Preparation method and application of cold plasma modified casein composite Chinese yam polysaccharide film

Abstract

The invention provides a preparation method of a cold plasma modified casein composite Chinese yam polysaccharide film. The method comprises the following steps: treating casein with cold plasma, preparing a casein solution, preparing a chitosan solution, preparing a Chinese yam polysaccharide solution, preparing an edible film solution, adding a plasticizer and forming a film. The invention further provides application for refrigeration and preservation of meat. The prepared cold plasma modified casein composite Chinese yam polysaccharide film has excellent light transmission, waterproof performance, barrier performance and mechanical performance to water vapor and oil, is high in tensile strength and good in elongation at break, and has antibacterial performance, oxidation resistance and antibacterial preservation capacity to cooled meat when being used for meat preservation.

Description

Technical Field

[0001] The invention belongs to the technical field of meat preservation, and particularly relates to a preparation method and application of a cold plasma modified casein composite yam polysaccharide film. Background Art

[0002] Meat and meat products are rich in nutritional matrix and are an important part of a balanced diet. However, meat and meat products are very prone to spoilage if they are not stored properly. Microbial reproduction, protein and fat oxidation, and enzyme digestion affect the shelf life of meat products, causing huge economic losses to companies. Moreover, once meat and meat products become spoiled, they will cause serious food safety problems. Studies have shown that biogenic amines such as tyramine, cadaverine, putrescine, and spermine in known livestock meat are potentially toxic, and 100 mg of tyramine can cause migraines, which is seriously harmful to human health. Therefore, the use of appropriate technology to maintain the safety and quality of meat and meat products is the main problem faced by meat companies, which is of great significance for ensuring the economic benefits and sustainable development of my country's meat industry.

[0003] Protein is not only an essential nutrient in human diet, but also an important raw material for food industry, with many functional properties related to food processing quality. However, the functional properties of natural protein may not be ideal, such as poor solubility and allergenicity, which limits its application in food industry. In the process of food processing, enzymatic, physical and chemical methods are usually used to improve the functional properties of protein to meet the requirements of actual production, but traditional modification methods usually require high energy consumption and cost, and the use of chemical reagents may have adverse effects on human health. Therefore, the use of eco-friendly non-thermal physical methods to modify protein structure and improve its functional properties has become a research hotspot in the field of food science. As an emerging non-thermal physical processing technology, cold plasma technology can bombard solid surfaces with high-energy particles, change the surface morphology of materials in the micrometer or nanometer range, and introduce various active groups. The use of cold plasma technology to improve the packaging performance of protein films is of great significance to promote the application of proteins in the field of food packaging. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a preparation method and application of a cold plasma modified casein composite yam polysaccharide film in view of the deficiencies of the above-mentioned prior art. The cold plasma modified casein composite yam polysaccharide film has excellent light transmittance, waterproof performance, barrier performance to water vapor and oil and mechanical properties, high tensile strength and good elongation at break. When used for meat preservation, it has antibacterial properties, antioxidant properties and antibacterial preservation ability for cooled meat.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for preparing a cold plasma modified casein composite yam polysaccharide film, the method comprising:

[0006] S1. Cold plasma treatment of casein:

[0007] Extracting milk casein from goat milk and freeze-drying it to obtain casein powder, and using cold plasma to modify the casein powder to obtain casein modified by cold plasma; the parameters of the cold plasma modification treatment are: frequency 10kHz, voltage 20kV, treatment temperature 28°C, relative humidity 34%, and treatment time 3min;

[0008] S2. Prepare casein solution:

[0009] The casein modified by cold plasma obtained in S1 was dissolved in distilled water, the pH of the system was adjusted to 11.5 with a 1 mol / L NaOH aqueous solution, and stirred at 37°C for 1 h to obtain a casein solution;

[0010] S3, preparing chitosan solution:

[0011] Add chitosan to an aqueous solution of acetic acid with a volume fraction of 1%, and stir at room temperature at 1000 r / min for 2 hours to obtain a chitosan solution;

