Application of fish skin gelatin composite coating as preservative in aquatic products

Abstract

The invention provides an application of a fish skin gelatin composite coating as a preservative in aquatic products, the aquatic products are oysters, the fish skin gelatin composite coating is an aqueous solution containing 1-1.5 g of fish skin gelatin and 20-30 mg of UVC-GA, and the fish skin gelatin composite coating is prepared by dissolving fish skin gelatin freeze-dried powder in water, adding a proper amount of a preservative, and uniformly stirring. The preparation method comprises the following steps: preparing a fish skin gelatin solution with the volume fraction of 5%, adding a UVC-GA antibacterial solution with the volume fraction of 0.2%, uniformly stirring, adjusting the pH value of the mixed solution to 5.5-6.5, placing the obtained mixed solution in a water bath with the temperature of 55 DEG C, keeping the temperature for 35 minutes, and carrying out ultrasonic treatment for 25 minutes for defoaming. The method effectively prolongs the shelf life of the oysters, provides a new method for aquatic products, is a promising method for delaying the deterioration of the quality of the aquatic products and reducing the loss after capturing, and has huge application potential.

Description

Technical Field

[0001] The invention relates to the technical field of aquatic product processing, and in particular to an application of a fish skin gelatin composite coating as a preservative in aquatic products. Background Art

[0002] Refrigeration can effectively inhibit the activity of endogenous enzymes in the muscle tissue of aquatic products and the growth and reproduction of surface microorganisms. It is widely used in the preservation of aquatic products, but it cannot completely inhibit microorganisms and is difficult to meet the needs of long-distance transportation and large-scale production. Spoilage caused by microbial contamination is an important problem that the food industry needs to solve. Food packaging is not only used to store food, but also to extend the shelf life of food, thereby ensuring food safety and improving food quality. A large number of new preservation technologies have been developed, and edible coatings are an effective strategy to improve the quality and shelf life of aquatic products. Therefore, the use of appropriate preservation methods, such as active packaging and natural phenolic compounds, can protect the quality of seafood products and prevent the generation of odors.

[0003] The addition of natural active substances is an effective way to improve the quality and performance of coatings. Bioactive substances, such as gelatin, whey, zein, chitosan, etc., are often used to prepare edible coatings to inhibit the growth of microorganisms in food. Considering the impact of food safety on consumer health, the development of safe and hygienic edible coatings has become increasingly popular in the field of food refrigeration and preservation. Gelatin has become a popular raw material for the development of edible coatings due to its good film-forming properties, gelling properties and air permeability. A variety of gelatin composite coatings have been developed in the food industry and have been widely used in food preservation. Fish gelatin has similar functional properties to mammalian gelatin and is an excellent substitute for mammalian gelatin. In addition, fish gelatin may be a more acceptable material than mammalian gelatin in terms of flavor and aroma.

[0004] Bioactive substances (such as organic acids, phenolic compounds, essential oils, etc.) are added to edible coatings to prevent oxidation, degradation and enzymatic browning of coated foods and improve food quality and safety. Gallic acid, GA, 3,4,5-trihydroxybenzoic acid, is a plant phenolic compound found in nuts, vegetables and tea leaves, a low molecular weight natural antioxidant and. It is considered to be an ideal bioactive ingredient with antibacterial and antioxidant activities that can extend the shelf life of seafood; in addition, it can improve the mechanical strength and antioxidant properties of the coating. The use of active packaging containing GA during food storage has a better preservation effect in delaying lipid oxidation or microbial growth.

[0005] So far, no research has reported the effects of edible composite coatings prepared from fish skin gelatin and gallic acid on the quality of refrigerated aquatic products. Summary of the invention

[0006] The purpose of the present invention is to provide an application of a fish skin gelatin composite coating as a preservative in aquatic products. The composite coating prepared by the method has good mechanical properties and antioxidant properties, has a good maintenance effect on the color, sensory organs and muscle structure of oysters, effectively hinders the decrease of oyster pH, inhibits the growth of microorganisms and the increase of TVB-N, reduces the production of biogenic amines, and further.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] The invention discloses an application of a fish skin gelatin composite coating as a preservative in aquatic products. The aquatic products are oysters. The fish skin gelatin composite coating is an aqueous solution containing 1-1.5 g of fish skin gelatin and 20-30 mg of UVC-GA. The fish skin gelatin composite coating is prepared by dissolving fish skin gelatin freeze-dried powder in water to prepare a fish skin gelatin solution with a volume fraction of 5%, adding a UVC-GA antibacterial liquid with a volume fraction of 0.2%, stirring evenly, adjusting the pH value of the mixed solution to 5.5-6.5, placing the obtained mixed solution in a water bath at 55°C for 35 minutes, and then ultrasonically treating the mixed solution for 25 minutes to defoam.

[0009] A method for preserving freshness of aquatic products comprises the following steps: immersing the aquatic products in a fish skin gelatin composite coating solution for 60-65 seconds, taking them out and air-drying them for 10 minutes at room temperature, then putting them into a food-grade PP plastic packaging box, sealing and marking them, and storing them in a refrigerator at 0°C.

[0010] Furthermore, the aquatic product is oyster meat.

[0011] Furthermore, the fish skin gelatin composite coating is an aqueous solution containing fish skin gelatin and UVC-GA.

[0012] Furthermore, 20-30 mg UVC-GA antibacterial liquid is added to every 1-1.5 g fish skin gelatin solution.

[0013] Furthermore, the fish skin gelatin solution is prepared by dissolving fish skin gelatin freeze-dried powder in water, stirring with a magnetic stirring bar at 200 rpm and 25° C. overnight to prepare a fish skin gelatin solution with a volume fraction of 5%.

