Coating film for salmon preservation, freshness detection indicating film and preparation method of coating film and freshness detection indicating film

Abstract

The invention provides a coating film for salmon preservation, a freshness detection indicating film and a preparation method thereof, and relates to the technical field of green intelligent food packaging. According to the membrane, natural extract tea polysaccharide is combined with epigallocatechin gallate (EGCG), chitosan and a plasticizer, and chitosan / tea polysaccharide-EGCG membrane liquid of nanoparticles, namely CS / CTE membrane liquid, is prepared; the CS / CTE film liquid is used for treating the surface of salmon meat to form a coating film with a fresh-keeping effect, so that the storage life of the salmon meat can be effectively prolonged, and the food safety performance and the practical value are improved; besides, the freshness indicating film prepared from the CS / CTE film liquid can reflect and respond to fish meat quality changes, is expected to be applied to real-time monitoring of freshness of other foods, and has a very good market prospect.

Description

Technical Field

[0001] The invention relates to the technical field of green and intelligent food packaging, and in particular to a coating film and a freshness detection indicator film for salmon preservation and a preparation method thereof. Background Art

[0002] Salmon, also known as salmon or king salmon, belongs to the order Salmoniformes among bony fish and is a high-quality aquatic resource. It is known for its high protein, low calories, and rich in various vitamins and minerals, so it has high food nutritional value and breeding economic benefits.

[0003] About 85% of salmon imported into the Chinese market are freshly caught, while more than 90% of imported salmon are whole fish that have not been deeply processed. Against the backdrop of the rapid development of China's fishery industry, the processing rate of salmon and other aquatic products is relatively low. This is mainly because the fat in salmon is easily oxidized, and its high protein and high water content make it more likely for microorganisms to reproduce. These factors work together to make salmon more susceptible to corruption during processing, storage and sales.

[0004] Although there are some methods of preserving seafood using plastic wrap liquid in current seafood preservation technology, these methods are often cumbersome to operate and the preservation effect is not satisfactory. In addition, the chemical reagents contained in some plastic wrap liquids are not completely harmless and may affect food safety. Therefore, the development of environmentally friendly and harmless preservation and freshness indicator materials has become an urgent need in the industry.

[0005] Epigallocatechin gallate (EGCG), as the main active ingredient in green tea, exhibits multiple mechanisms of action in meat preservation due to its potent antioxidant and antibacterial properties. EGCG has a significant inhibitory effect on fish spoilage bacteria (such as Pseudomonas), and combined with the oxygen barrier properties of the polysaccharide film, it can reduce the generation of TVB-N (volatile basic nitrogen). However, EGCG is easily degraded by pH, temperature and light, and the release rate of EGCG in the composite film needs to be precisely controlled to ensure that its biological activity can be fully and lastingly exerted.

[0006] In view of the problems existing in the application of EGCG in existing composite films, in order to more accurately control the effective release rate of EGCG to improve the effectiveness of its biological activity, thereby effectively improving the preservation effect of salmon, and at the same time developing an intelligent active indicator material for monitoring food quality and optimizing its commercial application value, the present invention provides a coating film and a freshness detection indicator film for salmon preservation and a preparation method thereof, in order to monitor the changes in fish quality while preserving the fish. Summary of the invention

[0007] One of the objectives of the present invention is to provide a coating film for salmon preservation. The raw materials for preparing the coating film include tea polysaccharide, epigallocatechin gallate (EGCG) powder, chitosan, and a plasticizer. The specific preparation steps are as follows:

[0008] ① Prepare a 2 - 8 mg / mL tea polysaccharide solution, add epigallocatechin gallate (EGCG) powder with a mass - to - volume ratio of 0.01% - 0.5% thereto, and mix evenly to obtain a tea polysaccharide - EGCG solution;

[0009] ② Add the tea polysaccharide - EGCG solution to a 0.2 - 1 mg / mL chitosan acetate solution at a volume ratio of 1:1 - 1:1.5 to obtain a tea polysaccharide - EGCG - chitosan acetate solution;