[0012] S4. Preparation of yam polysaccharide solution:

[0013] dissolving yam polysaccharide in distilled water to obtain yam polysaccharide solution;

[0014] S5, preparing edible film solution:

[0015] The chitosan solution obtained in S3 and the yam polysaccharide solution obtained in S4 are mixed to obtain a polysaccharide mixed solution;

[0016] The casein solution obtained in S2 is mixed with the polysaccharide mixed solution to obtain an edible film solution; the mass ratio of casein, chitosan and yam polysaccharide in the edible film solution is 1:1:1;

[0017] S6. Adding plasticizer:

[0018] adding glycerol to the edible film solution obtained in S5 to obtain a composite film solution;

[0019] S7, film forming:

[0020] The composite membrane solution obtained in S6 was stirred at 37° C. for 1 h, and then dried at 37° C. to obtain a cold plasma modified casein composite yam polysaccharide membrane.

[0021] Preferably, the ratio of casein after cold plasma modification treatment to distilled water in S2 is 2.5 g:100 mL.

[0022] Preferably, the amount ratio of chitosan to 1% acetic acid aqueous solution in S3 is 1 g:100 mL.

[0023] Preferably, the usage ratio of yam polysaccharide and distilled water in S4 is 1 g:100 mL.

[0024] Preferably, the mass ratio of glycerol to the total amount of casein, chitosan and yam polysaccharide in the composite membrane solution is 30%.

[0025] The present invention also provides application of the cold plasma modified casein composite yam polysaccharide film prepared by the above preparation method, wherein the cold plasma modified casein composite yam polysaccharide film is used for refrigerating and preserving meat.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] The cold plasma modified casein composite yam polysaccharide film prepared by the present invention has excellent light transmittance, waterproof performance, water vapor and oil barrier performance and mechanical properties, high tensile strength and good elongation at break. When used for meat preservation, it has antibacterial performance, antioxidant performance and antibacterial preservation ability for cooled meat.

[0028] The present invention is further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Graph showing the effect of cold plasma treatment on casein phosphorylation according to Example 2 of the present invention.

[0030] Figure 2 This is a graph showing the effect of cold plasma treatment on casein (A) water holding capacity (B) oil holding capacity (C) solubility (D) foaming property and foam stability in Example 2 of the present invention. (Note: Different letters indicate significant differences between treatments P<0.05)

[0031] Figure 3 The figure shows the effect of the cold plasma treatment of Example 2 of the present invention on casein (A) pH (B) carbonyl content (C) DPPH free radical scavenging ability (D) total antioxidant capacity.

[0032] Figure 4 This is the FTIR spectrum of casein treated with cold plasma in Example 2 of the present invention.

[0033] Figure 5 Graph showing the effect of cold plasma treatment on the secondary structure of casein according to Example 2 of the present invention.

[0034] Figure 6 Graph 2 shows the effect of cold plasma treatment of Example 2 of the present invention on (A) particle size, (B) PDI and (C) microstructure of casein.

[0035] Figure 7 The effects of different casein to polysaccharide ratios on (A) light transmittance, (B) L value, (C) a value and (D) b value of the edible film in Example 3 of the present invention are shown.

[0036] Figure 8 The effects of different casein to polysaccharide ratios on (A) thickness, (B) moisture content and (C) water solubility of edible films in Example 3 of the present invention are shown.

[0037] Fig. 9 The effects of different casein to polysaccharide ratios on (A) water vapor permeability, (B) oxygen permeability, (C) carbon dioxide permeability and (D) oil permeability of edible films according to Example 3 of the present invention are shown.

[0038] Fig.10 The following are the effects of different casein to polysaccharide ratios on (A) tensile strength and (B) elongation at break of edible films according to Example 3 of the present invention.

[0039] Fig.11 This is a graph showing the effects of different membrane treatments on the sensory scores of pork during refrigeration in Example 4 of the present invention.

