Starch-based composite film capable of releasing citrus essential oil in humidity response as well as preparation method and application of starch-based composite film

Abstract

The invention discloses a starch-based composite film capable of releasing citrus essential oil in response to humidity and a preparation method and application thereof.The method comprises the steps that a pectin-nano lignin compound is prepared to serve as a dispersion phase, the pectin-nano lignin compound and citrus essential oil are blended and then homogenized, and pectin-nano lignin stabilized citrus essential oil Pickering emulsion is obtained; and mixing the obtained emulsion with the gelatinized starch solution, homogenizing to obtain composite film-forming slurry, casting the composite film-forming slurry onto a mold, and airing to obtain the composite film. The starch-based composite film provided by the invention has excellent mechanical properties, water vapor and oxygen barrier properties, oxidation resistance, ultraviolet resistance and antibacterial properties, can maintain long-term slow release of essential oil in a normal environment, can respond in a humidity rising environment to accelerate release of the essential oil, can improve the quality of fruits in a storage period, and has good market prospects. The shelf life is obviously prolonged, and the fresh-keeping film can be used as a safe packaging material to be applied to fresh keeping of fruits.

Description

Technical Field

[0001] The invention belongs to the technical field of food packaging, and in particular relates to a starch-based composite film capable of releasing citrus essential oil in response to humidity, and a preparation method and application thereof. Background Art

[0002] Starch is a natural polymer derived from plants. They are biodegradable, transparent, and processable, and are considered to be one of the most promising alternatives to petrochemical plastics. However, starch-based packaging films have many defects, such as strong hydrophilicity, poor mechanical properties, and no antibacterial properties, which seriously hinder their practical application in the field of food preservation. At present, these defects are usually solved by physical mixing, chemical modification / cross-linking, etc., and have achieved certain success. Unfortunately, these methods still have some problems that need to be solved. Taking antibacterial properties as an example, some bioactive substances (such as essential oils) can be doped into the film matrix by simple physical mixing modification to obtain the target antibacterial properties. However, due to the elusive compatibility between materials, this physical mixing can easily lead to phase separation and even collapse of the film structure. In addition, essential oils are unstable and often lose during the preparation and storage of the film, which greatly reduces the antibacterial properties of the film. Chemical methods can avoid compatibility issues, but there is a potential risk of introducing biological toxicity, which is not allowed in the field of food packaging.

[0003] Pickering emulsion refers to an emulsion stabilized by solid particles as surfactants, in which the solid particles are dispersed between the droplets to form a physical barrier, thereby preventing the droplets from aggregating. Embedding essential oils as the continuous phase in Pickering emulsions can solve the above problems. However, there are several issues that need special attention when applied to film materials. The first problem is the release kinetics of essential oils. The instability and over-encapsulation of Pickering emulsions will lead to poor release kinetics, which will affect the actual antibacterial effect and long-term bioactive stability of the film. In recent years, the development of environmentally responsive release materials such as humidity, temperature and light has made it possible to regulate the release of active agents, thereby extending the shelf life of active agents. This requires the selection of a reliable and environmentally sensitive material as the dispersed phase of the Pickering emulsion. Fruits lose water due to natural transpiration and respiration during storage, which may cause a sudden increase in humidity inside the food packaging. However, fruits are more likely to breed microorganisms under such high humidity conditions, so it is meaningful to develop a packaging material that can release active antimicrobial ingredients according to humidity changes. In addition, due to direct intermixing with the starch matrix, it is also important to consider the compatibility between the dispersed phase of the Pickering emulsion and the starch, as poor compatibility can have an adverse effect on the film. Therefore, the selection of the dispersed phase of the Pickering emulsion is crucial not only to ensure the stability and compatibility of the emulsion, but also to ensure the environmental response properties. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a starch-based composite film that can release citrus essential oil with good physical properties and dual-mode release, and its preparation method and application.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions.

[0006] A preparation method of a starch-based composite film that can release citrus essential oil in response to humidity. Preferably, it includes the following steps:

[0007] (1) Dissolve lignin in acetone to obtain an acetone solution of lignin, pour the acetone solution of lignin into water, and after homogenization and concentration treatment, obtain a nano-lignin suspension;

[0008] (2) Mix the nano-lignin suspension obtained in step (1) with a pectin solution, and stir to form a pectin-nano-lignin complex;

[0009] (3) Mix the pectin-nano-lignin complex obtained in step (2) with citrus essential oil, and homogenize to obtain a pectin-nano-lignin-stabilized citrus essential oil Pickering emulsion;

[0010] (4) Add starch to water for gelatinization treatment to obtain a starch solution;

[0011] (5) Mix the citrus essential oil Pickering emulsion obtained in step (3) with the starch solution obtained in step (4), add glycerol, and homogenize to obtain a composite film-forming slurry;

[0012] (6) Pour the composite film-forming slurry obtained in step (5) into a mold and air-dry to obtain a starch-based composite film that can release citrus essential oil in response to humidity.

