Preparation method of blueberry fresh-keeping and freshness monitoring composite film

Abstract

The application discloses a kind of blueberry fresh-keeping and freshness monitoring composite film preparation method, using the following method is prepared: soybean protein isolate 28~32g, casein 25~28g, glycerol 24~26g, 2,5-furan dimethyl alcohol 2~3g are placed in beaker, with distilled water constant volume to 1000mL, after magnetic stirring, adjust pH value to 10, obtain mixed solution;Mixed solution is placed in ultrasonic constant-temperature water bath and is kept warm for 15~18min and then cooled, further add carvacol solution 5~6g, allicin 1~1.3g, anthocyanin 0.8~1.2g and mix homogeneously to obtain composite film solution;Composite film solution is poured into mold using flow casting method, placed in oven and baked to obtain composite film.The composite film prepared by the application can solve the problems of blueberry postharvest softening, rotting and deterioration, and can also monitor the freshness of blueberries.The composite film has the functions of blueberry fresh-keeping and freshness monitoring indication.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fruit preservation, in particular to a preparation method of a blueberry preservation and freshness monitoring composite film. BACKGROUND

[0002] Blueberry is scientifically known as Vaccinium spp of Ericaceae, and it mainly contains anthocyanins, vitamins, phenols, pectin, tannic acid, folic acid, flavonoids and other nutrients. It has the effects of preventing cardiovascular diseases, protecting vision, anti-aging, enhancing human immunity, lubricating intestines and defecation, etc. As a kind of berry with unique flavor and high nutritional value, blueberry is deeply loved by consumers. With the continuous expansion of global blueberry planting scale, consumers' understanding of the nutritional value of blueberry is deepening, and the demand for blueberry in the market is also gradually increasing.

[0003] Blueberry is harvested in summer with high temperature and much rain. The air humidity is high at the time of harvesting, and the microbial activity is vigorous. Blueberry has a high respiratory metabolism. In addition, the blueberry has thin skin and much juice, and has no nutrition and water supply, so the metabolism of the blueberry itself increases, the meat quality becomes soft, matures and ages, resulting in the decrease of the quality of the blueberry.

[0004] Blueberry has high nutritional value. The main problems after harvesting are softening and rotting, which cause the blueberry not to be stored and not to be transported, thereby seriously restricting the commodity value of the blueberry. Therefore, it is of great significance to study the preservation technology of blueberry for the development of the whole blueberry industry.

[0005] The post-harvest preservation of blueberry mainly includes physical preservation methods such as low-temperature cold storage, modified atmosphere preservation, and preservation film preservation, and chemical preservation methods such as chemical preservative spraying and soaking. However, low-temperature cold storage and modified atmosphere preservation are suitable for large-scale use by enterprises, but not suitable for the preservation of blueberry during the sales period in supermarkets, and have high energy consumption and cost. The preservation effect of the preservation film is relatively poor. The chemical preservative spraying and soaking for preserving blueberry can easily cause chemical residues and affect the taste, flavor and color of blueberry. At the same time, the emission of chemical preservatives can easily cause environmental pollution.

[0006] At present, the blueberry sold in supermarkets is mainly packaged in polyethylene terephthalate (PET) transparent packaging boxes. When consumers select and purchase blueberries, due to the factors such as storage temperature and transportation conditions, the freshness of blueberries cannot be accurately judged by only looking at the production date and surface appearance. Most consumers open the packaging box to observe, or even touch with their hands, which can easily cause physical damage to the blueberries, and repeatedly opening and closing the blueberry packaging box can affect the quality and shelf life of the blueberries.

[0007] Therefore, solving the problems of softening, rotting and metamorphic deterioration of blueberry after harvesting, and the freshness monitoring of blueberry during the sales period in supermarkets is a technical problem that needs to be solved in the blueberry post-harvest preservation industry at present. SUMMARY

[0008] The present application aims to provide a preparation method of a blueberry fresh-keeping and freshness monitoring composite film to solve the problems of postharvest softening, rotting and quality deterioration of blueberries and freshness monitoring of blueberries during supermarket sales.