[0040] Fig.12 This is a graph showing the effects of different membrane treatments on the pH of pork during refrigeration in Example 4 of the present invention.

[0041] Fig.13 This is a graph showing the effect of different membrane treatments on the TBARS value of pork during refrigeration in Example 4 of the present invention.

[0042] Fig.14 This is a graph showing the effect of different membrane treatments on the total bacterial count during refrigerated storage of pork in Example 4 of the present invention.

[0043] (Note: Figure 11-14 Different capital letters indicate significant differences between the CY group (P<0.05), different lowercase letters indicate significant differences between the PE group (P<0.05), ** indicates extremely significant differences between the CY and PE groups (P<0.01), * indicates significant differences between the CY and PE groups (P<0.05) DETAILED DESCRIPTION

[0044] Example 1

[0045] The preparation method of the cold plasma modified casein composite yam polysaccharide film of this embodiment is as follows:

[0046] S1. Cold plasma treatment of casein:

[0047] The goat milk used in the study was taken from fresh milk from a farm in Jinchang City, Gansu Province, China. After sampling, the samples were placed in an ice box and taken back, and stored at -80°C. The milk casein in the goat milk was extracted by ultracentrifugation and freeze-dried to obtain casein powder, which was modified by cold plasma of a dielectric barrier discharge reactor (DBD-50) to obtain casein modified by cold plasma; the parameters of the cold plasma modification treatment were: frequency 10kHz, voltage 20kV, treatment temperature 28°C, relative humidity 34%, and treatment time 3min;

[0048] S2. Prepare casein solution:

[0049] 2.5 g of the casein obtained in S1 after cold plasma modification was dissolved in 100 mL of distilled water, the pH of the system was adjusted to 11.5 with a 1 mol / L NaOH aqueous solution, and the mixture was stirred at 37°C for 1 h to obtain a casein solution;

[0050] S3, preparing chitosan solution:

[0051] 1 g of chitosan was added to 100 mL of 1% acetic acid aqueous solution, and stirred at 1000 r / min at room temperature for 2 h to obtain a chitosan solution;

[0052] S4. Preparation of yam polysaccharide solution:

[0053] Dissolve 1 g of yam polysaccharide in 100 mL of distilled water to obtain a yam polysaccharide solution;

[0054] S5, preparing edible film solution:

[0055] The chitosan solution obtained in S3 and the yam polysaccharide solution obtained in S4 are mixed to obtain a polysaccharide mixed solution;

[0056] The casein solution obtained in S2 is mixed with the polysaccharide mixed solution to obtain an edible film solution; the mass ratio of casein, chitosan and yam polysaccharide in the edible film solution is 1:1:1;

[0057] S6. Adding plasticizer:

[0058] Adding glycerol to the edible film solution obtained in S5 to obtain a composite film solution; the mass ratio of glycerol to the total amount of casein, chitosan and yam polysaccharide in the composite film solution is 30%;

[0059] S7, film forming:

[0060] The composite membrane solution obtained in S6 was stirred at 37° C. for 1 h, and then dried at 37° C. to obtain a cold plasma modified casein composite yam polysaccharide membrane.

[0061] Example 2

[0062] This example is to optimize the treatment time in the parameters of the cold plasma modification treatment in the method for preparing the cold plasma modified casein composite yam polysaccharide film in Example 1.

[0063] (1) Sample collection and processing

[0064] The goat milk used in the study was obtained from fresh milk from a farm in Jinchang City, Gansu Province, China. The samples were taken back in an ice box and stored at -80°C. Casein was extracted by ultracentrifugation. After extraction, the casein was freeze-dried. The casein freeze-dried samples were treated in a dielectric barrier discharge reactor (DBD-50) with a processing frequency of 10 kHz, a voltage of 20 kV, a processing temperature of 28°C, a relative humidity of 34%, and a processing time of 0, 2, 3, 4, and 5 min.