[0013] In the above preparation method of a starch-based composite film that can release citrus essential oil in response to humidity, preferably, in step (1), the mass-volume ratio of lignin to acetone is 1 g∶10 mL - 25 mL, the volume ratio of the acetone solution of lignin to water is 1∶5 - 10, and the mass-volume ratio of lignin to the suspension in the nano-lignin suspension is 0.5 g - 2.0 g∶100 mL.

[0014] In the above preparation method of a starch-based composite film that can release citrus essential oil in response to humidity, preferably, in step (2), the mass-volume ratio of pectin to the complex in the pectin-nano-lignin complex is 1 g - 2 g∶100 mL, and the mass-volume ratio of nano-lignin to the complex in the obtained pectin-nano-lignin complex is 0.25 g - 1.0 g∶100 mL.

[0015] For the preparation method of the above-mentioned starch-based composite film that responds to humidity and releases citrus essential oil, preferably, in step (3), the volume ratio of the pectin-nano lignin complex to the citrus essential oil is 5-9:5-1.

[0016] For the preparation method of the above-mentioned starch-based composite film that responds to humidity and releases citrus essential oil, preferably, in step (4), the mass-volume ratio of the starch to water is 4g-6g:100mL.

[0017] For the preparation method of the above-mentioned starch-based composite film that responds to humidity and releases citrus essential oil, preferably, in step (5), the volume ratio of the citrus essential oil Pickering emulsion to the starch solution is 1%-10%, and the mass ratio of the glycerol to the starch is 30%-40%.

[0018] For the preparation method of the above-mentioned starch-based composite film that responds to humidity and releases citrus essential oil, preferably, in step (1), the rotation speed of the homogenization is 8000rpm-12000rpm, and the time of the homogenization is 5min-15min; the concentration is carried out by a rotary evaporator.

[0019] For the preparation method of the above-mentioned starch-based composite film that responds to humidity and releases citrus essential oil, preferably, in step (2), the rotation speed of the stirring is 150rpm-300rpm, and the time of the stirring is 6h-10h.

[0020] For the preparation method of the above-mentioned starch-based composite film that responds to humidity and releases citrus essential oil, preferably, in step (3), the rotation speed of the homogenization is 10000rpm-12000rpm, and the time of the homogenization is 2min-5min.

[0021] For the preparation method of the above-mentioned starch-based composite film that responds to humidity and releases citrus essential oil, preferably, in step (4), the temperature of the gelatinization treatment is 85°C-95°C, the rotation speed of the gelatinization treatment is 150rpm-300rpm, and the time of the gelatinization treatment is 1h-2h.

[0022] For the preparation method of the above-mentioned starch-based composite film that responds to humidity and releases citrus essential oil, preferably, in step (5), the rotation speed of the homogenization is 3500rpm-5500rpm, and the time of the homogenization is 2min-5min.

[0023] As a general technical concept, the present invention also provides a starch-based composite film that responds to humidity and releases citrus essential oil prepared by the above-mentioned preparation method.

[0024] As a general technical concept, the present invention also provides an application of the above-mentioned starch-based composite film that responds to humidity and releases citrus essential oil in the field of food preservation.

[0025] For the above application, preferably, the starch-based composite film that releases citrus essential oil in response to humidity is used for fruit packaging and preservation to extend the shelf life of perishable fruits such as strawberries.

[0026] The main innovation of the present invention lies in:

[0027] Lignin has many good properties, including biodegradability, renewability, biosafety, natural antioxidant property, and ultraviolet resistance. Lignin can be nano-sized to be used as the dispersed phase of Pickering emulsion. However, the compatibility between lignin and the biopolymer matrix is often poor, and the stability and environmental responsiveness of Pickering emulsion stabilized only by lignin need to be improved. Pectin has good compatibility with both starch and lignin. In the present invention, pectin is used as part of the dispersed phase of nano-lignin Pickering emulsion, which can serve as a bridge connecting the emulsion and the starch matrix to ensure the compatibility between them. In addition, pectin can also act as a thickening agent and an emulsifying agent to further improve the stability of the emulsion, which is beneficial to maintaining biological activity in the long term. More importantly, as a hydrophilic colloid, pectin is very sensitive to changes in environmental humidity. Under the condition of humidity change, the pectin molecular chain will break and rearrange, showing humidity-responsive characteristics. Therefore, in the present invention, pectin-nano-lignin is used as the dispersed phase in Pickering emulsion, which shows good compatibility, stability, and humidity responsiveness, laying a foundation for the preparation of starch-based films with a balance of antibacterial and physical properties to achieve dual-mode essential oil release.