[0009] To achieve the above-mentioned purpose, the present application provides the following technical scheme: a preparation method of a blueberry fresh-keeping and freshness monitoring composite film, comprising the following steps:

[0010] Step one, weigh 28-32 g of soybean protein isolate, 25-28 g of casein, 24-26 g of glycerol, and 2-3 g of 2,5-furan dimethyl alcohol, and place them in a beaker. Add distilled water to make up to 1000 mL, and place it in a magnetic stirrer. Stir for 15-18 min, then adjust the pH value to 10 with NaOH to obtain a mixed solution.

[0011] Step two, place the mixed solution prepared in step one in an ultrasonic constant temperature water bath and heat for 15-18 min, then cool it down. The temperature of the ultrasonic constant temperature water bath is set to 80℃, and the ultrasonic frequency is 75-80 Hz.

[0012] Step three, add 5-6 g of carvyl alcohol solution, 1-1.3 g of allicin, and 0.8-1.2 g of anthocyanin to the cooled solution in step two, mix and stir, then place it in a homogenizer for 5-7 min to obtain a composite film solution.

[0013] Step four, pour the composite film solution prepared in step three into a mold using the flow casting method, and place it in an oven for baking to obtain a fresh-keeping and freshness monitoring composite film.

[0014] As a preferred technical scheme of the present application, the speed of the magnetic stirrer is 600-650 rpm.

[0015] As a preferred technical scheme of the present application, the concentration of NaOH is 1-1.2 mol / L.

[0016] As a preferred technical scheme of the present application, the concentration of carvyl alcohol solution is 99%.

[0017] As a preferred technical scheme of the present application, the speed of the homogenizer is 10000-12000 rpm.

[0018] As a preferred technical scheme of the present application, the temperature of the oven is set to 40-45℃, the time is 20-25 min, and the thickness of the composite film is 0.3-0.5 mm.

[0019] Compared with the prior art, the present application has the following advantages:

[0020] 1、The application adopts soybean protein isolate and casein as the film forming base material of the composite film, the protein purity of the soybean protein isolate is higher, and the film forming property is good. The casein is a calcium and phosphorus combined protein, and contains hydrophilic groups and hydrophobic groups in the molecular structure. The structure can make the casein form hydrogen bonds or other interactions with water molecules, enhance the binding force of the casein and water molecules, so that the carnosic acid, allicin and anthocyanin are more uniformly distributed in the protein molecules, and the flexibility, fresh-keeping function and sensitivity and accuracy of the color change indication of the composite film are improved. Meanwhile, the casein contains various bioactive peptides, the bioactive peptides can capture free radicals, prevent the generation of free radicals, and remove the formed free radicals and active oxygen, so as to help to maintain the color, flavor and nutritional value of the blueberries, and prolong the fresh-keeping period.

[0021] 2、The application adds 2,5-furan dimethyl alcohol, the 2,5-furan dimethyl alcohol is a biomass-based furan derivative compound containing two alcohol hydroxyl functional groups, which can interact with the soybean protein isolate and the casein as a surface active agent, so as to form a more dense film structure, and improve the flexibility, plasticity and barrier property of the composite film.

[0022] 3、The application adopts the ultrasonic constant temperature water bath to treat the mixed solution, which is beneficial to the uniform mixing of the soybean protein isolate, the casein, the glycerol and the 2,5-furan dimethyl alcohol, so as to ensure the uniformity and consistency of film forming, and improve the fresh-keeping and monitoring effects.