[0014] Furthermore, the fish skin gelatin freeze-dried powder is prepared by thawing the fish skin, washing the thawed fish skin with clean water, cutting it into blocks, soaking it in a 0.05 mol / L NaOH aqueous solution for 35-40 min, repeatedly washing it with clean water three times, and washing it to neutrality each time, swelling it with a 0.05 mol / L sulfuric acid solution for 3.5 h, washing it with water to neutrality and then mincing it, extracting it with 50°C hot water overnight, centrifuging it for 35 min, collecting the supernatant, concentrating it under reduced pressure and then freeze-drying it to obtain the grass carp fish skin gelatin freeze-dried powder.

[0015] Furthermore, the fish skin is any one of grass carp, sea bass and tilapia skin.

[0016] Furthermore, the UVC-GA antibacterial solution was prepared by diluting gallic acid with deionized water to a volume fraction of 0.2%, transferring it to a sterilized crystal culture dish, and irradiating it in a UV chamber at 4 J / cm 2 The irradiation was continued at high intensity for 12 h, and the irradiated gallic acid was filtered through a sterile filter with a pore size of 0.25 μm to obtain UVC-GA antibacterial liquid, which was stored in a dark place at room temperature.

[0017] Furthermore, a desktop UV-C intermittent light source with a wavelength of 254 nm and an average intensity of 4540 μW / cm 2 , liquid penetration is 17cm.

[0018] The present invention has the following beneficial effects due to the adoption of the above technical solution:

[0019] 1. In the present invention, the radiation-modified gallic acid can penetrate into the fish skin gelatin molecules, enhance the hydrogen bonding and van der Waals forces between the molecules, reduce the gaps between the molecules, and thus reduce the distance between the molecular chains. That is to say, the addition of the radiation-modified gallic acid improves the molecular cross-linking of the fish skin gelatin, forms hydrogen bonds, and makes the internal structure of the coating more compact, so that the fish skin gelatin composite coating has better mechanical properties; the mechanical properties of the fish skin gelatin composite coating are the key factors for the coating to effectively coat aquatic products. The coating with excellent mechanical properties can isolate the aquatic products from the external environment, thereby achieving the purpose of preserving the aquatic products.

[0020] 2. In the present invention, fish skin gelatin has antioxidant ability, and radiation-modified gallic acid has antioxidant activity of scavenging free radicals, which can delay lipid oxidation and reduce the oxygen permeability of the coating. Gallic acid added to fish skin gelatin plays a synergistic antibacterial and antioxidant role, further enhancing the antioxidant properties of the fish skin gelatin composite coating, thereby extending the shelf life of aquatic products.

[0021] 3. The present invention effectively prolongs the shelf life of oysters, provides a new method for aquatic products, delays the deterioration of aquatic product quality, reduces post-capture losses, is a promising method for maintaining the quality of refrigerated aquatic products, and has great application potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a graph showing the mechanical properties test results of the fish skin gelatin composite coating of the present invention;

[0023] Figure 2 This is a graph showing the antioxidant performance of the fish skin gelatin composite coating of the present invention;

[0024] Figure 3This is a graph showing the brightness change of oysters in Example 4 of the present invention;

[0025] Figure 4 This is a graph showing the change in redness of oysters in Example 4 of the present invention;

[0026] Figure 5 is a graph showing the trend of sensory score changes of oysters in Example 4 of the present invention;

[0027] Figure 6 is a graph showing the trend of pH value changes in oysters in Example 4 of the present invention;

[0028] Figure 7 This is a graph showing the TVC value variation of oysters in Example 4 of the present invention;

[0029] Figure 8 This is a graph showing the change in TVB-N value of oysters in Example 4 of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and preferred embodiments. However, it should be noted that many details listed in the specification are only for the purpose of enabling the reader to have a thorough understanding of one or more aspects of the present invention, and these aspects of the present invention can be implemented even without these specific details.

[0031] The invention discloses an application of a fish skin gelatin composite coating as a preservative in aquatic products. The aquatic products are oysters. The fish skin gelatin composite coating is an aqueous solution containing 1-1.5 g of fish skin gelatin and 20-30 mg of UVC-GA. The fish skin gelatin composite coating is prepared by dissolving fish skin gelatin freeze-dried powder in water to prepare a fish skin gelatin solution with a volume fraction of 5%, adding a UVC-GA antibacterial liquid with a volume fraction of 0.2%, stirring evenly, adjusting the pH value of the mixed solution to 5.5-6.5, placing the obtained mixed solution in a water bath at 55°C for 35 minutes, and then ultrasonically treating the mixed solution for 25 minutes to defoam.

[0032] A method for preserving freshness of aquatic products comprises the following steps: immersing the aquatic products in a fish skin gelatin composite coating solution for 60-65 seconds, taking them out and air-drying them for 10 minutes at room temperature, then putting them into a food-grade PP plastic packaging box, sealing and marking them, and storing them in a refrigerator at 0°C.

[0033] The seafood is oyster meat.

[0034] The fish skin gelatin composite coating is an aqueous solution containing fish skin gelatin and UVC-GA.

[0035] Add 20-30 mg UVC-GA antibacterial liquid to every 1-1.5 g fish skin gelatin solution.

[0036] The fish skin gelatin solution is prepared by dissolving fish skin gelatin freeze-dried powder in water, stirring with a magnetic stirring bar at 200 rpm and 25° C. overnight to prepare a fish skin gelatin solution with a volume fraction of 5%.

[0037] The fish skin gelatin freeze-dried powder is prepared by thawing the fish skin, washing the thawed fish skin with clean water, cutting it into blocks, soaking it in a 0.05 mol / L NaOH aqueous solution for 35-40 minutes, repeatedly washing it with clean water for three times, and washing it to neutrality each time, swelling it with a 0.05 mol / L sulfuric acid solution for 3.5 hours, washing it with water to neutrality and then mincing it, extracting it with 50°C hot water overnight, centrifuging it for 35 minutes, collecting the supernatant, concentrating it under reduced pressure and then freeze-drying it to obtain the grass carp fish skin gelatin freeze-dried powder.