[0010] ③ Adjust the pH of the tea polysaccharide - EGCG - chitosan acetate solution to 3.5 - 5.0, and continuously stir for 2 - 3 h to obtain a chitosan / tea polysaccharide - EGCG nano - suspension;

[0011] ④ Mix the chitosan / tea polysaccharide - EGCG nano - suspension and a 15 - 25 mg / mL matrix chitosan solution at a volume ratio of 3:2 - 2:1, stir for 1 - 2 h, and add the plasticizer glycerol during stirring, with the concentration accounting for 30% - 50% of the matrix chitosan concentration, to obtain a CS / CTE membrane solution. After attaching the CS / CTE membrane solution to the surface of salmon and drying, a coating film for salmon preservation is obtained.

[0012] Another objective of the present invention is to provide an indicator film for detecting the freshness of salmon. Cast the CS / CTE membrane solution in a mold to form a film with a thickness of 0.05 - 0.09 mm, demold after drying at 40 - 50 °C for 6 - 12 h, and dry and equilibrate at 20 - 25 °C and 50 - 55% RH for 36 - 48 h to obtain an indicator film for detecting the freshness of salmon.

[0013] Another objective of the present invention is to provide a preparation method for a coating film for salmon preservation, and the preparation method includes the following steps:

[0014] a. Prepare a 2 - 8 mg / mL tea polysaccharide solution, add epigallocatechin gallate (EGCG) powder with a mass - to - volume ratio of 0.01% - 0.5% thereto, and mix evenly to obtain a tea polysaccharide - EGCG solution;

[0015] b. Add the tea polysaccharide - EGCG solution to a 0.2 - 1 mg / mL chitosan acetate solution at a volume ratio of 1:1 - 1:1.5 to obtain a tea polysaccharide - EGCG - chitosan acetate solution;

[0016] c. Adjust the pH of the tea polysaccharide-EGCG-chitosan acetate solution to 3.5 - 5.0, and continuously stir for 2 - 3 h to obtain a chitosan / tea polysaccharide-EGCG nano-suspension;

[0017] d. Mix the chitosan / tea polysaccharide-EGCG nano-suspension and a 15 - 25 mg / mL matrix chitosan solution at a volume ratio of 3:2 - 2:1, and stir for 1 - 2 h. While stirring, add the plasticizer glycerol, with a concentration accounting for 30 - 50% of the matrix chitosan concentration, to obtain a CS / CTE membrane solution. After attaching the CS / CTE membrane solution to the surface of the salmon and drying, the coating film for salmon preservation is obtained.

[0018] The fourth object of the present invention is to provide a preparation method for an indicator film for detecting the freshness of salmon, and the specific steps are as follows:

[0019] 1. Under stirring conditions, prepare a tea polysaccharide solution, add EGCG powder thereto and mix evenly to obtain a tea polysaccharide-EGCG solution;

[0020] 2. Add the tea polysaccharide-EGCG solution to a 0.2 - 1 mg / mL chitosan acetate solution at a volume ratio of 1:1 - 1:1.5 to obtain a tea polysaccharide-EGCG-chitosan acetate solution; adjust the pH to 3.5 - 5.0 with sodium hydroxide, and then continuously stir for 2 - 3 h to obtain a chitosan / tea polysaccharide-EGCG nano-suspension;

[0021] 3. Under stirring conditions, add the chitosan / tea polysaccharide-EGCG nano-suspension to the matrix chitosan solution, mix and stir for 1 - 2 h. While stirring, add the plasticizer glycerol to obtain a chitosan / tea polysaccharide-EGCG membrane solution, abbreviated as CS / CTE membrane solution;

[0022] 4. Cast the CS / CTE membrane solution in a mold to form a membrane with a thickness of 0.05 - 0.09 mm. After drying and demolding, dry and equilibrate for 36 - 48 h under the conditions of 20 - 25 °C and 50 - 55% RH to obtain an indicator film for freshness detection.

[0023] Among them, the concentration of the prepared tea polysaccharide solution is 2 - 8 mg / mL, and the addition amount of EGCG is 0.01% - 0.5% according to the mass-to-volume ratio.

[0024] Among them, the mass concentration of the matrix chitosan solution is 15 - 25 mg / mL.

