emulsification index of the emulsions after standing for 1 day following treatment with different ion concentrations. The β-lactoglobulin and mixed group emulsions showed significant phase separation with increasing salt ion concentration, while the covalent group emulsion maintained good homogeneity throughout the entire ion concentration range, with no obvious stratification or flocculation observed. This phenomenon further indicates that covalent interfacial emulsions can effectively resist the adverse effects of salt ions, effectively delay emulsion flocculation, and ensure the long-term physical stability of the system.

[0033] (2) Thermal stability: To investigate the effect of different interfacial structures on the thermal stability of emulsions, the emulsification index, appearance changes, and curcumin retention of the three emulsions after heat treatment at 50°C, 70°C, and 90°C were measured. The three freshly prepared emulsions were placed in water baths at 50°C, 70°C, and 90°C and heated for 30 minutes, respectively, and then cooled to room temperature. The appearance changes, phase separation, and curcumin retention of each group were observed and recorded.

[0034] Depend on Figure 4(a) It can be seen that the retention rate of curcumin in all three emulsions decreased with increasing processing temperature, but the retention rate of the covalent group emulsion was significantly higher than that of the β-lactoglobulin and mixed group emulsions under various temperature conditions. This phenomenon is because high temperature induces irreversible denaturation of β-lactoglobulin, causing its molecular structure to partially unfold and expose hydrophobic groups, thereby causing protein flocculation and aggregation, weakening its ability to stabilize the emulsion. In contrast, the β-glucan added to the mixed group and covalent group emulsions can interact with β-lactoglobulin and effectively inhibit the thermal aggregation behavior of proteins. Especially in the covalent complex system, the polysaccharide chains covalently bound by glycosylation significantly enhance the steric hindrance between protein molecules, forming a denser adsorption layer at the oil-water interface, thereby effectively inhibiting droplet aggregation under high temperature conditions and giving the emulsion good thermal stability. Figure 4 (bc) shows the changes in the appearance and emulsification index of the emulsions after standing for 3 days following treatment at different temperatures. It can be observed that all three emulsions exhibited varying degrees of phase separation, with the β-lactoglobulin group showing the most severe stratification, while the covalent group showed the least. This result further indicates that heat treatment has a more significant destructive effect on emulsions stabilized by a single protein, while the introduction of polysaccharides, especially through covalent bonding, can significantly enhance the heat resistance of β-lactoglobulin, effectively delay emulsion flocculation, and ensure the long-term physical stability of the system.

[0035] (3) Storage stability: The three freshly prepared emulsions were stored in the dark at 4℃ and 25℃ for 15 days, and samples were taken at regular intervals on days 0, 3, 6, 9, 12 and 15 of the storage period to determine the remaining content of curcumin in the samples, so as to evaluate the effect of different interface emulsions on the storage stability of curcumin.

[0036] To investigate the effects of three interfacial emulsions on the curcumin retention capacity, the curcumin retention rate of the three emulsions was measured after 15 days of storage in the dark at 4℃ and 25℃. The results are as follows: Figure 5As shown in (ab), under storage conditions of 4℃, the retention rates of curcumin in the single β-lactoglobulin, physical mixture, and covalent complex stabilized emulsions were 78.16%, 81.24%, and 86.22%, respectively. The covalent group emulsion exhibited the highest retention effect, mainly due to the more dense spatial barrier constructed by the glycosylated covalent complex at the oil-water interface. This not only increased the thickness and steric hindrance of the interface layer but also effectively blocked the contact between curcumin and degradation-promoting factors such as ultraviolet light, oxygen, and free radicals. In addition, the particulate network structure formed by the covalent complex has higher mechanical strength and structural continuity, which can further inhibit the migration of curcumin to the aqueous phase through the physical barrier mechanism, thereby delaying its degradation process. When the storage temperature was increased to 25℃, the degradation of curcumin in all three emulsions was accelerated, but the retention rate of the covalent group emulsion was still significantly better than the other two groups. This result indicates that even under accelerated degradation conditions, the covalent interface structure can still provide continuous and effective protection. In summary, the emulsion delivery system based on protein-polysaccharide covalent bonds can significantly improve the storage stability of curcumin and effectively extend its shelf life of bioactivity, showing good application potential in the field of functional food delivery.

