A fresh-keeping coating film, preparation method and application thereof

Abstract

The invention discloses a fresh-keeping coating, a preparation method and application thereof, and belongs to the technical field of fruit and vegetable fresh-keeping. The preparation method of the fresh-keeping coating comprises: placing the pomace of the sago palm in a sodium carbonate aqueous solution, adding pectin methylesterase after heating and stirring, continuously stirring for enzymolysis and adjusting the degree of esterification to 40-45%, centrifuging, taking the supernatant, adding ferrous sulfate heptahydrate and hydrogen peroxide solution to the supernatant, stirring, producing precipitation, washing and freeze-drying the precipitation to obtain a sago palm pectin-polyphenol complex. The present invention is applied to the preservation of fruits and vegetables, has good antioxidant and antibacterial properties, and can better extend the storage period of fruits and vegetables.

Description

Technical Field

[0001] The invention belongs to the technical field of fruit and vegetable preservation, and in particular relates to a fresh-keeping coating film, a preparation method and application thereof. Background Art

[0002] The growth period of the heart-shaped fruit is short, and it matures in the hot season. Its skin is thin, not resistant to knocks, and difficult to store and transport. After being stored at natural temperature for only 8 to 10 days after being harvested, it will deteriorate in quality, such as becoming soft and fluffy, cracking of the skin, softening of the flesh due to water loss, browning of the tissue, and loss of flavor, causing great losses to fruit farmers and storers.

[0003] Currently, most commonly used fresh-keeping coatings are made by directly mixing or embedding natural compounds with antioxidant and antibacterial activity (such as plant polyphenols, flavonoids and plant essential oils), aiming to continuously release antibacterial compounds to enhance the antibacterial and fresh-keeping functions. However, these small molecule compounds have different tastes, odors and colors, and have poor stability, which usually leads to changes in fruit flavor and appearance. Covalent grafting of polyphenols to polysaccharides can achieve the purpose of stabilizing phenolic substances in the coating. However, although pectin molecules with excellent antioxidant and antibacterial properties can be obtained by covalently grafting polyphenols to pectin using laccase catalysis, the 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide introduced in this grafting reaction also increase safety risks. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention solves at least one of the above-mentioned technical problems of the existing fresh-keeping coatings, and proposes a fresh-keeping coating with good antioxidant and antibacterial properties that can better extend the storage period of fruits and vegetables, and a preparation method and application thereof.

[0005] In order to solve the technical problem, the technical solution adopted by the present invention is:

[0006] On one hand, the present invention provides a method for preparing a fresh-keeping coating, comprising: placing pomace of guava in a sodium carbonate aqueous solution, adding pectin methylesterase after heating and stirring, continuously stirring for enzymolysis and adjusting the degree of esterification to 40-45%, centrifuging, taking a supernatant, adding ferrous sulfate heptahydrate and a hydrogen peroxide solution to the supernatant, stirring to generate a precipitate, washing and freeze-drying the precipitate to obtain a guava pectin-polyphenol complex.

[0007] Preferably, the added amount of pectin methylesterase is 150-500 U / g of citron fruit pomace.

[0008] Preferably, the concentration of the sodium carbonate aqueous solution is 0.5%, and the mass / volume ratio of the pomace of the guava to the sodium carbonate aqueous solution is 1:20.

[0009] Preferably, the pomace of the guava is placed in a sodium carbonate aqueous solution, heated and stirred at 50° C., and then pectin methylesterase is added.

[0010] Preferably, the mass / volume ratio of ferrous sulfate heptahydrate to the supernatant is 0.25%; the concentration of the hydrogen peroxide solution is 30%, and the volume ratio of the hydrogen peroxide solution to the supernatant is 0.5%.

