A duck egg coating preservative based on a zein aqueous nanoparticle dispersion and a preparation method thereof

Abstract

The present invention provides a fresh-keeping agent for duck eggs based on a Zein aqueous nanoparticle dispersion liquid and a preparation method thereof. Step S1: Dissolve Zein powder in an ethanol solution, and dissolve chitosan CS powder in an acetic acid solution. After stirring evenly for a certain period of time, obtain a Zein solution and a CS solution respectively. Step S2: Dropwise add the Zein solution in Step S1 into the CS solution, continue to stir evenly for a certain period of time, vacuum concentrate and evaporate to remove ethanol, add distilled water to make up to the original volume, and centrifuge to remove insoluble substances to obtain a Zein / CS nanoparticle dispersion liquid. Step S3: Add glycerol to the Zein / CS nanoparticle dispersion liquid in Step S2, continue to stir evenly for a certain period of time to obtain a Zein / CS nanoparticle coating liquid, which is the fresh-keeping agent for duck eggs. The present invention uses an anti-solvent precipitation technique to prepare a Zein / CS nanoparticle aqueous dispersion liquid with a specific ratio, adds glycerol to the dispersion liquid, and after mixing, dip-coats duck eggs, which can extend the storage period of duck eggs and ensure their freshness.

Description

Technical Field

[0001] The present invention belongs to the technical field of food preservation, and particularly relates to a duck egg coating preservative based on a Zein aqueous nanoparticle dispersion liquid and a preparation method thereof. Background Art

[0002] With the improvement of people's living standards and the enhancement of health awareness, the requirements for the quality of duck eggs are increasing day by day. The quality of duck eggs is affected by various factors, including genetics, variety, nutrition, feeding environment, storage time, and storage environment, etc. The quality of duck eggs is closely related to their edible value and nutritional value. At the same time, the storage time of duck eggs also has an important impact on their quality, and thus has an impact on economic and social benefits. Therefore, scientifically controlling and prolonging the storage time of duck eggs can not only maintain their excellent characteristics but also increase the added value and market competitiveness of the products. Currently, most of the commercially available duck eggs are washed, but this step will damage the cutin membrane of the duck eggs and accelerate the deterioration of their quality.

[0003] For the preservation of duck eggs, domestic and foreign scholars have intervened by means such as temperature control and coating. The temperature control method requires the establishment of a special cold storage, and the later maintenance and upkeep costs of the cold storage are relatively high. In addition, when the duck eggs are taken out of the cold storage, the phenomenon of "sweating" of the duck eggs may occur, the quality will rapidly decline, and the breakage rate will also increase, making it impossible to continue storage. In comparison, the coating method has lower energy consumption. The research focus of the coating method is to find suitable coating raw materials. Coating raw materials are divided into chemical materials and green materials. However, the use of chemical materials does not meet the requirements of green environmental protection and edibility. In contrast, green materials are a more ideal choice.

[0004] Zein has the characteristics of non-toxicity, good biocompatibility, and biodegradability. Due to its excellent gas and moisture barrier properties, it is widely used in the coating preservation of fruits and vegetables, meats, and eggs. However, due to its hydrophobicity, Zein is often dissolved in ethanol aqueous solution or other organic solvents to prepare films or coatings, but the flammability and toxicity of most organic solvents cannot be ignored in the food field. Therefore, existing research has turned to the use of an anti-solvent precipitation method to prepare Zein nanoparticle aqueous dispersion liquids to further develop related packaging systems. However, during the preparation of nanoparticles, single Zein nanoparticles are prone to aggregate and form precipitates at the isoelectric point (pI≈6.2), resulting in poor dispersibility in aqueous solutions, and thus causing waste of Zein resources. When single Zein nanoparticles are used as coatings, the mechanical properties of the coatings are poor. Especially its brittle characteristics, as well as the uneven dispersion of nanoparticles in aqueous solutions, will have a negative impact on the uniformity of the coatings. At the same time, this pure Zein coating also lacks the ability of active antibacterial. Therefore, in coating applications, the performance of single Zein nanoparticles is not ideal. Summary of the Invention

[0005] In view of the above technical problems, the present invention provides a duck egg coating preservative based on a Zein aqueous nanoparticle dispersion and a preparation method thereof. By using an anti-solvent precipitation technique, a Zein / chitosan (CS) nanoparticle aqueous dispersion with a specific ratio is prepared, and glycerol is added to this dispersion. After mixing evenly, duck eggs are dip-coated, which can extend the storage period of duck eggs and ensure their freshness.

