Manufacturing method of edible carbon dot preservative film and application of edible carbon dot preservative film in poultry product preservation

Abstract

The invention discloses a preparation method of an edible carbon dot preservative film and application of the edible carbon dot preservative film in poultry product preservation, and the preparation method is characterized by comprising the following steps: mixing a sodium alginate solution with the mass concentration of 1-3% and a protamine solution with the mass concentration of 1-3% according to the volume ratio of (2-4): 1 to form an SA-PT solution; adding carbon dot powder of which the mass is 2-8% of that of the SA-PT solution into the SA-PT solution, and mixing to obtain the edible carbon dot preservative film; according to the application of the edible carbon dot preservative film in preparation of the antibacterial and antioxidant food preservative film and in preservation of poultry products, the poultry products are completely immersed in an edible carbon dot preservative film solution for 5 min, fished out and drained, and dried in an oven at the temperature of 30-40 DEG C for 20-40 min to form the film; the antibacterial plastic has the advantages of high oxidation resistance, strong antibacterial effect, safety and degradability.

Description

Technical Field

[0001] The present invention relates to the technical field of food functional packaging, and particularly to a preparation method of an edible carbon dot fresh-keeping film and its application in the preservation of poultry products. Background Art

[0002] Food safety has become a focus of wide social concern, especially the problem that meat products are prone to spoilage during storage and transportation. As an important part of daily diet, meat products require efficient preservation technologies to ensure food safety. However, although traditional petroleum-based plastic packaging materials have the advantages of convenience and cost-effectiveness, they are gradually regarded as unsustainable solutions due to their non-biodegradability and long-term environmental pollution problems. In addition, these plastics may release harmful chemical substances when in contact with food, posing a potential threat to human health. Therefore, the development of packaging materials with biocompatibility, safety and functionality has become an important research direction in the field of food science.

[0003] In recent years, as the core component of biodegradable active packaging, biopolymer films have received extensive attention due to their environmental friendliness, degradability, antibacterial properties and performance tunability. Natural polymers such as proteins, lipids and polysaccharides are often used as packaging substrates and are compounded with other components to optimize their performance. Among them, sodium alginate (SA) is widely used in food packaging due to its excellent film-forming property and biocompatibility. However, pure SA films have defects such as low mechanical strength, poor barrier properties, and limited antioxidant and antibacterial abilities. Protamine (PT) is a strongly basic protein extracted from the seminal vesicles of fish such as salmon and trout, and has significant antibacterial and biocompatible properties. As a functional additive, it can effectively improve the performance of composite films. However, PT will reduce the mechanical strength of the film, and the SA / PT composite film still has deficiencies in antioxidant and ultraviolet shielding properties, which limits its application in easily oxidized high-fat foods (such as meat products).

[0004] Carbon dots (CDs for short) are a new type of nanomaterial accidentally discovered during the purification process of single-walled carbon nanotubes in 2004. They have a core-shell structure composed of carbon atoms and usually have a size of about 10 nanometers. Its inner core is a graphitized crystal or amorphous carbon, and its outer shell is rich in functional groups such as hydroxyl and carboxyl groups, which endow CDs with excellent water solubility and stability, and can specifically bind to biomolecules. In recent years, based on the strong antioxidant, antibacterial activity and low toxicity of CDs, they have been introduced as active fillers into biopolymer films to improve packaging performance. At present, there are no relevant research reports at home and abroad on the preparation method and application of sodium alginate / protamine / carbon dot edible fresh-keeping films. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a preparation method of an edible carbon dot fresh-keeping film with high antioxidant and strong antibacterial effects and safe degradability, and its application in the fresh-keeping of poultry products.

[0006] The technical solution adopted by the present invention to solve the above technical problems is as follows: A preparation method of an edible carbon dot fresh-keeping film, comprising the following steps: Mix a sodium alginate solution with a mass concentration of 1-3% and a protamine solution with a mass concentration of 1-3% in a volume ratio of 2-4:1 to form a SA-PT solution, and add carbon dot (CDs) powder with a mass of 2-8% of the SA-PT solution to the SA-PT solution, and mix to obtain an edible carbon dot fresh-keeping film.