[0038] The fish skin is any one of grass carp, sea bass and tilapia skin.

[0039] The UVC-GA antibacterial solution was prepared by diluting gallic acid with deionized water to a volume fraction of 0.2%, transferring it to a sterilized crystal culture dish, and irradiating it in a UV chamber at 4 J / cm 2 The irradiation was continued at high intensity for 12 h, and the irradiated gallic acid was filtered through a sterile filter with a pore size of 0.25 μm to obtain UVC-GA antibacterial liquid, which was stored in a dark place at room temperature.

[0040] The UV chamber uses a desktop UV-C intermittent light source with a wavelength of 254nm and an average intensity of 4540μW / cm 2 , liquid penetration is 17cm.

[0041] Example 1

[0042] The invention discloses an application of fish skin gelatin composite coating as a preservative in aquatic products. Gallic acid is first diluted with deionized water to a volume fraction of 0.2%, transferred to a sterilized crystal culture dish, and then irradiated in a UV chamber at 4 J / cm 2 The irradiation was continued for 12 h, and the irradiated gallic acid was filtered through a sterile filter with a pore size of 0.25 μm to obtain UVC-GA antibacterial liquid, which was stored in the dark at room temperature. A desktop UV-C intermittent light source was used in the UV room with a wavelength of 254 nm and an average intensity of 4540 μW / cm 2, the liquid penetration is 17cm; then the grass carp skin is thawed, the thawed fish skin is rinsed with clean water and cut into blocks, and then soaked in 0.05mol / L NaOH aqueous solution (w / w=1:6) for 35-40min, and rinsed with clean water three times, and each time washed until neutral, and then swollen with 0.05mol / L sulfuric acid solution (w / w=1:6) for 3.5h, washed with water until neutral and minced, extracted with 50℃ hot water overnight, centrifuged for 35min, collected the supernatant, concentrated under reduced pressure and freeze-dried to obtain grass carp skin gelatin freeze-dried powder; the grass carp skin gelatin freeze-dried powder is dissolved in water, and 20 The method comprises the following steps: stirring the mixture at 0 rpm and 25° C. with a magnetic stirring bar overnight to prepare a grass carp skin gelatin solution with a volume fraction of 5%; then adding UVC-GA antibacterial liquid to the grass carp skin gelatin solution, adding 20-30 mg UVC-GA antibacterial liquid to every 1-1.5 g of the grass carp skin gelatin solution, and adjusting the pH value of the mixed solution to 5.5-6.5 after stirring evenly; placing the obtained mixed solution in a 55° C. water bath at a constant temperature for 35 min, and ultrasonically treating the mixed solution for 25 min to defoam, to obtain a grass carp skin gelatin composite coating solution, and marking the grass carp skin gelatin composite coating solution as GC-UVC-GA; finally taking 10 -12 g of the grass carp skin gelatin composite coating solution and casting it on a disposable sterile culture dish, and drying it in a 40-45° C. oven to form a film, to obtain a grass carp skin gelatin composite coating.

[0043] Example 2

[0044] The invention discloses an application of fish skin gelatin composite coating as a preservative in aquatic products. Gallic acid is first diluted with deionized water to a volume fraction of 0.2%, transferred to a sterilized crystal culture dish, and then irradiated in a UV chamber at 4 J / cm 2 The irradiation was continued for 12 h, and the irradiated gallic acid was filtered through a sterile filter with a pore size of 0.25 μm to obtain UVC-GA antibacterial liquid, which was stored in the dark at room temperature. A desktop UV-C intermittent light source was used in the UV room with a wavelength of 254 nm and an average intensity of 4540 μW / cm 2, the liquid penetration is 17cm; then the sea bass fish skin is thawed, the thawed fish skin is rinsed with clean water and cut into blocks, and then soaked in 0.05mol / L NaOH aqueous solution (w / w=1:6) for 35-40min, and rinsed with clean water three times, and each time washed until neutral, and then swollen with 0.05mol / L sulfuric acid solution (w / w=1:6) for 3.5h, washed with water until neutral and minced, extracted with 50℃ hot water overnight, centrifuged for 35min, collected the supernatant, concentrated under reduced pressure and freeze-dried to obtain sea bass fish skin gelatin freeze-dried powder; the sea bass fish skin gelatin freeze-dried powder is dissolved in water, and 200 rpm and a magnetic stirring bar at 25°C for overnight stirring to prepare a sea bass skin gelatin solution with a volume fraction of 5%; then, UVC-GA antibacterial liquid is added to the sea bass skin gelatin solution, 20-30 mg UVC-GA antibacterial liquid is added to every 1-1.5 g sea bass skin gelatin solution, and the pH value of the mixed solution is adjusted to 5.5-6.5 after stirring evenly, and the obtained mixed solution is placed in a 55°C water bath for 35 minutes and subjected to ultrasonic treatment for 25 minutes for defoaming to obtain a sea bass skin gelatin composite coating solution, and the sea bass skin gelatin composite coating solution is marked as SP-UVC-GA; finally, 10-12 g of the sea bass skin gelatin composite coating solution is cast on a disposable sterile culture dish, and dried in a 40-45°C oven to form a film, so as to obtain a sea bass skin gelatin composite coating.

