Edible protein film, method for preparing the same and use thereof

Abstract

The application relates to the field of food preservation technology, and discloses an edible protein film as well as a preparation method and application thereof. The preparation method comprises the following steps: step 1, solvent preparation: preparing an ethanol aqueous solution with a volume concentration of 55-65%, and preheating in a water bath at a temperature of 60-70 DEG C; step 2, preparation of a protein film forming solution: under the above water bath condition, sodium caseinate is added to the ethanol aqueous solution prepared in step 1 while stirring, and the solution is fully hydrated through stirring, then zein is added, after the protein is fully dissolved, glycerol and citric acid are added, the solution is stirred and then left to stand to remove bubbles, and a uniform protein film forming solution is obtained; and step 3, film forming: the protein film forming solution is taken out and placed on a plane substrate, the film is removed after drying treatment, and the film is balanced in a dryer, and the edible protein film is obtained. The edible protein film prepared by the preparation method has high barrier performance, waterproof performance and mechanical strength.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of food preservation technology, in particular to an edible protein film and a preparation method and application thereof. BACKGROUND

[0002] Packaging plays a key role in protecting food, extending shelf life and reducing food waste. Synthetic plastics are widely used in food packaging due to their high mechanical strength, high barrier property, low cost and other characteristics, but their toxicity, non-degradability, raw material (petroleum) cost and other problems limit their application in the field of food packaging. Edible film as a green packaging material has a wide range of sources and has incomparable safety and zero waste, etc., and has a good development prospect.

[0003] At present, the preparation of new degradable film is composed of protein, lipid, polysaccharide or their complex, and the inherent characteristics of protein make it an excellent choice for producing edible film. The characteristics of protein-based edible film (or called edible protein film) are: uniform texture and high transparency, moderate mechanical strength, high gas barrier property, protein network formation and the ability to induce plasticity and elasticity. Casting is a widely used process for forming edible protein-based film, which includes dissolving biopolymers in a suitable solvent to obtain a film-forming solution, and the solvents that can be used in this process are limited to water or ethanol, with the purpose of maintaining edibility. Protein-based edible film can be divided into animal isolated protein edible film (such as collagen, whey protein, casein, etc.) and plant isolated protein edible film (grain protein, legume protein, oilseed protein, green leaf protein, nut protein, etc.) according to the source.

[0004] Plant protein sources are popular due to their excellent film-forming properties, high mechanical properties, cohesive / adhesive properties, inexpensive edibility and fast biodegradability. Among the numerous edible films, only zein can form films without the need for additives (through the self-assembly process of molecules). Compared with other types of protein films, zein films exhibit good moisture barrier, oxygen barrier and volatile component barrier properties, and strong heat resistance, but have the disadvantages of brittleness and poor processability. Although the research on zein has been carried out for many years at home and abroad, so far, there is almost no industrial production and practical commercial application. As an animal-derived protein, casein has high nutritional value, water solubility and emulsifying ability, and is an ideal hydrocolloid for preparing edible films. Casein films can form a barrier to prevent food from contacting with external substances, so they are helpful to protect easily oxidized products. Although casein films have great potential in food packaging, some deficiencies still need to be solved before they are widely commercialized: on the one hand, casein-based films have high water sensitivity, absorbing and releasing water molecules, which greatly affects their mechanical and barrier properties. In addition, unlike synthetic polymer materials, plasticized casein films cannot provide high mechanical strength or good elasticity.

[0005] In recent years, there are more and more researches on the application of edible protein films in food packaging. In view of the limitations of edible protein films, the following aspects are mainly improved. On the one hand, plasticizers (glycerol, polyethylene glycol, fatty acids) are added to change the protein network and increase the flexibility of the film, however, their addition is often accompanied by the decrease of the water barrier property and the gas barrier property of the protein film. In addition, the incompatibility between some hydrophobic plasticizers and the protein matrix will also cause phase separation. Secondly, the performance of protein-based films is improved by laminating with additional lipid film or adding lipid coating. Laminating similar materials may help to reduce leakage through film pores and effectively improve the water resistance of the film, but waxes and fatty acids are usually temperature sensitive. So far, the performance (mechanical strength and water resistance) of protein-based edible films cannot completely compare with that of petroleum-based materials, and it is difficult to completely replace traditional plastic packaging films in a short period of time. On the one hand, the research on edible films generally focuses on the simple blending of components, which leads to poor interaction between matrices and easy microphase separation between matrix molecules, resulting in poor strength and barrier property of the formed film. In addition, the insolubility of some proteins limits their application as film-forming materials. To solve this problem, alkali or acid treatment is a traditional method for preparing low-solubility protein films, but it may cause the risk of corrosion burns and harmful substances.