[0037] Example 5: Determination of the Preservation Performance of Covalent Interfacial Emulsions Purchase ripe commercial apples and citrus fruits from a local supermarket and wash them with distilled water. Peel and core the apples using a pre-sterilized knife and cut them into uniform slices. Randomly divide the citrus fruits and apple slices into three groups and soak them for 2 minutes: CK group (distilled water), CE group (coating solution without curcumin), and CCE group (coating solution with curcumin). After the surface coating solution dries, store the samples at room temperature.

[0038] Color variation: The color parameters L (brightness), a (red-green), and b (yellow-blue) of the sliced ​​apples were measured using a colorimeter. Five different locations were randomly selected from each sample for color measurement, and the results were averaged. Color difference (ΔE) was used as the evaluation index. L*, a*, and b* represent the color characteristics of the background panel.

[0039]

[0040] Weight loss rate: The initial mass of the fruit was recorded as m0 on the first day of the experiment. After a certain storage period, the mass of the fruit was recorded as m1 on the testing day. The formula for calculating the weight loss rate is as follows:

[0041] like Figure 6As shown in (a), the morphological changes of citrus fruits under different treatments during 12 days of storage are illustrated. The CK group showed peel shrinkage during storage, indicating significant moisture loss. The coated samples maintained good morphology, while the CCE group showed significantly better preservation than the CE group. This is because the coating inhibited respiration and transpiration during storage, thus reducing moisture loss. The addition of curcumin further enhanced the water-retention properties of the coating. Figure 6 (b) shows the weight loss rates of the three treated sweet orange groups. Weight loss gradually increased with prolonged storage time; after 12 days of storage at room temperature, the weight loss rates of the CK, CE, and CCE groups were 24.84%, 22.86%, and 16.97%, respectively. The weight loss rates of the CE and CCE groups were significantly lower than those of the CK group, which is consistent with their appearance characteristics.

[0042] Figure 6 (ce) shows the effect of the emulsion coating on the preservation of sliced ​​apples. Browning and water loss occurred in all groups. However, the coated groups, especially the CCE group, showed significantly less browning and water loss than the CK group. This is because the barrier layer formed by the emulsion on the apple surface reduced surface evaporation and maintained the internal moisture balance. The addition of curcumin further protected the phenolic compounds in the fruit from oxidation, thereby reducing the formation of browning polymers and water loss.

[0043] Example 6: Determination of the Preservation Performance of Composite Film 1.6 g of gelatin particles were added to 40 mL of ultrapure water and stirred at 60 °C until completely dissolved to obtain a 4% gelatin solution. 0.4 mL of glycerol was added as a plasticizer, and the solution was stirred at room temperature for 20 min to obtain a film-forming solution. Freshly prepared curcumin emulsion (prepared in Example 1, denoted as CE) was then added to the film-forming solution at proportions of 0.625%, 1.25%, 1.875%, and 2.5% (v / v), respectively. After initial mixing at room temperature for 5 min, the mixture was homogenized at 12000 r / min for 3 min using a high-speed tissue disperser and sonicated for 5 min to remove air bubbles. A pure gelatin membrane without curcumin emulsion was used as a control. Finally, the film-forming solution was poured into square petri dishes (13 cm × 13 cm) and dried in an oven at 55 °C for 16 h to obtain a composite membrane. The dried composite membrane was then equilibrated for 24 h at temperatures of 35 °C and relative humidity of 75%. The pure gelatin film and the composite film with different emulsion contents were named GEL, G-CE-0.625, G-CE-1.25, G-CE-1.875, and G-CE-2.5, respectively.

[0044] (1) Color analysis and optical property determination of composite film: (a) The color parameters L (brightness), a (red-green), and b (yellow-blue) of the film were measured using a colorimeter. Five different locations were randomly selected from each film sample to measure the color, and the average value was taken. Color difference (ΔE) was used as the evaluation index. L0, a0, and b0 are the color characteristics of the background plate.

[0045]

[0046] (b) Cut the film into strips of 4×1 cm and fix them to one side of the inner wall of the cuvette. Using a blank cuvette as a control, measure the transmittance and absorbance of the film in the wavelength range of 200-800 nm using a UV-Vis spectrophotometer.