[0011] Preferably, after the enzymatic hydrolysis is completed, the enzyme is inactivated at 100°C, and after centrifugation, the supernatant is taken, ferrous sulfate heptahydrate and hydrogen peroxide solution are added to the supernatant, and stirred at 150 rpm for 20 minutes, the solution gradually produces precipitation, and after centrifugation, the pineapple pectin-polyphenol-iron complex is obtained.

[0012] Preferably, the guava pectin-polyphenol-iron complex is dissolved in a 70% ethanol solution at a mass / volume ratio of 1:10, and 1% hydrochloric acid is added, stirred at 150 rpm for 5 minutes, and after centrifugation, the precipitate is freeze-dried to obtain the guava pectin-polyphenol complex.

[0013] Preferably, the method further comprises: preparing 1-5% of pineapple pectin-polyphenol complex solution, ultrasonically degassing to obtain a uniform bubble-free coating liquid; and covering the surface of fruits and vegetables with the coating liquid to obtain a fresh-keeping coating film.

[0014] Another aspect of the present invention provides a fresh-keeping coating film prepared by the method for preparing the fresh-keeping coating film described in any of the above technical solutions.

[0015] The present invention also provides application of the fresh-keeping coating film in the fresh-keeping of fruits and vegetables, wherein the fruits and vegetables are fruits with fruit wax.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] The present invention provides a method for preparing a fresh-keeping coating film, which is prepared from pomace produced in the processing of heart-shaped fruit, and effectively utilizes byproducts produced in the processing of heart-shaped fruit, which is beneficial to the sustainable development of the heart-shaped fruit industry, simplifies the steps of separate extraction and grafting in the preparation process of pectin-polyphenol composite, and is safer to prepare the coating by free radical grafting of pectin and polyphenol. In the reaction, only ferrous ions and H2O2 need to be introduced as free radical activators, and no other organic reagents are needed, which is a green, safe and convenient combination method.

[0018] The present invention proposes an application of a fresh-keeping coating in the preservation of fruits and vegetables. The fresh-keeping coating is prepared by the above-mentioned free radical grafting, which avoids the influence of the taste and color of the polyphenols themselves on the fruits. At the same time, the antibacterial, antioxidant and hydrophobic properties of the polyphenols are also utilized to enhance the fresh-keeping effect of the coating and the adhesion ability to the fruit wax on the surface of fruits and vegetables, thereby achieving the purpose of extending the shelf life of fruits and vegetables. The technology of the invention can extend the storage period of the heart-shaped fruit at room temperature to more than 20 days. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the grafting degree of guava polyphenols in different embodiments;

[0020] Figure 2 Schematic diagram of antioxidant activity of different embodiments;

[0021] Figure 3 It is a schematic diagram of the weight loss rate change of different comparative examples and embodiments;

[0022] Figure 4 It is a schematic diagram of hardness change of different comparative examples and embodiments. DETAILED DESCRIPTION

[0023] The technical scheme in the specific embodiment of the present invention is described in detail and completely below. Obviously, the described embodiment is only a part of the specific implementation of the overall technical scheme of the present invention, rather than all implementations. Based on the overall concept of the present invention, all other embodiments obtained by ordinary technicians in this field fall within the scope of protection of the present invention.

[0024] On the one hand, the present invention proposes a method for preparing a fresh-keeping coating, comprising: placing the pomace of the heart-shaped fruit in a sodium carbonate aqueous solution, adding pectin methylesterase after heating and stirring, continuously stirring for enzymolysis and adjusting the degree of esterification to 40-45%, centrifuging, taking the supernatant, adding ferrous sulfate heptahydrate and hydrogen peroxide solution to the supernatant, stirring, generating a precipitate, washing and freeze-drying the precipitate to obtain a heart-shaped fruit pectin-polyphenol composite. The preparation method uses the pomace generated during the processing of the heart-shaped fruit as a raw material, efficiently utilizes the by-products generated during the processing of the heart-shaped fruit, is conducive to the sustainable development of the heart-shaped fruit industry, and simplifies the process of extracting pectin and polyphenols from the heart-shaped fruit by-products and then compounding, and directly obtains the composite by free radical grafting and iron ion precipitation, and the obtained coating is safer. In the reaction, only ferrous ions and H2O2 need to be introduced as free radical activators, and no other organic reagents are required. It is a green, safe and convenient combination method. At the same time, ferrous ions can also be used as cross-linking agents to enhance the cross-linking between pectin and polyphenols, making the coating structure more compact.