[0006] Alcohol-soluble proteins have high hydrophobicity and can be dispersed in 60-95% volume concentration of ethanol aqueous solution, 60-80% volume concentration of acetone aqueous solution or alkaline solution with pH>11, while sodium caseinate is a water-soluble polymer, so it is difficult for the two to be compatible in the same solvent under conventional conditions. The prior art mostly uses reverse solvent method or emulsification to form nanoparticles of casein and corn alcohol-soluble protein, and then adds excipients to prepare the casein-corn alcohol-soluble protein composite film, but this method will make the prepared film contain a large number of particles and have poor uniformity.

[0007] CN109280352A discloses a high-strength antibacterial preservative film and a preparation method thereof. The preservative film comprises high-strength preservative film raw materials, preservative barrier auxiliary agents, toughening additives and antibacterial agents. The preservative combination auxiliary agent is prepared by the following method: dissolving alcohol-soluble protein in ethanol solution, then adding it dropwise into sodium caseinate solution, and then performing rotary evaporation, low-temperature plasma treatment and freeze-drying. The preparation principle of the preservative barrier auxiliary agent is to form casein-alcohol-soluble protein composite particles by using reverse solvent. The film prepared by using the preservative barrier auxiliary agent has poor uniformity, poor water barrier property and poor gas barrier property, the film is not transparent, and the operation is complex, which cannot meet the actual demand.

[0008] CN108163369B discloses a one-way moisture-resistant multi-layer edible film and a preparation method thereof. In the method, 80% acetic acid-water solution is used as a co-solvent of alcohol-soluble protein and water-soluble protein, but the formed film has irritancy and high water vapor transmission rate, which cannot meet the demand of edibility and food preservation.

[0009] Therefore, it is of great significance to develop a solvent that can make the two proteins disperse together and has no corrosion and harm, and on this basis, to realize the interaction between the two protein matrices and improve the performance of the prepared edible protein film and broaden its application range in packaging. SUMMARY

[0010] The purpose of the present application is to overcome the problems existing in the prior art, and to provide an edible protein film and a preparation method and application thereof. The preparation method of the edible protein film of the present application can make sodium caseinate and alcohol-soluble protein disperse together, and no corrosive and harmful solvent is used, and the prepared edible protein film has high barrier property, water resistance and mechanical strength. The method of the present application overcomes the defects of low film strength and poor water resistance of the film formed by the traditional reverse solvent method or after emulsification. The purpose of the present application is to develop a solvent that can make hydrophilic and hydrophobic proteins disperse together and has no corrosion and harm, and to develop a preparation method of sodium caseinate-alcohol-soluble protein film with high barrier property, water resistance and mechanical strength.

[0011] In order to achieve the above object, the present application provides a method for preparing an edible protein film, comprising the following steps:

[0012] Step 1, solvent preparation: prepare an ethanol aqueous solution with a volume concentration of 55-65%, and preheat in a water bath at a temperature of 60-70℃;

[0013] Step 2, preparation of a protein film-forming solution: under the above water bath conditions, add sodium caseinate to the ethanol aqueous solution prepared in step 1 while stirring, and stir to fully hydrate the solution, then add zein, and after the protein is fully dissolved, add glycerol and citric acid, stir, and then stand to remove bubbles, to obtain a uniform protein film-forming solution;

[0014] Step 3, film formation: transfer the protein film-forming solution to a flat substrate, dry, remove the film, and then balance the film in a desiccator, to obtain the edible protein film.

[0015] The second aspect of the present application provides an edible protein film prepared by the method as described above.

[0016] The third aspect of the present application provides the use of the edible protein film as described above as a food preservative.

[0017] The fourth aspect of the present application provides the use of the edible protein film as described above in food preservation.

[0018] The fifth aspect of the present application provides a method for preserving food using the edible protein film as described above, comprising the following steps: preparing the edible protein film as described above into an edible packaging bag for a convenient food ingredient, and packaging the convenient food ingredient in the edible packaging bag.