[0007] Further, the preparation method of the sodium alginate solution is: Dissolve sodium alginate in distilled water, add glycerol and stir to obtain a sodium alginate solution with a mass concentration of 1-3%, wherein the mass percentage of glycerol added is 0.4-0.8%.

[0008] Further, the preparation method of the protamine solution is: Dissolve protamine in distilled water, add glycerol and stir to obtain a protamine solution with a mass concentration of 1-3%, wherein the mass percentage of glycerol added is 0.4-0.8%.

[0009] Further, the preparation method of the carbon dot powder is as follows: Wash and dry fresh orange peel, cut it into small pieces, mix it with distilled water in a mass-to-volume ratio of 1 g: 3 mL, heat it in a muffle furnace to 180-220 °C, and keep it for 2-6 h. After the reaction is completed, take it out and cool it to room temperature. After centrifuging the solution, filter the obtained supernatant with a 0.22 μm filter membrane, dialyze the filtered solution with a 500 Da dialysis membrane for 48 h, and then freeze-dry the purified solution to obtain carbon dot powder.

[0010] Further, mix a sodium alginate solution with a mass concentration of 2% and a protamine solution with a mass concentration of 2% in a volume ratio of 3:1 to form a SA-PT solution, and add carbon dot (CDs) powder with a mass of 4% of the SA-PT solution to the SA-PT solution, and mix to obtain an edible carbon dot fresh-keeping film.

[0011] The present invention also provides the application of the edible carbon dot fresh-keeping film prepared by the above method in the preparation of antibacterial and antioxidant food fresh-keeping films.

[0012] The present invention also provides the application of the edible carbon dot fresh-keeping film prepared by the above method in the fresh-keeping of poultry products.

[0013] Further, completely immerse the poultry products in the edible carbon dot fresh-keeping film solution for 5 min, take them out and drain, and dry them in an oven at 30-40 °C for 20-40 min to form a film.

[0014] Compared with the prior art, the advantages of the present invention are as follows: A method for preparing an edible carbon dot fresh-keeping film and its application in the fresh-keeping of poultry products. By combining orange peel-derived carbon dots with sodium alginate / protamine composite film, a new type of edible film material is developed. This material not only has higher antioxidant and antibacterial properties, but also maintains good degradability and biocompatibility. The production method is simple and easy to implement. First, carbon dots are prepared from orange peel, and then they are mixed with sodium alginate and protamine to form a composite film. The whole process does not require complex equipment and steps, facilitating industrial production. It performs excellently in the fresh-keeping of meat products. After immersing food in the SA / PT / CDs film solution for a short time and then drying to form a film, it can effectively extend the fresh-keeping period of meat products and reduce the risk of spoilage. Moreover, the present invention also improves the mechanical strength and barrier properties of sodium alginate film, enhances the antioxidant and ultraviolet shielding properties, makes up for the deficiencies of traditional SA / PT composite film in antibacterial antioxidant and ultraviolet shielding, and expands its application in easily oxidized high-fat foods. In addition, the present invention realizes the transformation from food waste (orange peel) to high-value-added packaging materials, reduces production costs and environmental pollution, and conforms to the concept of sustainable development.

[0015] In summary, for a method for preparing an edible carbon dot fresh-keeping film and its application in the fresh-keeping of poultry products of the present invention, the film has excellent antioxidant properties (the DPPH and ABTS radical scavenging rates reach 95.84% and 87.89% respectively), strong antibacterial properties (the inhibition rate against Pseudomonas fluorescens is 99.4%), high ultraviolet shielding ability, and is biodegradable at the same time. Applying it to the fresh-keeping of lion's head can significantly delay the rise of pH value, inhibit lipid oxidation (the TBARS value is reduced by 47%) and microbial proliferation (the TVC value is reduced by 43%), and extend the shelf life by 6 days. The present invention uses orange peel waste to prepare high-value-added packaging materials, with simple process, low cost and environmental friendliness, providing an efficient solution for the green fresh-keeping of perishable foods such as meat. Brief Description of the Drawings