[0025] Among them, the chitosan / tea polysaccharide-EGCG nano-suspension and the matrix chitosan solution are mixed at a volume ratio of 3:2 - 2:1.

[0026] Among them, the added plasticizer is glycerol, and its concentration is 30 - 50% of the mass of the matrix chitosan.

[0027] Among them, the drying temperature is 40 - 50 °C, and the drying time is 6 - 12 h. Description of the Drawings

[0028] Figure 1 This is the appearance of the fish meat stored at 4 °C after the control group and the membrane solutions of each group in the present invention are used to treat salmon.

[0029] Figure 2 This is the changes in pH, MDA, and TVB-N of the fish meat stored at 4 °C after the control group and the membrane solutions of each group in the present invention are used to treat salmon.

[0030] Figure 3 This is the response of the indicator membrane to the change in fish meat freshness in the present invention.

[0031] Figure 4 This is the correlation between TVB-N and ΔE values during the storage of CS / CTE - 0.1% and CS / CTE - 0.5% indicator membranes.

[0032] Figure 5 This is the change in color parameters of the CS / CTE - 0.5% indicator membrane during the storage of fish meat in the present invention.

[0033] Figure 6 This is the color parameters and photographic images of indicator membranes with different EGCG concentrations.

[0034] Advantageous Effects

[0035] The tea polysaccharide and chitosan used in the present invention are natural polysaccharides that can be extracted from tea leaves and crab shells. As common acidic and basic polysaccharides in nature, they have opposite charges. In addition, chitosan itself has excellent film-forming properties, but the single chitosan film lacks biological activity after formation. EGCG is a key bioactive component in green tea, which has a significant inhibitory effect on specific spoilage bacteria (SSO) such as Pseudomonas and Shewanella that cause fish spoilage, which are Gram-negative bacteria. Under refrigeration conditions, EGCG can reduce the decomposition of proteins by inhibiting the growth and reproduction of microorganisms, reduce the content of volatile basic nitrogen (TVB-N) in fish, reduce the odor substances (such as trimethylamine and putrescine) produced by the metabolism of spoilage bacteria, and reduce the spoilage odor of fish. EGCG indirectly reduces the favorable environment for bacterial growth by scavenging free radicals and inhibiting lipid oxidation. In addition, due to the different color changes of EGCG in different pH value environments and its high sensitivity, it can be used as a pH indicator to indicate the freshness of food. By means of the intermolecular hydrogen bond and electrostatic interaction between tea polysaccharide and chitosan, self-assembly forms nanoparticles, encapsulating EGCG to form chitosan / tea polysaccharide-EGCG nanoparticles, which can significantly improve the light and heat stability of EGCG and improve its biological activity. Mix the nanoparticles with the matrix chitosan to form a CS / CTE film; the CS / CTE film solution formed in the present invention not only delays the spoilage rate of salmon meat and has an obvious freshness preservation effect, but also the dried film has an indication stability for salmon;

[0036] The indicator film described in the present invention is an environmentally friendly edible coating prepared by using nanotechnology. The coating is prepared by incorporating chitosan / tea polysaccharide-EGCG nanoparticles into the matrix chitosan solution. Due to the encapsulation effect of the nanocarrier, the activity of EGCG is enhanced and extended, enabling it to play a role for a longer time. Therefore, the coating not only has a high freshness preservation effect, but also develops a smart active indicator material - freshness indicator film after drying and forming, which is used to monitor food quality. This kind of packaging not only helps to optimize the storage and sales process of food, reduce resource waste, but also helps to ensure the food safety of consumers, and has important practical value. In addition, the preparation method of the present invention has a simple process, wide raw material sources, and low energy consumption, which is conducive to large-scale industrial application. Detailed implementation manners

[0037] In order to make the purpose, features and advantages of the present invention clearer, the following will further clarify the present invention in combination with the drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. After reading the present invention, those skilled in the art's various equivalent modifications of the present invention fall within the scope defined by the appended claims of this application.