[0065] (2) Effect of cold plasma treatment on casein phosphorylation

[0066] PO4 3- The infrared characteristic absorption peak is between 1100-920cm -1 The characteristic absorption peak of PN in Lys is around 500 cm -1 Around. Figure 1 It can be seen that PO4 3- About 1080cm -1 The absorption peaks of casein at these two locations after cold plasma treatment are significantly increased, indicating that a new part of phosphate groups has been added to the protein after treatment, and most of these phosphate groups are connected to nitrogen atoms. In addition, at 950-870cm -1 A new absorption peak is added at the position of the absorption peak, which may be due to the addition of negative charges after cold plasma treatment, causing the position of the absorption peak to shift. It is speculated that PO4 3- In summary, low-temperature plasma treatment causes casein phosphorylation.

[0067] (3) Effects of cold plasma treatment on the functional properties of casein

[0068] Depend on Figure 2 It can be seen that within 3 minutes of cold plasma treatment, the water holding capacity of casein increased significantly ( Figure 2 A), while the oil holding capacity showed a decreasing trend (P<0.05; Figure 2B). The effect of plasma treatment on the water and oil holding capacity of casein may be related to the increase of active sites in plasma treatment, the loosening of the casein surface structure, and the exposure of hydrophobic groups. The solubility of casein increases first and then decreases with the increase of plasma treatment time. The solubility is the highest when the treatment time is 3min and 4min ( Figure 2 C), the cold plasma treatment may improve the solubility because the high-energy particles cause a large number of water groups to be excited, forming a water film on the protein surface, which increases the solubility of casein. In addition, within 3 minutes of cold plasma treatment, the foaming ability and foam stability of casein increased significantly. The foaming ability and foam stability of casein after treatment were higher than those of untreated ( Figure 2 D). In summary, cold plasma treatment can improve the water holding capacity, solubility and foaming ability of casein, and the protein properties are best when treated for 3-4 minutes.

[0069] (4) Effects of cold plasma treatment on pH and oxidative properties of casein

[0070] Depend on Figure 3 It can be seen that as the plasma treatment time of casein increased, the pH value decreased significantly (P<0.05; Figure 3 A) It may be because low-temperature plasma will produce a large number of high-energy particles during the discharge process, which will ionize oxygen molecules into single molecules of oxygen, oxidize NO to NO2, generate nitrous acid (HNO2) and nitric acid (HNO3), and water molecules react with hydrogen peroxide (H2O2) in the air or liquid to generate acidic H3O + ions. The carbonyl content showed an upward trend within 2-5 minutes of treatment ( Figure 3 B), the DPPH free radical scavenging ability showed a downward trend (P<0.05; Figure 3 C). The increase in carbonyl content is caused by the reactive oxygen and reactive nitrogen free radicals generated by cold plasma, which oxidize the side chains of amino acid residues, thereby causing casein oxidation. The total antioxidant capacity was significantly reduced after 3 minutes of treatment ( Figure 3 D). In summary, cold plasma treatment can lead to oxidation of casein, and a treatment time of 3 min is relatively more reasonable.

[0071] (5) Effect of cold plasma treatment on the secondary structure of casein

[0072] The ROS generated by cold plasma is very likely to cause protein oxidation and changes in amino acid side chain groups, thereby affecting the protein structure. -1 The Fourier transform infrared spectrum of represents the C=O stretching vibration of amide I, reflecting the changes in the secondary structure of the casein sample. Figure 4As shown in the figure, a significant blue shift was observed in the absorption amide I band at 2-3 minutes of treatment, while a red shift occurred at 4-5 minutes of treatment. Cold plasma reduced the density of C=O electrons, formed intermolecular hydrogen bonds, and caused the absorption peak to move to a lower wave number. The results show that the conformation of the protein becomes unstable as the treatment time increases.