[0045] Example 3

[0046] The invention discloses an application of fish skin gelatin composite coating as a preservative in aquatic products. Gallic acid is first diluted with deionized water to a volume fraction of 0.2%, transferred to a sterilized crystal culture dish, and then irradiated in a UV chamber at 4 J / cm 2 The irradiation was continued for 12 h, and the irradiated gallic acid was filtered through a sterile filter with a pore size of 0.25 μm to obtain UVC-GA antibacterial liquid, which was stored in the dark at room temperature. A desktop UV-C intermittent light source was used in the UV room with a wavelength of 254 nm and an average intensity of 4540 μW / cm 2, the liquid penetration is 17cm; then the tilapia skin is thawed, the thawed skin is rinsed with clean water and cut into blocks, and then soaked in a 0.05mol / L NaOH aqueous solution (w / w=1:6) for 35-40min, and rinsed with clean water three times, and each time washed until neutral, and then swollen with a 0.05mol / L sulfuric acid solution (w / w=1:6) for 3.5h, washed with water until neutral and minced, extracted with 50℃ hot water overnight, centrifuged for 35min, collected the supernatant, concentrated under reduced pressure and freeze-dried to obtain tilapia skin gelatin freeze-dried powder; the tilapia skin gelatin freeze-dried powder is dissolved in water, and 2 00rpm and 25 ℃ of magnetic stirring bars were stirred overnight to prepare a 5% volume fraction of tilapia fish skin gelatin solution; then UVC-GA antibacterial liquid was added to the tilapia fish skin gelatin solution, 20-30mg UVC-GA antibacterial liquid was added to every 1g of tilapia fish skin gelatin solution, and the pH of the mixed solution was adjusted to 5.5-6.5 after stirring evenly, the obtained mixed solution was placed in a 55 ℃ water bath and kept at constant temperature for 35min, and ultrasonic treatment was performed for 25min to defoam, so as to obtain a tilapia fish skin gelatin composite coating solution, and the tilapia fish skin gelatin composite coating solution was marked as TI-UVC-GA; finally, 10-12g of the tilapia fish skin gelatin composite coating solution was cast on a disposable sterile culture dish, and dried in a 40-45 ℃ oven to form a film, so as to obtain a tilapia fish skin gelatin composite coating.

[0047] Comparative Example 1

[0048] Gallic acid was diluted with deionized water to a volume fraction of 0.2%, transferred to a sterilized crystal culture dish, and continuously irradiated at an intensity of 4J / cm2 for 12 hours in a UV chamber. The irradiated gallic acid was filtered using a sterile filter with a pore size of 0.25μm to obtain a UVC-GA antibacterial liquid, which was stored in a dark place at room temperature; a desktop UV-C intermittent light source was used in the UV chamber with a wavelength of 254nm, an average intensity of 4540μW / cm2, and a liquid penetration of 17cm; the UVC-GA antibacterial liquid was marked as UVC-GA, and UVC-GA was only irradiated gallic acid; 10-12g of UVC-GA antibacterial liquid was cast on a disposable sterile culture dish, and dried in a 40-45℃ oven to form a film, and no film was formed after the UVC-GA antibacterial liquid was dried.

[0049] The composite coatings (GC-UVC-GA, SP-UVC-GA and TI-UVC-GA) in Examples 1-3 were subjected to mechanical property tests, and the tensile strength (TS, MPa) and elongation at break (E, %) of each coating were determined with reference to the ASTM standard method (2002). The films to be tested (GC-UVC-GA, SP-UVC-GA and TI-UVC-GA) were cut into strips of 550 mm × 100 mm using a paper cutter, and the experimental length was 200 mm. The test was measured using an automatic tensile testing machine, with a tensile speed of 10 mm / min and an effective clamping distance of 300 mm. The test was repeated 5 times to obtain the average value. The elongation at break and the tensile strength were calculated according to formula (1) and formula (2), respectively.

[0050]

[0051] In formula (1), D is the elongation at break of the specimen, %; L0 is the distance between the original marking lines of the specimen, mm; L is the distance between the marking lines when the specimen breaks, mm.

[0052] In formula (2), K is the tensile strength of the specimen, u / Mpa; p is the maximum load, N; b is the width of the specimen, mm; d is the thickness of the specimen, mm.

[0053] In the present invention, since the UVC-GA group coating did not form a film after drying, only the mechanical properties of the three composite gelatin-based coatings (GC-UVC-GA, SP-UVC-GA, and TI-UVC-GA) could be tested.

[0054] from Figure 1 It can be concluded that the three composite gelatin-based coatings (GC-UVC-GA, SP-UVC-GA and TI-UVC-GA) all have good mechanical properties, among which the elongation at break and tensile strength of the composite coatings prepared from tilapia and grass carp skin gelatin have no significant difference, and are significantly higher than those of the composite coatings prepared from sea bass skin gelatin.

[0055] In the present invention, the three composite gelatin-based coatings all have good mechanical properties, which are related to the functional properties of the coating matrix fish skin gelatin itself (such as film-forming properties, gel strength, viscosity, etc.) and the degree of crosslinking of the polymer network. The breaking elongation and tensile strength of the composite coatings prepared from tilapia and grass carp skin gelatin are significantly higher than those of the composite coatings prepared from sea bass skin gelatin. On the one hand, this is because the gel properties and viscosity of tilapia and grass carp skin gelatin are higher than those of sea bass skin gelatin. On the other hand, the addition of GA improves the intermolecular crosslinking of tilapia and grass carp skin gelatin, forms hydrogen bonds, and makes the internal structure of the coating more compact, thus having good mechanical properties.

[0056] The composite coatings (GC-UVC-GA, SP-UVC-GA, TI-UVC-GA and UVC-GA) in Examples 1-3 and Comparative Example 1 were tested for their antioxidant properties.

[0057] (1) DPPH free radical scavenging ability

[0058] Mix 3 ml of 5 mg / 100 mL DPPH ethanol solution with 1 mL of different coating solutions (GC-UVC-GA, SP-UVC-GA, TI-UVC-GA and UVC-GA), place the solution in a dark place to react for about 30 min, and then measure the absorbance of the solution at 517 nm using an ultraviolet spectrophotometer. Perform 5 parallel experiments for each sample and take the average value of the results. The free radical scavenging rate is calculated according to formula (3).

[0059]

[0060] In formula (3), DPPH free radical scavenging rate (%): DPPH free radical scavenging rate; ADPPH: absorbance of ethanol solution at 517 nm; A0: absorbance of chitosan and control sample solution at 517 nm.