[0038] The measurement methods of the key parameters of the film and fish involved in all embodiments of the present invention are as follows:

[0039] 1. Color parameters of the film

[0040] The color difference parameters of the thin film were measured using a color difference meter. A 36*36 mm film was observed for color parameters in the reflection mode of the color difference meter with a 30 mm aperture. Air was used as a blank control.

[0041] 2. Changes in pH of fish meat

[0042] Take 5 g of the treated fish meat, add 45 mL of pure water, homogenize for 1 min, let stand for 30 min, and detect the pH change after filtration.

[0043] 3. Changes in the content of malondialdehyde (MDA) in fish meat

[0044] Take 5 g of the treated fish meat, add 25 mL of 10% trichloroacetic acid mixture, homogenize for 1 min, centrifuge at 4 °C and 8000 rpm for 10 min. Take the supernatant after centrifugation, filter it, then mix it with an equal volume of 0.02 M thiobarbituric acid solution and heat it in a water bath at 95 °C for 20 min. After cooling to room temperature, detect the absorbance change using a spectrophotometer at 532 nm, and calculate the change in the content of malondialdehyde in fish meat using the malondialdehyde standard curve. The formula for calculating the content of malondialdehyde is as follows:

[0045]

[0046] Note: X = content of malondialdehyde (mg / kg);

[0047] c = concentration of malondialdehyde obtained from the standard curve (μg / mL);

[0048] V = volume of sample constant volume (mL);

[0049] m = weight of the sample represented by the final sample solution (g)

[0050] 4. Changes in total volatile basic nitrogen (TVB-N) in fish meat

[0051] Referring to the semi-micro Kjeldahl method in GB 5009.228-2016 of the national standard, use a Kjeldahl nitrogen analyzer to measure TVB-N, and the result is expressed as mg / 100 g. For 3 g of minced fish meat, add 15 mL of deionized water, shake for 30 min, filter, take 10 mL of the filtrate and add it to the distillation tube, add 5 mL of MgO suspension (10 g / L), and distill for 180 s. The formula for calculating TVB-N is as follows:

[0052]

[0053] Note: X = content of total volatile basic nitrogen in the sample, in milligrams per hundred grams (mg / 100 g) or milligrams per hundred milliliters (mg / 100 mL);

[0054] V1 = the volume of the hydrochloric acid standard titration solution consumed by the test solution, in milliliters (mL);

[0055] V2 = the volume of the hydrochloric acid standard titration solution consumed by the reagent blank, in milliliters (mL);

[0056] c = the concentration of the hydrochloric acid or sulfuric acid standard titration solution, in moles per liter (mol / L);

[0057] l4 = the mass of nitrogen equivalent to 1.0 mL of the hydrochloric acid [c(HCI) = 1,000 mol / L] standard titration solution, in grams per mole (g / mol);

[0058] m = the mass of the sample, in grams (g);

[0059] 100 = the conversion factor for converting the calculation result to milligrams per hundred grams (mg / 100g).

[0060] Example 1

[0061] The object of the present invention is to provide a coating film and a freshness detection indicating film for salmon preservation and their preparation methods, including the following steps:

[0062] 1. Under stirring conditions, prepare a 4 mg / mL tea polysaccharide solution, add EGCG powders with mass-volume ratios of 0.01%, 0.02%, 0.1%, and 0.5% thereto respectively, and continuously stir for 30 min to obtain four tea polysaccharide-EGCG solutions with different concentrations;

[0063] 2. Under stirring conditions, dissolve chitosan with acetic acid solution, the concentration of chitosan is 0.8 mg / mL, and the mass fraction of acetic acid is 1% to obtain a chitosan acetic acid solution. Add the tea polysaccharide-EGCG solution thereto, and mix the tea polysaccharide-EGCG solution and the chitosan acetic acid solution at a volume ratio of 1:1. Respectively prepare four tea polysaccharide-EGCG-chitosan acetic acid solutions with different concentrations of EGCG, adjust the pH to 4.0 with sodium hydroxide, and then continuously stir for 2 h to obtain four chitosan / tea polysaccharide-EGCG nano-suspensions loaded with different concentrations of EGCG;