[0023] 4、The anthocyanin is a water-soluble natural pigment extracted from plants, which will show different colors in different acid-base environments. During the postharvest storage of blueberries, CO2 is produced due to respiration, and carbonic acid is generated with the moisture in the air, which leads to the change of the pH of the storage environment of the blueberries. The composite film will change color during the storage of the blueberry fruits due to the anthocyanin, and then help the consumers to more accurately judge the freshness of the blueberries. The application proportionally compounds the carnosic acid, the allicin and the anthocyanin, and makes the composite film together with the film forming base material of the soybean protein isolate and the casein, so as to solve the problems of softening, rotting and deterioration of the postharvest blueberries, and monitor the freshness of the blueberries during the sale in supermarkets, and realize the functions of the blueberry fresh-keeping and the freshness monitoring indication. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is the preparation flow chart of the blueberry fresh-keeping and freshness monitoring composite film of the application;

[0025] Figure 2 It is the elongation at break column chart of the composite film of the application;

[0026] Figure 3 It is the weight loss rate change column chart of the blueberry during the storage process of the application;

[0027] Figure 4 The column chart of the change of the rot rate of the blueberries in the storage process of the application;

[0028] Figure 5 The line chart of the change of the hardness of the blueberries in the storage process of the application;

[0029] Figure 6 The line chart of the change of the color difference L value of the blueberries in the storage process of the application;

[0030] Figure 7 The square box for packaging the blueberries;

[0031] Figure 8 The round box for packaging the blueberries;

[0032] Figure 9 The honeycomb box for packaging the blueberries;

[0033] Figure 10 The column chart of the change of the L value of the blueberries in the storage process of the composite film;

[0034] Figure 11 The column chart of the change of the a value of the blueberries in the storage process of the composite film;

[0035] Figure 12 The column chart of the change of the b value of the blueberries in the storage process of the composite film;

[0036] Figure 13 The column chart of the change of the △E value of the blueberries in the storage process of the composite film;

[0037] Figure 14 The color change chart of the composite film of the square box packaging;

[0038] Figure 15 The color change chart of the composite film of the round box packaging;

[0039] Figure 16 The color change chart of the composite film of the honeycomb box packaging (the bottom background is shot with blueberries);

[0040] Figure 17 The color change chart of the composite film of the honeycomb box packaging (the bottom background is shot without blueberries). DETAILED DESCRIPTION

[0041] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the application.

[0042] The application adopts soybean protein isolate and casein as the film-forming base material of the composite film, the soybean protein isolate is a plant protein extracted from soybeans, has high protein purity and good film-forming property; the casein is a calcium and phosphorus binding protein, contains hydrophilic groups and hydrophobic groups in the molecular structure, which can form hydrogen bonds or other interactions with water molecules, thereby enhancing the binding force between the casein and water molecules, so that carvacol, allicin, anthocyanins and other substances are more uniformly distributed in the protein molecules, thereby improving the flexibility, preservation function, sensitivity and accuracy of the color change indication of the composite film. Meanwhile, the casein contains various bioactive peptides, which can capture free radicals, prevent free radical formation, and remove formed free radicals and active oxygen, thereby helping to maintain the color, flavor and nutritional value of blueberries and prolong their shelf life.

[0043] 2,5-furan dimethyl alcohol is a biomass-based furan derivative compound containing two alcohol hydroxyl functional groups, which can interact with soybean protein isolate and casein as a surfactant, thereby forming a denser film structure and improving the flexibility, plasticity and barrier properties of the composite film. The mixed solution composed of soybean protein isolate, casein, glycerol, 2,5-furan dimethyl alcohol and distilled water is treated by ultrasonic constant temperature water bath, which can cause collision and extrusion of micro-particles in the mixed solution, destroy bubbles and agglomerates in the solution, increase the gas-liquid exchange surface area in the liquid, thereby enhancing the reaction efficiency, the vibration and shearing action of ultrasonic waves can break the intermolecular interaction force, promote the uniform dispersion and rearrangement of substances, thereby improving the physical and chemical properties of the mixed solution, and the constant temperature water bath provides a stable temperature environment, which helps to optimize the chemical reaction conditions. This ultrasonic constant temperature water bath helps to mix soybean protein isolate, casein, glycerol and 2,5-furan dimethyl alcohol in distilled water more uniformly, ensures the uniformity and consistency of film formation, and improves the preservation and monitoring effect.