[0028] Compared with the prior art, the advantages of the present invention are:

[0029] (1) The present invention provides a method for preparing a starch-based composite film that releases citrus essential oil in response to humidity. First, lignin is dissolved in acetone and then poured into water, homogenized, and concentrated to obtain a nano-lignin suspension, i.e., lignin is transformed into nano-lignin by a high-speed homogenization-assisted anti-solvent precipitation method; it is mixed and incubated with a pectin solution to obtain a pectin-nano-lignin complex. On the one hand, pectin, as an emulsifier and thickener, enhances the long-term stability of the nano-lignin Pickering emulsion and acts as a bridge to connect nano-lignin and the starch matrix to ensure good compatibility between materials; on the other hand, pectin acts as a humidity-responsive switch, enabling the Pickering emulsion in the film to have the property of releasing essential oil in response to humidity. Subsequently, the pectin-nano-lignin complex and citrus essential oil are homogenously mixed to obtain a pectin-nano-lignin-stabilized citrus essential oil Pickering emulsion; the Pickering emulsion is mixed with a gelatinized starch solution, homogenized, cast, and dried to obtain the starch-based composite film of the present invention. The raw materials used in the preparation method of the present invention all come from the organic processing waste of agricultural products. For example, starch is derived from potatoes or cassava, pectin and citrus essential oil come from citrus peels, and lignin comes from sugarcane bagasse or wood chips. They are not only safe and biodegradable but also promote the comprehensive utilization and development of agricultural product processing waste resources and enhance their application value.

[0030] (2) The starch-based composite film of the present invention that releases citrus essential oil in response to humidity has good tensile strength, elongation at break, hydrophobicity, airtightness, and the essential oil in the composite film has good slow-release and humidity-responsive release properties. It also has good ultraviolet resistance, antioxidant properties, antibacterial properties, biosecurity, and biodegradability.

[0031] (3) The application of the starch-based composite film of the present invention that releases citrus essential oil in response to humidity in food preservation. Since the composite film of the present invention has good mechanical properties and barrier properties and also exhibits excellent ultraviolet resistance, antioxidant properties, and antibacterial properties, the film can release essential oil for a long time, and can respond to accelerate the release of essential oil in an environment with higher humidity. It also has biotoxicity and biodegradability. When used as a fresh-keeping film to package fruits (such as strawberries), it can significantly improve the quality of fruits during storage and extend their shelf life, and is very suitable for application as a substitute for petrochemical plastics in the field of food preservation. Description of the Drawings

[0032] Figure 1 It is a process flow chart of the preparation of the starch-based composite film that releases citrus essential oil in response to humidity in Example 1 of the present invention.

[0033] Figure 2 It is a comparison chart of the stability of nano-lignin Pickering emulsion (NLPE) and pectin-nano-lignin Pickering emulsion (PLCPE).

[0034] Figure 3 Mechanical strength (a) and elongation at break (b) of starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention.

[0035] Figure 4 Water contact angle of starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention.

[0036] Figure 5 Water vapor permeability (a) and oxygen permeability (b) of starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention.

[0037] Figure 6 Sustained release of essential oil (a) in starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention and humidity-responsive release (b) in starch-based composite films with humidity-responsive release of citrus essential oil in Example 1.

[0038] Figure 7 Anti-ultraviolet properties of starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention.

[0039] Figure 8 Antioxidant properties of starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention.

[0040] Figure 9 Antioxidant properties of starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention.

[0041] Figure 10 Antibacterial properties of starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention.

[0042] Figure 11 Evaluation of the fresh-keeping effect of starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention.

[0043] Figure 12 Biosafety of starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention.

[0044] Figure 13 Biodegradability of starch-based composite films with humidity-responsive release of citrus essential oil prepared with different ratios of PLCPE in Example 2 of the present invention. Detailed implementation mode

[0045] The present invention will be further described below in conjunction with the accompanying drawings of the specification and specific preferred embodiments, but the protection scope of the present invention is not limited thereby. The materials and instruments used in the following embodiments are all commercially available.