[0025] The above technical scheme places the pomace of the heart-shaped fruit in a sodium carbonate aqueous solution to provide an alkaline environment for the solution. Firstly, the combination between pectin and cellulose can be destroyed to dissolve pectin in the extract. Secondly, most phenolic substances are more soluble in an alkaline environment, which increases the concentration of phenolic substances in the solution. The pectin methylesterase is used to enzymatically hydrolyze the pomace of the heart-shaped fruit, and the degree of esterification is controlled to regulate the combination of pectin and polyphenols. At the same time, free radicals are generated by a hydrogen peroxide and ferrous ion system to promote the combination of pectin and polyphenols. Since iron ions are generated during the reaction of hydrogen peroxide and the ferrous ion system, the iron ions can be used as a cross-linking agent to precipitate the pectin complex. After washing, a pectin-polyphenol complex with better solubility is obtained. In addition, the technical solution also limits the use of the pomace of the heart-shaped fruit as the raw material, rather than other pomace that contains both pectin and polyphenols. The reason is that pectin and polyphenols from different sources will carry some characteristics of the raw materials such as aroma, color and taste after extraction. Using the pomace of the heart-shaped fruit for extraction can ensure the homology of the film and enhance the characteristics of the heart-shaped fruit.

[0026] In a preferred embodiment, the addition amount of pectin methylesterase is 150-500 U / g of guava pomace. The technical solution limits the addition amount of pectin methylesterase, because too much addition amount will cause excessive depolymerization of pectin, molecular weight decreases, thereby reducing its film-forming property, and too low addition will reduce the degree of pectin esterification, and the efficiency of free radical grafting reaction will also decrease. It is understandable that the addition amount of pectin methylesterase can also be 200 U / g guava pomace, 300 U / g guava pomace, 400 U / g guava pomace and any point value within the range thereof. It should be noted that the higher the polyphenol grafting degree is, the better its adhesion ability is, therefore, this ability is regulated by the addition amount of pectin methylesterase, the pectin methylesterase content increases, the degree of pectin esterification decreases, the lower the degree of esterification is, the higher the carboxyl content is, and it is easier to be attacked by free radicals to cause grafting reaction.

[0027] In a preferred embodiment, the concentration of the sodium carbonate aqueous solution is 0.5%, and the mass / volume ratio of the pomace of the heart-shaped fruit to the sodium carbonate aqueous solution is 1:20. The technical solution specifically limits the concentration and dosage of the sodium carbonate aqueous solution because, in order to limit the pH in the extraction environment, too high a pH will also lead to excessive depolymerization of pectin, and too low a pH will reduce the solubility of phenolic substances and form precipitation, making it difficult to separate.

[0028] In a preferred embodiment, the pomace of the pineapple is placed in a sodium carbonate aqueous solution, heated and stirred at 50° C., and then pectin methylesterase is added. The technical solution specifically limits the heating temperature because it is to accelerate the reaction efficiency and promote the dissolution of phenolic substances.

[0029] In a preferred embodiment, the mass / volume ratio of ferrous sulfate heptahydrate to the supernatant is 0.25%; the concentration of the hydrogen peroxide solution is 30%, and the volume ratio of the hydrogen peroxide solution to the supernatant is 0.5%. This technical solution specifically limits the amount of ferrous sulfate heptahydrate and hydrogen peroxide solution because, in this reaction, Fe 2+ First, it reacts with H2O2 to produce Fe 3+ , Fe 3+ It will react with H2O2 to generate Fe 2+ , and the cycle repeats until all the H2O2 is consumed. If the amount of H2O2 added is too low, it will be difficult to produce enough free radicals to promote the combination of pectin and phenolic substances. If the amount added is too high, it will also cause pectin to depolymerize and reduce film-forming properties.