[0016] Figure 1 It is a schematic diagram of the TEM image (A) and particle size distribution diagram (B) of CDs; Figure 2 It is a change diagram of the scavenging ability of different concentrations of carbon dots (12.5, 25, 50, 70 and 100 μg / mL) against ABTS and DPPH free radicals; Figure 3 It is a change diagram of the inhibition rate of different concentrations of carbon dots (0.0, 62.5, 125, 250, 500, 1000 and 2000 μg / mL) against four kinds of bacteria. Among them, A is the OD of Escherichia coli with different concentrations of carbon dots 600 value, and B is the OD of Salmonella typhimurium with different concentrations of carbon dots 600Value, C is the OD of Staphylococcus aureus with different concentrations of carbon dots 600 Value, D is the OD of Pseudomonas fluorescens with different concentrations of carbon dots 600 Value, E is the survival rate of four bacteria at different carbon dot concentrations; Figure 4 Schematic diagram of the microscopic morphology of different composite membranes; Figure 5 DSC change diagram measured by differential scanning calorimetry of different composite membranes; Figure 6 UV-visible light transmittance change diagram of different composite membranes; Figure 7 Change diagram of the scavenging ability of different composite membranes against ABTS and DPPH free radicals; Figure 8 Inhibition rate change diagram of different composite membranes against four bacteria (Escherichia coli, Salmonella typhimurium, Staphylococcus aureus and Pseudomonas fluorescens); Figure 9 Cytotoxicity change diagram of different composite membranes; Figure 10 pH value change diagram during the storage of lion's head before and after adding SA / PT / CDs-4% composite membrane; Figure 11 TBARS value change diagram during the storage of lion's head before and after adding SA / PT / CDs-4% composite membrane; Figure 12 TVB-N value change diagram during the storage of lion's head before and after adding SA / PT / CDs-4% composite membrane; Figure 13 TVC value change diagram during the storage of lion's head before and after adding SA / PT / CDs-4% composite membrane. Detailed implementation mode

[0017] The present invention will be further described in detail below with reference to the embodiments of the drawings.

[0018] Specific Example 1. A method for making an edible carbon dot fresh-keeping film includes the following steps: Step 1. Preparation of carbon dots from orange peel Weigh 8 g of fresh orange peel, wash, dry and cut it into small pieces, mix it with 24 mL of distilled water, heat the reaction container in a muffle furnace to 200 °C and keep it for 4 h; after the reaction is over, take it out and cool it to room temperature, centrifuge the solution at a speed of 8000 rpm / min for 5 min, and then filter the obtained supernatant with a 0.22 μm filter membrane; dialyze the filtered solution with a 500 Da dialysis membrane for 48 h; finally, freeze-dry the purified solution for 48 h to obtain carbon dot powder, and this powder needs to be stored in the dark before further use; Step 2: Preparation of sodium alginate / protamine / carbon dot composite membrane 2 g of sodium alginate SA and protamine PT were dissolved in 100 mL of distilled water (60 ℃, 2 h), 0.6 g of glycerol was added, and 100 mL of SA-PT solution was mixed according to SA:PT=3:1 (V:V). SA / PT / CDs composite membranes were prepared by mixing different mass percentages (w / w) of CDs (0%, 2%, 4%, 6% and 8% based on the mass of SA-PT) into the SA-PT solution, and then the obtained film-forming solution was continuously stirred for 2 h to ensure that the film-forming solution was mixed evenly. After vacuum defoaming, 20 mL of the film-forming solution was poured into a round culture dish with a diameter of 10 cm, dried at 40 ℃ for 4 h, and finally placed in a constant temperature and humidity incubator (25 ℃, relative humidity 50%) for 48 h for subsequent use. These membranes were named SA / PT (control group), SA / PT / CDs-2%, SA / PT / CDs-4%, SA / PT / CDs-6% and SA / PT / CDs-8%.

[0019] Specific Example 2. Performance analysis of the orange peel-derived carbon dots prepared in Specific Example 1.

[0020] 1. Morphological characterization of orange peel-derived carbon dots.

[0021] The morphology and size of carbon dots were characterized using a transmission electron microscope. 10 μL of carbon dot aqueous solution was taken with a pipette and dropped onto an ultra-thin carbon film copper grid, dried using an infrared lamp, and imaged and analyzed using a transmission electron microscope.