[0046] Example 1:

[0047] A preparation method of a starch-based composite film for humidity-responsive release of citrus essential oil of the present invention, as Figure 1 shown, includes the following steps:

[0048] (1) Dissolve alkali lignin in acetone sufficiently to obtain an acetone solution of lignin. The mass-volume ratio of alkali lignin to acetone is 1 g∶20 mL; pour the acetone solution of lignin into gently stirred water, perform high-speed homogenization treatment, and then remove the excess acetone and water through a rotary evaporator to obtain a nano-lignin suspension.

[0049] In this embodiment, water is used as the anti-solvent. Pour the acetone solution dissolved with lignin into water with the opposite polarity to precipitate lignin and form nanoparticles. Among them, the volume ratio of the acetone solution of lignin to water is 1∶10, the rotation speed of homogenization is 10000 rpm, the time of homogenization is 10 min, and the mass-volume ratio of lignin to the suspension in the nano-lignin suspension is 1 g∶100 mL.

[0050] (2) Mix the nano-lignin suspension obtained in step (1) with a 2% (mass-volume ratio, g / mL) aqueous solution of citrus peel pectin. The volume ratio of the nano-lignin suspension to the aqueous pectin solution is 1∶1, and stir at 150 rpm for 6 h at room temperature to form a pectin-nano-lignin complex. Among them, the mass-volume ratio of pectin to the complex in the pectin-nano-lignin complex is 1 g∶100 mL, and the mass-volume ratio of nano-lignin to the complex is 0.5 g∶100 mL.

[0051] (3) Mix the pectin-nano-lignin complex obtained in step (2) with citrus essential oil according to a volume ratio of 9∶1, and perform high-speed homogenization at a rotation speed of 12000 rpm for 2 min to form a stable Pickering emulsion of citrus essential oil with pectin-nano-lignin, denoted as PLCPE.

[0052] (4) Add starch to water for gelatinization treatment to obtain a starch solution. Among them, the mass-volume ratio of starch to water in the starch solution is 5 g∶100 mL, the temperature of gelatinization treatment is 90 °C, the rotation speed is 200 rpm, and the time is 1 h.

[0053] (5) Mix the PLCPE obtained in step (3) into the starch solution obtained in step (4), add glycerol, and perform low-speed homogenization to obtain a film-forming slurry. Among them, the volume of PLCPE is 10% of the volume of the starch solution, the mass of glycerol is 30% of the mass of the starch, the rotation speed of low-speed homogenization is 5000 rpm, and the time of low-speed homogenization is 2 min.

[0054] (6) Pour the film-forming slurry obtained in step (5) into a glass plate mold, and a starch-based composite film that can release citrus essential oil in response to humidity can be obtained after natural drying.

[0055] In this example, alkali lignin was purchased from Sigma-Aldrich, CAS 8068-05-1, moisture: 5.0%, carbon: 61.7%, sulfur: 1.9%, w / w; citrus peel pectin, molecular weight: 1564 KDa; galacturonic acid content 74.73%; degree of esterification: 54.85%, which can be commercially purchased or obtained by laboratory extraction; citrus essential oil was purchased from Macklin Biochemical Technology Co., Ltd., limonene > 90%; starch was purchased from Beijing Aoboxing Biotechnology Co., Ltd., containing 12.5% moisture, 0.41% protein, 0.15% fat, 0.12% ash and 31.8% amylose.

[0056] Example 2:

[0057] In this example, starch-based composite films that can release citrus essential oil in response to humidity with different dosages of PLCPE were prepared. The preparation method was basically the same as that of the starch-based composite film that can release citrus essential oil in response to humidity in Example 1, except that: in step (5), the volume of PLCPE used was 1%, 5%, 10%, 25% of the volume of the starch solution, and the starch-based composite films that can release citrus essential oil in response to humidity prepared were denoted as 1% PLCPE, 5% PLCPE, 10% PLCPE, 25% PLCPE respectively.

[0058] Control group (0% PLCPE): Only use the gelatinized starch solution to produce starch-based films without adding any PLCPE.

[0059] Example 3:

[0060] In this example, starch-based composite films with humidity-responsive release of citrus essential oil were prepared with different ratios of pectin-nano-lignin complex to citrus essential oil. The preparation method was basically the same as that of the starch-based composite film with humidity-responsive release of citrus essential oil in Example 1, except that in step (3), the volume ratios of pectin-nano-lignin to citrus essential oil were 5:5, 7:3, and 9:1, respectively. There were differences in the physical properties and antibacterial properties of the starch-based composite films with humidity-responsive release of citrus essential oil produced by adding emulsions with different volume ratios of pectin-nano-lignin to citrus essential oil to the starch matrix. As the content of citrus essential oil increased, the antibacterial property of the composite film improved, the elongation at break increased, and the mechanical property decreased slightly.