[0061] (2)Fe 3+ Reduction power

[0062] Take 2.5mL of different coating solutions (GC-UVC-GA, SP-UVC-GA, TI-UVC-GA and UVC-GA), add 2.5mL of phosphate buffer solution (20mmol / L, pH=6.69) and 2.5mL of 1% potassium ferricyanide solution, mix thoroughly, place in a 50℃ water bath for 20min, and then quickly add 2.5mL of 10% trichloroacetic acid solution to the mixed system after ice bath. Centrifuge at 5000r / min for 15min, then take out 2.5mL of supernatant from the mixed system and place in a test tube, add 2.5mL of distilled water and 1mL of 0.1% ferric chloride solution, react at room temperature for 5min, and measure the absorbance at 700nm. Perform 5 parallel experiments for each sample, and take the average value of the results.

[0063] from Figure 2It can be concluded that in the present invention, each test group coating (GC-UVC-GA, SP-UVC-GA, TI-UVC-GA and UVC-GA) has the ability to remove DPPH, wherein the composite coating (GC-UVC-GA, SP-UVC-GA and TI-UVC-GA) prepared by fish skin gelatin and UVC-GA has the ability to remove DPPH much higher than that of UVC-GA coating, and there is no significant difference between the three composite coatings (GC-UVC-GA, SP-UVC-GA and TI-UVC-GA). The reducing ability of the composite coating (GC-UVC-GA, SP-UVC-GA and TI-UVC-GA) is also much higher than that of UVC-GA coating, and the composite coating prepared by tilapia skin has the strongest reducing ability. It can be seen that after adding fish skin gelatin, the antioxidant capacity of the coating has been significantly improved.

[0064] The main reason for the deterioration of aquatic products is protein oxidation. The antioxidant properties of the coating determine whether the protein and lipid are oxidized and the degree of oxidation, and then determine the shelf life of the aquatic products. Fish skin gelatin itself has antioxidant capacity, and GA is also an antioxidant. Due to its antioxidant activity of scavenging free radicals, it can delay lipid oxidation and reduce the oxygen permeability of the coating. In the present invention, the composite coating prepared from irradiated GA and fish skin gelatin has good antioxidant properties, and the antioxidant capacity of the composite coating is much higher than that of the UVC-GA coating. It can be seen that the addition of gallic acid further enhances the antioxidant properties of the coating. There is a synergistic antibacterial and antioxidant effect between GA and fish skin gelatin, which further improves the fresh-keeping effect of the composite coating.

[0065] Example 4

[0066] The composite coatings of Examples 1-3 (GC-UVC-GA, SP-UVC-GA and TI-UVC-GA) and the coating solution (UVC-GA) in Comparative Example 1 were designed for freshness preservation.

[0067] 150 fresh oysters from the Beibu Gulf were purchased from the Tancun Seafood Wholesale Market in Nanning. The oysters were shelled and the meat was kept. All oyster meat was stored in ice at 0℃ and used within 2 hours. Treatment method of coating group: oyster meat was immersed in each coating solution (GC-UVC-GA, SP-UVC-GA, TI-UVC-GA and UVC-GA) for 60-65s, taken out and air-dried for 10min at room temperature (25℃), then placed in a food-grade PP plastic packaging box, sealed and marked, and stored in a refrigerator at 0℃; 30 oysters per group, 3 oysters per plastic box, a total of 10 plastic boxes, stored for 8 days. Oyster meat without coating was marked as CON as the control group. Treatment method of control group: except that the immersion solution was changed to pure water, the other operation steps were the same as the above steps. 30 oysters per group, 3 oysters per plastic box, a total of 10 plastic boxes, stored for 8 days.

[0068] After the start of the trial, oysters were taken on days 0, 2, 4, 6, and 8 to evaluate their surface color to assess the preservation effect of the coating on the oysters. All measurements were performed on three independent samples.

[0069] The color of oysters was analyzed using a Minolta colorimeter (CR-400, Minolta, Japan). After calibration with a specified white plate, the surface layer of the oyster (without removing the coating) was spread flat on a culture dish, and the measuring head was placed on the sample surface for measurement. The L* value (brightness value), a* value (redness value) and b* value (yellowness value) were recorded. The test was repeated 3 times.

[0070] from Figure 3-4 As can be seen from the results, the coating has a direct effect on the L* and a* values. On day 0, there was no difference in the L* and a* values ​​of oysters in each group (P>0.05). As the storage time increased, the L* and a* values ​​of oysters in each group decreased (P<0.05), but the degree of decrease was different. The L* and a* values ​​of oysters in the control group and UVC-GA group decreased sharply, while the L* and a* values ​​of oysters in the composite coating groups (GC-UVC-GA, SP-UVC-GA, and TI-UVC-GA) decreased less. During the entire storage period, the L* and a* values ​​of oysters in the coating group were significantly higher than those in the UVC-GA group (P<0.05), and the L* and a* values ​​of oysters in the composite coating groups (GC-UVC-GA, SP-UVC-GA, and TI-UVC-GA) were higher than those in the UVC-GA group, indicating that the composite coating effectively prevented the decrease in the brightness of oyster meat. It can be seen that the composite coatings (GC-UVC-GA, SP-UVC-GA and TI-UVC-GA) prepared by fish skin gelatin and UVC-GA have a good effect on maintaining the color of oyster meat.

[0071] After the start of the trial, oysters were taken and their sensory evaluation was performed on days 0, 2, 4, 6, and 8 to evaluate the preservation effect of the coating on the oysters. All measurements were performed on three independent samples.

[0072] A team of 10 trained personnel conducted sensory evaluation and scoring of oysters stored for different periods of time according to the evaluation criteria of GB / T 30762-2018 "Guidelines for Sensory Evaluation of Aquatic Products". The final evaluation result was the average of all the scores.