[0030] In a preferred embodiment, after the enzymatic hydrolysis is completed, the enzyme is inactivated at 100°C, and after centrifugation, the supernatant is taken, ferrous sulfate heptahydrate and hydrogen peroxide solution are added to the supernatant, and then stirred at 150 rpm for 20 minutes, and the solution gradually produces precipitation. After centrifugation, the pineapple pectin-polyphenol-iron complex is obtained.

[0031] In a preferred embodiment, the guava pectin-polyphenol-iron complex is dissolved in a 70% ethanol solution at a ratio of 1:10 (w / v), and 1% hydrochloric acid is added, stirred at 150 rpm for 5 minutes, and centrifuged at 5000 g, and the precipitate is freeze-dried to obtain the guava pectin-polyphenol complex. 2+ The solubility of the precipitated complex is low, and this step is used to perform desalting to improve the solubility of the complex. In this step, the iron ions are washed away by stirring, and then the separation is accelerated by centrifugation.

[0032] In a preferred embodiment, it also includes: preparing a 1-5% solution of the pectin-polyphenol complex of the heart of the fruit, ultrasonically degassing to obtain a uniform and bubble-free coating liquid; covering the coating liquid on the surface of fruits and vegetables to obtain a fresh-keeping coating film. The technical solution specifically limits the concentration of the pectin-polyphenol-iron complex of the heart of the fruit in the coating liquid, because if the concentration is too high, the film will be thicker, and the carbon dioxide exhaled by the fruit will not be discharged, aggravating the quality deterioration. If the concentration is too low, it is difficult to achieve a fresh-keeping effect; it is further limited; using ultrasound for degassing, because it avoids the influence of bubbles in the film-forming liquid on the coating film and ensures the uniformity of the coating film. Further, degassing with 100 W ultrasound for 10 min, a uniform and bubble-free solution is obtained.

[0033] Another aspect of the present invention provides a fresh-keeping coating film prepared by the method for preparing the fresh-keeping coating film described in any of the above technical solutions.

[0034] The present invention also provides the application of the above-mentioned fresh-keeping coating in the preservation of fruits and vegetables. Furthermore, the fruits and vegetables are fruits with fruit wax. It should be noted that the above-mentioned coating can also be used for the preservation of other fruits and vegetables, but its effect can be better reflected when it is applied to fruits with fruit wax. The hydrophobicity of traditional polysaccharide coatings is poor, and the adhesion effect on the surface of fruits with fruit wax is poor. The preparation method of the fresh-keeping coating of the present invention adjusts the hydrophobicity of the coating and enhances its adhesion ability. Specifically, pectin is a hydrophilic polysaccharide, which is difficult to be tightly combined with fruit wax with better hydrophobicity, but by grafting phenolic substances, the hydrophobicity of pectin is improved, which is conducive to the combination between the coating and the fruit wax.

[0035] Furthermore, the fruit with fruit wax is a heart-shaped fruit, and the heart-shaped fruit indicates the presence of fruit wax. A single pectin coating is difficult to adhere tightly to the surface of the heart-shaped fruit due to its good hydrophilicity. By adding polyphenols, its hydrophobicity will also be increased, thereby enhancing the adhesion ability of the coating. When in use, a fresh, clean, and smooth heart-shaped fruit is placed in the coating liquid for 60 seconds, taken out and dried naturally, and stored at 25°C. Furthermore, the surface of the heart-shaped fruit can also be covered by spraying or infiltration. The above-mentioned coating has good antioxidant and antibacterial properties, can better extend the storage period of the heart-shaped fruit, and maintain the fruit hardness of the heart-shaped fruit.