[0061] Comparative Example 1:

[0062] A starch-based composite film with humidity-responsive release of citrus essential oil was prepared. The preparation method was basically the same as that of the starch-based composite film with humidity-responsive release of citrus essential oil in Example 1, except that in steps (2) and (3), pectin solution was not used, and only nano-lignin and citrus essential oil were used to prepare Pickering emulsion.

[0063] In Comparative Example 1, the emulsifying property of nano-lignin alone and its ability to stabilize Pickering emulsion were poor, which would lead to premature demulsification or phase separation of the emulsion, resulting in poor stability of the Pickering emulsion. This would make the essential oil in the composite film unstable and release prematurely, greatly affecting the humidity-responsive antibacterial effect of the composite film.

[0064] Performance investigation and evaluation

[0065] (I) Preparation and stability of nano-lignin Pickering emulsion and pectin-nano-lignin Pickering emulsion

[0066] Using nano-lignin and pectin-nano-lignin complex as the dispersed phase respectively, and mixing them with different ratios of citrus essential oil. The volume ratios of the dispersed phase to citrus essential oil were 5:5, 7:3, and 9:1, respectively. Nano-lignin Pickering emulsion (NLPE) and pectin-nano-lignin Pickering emulsion (PLCPE) were prepared by high-speed homogenization. Store and observe the stability of the above emulsions. The results are as Figure 2 shown, Figure 2Digital photographs of the appearance of nano-lignin Pickering emulsions (NLPE) and pectin-nano-lignin Pickering emulsions (PLCPE) with different ratios during storage. As can be seen from the figure, after 21 days, severe phase separation and demulsification occurred in all NLPE, indicating that it is difficult to prepare stable Pickering emulsions with single nano-lignin. In contrast, PLCPE hardly changed after 21 days, remaining in their original state without phase separation or demulsification. This indicates that the pectin-nano-lignin complex exhibits better emulsifying ability than single nano-lignin and can greatly extend the stability of Pickering emulsions.

[0067] (II) Determination of the physical properties of composite films

[0068] Good tensile strength (TS) and elongation at break (EAB) are very important for the application of starch films in the packaging field. Figure 3 Shows the TS and EAB of the film prepared in Example 2. Generally speaking, the mechanical properties of pure starch films are poor, which severely limits their practical applications. As Figure 3 shown in a of, adding PLCPE can significantly improve the TS performance of starch films and gradually increase with the increase of the addition amount (up to 2.5 times). PLCPE has good compatibility with the starch matrix. They fill the voids between starch molecules and make the intermolecular cross-linking and entanglement tighter through hydrogen bonds, thus greatly improving the tensile strength of the film. However, when the addition amount of PLCPE reaches 25%, the tensile strength of the film decreases, which may be related to the easier agglomeration caused by the excessive addition of PLCPE. In addition, a large amount of PLCPE will hinder the continuity of the starch matrix and is not conducive to its film-forming stability. This destruction of continuity will affect the mechanical properties and overall performance of the film. The elongation at break of starch films is similar to the change of tensile strength, as Figure 3 shown in b of. These results indicate that PLCPE makes a positive contribution to improving the mechanical properties of starch-based films. Considering that excessive PLCPE will cause adverse reactions, its addition amount should not exceed 25%.

[0069] The strong hydrophilicity of starch-based films limits their application in food packaging. The water contact angle (WCA) is an important index to evaluate the hydrophilicity / hydrophobicity of the film surface. Figure 4The WCA of the film prepared in Example 2 is shown. As shown in the figure, the water contact angle of the pure starch film is 69.00°, indicating poor hydrophobicity. Adding a small amount (1%) of PLCPE hardly changes the WCA of the starch film. However, as the addition amount of PLCPE increases, the WCA of the film increases significantly, and the water contact angle reaches 90.75°, showing a concentration dependence. This indicates that PLCPE plays an important role in enhancing the hydrophobicity of the starch-based film. PLCPE and starch form new hydrogen bonds, which may hinder the formation of hydrogen bonds between starch and water, thus increasing the surface hydrophobicity. In addition, the lignin in PLCPE itself has hydrophobicity, and they are evenly dispersed in the starch film, providing a good opportunity to increase the hydrophobicity of the film.