[0006] Note that the recitation of these objectives does not preclude the existence of other objectives. One embodiment of the present invention does not necessarily need to achieve all of the above objectives. Objectives other than the above can be extracted from the descriptions in the specification, drawings, and claims.

[0007] The present invention achieves the above technical objectives through the following technical means.

[0008] A preparation method of a duck egg coating preservative based on a Zein aqueous nanoparticle dispersion includes the following steps:

[0009] Step S1: Dissolve Zein powder in an ethanol solution and dissolve CS powder in an acetic acid solution. After stirring evenly for a certain period of time at a constant speed, a Zein solution and a CS solution are obtained respectively.

[0010] Step S2: Dropwise add the Zein solution in Step S1 into the CS solution, continue to stir evenly for a certain period of time, vacuum concentrate and evaporate to remove ethanol, add distilled water to make up the original volume, and centrifuge to remove insoluble substances to obtain a Zein / CS nanoparticle dispersion.

[0011] Step S3: Add glycerol to the Zein / CS nanoparticle dispersion in Step S2, continue to stir evenly for a certain period of time to obtain a Zein / CS nanoparticle coating solution, which is the duck egg coating preservative.

[0012] In the above solution, in the Zein solution in Step S1, the solid-liquid ratio of Zein powder to the ethanol solution is 0.05:100 - 2:100; in the CS solution, the solid-liquid ratio of CS powder to the acetic acid solution is 0.05:100 - 2:100; the volume fraction of the ethanol solution is 60 - 80%, the volume fraction of the acetic acid solution is 0.05 - 2%, and the mass ratio of Zein to CS is 20:1 - 1:2.

[0013] Further, in the Zein solution, the solid-liquid ratio of Zein powder to the ethanol solution is 1:100; in the CS solution, the solid-liquid ratio of CS powder to the acetic acid solution is 0.2:100; the volume fraction of the ethanol solution is 70%; the volume fraction of the acetic acid solution is 1%, and the mass ratio of Zein to CS is 5:1.

[0014] In the above solution, the stirring speed in step S1 is 500 - 700 r / min, and the stirring time is 1 - 12 h.

[0015] In the above solution, the stirring speed in step S2 is 500 - 700 r / min, and the stirring time is 1 - 3 h.

[0016] In the above solution, the vacuum concentration temperature in step S2 is 40 - 50 °C, the vacuum degree is -0.06 - -0.09 MPa, the centrifugal speed is 4000 - 6000 r / min, and the centrifugal time is 10 - 20 min.

[0017] In the above solution, the mass ratio of glycerol to Zein in step S3 is 0.3:1 - 0.5:1.

[0018] In the above solution, the mass ratio of glycerol to Zein in step S3 is 0.4:1.

[0019] In the above solution, the stirring speed in step S3 is 500 - 700 r / min, and the stirring time is 20 - 40 min.

[0020] A duck egg coating preservative based on Zein aqueous nanoparticle dispersion liquid, which is prepared according to the preparation method of the duck egg coating preservative based on Zein aqueous nanoparticle dispersion liquid.

[0021] The determination of the duck egg coating preservative based on Zein aqueous nanoparticle dispersion liquid includes the following steps:

[0022] Step S1): Directly add the Zein solution in step S1 to the CS solution, add glycerol, and continue to stir evenly for a certain time to obtain a Zein / CS blend coating solution.

[0023] Step S2): Pretreat the duck eggs. Wash the fresh duck eggs with clean water and drain them. Then evenly dip the coating solutions in steps S3 and S1) on the surface of the duck eggs, dry them and store them, and take samples to observe the preservation effect.

[0024] In step S1), the mass ratio of glycerol to Zein is 0.3:1 - 0.5:1, preferably 0.4:1, the stirring speed is 500 - 700 r / min, and the stirring time is 20 - 40 min.

[0025] In step S2), an LED cold light egg illuminator is selected to inspect fresh duck eggs, and unqualified eggs such as damaged ones are picked out. Subsequently, the fresh eggs that pass the inspection are placed in clean water for washing, drained after washing and left for use. The dipping time is 1 - 3 minutes. After dipping, they are dried and stored for 30 - 40 days. When sampling and measuring, 5 duck eggs are taken out, and the physical and chemical properties of each duck egg are determined. The egg weight is measured before opening the egg, and the Haugh unit is measured after opening the egg. Then, the egg white and egg yolk are separated with an egg separator and stored in centrifuge tubes and petri dishes respectively for preparing to measure the pH value of the egg white and the yolk index. After the eggshell is washed and naturally dried, its microscopic morphology and contact angle are measured.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] (1) Compared with traditional duck egg coating agents such as vegetable oil, mineral oil, paraffin wax, etc., the coating matrix zein used in the present invention has the advantages of being natural, non-toxic, edible, good biocompatibility and biodegradability, excellent gas / oil-water barrier performance and tough texture.

