Fresh-cut apple fresh-keeping composite coating film as well as preparation method and application thereof

Abstract

The preparation method comprises the following steps: uniformly mixing sodium alginate, whey protein, glycerol, L-malic acid and ferulic acid to obtain a film coating solution, immersing the fresh-cut apples into the film coating solution for 5 minutes, and drying the fresh-cut apples in a ventilation environment at 25 DEG C to ensure that uniform films are formed on the surfaces of the apples so as to obtain the fresh-keeping composite film for the fresh-cut apples. And storing in a plastic preservation box at a low temperature of 4 DEG C. The L-malic acid / ferulic acid-sodium alginate / whey protein fresh-keeping composite coating film prepared by the preparation method can be used for fruit fresh keeping; the fresh-keeping composite coating film can effectively inhibit the growth and reproduction of Escherichia coli O157: H7, prolongs the preservation time of the apples, solves the problem that the fresh-cut apples are very easy to brown, delays the quality deterioration of the fresh-cut apples, and significantly prolongs the storage period of the fresh-cut apples.

Description

Technical Field

[0001] The present invention relates to the technical field of fresh - keeping technology for fruits and vegetables, and particularly relates to a preparation method and application of a fresh - cut apple fresh - keeping composite coating film. Background Art

[0002] Due to the production process of fresh - cut fruits requiring steps such as washing, peeling, pitting, cutting, and packaging, these steps make the fruit quality more prone to deterioration, thereby reducing its commercial value. At the same time, the low - acid environment and nutrient - rich surface of fresh - cut fruits create a favorable growth environment for microorganisms, which is more likely to promote the growth of pathogenic microorganisms. Traditional physical and chemical sterilization methods such as ultraviolet irradiation, chlorine - based disinfectants, ozone (O 3 ) and hydrogen peroxide (H 2 O 2 ) are the most widely used disinfectants. Although these methods may be effective, they are costly and can affect the nutritional components and overall food quality. In addition, consumers are also concerned about the potential carcinogenicity of certain chemical preservatives. Therefore, it has become particularly important to find efficient, safe, and natural food preservatives in the food industry to inhibit or eliminate food - borne pathogens and extend the shelf life of food.

[0003] In fresh - cut fruit and vegetable products, the significant and rapid enzymatic browning phenomenon after cutting apples has significantly restricted their market promotion and supply stability. Enzymatic browning is mainly caused by the activities of polyphenol oxidase (PPO) and peroxidase (POD) naturally present in foods. It shortens the shelf life of fruits and vegetables. The polyphenol oxidase (PPO) and peroxidase (POD) in apples are highly active and can rapidly catalyze the oxidation of phenolic substances into quinone substances, and the quinone substances condense with amino acids, proteins, and other compounds to produce brown pigments, thereby reducing the acceptability and nutritional quality of the product. Nowadays, various physical and chemical technical methods can slow down the browning of fresh - cut apples. Modified atmosphere packaging (MAP), hot water, ultraviolet irradiation (UV - C), and plasma - activated water (PAM) are common physical techniques for delaying the browning of fresh - cut apples. However, physical techniques have disadvantages such as high equipment costs, usually unsatisfactory effects, and reduced freshness. The use of chemical anti - browning agents has been controversial due to safety issues. Therefore, the development of healthy and environmentally friendly browning inhibitors is becoming an important research field.

[0004] Coating technology has been attracting people's attention as a preservation method in the past few decades. On the one hand, some coating materials themselves have certain antibacterial or preservation functions. On the other hand, it also reduces the ecological damage caused by non-biodegradable packaging materials. Sodium alginate (SA) is an anionic linear polysaccharide extracted from brown algae or seaweeds. Sodium alginate has excellent biocompatibility, degradability, antioxidant activity and film-forming ability; therefore, it is widely used in various food preservation applications. However, sodium alginate contains several hydrophilic groups (hydroxyl and carboxyl groups), which can bind to water and result in poor water resistance. In addition, the mechanical strength of sodium alginate is relatively poor. Therefore, improving the water resistance and mechanical strength of sodium alginate-based edible coatings can enhance their ability to preserve fruits and vegetables. These problems can be improved by combining with other polymers (such as proteins or other polysaccharides). Whey protein is an excellent film-forming polymer due to its good emulsification, gelation and water retention properties. The incorporation of whey protein increases the opacity, hydrophobicity and structural density of sodium alginate-based films, with good mechanical strength and UV barrier properties. The addition of L-malic acid and ferulic acid further enhances the antibacterial and preservation effects of the coating, extends the shelf life of apples, and obtains an effective method for eliminating important pathogenic bacteria and maintaining the quality in fresh-cut fruits. Summary of the Invention