[0019] By the above technical solution, the present application has the following beneficial effects:

[0020] 1. Due to the different solubility characteristics of sodium caseinate and zein, existing technologies for forming casein-zein composite films mostly use reverse-phase solvent methods or emulsification to form casein and zein into nanoparticles, followed by the addition of glycerol, emulsifiers, and other excipients. This results in films containing a large number of particles, leading to poor homogeneity, incompatibility between the two proteins, and low protein strength and tensile strength. The preparation method of this invention allows for the co-dispersion of zein and sodium caseinate in the same solvent by adjusting the temperature and composition of the solvent, resulting in complete compatibility between the two proteins. The inventors of this invention have discovered that the method allows sodium caseinate and zein to interact during film formation, forming a structure with higher compatibility and density, avoiding the microphase separation phenomenon caused by emulsification or reverse-phase solvent methods. Furthermore, the solvent used in this method is non-irritating and easily volatilizes during film formation, avoiding the use and residue of irritating and harmful components such as acetic acid and high-concentration alkali.

[0021] 2. Existing protein-based membrane structures contain numerous microphase separation structures, resulting in poor barrier properties against gases and moisture, thus limiting their applications. The membrane formed by this invention exhibits no microphase separation, has a dense microstructure, and good compatibility. Therefore, its barrier properties against gases and moisture are superior to membranes formed by combining alcohol-soluble proteins with other proteins through reverse-phase solvents or emulsification.

[0022] 3. The edible protein film described in this invention is a biodegradable material. Its development and promotion help retain more resources in the consumption and production cycle, reduce waste generation, and alleviate the environmental problems caused by traditional plastics. At the same time, the edible protein film described in this invention, as a green packaging material, has broad application prospects due to its wide availability of raw materials, low cost, safety, and zero waste, which is unmatched by other materials.

[0023] 4. The preparation method of this invention combines hydrophobic and water-insoluble zein with other more hydrophilic proteins to prepare a protein-based edible protein film. Edible protein films prepared by the method of this invention with different ratios of sodium caseinate and zein exhibit high water resistance, tensile strength, and elasticity. The method of this invention results in a synergistic combination of the physicochemical properties of sodium caseinate (CN) and zein, compensating for their respective shortcomings (e.g., the brittleness of the zein film and the high hydrophilicity of the casein film), thereby forming a compact structure with excellent water-blocking properties.

[0024] 5、Compared with the rigid zein film, the elasticity of the edible protein film prepared by the preparation method of the application is greatly improved, which is significantly better than the traditional composite film formed by zein-casein sodium. At the same time, the preparation method of the application overcomes the defects of low strength and poor water resistance of the film prepared by the traditional reverse solvent method (forming a zein-casein suspension system or forming a film after emulsification). The inventors found that when the addition ratio of casein sodium and zein is 0.75:1 in the method of the application, the edible protein film prepared is most suitable for food packaging application, has the highest transparency, tensile strength and elasticity, and the lowest water vapor permeability, i.e. the best water resistance.

[0025] 6、The edible protein film prepared in the application dissolves quickly in hot water, combined with its edibility and good water resistance at room temperature, should be an ideal material for convenient food packaging. In summary, the application provides a safe, food compatible and environmentally friendly method for manufacturing edible protein-based food packaging film. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 State diagram of casein sodium dispersed in liquid (from left to right, the liquid is water, 40% volume concentration of ethanol aqueous solution, 60% volume concentration of ethanol aqueous solution, 80% volume concentration of ethanol aqueous solution);

[0027] Figure 2 Macrograph of the edible protein film prepared in Comparative Example 1 and Examples 1-5;

[0028] Figure 3 Macrograph of the edible protein film prepared in Comparative Example 2;

[0029] Figure 4 Scanning electron microscope graph of the cross section of the edible protein film prepared in Comparative Example 1 and Examples 1-5, the magnification of a-f is 500 times, and the magnification of A-F is 5000 times; wherein a, A is CN-0 edible protein film; b, B is CN-0.25 edible protein film; c, C is CN-0.5 edible protein film; d, D is CN-0.75 edible protein film; e, E is CN-1.0 edible protein film; f, F is CN-1.25 edible protein film;

[0030] Figure 5 Differential scanning calorimetry curve comparison diagram of the edible protein film prepared in Comparative Examples 1-2 and Examples 1-5. DETAILED DESCRIPTION

[0031] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as the exact dimensions are not considered critical for the purposes of the application. The endpoints of the ranges and any values are provided as approximations only and are understood to encompass a range of values around the endpoints. Various exemplary dimensions are provided in the specification for the purpose of clarity and merit only and are not intended to be limiting.