[0064] 3. Under stirring conditions, add the four chitosan / tea polysaccharide-EGCG nano-suspensions loaded with different concentrations of EGCG to a 20 mg / mL matrix chitosan solution respectively, the volume ratio of the chitosan / tea polysaccharide-EGCG nano-suspension to the matrix chitosan solution is 3:2, mix and stir for 1 h, and add the plasticizer glycerol with a concentration of 30% of the mass of the matrix chitosan while stirring to obtain four chitosan / tea polysaccharide-EGCG membrane solutions (CS / CTE membrane solutions) with different concentrations of EGCG;

[0065] 4. Take four equal amounts of salmon fish blocks, attach four portions of the CS / CTE film solution to the surface of the salmon respectively, and then dry. Four coating film samples for preservation are formed on the surface of the salmon, namely coating film CS / CTE-0.01%, coating film CS / CTE-0.02%, coating film CS / CTE-0.1%, and coating film CS / CTE-0.5%.

[0066] 5. Take 30 g of each of the four CS / CTE film solutions with different concentrations, cast them in a mold to form a film with a thickness of 0.08 mm, dry at 50 °C for 6 h, then demold, and dry and equilibrate at 25 °C and 50% RH for 48 h to obtain four freshness detection indicator film samples respectively, namely indicator film CS / CTE-0.01%, indicator film CS / CTE-0.02%, indicator film CS / CTE-0.1%, and indicator film CS / CTE-0.5%.

[0067] Verification Experiment 1:

[0068] Set up a control experiment, measure relevant indicators of the four groups of coating film samples obtained from the control group and Example 1 under the same conditions at 0, 2, 4, 6, and 8 days respectively, and evaluate and analyze the results as follows:

[0069] Blank control group: Salmon meat samples without any coating treatment;

[0070] CS / CT (coating control group): Without adding EGCG, and other conditions are the same as in Example 1;

[0071] CS / CTE-0.01% (coating experimental group 1): The concentration of added EGCG is 0.01%;

[0072] CS / CTE-0.02% (coating experimental group 2): The concentration of added EGCG is 0.02%;

[0073] CS / CTE-0.1% (coating experimental group 3): The concentration of added EGCG is 0.1%;

[0074] CS / CTE-0.5% (coating experimental group 4): The concentration of added EGCG is 0.5%.

[0075] The samples are all stored at 4 °C.

[0076] Detection results:

[0077] For the appearance changes of the fish meat by the blank control group and the coatings of each experimental group, see Figure 1, the appearance of each group of samples can be seen on the 0th, 2nd, 4th, 6th, and 8th days. It can be seen that the freshness of the salmon treated with the film-forming solution in the experimental group is better than that of the blank control group and the control group samples. Among them, the freshness of the salmon slices treated with the CS / CTE-0.5% and CS / CTE-0.1% film-forming solutions in the experimental group is significantly better than that of the blank control group and the control group samples.

[0078] For the changes in the chemical indicators of the fish meat with the coating of the control examples and each example, see Figure 2 , the pH value of the fish meat is an indicator of freshness; malondialdehyde is the main degradation product of unsaturated fatty acids and an important indicator for evaluating the freshness of fish meat. Total volatile basic nitrogen is usually used as a marker for the degradation of proteins and amines. Generally speaking, fish meat with TVB-N ≥ 20 mg / 100 g is considered spoiled, fish meat with 15 - 20 mg / 100 g is sub-fresh, and fish meat with ≤ 15 mg / 100 g is fresh; it can be seen that whether it is pH, malondialdehyde or the content of total volatile basic nitrogen will gradually increase with the storage of the fish meat. With the increase in the concentration of EGCG in the film, compared with the blank control group and the control group, the spoilage rate of the fish meat will be delayed after the coating treatment in the experimental group.