[0005] The present invention provides a preparation method and application of an L-malic acid / ferulic acid-sodium alginate / whey protein fresh-keeping composite coating film. Using sodium alginate and whey protein as coating materials and adding L-malic acid and ferulic acid to achieve antibacterial and fresh-keeping effects. The experimental results show that the coating has a significant inhibitory effect on the foodborne pathogenic bacterium Escherichia coli O157:H7, and reduces the water loss rate of fresh-cut apples, significantly reduces the activities of polyphenol oxidase and peroxidase, and improves the activity of catalase during storage, thereby improving the scavenging ability of reactive oxygen species, reducing the content of malondialdehyde, delaying the quality deterioration of fresh-cut apples, solving the problem that fresh-cut apples are extremely prone to browning, maintaining their freshness, and can also significantly extend the shelf life.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] The present invention first provides a preparation method of the above-mentioned L-malic acid / ferulic acid-sodium alginate / whey protein fresh-keeping composite coating film, including the following steps:

[0008] S1. Prepare a sodium alginate / whey protein composite coating solution: Disperse sodium alginate in water, add glycerol, mix evenly, add whey protein, and stir magnetically at 80 °C until completely dissolved to obtain a sodium alginate / whey protein composite coating solution. The mass-volume ratio of sodium alginate, whey protein, glycerol and water is 1.5 g:0.5 g:0.3 g:100 mL

[0009] S2. Dissolve L - malic acid and ferulic acid in the sodium alginate / whey protein composite coating solution until completely dissolved. Let it stand for 1.5 - 2.5 h, and then perform ultrasonic degassing to obtain the L - malic acid / ferulic acid composite coating solution. The mass - volume ratio of L - malic acid, ferulic acid, and water is 3.125 mg:0.1 g:100 mL.

[0010] S3. Prepare the L - malic acid / ferulic acid - sodium alginate / whey protein fresh - keeping composite coating by the casting method: Pour the prepared coating solution onto a glass petri dish and dry it at 25°C. Equilibrate the obtained film at 25°C for 24 h to obtain the fresh - keeping composite coating. The film - forming thickness is between 0.02 and 0.09 mm.

[0011] The present invention also provides a method for using the above - mentioned L - malic acid / ferulic acid - sodium alginate / whey protein fresh - keeping composite coating on fresh - cut apples.

[0012] Specifically, the method for use is as follows: Apples with similar shapes, sizes, colors, and textures are peeled with a peeler, immersed in the coating solution prepared in S2 for 5 min, taken out, placed in a ventilated place to dry, and then put into a plastic fresh - keeping box and stored at 4°C.

[0013] The beneficial effects of the present invention are as follows:

[0014] (1) The present invention prepares the L - malic acid / ferulic acid - sodium alginate / whey protein fresh - keeping composite coating for the first time. The prepared fresh - keeping composite coating has good antibacterial activity against the food - borne pathogen Escherichia coli O157:H7.

[0015] (2) The L - malic acid / ferulic acid - sodium alginate / whey protein fresh - keeping composite coating provided by the present invention solves the problem of rapid browning of fresh - cut apples, delays the deterioration of the quality of fresh - cut apples, improves the antioxidant capacity of fresh - cut apples during storage at 4°C, and extends the shelf life. Description of the Drawings

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings.

[0017] Figure 1 It is the photographed observation result diagram of each fresh - keeping composite coating in Experimental Example 2 of the present invention;

[0018] a is sodium alginate coating (SA), b is sodium alginate / whey protein coating (SA / WP), c is sodium alginate / whey protein / L - malic acid coating (SA / WP / LMA), d is sodium alginate / whey protein / ferulic acid coating (SA / WP / FA), and e is sodium alginate / whey protein / L - malic acid / ferulic acid coating (SA / WP / LMA / FA).