[0032] The first aspect of the application provides a method for preparing an edible protein film, the method comprising the following steps:

[0033] Step 1, solvent preparation: prepare an aqueous ethanol solution with a volume concentration of 55-65%, and preheat in a water bath at a temperature of 60-70°C;

[0034] Step 2, preparation of protein film-forming solution: under the above water bath conditions, add sodium caseinate to the aqueous ethanol solution prepared in step 1 while stirring, and stir to fully hydrate the solution. Then add zein, and after the protein is fully dissolved, add glycerol and citric acid. After stirring, let stand to remove bubbles, and obtain a uniform protein film-forming solution;

[0035] Step 3, film formation: transfer the protein film-forming solution to a flat substrate, dry and remove the film, and then place the film in a desiccator for equilibration, to obtain the edible protein film.

[0036] In some embodiments of the application, the flat substrate is a culture dish.

[0037] In some embodiments of the application, in step 3, the drying is performed at a temperature of 35-55°C.

[0038] In some embodiments of the application, in step 1, the aqueous ethanol solution has a volume concentration of 60-65%, and the water bath temperature is 65-70°C. Preferably, in step 1, the aqueous ethanol solution has a volume concentration of 60%, and the water bath temperature is 65°C.

[0039] In some embodiments of the application, in step 2, the addition ratio of sodium caseinate to zein is 0.05-1.25:1, and the concentration of the total fixed protein is 0.06-0.08 g / mL. The addition ratio of sodium caseinate to zein is a mass ratio.

[0040] In the application, the total fixed protein refers to sodium caseinate and zein, and the concentration of the total fixed protein refers to the sum of the concentrations of sodium caseinate and zein in the protein film-forming solution.

[0041] In some embodiments of the present application, the ratio of sodium caseinate to zein added in step 2 is 0.75-1.25:1, and the concentration of total protein is 0.07-0.08 g / mL. Here, the ratio of sodium caseinate to zein is by mass. The inventors have found in research that when the ratio of sodium caseinate to zein is 0.75:1, the edible protein film prepared has higher transparency, tensile strength and elasticity, and lower water vapor permeability, i.e. better water resistance. Therefore, preferably, the ratio of sodium caseinate to zein is 0.75:1. Preferably, the ratio of sodium caseinate to zein is 0.75:1, and the concentration of total protein is 0.07 g / mL.

[0042] In some embodiments of the present application, the stirring conditions in step 2 can be selected within a wide range, for example, 300-800 rpm for 5-35 min, and can be selected by a person skilled in the art according to actual conditions, which will not be described here.

[0043] In the present application, hydration refers to the full action of sodium caseinate and water in the solvent, so that sodium caseinate is well dispersed in the film-forming solvent.

[0044] In some embodiments of the present application, the time for standing and defoaming in step 2 can be selected within a wide range, for example, 30-120 s.

[0045] In some embodiments of the present application, the concentration of glycerol in step 2 is 0.005-0.015 g / mL, and the mass concentration of citric acid is 0.015-0.03 g / mL. It should be understood that the concentration of glycerol refers to the concentration of glycerol in the protein film-forming solution, and the concentration of citric acid refers to the concentration of citric acid in the protein film-forming solution.

[0046] In some embodiments of the present application, the drying conditions in step 3 are 37-45°C for 3-7 h.

[0047] In some embodiments of the present application, the drying conditions in step 3 are 40-45°C for 3-5 h.

[0048] In some embodiments of the present application, in step 3, the bottom of the dryer is provided with saturated potassium carbonate solution, and the relative humidity therein is maintained at 40-45% for at least 48 h.

[0049] The second aspect of the present application provides an edible protein film prepared by the method as described above.

[0050] In the present application, the edible protein film has high gas and moisture barrier properties.

[0051] The third aspect of the present application provides the use of the edible protein film as described above as a food preservative.

[0052] The fourth aspect of the present application provides the use of the edible protein film as described above in food preservation.

[0053] In some embodiments of the present application, the use is the use of the edible protein film as an edible coating and / or an edible packaging film.

[0054] The fifth aspect of the present application provides a method for preserving food using the novel edible protein film as described above, which comprises the steps of: preparing the edible protein film as described above into an edible packaging bag for a convenient food ingredient, and packaging the convenient food ingredient in the edible packaging bag. When used, the edible packaging bag containing the convenient food ingredient can be directly added into hot water.

[0055] The present application will be described in detail below through examples. In the following examples, unless otherwise specified, the reagents are commercially available, and the methods used are conventional methods in the art.

[0056] Examples 1-5

[0057] This example is used to illustrate the method for preparing the film of the present application.

[0058] The preparation of the film is carried out according to the following steps:

[0059] Step 1, solvent preparation: prepare a 60% volume concentration of ethanol aqueous solution as an environmentally friendly solvent, and preheat in a water bath at a temperature of 65°C.