[0028] (2) The present invention first uses the aqueous dispersion of zein nanoparticles prepared by the anti-solvent precipitation method as a coating agent for duck egg preservation, avoiding the safety hazards brought by organic solvents.

[0029] (3) The present invention improves the traditional anti-solvent precipitation technology. By introducing CS as a stabilizer for the aqueous dispersion of zein nanoparticles, the utilization rate of zein is improved, the uniformity of the obtained coating is improved, and CS also endows the coating with certain active antibacterial properties, and the raw materials are widely sourced and easily obtained.

[0030] (4) Under the same formulation, compared with the traditional blended ethanol-based coating, the film-forming characteristics of the coating with a nanostructure and the preservation effect on duck eggs are not weakened.

[0031] Note that the recording of these effects does not prevent the existence of other effects. A mode of the present invention does not necessarily have all the above effects. Effects other than the above can be obviously seen and extracted from the descriptions in the specification, drawings, claims, etc. Description of the Drawings

[0032] Figure 1 It is a model for measuring the water vapor, carbon dioxide and oxygen permeation rates of uncoated, blended coating and nano-coating used in the specific embodiment of the present invention;

[0033] Figure 2SEM images of the cross-section and surface of uncoated, blend-coated, and nano-coated eggshells in the specific embodiments of the present invention; where A1 is the cross-section SEM image of an uncoated duck eggshell, A2 is the cross-section SEM image of a blend-coated duck eggshell, A3 is the cross-section SEM image of a nano-coated duck eggshell; B1 is the surface SEM image of an uncoated duck eggshell, B2 is the surface SEM image of a blend-coated duck eggshell, B3 is the surface SEM image of a nano-coated duck eggshell;

[0034] Figure 3 Column chart of the contact angle of uncoated, blend-coated, and nano-coated eggshells in the specific embodiments of the present invention;

[0035] Figure 4 Schematic diagram of the contact angle of uncoated, blend-coated, and nano-coated eggshells in the specific embodiments of the present invention;

[0036] Figure 5 Transmittance diagram of the blend coating solution and nano coating solution in the specific embodiments of the present invention. Specific embodiments

[0037] The present invention will be further described in detail below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto. The reagents, materials, etc. used in the following examples can be obtained from commercial sources without special instructions.

[0038] Example 1

[0039] Dissolve 1.00 g of Zein in 100 mL of 70% ethanol and stir at 600 r / min for 1 h to ensure complete dissolution of Zein, obtaining a Zein solution. Dissolve 0.05 g of CS in 100 mL of 1% (v / v) acetic acid aqueous solution and place it in the refrigerator for overnight hydration to obtain a CS solution. Under continuous stirring (600 r / min), slowly add the Zein solution dropwise to the CS solution and then stir at 600 r / min for another 30 min to obtain a Zein / CS nano-dispersion. Vacuum concentrate the Zein / CS nano-dispersion (45 °C and -0.08 MPa) to remove ethanol, add distilled water to supplement to the original volume, and centrifuge at 5000 r / min for 15 min. The obtained supernatant is the Zein / CS nano-aqueous dispersion. The specific detection methods are as follows:

[0040] Particle size, PDI, and Zeta-potential: At room temperature, the particle size, PDI, and Zeta-potential of the composite nanoparticles are measured by dynamic light scattering. The particle size of the sample is calculated based on the Stokes-Einstein equation and the Smoluchowski model respectively. Dilute the nanoparticle dispersion to a suitable concentration before measurement to avoid multiple scattering effects. Each sample is measured in parallel three times and the average value is taken.

[0041] As shown in Table 1, the particle size of the obtained composite nanoparticles was 172.21 nm, the PDI was 0.16, and the Zeta-potential was 39.56 mV.

[0042] Example 2

[0043] Dissolve 1.00 g of Zein in 100 mL of 70% ethanol and stir at 600 r / min for 1 h to ensure complete dissolution of Zein to obtain a Zein solution. Dissolve 0.20 g of CS in 100 mL of 1% (v / v) acetic acid aqueous solution and place it in the refrigerator for overnight hydration to obtain a CS solution. Under continuous stirring (600 r / min), slowly add the Zein solution dropwise to the CS solution and stir at 600 r / min for another 30 min to obtain a Zein / CS nano-dispersion. Vacuum concentrate the Zein / CS nano-dispersion (45 °C and -0.08 MPa) to remove ethanol, add distilled water to make up the original volume, and centrifuge at 5000 r / min for 15 min. The obtained supernatant is the Zein / CS nano-aqueous dispersion. The specific detection method refers to Example 1.