[0079] Verification Experiment 2:

[0080] Four indicator film samples were respectively pasted on the inner wall of the fresh-keeping box containing fresh salmon meat samples and stored at 4°C. A control experiment was set up. The four groups of indicator film samples obtained from the treatment of Example 1 were measured for relevant indicators at 0, 1, 2, 3, and 4 days under the same conditions, and the results were evaluated and analyzed as follows:

[0081] CS / CT (indicator film control group): No EGCG was added, and other conditions were the same as in Example 1;

[0082] CS / CTE-0.01% (indicator film experimental group 1): The added EGCG concentration was 0.01%;

[0083] CS / CTE-0.02% (indicator film experimental group 2): The added EGCG concentration was 0.02%;

[0084] CS / CTE-0.1% (indicator film experimental group 3): The added EGCG concentration was 0.1%;

[0085] CS / CTE-0.5% (indicator film experimental group 4): The added EGCG concentration was 0.5%.

[0086] The samples were all stored at 4°C.

[0087] Test Results

[0088] From Figure 6It can be seen that with the increase in the concentration of EGCG, the decrease in the L* (brightness) value and the increase in the a* (red) and b* (yellow) values indicate that the addition of EGCG-loaded nanoparticles weakens the lightness of the film and enhances the red-yellow color. When the EGCG addition amount is 0.5%, compared with the 0.1% addition amount, the b* value slightly decreases.

[0089] The indication of the freshness of fish by indicator films containing different EGCG concentrations is shown in Figure 3 , it can be seen that as the storage days increase, obvious shrinkage appears on the surface of the composite film (except for EGCG concentrations of 0.1% and 0.5%). This is because when the EGCG concentration is low, the hydrophilicity of the film is strong. On the 4th day for all indicator films added with EGCG, compared with the 0th day, the reddish-brown color on the film surface increases.

[0090] From Figure 2 it is known that various chemical indicators show that when the EGCG concentration is 0.1% and 0.5%, the preservation effect on fish is the best. Therefore, in Figure 4 , the correlation between TVB-N and ΔE values during the storage of CS / CTE-0.1% and CS / CTE-0.5% indicator films was observed. The correlation analysis between the ΔE value and TVB-N of the indicator film follows a linear model. For the CS / CTE-0.1% film, the coefficient is 0.736, and for the CS / CTE-0.5% film, the coefficient is 0.9389. Among them, the CS / CTE-0.5% indicator film shows an obvious color change on the 4th day, and at this time the fish is in the spoilage stage. This is because during the spoilage process of fish, protein degradation will form volatile alkaline gases, and the EGCG molecules contained in the film contact these gases and autoxidize to form colored dimers, causing the color change of the film, thus achieving the indication effect. Usually, the greater the difference in ΔE, the easier it is to be observed by the naked eye. Therefore, CS / CTE-0.5% is most suitable for indicating the freshness of fish.

[0091] The change in color parameters of the CS / CTE-0.5% indicator film during the storage of fish is shown in Figure 5 , it can be seen that with the increase in storage days, the increase in red (a* value) and yellow (b* value) is promoted, but the brightness (L* value) decreases. This is because as the storage time of salmon increases, the large production of volatile nitrogen compounds leads to an increase in pH value, causing the accumulation of red color in the composite film. In the later stage of storage, due to the increase in alkalinity, a large amount of volatile nitrogen-containing compounds accumulate, resulting in the composite film becoming gray and dull. In this way, the freshness of fish can be judged according to the change in the color parameters of the film, making the CS / CTE film have a significant indication effect.

[0092] Example 2

[0093] 1. Under stirring conditions, a 2 mg / mL tea polysaccharide solution was prepared, and EGCG powder with a mass-volume ratio of 0.02% was added thereto and mixed evenly to obtain a tea polysaccharide-EGCG solution;

[0094] 2. Under stirring conditions, chitosan was dissolved in acetic acid solution to obtain a chitosan acetic acid solution with a chitosan concentration of 0.2 mg / mL. The tea polysaccharide-EGCG solution was added thereto, and the tea polysaccharide-EGCG solution and the chitosan acetic acid solution were mixed at a volume ratio of 1:1.2 to obtain a tea polysaccharide-EGCG-chitosan acetic acid solution. The pH was adjusted to 3.5 with sodium hydroxide, and then stirring was continued for 2.5 h to obtain a chitosan / tea polysaccharide-EGCG nano-suspension;

