A multi-layer water-resistant edible fresh-keeping coating with antibacterial and antioxidant functions and its preparation method

Abstract

The present invention provides a multi-layer water-resistant edible fresh-keeping coating with antibacterial and antioxidant functions and a preparation method thereof. The multi-layer water-resistant edible fresh-keeping coating includes at least one cellulose-based fresh-keeping inner layer and at least one chitin-based fresh-keeping outer layer that are alternately stacked on the surface of fruits and vegetables in sequence; the cellulose-based fresh-keeping inner layer includes a curcumin / cyclodextrin inclusion complex. The multi-layer water-resistant edible fresh-keeping coating provided by the present invention is composed of multiple layers of cellulose-based fresh-keeping inner layers and multiple layers of chitin-based fresh-keeping outer layers that are alternately stacked in sequence. Through the layer-by-layer deposition self-assembly technology, the electrostatic attraction between negatively charged nanocellulose and positively charged nanochitin is utilized to tightly combine the cellulose-based fresh-keeping inner layer and the chitin-based fresh-keeping outer layer, thereby forming a dense and uniform multi-layer water-resistant edible fresh-keeping coating on the surface of fruits and vegetables.

Description

Technical Field

[0001] The present invention belongs to the technical field of fruit and vegetable preservation, and relates to a multi-layer water-resistant edible preservation coating with antibacterial and antioxidant functions and a preparation method thereof. Background Art

[0002] Fresh agricultural products such as fruits and vegetables suffer particularly serious losses in the post-harvest supply chain due to factors such as dehydration and respiratory metabolism. China is a large consumer of fruits and also a large importer of tropical fruits. In the post-harvest supply chain, 20 - 30% of fruits are wasted every year, causing serious economic losses. In order to extend the shelf life, various fruit preservation technologies have been developed, such as refrigeration, modified atmosphere packaging (MAP), preservation coatings, etc. Among them, a preservation coating refers to a transparent and uniform inert barrier substance directly coated on the fruit epidermis, which can extend the fruit shelf life by blocking gases, inhibiting respiration, reducing water loss and fruit shrinkage. At the same time, the preservation coating also has the functions of delaying fruit color change, retaining fruit fragrance, and inhibiting microbial growth. Due to the excellent performance of the preservation coating, simple operation and controllable cost, it is increasingly favored by the food industry and consumers. Layer-by-layer assembly is a common method for preparing fruit and vegetable preservation coatings. By combining components with different functions, multifunctional edible preservation coatings can be prepared.

[0003] In recent years, nano-cellulose has been used as a coating matrix component to prepare fruit and vegetable preservation / packaging materials due to its good film-forming property, excellent gas barrier property and outstanding biocompatibility. On the one hand, due to the inherent hydrophilicity of nano-cellulose, the barrier performance of the nano-cellulose preservation coating deteriorates or even the integrity is damaged under high relative humidity conditions, and it cannot play the preservation role. Therefore, the water resistance of nano-cellulose is enhanced by surface hydrophobic modification of nano-cellulose or physical blending with soybean oil, oleic acid, silica and sunflower oil, etc. On the other hand, the nano-cellulose material often cannot meet the requirements of fruit and vegetable preservation simultaneously due to its single function. Therefore, systems such as nano-chitin-TEMPO oxidized cellulose nanofibers, methyl cellulose / carboxymethyl cellulose / hydroxypropyl methyl cellulose-chitosan are used for layer-by-layer assembly to prepare packaging materials with antibacterial functions.

[0004] On the one hand, although the layer-by-layer self-assembly of nano-cellulose-nano-chitin to prepare packaging materials is simple in operation, there is no report on the actual preservation effect of nano-cellulose-nano-chitin double-layer preservation materials at present due to the inherent defects of polysaccharide materials (inability to be heat-bonded). On the other hand, for materials with irregular shapes such as fruits and vegetables, coating preservation is a simpler and more feasible method, but the current related research is still lacking. In addition, although the two-component (nano-cellulose and nano-chitin) has excellent film-forming performance, barrier performance and antibacterial performance, it lacks characteristics such as water resistance and antioxidant properties required for fruit and vegetable preservation. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the purpose of the present invention is to provide a multi-layer water-resistant edible fresh-keeping coating with antibacterial and antioxidant functions and a preparation method thereof. The multi-layer water-resistant edible fresh-keeping coating provided by the present invention is composed of multiple layers of cellulose-based fresh-keeping inner layers and multiple layers of chitin-based fresh-keeping outer layers alternately stacked in sequence. Through the layer-by-layer deposition self-assembly technology, the electrostatic attraction between negatively charged nanocellulose and positively charged nanochitin is utilized to tightly combine the cellulose-based fresh-keeping inner layer and the chitin-based fresh-keeping outer layer, thereby forming a dense and uniform multi-layer water-resistant edible fresh-keeping coating on the surface of fruits and vegetables.

[0006] To achieve this purpose, the present invention adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a multi-layer water-resistant edible fresh-keeping coating with antibacterial and antioxidant functions. The multi-layer water-resistant edible fresh-keeping coating includes at least one cellulose-based fresh-keeping inner layer and at least one chitin-based fresh-keeping outer layer alternately stacked in sequence from the surface of fruits and vegetables;

[0008] The cellulose-based fresh-keeping inner layer includes a curcumin / cyclodextrin inclusion complex.

[0009] The multi-layer water-resistant edible fresh-keeping coating provided by the present invention is composed of multiple layers of cellulose-based fresh-keeping inner layers and multiple layers of chitin-based fresh-keeping outer layers alternately stacked in sequence. Through the layer-by-layer deposition self-assembly technology, the electrostatic attraction between negatively charged nanocellulose and positively charged nanochitin is utilized to tightly combine the cellulose-based fresh-keeping inner layer and the chitin-based fresh-keeping outer layer, thereby forming a dense and uniform multi-layer water-resistant edible fresh-keeping coating on the surface of fruits and vegetables. The intertwined nanocellulose in the cellulose-based fresh-keeping inner layer forms a three-dimensional network structure, which prolongs the diffusion path of gas in the fresh-keeping coating, thereby reducing the gas permeability of the fresh-keeping coating and reducing the loss of moisture and nutrients on the surface and inside of fruits and vegetables.

[0010] The cellulose-based fresh-keeping inner layer not only reduces gas permeability but also plays an antioxidant function. In the present invention, curcumin / cyclodextrin inclusion complex is added to the cellulose-based fresh-keeping inner layer. The "hydrophobic inside and hydrophilic outside" three-dimensional structure of cyclodextrin is used to encapsulate curcumin, enabling curcumin and cyclodextrin to undergo host-guest interaction through non-covalent bonds. The insoluble curcumin is included in its hydrophobic cavity to form curcumin / cyclodextrin inclusion complex, which can not only improve the water solubility of curcumin but also overcome the defect that curcumin is easily decomposed by light, greatly enhancing the water solubility, bioavailability and antioxidant ability of curcumin. At the same time, the curcumin / cyclodextrin inclusion complex is fixed by the hydrogen bond interaction between nanocellulose and cyclodextrin molecules, allowing the curcumin / cyclodextrin inclusion complex to be evenly dispersed in the cellulose-based fresh-keeping inner layer. As the storage time of fruits and vegetables extends, the curcumin in the hydrophobic cavity of cyclodextrin is gradually released, exerting a long-term antioxidant effect.

[0011] The chitin-based fresh-keeping outer layer mainly plays an antibacterial function. When bacteria in the external environment invade the surface of fruits and vegetables, nano-chitin in the chitin-based fresh-keeping outer layer can adsorb on the surface of bacterial cells to form a polymer film, preventing the transport of nutrients into the bacterial cells, thereby exerting an antibacterial function. In addition, nano-chitin can also penetrate into the bacterial cells. Since nano-chitin is positively charged, it can interact with acidic substances such as lipopolysaccharide, teichoic acid, teichuronic acid, and capsular polysaccharide produced on the bacterial surface, disrupting its physiological functions; at the same time, it can also adsorb the anionic cytoplasm in the bacterial cells and cause flocculation, disturbing the normal physiological activities of bacteria.

[0012] The multilayer water-resistant and edible fresh-keeping coating prepared in the present invention still has extremely high barrier performance under high humidity conditions, has excellent antibacterial and antioxidant properties, can significantly extend the shelf life of fruits and vegetables, and can better retain the hardness and VC content.

[0013] As a preferred technical solution of the present invention, the cellulose-based fresh-keeping inner layer is formed by coating the cellulose-based fresh-keeping liquid on the surface of fruits and vegetables and then drying.

[0014] In some alternative examples, the cellulose-based fresh-keeping liquid includes a nanocellulose solution, a film-forming agent, a plasticizer, and a curcumin / cyclodextrin inclusion complex.

[0015] In some alternative examples, the film-forming agent includes beeswax and / or coconut oil, and the plasticizer includes glycerol and / or potassium sorbate.

[0016] The cellulose-based fresh-keeping solution provided by the present invention also adds a film-forming agent and a plasticizer to improve the wettability, film-forming property of the fresh-keeping solution and the integrity of the fresh-keeping coating. The specific substances of the selected film-forming agent and plasticizer are all edible components, which have no toxic or side effects on human health and can be directly eaten together with fruits and vegetables or can be eaten after washing.