[0044] As shown in Table 1, the particle size of the obtained composite nanoparticles was 140.64 nm, the PDI was 0.17, and the Zeta-potential was 46.90 mV.

[0045] Example 3

[0046] Dissolve 1.00 g of Zein in 100 mL of 70% ethanol and stir at 600 r / min for 1 h to ensure complete dissolution of Zein to obtain a Zein solution. Dissolve 2.00 g of CS in 100 mL of 1% (v / v) acetic acid aqueous solution and place it in the refrigerator for overnight hydration to obtain a CS solution. Under continuous stirring (600 r / min), slowly add the Zein solution dropwise to the CS solution and stir at 600 r / min for another 30 min to obtain a Zein / CS nano-dispersion. Vacuum concentrate the Zein / CS nano-dispersion (45 °C and -0.08 MPa) to remove ethanol, add distilled water to make up the original volume, and centrifuge at 5000 r / min for 15 min. The obtained supernatant is the Zein / CS nano-aqueous dispersion. The specific detection method refers to Example 1.

[0047] As shown in Table 1, the particle size of the obtained composite nanoparticles was 711.50 nm, the PDI was 0.22, and the Zeta-potential was 47.82 mV. With the increase in the amount of CS, the particle size of the Zein / CS nanoparticles showed an overall increasing trend. This was because the excessive addition of CS and its relatively large molecular weight could form larger self-assembled nanospheres with proteins. The composite particles were polydisperse, resulting in serious flocculation phenomena. With the increase in the CS addition amount, the potential value of the composite also increased. The negatively charged sites on the Zein molecules were not sufficient to neutralize the positive charges of the CS molecules, thus increasing the number of positive charges in the system. Maintaining a smaller particle size of the nanocomposite was beneficial for exerting the advantage of its high surface area, forming a denser structure, and thus improving the performance of the nanocomposite material. The Zeta-potential is a key indicator of the stability of a colloidal dispersion system, reflecting the electrostatic repulsion degree between adjacent charged particles. The higher the absolute value, the higher the stability of the dispersion system. Therefore, the composite with a Zein and CS ratio of 5:1 was selected for subsequent research.

[0048] Table 1 Colloidal properties of Zein / CS nanoparticles with different mass ratios

[0049]

[0050] Example 4

[0051] According to the method of Example 2, 1.00 g of Zein was dissolved in 100 mL of 70% ethanol and stirred at 600 r / min for 1 h to ensure complete dissolution of Zein, obtaining a Zein solution. 0.20 g of chitosan was dissolved in 100 mL of 1% (v / v) acetic acid aqueous solution and hydrated overnight in the refrigerator to obtain a chitosan solution. Under continuous stirring (600 r / min), the Zein solution was slowly added dropwise to the chitosan solution and stirred at 600 r / min for another 30 min to obtain a Zein / CS nano-dispersion. The Zein / CS nano-dispersion was vacuum concentrated (45 °C and -0.08 MPa) to remove ethanol, distilled water was added to make up the original volume, and it was centrifuged at 5000 r / min for 15 min. The resulting supernatant was the Zein / CS nano-aqueous dispersion. The specific detection methods are as follows:

[0052] Storage stability of the composite nanoparticles: The composite nanoparticles were stored at 4 °C and 25 °C for 21 days. Referring to the detection method in Example 1, the particle size, PDI, and Zeta-potential of the composite nanoparticles were measured.

[0053] Table 2 Effects of storage at 4 °C and 25 °C for 21 days on the colloidal properties of nanoparticles

[0054]

[0055] As shown in Table 2, the entire storage process had little effect on the particle size, PDI, and Zeta-potential of the composite nanoparticles. Under different conditions, the nanoparticles were relatively stable against aggregation during the entire storage process, which was attributed to the improvement of the properties of Zein nanoparticles by CS. Zein and CS could cooperate to prevent particle aggregation through hydrogen bonding and electrostatic interactions. The environments that the nanoparticles contacted under the two storage conditions were different, which led to the difference in Zeta-potential. When the nanoparticles were stored at high temperature or for a long time, their conformation might change, which might be due to the decomposition of the polymer network, resulting in a slight decrease in their stability. On the contrary, the nanoparticles showed better stability under low-temperature storage, indicating that the composite system had stronger stabilizing ability.