[0073] The sensory scores of oysters in each group showed the following trend: Figure 5As shown. With the extension of storage time, the sensory scores of oysters showed a significant downward trend (p<0.05), among which the sensory scores of oysters in the control group decreased the fastest (p<0.05). Fresh oyster meat is milky white, the mantle is brown or black, the fresh and sweet taste is strong, the meat is tight and elastic, and the sensory score is close to the full score of 15 points. During the storage process, the scores of the coating groups were greater than those of the control group. After 4 days of storage, the sensory score of oysters in the composite coating group was higher than that of the UVC-GA group, among which the sensory score of the TI-UVC-GA group was higher than that of the GC-UVC-GA and SP-UVC-GA groups. On the 8th day, the oysters in the control group and UVC-GA group were no longer edible, and the oysters in the composite coating groups (GC-UVC-GA, SP-UVC-GA and TI-UVC-GA) were still within the acceptable range. This shows that the composite coating can effectively maintain the sensory properties of oysters during cold storage.

[0074] After the start of the experiment, oysters were taken on days 0, 2, 4, 6, and 8 to test their pH values ​​to evaluate the preservation effect of the coating on oysters. All measurements were performed on three independent samples.

[0075] 2 g of oyster meat sample was chopped and placed in a beaker. Then 100 mL of ultrapure water was boiled and cooled to room temperature and poured into the beaker containing the sample. After homogenization using a homogenizer, the water was allowed to stand for 30 minutes. Finally, the pH value of the oyster meat extract was read using a pH meter.

[0076] like Figure 6 As shown in the figure, the initial pH value of oysters in each group was 6.36. As the storage time increased, the pH value of oysters in each group showed a downward trend. The pH value of oysters in the coating group was higher than that in the control group, indicating that the coating effectively prevented the decrease of pH in oysters. The protective effect of the coating began to appear on the second day of refrigeration. On the sixth day, the pH value of oysters in all coating groups was still greater than 6, while that of the control group samples was less than 6. The pH value of oysters in the composite coating group on the eighth day was higher than that of oysters in the UVC-GA group on the sixth day. Obviously, the coating has a good effect on preventing the corruption of refrigerated oysters, and the composite coating prepared with fish skin gelatin and UVC-GA has a better effect.

[0077] After the start of the experiment, oysters were taken on days 0, 2, 4, 6, and 8, and their muscle tissue structure was observed by sectioning to evaluate the preservation effect of the coating on oysters. All measurements were performed on three independent samples.

[0078] Oyster adductor muscle (1cm×1cm×5mm) was fixed in 4% paraformaldehyde fixative for more than 24h. Then it was dehydrated with ethanol dilutions (70%, 80%, 90% and 100%). After being transparentized with xylene, the sample was placed in a 60℃ paraffin bath for at least 1h, and the sample was embedded in a cooled solid paraffin mold (20mm×15mm×15mm) and cut into 4μm thick sections using a paraffin slicer (LEICA RM2016). After the sections were stained with hematoxylin-eosin, the microstructural characteristics of the muscle tissue were observed and images were collected using a microscope (NikonTiE, Japan).

[0079] The muscle fibers of fresh (0d) oyster muscles are long and tightly arranged, and the endomysium tightly wraps the muscle fibers. After the oysters in the control group were refrigerated, the gaps between the muscle fibers gradually increased, and the myofibrils broke after 6d of refrigeration. As the refrigeration time increased, the gaps continued to increase, the breaks became more obvious, the endomysium gradually disappeared, and the boundaries between the muscle fibers were unclear, indicating that both myofibrillar proteins and connective tissue proteins had been degraded. The shape of the oyster muscle fibers in the coating group was well maintained, but as the refrigeration time increased, the gaps between the muscle fibers also began to increase. Among the coating groups, the TI-UVC-GA coating had the best effect on maintaining the muscle fiber structure of the oyster muscles, followed by the GC-UVC-GA, SP-UVC-GA and UVC-GA groups. On the 8th day of refrigeration, the boundaries between the oyster muscle fibers in the TI-UVC-GA coating group were still clear.

[0080] After the start of the experiment, oysters were taken on days 0, 2, 4, 6, and 8 to count the total number of colonies to evaluate the preservation effect of the coating on oysters. All measurements were performed on three independent samples.

[0081] 5 g of oyster meat stored at different times was mixed with 45 mL of sterile saline and the bacteria were counted using the 10-fold dilution method. A 10-fold dilution was prepared and 100 μL of each dilution was applied to the counting agar plate. The plates were incubated at 37°C under aerobic conditions for 48 ± 1 h. The TVC values ​​were recorded and the results were expressed as Log CFU / g oyster.

[0082] Figure 7 It reflects the changes in the total colony count of oysters during cold storage. The TVC values ​​of oysters in each group increased with the extension of storage time. Compared with the control group, the TVC value of the coating group was lower, indicating that the reproduction of microorganisms in the coating group was inhibited. Among the coating groups, the TVC value of oysters in the composite coating group was significantly lower than that in the UVC-GA group, indicating that adding UVC-GA to fish skin gelatin can enhance its antibacterial effect. There was no significant difference between the composite coating groups.

[0083] After the start of the experiment, oysters were collected on days 0, 2, 4, 6, and 8 to measure their total volatile basic nitrogen (TVB-N) to evaluate the preservation effect of the coating on oysters. All measurements were performed on three independent samples.

[0084] According to the Chinese national standard GB 5009.228-2016, the TVB-N content in oyster samples was determined by semi-micro nitrogen measurement. Weigh 5g of oyster meat with different storage times, add 100mL of distilled water, soak for 30min and filter. Add 10ml of filtrate and 5ml of magnesium oxide suspension (10g / L) to the reaction chamber, distill for 5min, and place the condensate in a flask containing 200mL of 2.0% boric acid solution to receive. Titrate to the endpoint with 0.01mol / L hydrochloric acid standard titration solution, the indicator is a mixture of 1g / L methyl red ethanol solution and 5g / L bromocresol green ethanol solution, and the endpoint color is purple-red. TVB-N is expressed in mg / 100g sample.