[0017] The present invention uses beeswax and / or coconut oil as the film-forming agent. The hydrophobic beeswax and coconut oil can keep the fresh-keeping coating intact under low-temperature and high-humidity conditions, playing a role in enhancing the water resistance of the coating. Glycerol and / or potassium sorbate are used as the plasticizer to regulate the interfacial interaction between components.

[0018] As a preferred technical solution of the present invention, the mass fraction of the nano-cellulose solution is 0.5-1.5 wt%, for example, it can be 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt% or 1.5 wt%, but it is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0019] Fruits and vegetables release CO2 through respiration inside the fresh-keeping coating, resulting in an atmosphere with a relatively high CO2 content and a relatively low O2 content on the surface of fruits and vegetables, thereby inhibiting the respiratory activities of the fresh-keeping objects and delaying the ripening process of the fresh-keeping objects, achieving the purpose of modified atmosphere fresh-keeping. However, the higher the CO2 concentration is not necessarily better. For example, when preserving some leafy vegetables, too high a CO2 concentration on the surface of the vegetables will induce the leafy vegetables to carry out anaerobic respiration, resulting in a large accumulation of harmful toxins and making them unfit for consumption.

[0020] The fresh-keeping solution provided by the present invention can block external oxygen from passing through the fresh-keeping coating into the interior of fruits and vegetables, and at the same time, can also discharge a certain amount of excessive CO2 accumulated by fruits and vegetables due to respiration. This is because CO2 has a relatively high induced polarity and can generate a relatively large intermolecular force with the nano-cellulose molecules containing a large number of polar groups, making the solubility of CO2 in the fresh-keeping coating relatively large. Therefore, it shows a relatively high CO2 permeability, thus effectively preventing the problem of toxin accumulation caused by anaerobic respiration of fruits and vegetables.

[0021] In addition, the water vapor transmission rate of the fresh-keeping coating affects the mass loss rate of fruits and vegetables. An appropriate water vapor transmission rate is an important characteristic of the fresh-keeping coating when applied to controlled atmosphere fresh-keeping. In the present invention, a cellulose-based fresh-keeping inner layer is formed by coating and film-forming, so that the prepared fresh-keeping coating can not only adjust the O2 transmission rate and CO2 transmission rate, but also achieve the water retention effect on fruits and vegetables. This is because the nano-cellulose molecules have a microscopic network structure and strong water retention performance; at the same time, the nano-cellulose molecular chains carry hydrophilic groups such as hydroxyl groups, so that the fresh-keeping coating has a strong acting force on water molecules, can slow down the water transpiration on the surface of fruits and vegetables, reduce the mass loss of fruits and vegetables, and thus effectively adjust the air humidity on the surface of fruits and vegetables; at the same time, the structure of the fresh-keeping coating after dip-coating and film-forming is softer and denser, can serve as a barrier for water transportation, reduce the water loss on the surface of fruits, and can effectively inhibit the water loss rate and the respiratory activity degree of fruits and vegetables during storage, so as to reduce the respiratory consumption of fruits and achieve the fresh-keeping effect on fruits and vegetables in a low-temperature refrigerated environment.

[0022] The present invention specifically limits the mass fraction of the nano-cellulose solution to 0.5-1.5 wt%. When the mass fraction of the nano-cellulose solution is lower than 0.5 wt%, due to the too low concentration of nano-cellulose, the viscosity of the fresh-keeping solution is too small, which affects the adhesion of the fresh-keeping solution on the surface of fruits and vegetables. Therefore, it is not easy to form a complete fresh-keeping coating on the surface of fruits and vegetables by the dip-coating method, and the fresh-keeping effect cannot be achieved; when the mass fraction of the nano-cellulose solution is higher than 1.5 wt%, due to the too high concentration of nano-cellulose, the viscosity of the fresh-keeping solution is too large, the fruit and vegetable fresh-keeping solution gels, affecting its processing performance, and it is impossible to prepare a fresh-keeping coating by the dip-coating method and the thickness of the prepared fresh-keeping coating is uneven, affecting the actual fresh-keeping effect.

[0023] It should be noted that the nano-cellulose used in the nano-cellulose solution provided by the present invention can be cellulose nanocrystals, or cellulose nanofibers, or a mixture of cellulose nanocrystals and cellulose nanofibers.

[0024] The present invention does not make specific requirements and special limitations on the preparation methods of cellulose nanocrystals and cellulose nanofibers. Preferably, cellulose nanocrystals are prepared by the sulfuric acid hydrolysis method, and the prepared cellulose nanocrystals have a length of 50-500 nm and a diameter of 5-20 nm; cellulose nanofibers are prepared by the TEMPO oxidation method, and the prepared cellulose nanofibers have a length of more than 600 nm, a diameter of 3-10 nm, and a surface charge amount of 1.0-1.8 mmol / g; the length of nano-chitin is more than 550 nm, the diameter is 5-30 nm, and the charge amount is 1.5-3 mmol / g.

[0025] In some optional examples, based on the dry weight of nanocellulose in the nanocellulose solution, the added amount of the film-forming agent is 20-50wt%, for example, it can be 20wt%, 22wt%, 24wt%, 26wt%, 28wt%, 30wt%, 32wt%, 34wt%, 36wt%, 38wt%, 40wt%, 42wt%, 44wt%, 46wt%, 48wt% or 50wt%; the added amount of the plasticizer is 20-50wt%, for example, it can be 20wt%, 22wt%, 24wt%, 26wt%, 28wt%, 30wt%, 32wt%, 34wt%, 36wt%, 38wt%, 40wt%, 42wt%, 44wt%, 46wt%, 48wt% or 50wt%; the addition amount of the curcumin / cyclodextrin inclusion compound is 5-10wt%, for example, it can be 5.0wt%, 5.5wt%, 6.0wt%, 6.5wt%, 7.0wt%, 7.5wt%, 8.0wt%, 8.5wt%, 9.0wt%, 9.5wt% or 10.0wt%, but is not limited to the enumerated values, and other values not listed within the numerical range are also applicable.

[0026] The present invention uses beeswax and coconut oil as film-forming agents. The amount of film-forming agent added directly affects the internal structure of the fresh-keeping coating, thereby affecting the comprehensive properties of the fresh-keeping coating, such as water vapor permeability, oxygen permeability, elongation at break, and tensile strength. Because beeswax and coconut oil contain fatty acids, the hydroxyl groups in the nanocellulose molecular chains generate hydrogen bonds and electrostatic interactions with the carboxylic acids in the fatty acids, allowing the nanocellulose in the fresh-keeping coating to be tightly and orderly arranged, giving the fresh-keeping coating a dense network structure and improving the oxygen barrier and water retention capabilities of the fresh-keeping coating.

[0027] The present invention particularly limits the addition amount of film-forming agent to 20-50wt%. When the addition amount of film-forming agent is less than 20wt%, it cannot play the purpose of enhancing moisture barrier performance and water resistance. When the addition amount of film-forming agent exceeds 50wt%, the hydrophobic effect and emulsification of beeswax and coconut oil weaken the bonding effect between nanocellulose molecules, resulting in a significant decrease in the tensile strength and elongation at break of the fresh-keeping coating; At the same time, when the film-forming agent is added in excess, it will cause its crystallization in the fresh-keeping coating to be discontinuous, destroying the three-dimensional network formed by nanocellulose, resulting in a deterioration in oxygen barrier performance; It will also cause the stress distribution inside the fresh-keeping coating to be uneven and the mechanical properties to deteriorate; At the same time, accompanied by the precipitation of some lipids, the smoothness of the fresh-keeping coating surface is reduced, ultimately resulting in a significant decrease in the tensile strength and elongation at break of the fresh-keeping coating. The present invention comprehensively considers the impact of the addition amount of film-forming agent on the various properties of the fresh-keeping coating, and particularly preferably the addition amount of film-forming agent is 20-50wt%.

[0028] The present invention uses glycerol and potassium sorbate as plasticizers, and the addition amounts of glycerol and potassium sorbate have a significant impact on the performance of the fresh-keeping coating. First of all, since glycerol and potassium sorbate are organic small-molecule substances, adding appropriate amounts of glycerol and potassium sorbate can increase the interaction between nanocellulose molecules and film-forming agent molecules, and can also fill the voids in the nanocellulose network structure, enhancing the compactness of the fresh-keeping coating, making it difficult for water and oxygen to penetrate through the fresh-keeping coating, and further improving the water retention ability and oxygen barrier ability of the fresh-keeping coating. Secondly, glycerol and potassium sorbate can penetrate into the nanocellulose matrix, reducing the force between nanocellulose molecular chains, activating the nanocellulose molecular chains and making them easy to slide, increasing the fluidity of the nanocellulose molecular chains, and improving the toughness and elasticity of the fresh-keeping coating. Thirdly, the hydroxyl groups in glycerol and potassium sorbate molecules will form hydrogen bonds with the carboxyl groups on the nanocellulose molecular chains, enhancing the intermolecular interaction of the fresh-keeping coating and increasing the tensile strength and elongation at break of the fresh-keeping coating.