[0044] Carvacol has strong antibacterial ability and antioxidant effect, can inhibit the growth of bacteria in food, delay the oxidation process of ingredients in food, reduce the corruption of food, help to keep the freshness and taste of food. Allicin is the main active ingredient of garlic, which can prevent the invasion of external harmful microorganisms and reduce the rot rate of fruits and vegetables. Anthocyanins are a kind of water-soluble natural pigment extracted from plants, which can show different colors in different acid-base environments. In acidic conditions, it shows red color, and in alkaline conditions, it shows blue color. During the storage period of blueberries, CO2 is produced due to respiration, and carbonic acid is produced with the moisture in the air, resulting in changes in the pH of the storage environment of blueberries. The composite film will discolor during the storage period of blueberry fruits due to the presence of anthocyanins, which can help consumers more accurately judge the freshness of blueberries. The present application proportionally compounds carvacol, allicin and anthocyanin, and makes a composite film with soybean protein isolate and casein film-forming substrate, which solves the problems of softening, rotting and deterioration of blueberries after harvesting, and monitors the freshness of blueberries during the sale period in supermarkets, which plays a dual role of blueberry preservation and freshness monitoring.

[0045] The raw materials used in the present application are all commercially available.

[0046] Example 1:

[0047] A preparation method of a blueberry preservation and freshness monitoring composite film, comprising the following steps:

[0048] Step one, weigh 28g of soybean protein isolate, 25g of casein, 24g of glycerol and 2g of 2,5-furandimethanol, and place them in a beaker, and then add distilled water to make up to 1000ml, and then place them in a magnetic stirrer and stir for 15 minutes, then adjust the pH value to 10 with NaOH, and then get a mixed solution, the stirring speed of the magnetic stirrer is 600rpm, and the concentration of NaOH is 1mol / L.

[0049] Step two, place the mixed solution prepared in step one in an ultrasonic constant temperature water bath and heat for 15 minutes, then cool it down, the temperature of the ultrasonic constant temperature water bath is set to 80℃, and the ultrasonic frequency is 75Hz.

[0050] Step three, add 5g of carvacol solution, 1g of allicin and 0.8g of anthocyanin to the cooled solution of step two, mix and stir, then place it in a homogenizer and homogenize for 5 minutes to get a composite film solution, the concentration of carvacol solution is 99%, and the rotating speed of the homogenizer is 10000rpm.

[0051] Step four, pour the composite film solution prepared in step three into a mold by using a flow casting method, and then place it in an oven to bake, and then get a preservation and freshness monitoring composite film, the temperature of the oven is set to 40℃, the time is 20 minutes, and the thickness of the composite film is 0.3mm.

[0052] Example 2:

[0053] A method for preparing a blueberry fresh-keeping and freshness monitoring composite film, comprising the following steps:

[0054] Step one, weigh 32g of soybean protein isolate, 28g of casein, 26g of glycerol, and 3g of 2,5-furan dimethyl alcohol, and place them in a beaker, and then add distilled water to make up to 1000ml, and then place them in a magnetic stirrer and stir for 18 minutes, then adjust the pH value to 10 with NaOH, and then obtain a mixed solution, the stirring speed of the magnetic stirrer is 650rpm, and the concentration of NaOH is 1.2mol / L.

[0055] Step two, place the mixed solution prepared in step one in an ultrasonic constant temperature water bath and heat for 18 minutes, then cool it down, set the temperature of the ultrasonic constant temperature water bath to 80℃, and the ultrasonic frequency to 80Hz.

[0056] Step three, add 6g of carvacol solution, 1.3g of allicin, and 1.2g of anthocyanin to the cooled solution in step two, mix and stir, then place it in a homogenizer and homogenize for 7 minutes to obtain a composite film solution, the concentration of the carvacol solution is 99%, and the rotation speed of the homogenizer is 12000rpm.

[0057] Step four, pour the composite film solution prepared in step three into a mold using the flow casting method, and then place it in an oven to bake, and then obtain a fresh-keeping and freshness monitoring composite film, the temperature of the oven is set to 45℃, the time is 25 minutes, and the thickness of the composite film is 0.5mm.