[0085] Changes in TVB-N values ​​during oyster cold storage Figure 8 As shown. During the cold storage period, the TVB-N content of oysters in each group showed an increasing trend. The TVB-N content of oysters in the control group and the UVC-GA coating group showed a linear upward trend and reached a maximum value on the 8th day, which was 30.72 mg / 100g and 27.08 mg / 100g, respectively. The TVB-N content of oysters in the composite coating group increased slowly and also reached a maximum value on the 8th day, with a maximum value of 16.17 mg / 100g. The TVB-N content of oysters in the composite coating group was significantly lower than that in the control group and the UVC-GA coating group, and there was no significant difference between the composite coating groups. The results show that the composite coating has excellent barrier properties and can effectively slow down the increase of TVB-N in oysters and the corruption of oysters during cold storage.

[0086] After the start of the experiment, oysters were collected on days 0, 2, 4, 6, and 8 to measure their biogenic amines to evaluate the preservation effect of the coating on oysters. All measurements were performed on three independent samples.

[0087] The histamine, cadaverine, putrescine and cadaverine of oyster meat at different storage times were detected according to the National Food Safety Standard of China - Determination of Biogenic Amines in Food (GB 5009.208-2016). 5 g of homogenized oyster meat was accurately weighed and placed in a 100 ml stoppered conical flask, 500 μL of internal standard solution (1.0 mg / mL) was added to the sample and mixed thoroughly, 20 ml of 5% trichloroacetic acid solution was added and extracted by shaking for 30 min, transferred to a 50 ml stoppered centrifuge tube, centrifuged at 5 000 r / min for 10 min, the supernatant was transferred to a 50 ml volumetric flask, the residue was extracted again with 20 ml of 5% trichloroacetic acid solution, the supernatant was combined and diluted to 25 ml with 5% trichloroacetic acid. Thermo Fisher Scientific Hypersil GOLD C18 column (250 mm × 4.6 mm, 5 μm) was used for separation, the UV detection wavelength was 250 nm, the injection volume was 20 μL, the column temperature was 35 ° C, the mobile phase A was 0.01 mol / L ammonium acetate solution (containing 0.1% acetic acid)-acetonitrile (9:1, V / V), and the mobile phase B was acetonitrile 0.01 mol / L ammonium acetate solution (9:1, V / V), and the flow rate was 0.8 mL / min. The concentration range of the standard curve was 0.1-50 mg / kg. The biogenic amines were qualitatively and quantitatively determined based on the peak area and retention time, with reference to the standard solution (Sigma-Aldrich Trading Co., Ltd, Shanghai, China).

[0088] In the present invention, the contents of the four biogenic amines in each group of oysters showed an upward trend during the cold storage period. In the first four days of cold storage, the content of biogenic amines increased slowly, but began to increase rapidly after the fourth day, which was basically consistent with the change trend of TVC. Compared with the experimental groups, the content of biogenic amines in the control group and the UVC-GA group was always higher than that in the composite coating group, especially after the fourth day of cold storage. On the eighth day of cold storage, the cadaverine and putrescine contents of the oysters in the control group were twice the maximum values ​​of the composite coating group. In the composite coating group, the content of biogenic amines in the TI-UVC-GA group and the GC-UVC-GA group was lower than that in the SP-UVC-GA group. The results show that the fish skin gelatin composite coating prepared by fish skin gelatin and UVC-GA has the effect of inhibiting the production of biogenic amines in cold-stored oysters, and the inhibitory effect of freshwater fish skin gelatin is more significant.

[0089] In the present invention, the L* value of oysters decreased significantly with the extension of refrigeration time, the L* value and a* value of oysters in the coating group were significantly higher than those in the control group, and the L* and a* values ​​of oysters in the composite coating group were higher than those in the UVC-GA group, indicating that the addition of fish skin gelatin effectively improved the brightness of the oyster surface. Gelatin has many advantages as a film-forming matrix in edible coatings, which can maintain the gloss of food surface, inhibit food oxidation, prevent product shrinkage and juice loss, etc. On the other hand, the color of refrigerated aquatic products is a comprehensive reflection of the degree of oxidation of proteins and pigments. The antioxidant capacity of GA can inhibit the oxidation of proteins. The composite coating in the present invention has good antioxidant properties, effectively alleviates the oxidation of myoglobin, and causes the a* value of oysters to decrease during refrigeration.

[0090] In the present invention, on the 6th day of refrigeration, the control group sample had an obvious spoilage smell, and the comprehensive sensory score was reduced to 2.98. At the same time, the composite coating group had a higher sensory score, mainly because the composite coating had good antibacterial and antioxidant properties.

[0091] In the present invention, as the cold storage time increases, the pH value of the oysters shows a downward trend. After the oysters die, different biochemical reactions occur in the body, and the lactic acid and phosphoric acid produced by the decomposition of ATP and glycogen accumulate. The pH values ​​of oysters in all coating groups are higher than those in the control group, indicating that the coating effectively hinders the decrease of the pH of the oysters, and the composite coating has a better preservation effect. Due to the excellent gas barrier properties of the gelatin-based coating and the antioxidant and antibacterial properties of GA, the gallic acid cross-linked gelatin film has better mechanical properties, antioxidant properties and antibacterial properties. During the cold storage period, the TVB-N content of oysters in each group generally showed an upward trend. This is due to the combined action of endogenous enzymes and microbial enzymes, which generate nitrogen-containing compounds such as ammonia, trimethylamine, and histamine. The TVB-N content in the composite coating group was significantly lower than that in the control group and the UVC-GA coating group. This significant difference is related to the antibacterial properties of the composite coating, indicating that the gelatin-based coating with the addition of GA has the effect of inhibiting the growth of microorganisms, thereby delaying the decomposition of proteins caused by microorganisms, and effectively reducing the production rate of TVB-N.