[0029] The present invention specifically limits the addition amount of the plasticizer to 20-50 wt%. When the addition amount of the plasticizer is less than 20 wt%, due to the large brittleness of nanocellulose, the fresh-keeping coating will crack and fail to achieve the fresh-keeping effect. When the addition amount of the plasticizer exceeds 50 wt%, the fluidity of the nanocellulose molecular chains is further enhanced, which will lead to an increase in the pores between the nanocellulose molecular chains, reducing the compactness of the fresh-keeping coating, and then playing a certain role in promoting the increase of the oxygen transmission rate, resulting in a decrease in the oxygen barrier ability of the fresh-keeping coating. In addition, when the addition amount of the plasticizer is too high, the softening effect of the plasticizer on the fresh-keeping coating is enhanced, resulting in a decrease in the rigidity and tensile strength of the fresh-keeping coating. The present invention comprehensively considers the influence of the addition amount of the plasticizer on the various performances of the fresh-keeping coating, and particularly preferably the addition amount of the plasticizer is 20-50 wt%.

[0030] Curcumin is an active ingredient extracted from the rhizomes of Zingiberaceae plants. It has antioxidant effects and can greatly extend the fresh-keeping time of the fresh-keeping coating for fruits and vegetables. At the same time, curcumin has low toxicity and a low probability of adverse reactions such as allergies, and has no toxic side effects on human health and meets the edible conditions. However, curcumin is a fat-soluble substance and is insoluble in water, so it is not easily dispersed in an aqueous system, has a low utilization rate, and is difficult to fully exert its antioxidant and antibacterial abilities.

[0031] Cyclodextrin is a water-soluble, non-reducing, white crystal that is not easily hydrolyzed by acid. It is extracted from starch-containing raw materials such as corn or potatoes through catalytic enzymes. It has a pure plant origin, is non-toxic and edible, and has a very low risk of causing allergic and other adverse reactions. There are three common types of cyclodextrins, namely α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin, which are formed by combining 6, 7, or 8 glucose units through 1,4-glycosidic bonds. The unique feature of the cyclodextrin molecule lies in its cyclic three-dimensional structure. A hydrophobic cavity of a specific size can be formed inside its molecular structure, which can absorb lipophilic molecules of compatible size and shape as "guests", and its hydrophilic surface can ensure the tolerance of the molecule in the water-based system.

[0032] By virtue of the "hydrophobic inside and hydrophilic outside" property of cyclodextrin, the present invention uses it as a sustained-release carrier, and curcumin is included in its hydrophobic cavity to form a cyclodextrin / curcumin inclusion complex. The inclusion of curcumin by cyclodextrin can improve the stability and solubility of curcumin, reduce the loss of active ingredients during the production and storage of the cellulose-based fresh-keeping liquid, and can also extend the shelf life of fruits and vegetables, reduce the change of the sensory characteristics of fruits and vegetables, and effectively prevent ultraviolet-induced deterioration and oxidation by controlling the release time and release amount of curcumin.

[0033] The present invention preferably uses β-cyclodextrin. This is because, from the structure of cyclodextrin, the pore gap of the α-cyclodextrin molecule is relatively small, usually only capable of including guest substances with relatively small molecules, and currently there is no industrial-grade product of α-cyclodextrin; while the pore gap of the γ-cyclodextrin molecule is relatively small, capable of including guest substances with relatively large molecules, but also due to the current high production cost, it cannot be mass-produced industrially, resulting in limited industrial application; the pore gap of the β-cyclodextrin molecule is moderate, with a wide application range and low production cost, and it is currently the only cyclodextrin product that can be mass-produced in industrial applications.

[0034] The present invention specifically limits the addition amount of the curcumin / cyclodextrin inclusion complex to 5-10 wt%. When the addition amount of the curcumin / cyclodextrin inclusion complex is lower than 5 wt%, due to the too low effective content of the functional components, the antibacterial, antioxidant effect and fresh-keeping effect of the fresh-keeping coating become poor; when the addition amount of the curcumin / cyclodextrin inclusion complex is higher than 10 wt%, due to the crystallization and precipitation of cyclodextrin molecules during the formation of the fresh-keeping coating, the uniformity and integrity of the fresh-keeping coating are damaged, affecting the barrier performance of the fresh-keeping coating, and further affecting the fresh-keeping effect. In addition, too high an addition amount of curcumin will cause the transparency of the fresh-keeping coating to decrease, and further affect the acceptability of the fresh-keeping liquid by consumers.

[0035] As a preferred technical solution of the present invention, the chitin-based fresh-keeping outer layer is formed by coating the chitin-based fresh-keeping liquid on the surface of the cellulose-based fresh-keeping inner layer and then drying it.

[0036] In some alternative embodiments, the chitin-based fresh-keeping liquid comprises a nano-chitin solution, a film-forming agent, and a plasticizer.

[0037] In some alternative embodiments, the film-forming agent comprises beeswax and / or coconut oil, and the plasticizer comprises glycerol and / or potassium sorbate.

[0038] As a preferred technical solution of the present invention, the mass fraction of the nano-chitin solution is 0.5 - 1.5 wt%, for example, it can be 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt% or 1.5 wt%, but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0039] The present invention does not make specific requirements and special limitations on the preparation method of nano-chitin. Due to the strong hydrogen bond force between chitin molecules, it is difficult to obtain nano-chitin by traditional simple treatment methods, and strong mechanical force is required to obtain nano-chitin. Currently, the commonly used strong mechanical force treatment method is high-pressure homogenization. Under acidic conditions, chitin molecules carry a positive charge, and high-pressure homogenization can break up chitin to form nanofibers. In addition, nano-chitin crystals containing crystalline regions can be obtained by removing the amorphous regions of chitin through strong acid hydrolysis, and nano-chitin can also be obtained by oxidizing chitin with chemical reagents.

[0040] The present invention specifically limits the mass fraction of the nano-chitin solution to 0.5 - 1.5 wt%. The surface of the antibacterial non-woven fabric prepared within this range is smooth, uniform, and dense, and can also meet certain antibacterial requirements. When the mass fraction of the nano-chitin solution is lower than 0.5 wt%, the content of nano-chitin in the fresh-keeping coating is too small to meet the antibacterial requirements of the antibacterial non-woven fabric; in addition, due to the low concentration of nano-chitin, the viscosity of the fresh-keeping liquid is too low, resulting in a decrease in the adhesion of the fresh-keeping liquid on the surface of fruits and vegetables or the inner layer of cellulose-based fresh-keeping, and it is difficult to form a complete fresh-keeping coating on the surface of fruits and vegetables by dip coating method, resulting in a reduction in the fresh-keeping effect of the fresh-keeping coating; when the mass fraction of the nano-chitin solution exceeds 1.5 wt%, due to the small size, large specific surface area, and many active groups of nano-chitin, the hydrogen bond interaction and adsorption effect between nano-chitin molecules are further enhanced, and the aggregation degree of nano-chitin is aggravated, resulting in a decrease in the flatness of the surface of the antibacterial non-woven fabric and an increase in the surface area of fruits and vegetables, thereby increasing the effective transpiration area of water vapor and causing rapid water loss on the surface of fruits and vegetables; in addition, due to the too high concentration of nano-chitin, the viscosity of the chitin-based fresh-keeping liquid is too large, resulting in gelation of the chitin-based fresh-keeping liquid and affecting its processing performance.

[0041] In some alternative embodiments, based on the dry weight of nano-chitin in the nano-chitin solution, the addition amount of the film-forming agent is 20-50 wt%, for example, it can be 20 wt%, 22 wt%, 24 wt%, 26 wt%, 28 wt%, 30 wt%, 32 wt%, 34 wt%, 36 wt%, 38 wt%, 40 wt%, 42 wt%, 44 wt%, 46 wt%, 48 wt% or 50 wt%; the addition amount of the plasticizer is 20-50 wt%, for example, it can be 20 wt%, 22 wt%, 24 wt%, 26 wt%, 28 wt%, 30 wt%, 32 wt%, 34 wt%, 36 wt%, 38 wt%, 40 wt%, 42 wt%, 44 wt%, 46 wt%, 48 wt% or 50 wt%, but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0042] In a second aspect, the present invention provides a method for preparing the multi-layer water-resistant edible fresh-keeping coating described in the first aspect, and the preparation method includes:

[0043] (I) Mix the cyclodextrin solution and the curcumin solution evenly, then cool and crystallize to form a precipitate, filter the precipitate and dry it to obtain a curcumin / cyclodextrin inclusion complex; mix the nano-cellulose solution, the film-forming agent, the plasticizer and the curcumin / cyclodextrin inclusion complex in proportion and heat to melt the film-forming agent to obtain a cellulose precursor solution; emulsify and vacuum degas the cellulose precursor solution in sequence to obtain a cellulose-based fresh-keeping solution;

[0044] (II) Mix the nano-chitin solution, the film-forming agent and the plasticizer in proportion and heat to melt the film-forming agent to obtain a chitin precursor solution; emulsify and vacuum degas the chitin precursor solution in sequence to obtain a chitin-based fresh-keeping solution;

[0045] (III) Immerse the fruits and vegetables to be fresh-keeping treated in the cellulose-based fresh-keeping solution obtained in step (I), or spray the cellulose-based fresh-keeping solution obtained in step (I) on the surface of the fruits and vegetables to be fresh-keeping treated, and then air-dry the fruits and vegetables to form a cellulose-based fresh-keeping inner layer on the surface of the fruits and vegetables;

[0046] (IV) Immerse the fruits and vegetables with a cellulose-based fresh-keeping inner layer formed thereon in the chitin-based fresh-keeping solution obtained in step (II), or spray the chitin-based fresh-keeping solution obtained in step (II) on the surface of the cellulose-based fresh-keeping inner layer, and then air-dry the fruits and vegetables to form a chitin-based fresh-keeping outer layer on the surface of the cellulose-based fresh-keeping inner layer;

[0047] (V) Alternately repeat steps (III) and (IV) at least once.