[0058] Comparative Example One:

[0059] Remove the casein in Example One, and follow the same method steps as in Example One.

[0060] Comparative Example Two:

[0061] Remove the 2,5-furan dimethyl alcohol in Example One, and follow the same method steps as in Example One.

[0062] Comparative Example Three:

[0063] Change step two in Example One to: place the mixed solution prepared in step one in an 80℃ constant temperature water bath and heat for 15 minutes, then cool it down, and follow the same method steps as in Example One.

[0064] Comparative Example Four:

[0065] Remove the carvacol solution, allicin, and anthocyanin in Example One, and follow the same method steps as in Example One.

[0066] Test the elongation at break of the composite films prepared in Examples One and Two and Comparative Examples One, Two, Three, and Four, the elongation at break = (length after stretching to break - original length) x 100% / original length, and the results are shown in Table 1.Figure 2 As shown, the elongation at break of the composite films of Example One, Two and Comparative Examples Three and Four are 20.17%, 19.55%, 19.23% and 19.79% respectively, and the elongation at break of the composite films of Comparative Example One and Comparative Example Two are 15.81% and 15.63% respectively, which are 78.4% and 77.5% of that of Example One respectively. The elongation at break refers to the ratio of the length after stretching to the length before stretching of the composite film when the composite film is stretched to break under external force, and is an important index for measuring the flexibility of the composite film. The composite films of Comparative Example One and Comparative Example Two are not added with casein and 2,5-furandimethanol. Casein is a calcium and phosphorus binding protein, and contains hydrophilic groups and hydrophobic groups in the molecular structure. Such structure can form hydrogen bonds or other interactions with water molecules, enhance the binding force between casein and water molecules, and improve the flexibility of the composite film. 2,5-furandimethanol is a biomass-based furan derivative compound containing two alcohol hydroxyl functional groups, which can interact with soy protein isolate and casein to form a more compact film structure, thereby improving the flexibility and plasticity of the composite film.

[0067] The composite films prepared in Example One, Two and Comparative Examples One, Two, Three and Four were attached to the inside of the PET transparent packaging box cover. After the blueberries were picked, they were pre-cooled at 0°C for 3h to dissipate the field heat. The blueberries with dissipated field heat were placed inside the PET packaging box cover with the composite film attached, and the CK group was the PET packaging box without the composite film. Each box of blueberries weighed (125±10)g, and the treatment process was carried out under natural light. After treatment, the samples were stored at a temperature of (20±0.5)℃ and a humidity of (95-99)% RH. Samples were taken every 2 days, and the 0-day index was measured using the fruits with dissipated field heat picked on the same day.

[0068] Figure 3 The weight loss rate of the blueberries was calculated by weighing method according to the following formula: weight loss rate (%) = (weight before storage - weight after storage) x 100% / weight before storage. For example, Figure 3As shown, the weight loss rate of blueberry fruit showed an upward trend with the extension of storage time. Within the first 2 days, there was no significant difference in the results of the CK group and the examples and comparative examples. From the 4th day, the weight loss rate of blueberry fruit in the CK group was significantly lower than that of the examples and comparative examples (P<0.05). By the 8th day of storage, the weight loss rate of blueberry in the CK group reached 3.53%, which was 72.4% higher than that of Example 1, and the difference in weight loss rate reached a maximum, indicating that the composite film treatment could effectively inhibit the weight loss of blueberry fruit. The difference in weight loss rate of blueberry treated by the composite films prepared in Examples 1 and 2 and Comparative Examples 3 and 4 was not significant. By the 10th day of storage, the weight loss rate of blueberry in Example 1 was 2.7%, and the weight loss rates of Comparative Example 1 and Comparative Example 2 were 3.01% and 3.47%, respectively, which were 11.6% and 28.7% higher than that of Example 1, respectively. The addition of casein and 2,5-furandimethanol during the preparation of the composite film helped to form a more compact film structure, thereby delaying the loss of water in blueberries.