[0073] Typically, the amide I region reflects conformational changes in proteins, including the α-helix (1650–1660 cm -1 ), β-folding (1610~1640cm -1 ), β angle (1661~1681cm -1 ) and random curl (1640~1650cm -1 ).like Figure 5 As shown, the proportion of α-helix increased during treatment for 0 to 3 minutes, and decreased during treatment for 3 to 5 minutes (P<0.05). In addition, with the extension of treatment time, the β-folding of the sample decreased, and the β-turn and random coiling increased. The extension of cold plasma treatment time will inevitably cause protein aggregation, resulting in the conversion of β-folding to random coiling and β-turn. The results show that the optimal time for cold plasma treatment of sheep milk casein is 3 minutes, and the protein conformation is destroyed as the treatment time increases.

[0074] (6) Effect of cold plasma treatment on casein particle size and microstructure

[0075] like Figure 6 A and 6B show that the casein particle size increased significantly after 2-4 min of treatment (P<0.05). Protein oxidation caused by cold plasma treatment will lead to an increase in carbonyl groups and a decrease in free -SH groups, which will then form disulfide crosslinks and cause protein aggregation. No changes were observed after 4-5 min of treatment, which may be because the driving force generated by the plasma caused the depolymerization and decomposition of casein micelles. In addition, the PDI of the untreated sample and the sample treated for 2 min was less than 0.2, and the PDI was significantly increased after 3 min of treatment (P<0.05).

[0076] Effects of cold plasma treatment on casein microstructure Figure 6 C. Before 3 minutes of cold plasma treatment, many dense and uniform monomers were observed, and the surface morphology was smooth and complete. At 4 and 5 minutes of treatment, obvious irregular protein aggregation and some holes were found, which significantly changed the microstructure of casein.

[0077] (7) Summary

[0078] Cold plasma treatment for 0 to 4 minutes increased the phosphorylation degree of casein. After treatment for 2 to 3 minutes, the water holding capacity, solubility, foaming and foam stability of casein increased, while the pH value, DPPH free radical scavenging ability and antioxidant activity decreased. As the treatment time increased, the protein conformation became unstable, and obvious irregular protein aggregation and some holes were found, which significantly changed the microstructure of casein. In summary, cold plasma treatment for 3 minutes (10kHz, 20kV) is the best parameter to be selected.

[0079] Example 3

[0080] This example is a method for preparing a cold plasma modified casein composite yam polysaccharide film in Example 1, in which the weight ratio of casein and polysaccharide in preparing an edible film solution is optimized.

[0081] (1) Sample collection and processing

[0082] Casein treated with cold plasma (frequency 10kHz, voltage 20kV, time 3min), yam polysaccharide with purity 75%, glycerol, sodium alginate, chitosan, and acetic acid.

[0083] (2) Preparation process of cold plasma modified goat milk casein composite yam polysaccharide edible film

[0084] The film was prepared by solution casting. Casein (2.5 g) after cold plasma modification was dissolved in 100 mL of distilled water, and the pH was adjusted to 11.5 with 1 mol / L NaOH. The solution was stirred continuously for 1 h at 37 °C to prepare a casein solution with a concentration of 2.5% (w / v). Chitosan (1 g) was mixed in an aqueous solution of acetic acid (1% v / v, 100 mL) and stirred vigorously (1000 r / min) for 2 h at room temperature until the solvation was complete. 1 g of yam polysaccharide was dissolved in 100 mL of distilled water to prepare a 1% (w / v) yam polysaccharide solution. The edible film solution was prepared with a weight ratio of casein: polysaccharide (yam polysaccharide: chitosan = 1:1) of 1:5, 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, the total weight of casein and polysaccharide in the edible film solution was 0.5g, and 30% (based on the total weight of casein and polysaccharide in the composite film solution) of glycerol was added as a plasticizer. The composite film solution was stirred continuously at 37°C for 1h, poured into a culture dish, and placed in an electric constant temperature incubator at 37°C to dry for 12h.