[0036] The invention provides a method for preparing a fresh-keeping coating film, wherein the fresh-keeping coating film is prepared from jackfruit pectin and jackfruit polyphenols, and the jackfruit pectin is modified to have a higher polyphenol loading rate and good film-forming properties of pectin, thereby slowing down the water loss rate of the jackfruit, providing a good antibacterial effect, and allowing the jackfruit to maintain good flavor and color during the shelf life.

[0037] In order to more clearly and in detail introduce the fresh-keeping coating provided by the embodiments of the present invention, its preparation method and application, the following will be described in conjunction with specific embodiments.

[0038] Example 1

[0039] Step 1: Place the guava pomace in a 0.5% sodium carbonate aqueous solution at a ratio of 1:20 (w / v) at 50°C, heat and stir for 1 h, then add pectin methylesterase (150 U / g guava pomace) and continue stirring for 2 h for enzymolysis. After the enzymolysis is completed, the enzyme is inactivated at 100°C, the mixture is centrifuged at 8000 g, and the supernatant is taken.

[0040] Step 2: After adding 0.25% (w / v) ferrous sulfate heptahydrate and 0.5% (v / v) 30% hydrogen peroxide solution to the supernatant and stirring at 150 rpm for 20 minutes, a precipitate gradually formed in the solution. After centrifugation of the precipitate, a pineapple pectin-polyphenol-iron complex was obtained.

[0041] Step 3: The guava pectin-polyphenol-iron complex was dissolved in 70% ethanol solution at a ratio of 1:10 (w / v), and 1% hydrochloric acid was added. The mixture was stirred at 150 rpm for 5 minutes. After centrifugation at 5000 g, the precipitate was freeze-dried to obtain the guava pectin-polyphenol complex.

[0042] Step 4: Prepare the coating solution by dissolving 3% guava pectin-polyphenol complex, and perform ultrasonic degassing at 100 W for 10 min to obtain a uniform bubble-free solution.

[0043] Step 5: Place fresh, clean and smooth pineapple in the coating solution for 60 seconds, take it out and dry it naturally, and store it at 25℃.

[0044] Example 2

[0045] Step 1: Place the guava pomace in a 0.5% sodium carbonate aqueous solution at a ratio of 1:20 (w / v) at 50°C, heat and stir for 1 h, then add pectin methylesterase (300 U / g guava pomace) and continue stirring for 2 h for enzymolysis. After the enzymolysis is completed, the enzyme is inactivated at 100°C, the mixture is centrifuged at 8000 g, and the supernatant is taken.

[0046] Step 2: After adding 0.25% (w / v) ferrous sulfate heptahydrate and 0.5% (v / v) 30% hydrogen peroxide solution to the supernatant and stirring at 150 rpm for 20 minutes, a precipitate gradually formed in the solution. After centrifugation of the precipitate, a pineapple pectin-polyphenol-iron complex was obtained.

[0047] Step 3: The guava pectin-polyphenol-iron complex was dissolved in 70% ethanol solution at a ratio of 1:10 (w / v), and 1% hydrochloric acid was added. The mixture was stirred at 150 rpm for 5 minutes. After centrifugation at 5000 g, the precipitate was freeze-dried to obtain the guava pectin-polyphenol complex.

[0048] Step 4: Prepare the coating solution by dissolving 3% serrata pectin-polyphenol complex, and degas with 100 W ultrasound for 10 min to obtain a uniform solution without bubbles. Step 5: Place fresh, clean and smooth serrata in the coating solution for 60 s, take it out and dry it naturally, and store it at 25 °C.

[0049] In this embodiment, except step 1, the remaining steps are the same as those in embodiment 1.