[0070] Water vapor permeability (WVP) and oxygen permeability (OP) are important evaluation indicators for starch-based films used in food packaging. Since there are many cracks in the 25% PLCPE-starch composite film and it is impossible to completely obtain the film required for testing, the test was not carried out. Figure 5 The WVP and OP of the film prepared in Example 2 are shown. As Figure 5 shown in a of, the WVP value of the PLCPE-starch composite film is slightly lower than that of the pure starch film, indicating that adding PLCPE can enhance the water vapor barrier property of the film. The enhanced surface hydrophobicity of the PLCPE-starch composite film will hinder the formation of hydrogen bonds between starch and water vapor, thus reducing the absorption and permeation of water vapor by the film. The change result of the OP of the composite film is similar to that of the WVP, as Figure 5 shown in b of. PLCPE and starch are tightly cross-linked by hydrogen bonds, improving the dense structure of the film and the curvature of the internal network voids, thus increasing the diffusion path of gas in the film and preventing the gas from passing through the film quickly.

[0071] (III) Sustained release and humidity-responsive release properties of essential oils in the composite film

[0072] The release of citrus essential oil in the composite film prepared in Example 2 was monitored by headspace gas chromatography. Figure 6Figure a shows the release rate of citrus essential oil in the film prepared in Example 2 over time. As shown in the figure, during the observation period, the citrus essential oil in all films was gradually released over time, showing typical sustained-release characteristics. The release of citrus essential oil in the film mainly occurred in the early stage. Finally, the residual amount of citrus essential oil in the 5% PLCPE starch composite film (13.00%) was the highest, showing the best sustained-release characteristics. These results indicate that PLCPE in the film can prevent the volatilization and release of citrus essential oil from the film, and when the addition ratio of PLCPE is moderate (5%-10%), the protection effect is the best. Pectin and lignin nanoparticles in the PLCPE dispersed phase provide a natural barrier for citrus essential oil, preventing them from being disturbed by the outside world and released rapidly. In addition, PLCPE and the starch matrix are cross-linked and entangled with each other through hydrogen bonds, making the film form a denser network structure, thereby slowing down the release rate of citrus essential oil from the film. This sustained-release characteristic of citrus essential oil can ensure that the film maintains its biological activity for a long time.

[0073] The composite film prepared in Example 1 was placed under different humidity levels, and the release of citrus essential oil was monitored to examine its release characteristics in response to humidity. The results are as Figure 6 shown in Figure b. On the first day, the release order of citrus essential oil in the film was: 98% RH > 75% RH > 43% RH > 23% RH, showing a positive correlation with the increase in humidity. Generally speaking, at each observation stage, the release amount of citrus essential oil in the film increased with the increase in humidity. Under low humidity conditions (<43% RH), the change in humidity had little effect on the final release amount of the essential oil (about 86.8%), while when the humidity increased further (>75% RH), the release rate of the essential oil increased significantly, and almost all the essential oils were released in advance. These results prove the humidity-responsive release characteristics of citrus essential oil of the composite film. Under low humidity conditions, pectin can maintain its complete structure, which ensures the stability of PLCPE in the composite film. At this time, the citrus essential oil in the film can be slowly released for a long time. As the humidity increases, the interaction between pectin molecular chains weakens, resulting in depolymerization and breakage. At this time, the citrus essential oil in the film will be rapidly released. Therefore, this composite film releases slowly at low humidity and is sensitive to humidity at high humidity, showing interesting dual-mode release characteristics, which are very beneficial for it to cope with different application scenarios. For example, in fruit packaging, the citrus essential oil in the film will be slowly released initially to maintain the antibacterial activity inside the package, while when the fruit loses water, the citrus essential oil will be rapidly released to hinder the growth of microorganisms and ultimately prevent the fruit from spoiling.

[0074] (IV) Anti-UV, antioxidant and antibacterial properties of the composite film

[0075] The anti-ultraviolet performance is an important functional factor to be considered in food packaging, as it helps prevent nutrient loss or food spoilage caused by ultraviolet radiation. The film prepared in Example 2 was subjected to anti-ultraviolet test, as Figure 7 shown, the starch film had a relatively high transmittance in the ultraviolet absorption wavelength range of 400 nm - 200 nm, indicating its low anti-ultraviolet ability. After adding PLCPE, the transmittance of the composite film decreased in the wavelength range of 400 nm - 200 nm, which indicated that the anti-ultraviolet ability of the film was enhanced. Subsequently, with the continuous increase of the PLCPE addition amount, the ultraviolet transmittance of the starch film gradually decreased. Finally, the PLCPE-starch composite film blocked most of UVA (400 nm - 320 nm), almost the entire UVB (320 nm - 275 nm) and UVC spectrum (275 nm - 200 nm), showing excellent anti-ultraviolet performance. We attribute this to the nano-lignin in PLCPE. The structure of lignin contains specific phenolic hydroxyl groups and chromophores, which have a strong absorption ability for ultraviolet wavelengths, so it can block most of the ultraviolet rays. This excellent anti-ultraviolet performance is very useful for the stability of citrus essential oil and the functional application of the composite film.