[0060] Step 2, preparation of protein film-forming solution: under the above water bath conditions, sodium caseinate is added to the ethanol aqueous solution prepared in step 1 while stirring with an electric stirrer (400 rpm), and the solution is stirred for 10 min to fully hydrate, then zein is added, and after the proteins are fully dissolved, glycerol and citric acid are added, and after stirring for 5 min, the solution is left to stand for 120 s to remove bubbles, obtaining a uniform protein film-forming solution, achieving the co-dispersion of sodium caseinate and zein.

[0061] In the above steps, the amounts of sodium caseinate, zein, glycerol, and citric acid added are such that the contents of the components in the protein film-forming solution are as shown in Table 1.

[0062] Table 1

[0063]

[0064] Step 3, film formation by casting method: 7 mL of the protein film forming solution was taken in a 90 mm plastic petri dish, and the film was dried in a 37 °C incubator for 5 h, then removed and placed in a desiccator (with saturated potassium carbonate solution at the bottom) with a relative humidity of 43% for 48 h. The edible protein film was obtained and named CN-0.25, CN-0.5, CN-0.75, CN-1.0, CN-1.25, respectively.

[0065] Comparative Example 1

[0066] The film was prepared according to the following steps:

[0067] Step 1, solvent preparation: prepare a 60% (v / v) ethanol aqueous solution as an environmentally friendly solvent and preheat in a water bath at 65 °C.

[0068] Step 2, preparation of protein film forming solution: under the above water bath conditions, add sodium caseinate to the ethanol aqueous solution prepared in step 1 while stirring with an electric stirrer (400 rpm), stir for 10 min to fully hydrate the solution, then add glycerol and citric acid, stir for 5 min, and then stand for 120 s to remove bubbles, obtaining the protein film forming solution.

[0069] In the above steps, the amounts of sodium caseinate, glycerol, and citric acid added were such that the contents of each component in the protein film forming solution were as shown in Table 1.

[0070] Step 3, film formation by casting method: 7 mL of the protein film forming solution was taken in a 90 mm plastic petri dish, and the film was dried in a 37 °C incubator for 5 h, then removed and placed in a desiccator (with saturated potassium carbonate solution at the bottom) with a relative humidity of 43% for 48 h. The edible protein film was obtained and named CN-0.25, CN-0.5, CN-0.75, CN-1.0, CN-1.25, respectively.

[0071] Comparative Example 2

[0072] Step 1, solvent preparation: prepare a 60% (v / v) ethanol aqueous solution as an environmentally friendly solvent and preheat in a water bath at 65 °C.

[0073] Step 2, preparation of protein film forming solution: under the above water bath conditions, add sodium caseinate to the ethanol aqueous solution prepared in step 1 while stirring with an electric stirrer (400 rpm), stir for 10 min to fully hydrate the solution, then add glycerol and citric acid, stir for 5 min, and then stand for 120 s to remove bubbles, obtaining the protein film forming solution.

[0074] In the above steps, the amounts of sodium caseinate, glycerol, and citric acid added were such that the contents of each component in the protein film forming solution were as shown in Table 1.

[0075] Step 3, film formation by casting method: 7 mL of the protein film forming solution was transferred to a 90 mm plastic petri dish, and the film was dried in a 37 °C constant temperature incubator for 5 h, then removed, and placed in a desiccator (with saturated potassium carbonate solution at the bottom) with a relative humidity of 43% for 48 h to obtain an edible protein film, named Zein-0.

[0076] Comparative Example 3

[0077] Step 1, a stock solution was prepared by dissolving zein in a 60% (v / v) aqueous ethanol solution and stirring on a magnetic stirrer for 1 h, and the content of zein in the stock solution was 0.08 g / mL. The obtained stock solution was added dropwise to water (the volume ratio of the stock solution to water was 1:4) under gentle stirring on a magnetic stirrer. Stirring was continued for 30 min to completely precipitate the zein to obtain a zein dispersion. Ethanol was removed from the zein dispersion by rotary evaporation (45 °C, 30 min), and finally 25 mL of a zein nanoparticle dispersion was obtained.

[0078] Step 2, preparation of the protein film forming solution: sodium caseinate was added to the zein nanoparticle dispersion prepared in Step 1 while stirring with an electric stirrer (400 rpm), and stirring was continued for 30 min. After the protein was completely dissolved, glycerol and citric acid were added, stirring was continued for 5 min, and then the mixture was left to stand for 120 s to remove bubbles, to obtain a protein film forming solution.