[0048] Compared with other physical or chemical fresh-keeping materials, the main materials in the cellulose-based fresh-keeping solution and chitin-based fresh-keeping solution provided by the present invention are all biomass materials. Therefore, the double-layer water-resistant and edible fresh-keeping coating formed after soaking or spraying has excellent biodegradability. No toxic or harmful chemical reagents need to be added during the preparation process, which is environmentally friendly and conforms to the development concept of green chemistry.

[0049] During the alternating soaking process of the cellulose-based fresh-keeping solution and the chitin-based fresh-keeping solution, it includes both chemical adsorption and physical adsorption. On the one hand, the nanocellulose in the cellulose-based fresh-keeping solution can modify the surface of fruits and vegetables through electrostatic force, hydrogen bond and coordination. At the same time, the nanochitin in the chitin-based fresh-keeping solution contains a large number of amide groups, and the amide groups contain lone pairs of electrons, which can form a coordination bond NH3 + with the hydrogen ions in the chitin-based fresh-keeping solution, while the carboxylate ions on the surface of nanocellulose can react with NH3 + to form a new chemical bond, thus realizing the chemical adsorption. On the other hand, the surface of the nanochitin in the chitin-based fresh-keeping solution is positively charged, and the surface of the nanocellulose in the cellulose-based fresh-keeping solution is negatively charged. The two can achieve physical adsorption through electrostatic attraction. In addition, the nanochitin and nanocellulose can also be combined through hydrogen bond to achieve in-situ layer-by-layer self-assembly.

[0050] As a preferred technical solution of the present invention, in step (Ⅰ), the preparation process of the cyclodextrin solution includes:

[0051] At an environmental temperature of 60-70 °C, dissolve cyclodextrin in deionized water to obtain the cyclodextrin solution. For example, it can be 60 °C, 61 °C, 62 °C, 63 °C, 64 °C, 65 °C, 66 °C, 67 °C, 68 °C, 69 °C or 70 °C, but it is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0052] In some optional examples, the mass fraction of the cyclodextrin solution is 3-6 wt%, for example, it can be 3.0 wt%, 3.2 wt%, 3.4 wt%, 3.6 wt%, 3.8 wt%, 4.0 wt%, 4.2 wt%, 4.4 wt%, 4.6 wt%, 4.8 wt%, 5.0 wt%, 5.2 wt%, 5.4 wt%, 5.6 wt%, 5.8 wt% or 6.0 wt%, but it is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0053] The preparation process of the curcumin solution includes:

[0054] Dissolve curcumin in ethanol at an ambient temperature of 60 - 70 °C to obtain the curcumin solution. For example, the temperature can be 60 °C, 61 °C, 62 °C, 63 °C, 64 °C, 65 °C, 66 °C, 67 °C, 68 °C, 69 °C, or 70 °C, but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0055] In some alternative examples, the concentration of the curcumin solution is 0.005 - 0.01 g / mL. For example, it can be 0.005 g / mL, 0.0055 g / mL, 0.006 g / mL, 0.0065 g / mL, 0.007 g / mL, 0.0075 g / mL, 0.008 g / mL, 0.0085 g / mL, 0.009 g / mL, 0.0095 g / mL, or 0.01 g / mL, but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0056] The mixing process of the cyclodextrin solution and the curcumin solution includes:

[0057] At an ambient temperature of 60 - 70 °C, slowly add the curcumin solution drop by drop into the cyclodextrin solution. After all the drops are added, keep the ambient temperature constant and continue stirring for 2 - 4 h. Here, the ambient temperature can be 60 °C, 61 °C, 62 °C, 63 °C, 64 °C, 65 °C, 66 °C, 67 °C, 68 °C, 69 °C, or 70 °C, and the stirring time can be 2.0 h, 2.2 h, 2.4 h, 2.6 h, 2.8 h, 3.0 h, 3.2 h, 3.4 h, 3.6 h, 3.8 h, or 4.0 h, but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0058] In some alternative examples, the temperature for cooling crystallization is 1 - 10 °C. For example, it can be 1 °C, 2 °C, 3 °C, 4 °C, 5 °C, 6 °C, 7 °C, 8 °C, 9 °C, or 10 °C, but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0059] In some alternative examples, the time for cooling crystallization is 8 - 12 h. For example, it can be 8.0 h, 8.5 h, 9.0 h, 9.5 h, 10.0 h, 10.5 h, 11.0 h, 11.5 h, or 12.0 h, but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0060] In some alternative examples, the temperature for drying is 40 - 60 °C. For example, it can be 40 °C, 42 °C, 44 °C, 46 °C, 48 °C, 50 °C, 52 °C, 54 °C, 56 °C, 58 °C, or 60 °C, but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0061] In some alternative examples, the heating temperature is 80 - 90 °C. For example, it can be 80 °C, 81 °C, 82 °C, 83 °C, 84 °C, 85 °C, 86 °C, 87 °C, 88 °C, 89 °C or 90 °C, but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0062] In some alternative examples, the emulsification time is 1 - 10 min. For example, it can be 1 min, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min or 10 min, but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0063] The present invention does not make specific requirements and special limitations on the emulsification method. For example, it can be ultrasonic emulsification method, homogenization method, ultra-high pressure homogenization method or grinding method, etc.

[0064] The ultrasonic emulsification method is carried out in an ultrasonic emulsifier, using the cavitation effect of ultrasound to break, mix and emulsify the materials. The homogenization method is carried out in a high-pressure homogenizer, and the materials are refined under the triple action of extrusion, strong impact and decompression expansion, so that the materials can be more evenly mixed with each other. The ultra-high pressure homogenization method is carried out in a microfluidizer or a high-pressure homogenizer, using the high shear, high collision and cavitation effects generated when the fluid flows through the interaction chamber at high speed to disperse the materials. The grinding method is carried out in a ball mill or a nano sand mill, using the strong frictional force and impact force generated by the speed difference between the grinding tank and the grinding balls to crush the materials.

[0065] In some alternative examples, the vacuum degassing time is 5 - 10 min. For example, it can be 5.0 min, 5.5 min, 6.0 min, 6.5 min, 7.0 min, 7.5 min, 8.0 min, 8.5 min, 9.0 min, 9.5 min or 10.0 min, but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0066] As a preferred technical solution of the present invention, in step (II), the heating temperature is 80 - 90 °C. For example, it can be 80 °C, 81 °C, 82 °C, 83 °C, 84 °C, 85 °C, 86 °C, 87 °C, 88 °C, 89 °C or 90 °C, but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0067] In some alternative examples, the emulsification time is 1 - 10 min. For example, it can be 1 min, 2 min, 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, 9 min, or 10 min, but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0068] In some alternative examples, the vacuum degassing time is 5 - 10 min. For example, it can be 5.0 min, 5.5 min, 6.0 min, 6.5 min, 7.0 min, 7.5 min, 8.0 min, 8.5 min, 9.0 min, 9.5 min, or 10.0 min, but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0069] As a preferred technical solution of the present invention, in step (Ⅲ), the single soaking time is 5 - 60 s. For example, it can be 5 s, 10 s, 15 s, 20 s, 25 s, 30 s, 35 s, 40 s, 45 s, 50 s, 55 s, or 60 s, but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0070] In some alternative examples, the number of soaking times is 2 - 5 times. For example, it can be 2 times, 3 times, 4 times, or 5 times, but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0071] In some alternative examples, the spraying flow rate is 5 - 20 mL / min. For example, it can be 5 mL / min, 6 mL / min, 7 mL / min, 8 mL / min, 9 mL / min, 10 mL / min, 11 mL / min, 12 mL / min, 13 mL / min, 14 mL / min, 15 mL / min, 16 mL / min, 17 mL / min, 18 mL / min, 19 mL / min, or 20 mL / min, but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0072] As a preferred technical solution of the present invention, in step (Ⅳ), the single soaking time is 5 - 60 s. For example, it can be 5 s, 10 s, 15 s, 20 s, 25 s, 30 s, 35 s, 40 s, 45 s, 50 s, 55 s, or 60 s, but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0073] In some alternative examples, the number of soaking times is 2 - 5 times. For example, it can be 2 times, 3 times, 4 times, or 5 times, but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0074] In some alternative embodiments, the flow rate of the spraying is 5 - 20 mL / min. For example, it can be 5 mL / min, 6 mL / min, 7 mL / min, 8 mL / min, 9 mL / min, 10 mL / min, 11 mL / min, 12 mL / min, 13 mL / min, 14 mL / min, 15 mL / min, 16 mL / min, 17 mL / min, 18 mL / min, 19 mL / min, or 20 mL / min. However, it is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0075] Exemplarily, the present invention provides a method for preparing a multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions. The preparation method specifically includes the following steps:

[0076] (1) At an environmental temperature of 60 - 70 °C, dissolve cyclodextrin in 100 g of deionized water to obtain a 3 - 6 wt% cyclodextrin solution; at an environmental temperature of 60 - 70 °C, dissolve curcumin in 210 mL of ethanol to obtain a 0.005 - 0.01 g / mL curcumin solution; at an environmental temperature of 60 - 70 °C, gradually add the curcumin solution dropwise to the cyclodextrin solution. After all the addition is completed, keep the environmental temperature unchanged and continue stirring for 2 - 4 h. Then, cool and crystallize at 1 - 5 °C for 8 - 12 h to form a precipitate. Filter the precipitate and dry it at 40 - 60 °C to obtain a curcumin / cyclodextrin inclusion complex;

[0077] (2) Mix a 0.5 - 1.5 wt% nano-cellulose solution, a film-forming agent, a plasticizer, and the curcumin / cyclodextrin inclusion complex obtained in step (1) in proportion. Based on the dry weight of nano-cellulose in the nano-cellulose solution, the addition amount of the film-forming agent is 20 - 50 wt%, the addition amount of the plasticizer is 20 - 50 wt%, and the addition amount of the curcumin / cyclodextrin inclusion complex is 5 - 10 wt%; heat the mixture to 80 - 90 °C until the film-forming agent melts to obtain a pre-cellulose solution; emulsify the pre-cellulose solution for 1 - 10 min, and then perform vacuum degassing for 5 - 10 min to obtain a cellulose-based fresh-keeping solution;

[0078] (3) Mix a 0.5 - 1.5 wt% nano-chitin solution, a film-forming agent, and a plasticizer in proportion. Based on the dry weight of nano-chitin in the nano-chitin solution, the addition amount of the film-forming agent is 20 - 50 wt%, and the addition amount of the plasticizer is 20 - 50 wt%; heat the mixture to 80 - 90 °C until the film-forming agent melts to obtain a pre-chitin solution; emulsify the pre-chitin solution for 1 - 10 min, and then perform vacuum degassing for 5 - 10 min to obtain a chitin-based fresh-keeping solution;

[0079] (4) Soak the fruits and vegetables to be preserved in the cellulose-based preservation solution obtained in step (2) for 2 - 5 times, with each soaking lasting for 5 - 60 s; or spray the cellulose-based preservation solution obtained in step (2) onto the surface of the fruits and vegetables to be preserved at a flow rate of 5 - 20 mL / min, and then air-dry the fruits and vegetables to form a cellulose-based preservation inner layer on the surface of the fruits and vegetables;

[0080] (5) Soak the fruits and vegetables with the cellulose-based preservation inner layer formed thereon in the chitosan-based preservation solution obtained in step (3) for 2 - 5 times, with each soaking lasting for 5 - 60 s, or spray the chitosan-based preservation solution obtained in step (3) onto the surface of the cellulose-based preservation inner layer at a flow rate of 5 - 20 mL / min, and then air-dry the fruits and vegetables to form a chitosan-based preservation outer layer on the surface of the cellulose-based preservation inner layer;

[0081] (6) Alternately repeat step (4) and step (5) at least once to form at least one cellulose-based preservation inner layer and at least one chitosan-based preservation outer layer that are alternately stacked on the surface of the fruits and vegetables.

[0082] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0083] The multi-layer water-resistant and edible preservation coating provided by the present invention is composed of multiple cellulose-based preservation inner layers and multiple chitosan-based preservation outer layers stacked alternately in sequence. Through the layer-by-layer deposition self-assembly technique, the electrostatic attraction between negatively charged nanocellulose and positively charged nanochitosan is utilized to closely combine the cellulose-based preservation inner layer and the chitosan-based preservation outer layer, thereby forming a dense and uniform multi-layer water-resistant and edible preservation coating on the surface of the fruits and vegetables. The intertwined nanocellulose in the cellulose-based preservation inner layer forms a three-dimensional network structure, extending the diffusion path of gas in the preservation coating, thereby reducing the gas permeability of the preservation coating and reducing the loss of moisture and nutrients on the surface and inside of the fruits and vegetables.

[0084] While reducing the gas permeability, the cellulose-based preservation inner layer also exerts an antioxidant function. In the cellulose-based preservation inner layer of the present invention, curcumin / cyclodextrin inclusion complex is added. The "inner hydrophobic and outer hydrophilic" three-dimensional structure of cyclodextrin is used to encapsulate curcumin, enabling curcumin and cyclodextrin to undergo host-guest interaction through non-covalent bonds, encapsulating insoluble curcumin into its hydrophobic cavity to form curcumin / cyclodextrin inclusion complex, which not only improves the water solubility of curcumin but also overcomes the defect that curcumin is easily decomposed when exposed to light, greatly enhancing the water solubility, bioavailability, and antioxidant capacity of curcumin. At the same time, the curcumin / cyclodextrin inclusion complex is fixed by the hydrogen bond interaction between nanocellulose and cyclodextrin molecules, enabling the curcumin / cyclodextrin inclusion complex to be uniformly dispersed in the cellulose-based preservation inner layer. As the storage time of the fruits and vegetables extends, curcumin in the hydrophobic cavity of cyclodextrin is gradually released, exerting a long-term antioxidant effect.

[0085] The chitin-based fresh-keeping outer layer mainly plays an antibacterial function. When bacteria in the external environment invade the surface of fruits and vegetables, nano-chitin in the chitin-based fresh-keeping outer layer can adsorb on the surface of bacterial cells to form a polymer film, which prevents nutrients from being transported into the bacterial cells, thus playing an antibacterial function. In addition, nano-chitin can also penetrate into the bacterial cells. Since nano-chitin is positively charged, it can interact with acidic substances such as lipopolysaccharide, teichoic acid, teichuronic acid, and capsular polysaccharide produced on the surface of bacteria, disturbing their physiological functions; at the same time, it can also adsorb the anionic cytoplasm in the bacterial cells and cause flocculation, disrupting the normal physiological activities of bacteria.

[0086] The multi-layer water-resistant and edible fresh-keeping coating prepared by the present invention still has extremely high barrier performance under high humidity conditions, has excellent antibacterial and antioxidant properties, can significantly extend the shelf life of fruits and vegetables, and can better retain the hardness and VC content. Description of the Drawings

[0087] Figure 1 It is an optical micrograph of the fresh-keeping coating prepared in Example 1 of the present invention;

[0088] Figure 2 It is a photo of the appearance state of the banana storage fresh-keeping test carried out in Example 1 of the present invention and the comparative example. Detailed Embodiments

[0089] The technical solution of the present invention will be described in detail below in conjunction with specific embodiments and their accompanying drawings. The embodiments described here are specific specific embodiments of the present invention and are used to illustrate the concept of the present invention; these descriptions are all explanatory and exemplary, and should not be construed as limiting the embodiments of the present invention and the protection scope of the present invention. Except for the embodiments described here, those skilled in the art can also adopt other obvious technical solutions based on the content disclosed in the claims of this application and its specification. These technical solutions include technical solutions that make any obvious substitutions and modifications to the embodiments described here.

[0090] Example 1

[0091] This embodiment provides a preparation method of a multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions. The preparation method specifically includes the following steps:

[0092] (1) At an ambient temperature of 60 °C, dissolve β-cyclodextrin in 100 g of deionized water to obtain a 3 wt% β-cyclodextrin solution; at an ambient temperature of 60 °C, dissolve curcumin in 210 mL of ethanol to obtain a 0.005 g / mL curcumin solution; at an ambient temperature of 60 °C, add the curcumin solution dropwise to the β-cyclodextrin solution. After all the addition is completed, keep the ambient temperature unchanged and continue stirring for 2 h. Then, cool and crystallize at 1 °C for 8 h to form a precipitate. Filter the precipitate and dry it at 40 °C to obtain a curcumin / cyclodextrin inclusion complex;

[0093] (2) Mix a 0.5 wt% cellulose nanofiber solution, beeswax, glycerol, and the curcumin / cyclodextrin inclusion complex obtained in step (1) in proportion. Based on the dry weight of cellulose nanofibers in the cellulose nanofiber solution, the addition amount of beeswax is 20 wt%, the addition amount of glycerol is 20 wt%, and the addition amount of the curcumin / cyclodextrin inclusion complex is 5 wt%; heat the mixture to 80 °C until the beeswax melts to obtain a cellulose precursor solution; emulsify the cellulose precursor solution for 1 min, and then perform vacuum degassing for 5 min to obtain a cellulose-based fresh-keeping solution;

[0094] (3) Mix a 0.5 wt% nanochitin solution, beeswax, and glycerol in proportion. Based on the dry weight of nanochitin in the nanochitin solution, the addition amount of beeswax is 20 wt%, and the addition amount of glycerol is 20 wt%; heat the mixture to 80 °C until the beeswax melts to obtain a chitin precursor solution; emulsify the chitin precursor solution for 1 min, and then perform vacuum degassing for 5 min to obtain a chitin-based fresh-keeping solution;

[0095] (4) Immerse the freshly picked bananas in the cellulose-based fresh-keeping solution obtained in step (2) twice, each time for 60 s. Then, dry the fruits and vegetables to form a cellulose-based fresh-keeping inner layer on the surface of the fruits and vegetables;

[0096] (5) Immerse the fruits and vegetables with the cellulose-based fresh-keeping inner layer formed in step (4) in the chitin-based fresh-keeping solution obtained in step (3) twice, each time for 60 s. Then, dry the fruits and vegetables to form a chitin-based fresh-keeping outer layer on the surface of the cellulose-based fresh-keeping inner layer;

[0097] (6) Alternately repeat steps (4) and (5) twice to form two alternating layers of cellulose-based fresh-keeping inner layers and two layers of chitin-based fresh-keeping outer layers on the surface of the fruits and vegetables.