[0076] The antioxidant ability can enable the packaging material to reduce the oxidative browning of food. The antioxidant activities of the films prepared in Example 2 were evaluated by measuring the DPPH radical scavenging rate and the total antioxidant capacity (T-AOC). As Figure 8 shown in a and b, the DPPH radical scavenging rate and T-AOC values of the pure starch film were low, indicating its poor antioxidant activity. Even with the addition of a small amount of PLCPE, the DPPH value and T-AOC value of the starch film increased significantly, which indicated that their antioxidant activities were enhanced. With the further increase of the PLCPE content, the antioxidant activity of the starch film also gradually increased and showed a characteristic related to the PLCPE concentration, which indicated that PLCPE played an important role in the antioxidant activity of the starch film. The citrus essential oil in PLCPE itself has good antioxidant properties and is the key factor for enhancing the antioxidant activity of the starch film. In addition, the lignin in the dispersed phase of PLCPE contains a large number of phenolic structures and can also be used as an antioxidant.

[0077] Figure 9 a of Figure 9As shown in b and c, this may be because the organic components in starch provide some nutrients for bacterial growth. This result indicates the deficiency of pure starch films in antibacterial performance. Therefore, improving their antibacterial activity is a meaningful task. Next, the antibacterial activity of the composite films was tested. As Figure 9 shown in b and c, compared with the pure starch film, the antibacterial rate of the composite film was significantly increased (p < 0.05), and it was positively correlated with the addition amount of PLCPE, indicating that PLCPE can effectively kill or inhibit the growth of bacteria. This antibacterial activity is mainly attributed to the active ingredients contained in citrus essential oil, such as monoterpenes and terpenes, which can penetrate and destroy the cell membrane of microorganisms, resulting in the leakage of important substances inside the bacteria, thus leading to the death of bacteria. The active compounds in citrus essential oil may also interfere with the metabolic pathways of microorganisms, affecting their production and biosynthesis ability, thereby inhibiting their growth. These results indicate that the PLCPE-starch composite film has excellent antibacterial activity, which is of great significance for its application in preventing food microbial contamination.

[0078] (V) Fruit preservation, biosafety and degradability of the composite film

[0079] The preservation effect of the PLCPE-starch composite film prepared in Example 2 on strawberries was measured to evaluate its potential in practical applications as an active packaging film. As Figure 10 shown, the strawberries packaged with commercially available fresh-keeping films and pure starch films showed deterioration / browning during storage. In contrast, the strawberries packaged with the composite film of the present invention maintained their original form throughout the storage period without any rot or browning, showing excellent preservation effect. The composite film of the present invention has the characteristics of antioxidant, antibacterial and anti-ultraviolet, which provides favorable conditions for preventing fruit deterioration or browning. In addition, the citrus essential oil in the composite film of the present invention has a dual-mode release characteristic (long-term continuous release and humidity-responsive release), ensuring the stability of the essential oil release, which is beneficial to the continuous presence and long-term biological activity of the essential oil in the packaging.

[0080] Next, we measured the hardness and total soluble solid (TSS) retention rate of strawberries during storage to further evaluate their quality changes. Figure 11 shows the changes in hardness and TSS of strawberries during storage. The hardness and TSS retention rate of strawberries packaged with fresh-keeping films and pure starch films were lower on the 7th day, which was consistent with the phenomenon of strawberry deterioration. In contrast, the strawberries packaged with the composite film prepared in Example 2 almost maintained their original hardness (79.19%-89.45%) and TSS (84.52%-96.85%), indicating that they maintained a high quality throughout the storage period. These results verified the potential of the composite film in strawberry preservation.

[0081] The biosafety of food packaging is one of the most concerned issues. Therefore, we evaluated its safety by measuring the effect of the composite film prepared in Example 2 on the viability of GES-1 cells. Since the content of PLCPE in these films is different, we selected the composite film with 10% PLCPE for food packaging application testing. As Figure 12 shown, GES-1 cells maintained a high survival rate (>85%) within 24 hours and 48 hours, indicating that the composite film has little biotoxicity. This result was also expected because all the raw materials used in the composite film are natural components from plants, and this biosafety is particularly attractive for its application in the food field.