[0095] 3. Under stirring conditions, the chitosan / tea polysaccharide-EGCG nano-suspension was added to a matrix chitosan solution with a mass concentration of 15 mg / mL. The volume ratio of the chitosan / tea polysaccharide-EGCG nano-suspension to the matrix chitosan solution was 1.6:1, and stirring was carried out for 1.5 h. During stirring, a plasticizer glycerol with a concentration of 40% of the mass of the matrix chitosan was added to obtain a CS / CTE membrane solution;

[0096] 4. After the CS / CTE membrane solution was attached to the surface of salmon and dried, the coating film for preservation was formed on the surface of salmon;

[0097] 5. 20 g of the CS / CTE membrane solution was cast in a mold to form a 0.05 mm film. After drying at 40 °C for 9 h, the film was demolded. After drying and equilibrating at 20 °C and 55% RH for 36 h, an indicator film for freshness detection was obtained.

[0098] Example 3

[0099] 1. Under stirring conditions, an 8 mg / mL tea polysaccharide solution was prepared, and EGCG powder with a mass-volume ratio of 0.3% was added thereto and mixed evenly to obtain a tea polysaccharide-EGCG solution;

[0100] 2. Under stirring conditions, chitosan was dissolved in acetic acid solution to obtain a chitosan acetic acid solution with a chitosan concentration of 1 mg / mL. The tea polysaccharide-EGCG solution was added thereto, and the tea polysaccharide-EGCG solution and the chitosan acetic acid solution were mixed at a volume ratio of 1:1.5 to obtain a tea polysaccharide-EGCG-chitosan acetic acid solution. The pH was adjusted to 5.0 with sodium hydroxide, and then stirring was continued for 3 h to obtain a chitosan / tea polysaccharide-EGCG nano-suspension;

[0101] 3. Under stirring conditions, add the chitosan / tea polysaccharide-EGCG nano-suspension to the matrix chitosan solution with a mass concentration of 25 mg / mL. The volume ratio of the chitosan / tea polysaccharide-EGCG nano-suspension to the matrix chitosan solution is 2:1, and mix and stir for 2 h. While stirring, add the plasticizer glycerol with a concentration of 30-50% of the mass of the matrix chitosan to obtain the chitosan / tea polysaccharide-EGCG film solution, referred to as the CS / CTE film solution for short;

[0102] 4. After attaching the CS / CTE film solution to the surface of the salmon and drying, the coating film for preservation is formed on the surface of the salmon;

[0103] 5. Cast 45 g of the CS / CTE film solution in a mold to form a film with a thickness of 0.09 mm. After drying at 45 °C for 12 h, demold it. After drying and equilibrating at 23 °C and 52% RH for 42 h, an indicator film for freshness detection is obtained.

Claims

1. A coating film for preserving salmon, characterized in that, The preparation raw materials of the coating film for salmon freshness preservation include tea polysaccharide, epigallocatechin gallate (EGCG) powder, chitosan and plasticizer, and are specifically prepared by the following steps; ① Prepare a tea polysaccharide solution with a concentration of 2 - 8 mg / mL, add epigallocatechin gallate (EGCG) powder with a mass - to - volume ratio of 0.01% - 0.5% thereto and mix evenly to obtain a tea polysaccharide - EGCG solution; ② Add the tea polysaccharide - EGCG solution to a chitosan acetate solution with a concentration of 0.2 - 1 mg / mL according to a volume ratio of 1:1 - 1:1.5 to obtain a tea polysaccharide - EGCG - chitosan acetate solution; ③ Adjust the pH of the tea polysaccharide - EGCG - chitosan acetate solution to 3.5 - 5.0, and continuously stir for 2 - 3 h to obtain a chitosan / tea polysaccharide - EGCG nano - suspension; ④ Mix and stir the chitosan / tea polysaccharide - EGCG nano - suspension and a matrix chitosan solution with a concentration of 15 - 25 mg / mL according to a volume ratio of 3:2 - 2:1 for 1 - 2 h. While stirring, add plasticizer glycerol with a concentration accounting for 30% - 50% of the matrix chitosan concentration to obtain a CS / CTE membrane solution. After attaching the CS / CTE membrane solution to the surface of salmon and drying, the coating film for salmon freshness preservation is obtained.