[0098] The surface morphology of the multi-layer water-resistant and edible fresh-keeping coating provided in this example is as Figure 1 shown.

[0099] Example 2

[0100] This embodiment provides a preparation method of a multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions. The preparation method specifically includes the following steps:

[0101] (1) At an ambient temperature of 62 °C, dissolve β-cyclodextrin in 100 g of deionized water to obtain a 4 wt% β-cyclodextrin solution; at an ambient temperature of 62 °C, dissolve curcumin in 210 mL of ethanol to obtain a 0.006 g / mL curcumin solution; at an ambient temperature of 62 °C, dropwise add the curcumin solution to the β-cyclodextrin solution. After all the addition is completed, keep the ambient temperature unchanged and continue stirring for 2.5 h. Then cool and crystallize at 3 °C for 9 h to form a precipitate. Filter the precipitate and dry it at 45 °C to obtain a curcumin / cyclodextrin inclusion complex;

[0102] (2) Mix an 0.8 wt% cellulose nanofiber solution, beeswax, glycerol, and the curcumin / cyclodextrin inclusion complex obtained in step (1) in proportion. Based on the dry weight of cellulose nanofibers in the cellulose nanofiber solution, the addition amount of beeswax is 30 wt%, the addition amount of glycerol is 30 wt%, and the addition amount of the curcumin / cyclodextrin inclusion complex is 6 wt%; heat the mixture to 82 °C until the beeswax melts to obtain a cellulose precursor solution; emulsify the cellulose precursor solution for 3 min, and then perform vacuum degassing for 6 min to obtain a cellulose-based fresh-keeping solution;

[0103] (3) Mix an 0.8 wt% nano-chitin solution, beeswax, and glycerol in proportion. Based on the dry weight of nano-chitin in the nano-chitin solution, the addition amount of beeswax is 30 wt%, and the addition amount of glycerol is 30 wt%; heat the mixture to 82 °C until the beeswax melts to obtain a chitin precursor solution; emulsify the chitin precursor solution for 3 min, and then perform vacuum degassing for 6 min to obtain a chitin-based fresh-keeping solution;

[0104] (4) Immerse the freshly picked bananas in the cellulose-based fresh-keeping solution obtained in step (2) 4 times, each time for 30 s, and then air-dry the bananas to form a cellulose-based fresh-keeping inner layer on the banana surface;

[0105] (5) Immerse the bananas with the cellulose-based fresh-keeping inner layer formed in step (4) in the chitin-based fresh-keeping solution obtained in step (3) 4 times, each time for 30 s, and then air-dry the bananas to form a chitin-based fresh-keeping outer layer on the surface of the cellulose-based fresh-keeping inner layer;

[0106] (6) Alternately repeat steps (4) and (5) twice to form 2 alternating layers of cellulose-based fresh-keeping inner layers and 2 layers of chitin-based fresh-keeping outer layers on the surface of the fruits and vegetables.

[0107] Example 3

[0108] This embodiment provides a preparation method of a multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions. The preparation method specifically includes the following steps:

[0109] (1) At an ambient temperature of 65 °C, dissolve β-cyclodextrin in 100 g of deionized water to obtain a 5 wt% β-cyclodextrin solution; at an ambient temperature of 65 °C, dissolve curcumin in 210 mL of ethanol to obtain a 0.007 g / mL curcumin solution; at an ambient temperature of 65 °C, add the curcumin solution dropwise to the β-cyclodextrin solution. After all the addition is completed, keep the ambient temperature unchanged and continue stirring for 3 h. Then, cool and crystallize at 5 °C for 10 h to form a precipitate. Filter the precipitate and dry it at 50 °C to obtain curcumin / cyclodextrin inclusion complex;

[0110] (2) Mix a 1 wt% nanocellulose solution (including cellulose nanofibers and cellulose nanocrystals with a mass ratio of 1:1), beeswax, glycerol, and the curcumin / cyclodextrin inclusion complex obtained in step (1) in proportion. Based on the dry weight of nanocellulose in the nanocellulose solution, the addition amount of beeswax is 35 wt%, the addition amount of glycerol is 35 wt%, and the addition amount of curcumin / cyclodextrin inclusion complex is 7 wt%; heat the mixture to 85 °C until the beeswax melts to obtain a cellulose precursor solution; emulsify the cellulose precursor solution for 5 min, and then perform vacuum degassing for 7 min to obtain a cellulose-based fresh-keeping solution;

[0111] (3) Mix a 1 wt% nanochitin solution, beeswax, and glycerol in proportion. Based on the dry weight of nanochitin in the nanochitin solution, the addition amount of beeswax is 35 wt%, and the addition amount of glycerol is 35 wt%; heat the mixture to 85 °C until the beeswax melts to obtain a chitin precursor solution; emulsify the chitin precursor solution for 5 min, and then perform vacuum degassing for 7 min to obtain a chitin-based fresh-keeping solution;

[0112] (4) Immerse the freshly picked bananas in the cellulose-based fresh-keeping solution obtained in step (2) 5 times, each time for 5 s, and then air-dry the bananas to form a cellulose-based fresh-keeping inner layer on the banana surface;

[0113] (5) Immerse the bananas with the cellulose-based fresh-keeping inner layer formed in step (4) in the chitin-based fresh-keeping solution obtained in step (3) 5 times, each time for 5 s, and then air-dry the bananas to form a chitin-based fresh-keeping outer layer on the surface of the cellulose-based fresh-keeping inner layer;

[0114] (6) Alternately repeat steps (4) and (5) at least three times to form 3 alternating layers of cellulose-based fresh-keeping inner layers and 3 layers of chitin-based fresh-keeping outer layers on the banana surface.

[0115] Example 4

[0116] This embodiment provides a method for preparing a multi-layer water-resistant edible fresh-keeping coating with antibacterial and antioxidant functions. The preparation method specifically includes the following steps:

[0117] (1) At an environmental temperature of 68 °C, dissolve β-cyclodextrin in 100 g of deionized water to obtain a 5 wt% β-cyclodextrin solution; at an environmental temperature of 68 °C, dissolve curcumin in 210 mL of ethanol to obtain a 0.008 g / mL curcumin solution; at an environmental temperature of 68 °C, gradually add the curcumin solution dropwise to the β-cyclodextrin solution. After all the addition is completed, keep the environmental temperature unchanged and continue stirring for 3.5 h. Then, cool and crystallize at 7 °C for 11 h to form a precipitate. Filter the precipitate and dry it at 55 °C to obtain a curcumin / cyclodextrin inclusion complex;

[0118] (2) Mix a 1.2 wt% nanocellulose solution (including cellulose nanofibers and cellulose nanocrystals with a mass ratio of 1:2), coconut oil, potassium sorbate, and the curcumin / cyclodextrin inclusion complex obtained in step (1) in proportion. Based on the dry weight of the nanocellulose in the nanocellulose solution, the addition amount of coconut oil is 40 wt%, the addition amount of potassium sorbate is 40 wt%, and the addition amount of the curcumin / cyclodextrin inclusion complex is 8 wt%; heat the mixture to 88 °C until the coconut oil melts to obtain a cellulose precursor liquid; emulsify the cellulose precursor liquid for 7 min, and then vacuum degas for 8 min to obtain a cellulose-based fresh-keeping liquid;

[0119] (3) Mix a 1.2 wt% nanochitin solution, coconut oil, and potassium sorbate in proportion. Based on the dry weight of the nanochitin in the nanochitin solution, the addition amount of coconut oil is 40 wt%, and the addition amount of potassium sorbate is 40 wt%; heat the mixture to 88 °C until the coconut oil melts to obtain a chitin precursor liquid; emulsify the chitin precursor liquid for 7 min, and then vacuum degas for 8 min to obtain a chitin-based fresh-keeping liquid;

[0120] (4) Spray the cellulose-based fresh-keeping liquid obtained in step (2) onto the surface of freshly picked bananas at a flow rate of 10 mL / min, and then dry the bananas to form a cellulose-based fresh-keeping inner layer on the banana surface;

[0121] (5) Spray the chitin-based fresh-keeping liquid obtained in step (3) onto the surface of the cellulose-based fresh-keeping inner layer at a flow rate of 10 mL / min, and then dry the bananas to form a chitin-based fresh-keeping outer layer on the surface of the cellulose-based fresh-keeping inner layer;

[0122] (6) Alternately repeat steps (4) and (5) at least three times to form 3 alternating layers of cellulose-based fresh-keeping inner layers and 3 layers of chitin-based fresh-keeping outer layers on the banana surface.