[0056] Example 5

[0057] According to the method of Example 2, 1.00 g of Zein was dissolved in 100 mL of 70% ethanol, and stirred at 600 r / min for 1 h to ensure complete dissolution of Zein, obtaining a Zein solution. 0.20 g of chitosan was dissolved in 100 mL of 1% (v / v) acetic acid aqueous solution and hydrated overnight in the refrigerator to obtain a chitosan solution. Under continuous stirring (600 r / min), the Zein solution was slowly added dropwise to the chitosan solution, and then stirred at 600 r / min for another 30 min to obtain a Zein / CS nano-dispersion. The Zein / CS nano-dispersion was vacuum concentrated (45 °C and -0.08 MPa) to remove ethanol, distilled water was added to make up the original volume, and centrifuged at 5000 r / min for 15 min. The obtained supernatant was the Zein / CS nano-aqueous dispersion. The specific detection methods are as follows:

[0058] Redispersibility of the composite nanoparticles: 50 mg of freeze-dried powdered composite nanoparticles were dispersed in 10 mL of deionized water to reach a protein concentration of 5 mg / mL. After the solution was stirred at 600 rpm for 2 h. Refer to the detection method in Example 1 to measure the particle size, PDI, and Zeta-potential of the composite nanoparticles.

[0059] The Zeta-potential of the nanoparticles after freeze-drying and reconstitution was 45.98 mV, and the particle size and PDI were 278.22 nm and 0.28, respectively, which were close to the original values described in Example 2. The colloidal properties of the nanoparticles remained stable after redispersion, indicating that freeze-drying did not damage the structural composition of the nanoparticles.

[0060] Example 6

[0061] Preparation of nano - coating: Dissolve 1.00 g of Zein in 100 mL of 70% ethanol, stir at 600 r / min for 1 h to ensure complete dissolution of Zein, and obtain a Zein solution. Dissolve 0.20 g of CS in 100 mL of 1% (v / v) acetic acid aqueous solution, and place it in the refrigerator for overnight hydration to obtain a CS solution. Under continuous stirring (600 r / min), slowly add the Zein solution drop - by - drop to the CS solution, and then stir at 600 r / min for another 30 min to obtain a Zein / CS nano - dispersion. Vacuum - concentrate the Zein / CS nano - dispersion (at 45 °C and - 0.08 MPa) to remove ethanol, add distilled water to make up to the original volume, and centrifuge at 5000 r / min for 15 min. The obtained supernatant is the Zein / CS nano - aqueous dispersion. Add 0.4 g of glycerol to the Zein / CS nanoparticle dispersion, continue to stir at a constant speed for a certain time to obtain a Zein / CS nanoparticle coating solution.

[0062] Preparation of blend coating: Dissolve 1.00 g of Zein in 100 mL of 70% ethanol, stir at 600 r / min for 1 h to ensure complete dissolution of Zein, and obtain a Zein solution. Dissolve 0.20 g of CS in 100 mL of 1% (v / v) acetic acid aqueous solution, and place it in the refrigerator for overnight hydration to obtain a CS solution. Under continuous stirring (600 r / min), directly add the Zein solution to the CS solution, then add 0.4 g of glycerol, and stir at 600 r / min for another 30 min to obtain a Zein / CS blend coating solution.

[0063] After fresh duck eggs are candled and unqualified eggs are removed, they are subjected to coating for preservation. Immerse the cleaned duck eggs in the coating solution for 2 min and then take them out to dry naturally. After coating, the duck eggs are stored in a constant - temperature and humidity - controlled chamber at 25 °C and a relative humidity of 70%. Egg white samples are taken from each duck egg on the 0th, 7th, 14th, 21st, 28th, and 35th days. After each sample is stirred evenly, the pH value is directly measured with a pH meter (Table 3); Open each egg in the middle and pour the contents onto a horizontal glass plane. Measure the height of the widest part of the thick egg white 1 cm away from the egg yolk as the height of the egg white. Calculate the Haugh unit according to the following formula: Haugh unit = 100Lg(H - 1.7W 0.37 + 7.6), where H is the height of the egg white, in mm; W is the weight of the egg, in g; 100, 1.7, and 7.6 are conversion coefficients (Table 4); Place the egg yolk of each duck egg on a clean petri dish, let it stand for 3 min, measure the height and diameter of the egg yolk, calculate the yolk index, and take the average value. The calculation formula is as follows: where h is the height of the egg yolk, in mm; d is the diameter of the egg yolk, in mm (Table 5). Uncoated duck eggs are used as positive controls.