[0085] (3) Optical properties of edible films

[0086] The color and opacity of edible films are important indicators for judging the quality of edible films. They can protect packaged food from ultraviolet radiation that may cause food oxidation, but darker edible films will blur consumers' vision. Figure 7It can be seen that with the decrease of polysaccharide content, the transmittance of edible film showed a significant increasing trend, and the transmittance was the highest when the ratio of protein to polysaccharide was 1:2, 1:1, 2:1 and 3:1 ( Figure 7 A), this is because in the process of composite film formation, the higher the polysaccharide content, the stronger the molecules absorb yellow light and the worse the light transmittance. In addition, the brightness value L increases with the increase of protein content ( Figure 7 B); the a values ​​are all negative, indicating that the edible films are all green, and there is no difference between different proportions ( Figure 7 C); the larger the b value, the yellower the color of the edible film. As the polysaccharide content decreases, the b value of the edible film shows a significant downward trend ( Figure 7 D).

[0087] (4) Edible film thickness, moisture content, and water solubility

[0088] The thickness of edible film is Figure 8 As shown in A, when the solid weight of the edible film solution was kept consistent, there was no significant difference in the thickness of the edible films with different protein and polysaccharide ratios, indicating that the thickness of the edible film was not a key influencing factor. Figure 8 B represents the moisture content of the edible film. The addition of yam polysaccharide can enhance the hydrophobic interaction of the film. As the polysaccharide content decreases, the moisture content of the edible film increases. However, there is no significant difference in the moisture content of the edible film when the ratio of casein to polysaccharide is 1:3, 1:2, 1:1, 2:1, and 3:1, which also shows that the casein content has no effect on the moisture content of the edible film. Figure 8 As shown in Figure C, the water solubility determines whether the film is suitable for packaging foods with high water content or storing them in a high humidity environment, which can affect its water resistance. As the polysaccharide content decreases, the water solubility of the edible film decreases. The water solubility is lowest when the casein to polysaccharide ratio is 1:2 and 1:1, and there is no difference in water solubility with the edible film with increased casein content, indicating that the edible film under this ratio has excellent waterproof properties.

[0089] (5) Barrier properties of edible films

[0090] As a barrier between food and the outside environment, food packaging film plays an important role in preventing the exchange of substances between the two and contamination and damage to food quality. Water vapor permeability is an important indicator to measure the water barrier performance of the film. Fig. 9As shown in A, with the decrease of polysaccharide content, water vapor permeability shows a trend of first decreasing and then increasing (P<0.05). When the ratio of protein to polysaccharide is 1:5 and 1:4, the water vapor permeability is the highest and the barrier performance is the worst. When the ratio of protein to polysaccharide is 1:1, 2:1 and 3:1, the barrier performance is second. When the ratio of protein to polysaccharide is 1:3 and 1:2, the water vapor permeability is the lowest and the barrier performance is the best. Too high or too low polysaccharide concentration will make the membrane structure loose, the gap between polymers will increase, and water molecules will pass through the membrane more easily, resulting in increased water vapor permeability and reduced barrier performance. Oxygen permeability is an important indicator to measure the oxygen barrier performance of the membrane. Fig. 9 As shown in Figure B, when the ratio of protein to polysaccharide is 1:5 and 1:4, the oxygen permeability is the highest and the barrier performance is the worst. When the ratio of protein to polysaccharide is 1:2 and 1:1, the barrier performance is the best. The carbon dioxide permeability of the edible film has the same trend as the oxygen permeability ( Fig. 9 C). Oil permeability is an important indicator to measure the oil barrier performance of the membrane. Fig. 9 As shown in D, when the ratio of protein to polysaccharide is 1:1, 2:1 and 3:1, the oil permeability is the lowest and the barrier performance is the best. When the ratio of protein to polysaccharide is 1:2, the barrier performance is second. When the ratio of protein to polysaccharide is 1:5 and 1:4, the oil permeability is the highest and the barrier performance is the worst.