[0019] Figure 2 It is the test result graph of the water contact angle of the fresh - keeping composite coating in Example 2;

[0020] Figure 3 It is the test result graph of the ultraviolet - blocking performance and transparency of the fresh - keeping composite coating in Example 2;

[0021] Figure 4 It is the test result graph of the antibacterial rate of each fresh - keeping composite coating against Escherichia coli O157:H7 in Experimental Example 4 of the present invention;

[0022] Figure 5 It is the graph of the influence of different coating treatments on the appearance of fresh - cut apples during storage in Experimental Example 4 of the present invention

[0023] Figure 6 It is the test result graph of the weight loss rate of fresh - cut apples within 9 days of fresh - keeping in Experimental Example 4 of the present invention

[0024] Figure 7 It is the test result graph of the polyphenol oxidase content of fresh - cut apples in Experimental Example 4 of the present invention

[0025] Figure 8 It is the test result graph of the peroxidase content of fresh - cut apples in Experimental Example 4 of the present invention

[0026] Figure 9 It is the test result graph of the catalase content of fresh - cut apples in Experimental Example 4 of the present invention

[0027] Figure 10 It is the test result graph of the malondialdehyde content of fresh - cut apples in Experimental Example 4 of the present invention Detailed implementation manners

[0028] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further introduced in detail below in conjunction with embodiments. The following embodiments are used to illustrate the present invention, but are not used to limit the scope of the present invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well - known to those skilled in the art, and the raw materials used are all commercially available products.

[0029] Example 1 Preparation method of L - malic acid / ferulic acid - sodium alginate / whey protein fresh - keeping composite coating

[0030] Preparation of L - malic acid / ferulic acid - sodium alginate / whey protein fresh - keeping composite coating film: Mix 1.5 g of sodium alginate, 0.5 g of whey protein, 0.3 g of glycerol and 100 mL of distilled water, place it on a magnetic stirrer and stir at 80 °C at 400 r / min for 1 h, add 3.125 mg of L - malic acid and 0.1 g of ferulic acid and mix evenly to obtain a uniform coating solution; place the uniform coating solution in an ultrasonic cleaner and ultrasonicate at 200 w for 30 min, after ultrasonication, let it stand for 10 min, pour it into a glass petri dish for casting, and place it at 25 °C to dry for 24 h until completely dried. After the film softens, peel off the film to obtain the L - malic acid / ferulic acid - sodium alginate / whey protein fresh - keeping composite coating film.

[0031] Example 2 Characteristics of L - malic acid / ferulic acid - sodium alginate / whey protein fresh - keeping composite coating film

[0032] (1) Appearance and thickness measurement

[0033] Use a micrometer to measure the thickness at five randomly selected points on each layer of the film. Each coating film is as Figure 1 shown, a is sodium alginate coating film (SA), b is sodium alginate / whey protein coating film (SA / WP), c is sodium alginate / whey protein / L - malic acid coating film (SA / WP / LMA), d is sodium alginate / whey protein / ferulic acid coating film (SA / WP / FA). e is sodium alginate / whey protein / L - malic acid / ferulic acid coating film (SA / WP / LMA / FA). The results show that due to the addition of whey protein, the color of the coating film turns yellow, and the thickness of each coating film also increases significantly (P < 0.05) due to the addition of antibacterial agents and preservatives.

[0034] (2) Water contact angle test

[0035] Use a contact angle measuring instrument to measure the hydrophobicity of the fresh - keeping composite coating film. The water contact angle test results of the fresh - keeping composite coating film are as Figure 2 shown, the angle of the SA / WP / LMA coating film is up to 97° at most, and the angle of the SA / WP / LMA / FA coating film is 88.6°, with good hydrophobicity. The size of the hydrophobic angle of the film is related to the number of hydrophobic and hydrophilic groups on its surface. This may be due to the enhanced interaction between L - malic acid and ferulic acid and sodium alginate and whey protein, resulting in the formation of more hydrogen bonds, thus reducing the number of –OH exposed on the film surface.