[0050] Example 3

[0051] Step 1: Place the guava pomace in a 0.5% sodium carbonate aqueous solution at a ratio of 1:20 (w / v) at 50°C, heat and stir for 1 h, then add pectin methylesterase (450 U / g guava pomace) and continue stirring for 2 h for enzymolysis. After the enzymolysis is completed, the enzyme is inactivated at 100°C, the mixture is centrifuged at 8000 g, and the supernatant is taken.

[0052] Step 2: After adding 0.25% (w / v) ferrous sulfate heptahydrate and 0.5% (v / v) 30% hydrogen peroxide solution to the supernatant and stirring at 150 rpm for 20 minutes, a precipitate gradually formed in the solution. After centrifugation of the precipitate, a pineapple pectin-polyphenol-iron complex was obtained.

[0053] Step 3: The guava pectin-polyphenol-iron complex was dissolved in 70% ethanol solution at a ratio of 1:10 (w / v), and 1% hydrochloric acid was added. The mixture was stirred at 150 rpm for 5 minutes. After centrifugation at 5000 g, the precipitate was freeze-dried to obtain the guava pectin-polyphenol complex.

[0054] Step 4: Prepare the coating solution by dissolving 3% guava pectin-polyphenol complex, and perform ultrasonic degassing at 100 W for 10 min to obtain a uniform bubble-free solution.

[0055] Step 5: Place fresh, clean and smooth pineapple in the coating solution for 60 seconds, take it out and dry it naturally, and store it at 25℃.

[0056] In this embodiment, except step 1, the remaining steps are the same as those in embodiment 1.

[0057] Example 4

[0058] Step 1: Place the guava pomace in a 0.5% sodium carbonate aqueous solution at a ratio of 1:20 (w / v) at 50°C, heat and stir for 1 h, then add pectin methylesterase (450 U / g guava pomace) and continue stirring for 2 h for enzymolysis. After the enzymolysis is completed, the enzyme is inactivated at 100°C, the mixture is centrifuged at 8000 g, and the supernatant is taken.

[0059] Step 2: After adding 0.25% (w / v) ferrous sulfate heptahydrate and 0.5% (v / v) 30% hydrogen peroxide solution to the supernatant and stirring at 150 rpm for 20 minutes, a precipitate gradually formed in the solution. After centrifugation of the precipitate, a pineapple pectin-polyphenol-iron complex was obtained.

[0060] Step 3: The guava pectin-polyphenol-iron complex was dissolved in 70% ethanol solution at a ratio of 1:10 (w / v), and 1% hydrochloric acid was added. The mixture was stirred at 150 rpm for 5 minutes. After centrifugation at 5000 g, the precipitate was freeze-dried to obtain the guava pectin-polyphenol complex.

[0061] Step 4: Prepare a coating suspension by dissolving 1% guava pectin-polyphenol complex, and perform ultrasonic degassing at 100 W for 10 min to obtain a coating solution.

[0062] Step 5: Place fresh, clean and smooth pineapple in the coating solution for 60 seconds, take it out and dry it naturally, and store it at 25℃.

[0063] Except step 4, the remaining steps are the same as those in Example 3.

[0064] Example 5

[0065] Step 1: Place the guava pomace in a 0.5% sodium carbonate aqueous solution at a ratio of 1:20 (w / v) at 50°C, heat and stir for 1 h, then add pectin methylesterase (450 U / g guava pomace) and continue stirring for 2 h for enzymolysis. After the enzymolysis is completed, the enzyme is inactivated at 100°C, the mixture is centrifuged at 8000 g, and the supernatant is taken.

[0066] Step 2: After adding 0.25% (w / v) ferrous sulfate heptahydrate and 0.5% (v / v) 30% hydrogen peroxide solution to the supernatant and stirring at 150 rpm for 20 minutes, a precipitate gradually formed in the solution. After centrifugation of the precipitate, a pineapple pectin-polyphenol-iron complex was obtained.