[0047] (2) Characterization of composite membrane structure: The microstructure of the thin film was analyzed using scanning electron microscopy. Double-sided conductive tape was used to cut the thin film sample into strips of 10 mm × 5 mm, which were then fixed on the sample stage and sputtered with gold. The accelerating voltage was set to 3 kV, and the microstructure of the surface (×3000) and cross-section (×500) of the thin film was observed. The thin film sample was then immersed in liquid nitrogen for cryogenic treatment to achieve natural fracture under low-temperature conditions, thus obtaining the cross-section of the thin film.

[0048] (3) Fruit preservation characteristics: Ripe commercial apples were purchased from local supermarkets, washed with distilled water, peeled and cored using pre-sterilized knives, and cut into uniform apple slices. The apple slices were randomly grouped and placed in sterile plastic petri dishes, sealed with pure gelatin film and composite films with different curcumin contents. An unpackaged group served as a blank control. All samples were stored at room temperature (25±1)°C, and indicators were measured regularly: the appearance changes of the samples were recorded using a digital camera to visually assess sensory quality; the mass was weighed regularly using a precision electronic balance, and the weight loss rate was calculated to quantify the degree of moisture loss; the surface color parameters (L, a, b) of the samples were measured using a colorimeter, and the browning index was calculated accordingly to evaluate the inhibitory effect of the film on browning of the sliced ​​apples. The above indicators comprehensively reflect the preservation performance of different composite films on sliced ​​apples.

[0049] (a) Browning Index: The browning index (BI) quantifies the degree of browning of sliced ​​apples during storage. After calibration with a standard white plate, the color values ​​(L, a, and b) of each sample were measured using a colorimeter. The BI measurements for sliced ​​apples are as follows:

[0050]

[0051] (b) Weight loss: The initial mass of the fruit was recorded as m0 on the first day of the experiment. After a certain storage period, the mass of the fruit was recorded as m1 on the testing day. The formula for calculating the weight loss rate is as follows:

[0052] The effects of adding curcumin emulsion at different volume fractions on the appearance, color parameters, and thickness of the films are shown in Table 1. All films exhibited a uniform and smooth appearance. Regarding color, gelatin had the highest brightness (L) value. With increasing curcumin volume fraction, the visual chromaticity of the films changed significantly, gradually transitioning from light yellow to dark yellowish-brown. Corresponding characterization showed that the L value of the films gradually decreased, while the a (red / green) and b (yellow / blue) values ​​significantly increased. This is because the yellowish-brown color of the curcumin emulsion itself, when introduced into the film-forming matrix, reduced the overall brightness of the film and increased the reddish-yellow hue, thus significantly increasing ΔE. This is consistent with the change in the film's appearance color. The macroscopic changes in appearance also indicate that the curcumin emulsion was well dispersed in the gelatin matrix, and the two exhibit good biocompatibility. Regarding film thickness, the average thickness of the gelatin film was (0.118 ± 0.019) mm. As the liquid fraction of curcumin emulsion increases, the thickness of the film increases, eventually reaching (0.197±0.015) mm, which is attributed to the increase in the total solid content in the film-forming system.

[0053] Table 1

[0054] Note: Different letters in the same row indicate significant differences between composite membranes (P<0.05).

[0055] The microstructure of the surface and cross-section of gelatin-based emulsion films can be observed using SEM. The microstructures of composite films containing different CEs are shown below. Figure 7 As shown in the figure, the GEL membrane surface generally exhibits a smooth, uniform, dense, and non-porous characteristic. The addition of CE did not change the morphology of the membrane; however, with increasing addition, some raised particles appeared on the membrane surface, and micropores appeared in the cross-section, indicating that the emulsion had a certain influence on the membrane. Furthermore, the network structure formed by the cross-sectional pores of the composite membrane is beneficial to improving the membrane's barrier properties against water vapor and oxygen, and is conducive to the membrane's toughness and the sustained-release effect of curcumin. SEM image results show that the emulsion has been successfully incorporated into the gelatin-based membrane, and the emulsion droplets are sufficiently and uniformly dispersed in the membrane.