[0022] The uniform, non-agglomerated and stable carbon dots were synthesized from orange peel by a one-step hydrothermal method. The TEM images of CDs are shown in Figure 2. Figure 1 As shown in Figure A, these CDs appear as spherical particles; the particle size distribution is as follows Figure 1 As shown in B, the average particle size is 4.77 nm and the lattice spacing is 0.34 nm, corresponding to the (002) crystal plane of graphitic carbon.

[0023] 2. Study on the antioxidant capacity of orange peel derived carbon dots.

[0024] The antioxidant activities of CDs and composite films were evaluated by DPPH scavenging rate and ABTS scavenging rate. The absorbance was measured at 517 nm and 734 nm, respectively, and the DPPH and ABTS scavenging rates (%) were calculated using the following formula: DPPH clearance rate (%) = [(A0-A1) / A0]*100%, ABTS clearance rate (%) = [(B0-B1) / B0]*100%, Among them, A0 represents the DPPH absorbance value of the blank group, and A1 represents the absorbance value of the experimental group. B0 represents the ABTS absorbance value of the blank group, and B1 represents the absorbance value of the experimental group.

[0025] Figure 2 The results show that the antioxidant ability of CDs is related to the concentration. When the concentration of CDs increases from 12.5 μg / mL to 100 μg / mL, the scavenging rate of ABTS radicals increases from 54.05% to 88.62%, while the scavenging rate of DPPH radicals increases from 22.06% to 83.04%. The scavenging ability of CDs for DPPH radicals is higher than that for ABTS radicals, which may be due to the hydrophilicity of CDs making it easier to interact with the aqueous solution of ABTS. CDs exhibit excellent ABTS and DPPH radical scavenging abilities, which may be attributed to the presence of a large number of oxygen-containing functional groups and amide functional groups on their surface. These functional groups quench or reduce free radicals by transferring protons from the hydroxyl and amide groups of CDs to DPPH-H and ABTS-H.

[0026] 3. Study on the antibacterial performance of orange peel-derived carbon dots.

[0027] Antibacterial activity test method of CDs: The antibacterial effects of CDs were tested in liquid and solid media respectively. Escherichia coli ( Escherichia coli, E. coli ), Salmonella typhimurium ( Salmonella typhimurium, S. typhimurium typhimurium ), Staphylococcus aureus ( Staphylococcus aureus, S. aureus ), and Pseudomonas fluorescens ( Pseudomonas fluorescens, P. fluorescens ) were inoculated into sterile LB liquid medium respectively. After culturing in a shaking incubator at 37 °C for 12 h, the concentration of the bacterial suspension was diluted to 10 5 CFU / mL. Then, all bacterial suspensions were treated with different concentrations of CDs (0.0, 62.5, 125, 250, 500, 1000, and 2000 μg / mL), and further cultured for 10 h. The absorbance at 600 nm was measured every two hours to evaluate the growth of the bacterial species. The bacterial suspensions of the four bacterial species (10 5 CFU / mL) were respectively spread on the surface of agar plates with CDs concentrations of 0.0, 62.5, 125, 250, 500, 1000, and 2000 μg / mL. After incubating at 37 °C for 24 h, the survival rate of the bacterial species was calculated.

[0028] To evaluate the antibacterial properties of CDs, four representative bacteria were selected in this study: E. coli, S. typhimurium typhimurium, S. aureus, P. fluorescens. The growth activity of bacteria was evaluated by measuring the OD600 value of the bacterial suspension after co-incubation with different concentrations of CDs (0, 62.5, 125, 250, 500, 1000, and 2000 μg / mL) for 10 h ( Figure 3 A-D). The results showed that as the concentration of CDs increased from 0 to 2000 μg / mL, the inhibitory effect on bacterial growth gradually enhanced. It is worth noting that when the concentration of CDs reached 2000 μg / mL, the growth of bacteria was almost completely inhibited after 8 h of co-culture, indicating that CDs had extremely strong antibacterial ability at this concentration. Further analysis found that there were significant differences in the sensitivity of different types of bacteria to CDs, and Gram-positive bacteria ( S. aureus ) were more sensitive to CDs than Gram-negative bacteria ( E. coli, S. typhimurium typhimurium, P. fluorescens ). This difference may be related to the differences in the cell wall structures of the two types of bacteria. The cell wall of Gram-positive bacteria is mainly composed of a relatively thick peptidoglycan layer, while the cell wall of Gram-negative bacteria contains a relatively thin peptidoglycan layer and an outer membrane structure. Therefore, CDs may be more likely to penetrate and damage the cell wall of Gram-positive bacteria, thus exerting a stronger antibacterial effect. In addition, in this study, the bacterial survival rate after 10 h of treatment with CDs was further measured ( Figure 3 E). The results showed that as the concentration of CDs increased, the bacterial viability decreased from 100% to 13% - 26%. This result was consistent with the measurement result of the OD 600 value, further confirming the strong inhibitory effect of CDs on bacterial growth and the differences in the tolerance of different bacterial species to CDs.