[0082] The starch-based film prepared in Example 2 and polyethylene plastic (control group) were buried in moist soil to observe their degradability. As Figure 13 shown, the starch-based films showed slight swelling and deformation on the second day, indicating that their internal molecular structure might have started to decompose and become loose at this time. On the 5th day, the starch-based films showed signs of cracking and partial degradation, which is related to the ability of microorganisms in the soil to directly attack and digest large organic starch molecules. All starch-based films were mostly or almost completely degraded on the 10th day, indicating that even at the end of their life cycle, they will not impose an additional burden on the environment. As expected, the polyethylene plastic showed little change throughout the observation period, verifying the defect of traditional petrochemical plastics in terms of degradation. In conclusion, this degradability of the starch-based film makes it particularly promising as a substitute for petrochemical plastics.

[0083] As described above, it is only the preferred embodiments of the present invention, and there is no limitation to the present invention in any form. Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make many possible changes and modifications to the technical solution of the present invention, or modify it into an equivalent embodiment with equivalent changes, without departing from the spirit and technical solution of the present invention. Therefore, any simple modification, equivalent replacement, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the technical solution of the present invention still fall within the scope of the protection of the technical solution of the present invention.

Claims

1. A method for preparing a starch-based composite film for releasing citrus essential oil in response to humidity, characterized in that: The following steps are involved: (1) dissolving lignin in acetone to obtain an acetone solution of lignin, pouring the acetone solution of lignin into water, and performing homogenization and concentration treatment to obtain a nano-lignin suspension; (2) mixing the nano-lignin suspension obtained in step (1) with the pectin solution and stirring to form a pectin-nano-lignin complex; (3) mixing the pectin-nanolignin complex obtained in step (2) with citrus essential oil and homogenizing to obtain a pectin-nanolignin stabilized citrus essential oil Pickering emulsion; (4) adding starch into water for gelatinization to obtain a starch solution; (5) mixing the citrus essential oil Pickering emulsion obtained in step (3) with the starch solution obtained in step (4), adding glycerol, and homogenizing to obtain a composite film-forming slurry; (6) Casting the composite film-forming slurry obtained in step (5) into a mold and drying it to obtain a starch-based composite film that releases citrus essential oil in response to humidity.

2. The method for preparing the starch-based composite film for releasing citrus essential oil in response to humidity according to claim 1, characterized in that: In step (1), the mass volume ratio of the lignin to acetone is 1 g: 10 mL to 25 mL, the volume ratio of the acetone solution of the lignin to water is 1: 5 to 10, and the mass volume ratio of the lignin to the suspension in the nano-lignin suspension is 0.5 g to 2.0 g: 100 mL.

3. The method for preparing a starch-based composite film for releasing citrus essential oil in response to humidity according to claim 2, characterized in that: In step (2), the mass volume ratio of pectin to the complex in the pectin-nanolignin complex is 1 g to 2 g: 100 mL, and the mass volume ratio of nanolignin to the complex in the obtained pectin-nanolignin complex is 0.25 g to 1.0 g: 100 mL.

4. The method for preparing a starch-based composite film for releasing citrus essential oil in response to humidity according to claim 3, characterized in that: In step (3), the volume ratio of the pectin-nano lignin complex to the citrus essential oil is 5-9:5-1.

5. The method for preparing a starch-based composite film for releasing citrus essential oil in response to humidity according to claim 4, characterized in that: In step (4), the mass volume ratio of starch to water is 4 g to 6 g: 100 mL.

6. The method for preparing the starch-based composite film for releasing citrus essential oil in response to humidity according to claim 5, characterized in that: In step (5), the volume ratio of the citrus essential oil Pickering emulsion to the starch solution is 1% to 10%, and the mass ratio of the glycerol to the starch is 30% to 40%.

7. The method for preparing the starch-based composite film for releasing citrus essential oil in response to humidity according to claim 6, characterized in that: In step (1), the homogenization speed is 8000 rpm to 12000 rpm, and the homogenization time is 5 min to 15 min; the concentration is carried out by a rotary evaporator; In step (2), the stirring speed is 150 rpm to 300 rpm, and the stirring time is 6 h to 10 h; In step (3), the homogenization speed is 10000 rpm to 12000 rpm, and the homogenization time is 2 min to 5 min; In step (4), the gelatinization temperature is 85°C to 95°C, the gelatinization speed is 150rpm to 300rpm, and the gelatinization time is 1h to 2h; In step (5), the homogenization speed is 3500 rpm to 5500 rpm, and the homogenization time is 2 min to 5 min.

8. A starch-based composite film for releasing citrus essential oil in response to humidity, prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the starch-based composite film for releasing citrus essential oil in response to humidity as claimed in claim 8 in the field of food preservation.

10. The use according to claim 9, characterized in that: The starch-based composite film capable of releasing citrus essential oil in response to humidity is used for fruit packaging and freshness preservation.

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