[0036] (3) UV - blocking and transparency test

[0037] Use a UV - visible spectrophotometer to evaluate the UV - blocking performance and transparency of the composite film in the range of 200 - 800 nm. The UV - blocking and transparency test results of the fresh - keeping composite coating film are as Figure 3As shown, the coating film can effectively protect fruits and extend their shelf life by reducing the oxidation and degradation of food components through good ultraviolet and light barrier properties. The transparency of the coating film decreases with the addition of the composite bacteriostatic agent. The addition of L-malic acid and ferulic acid significantly reduces the transparency of the coating film (P < 0.05). Generally speaking, the SA / WP / LMA / FA coating film with good transparency is considered suitable for preserving fresh-cut apples and can effectively block ultraviolet rays within 200 - 300 nm.

[0038] Example 3 Method of Using L-Malic Acid / Ferulic Acid-Sodium Alginate / Whey Protein Fresh-Keeping Composite Coating Film

[0039] The apples used in the experiment were consistent in shape, size, color, and texture, without mechanical damage, diseases, or pests. For the treatment of apples, apples with similar shape, size, color, and texture and without mechanical damage, diseases, or pests were selected. The apples were washed with clean water, the outer skins were peeled off with a peeler, the apple fruits were immersed in the coating liquid shown in Example 1 for 5 minutes, taken out and placed in a ventilated place to dry, and then put into a plastic fresh-keeping box and stored at 4°C.

[0040] Example 4 Fresh-Keeping Effect of L-Malic Acid / Ferulic Acid-Sodium Alginate / Whey Protein Fresh-Keeping Composite Coating Film

[0041] 1. Determination of the Bacteriostatic Performance of the Fresh-Keeping Composite Coating Film

[0042] Strain activation: Escherichia coli O157:H7 was inoculated into liquid LB medium at an inoculation amount of 2% (v / v) respectively, cultured in a constant temperature incubator at 37°C for 24 hours to obtain the first-generation Escherichia coli O157:H7, and then inoculated into liquid LB medium at an inoculation amount of 2% (v / v) and cultured in a constant temperature incubator at 37°C for 24 hours to obtain the activated strain.

[0043] Bacteriostatic rate test: The activated Escherichia coli O157:H7 was diluted into a bacterial suspension with a concentration of 1.0×10 6 CFU / mL. 0.3 g of the prepared fresh-keeping composite film was immersed in 10 mL of the bacterial suspension after being irradiated with ultraviolet light for 1 hour, placed in a shaking incubator at 37°C and shaken at 150 r / min for 3 hours to obtain a culture solution. 100 μL of the culture solution was evenly spread on an LB agar plate, inverted and cultured in a constant temperature incubator at 37°C for 24 hours. After the culture was completed, the culture dish was observed for colony counting. The bacteriostatic rate of the fresh-keeping composite coating film against Escherichia coli O157:H7 was calculated according to formula (1.1);

[0044]

[0045] As Figure 4As shown, the flora in the SA / WP / LMA treatment group and the SA / WP / LMA / FA treatment group decreased significantly (P < 0.05). The L-malic acid / ferulic acid-sodium alginate / whey protein fresh-keeping composite coating can reduce the growth and reproduction of Escherichia coli O157:H7 and has excellent antibacterial properties.

[0046] 2. Fresh-keeping effect of the fresh-keeping composite coating

[0047] (1) Determination of appearance quality

[0048] As Figure 5 shown: Under the storage condition of 4°C, there were significant differences in the browning process of fresh-cut apples and the effects of different treatment groups. At the initial stage of storage (day 0), there was no obvious browning in the blank group, SA treatment group, SA / WP treatment group, and SA / WP / LMA treatment group. However, from day 1, the browning phenomenon began to appear and intensified with the extension of storage time: the browning degree increased sharply on day 5 and completely lost its edible value by day 9. The SA / WP / FA treatment group and the SA / WP / LMA / FA treatment group still maintained a good appearance throughout the storage period (9 days in total). The combination of L-malic acid and ferulic acid promoted the effect of inhibiting browning and extended the shelf life of fresh-cut apples to 9 days. The L-malic acid / ferulic acid-sodium alginate / whey protein fresh-keeping composite coating has a good fresh-keeping effect.