[0029] Specific Example 3. Performance analysis of the edible carbon dot fresh-keeping film SA / PT / CDs prepared in Specific Example 1.

[0030] 1. Morphological characterization of the SA / PT / CDs composite film.

[0031] The surface and cross-section (quenched by liquid nitrogen) of the composite film were observed by SEM. The film sample was adhered to the sample stage, the conductivity was improved by spraying gold, and then observed with an electron microscope.

[0032] The abundant hydroxyl and carboxyl groups on the surface of CDs and the hydrogen bond interaction between SA and PT molecules enabled their uniform dispersion in the polymer matrix, effectively avoiding the aggregation of nanoparticles. As Figure 4As shown, the cross-sectional morphology of the pure SA / PT film is rough, showing obvious pores and transverse cracks. As the CDs content increases from 2% to 4%, the pores and cracks gradually decrease, and the cross-sectional morphology becomes more compact and smooth. This indicates that CDs are uniformly dispersed in the polymer matrix and have good compatibility with the SA / PT composite. However, when the CDs content further increases to 6% and 8%, the surface of the composite film is slightly rough, and some tiny cracks also appear on the cross-section, which may be due to the poor compatibility between the excessive CDs and the biopolymer matrix. In addition, the vertical cracks on the cross-section may be caused by the evaporation of moisture during the air-drying process of the film. Generally speaking, the SEM results show that the addition of an appropriate amount of CDs can significantly improve the microstructure of the composite film, while an excessive CDs content may have an adverse effect on the performance of the composite film.

[0033] 2. Thermal property study of SA / PT / CDs composite films.

[0034] A differential scanning calorimeter was used to evaluate the thermal stability of the films. The scanning temperature was 25 - 250 °C, and the heating rate was 20 °C / min.

[0035] The DSC curves of the six groups of films were generally similar ( Figure 5 ), and the melting points of the composite films containing CDs were all higher than that of the pure SA / PT film (118 °C). The melting point of the film increased with the increase of the CDs content (2% - 6%), which may be related to the enhanced hydrogen bond interaction between CDs and the SA / PT matrix. As nano-fillers, CDs restrict the movement of molecular chains through physical cross-linking, thus improving the thermal stability of the material. However, when the CDs content was added to 8%, the melting point of the film dropped back to 123 °C, which may be because excessive CDs caused agglomeration, thus weakening the thermal stability of the film.

[0036] 3. Determination of ultraviolet-visible light transmittance of SA / PT / CDs composite films.

[0037] Ultraviolet rays have a significant impact on the quality, nutritional value, and biochemical reactions of foods. The ultraviolet protection ability of packaging films is crucial for preventing photochemical reactions in packaged foods and ensuring food quality. An ultraviolet-visible spectrophotometer was used to measure the barrier rate of the film to ultraviolet and visible light at wavelengths of 200 - 800 nm.

[0038] As Figure 6As shown, when CDs are incorporated into the SA / PT matrix, the ultraviolet blocking ability is enhanced with the increase of the doping concentration. The transmittance of ultraviolet light (<400 nm) and visible light (400 - 700 nm) both decreases significantly. This excellent ultraviolet blocking ability enables it to prevent ultraviolet light from penetrating the thin film matrix. Thus, the sustainable and environmentally friendly SA / PT / CDs thin film developed in this study can effectively shield UV-A and UV-B without the need for additional materials.

[0039] 4. Study on the antioxidant ability of the SA / PT / CDs composite film.