[0092] In the present invention, the TVC value of each group of oysters increases with the extension of storage time, and the TVC value of the oysters in the composite coating group is significantly lower than that in the UVC-GA group, indicating that the composite coating has good antibacterial properties and effectively inhibits the reproduction of microorganisms. Microorganisms decompose oyster proteins and amino acids, leading to the formation of biogenic amines. Histamine, tryptamine, putrescine and cadaverine are usually used as indicators for determining the degree of corruption of aquatic products, and are formed by decarboxylation of histidine, tryptophan, ornithine and lysine, respectively. In the present invention, the contents of the four biogenic amines in oysters all show an upward trend during cold storage, which is basically consistent with the change trend of TVC. The content of biogenic amines in the control group and the UVC-GA group is always higher than that in the composite coating group, indicating that the composite coating has good antibacterial properties. In the composite coating group, the content of biogenic amines in the TI-UVC-GA group and the GC-UVC-GA group is lower than that in the SP-UVC-GA group because of the good mechanical properties of the gelatin-based coating of freshwater fish skin.

[0093] The present invention adds gallic acid to fish skin gelatin to prepare a coating, and uses it for refrigerated aquatic product preservation to examine its potential for practical application. Fish skin gelatin and UVC-treated gallic acid (UVC-GA) are combined to prepare a composite coating as a preservative, which is a promising method for maintaining the quality of refrigerated aquatic products. The prepared composite coating has great application potential and will provide a useful reference for using fish skin gelatin for food packaging and extending the shelf life of food, and can extend the shelf life of aquatic products.

[0094] In the present invention, the fish skin gelatin-UVC-GA composite coating has good mechanical properties and antioxidant properties, has a good preserving effect on the color, sensory organ and muscle structure of oysters, and effectively hinders the decrease in the pH of oyster meat. The fish skin gelatin-UVC-GA composite coating inhibits the growth of microorganisms and the increase of TVB-N, and reduces the production of biogenic amines. Compared with the use of UVC-GA alone, the composite coating has a more significant effect on maintaining the quality of oysters during refrigeration due to the excellent mechanical properties and gas barrier properties of the composite gelatin-based coating, the antioxidant and antibacterial effects of GA, and the synergistic effect of the two. In the present invention, the fish skin gelatin-UVC-GA composite coating can extend the shelf life of oysters by about 4 days. The preparation of a composite coating by combining fish skin gelatin and GA is a promising method for maintaining the storage quality of aquatic products and has great application potential.

[0095] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. An application of a fish skin gelatin composite coating as a preservative in aquatic products, characterized in that: The aquatic product is oyster, the fish skin gelatin composite coating is an aqueous solution containing 1-1.5g of fish skin gelatin and 20-30mg of UVC-GA, and the fish skin gelatin composite coating is prepared by dissolving fish skin gelatin freeze-dried powder in water to prepare a fish skin gelatin solution with a volume fraction of 5%, then adding a UVC-GA antibacterial liquid with a volume fraction of 0.2%, stirring evenly, adjusting the pH value of the mixed solution to 5.5-6.5, placing the obtained mixed solution in a 55°C water bath to maintain the temperature for 35 minutes, and then ultrasonically treating the mixed solution for 25 minutes to defoam.

2. A method for preserving freshness in aquatic products, characterized in that: The following steps are involved: Immerse the aquatic product in the fish skin gelatin composite coating solution for 60-65 seconds, take it out and air-dry it at room temperature for 10 minutes, then put it into a food-grade PP plastic packaging box, seal it, mark it, and store it in a 0℃ refrigerator.

3. A method for preserving freshness in aquatic products according to claim 2, characterized in that: The seafood is oyster meat.

4. A method for preserving freshness in aquatic products according to claim 2, characterized in that: The fish skin gelatin composite coating is an aqueous solution containing fish skin gelatin and UVC-GA.

5. A method for preserving freshness in aquatic products according to claim 4, characterized in that: Add 20-30 mg UVC-GA antibacterial liquid to every 1-1.5 g fish skin gelatin solution.

6. A method for preserving freshness in aquatic products according to claim 4, characterized in that: The fish skin gelatin solution is prepared by dissolving fish skin gelatin freeze-dried powder in water, stirring with a magnetic stirring bar at 200 rpm and 25° C. overnight to prepare a fish skin gelatin solution with a volume fraction of 5%.

7. A method for preserving freshness in aquatic products according to claim 6, characterized in that: The fish skin gelatin freeze-dried powder is prepared by thawing the fish skin, washing the thawed fish skin with clean water, cutting it into blocks, soaking it in a 0.05 mol / L NaOH aqueous solution for 35-40 minutes, repeatedly washing it with clean water for three times, and washing it to neutrality each time, swelling it with a 0.05 mol / L sulfuric acid solution for 3.5 hours, washing it with water to neutrality and then mincing it, extracting it with 50°C hot water overnight, centrifuging it for 35 minutes, collecting the supernatant, concentrating it under reduced pressure and then freeze-drying it to obtain the grass carp fish skin gelatin freeze-dried powder.

8. A method for preserving freshness in aquatic products according to claim 7, characterized in that: The fish skin is any one of grass carp, sea bass and tilapia skin.

9. A method for preserving freshness in aquatic products according to claim 4, characterized in that: The UVC-GA antibacterial solution was prepared by diluting gallic acid with deionized water to a volume fraction of 0.2%, transferring it to a sterilized crystal culture dish, and irradiating it in a UV chamber at 4 J / cm 2 The irradiation was continued at high intensity for 12 h, and the irradiated gallic acid was filtered through a sterile filter with a pore size of 0.25 μm to obtain UVC-GA antibacterial liquid, which was stored in a dark place at room temperature.

10. A method for preserving freshness in aquatic products according to claim 9, characterized in that: The UV chamber uses a desktop UV-C intermittent light source with a wavelength of 254nm and an average intensity of 4540μW / cm 2 , liquid penetration is 17cm.

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