[0056] The effects of different G-CE combined treatments on the browning process of sliced ​​apples during storage (0-3 days) were evaluated by visual observation and browning index (BI) measurement. Figure 8As shown in (a), there was no significant difference in the color of the cut surfaces of apples in each group after 0 days of storage; after 1 day, the cut surfaces of the control group began to show obvious yellow-brown changes, while the treated groups still maintained good color; after 3 days of storage, the cut surfaces of the control group showed severe browning, followed by the GEL-treated group, while the G-CE composite treatment groups (especially G-CE-1.25, G-CE-1.875, and G-CE-2.5) had the lightest cut surface color, still maintaining a good bright white or pale yellow appearance. BI ( Figure 8 (b) The measurement results were highly consistent with the changes in appearance: the BI value of the control group increased significantly after 1 day of storage, and increased sharply after 3 days, while the BI value of the G-CE composite treatment group remained at the lowest level, demonstrating excellent anti-browning ability. This is mainly attributed to the physical barrier formed by the film maintaining the internal moisture balance of the fruit, while the addition of CE enhanced the ability to scavenge reactive oxygen species and effectively protected phenolic compounds from oxidation, thus synergistically delaying the browning process.

[0057] Figure 9 This study demonstrates the effect of different CE composite film packaging on the weight loss rate of sliced ​​apples during storage. It was found that the weight loss rate of all samples increased continuously with prolonged storage time. On day 1 of storage, there was no significant difference in weight loss rate among the groups, all remaining between 25-26%. By day 3, the weight loss rate of the control group was significantly higher than that of the other groups. This was mainly because the unprotected sliced ​​apples exhibited high respiration activity, and the combined effects of dry matter consumption and transpiration exacerbated moisture loss. The weight loss rate of the pure GEL group was slightly lower than that of the control group, while all CE composite film groups were able to delay weight loss to varying degrees, with the G-CE-1.25 group showing the most significant effect. This is partly due to the hydrogen bonding between the emulsion and the gelatin matrix, which enhanced the film's water vapor barrier capacity, effectively preventing the escape of internal moisture from the apple; and partly due to the introduction of an appropriate amount of CE, which helped maintain the continuity and density of the film structure. However, when the amount of CE added is too high, such as in the G-CE-2.5 group, the emulsion droplets may aggregate and flocculate, disrupting the uniform network structure of the polymer and even creating micropores or interface defects, which in turn accelerates moisture loss. In summary, composite films with an appropriate amount of CE can effectively delay the moisture loss of sliced ​​apples.

[0058] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing a curcumin emulsion, characterized in that, Follow these steps: (1) Dissolve β-lactoglobulin and β-glucan in water at a ratio of 1:(0.5-6) ​​by mass to obtain a mixed solution with a protein concentration of 5-10 mg / mL, which is used as the aqueous phase; (2) The mixed solution described in step (1) is reacted at 50-90℃ for 0.5-4 h to carry out a wet heat glycosylation reaction. After the reaction is completed, it is rapidly cooled and freeze-dried to obtain a covalent complex. (3) Dissolve curcumin in the oil phase, use the covalent complex as an emulsifier, mix it with the aqueous phase at a volume ratio of 1:(2-4), and then homogenize and sonicate to obtain curcumin emulsion.

2. The method for preparing curcumin emulsion according to claim 1, characterized in that, The mass ratio of β-lactoglobulin to β-glucan is 1:

2.

3. The method for preparing curcumin emulsion according to claim 1, characterized in that, The high-speed homogenization conditions are 8000-12000 r / min for 5-10 min, and the ultrasonic treatment time is 5-15 min.

4. The method for preparing curcumin emulsion according to claim 1, characterized in that, The oil phase is edible oil, and the mass ratio of curcumin to edible oil is (1-5):

100.

5. The method for preparing curcumin emulsion according to claim 4, characterized in that, The edible oil is one or more of soybean oil, corn oil, peanut oil, rapeseed oil, and hemp seed oil.

6. The method for preparing curcumin emulsion according to claim 4, characterized in that, The amount of the covalent complex used in step (3) is (1-4) of the mass of the aqueous phase.

7. The use of the curcumin emulsion according to claim 1 in the preparation of gelatin active film.

8. The application of the curcumin emulsion according to claim 7 in the preparation of gelatin active films, characterized in that, The method for preparing the gelatin active film is as follows: (1) Dissolve gelatin in water to prepare a gelatin solution with a mass concentration of 3-5%, and add 10-30% glycerin by mass of gelatin; (2) Add the curcumin emulsion of claim 1 to the gelatin solution, the amount of addition is 0.5-3%, and defoam after homogenization; (3) Pour the film-forming solution into the mold and dry it at 50-60℃ to obtain a gelatin active film.

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