[0040] The test method for the antioxidant ability of the composite film is the same as that in Example 2. As Figure 7 shown, the SA / PT film shows the lowest antioxidant activity, with the scavenging rate of DPPH free radicals being 9.44% and the scavenging rate of ABTS free radicals being 5.15%. With the increase of the CDs content, the antioxidant activity of the SA / PT / CDs film increases significantly (p < 0.05). Especially when the CDs content reaches 8%, the highest antioxidant activity is observed, with the scavenging rate of DPPH free radicals being 95.84% (an increase of 86.40% compared to the control group) and the scavenging rate of ABTS free radicals being 87.89% (an increase of 82.74% compared to the control group). The enhanced antioxidant activity is attributed to the strong antioxidant properties of CDs. Thus, these thin films can be used as antioxidant packaging materials to prevent food from deteriorating due to oxidation during storage, thereby extending the shelf life of food.

[0041] 5. Study on the antibacterial performance of the SA / PT / CDs composite film.

[0042] Test method for the antibacterial activity of the composite film: The antibacterial activities of the composite film against four kinds of bacteria were tested. 5 mL of bacterial solution (10 5 CFU / mL) was respectively added to centrifuge tubes containing 50 mg of the thin film solution and cultured at 37 °C for 10 h. Then, 100 μL of the above mixture was evenly spread on the solid medium and cultured at 37 °C for 24 h. Finally, the growth inhibition rate was calculated based on the number of colonies.

[0043] Microbial contamination is one of the main problems of food packaging materials. Figure 8 shows the inhibition rates of the composite film against four kinds of bacteria ( E. coli, S. typhimurium, S. aureus and P. fluorescens ). The weak antibacterial ability of the SA / PT film comes from protamine. With the addition of CDs, the antibacterial ability of the film is significantly enhanced. When the CDs concentration reaches 4%, the inhibition rate against E. coli is 91.4% (an increase of 84.6% compared to the control group), the inhibition rate against S. typhimurium is 79.4% (an increase of 55.9% compared to the control group), and the inhibition rate against S. aureusThe inhibition rate was 95.3% (70.4% higher than the control group), and for P. fluorescens the inhibition rate was 99.4% (88.9% higher than the control group). Overall, the SA / PT / CDs film exhibited excellent antibacterial activity and had broad application potential in food preservation.

[0044] 6. Cytotoxicity study of SA / PT / CDs composite film.

[0045] To ensure the safety of the synthesized film in this study, an in vitro toxicity test was conducted. RAW 246.7 cells were placed in a 96-well plate containing DMEM medium (1×10 3 cells per well) and grown in an incubator at 37 °C for 24 h, and then co-cultured with thin film solutions with different CDs contents. The control group did not add thin film solution. The cell viability (%) was detected by the CCK-8 assay method, and the absorbance value was measured at 450 nm. The formula is as follows: Cell viability (%) = (A / B) * 100%, where A represents the absorbance of the sample and B represents the absorbance of the control group.

[0046] As Figure 9 shown, the survival rate of RAW 264.7 cells decreased with the increase of CDs concentration. After exposure to low-dose carbon dots (2% and 4%) for 24 h, the cell survival rate remained above 95%, showing no significant difference compared with the CK group (p > 0.05). Although higher doses of carbon dots (6% and 8%) slightly inhibited cell proliferation, the cell survival rate was still higher than 80%. Overall, the synthesized film in this study showed low toxicity to RAW 264.7 cells and exhibited good biocompatibility.

[0047] Specific Example 4. Analysis of the freshness preservation performance of the SA / PT / CDs-4% composite film prepared in Specific Example 1 for lion's head.

[0048] The lion's head was completely immersed in the SA / PT / CDs film solution for 5 min, fished out and drained, dried in an oven at 35 °C for 30 min to form a film, placed in a fresh-keeping bag, and stored in a refrigerator at 4 °C. Samples at 0, 3, 6, 9, and 12 days were taken for the determination of pH value, TBARS value, TVB-N value, and TVC value.