[0091] (6) Mechanical properties of edible films

[0092] The tensile strength of the composite film can reflect the force that the composite film can withstand. The higher the tensile strength, the greater the force that the composite film can withstand. The elongation at break of the composite film can reflect the flexibility of the composite film. The higher the elongation at break, the better the flexibility of the composite film. Fig.10 A and 10B show the effects of different casein to polysaccharide ratios on the mechanical properties of edible films (including tensile strength and elongation at break). Fig.10 It can be seen that the tensile strength of the edible film is the highest when the ratio of protein to polysaccharide is 1:5, 1:4, 1:3 and 1:2. The elongation at break of the edible film is the best when the ratio of protein to polysaccharide is 1:4, 1:3 and 1:2.

[0093] (7) Summary

[0094] The preparation process of cold plasma modified goat milk casein composite yam polysaccharide edible film was determined, and the optical properties, film thickness, moisture content, water solubility, barrier properties and mechanical properties of the edible film were measured. It was found that the edible film with the best performance was made when the ratio of casein to polysaccharide was 1:2.

[0095] Example 4

[0096] This example is a study on the application of the cold plasma modified casein composite yam polysaccharide film prepared in Example 1 in meat preservation.

[0097] The cold plasma modified casein composite yam polysaccharide film is used for refrigeration and preservation of meat; the cold plasma modified casein composite yam polysaccharide wraps the meat and is stored at a temperature of 4° C. for 10 days.

[0098] (1) Sample collection and processing

[0099] Fresh pork was purchased from a local supermarket, rinsed carefully with sterile water, placed on a clean bench, and the fascia and excess fat were removed with a sterile knife, and cut into 3cm×3cm×3cm small pieces. Then, the pork was packaged with PE plastic wrap (PE) and modified casein composite yam polysaccharide film (CY), respectively, and stored at 4°C for 10 days. The relevant indicators were measured on the 0th, 2nd, 4th, 6th, 8th, and 10th days.

[0100] (2) Effects of different membrane treatments on the sensory quality of pork

[0101] The two groups of pork were scored by sensory evaluation based on smell, color and texture, with a maximum score of 10 and a score below 6.0 being considered sensory unacceptable. Fig.11 It can be seen that the sensory scores of the two groups of pork decreased to varying degrees with the extension of storage time. The sensory score of the PE group was greater than 6.0 points in the first 2 days. From the 4th to the 10th day, the freshness of the pork decreased rapidly, and the rancid smell was obviously felt. The sensory score of the CY group was higher in the first 4 days, and it was still within the acceptable range on the 6th day, with no obvious odor.

[0102] (3) Effects of different membrane treatments on pork pH

[0103] pH change is an important indicator of the degree of spoilage of meat products. Freshly slaughtered meat is generally slightly acidic. As the maturation time increases, muscle glycogen breaks down to produce lactic acid under the action of glycolysis, and the pH of the meat decreases, which can be as low as 5.4-5.6. After that, under the action of microorganisms and endogenous enzymes, protein breaks down to produce alkaline substances such as amines, the pH rises, and the meat deteriorates. According to GB / T9695.5-2008 "Determination of pH of Meat and Meat Products", meat with a pH greater than 6.7 is considered spoiled meat. Fig.12 It can be seen that the pH of pork decreased after 2 days of storage, and then showed an overall upward trend. The pH of the PE group increased significantly and exceeded 6.7 on the 6th day. There was no difference in the CY group during storage for 2 to 4 days, and the pH was still below 6.7 on the 8th day of storage, indicating that the edible film has a strong antibacterial ability.

[0104] (4) Effects of different membrane treatments on TBARS values ​​of pork

[0105] Unsaturated fatty acids in meat are easily oxidized by air. The secondary product malondialdehyde generated after oxidation can react with thiobarbituric acid to form a stable red complex, which can be used to judge the degree of lipid oxidation in meat. When TBARS (thiobarbituric acid reactants) is between 0.202 and 0.664 mg / kg, it is good quality meat; when it is greater than 1 mg / kg, it is inferior quality meat. Fig.13 It can be seen that the TBARS values ​​of pork treated with different films increased significantly during storage. The PE group increased very quickly, and the rancid taste was clearly felt on the 8th day of storage. The TBARS value of the CY group was significantly lower than that of the PE group, indicating that edible films can delay lipid oxidation to a certain extent.