[0064] As shown in Table 3, during storage, the pH value of the egg white in the coated group was significantly lower than that in the uncoated group. This may be because the coating can enhance the barrier performance of the eggshell, thereby retaining carbon dioxide and water, and also preventing the oxidation of the duck egg contents and the invasion of microorganisms. Among them, there was no significant difference in the fresh-keeping effect of the nano-coated group and the blend-coated group on duck eggs. In addition, the pH value of the egg white in all duck egg samples showed a trend of first increasing and then decreasing. The carbonic acid content in the fresh duck egg white was about 1.44 - 2.05 mg / g. At the initial stage of storage, the diffusion rate of carbon dioxide was relatively fast, which led to an increase in the pH value of the egg white. However, as the storage time extended, the diffusion rate of carbon dioxide gradually slowed down, resulting in a decrease in the pH value of the egg white. Therefore, although the pH value of the egg white in the nano-coated and blend-coated groups still increased, its rate was effectively controlled.

[0065] Table 3 Effects of coatings on the pH of duck egg white during storage

[0066]

[0067] As shown in Table 4, during storage, the Haugh unit of all duck egg samples decreased, which was due to the physiological activities of the duck eggs themselves and the action of various enzymes, resulting in the destruction of the colloidal structure of the thick egg white. At the same time, the decomposition of ovomucin and its complexes also led to a decrease in the viscosity of the egg white, thereby reducing the content of the thick egg white. The Haugh unit of the coated duck eggs was significantly higher than that of the uncoated group. It was because the coating could cover the tiny pores on the eggshell surface, slow down the diffusion of carbon dioxide generated by the decomposition of organic compounds, and at the same time, in an alkaline environment, the disulfide bonds of ovomucin in the egg white depolymerized, resulting in the destruction of the structure of the ovomucin-lysozyme complex, and then leading to a decrease in the Haugh unit, which was consistent with the result of the egg white pH value. The relatively stable pH value of the egg white in the coated duck eggs was helpful to maintain the content of the thick egg white and a relatively high viscosity of the egg white, thereby delaying the decline of the Haugh unit value.

[0068] Table 4 Effects of coatings on the Haugh unit of duck eggs during storage

[0069]

[0070] As shown in Table 5, the yolk index can be used as an indirect measure of the yolk membrane strength, reflecting the overall properties of the yolk and the egg quality. The yolk index of the coated duck eggs was significantly higher than that of the uncoated group because the coating inhibited the rate of water penetration from the egg white into the yolk and slowed down the weakening rate of the yolk membrane. Therefore, the water molecule activity of the duck eggs in the coated group was inhibited, oxygen was blocked, and the lipids in the yolk were difficult to oxidize. These factors jointly delayed the hydration of the egg white and the softening of the yolk, ultimately resulting in a slow decline in the yolk index. This was consistent with the results of the egg white pH value and the Haugh unit. Therefore, the relatively stable water penetration and oxygen barrier in the coated duck eggs could delay the decline of the yolk index, indicating that the fresh-keeping effect of the nano-coated duck eggs was similar to that of the blend-coated ones.

[0071] Table 5 Effect of coatings on the yolk index of duck eggs during storage

[0072]

[0073] Example 7

[0074] The preparation of the nano - coating solution and the blend - coating solution and the coating of duck eggs refer to Example 6, and the water vapor permeability, carbon dioxide and oxygen permeability of the coated eggshells were measured. The specific detection methods are as follows: (1) Water vapor permeability: The pretreatment of the eggshell is as Figure 1 shown. Mark the duck eggs according to the size of the conical flask mouth, then cut out the top eggshell of the duck eggs with a cutter, wash and air - dry them naturally for standby. The eggshell fits the mouth of a 50 mL conical flask with a sealing film on the inner wall. Add 3.0 g of anhydrous calcium chloride into the conical flask, and cover the mouth of the conical flask with the prepared eggshell. Apply vaseline to seal the gap between the eggshell and the conical flask. Weigh the whole as a unit, put it into a constant temperature and humidity chamber at 25 °C and a relative humidity of 83%, and measure the sample mass on the 7th day. The calculation formula for water vapor permeability is as follows where the water vapor permeability, g·mm·m -2 ·d -1 ·KPa -1 ; W is the total increased mass, g; D is the coating thickness, mm; t is the test time, d; A is the coating area, m 2 ; ΔP is the water vapor pressure difference inside and outside the film. In this experiment, ΔP is 2.52936 KPa (Table 6).

[0075] (2) Carbon dioxide and oxygen permeability: The treatment method of the eggshell is the same as that of water vapor permeability, but saturated potassium hydroxide solution and FeSO4·7H2O are used instead of anhydrous calcium chloride. Place the sample in a constant temperature and humidity chamber at 25 °C and a relative humidity of 83% for 7 days, and record the increased mass of the saturated potassium hydroxide solution and FeSO4·7H2O. The calculation formulas for carbon dioxide and oxygen permeability are as follows:

[0076]

[0077]

[0078] In the formula: The carbon dioxide and oxygen permeability, g·mm·m -2 ·d -1 ; Δm is the total weight gain, g; L is the coating thickness, mm; t is the test time, d; A is the coating area, m 2 .