[0049] The preparation method of lion's head is as follows: (1) Raw material pretreatment: Cut the duck breast meat into small pieces, remove visible fat and connective tissue, cut the pork fat into pieces, wash and drain the water for later use; (2) Minced meat preparation: Mix the duck meat pieces with an appropriate amount of pork fat (mass ratio of 7:3), and add the following auxiliary materials: the starch addition amount is 4wt% of the minced meat quality; the addition amounts of salt, white granulated sugar, and cooking wine are all 2% of the minced meat quality; the addition amounts of ginger powder and garlic powder are 0.2% of the minced meat quality; the addition amount of dark soy sauce is 1% of the minced meat quality. Put them into a household meat grinder and beat until the minced meat is uniform; (3) Shaping and steaming: Make the mixed minced meat into meatballs with a diameter of about 5 cm, place them in a steamer, steam at 100 °C for 20 min, and cool to room temperature for later use.

[0050] 1. Effect of SA / PT / CDs-4% composite film on the pH value of lion's head during storage.

[0051] The pH value is a key indicator to measure the freshness of meat. At room temperature, mix 3 g of lion's head with 17 mL of distilled water, and homogenize and disperse it in a homogenizer under ice bath conditions (10,000 r / min, 6 s each time). After centrifugation, take the supernatant and measure it with a pH meter, record the pH value, and repeat each sample three times.

[0052] Figure 10 It shows the change of pH value of each group of lion's heads during storage, and the pH value shows a trend of first decreasing and then increasing. In the initial stage (0 - 6 days), the pH value of the CK group decreased from 6.1 to 5.2 (the pH value of the experimental group decreased from 6.0 to 5.7), mainly due to the degradation of ATP in muscle tissue to generate phosphoric acid and the anaerobic glycolysis of glycogen to produce lactic acid; in the middle and late storage period (6 - 9 days), the pH value of the CK group rebounded to 6.8 (the pH value of the experimental group rebounded to 6.4), which is closely related to the production of alkaline amines (such as ammonia, trimethylamine) by proteolysis and the enhanced microbial metabolic activity. However, compared with the CK group, the rising rate of the pH value of the group treated with the film was significantly reduced, which indicates that the strong antioxidant effect and low oxygen permeability of the SA / PT / CDs-4% group synergistically inhibited lipid oxidation, indirectly inhibited the activity of endogenous proteases, reduced the decomposition rate of proteins by aerobic bacteria, and reduced the accumulation of alkaline metabolites, thus playing a role in delaying the deterioration of lion's head.

[0053] 2. Effect of SA / PT / CDs-4% composite film on the TBARS value of lion's head during storage.

[0054] Mix 5 g of lion's head sample with 25 mL of 7.5% (v / v) trichloroacetic acid solution, homogenize thoroughly and filter with filter paper. Take the supernatant (2.5 mL) and mix it with 2.5 mL of 0.02 mol / L thiobarbituric acid solution. Heat the mixed solution in a boiling water bath for 40 min, cool to room temperature, measure its absorbance at 532 nm, and calculate the TBARS value.

[0055] Lipid oxidation can deteriorate the sensory properties and color stability of meat products, leading to undesirable flavor, odor, and color changes. In addition, oxidation products can also damage the nutritional value of amino acids and reduce the functionality of proteins. As Figure 11 shown, with the increase of storage time, the TBARS values of the two groups of lion's heads gradually increased, indicating an intensification of lipid oxidation. However, on the 12th day, the TBARS value of the group treated with the film (0.78 ± 0.09 mg MDA / kg) was significantly lower than that of the CK group (1.47 ± 0.08 mg MDA / kg). This indicates that the film inhibited the growth of spoilage microorganisms, thus reducing the amount of lipase produced by bacteria, and further reducing the degree of lipid oxidation.

[0056] 3. Effect of SA / PT / CDs-4% composite film on TVB-N value of lion's head during storage.

[0057] The determination was carried out according to the method in GB / T 5009.228-2016, and each group had three parallel tests, and the average value was taken.

[0058] An increase in TVB-N level indicates the degradation of nitrogen-containing compounds due to the action of microorganisms and enzymes. Figure 12 The changes in TVB-N values of lion's head during storage are shown. The TVB-N values of both groups gradually increased, but the increase rate of the film-treated group was significantly lower than that of the CK group. By the 12th day, the TVB-N value of the CK group reached 16.40 ± 0.90 mg / 100 g, exceeding the spoilage threshold of 15 mg / 100 g, while the film-treated group was only 9 ± 0.76 mg / 100 g (45.5% lower than the CK group). These data indicate that the SA / PT / CDs-4% film inhibited the protein degradation caused by microorganisms, thus reducing the rapid increase in TVB-N value related to bacterial proliferation.