[0079] In the above steps, the amounts of sodium caseinate, zein, glycerol, and citric acid added were such that the contents of the components in the protein film forming solution were as shown in Table 1.

[0080] Step 3, film formation by casting method: 7 mL of the protein film forming solution was transferred to a 90 mm plastic petri dish, and the film was dried in a 37 °C constant temperature incubator for 5 h, then removed, and placed in a desiccator (with saturated potassium carbonate solution at the bottom) with a relative humidity of 43% for 48 h to obtain an edible protein film, named Zein-0.

[0081] Test Example 1

[0082] Aqueous ethanol solutions with a volume concentration of 40%, a volume concentration of 60%, and a volume concentration of 80% were prepared.

[0083] Water, 40% ethanol aqueous solution, 60% ethanol aqueous solution, 80% ethanol aqueous solution were preheated in a 65°C water bath, then sodium caseinate was added under stirring at 500 rpm, and the dispersion was prepared after 20 min of hydration. The amount of sodium caseinate added was such that the content of sodium caseinate in the system was 0.03 g / mL. After the dispersion was cooled to room temperature, the state of dispersion of sodium caseinate in different liquids was observed, and the results are shown in Table 1. Figure 1 .

[0084] As shown in Table 1, sodium caseinate can form a stable dispersion in 60% ethanol aqueous solution, and co-disperse sodium caseinate and zein. Figure 1

[0085] Test Example 2

[0086] In this test example, the sensory properties (thickness, color, transparency), mechanical properties, moisture sensitivity (water vapor transmission rate, hygroscopicity, solubility, swelling degree), thermal properties (glass transition temperature, thermal degradation) of the edible protein films prepared in Examples 1-5 and Comparative Examples were characterized; and the mechanism of improving the processability of the edible protein films was preliminarily evaluated by microstructure observation (scanning electron microscopy).

[0087] The detection methods involved in this test example include:

[0088] I. Sensory properties of edible protein films

[0089] 1. Color: The color of the edible protein film was measured using a colorimeter, which was set to transmission measurement mode. The edible protein film was fixed at the transmission measurement position. The instrument was calibrated using white A4 paper. The color difference values (L*, a* and b*) were directly read from the instrument. Each group was measured 5 times, and the test results were expressed as the average value.

[0090] The calculation formula of whiteness (WI) is as follows:

[0091]

[0092] 2. Film thickness: The sample (edible protein film) was measured for film thickness using an electronic digital caliper, and 9 points were randomly measured for each disc, with the average value taken as the film thickness.

[0093] 3. Opacity: Opacity is defined as the absorbance of the edible protein film at 600 nm per unit thickness.

[0094]

[0095] In the formula, Abs represents the absorbance value at 600 nm; T is the thickness of the edible protein film (mm). ​

[0096] II. Mechanical Properties of Edible Protein Membranes

[0097] Tensile tests were performed on 1cm × 3cm rectangular specimens using an Instron tensile tester. The clamping distance was 2cm, and the test was conducted at a constant deformation rate of 1mm / s at 25℃. Five parallel analyses were performed on each sample (edible protein membrane), and the average value was taken. The formulas for calculating tensile strength (TS) and elongation at break (EB) are as follows:

[0098]

[0099]

[0100] In the formula, F m The maximum force recorded is in Newtons (N); S is the cross-sectional area of ​​the tested sample (edible protein membrane) in square millimeters (mm). 2 );L o The initial sample length is in millimeters (mm); L b The length of the specimen at fracture is expressed in millimeters (mm).

[0101] III. Moisture Sensitivity of Edible Protein Membranes

[0102] 1. Water vapor transmission rate (WVP): Performed continuously for 12 hours at 25°C and 90 (±2)% humidity, according to ASTM standard method E96 / E96M. The calculation formula is as follows:

[0103]

[0104] In the formula, A is the effective test area of ​​the sample (edible protein membrane) (2.83 × 10⁻⁶). -3 m 2 S is the saturated vapor pressure of water at 25℃ (3169Pa), ΔR is the relative humidity on both sides of the sample (edible protein membrane), t is the test time (h), m1 is the weight of anhydrous calcium chloride at the end of the test (g), and m0 is the initial weight of anhydrous calcium chloride (g).

[0105] 2. Swelling Degree: A 50mm × 50mm sample (edible protein membrane) was first dried in a 50℃ oven to constant weight, weighed, and marked as m0. Then, it was immersed in 40mL of deionized water at 23±1℃. After 24 hours, the edible protein membrane was removed from the water, dried on a paper towel, weighed, and marked as m1. The swelling degree was calculated using the following formula. Three replicates were performed for each sample.