[0079] As shown in Table 6, there is no significant difference in the barrier performance between the nano-coating and the blend coating. The water vapor transmission rate, oxygen transmission rate, and carbon dioxide transmission rate of the coated group are all lower than those of the uncoated group. This is because the nanoparticles occupy the pores of the coating, forming a denser network structure, which is beneficial to improving the water resistance of the coating. Generally speaking, the hydroxyl groups of biopolymers will combine with water molecules, thus promoting the transmission of water vapor in the coating. In addition, the solubility of biopolymers is also closely related to the water vapor permeability. Compared with pure CS and pure Zein, CS may increase the cross-linking of Zein and reduce the free volume of the polymer mechanism. Moreover, the reduction of the electrostatic repulsion force between CS and Zein and the enhancement of the intermolecular interaction reduce the diffusion rate of water molecules in the coating, thereby resulting in a low water vapor transmission rate of the coating. There is no significant difference in the carbon dioxide and oxygen transmission rates between the coated groups for the following reasons: The blend coating has a better gas barrier due to the presence of polar interactions in its structure and generates vertical layers to prevent gas transfer in the blend coating. The nano-coating may limit the chain mobility due to the interaction between nanoparticles, forming a dense structure, thus reducing gas diffusion. The oxygen transmission rate and carbon dioxide transmission rate values of the coating are different. The carbon dioxide transmission rate is about seven times higher than its corresponding oxygen transmission rate, which can be attributed to the differences in the resistance to the diffusion of different gases caused by polymer interactions.

[0080] Table 6 Water vapor, carbon dioxide, and oxygen transmission rates of blend coatings and nanoparticle coatings

[0081]

[0082] Example 8

[0083] The preparation of the nano-coating solution and the blend coating solution and the duck egg coating refer to Example 6. After the eggshells are washed and air-dried naturally, the eggshells are cut into small pieces about 5 mm in size, stuck on the sample plate, sputter-coated with gold, and the cross-section and surface morphology are observed by scanning electron microscopy (SEM).

[0084] As Figure 2 shown in B1, the surface of the untreated eggshell is covered with fine cracks. As Figure 2 shown in B2-3, the cracks on the surface of the eggshell after coating are reduced. Among them, the surface of the eggshell in the nano-coating group is smoother and denser. As Figure 2 shown in A1-3, a more uniform film is formed on the cross-section of the eggshell after being treated with the coating prepared by the method of the present invention, thereby enhancing the adhesion between the coating and the duck egg. In addition to the coating sealing the eggshell, the change in the structure of the eggshell itself can also reduce gas exchange. The coating solution contains a certain amount of acetic acid. Due to the high calcium content in the eggshell, the acetic acid in the coating solution will react with calcium, resulting in a possible reduction in the thickness of the outer shell surface. However, the product of this reaction will fill the pores of the eggshell, thereby reducing the gas penetration between the inside of the egg and the external environment.

[0085] Example 9

[0086] The preparation of the nano - coating solution and the blend - coating solution and the duck - egg coating refer to Example 6. After the eggshells are washed and air - dried naturally, the eggshells are fixed on a movable platform, and 2 μL of water droplets are placed on the coating surface. The contact - angle images are captured by a camera, and the contact angles on each side of the water droplet are calculated using the Young / Laplace method of CAM 2008 software.

[0087] As Figure 3 and Figure 4 shown, the contact angles of the untreated, blend - coated, and nano - coated eggshells are 92.13°, 94.15°, and 107.93° respectively. The large contact angle of the water droplet on the nano - coating surface indicates that the interaction between the nano - coating and the water droplet is weak, and the nano - coating has excellent waterproof performance. The hydrophobicity of the nano - coating is stronger than that of the blend - coating, which may be because the small - sized nanoparticles can fill the larger pores in the coating, thus reducing the pores on the coating surface and making the structure more compact. In addition, the nanoparticles on the nano - coating surface can form more tiny protrusions and depressions, thus increasing the surface roughness. This rough surface can make the water droplets more likely to form spherical shapes on the surface and less likely to flow or penetrate on the surface, which is beneficial to resisting the water loss in the duck - egg contents during storage.

[0088] Example 10

[0089] The preparation of the nano - coating solution and the blend - coating solution and the duck - egg coating refer to Example 6. The above solutions are coated on one side of a standard cuvette and placed in a sample cell after complete drying. The transmittance of the coating in the wavelength range of 200 - 800 nm is measured using a UV - Vis spectrophotometer.