[0049] The weight loss rate of apples was measured by the weighing method. The test results are as Figure 6 shown. During the storage of fresh-cut apples, the weight loss rate of each group of fruits showed an upward trend with the extension of the fresh-keeping time. Among them, the weight loss rate of the blank group was significantly higher than that of other experimental groups. This phenomenon can be attributed to the direct exposure of fresh-cut apples to the air, resulting in an increase in the water film permeability on the apple surface and accelerating the loss of water. The weight loss rate of the SA / WP / LMA / FA treatment group was the lowest, which may be attributed to the excellent water barrier performance of the composite film formed by sodium alginate and whey protein. By reducing the water vapor transmission rate, it delays water loss, and the synergistic effect of L-malic acid and ferulic acid can inhibit respiratory metabolism and reduce the consumption caused by respiration. The L-malic acid / ferulic acid-sodium alginate / whey protein fresh-keeping composite coating can effectively slow down the water loss of fresh-cut apples and maintain the good quality of fresh-cut apples.

[0050] (2) Determination of browning-related indicators

[0051] During the processing and storage of fruits and vegetables, the browning phenomenon that occurs in their tissues is closely related to the activity of polyphenol oxidase. The high activity of polyphenol oxidase (PPO) often leads to easier browning of fruit and vegetable tissues, affecting the color and quality of fruits and vegetables. Polyphenol oxidase (PPO) is an oxidoreductase that catalyzes the oxidation of phenolic compounds into quinone compounds, which then form brown polymers. The activity of polyphenol oxidase (PPO) was measured by the catechol method, and the test results are as Figure 7 shown. Due to the accumulation of phenolic substances in each group, the PPO activity in each group was increasing. During the 9-day storage period, the activity of each group showed a trend of first increasing and then decreasing, and the activity of each group at the end of storage increased compared with that on the 0th day. On the 5th day, the activity of the blank group, SA treatment group, SA / WP treatment group, and SA / WP / LMA treatment group reached the highest, which was the same as the browning of the pulp. It should be noted that the activity of the SA / WP / FA treatment group and the SA / WP / LMA / FA treatment group was significantly lower than that of the blank group (P < 0.05), and the overall activity was at a relatively low level, with the SA / WP / LMA / FA treatment group having the lowest activity.

[0052] The browning phenomenon is also related to the activity of peroxidase. In addition, peroxidase (POD) can also utilize H 2 O 2 to release free radicals, accelerate the lipid peroxidation of cell membranes, and further damage the cell structure. Therefore, inhibiting the activity of peroxidase can effectively delay browning. The activity of peroxidase (POD) was measured by the guaiacol method, and the test results are as Figure 8 shown. During storage, the POD activity of the blank group and the SA treatment group was higher than that of other experimental groups. The POD activity of the SA / WP / FA treatment group and the SA / WP / LMA / FA treatment group remained at a relatively low level, indicating that the L-malic acid / ferulic acid-sodium alginate / whey protein treatment can effectively reduce the POD activity of apples during storage, and can prevent the physiological toxicity caused by the accumulation of H 2 O 2 to a certain extent, enhance the antioxidant capacity of apples, and thus protect the color and quality during storage.

[0053] (3) Determination of antioxidant indexes

[0054] Catalase (CAT) can catalyze the decomposition of hydrogen peroxide accumulated in plants into water and molecular oxygen, thereby reducing the possible oxidative damage to fruit and vegetable tissues caused by H 2 O 2 The activity of catalase was measured by ultraviolet spectrophotometry, and the test results are as Figure 9As shown, during the first 3 days of storage, the activities of all groups showed an upward trend. After that, the CAT activities of the control group and the SA treatment group both showed a downward trend, which may be related to the excessive hydrolysis and consumption of hydrogen peroxide. The SA / WP / FA treatment group, the SA / WP / LMA treatment group, and the SA / WP / LMA / FA treatment group showed an upward trend throughout the storage period and were significantly higher than other groups on the 10th day (P < 0.05), indicating that the compounding of L-malic acid and ferulic acid significantly increased the CAT activity during storage, thereby improving the ability to scavenge reactive oxygen species, and the compounding effect improved the antioxidant capacity of the fresh-keeping composite coating.