[0059] 4. Effect of SA / PT / CDs-4% composite film on TVC value of lion's head during storage.

[0060] Weigh 5 g of meat sample, mix it with 45 mL of sterile normal saline respectively, and put it into a sterile homogenization bag. Use a homogenizer to beat to ensure thorough mixing. Take 1 mL of the above mixture and coat it on PCA solid medium, and incubate it at 30 °C for 48 h, and record the number of colonies generated.

[0061] The rapid reproduction of microorganisms is one of the main factors leading to the deterioration of food quality. The TVC of lion's head products should not exceed 6 log CFU / g, and this value is considered as the acceptable threshold of freshness. As Figure 13As shown, with the extension of storage time, the TVC value gradually increases. On the 6th day, the TVC value of the CK group (7.10 ± 0.04 log CFU / g) has exceeded the spoilage threshold, while the value of the film-coated group is close to the spoilage threshold (6 ± 0.05 log CFU / g) on the 12th day. This indicates that the SA / PT / CDs-4% film treatment significantly inhibits the growth of microorganisms and extends the shelf life of the lion's head by 6 days.

[0062] The above description is not a limitation of the present invention, nor is the present invention limited to the above examples. Changes, modifications, additions or substitutions made by those of ordinary skill in the art within the scope of the essence of the present invention shall also fall within the protection scope of the present invention.

Claims

1. A method for preparing an edible carbon dot fresh-keeping film, characterized in that It includes the following steps: Mix a sodium alginate solution with a mass concentration of 1-3% and a protamine solution with a mass concentration of 1-3% at a volume ratio of 2-4:1 to form a SA-PT solution. Add carbon dot powder accounting for 2-8% of the mass of the SA-PT solution to the SA-PT solution and mix to obtain an edible carbon dot fresh-keeping film.

2. The manufacturing method of an edible carbon dot fresh-keeping film according to claim 1, characterized in that: The preparation method of the sodium alginate solution is as follows: Dissolve sodium alginate in distilled water, add glycerol and stir to obtain a sodium alginate solution with a mass concentration of 1-3%, wherein the mass percentage of glycerol added is 0.4-0.8%.

3. The manufacturing method of an edible carbon dot fresh-keeping film according to claim 1, characterized in that: The preparation method of the protamine solution is as follows: Dissolve protamine in distilled water, add glycerol and stir to obtain a protamine solution with a mass concentration of 1-3%, wherein the mass percentage of glycerol added is 0.4-0.8%.

4. The manufacturing method of an edible carbon dot fresh-keeping film according to claim 1, characterized in that The preparation method of the carbon dot powder is as follows: Wash and dry fresh orange peels and cut them into small pieces. Mix them with distilled water at a mass-to-volume ratio of 1 g: 3 mL, heat in a muffle furnace to 180-220 °C and keep for 2-6 h. After the reaction ends, take out and cool to room temperature. Centrifuge the solution, filter the obtained supernatant with a 0.22 μm filter membrane, dialyze the filtered solution with a 500 Da dialysis membrane for 48 h, and then freeze-dry the purified solution to obtain carbon dot powder.

5. The manufacturing method of an edible carbon dot fresh-keeping film according to any one of claims 1-4, characterized in that: Mix a sodium alginate solution with a mass concentration of 2% and a protamine solution with a mass concentration of 2% at a volume ratio of 3:1 to form a SA-PT solution. Add carbon dot powder accounting for 4% of the mass of the SA-PT solution to the SA-PT solution and mix to obtain an edible carbon dot fresh-keeping film.

6. Application of the edible carbon dot fresh-keeping film prepared by the method described in claim 1 in the preparation of an antibacterial and antioxidant food fresh-keeping film.

7. Application of the edible carbon dot fresh-keeping film prepared by the method described in claim 1 in the preservation of poultry products.

8. The application according to claim 6 or 7, characterized in that: Completely immerse the poultry products in the edible carbon dot fresh-keeping film solution for 5 min, take them out and drain, and dry them in an oven at 30-40 °C for 20-40 min to form a film.

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