[0090] The CS coating has the highest transmittance. Compared with the Zein coating, the nano - coating has a high transmittance, and the blend - coating has a low transmittance ( Figure 5)。The transmittance is used to characterize the transparency of the coating to judge the compatibility degree of Zein and CS. The higher transmittance of the nano - coating may be due to the small average particle size, which enhances its Tyndall effect. At the same time, with a high compatibility degree of Zein and CS, the coating network structure becomes more orderly and uniform, improving the transmittance. Meanwhile, the transmittance of the coating can prevent or reduce the adverse effects of ultraviolet or visible light, such as lipid oxidation, color change, bacterial growth, the formation of degradation reactions, and odor. Ultraviolet light can be divided into UV - A (320 - 400 nm), UV - B (290 - 320 nm), and UV - C (100 - 290 nm), and the most energetic ultraviolet light that causes the greatest damage to the coating is UV - B

[171] . Compared with the blend coating, the transmittance of the nano - coating has been significantly improved. This means that by modifying Zein into nanoparticles, the ultraviolet - blocking performance of the coating can be significantly changed. This is mainly because Zein contains a large number of aromatic amino acids, and the benzene ring structure can effectively absorb ultraviolet light. In summary, the high transmittance of the nano - coating can be used to improve the product appearance.

[0091] It should be understood that although this specification is described according to each embodiment, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0092] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present invention, and they are not used to limit the protection scope of the present invention. Any equivalent embodiments or changes made without departing from the technical spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A preparation method of a fresh-keeping agent for duck eggs coated with a Zein aqueous nanoparticle dispersion liquid, characterized in that, It includes the following steps: Step S1: Dissolve zein powder in an ethanol solution and dissolve chitosan CS powder in an acetic acid solution. After stirring evenly for a certain period of time, a zein solution and a CS solution are obtained respectively; Step S2: Dropwise add the zein solution in Step S1 into the CS solution, continue to stir evenly for a certain period of time, vacuum concentrate to evaporate ethanol, add distilled water to make up to the original volume, and centrifuge to remove insoluble substances to obtain a zein / CS nanoparticle dispersion; Step S3: Add glycerol to the zein / CS nanoparticle dispersion in Step S2, continue to stir evenly for a certain period of time to obtain a zein / CS nanoparticle coating solution, which is the fresh-keeping agent for duck eggs; In Step S1, the solid-liquid ratio of zein powder to ethanol solution in the zein solution is 1:100; the solid-liquid ratio of chitosan CS powder to acetic acid solution in the CS solution is 0.2:100; the volume fraction of the ethanol solution is 70%; the volume fraction of the acetic acid solution is 1%, and the mass ratio of zein to CS is 5:

1.

2. The preparation method of the duck egg coating preservative based on the Zein aqueous nanoparticle dispersion liquid according to claim 1, characterized in that, In Step S1, the stirring speed is 500 - 700 r / min, and the stirring time is 1 - 12 h.

3. The preparation method of the duck egg coating preservative based on the Zein aqueous nanoparticle dispersion according to claim 1, characterized in that In Step S2, the stirring speed is 500 - 700 r / min, and the stirring time is 1 - 3 h.

4. The preparation method of the duck egg coating preservative based on the Zein aqueous nanoparticle dispersion liquid according to claim 1, characterized in that, In Step S2, the vacuum concentration temperature is 40 - 50 °C, the vacuum degree is -0.06 - -0.09 MPa, the centrifugal speed is 4000 - 6000 r / min, and the centrifugal time is 10 - 20 min.

5. The preparation method of the duck egg coating preservative based on the Zein aqueous nanoparticle dispersion liquid according to claim 1, characterized in that, In Step S3, the mass ratio of glycerol to zein is 0.3:1 - 0.5:

1.

6. The preparation method of the duck egg coating preservative based on the Zein aqueous nanoparticle dispersion according to claim 5, characterized in that, In Step S3, the mass ratio of glycerol to zein is 0.4:

1.

7. The preparation method of the duck egg coating preservative based on the Zein aqueous nanoparticle dispersion liquid according to claim 1, characterized in that In Step S3, the stirring speed is 500 - 700 r / min, and the stirring time is 20 - 40 min.

8. A fresh-keeping agent for duck eggs based on a Zein aqueous nanoparticle dispersion, characterized in that, It is prepared by the preparation method of the fresh-keeping agent for duck egg coatings based on zein aqueous nanoparticle dispersion according to any one of claims 1 - 7.

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