[0055] In addition, malondialdehyde (MDA) is one of the main products of membrane lipid peroxidation. Usually, its content is used as an index of lipid peroxidation to reflect the degree of membrane lipid peroxidation in plant cells. Therefore, the accumulation of MDA can cause certain damage to the cytoplasmic membrane and cell organelles of fruits and vegetables. The activity of malondialdehyde (MDA) was determined by the thiobarbituric acid colorimetric method. The test results are as Figure 10 shown. During the storage of apples, the MDA contents of the SA / WP / FA treatment group and the SA / WP / LMA / FA treatment group were always significantly lower than those of the control group, the SA treatment group, the SA / WP treatment group, and the SA / WP / LMA treatment group (P < 0.05), indicating that the composite coating treatment could effectively inhibit the membrane lipid peroxidation reaction. In the first 4 days of storage, the MDA contents of the control group and the SA treatment group were significantly higher than those of other experimental groups, which may be related to the excessive accumulation of reactive oxygen species (ROS) caused by mechanical damage during fresh-cutting. The fresh-keeping composite coating significantly delayed the rising rate of MDA content through forming a physical barrier and antioxidant action. The L-malic acid / ferulic acid-sodium alginate / whey protein fresh-keeping composite coating could effectively inhibit the membrane lipid peroxidation reaction and extend the shelf life of fresh-cut apples.

[0056] Only some exemplary embodiments of the present invention have been described by way of illustration above. Undoubtedly, for those of ordinary skill in the art, the described embodiments can be modified in various different ways without departing from the spirit and scope of the present invention. Therefore, the above description is illustrative in nature and should not be construed as a limitation on the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a L-malic acid / ferulic acid-sodium alginate / whey protein fresh-keeping composite coating liquid for fresh-cut apples, characterized in that: The following steps are involved: S1. Prepare sodium alginate / whey protein composite coating liquid: disperse sodium alginate in water, add glycerol, mix well, add whey protein, stir magnetically at 80°C until completely dissolved, and obtain sodium alginate / whey protein composite coating liquid. S2. Dissolve L-malic acid / ferulic acid in the sodium alginate / whey protein composite coating solution until it is completely dissolved, let stand for 1.5-2.5 hours, and perform ultrasonic degassing to obtain the L-malic acid / ferulic acid composite coating solution.

2. An L-malic acid / ferulic acid composite coating liquid, characterized in that: It is prepared by the method described in claim 1.

3. The preparation method according to claim 1, characterized in that The mass volume ratio of sodium alginate, whey protein, glycerol and water in step S1 is 0.5-1.5 g: 0.5-0.7 g: 0.2-0.4 g: 100 mL.

4. The preparation method according to claim 1, characterized in that , the mass volume ratio of sodium alginate, whey protein, glycerol and water in step S1 is 1.5g:0.5g:0.3g:100mL.

5. The preparation method according to claim 1, characterized in that: The mass volume ratio of L-malic acid, ferulic acid and water in step S2 is 3.125 mg:0.1 g:100 mL.

6. An L-malic acid / ferulic acid-sodium alginate / whey protein fresh-keeping composite coating, characterized in that: According to claim 2, the composite coating is prepared by the casting method: the prepared coating liquid is evenly spread on a glass mold, dried at 25°C, and the obtained film is balanced at 25°C for 24 hours to obtain a fresh-keeping composite coating.

7. Use of the L-malic acid / ferulic acid-sodium alginate / whey protein fresh-keeping composite coating liquid according to claim 1 in the preservation of fresh-cut fruits.

8. The use of the fresh-keeping composite coating liquid in fresh-cut apples according to claim 7, characterized in that: Peel apples of similar shape, size, color and texture with a peeler, immerse them in the coating solution for 5 minutes, take them out and dry them in a ventilated environment at 25°C to ensure that a uniform coating is formed on the surface of the apples, and store them in plastic fresh-keeping boxes at 4°C.

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