2. A freshness detection indicating film for salmon, characterized in that, Cast the CS / CTE membrane solution in a mold to form a membrane with a thickness of 0.05 - 0.09 mm, dry it at 40 - 50 °C for 6 - 12 h and then demold it. After drying and equilibrating for 36 - 48 h under the conditions of 20 - 25 °C and 50 - 55% RH, a membrane for salmon freshness detection indication is obtained.

3. A preparation method of a coating film for salmon preservation, characterized in that, The preparation method of the coating film for salmon freshness preservation includes the following steps; ① Prepare a tea polysaccharide solution with a concentration of 2 - 8 mg / mL, add epigallocatechin gallate (EGCG) powder with a mass - to - volume ratio of 0.01% - 0.5% thereto and mix evenly to obtain a tea polysaccharide - EGCG solution; ② Add the tea polysaccharide - EGCG solution to a chitosan acetate solution with a concentration of 0.2 - 1 mg / mL according to a volume ratio of 1:1 - 1:1.5 to obtain a tea polysaccharide - EGCG - chitosan acetate solution; ③ Adjust the pH of the tea polysaccharide - EGCG - chitosan acetate solution to 3.5 - 5.0, and continuously stir for 2 - 3 h to obtain a chitosan / tea polysaccharide - EGCG nano - suspension; ④ Mix and stir the chitosan / tea polysaccharide - EGCG nano - suspension and a matrix chitosan solution with a concentration of 15 - 25 mg / mL according to a volume ratio of 3:2 - 2:1 for 1 - 2 h. While stirring, add plasticizer glycerol with a concentration accounting for 30% - 50% of the matrix chitosan concentration to obtain a CS / CTE membrane solution. After attaching the CS / CTE membrane solution to the surface of salmon and drying, the coating film for salmon freshness preservation is obtained.

4. A preparation method for a freshness detection indicating film for salmon, characterized in that, The method includes the following steps: A. Under stirring conditions, prepare a tea polysaccharide solution, add EGCG powder thereto and mix evenly to obtain a tea polysaccharide - EGCG solution; B. Add the tea polysaccharide-EGCG solution to the chitosan acetate solution at a volume ratio of 1:1 to 1:1.5 to obtain a tea polysaccharide-EGCG-chitosan acetate solution; adjust the pH to 3.5 - 5.0 with sodium hydroxide, and then continuously stir for 2 - 3 h to obtain a chitosan / tea polysaccharide-EGCG nano-suspension; C. Under stirring conditions, add the chitosan / tea polysaccharide-EGCG nano-suspension to the matrix chitosan solution, mix and stir for 1 - 2 h, and add the plasticizer glycerol while stirring to obtain a CS / CTE membrane solution; D. Cast the CS / CTE membrane solution in a mold to form a membrane with a thickness of 0.05 - 0.09 mm, demold after drying, and dry and equilibrate for 36 - 48 h under the conditions of 20 - 25 °C and 50 - 55% RH to obtain the freshness detection indicating membrane.

5. The preparation method of the salmon freshness detection indicating film according to claim 4, characterized in that, The concentration of the prepared tea polysaccharide solution is 2 - 8 mg / mL, and the addition amount of EGCG is 0.01% - 0.5% by mass / volume ratio.

6. The preparation method of the salmon freshness detection indicating film according to claim 4, characterized in that, The mass concentration of the matrix chitosan solution is 15 - 25 mg / mL.

7. The preparation method of the salmon freshness detection indicating film according to claim 4, characterized in that, The chitosan / tea polysaccharide-EGCG nano-suspension and the matrix chitosan solution are mixed at a volume ratio of 3:2 to 2:

1.

8. The preparation method of the salmon freshness detection indicating film according to claim 4, characterized in that, The added plasticizer is glycerol, and its concentration is 30 - 50% of the mass of the matrix chitosan.

9. The preparation method of the salmon freshness detection indicating film according to claim 4, wherein, The drying temperature is 40 - 50 °C, and the drying time is 6 - 12 h.

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