[0106] (5) Effects of different membrane treatments on total colony count in pork

[0107] Depend on Fig.14 It can be seen that the total number of colonies in pork treated with different films increased significantly during storage. The total number of colonies in the PE group and the CY group exceeded the national standard limit (6log CFU / g) on ​​the 6th and 10th day of storage, respectively, indicating that it had deteriorated. The total number of colonies in the CY group was significantly lower than that in the PE group, indicating that the edible film has a certain antibacterial effect.

[0108] (6) Summary

[0109] Compared with PE cling film, the cold plasma modified casein composite yam polysaccharide film prepared in Example 1 significantly delayed the increase of pork pH, TBARS value and total bacterial count, and had strong antioxidant and antibacterial capabilities.

[0110] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent change made to the above embodiment according to the technical essence of the invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a cold plasma modified casein composite yam polysaccharide film, characterized in that: The method is: S1. Cold plasma treatment of casein: Extracting milk casein from goat milk and freeze-drying it to obtain casein powder, and using cold plasma to modify the casein powder to obtain casein modified by cold plasma; the parameters of the cold plasma modification treatment are: frequency 10kHz, voltage 20kV, treatment temperature 28°C, relative humidity 34%, and treatment time 3min; S2. Prepare casein solution: The casein modified by cold plasma obtained in S1 was dissolved in distilled water, the pH of the system was adjusted to 11.5 with a 1 mol / L NaOH aqueous solution, and stirred at 37°C for 1 h to obtain a casein solution; S3, preparing chitosan solution: Add chitosan to an aqueous solution of acetic acid with a volume fraction of 1%, and stir at room temperature at 1000 r / min for 2 hours to obtain a chitosan solution; S4. Preparation of yam polysaccharide solution: dissolving yam polysaccharide in distilled water to obtain yam polysaccharide solution; S5, preparing edible film solution: The chitosan solution obtained in S3 and the yam polysaccharide solution obtained in S4 are mixed to obtain a polysaccharide mixed solution; The casein solution obtained in S2 is mixed with the polysaccharide mixed solution to obtain an edible film solution; the mass ratio of casein, chitosan and yam polysaccharide in the edible film solution is 1:1:1; S6. Adding plasticizer: adding glycerol to the edible film solution obtained in S5 to obtain a composite film solution; S7, film forming: The composite membrane solution obtained in S6 was stirred at 37° C. for 1 h, and then dried at 37° C. to obtain a cold plasma modified casein composite yam polysaccharide membrane.

2. The method for preparing a cold plasma modified casein composite yam polysaccharide film according to claim 1, characterized in that: The usage ratio of casein after cold plasma modification treatment and distilled water in S2 is 2.5 g:100 mL.

3. The method for preparing a cold plasma modified casein composite yam polysaccharide film according to claim 1, characterized in that: The amount ratio of chitosan and 1% acetic acid aqueous solution in S3 is 1 g:100 mL.

4. The method for preparing a cold plasma modified casein composite yam polysaccharide film according to claim 1, characterized in that: The dosage ratio of yam polysaccharide and distilled water in S4 is 1g:100mL.

5. The method for preparing a cold plasma modified casein composite yam polysaccharide film according to claim 1, characterized in that: The mass ratio of glycerol to the total amount of casein, chitosan and yam polysaccharide in the composite membrane solution is 30%.

6. An application of a cold plasma modified casein composite yam polysaccharide film prepared by the preparation method according to any one of claims 1 to 5, characterized in that: The cold plasma modified casein composite yam polysaccharide film is used for refrigerating and preserving meat.

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