[0106]

[0107] 3. Moisture absorption rate: The sample (edible protein film) was fixed in an aluminum moisture permeable cup (without anhydrous calcium chloride), weighed, and labeled as ml; after being placed at 25°C and a relative humidity of 100 ± 2% for 12 h, it was weighed and labeled as m2; the weight of the aluminum moisture permeable cup was mo, and each sample was measured in triplicate. The moisture absorption rate was calculated according to the following formula:

[0108]

[0109] 4. Solubility: The sample (edible protein film) was cut into a 4 cm x 4 cm rectangular strip, dried to a constant weight in a 50°C oven, and accurately weighed, with the mass recorded as mo; after weighing, it was placed in a glass dish, 40 mL of deionized water was added, and it was soaked at room temperature (25 ± 1°C) for 24 h. Then the edible protein film was taken out and dried at 50°C to a constant weight, and its mass was accurately weighed and recorded as m s . The solubility was calculated by the following formula:

[0110]

[0111] 5. Water contact angle (WCA) test: The static contact angle of the edible protein film was measured using an optical contact angle measuring instrument. The edible protein film was laid flat on the sample plate, and deionized water droplets were automatically dropped onto the film surface using a sample injection needle. The liquid was magnified and displayed on the screen through an optical reflection system and a magnification system, and the droplet state was recorded by taking a photo in time. The static contact angle of the droplet on the edible protein film was measured by the optical contact angle measuring instrument, and 5 points were randomly measured for each film.

[0112] Four, thermal property analysis of the edible protein film

[0113] Thermal stability - DSC analysis: DSC analysis was performed using a heat / cold / heat cycle at a speed of 10°C / min and a temperature range of -10°C-200°C using a DSC Q2000 equipped with a cooling system. A fully sealed aluminum pan was used, nitrogen was used as the purging gas, the sample chamber was flushed at a flow rate of 50 mL / min, and the glass transition temperature of the sample (edible protein film) was obtained using TA software analysis.

[0114] Five, scanning electron microscope

[0115] The cross-sectional scanning of the dried composite film was observed using a scanning electron microscope.

[0116] The cross-sectional scanning was detected according to the following method: the edible protein film sample strip was broken by liquid nitrogen, the fracture surface was upward, and the conductive glue was used to fix it on the sample stage, and vacuum gold plating treatment was performed to obtain the edible protein film sample stage. The edible protein film sample stage was placed in the observation device, and the surface was observed at an acceleration voltage of 20 kV and a magnification of 500 and 5000, respectively.

[0117] Detection results:

[0118] The detection results of the color, film thickness, and opacity of the edible protein films prepared in Examples 1-5 and Comparative Examples 1-3 are shown in Table 2.

[0119] Table 2

[0120]

[0121] ND indicates that testing was not possible.

[0122] In each column, different lower case superscripts indicate a significant difference (p < 0.05) between data.

[0123] As can be seen from the results shown in Table 2, the brightness and whiteness of the edible protein films prepared using the method of the present application are improved due to the synergistic effect of sodium caseinate and zein. At the same time, as can be seen from the opacity results of the edible protein films shown in Table 2, the transparency of the films is also improved due to the good interaction between sodium caseinate and zein.

[0124] The results of the tensile strength and elongation at break of the edible protein films prepared in Examples 1-5 and Comparative Examples 1-3 are shown in Table 3.

[0125] Table 3

[0126]

[0127]

[0128] ND indicates that testing was not possible.

[0129] In each column, different lower case superscripts indicate a significant difference (p < 0.05) between data.

[0130] As can be seen from Table 3, the tensile strength and elongation at break of the edible protein films prepared in the examples are superior to those of the edible protein films prepared in the comparative examples, indicating that the interaction between sodium caseinate and zein in the preparation method of the present application improves the flexibility of the edible protein films.

[0131] The solubility, swelling degree, moisture absorption rate, water contact angle, and water vapor permeability of the edible protein films prepared in Examples 1-5 and Comparative Examples 1-3 are shown in Table 4, respectively. As can be seen from Table 4, the solubility, swelling degree, moisture absorption rate, water contact angle, and water vapor permeability of the edible protein films prepared in the examples are superior to those of the edible protein films prepared in the comparative examples, indicating that the interaction between sodium caseinate and zein in the preparation method of the present application reduces the moisture sensitivity of the prepared edible protein films.

[0132] Table 4

[0133]

[0134] ND indicates that it cannot be tested.

[0135] In each column, different lowercase superscripts indicate significant differences between data (p<0.05).