[0123] Example 5

[0124] This embodiment provides a preparation method of a multi-layer water-resistant edible fresh-keeping coating with antibacterial and antioxidant functions. The preparation method specifically includes the following steps:

[0125] (1) At an environmental temperature of 70 °C, dissolve β-cyclodextrin in 100 g of deionized water to obtain a 6 wt% β-cyclodextrin solution; at an environmental temperature of 70 °C, dissolve curcumin in 210 mL of ethanol to obtain a 0.01 g / mL curcumin solution; at an environmental temperature of 70 °C, add the curcumin solution dropwise to the β-cyclodextrin solution. After all the addition is completed, keep the environmental temperature unchanged and continue stirring for 4 h. Then cool and crystallize at 10 °C for 12 h to form a precipitate. Filter the precipitate and dry it at 60 °C to obtain a curcumin / cyclodextrin inclusion complex;

[0126] (2) Mix a 1.5 wt% nanocellulose solution (including cellulose nanofibers and cellulose nanocrystals with a mass ratio of 1:3), coconut oil, potassium sorbate, and the curcumin / cyclodextrin inclusion complex obtained in step (1) in proportion. Based on the dry weight of nanocellulose in the nanocellulose solution, the addition amount of coconut oil is 50 wt%, the addition amount of potassium sorbate is 50 wt%, and the addition amount of the curcumin / cyclodextrin inclusion complex is 10 wt%; heat the mixture to 90 °C until the coconut oil melts to obtain a cellulose precursor solution; emulsify the cellulose precursor solution for 10 min, and then vacuum degas for 10 min to obtain a cellulose-based fresh-keeping liquid;

[0127] (3) Mix a 1.5 wt% nanochitin solution, coconut oil, and potassium sorbate in proportion. Based on the dry weight of nanochitin in the nanochitin solution, the addition amount of coconut oil is 50 wt%, and the addition amount of potassium sorbate is 50 wt%; heat the mixture to 90 °C until the coconut oil melts to obtain a chitin precursor solution; emulsify the chitin precursor solution for 10 min, and then vacuum degas for 10 min to obtain a chitin-based fresh-keeping liquid;

[0128] (4) Spray the cellulose-based fresh-keeping liquid obtained in step (2) onto the surface of freshly picked bananas at a flow rate of 20 mL / min, and then dry the bananas to form a cellulose-based fresh-keeping inner layer on the banana surface;

[0129] (5) Spray the chitin-based fresh-keeping liquid obtained in step (3) onto the surface of the cellulose-based fresh-keeping inner layer at a flow rate of 20 mL / min, and then dry the bananas to form a chitin-based fresh-keeping outer layer on the surface of the cellulose-based fresh-keeping inner layer;

[0130] (6) Alternately repeat steps (4) and (5) at least four times to form an alternately stacked 4-layer cellulose-based fresh-keeping inner layer and a 4-layer chitin-based fresh-keeping outer layer on the banana surface.

[0131] Example 6

[0132] This example provides a preparation method of a multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions. The difference between this preparation method and that of Example 1 is that in step (2), the mass fraction of the cellulose nanofiber solution is adjusted to 0.3 wt%, and other process parameters and operation steps are exactly the same as those of Example 1.

[0133] Example 7

[0134] This example provides a preparation method of a multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions. The difference between this preparation method and that of Example 1 is that in step (2), the mass fraction of the cellulose nanofiber solution is adjusted to 1.8 wt%, and other process parameters and operation steps are exactly the same as those of Example 1.

[0135] Example 8

[0136] This example provides a preparation method of a multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions. The difference between this preparation method and that of Example 1 is that in step (2), the addition amount of the curcumin / cyclodextrin inclusion complex is adjusted to 2 wt%, and other process parameters and operation steps are exactly the same as those of Example 1.

[0137] Example 9

[0138] This example provides a preparation method of a multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions. The difference between this preparation method and that of Example 1 is that in step (2), the addition amount of the curcumin / cyclodextrin inclusion complex is adjusted to 12 wt%, and other process parameters and operation steps are exactly the same as those of Example 1.

[0139] Example 10

[0140] This example provides a preparation method of a multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions. The difference between this preparation method and that of Example 1 is that in step (3), the mass fraction of the nano-chitin solution is adjusted to 0.3 wt%, and other process parameters and operation steps are exactly the same as those of Example 1.

[0141] Example 11

[0142] This embodiment provides a preparation method of a multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions. The difference between this preparation method and that of Example 1 lies in that in step (3), the mass fraction of the nano-chitin solution is adjusted to 1.8 wt%, and other process parameters and operation steps are exactly the same as those of Example 1.

[0143] Comparative Example

[0144] This comparative example is a freshly picked banana without any fresh-keeping treatment.

[0145] The inclusion rates of the curcumin / cyclodextrin inclusion complexes provided in Examples 1-11 were tested. The specific test steps are as follows:

[0146] Prepare curcumin ethanol standard solutions (with concentrations of 1 μg / mL, 5 μg / mL, 10 μg / mL, 25 μg / mL, and 50 μg / mL respectively). Measure the absorbance at 300-800 nm, and draw a standard curve at the maximum absorption (around 426 nm). Accurately weigh a certain mass of the inclusion complex, add anhydrous ethanol and ultrasonically dissolve it for 20 min. After completion, transfer it to a 100 mL brown volumetric flask, and dilute it to the scale with anhydrous ethanol. Using anhydrous ethanol as the blank, measure the absorbance of the inclusion complex at a wavelength of 423 nm, so as to calculate the inclusion rate of the curcumin / cyclodextrin inclusion complex.

[0147] The water vapor transmission coefficient and oxygen permeability coefficient of the fresh-keeping coatings provided in Examples 1-11 were tested. The specific test steps are as follows:

[0148] (1) Water vapor transmission coefficient of the fresh-keeping coating

[0149] Refer to the national standard GB / T 1037-2021 "Determination of Water Vapor Transmission Performance of Plastic Films and Sheets - Cup Method for Weight Gain and Weight Loss" to test the water vapor barrier performance of the fresh-keeping coating.

[0150] (2) Oxygen permeability coefficient of the fresh-keeping coating

[0151] An oxygen permeation tester was used to test the oxygen barrier performance. The specific method is as follows: One side of the coated film is pure oxygen, and the other side is evacuated. Due to the osmotic pressure difference on both sides of the film, oxygen permeates through the film from the pure oxygen side into the vacuum side, and the oxygen barrier performance of the fresh-keeping coating can be obtained by monitoring the pressure change on the vacuum side.

[0152] The test results are shown in Table 1.

[0153] Table 1 Various test results of the fresh-keeping liquid and the fresh-keeping coating performance

[0154] Inclusion rate of curcumin / cyclodextrin inclusion complex % <![CDATA[Water vapor transmission coefficient kg·m / (s·m 2 ·Pa)]]> <![CDATA[Oxygen permeability coefficient cm 3 ·μm / (m 2 ·day·kPa)]]> Example 1 8.5 <![CDATA[9.8×10 -12 > 148 Example 2 9.3 <![CDATA[8.6×10 -12 > 142 Example 3 10.4 <![CDATA[7.5×10 -12 > 133 Example 4 11.5 <![CDATA[5.2×10 -12 > 115 Example 5 12.8 <![CDATA[6.8×10 -12 > 126 Example 6 8.5 <![CDATA[13.5×10 -12 > 168 Example 7 8.5 <![CDATA[9.5×10 -12 > 139 Example 8 8.5 <![CDATA[12.8×10 -12 > 163 Example 9 8.5 <![CDATA[10.3×10 -12 > 157 Example 10 8.5 <![CDATA[13.3×10 -12 > 165 Example 11 8.5 <![CDATA[9.6×10 -12 > 146

[0155] The bananas treated with the preservation method provided in Example 1 and the bananas not treated with the preservation method were stored in a constant temperature and humidity environment (ambient temperature 25°C, relative humidity 50%) for 9 days. The appearance of the bananas before and after the preservation method was as follows: Figure 2 As shown, after 5 days of storage, the surface of the bananas provided in the comparative example began to brown, with numerous, large black patches appearing. However, the bananas provided in Example 1, which had undergone the preservation treatment, only showed a small number of black spots, maintaining their good appearance and quality. By the 9th day of storage, the surface of the bananas provided in the comparative example had completely browned, softened, and begun to rot, making them inedible. However, the bananas provided in Example 1, which had undergone the preservation solution treatment, showed some browning but no obvious rot and remained edible.

[0156] The weight loss rate, VC content, and firmness of the bananas provided in Examples 1-11 and the comparative example were tested under the following test conditions: an ambient temperature of 25°C, a relative humidity of 50%, and a storage time of 15 days. The specific test steps are as follows:

[0157] (1) Weight loss rate

[0158] The weight was measured during storage. The bananas were weighed on the 0th day of storage and the weight m0 was recorded. The bananas were stored for 15 days in a constant temperature and humidity environment (ambient temperature 25℃, relative humidity 50%) and the weight m was recorded. n , the weight loss rate w (%) is calculated using the following formula:

[0159] .

[0160] (2) VC content

[0161] The VC content in bananas was tested with reference to the third method 2,6-dichloroindophenol titration method in accordance with the national standard GB 5009.86-2016 "National Food Safety Standard - Determination of Ascorbic Acid in Foods".