[0067] Step 3: The guava pectin-polyphenol-iron complex was dissolved in 70% ethanol solution at a ratio of 1:10 (w / v), and 1% hydrochloric acid was added. The mixture was stirred at 150 rpm for 5 minutes. After centrifugation at 5000 g, the precipitate was freeze-dried to obtain the guava pectin-polyphenol complex.

[0068] Step 4: Prepare a coating suspension by dissolving 5% guava pectin-polyphenol complex, and degas with 100 W ultrasound for 10 min to obtain a coating solution.

[0069] Step 5: Place fresh, clean and smooth pineapple in the coating solution for 60 seconds, take it out and dry it naturally, and store it at 25℃.

[0070] Except step 4, the remaining steps are the same as those in Example 3.

[0071] Comparative Example 1

[0072] The pineapple was stored at 25℃ without any treatment.

[0073] Comparative Example 2

[0074] Soak the guava in pure water for 60 s, air dry naturally, and then store at 25°C.

[0075] Performance Testing

[0076] (1) Grafting degree of guava polyphenol: The grafting degree was determined by the Folin phenol method. 1 mL of pectin solution grafted with guava polyphenol (3 mg / mL) was added to 2.5 mL of 10% Folin phenol for 3 minutes, and then 2 mL of 7.5% Na2CO3 was added to the mixture and reacted in the dark for 1 hour. Finally, the absorbance of the reaction solution was measured at 765 nm, and the grafting degree was calculated based on the standard curve of gallic acid. The results are shown in the figure. Figure 1 As shown, the polyphenol grafting degree of Example 3 is the highest. This is because the higher the amount of pectin methylesterase added, the lower the pectin esterification degree, and the free radicals esterify the pectin and polyphenols by oxidizing the carboxyl groups to form grafts. Therefore, low-ester pectin is more conducive to polyphenol grafting.

[0077] (2) Antioxidant activity determination: 0.5 mL of sample solution and 0.5 mL of DPPH solution were mixed thoroughly in a 5 mL centrifuge tube and protected from light reaction for 0.5 h. The absorbance value (A1) of the mixture was measured at 517 nm. The absorbance value A2 was measured with anhydrous ethanol. Ethanol was used to replace the DPPH solution as the blank group, and distilled water was used to replace the sample as the control group, and the absorbance value A0 was measured. The DPPH free radical scavenging rate of the sample was calculated according to the following formula:

[0078]

[0079] The results are as follows Figure 2 As shown, the antioxidant properties of the samples are mainly related to the polyphenol content. Therefore, this experiment verifies that Example 3 with a high polyphenol grafting rate has the best antioxidant property.

[0080] (3) Determination of weight loss rate: Measure the weight of the fruit on the 0th, 2nd, 4th, 6th, 8th and 10th day of storage, and calculate the weight loss rate according to the following formula and express it as a percentage.

[0081]

[0082] The results are as follows Figure 3 As shown, the coating isolates the fruit from the atmosphere, reduces the respiration of the fruit, and thus reduces the weight loss of the grapes. Compared with Examples 1 and 2, Example 3 has a better protective effect, which may be due to the fact that after more polyphenols are grafted, a large number of hydroxyl groups can form intermolecular or intramolecular hydrogen bonds, forming a denser coating, thereby more effectively reducing the weight loss of the fruit.

[0083] (4) Hardness test: Three points were evenly selected on the equator of the fruit, and the hardness of the fruit on the 0th, 2nd, 4th, 6th, 8th and 10th day of storage was measured using a GY-3 digital fruit hardness tester. Figure 4As shown, with the extension of storage period, the hardness of the fruit decreased, but the hardness of the embodiment group was better than that of the comparative group, among which the effect of embodiment 3 was the best. The results show that the coating can delay the quality decline of the heart fruit during storage, which is mainly attributed to the coating effectively delaying the loss of water and ripening of the heart fruit.