[0136] Macroscopic images of the edible protein films prepared in Examples 1-5 and Comparative Example 1 are shown below. Figure 2 As shown, a macroscopic image of the edible protein membrane prepared in Comparative Example 2 is shown below. Figure 3 As shown. (Through) Figure 2 The results show that the edible protein film prepared by the method of the present invention has high uniformity and transparency, indicating that the addition of sodium caseinate in the preparation method of the present invention significantly improves the uniformity and transparency of the obtained edible protein film. Figure 3 The fact that Comparative Example 2 could not produce a complete edible protein film indicates that the edible protein film produced without using the preparation method of the present invention has poor performance. It also shows that the interaction between sodium caseinate and zein in the preparation method of the present invention plays an important role in the performance of the edible protein film.

[0137] Figure 4 As shown in the cross-sectional scanning electron microscope images of the dried edible protein films of Examples 1-5 and Comparative Example 1, it can be seen that the method of the present invention enables sodium caseinate to have high compatibility with zein, thereby producing an edible protein film with a dense structure. Figure 4 In the diagram, the magnification of af is 500x, and the magnification of AF is 5000x; a, A is CN-0; b, B is CN-0.25; c, C is CN-0.5; d, D is CN-0.75; e, E is CN-1.0; f, F is CN-1.25.

[0138] The differential scanning calorimetry (DSC) curves of the edible protein films prepared in Examples 1-5 and Comparative Examples 1-2 are shown below. Figure 5 As shown, through Figure 5 It is understood that the interaction between sodium caseinate and zein in the method of the present invention improves the thermal stability of the prepared edible protein film.

[0139] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing an edible protein membrane, characterized in that, The preparation method includes the following steps: Step 1, Solvent preparation: Prepare an ethanol aqueous solution with a volume concentration of 55-65% and preheat it in a water bath at a temperature of 60-70℃; Step 2, Preparation of protein film-forming solution: Under the above water bath conditions, sodium caseinate is added to the ethanol aqueous solution prepared in step 1 while stirring, and the solution is stirred to fully hydrate it. Then, zein is added. After the protein is fully dissolved, glycerol and citric acid are added, stirred and allowed to stand to defoam, and a uniform protein film-forming solution is obtained. Step 3, film formation: Transfer the protein film-forming solution into a planar matrix, dry it, peel off the film, and place the film in a desiccator for equilibration to obtain the edible protein film. In step 2, the ratio of sodium caseinate to zein added is 0.05-1.25:1, wherein the total protein concentration is fixed at 0.06-0.08 g / mL; In step 2, the concentration of glycerol is 0.005-0.015 g / mL and the concentration of citric acid is 0.015-0.03 g / mL.

2. The method for preparing the edible protein membrane according to claim 1, characterized in that, In step 1, the volume concentration of the ethanol-water solution is 60-65%, and the water bath temperature is 65-70℃. And / or, in step 3, the planar substrate is a petri dish.

3. The method for preparing the edible protein membrane according to claim 1, characterized in that, In step 1, the volume concentration of the ethanol aqueous solution is 60%, and the water bath temperature is 65℃.

4. The method for preparing the edible protein membrane according to claim 1, characterized in that, In step 2, the ratio of sodium caseinate to zein added is 0.75-1.25:1, and the total protein concentration is fixed at 0.07-0.08 g / mL.

5. The method for preparing the edible protein membrane according to claim 1 or 4, characterized in that, In step 2, the ratio of sodium caseinate to zein added is 0.75:1, with the total protein concentration fixed at 0.07 g / mL.

6. The method for preparing the edible protein membrane according to claim 1, characterized in that, The drying conditions in step 3 are 37-45℃ and 3-7 hours; the bottom of the dryer is filled with a saturated potassium carbonate solution, and the relative humidity is maintained at 40-45% for at least 48 hours.

7. The method for preparing the edible protein membrane according to claim 1, characterized in that, The drying conditions in step 3 are 40-45℃ and 3-5 hours.

8. An edible protein membrane, characterized in that, The edible protein membrane is prepared by the method according to any one of claims 1-7.

9. The application of the edible protein film according to claim 8 as a food preservative.

10. The application of the edible protein film according to claim 8 in food preservation.

11. The application according to claim 10, characterized in that, The application refers to the use of edible protein films in edible coatings and / or edible packaging films.

12. The method for preserving food using the edible protein film as described in claim 8, characterized in that, The method includes the following steps: preparing the edible protein film of claim 8 into an edible packaging bag for convenient food ingredients, and packaging the convenient food ingredients in the edible packaging bag.

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