[0162] (3) Hardness

[0163] The hardness of bananas after fresh-keeping treatment during storage was measured using a GY-4 digital fruit hardness tester. A flat probe with a diameter of 5 mm was inserted into five circumferential positions of the largest longitudinal section of the banana to obtain five groups of hardness values, and the average value was taken.

[0164] The test results are shown in Table 2.

[0165] Table 2 Test results of banana storage and freshness test

[0166] Weight loss rate % VC content mg / 100g Hardness N Example 1 19.3 6.2 8.5 Example 2 17.6 7.6 9.1 Example 3 16.4 8.8 10.3 Example 4 14.0 9.7 11.6 Example 5 15.4 8.5 10.5 Example 6 22.8 4.3 6.2 Example 7 20.4 5.2 7.5 Example 8 21.6 4.6 7.0 Example 9 20.9 4.8 7.2 Example 10 22.2 4.4 6.7 Example 11 20.5 5.0 7.3 Comparative Example 23.8 3.8 5.4

[0167] It can be seen from the test data provided in Table 1 that the water vapor transmission coefficient of the fresh-keeping coatings prepared in Examples 1-5 is 5×10 -12 -10×10 -12 kg·m / (s·m 2 ·Pa), and the oxygen permeability coefficient is 110-150 cm 3 ·μm / (m 2 ·day·kPa), indicating that the fresh-keeping coatings prepared by the present invention have excellent oxygen barrier ability and water retention ability.

[0168] It can be seen from the test data provided in Table 2 that the weight loss rate of bananas after 15 days of storage is 14-20%, the VC content is 6-10 mg / 100 g, and the hardness is 8-12 N.

[0169] It can be seen from the test data provided in Examples 1, 6 and 7 that the mass fraction of the nano-cellulose solution in Example 6 is too low, resulting in too low viscosity of the fresh-keeping liquid, which affects the adhesion of the fresh-keeping liquid on the surface of bananas. Therefore, it is not easy to form a complete fresh-keeping coating on the surface of bananas by the dip-coating method, ultimately affecting the fresh-keeping effect on bananas; the mass fraction of the nano-cellulose solution in Example 7 is too high, resulting in gelation of the fruit and vegetable fresh-keeping liquid, affecting its processing performance, and the thickness of the fresh-keeping coating prepared by the dip-coating method is uneven, affecting the actual fresh-keeping effect.

[0170] It can be seen from the test data provided in Examples 1, 8 and 9 that the addition amount of the curcumin / cyclodextrin inclusion complex in Example 8 is too low. Due to the too low effective content of the functional components, the antibacterial and antioxidant effects and the fresh-keeping effect of the fresh-keeping coating become poor, ultimately affecting the fresh-keeping effect on bananas; the addition amount of the curcumin / cyclodextrin inclusion complex in Example 9 is too high. Due to the crystallization and precipitation of cyclodextrin molecules during the formation of the fresh-keeping coating, the uniformity and integrity of the fresh-keeping coating are damaged, affecting the barrier performance of the fresh-keeping coating, and further affecting the fresh-keeping effect of the fresh-keeping coating.

[0171] It can be seen from the test data provided in Examples 1, 10 and 11 that the mass fraction of the nano-chitin solution in Example 10 is too low, resulting in too low viscosity of the fresh-keeping liquid, which affects the adhesion of the fresh-keeping liquid on the surface of bananas. Therefore, it is not easy to form a complete fresh-keeping coating on the surface of bananas by the dip-coating method, ultimately affecting the fresh-keeping effect on bananas; the mass fraction of the nano-chitin solution in Example 11 is too high, resulting in gelation of the fruit and vegetable fresh-keeping liquid, affecting its processing performance, and the thickness of the fresh-keeping coating prepared by the dip-coating method is uneven, affecting the actual fresh-keeping effect.

[0172] The applicant declares that the above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A multi-layer water-resistant and edible fresh-keeping coating with antibacterial and antioxidant functions, characterized in that, The multi-layer water-resistant edible fresh-keeping coating comprises at least one cellulose-based fresh-keeping inner layer and at least one chitin-based fresh-keeping outer layer which are alternately stacked on the surface of fruits and vegetables in sequence; The cellulose-based fresh-keeping inner layer contains curcumin / cyclodextrin inclusion compound; The cellulose-based fresh-keeping inner layer is formed by coating a cellulose-based fresh-keeping liquid on the surface of fruits and vegetables and then drying; The cellulose-based fresh-keeping liquid comprises a nano-cellulose solution, a film-forming agent, a plasticizer and curcumin / cyclodextrin inclusion compound; The film-forming agent comprises beeswax and / or coconut oil, and the plasticizer comprises glycerol and / or potassium sorbate; The mass fraction of the nano-cellulose solution is 0.5-1.5 wt%; Based on the dry weight of nano-cellulose in the nano-cellulose solution, the addition amount of the film-forming agent is 20-50 wt%, the addition amount of the plasticizer is 20-50 wt%, and the addition amount of curcumin / cyclodextrin inclusion compound is 5-10 wt%; The chitin-based fresh-keeping outer layer is formed by coating a chitin-based fresh-keeping liquid on the surface of the cellulose-based fresh-keeping inner layer and then drying; The chitin-based fresh-keeping liquid comprises a nano-chitin solution, a film-forming agent and a plasticizer; The film-forming agent comprises beeswax and / or coconut oil, the plasticizer comprises glycerol and / or potassium sorbate; the mass fraction of the nano-chitin solution is 0.5-1.5 wt%; Based on the dry weight of nano-chitin in the nano-chitin solution, the addition amount of the film-forming agent is 20-50 wt%, and the addition amount of the plasticizer is 20-50 wt%.

2. The preparation method of the multi-layer water-resistant edible fresh-keeping coating according to claim 1, characterized in that, The preparation method comprises: (Ⅰ) Mix the cyclodextrin solution and the curcumin solution evenly, then cool and crystallize to form a precipitate. Filter the precipitate and dry it to obtain curcumin / cyclodextrin inclusion compound; mix the nano-cellulose solution, the film-forming agent, the plasticizer and curcumin / cyclodextrin inclusion compound in proportion and heat to melt the film-forming agent to obtain a cellulose precursor liquid; emulsify and vacuum degas the cellulose precursor liquid in sequence to obtain a cellulose-based fresh-keeping liquid; (Ⅱ) Mix the nano-chitin solution, the film-forming agent and the plasticizer in proportion and heat to melt the film-forming agent to obtain a chitin precursor liquid; emulsify and vacuum degas the chitin precursor liquid in sequence to obtain a chitin-based fresh-keeping liquid; (Ⅲ) Immerse the fruits and vegetables to be fresh-keeping treated in the cellulose-based fresh-keeping liquid obtained in step (Ⅰ), or spray the cellulose-based fresh-keeping liquid obtained in step (Ⅰ) on the surface of the fruits and vegetables to be fresh-keeping treated, and then dry the fruits and vegetables to form a cellulose-based fresh-keeping inner layer on the surface of the fruits and vegetables; (Ⅳ) Immerse the fruits and vegetables with a cellulose-based fresh-keeping inner layer formed in them in the chitin-based fresh-keeping liquid obtained in step (Ⅱ), or spray the chitin-based fresh-keeping liquid obtained in step (Ⅱ) on the surface of the cellulose-based fresh-keeping inner layer, and then dry the fruits and vegetables to form a chitin-based fresh-keeping outer layer on the surface of the cellulose-based fresh-keeping inner layer; (Ⅴ) Alternately repeat step (Ⅲ) and step (Ⅳ) at least once.

3. The preparation method according to claim 2, characterized in that, In step (Ⅰ), the preparation process of the cyclodextrin solution comprises: Dissolve cyclodextrin in deionized water at an ambient temperature of 60-70 °C to obtain the cyclodextrin solution; The mass fraction of the cyclodextrin solution is 3-6 wt%; The preparation process of the curcumin solution comprises: Dissolve curcumin in ethanol at an ambient temperature of 60 - 70 °C to obtain the curcumin solution; The concentration of the curcumin solution is 0.005 - 0.01 g / mL; The mixing process of the cyclodextrin solution and the curcumin solution includes: At an ambient temperature of 60 - 70 °C, add the curcumin solution dropwise to the cyclodextrin solution. After all the addition is completed, keep the ambient temperature unchanged and continue stirring for 2 - 4 h; The temperature for cooling crystallization is 1 - 10 °C; The time for cooling crystallization is 8 - 12 h; The temperature for drying is 40 - 60 °C; The temperature for heating is 80 - 90 °C; The time for emulsification is 1 - 10 min; The time for vacuum degassing is 5 - 10 min.

4. The preparation method according to claim 2, characterized in that, In step (Ⅱ), the temperature for heating is 80 - 90 °C; The time for emulsification is 1 - 10 min; The time for vacuum degassing is 5 - 10 min.

5. The preparation method according to claim 2, characterized in that, In step (Ⅲ), the single soaking time is 5 - 60 s; The number of soaking times is 2 - 5 times; The spraying flow rate is 5 - 20 mL / min.

6. The preparation method according to claim 2, characterized in that, In step (Ⅳ), the single soaking time is 5 - 60 s; The number of soaking times is 2 - 5 times; The spraying flow rate is 5 - 20 mL / min.

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