[0084] (5) Hydrophobicity measurement: The peel of the sago palm was cut into small pieces of 1 cm × 1 cm and dried on a glass slide at room temperature. The surface tension and hydrophobicity of the sago palm surface were measured using an optical contact angle meter. The structure is shown in Table 1.

[0085] Table 1 Contact angles of different embodiments

[0086]

[0087] (6) Total colony count: Record the growth of microorganisms in different comparative examples and embodiments. On the 15th day, 10 g of fruit was added to 90 g of sterile deionized water and homogenized at 1000 r / min for 5 minutes to prepare a 1:10 (w / w) sample solution. The sample solution was diluted to a solution of a certain concentration, 1 mL of the diluted sample solution was fully mixed with 20 mL of PCA medium solution, and incubated at 37°C for 72 hours. The total colony count on each plate was recorded. The results are shown in Table 2.

[0088] Table 2 Total colony counts of different comparative examples and embodiments

[0089]

[0090] It can be found from Table 2 that compared with Comparative Example 1 and Comparative Example 2, Example 3, Example 4 and Example 5 have better antibacterial effects.

Claims

1. A method for preparing a fresh-keeping coating, characterized in that: include: The pomace of the guava is placed in a sodium carbonate aqueous solution, heated and stirred, and then pectin methylesterase is added, and the mixture is continuously stirred for enzymolysis and the degree of esterification is adjusted to 40-45%, and the mixture is centrifuged to obtain a supernatant, and ferrous sulfate heptahydrate and a hydrogen peroxide solution are added to the supernatant, and the mixture is stirred to produce a precipitate, and the precipitate is washed and freeze-dried to obtain a guava pectin-polyphenol complex; The mass / volume ratio of ferrous sulfate heptahydrate to the supernatant is 0.25%; the concentration of the hydrogen peroxide solution is 30%, and the volume ratio of the hydrogen peroxide solution to the supernatant is 0.5%.

2. The method for preparing the fresh-keeping coating according to claim 1, characterized in that: The added amount of pectin methylesterase is 150-500 U / g of guava pomace.

3. The method for preparing the fresh-keeping coating according to claim 1, characterized in that: The concentration of the sodium carbonate aqueous solution is 0.5%, and the mass / volume ratio of the pomace of the guava to the sodium carbonate aqueous solution is 1:

20.

4. The method for preparing the fresh-keeping coating according to claim 1, characterized in that: Place the pomace of the guava in a sodium carbonate aqueous solution, heat and stir at 50°C, and then add pectin methylesterase.

5. The method for preparing the fresh-keeping coating according to claim 1, characterized in that: After the enzymatic hydrolysis, the enzyme was inactivated at 100°C. After centrifugation, the supernatant was taken. Ferrous sulfate heptahydrate and hydrogen peroxide solution were added to the supernatant. The solution was stirred at 150 rpm for 20 minutes. Precipitation gradually occurred in the solution. After centrifugation, the pectin-polyphenol-iron complex of citrus fruit was obtained.

6. The method for preparing the fresh-keeping coating according to claim 5, characterized in that: The guava pectin-polyphenol-iron complex was dissolved in an ethanol solution at a mass / volume ratio of 1:10, and hydrochloric acid was added. The mixture was stirred and centrifuged, and then the precipitate was freeze-dried to obtain the guava pectin-polyphenol complex.

7. The method for preparing the fresh-keeping coating according to claim 1, characterized in that: Also includes: Prepare 1-5% pineapple pectin-polyphenol complex solution, and perform ultrasonic degassing to obtain a uniform coating solution without bubbles; The coating liquid is covered on the surface of fruits and vegetables to obtain a fresh-keeping coating film.

8. The fresh-keeping coating prepared according to the method for preparing the fresh-keeping coating according to any one of claims 1 to 7.

9. The use of the fresh-keeping coating according to claim 8 in the preservation of fruits and vegetables, characterized in that: Fruits and vegetables are fruits with fruit wax.

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