[0069] Figure 4 The change in the rot rate of blueberries was determined by sensory evaluation. Fruits with rot spots, mold, and soft rot on the surface were considered rotten fruits. Forty fruits were randomly selected from each group, and their average values were calculated according to the following formula: rot rate (%) = (rotten fruits / total number of fruits surveyed) x 100%. For example, Figure 4 As shown, under normal temperature conditions, the fruit rot rate gradually increased with the extension of storage time. The rot rate of blueberry fruit treated by the composite films prepared in Examples 1 and 2 was lower than that of Comparative Examples 1, 2, 3, and 4 and the CK group. From the 8th day of storage, the rot rate of blueberry fruit treated by the composite films prepared in Examples 1 and 2 and Comparative Examples 1, 2, and 3 was significantly lower than that of the CK group and Comparative Example 4 (P<0.05). By the 10th day of storage, the rot rate of blueberry fruit in the CK group reached 24.15%, while the rot rate of blueberry in the examples was below 15%. The rot rates of blueberry treated by the composite films prepared in Comparative Examples 1, 2, 3, and 4 were 16.11%, 15.25%, 15.61%, and 19.19%, respectively, which were higher than those of the examples. This indicates that the addition of casein, 2,5-furandimethanol, carvacrol solution, garlicin, and anthocyanin during the preparation of the composite film can reduce the rot rate of blueberry fruit. Ultrasonic constant temperature water bath treatment of the mixed solution can make the reaction and mixing between the substances more uniform, thereby playing a better role in preserving blueberries and reducing the rot rate of blueberries.

[0070] Figure 5 The change in the hardness of blueberries was determined using a texture analyzer. The hardness of blueberries was measured at two equatorial points of the blueberry fruit with a measurement depth of 7 mm and a measurement speed of 0.5 mm / s. The hardness of blueberry fruit was expressed in Newton (N). For example, Figure 5As shown, the hardness of blueberries under normal temperature conditions showed a downward trend with the extension of storage time. The hardness of blueberries treated with the composite film prepared in Example 1 and Example 2 was relatively high, being 1.56 N and 1.60 N respectively, while the hardness of blueberries in the CK group was only 1.31 N. The hardness of blueberries treated with the composite film prepared in Comparative Examples 1, 2, 3 and 4 was respectively 0.16 N, 0.2 N, 0.09 N and 0.17 N lower than that of Comparative Example 1. The addition of casein and 2,5-furandimethanol helped to form a more compact film structure, reduce the loss of water in blueberries, and thus reduce the hardness of blueberries. Carvacol solution, allicin and anthocyanin can reduce the rot rate of blueberry fruits. Therefore, the composite film treatment can effectively prevent the decrease of the hardness of blueberries and ensure the freshness of blueberries.

[0071] Figure 6 The change of the skin color difference L value of blueberry fruits was measured. The L value represents the lightness and darkness of the color, wherein a higher L value indicates a brighter color and a lower L value indicates a darker color. The change of the L value can reflect the freshness and maturity of blueberries, which is of great significance for evaluating the quality of blueberries. The L value was measured by a colorimeter. Fifteen blueberries with consistent size and shape were randomly selected. The colorimeter was first calibrated with a white board. The lens of the colorimeter was aimed at the equator of the blueberries for measurement. Figure 6 As shown, with the extension of storage time, the skin brightness L value of blueberry fruits showed a downward trend as a whole. When the blueberries were stored for 10 days, the L value of blueberries in Example 1 was respectively 4.7%, 6.3%, 3.4%, 7.4% and 11.1% higher than that of Comparative Examples 1, 2, 3, 4 and the CK group. The blueberries treated with the composite film prepared in the application can effectively delay the decrease of the skin brightness L value of blueberry fruits and ensure the freshness of blueberries.

[0072] PET square boxes (as shown in Figure 7 ), round boxes (as shown in Figure 8 ) and honeycomb boxes (as shown in Figure 9 ) were used to package blueberries. The composite film prepared in Example 1 was pasted on the lids of the three kinds of PET packaging boxes for observation of the monitoring effect of the composite film on the freshness of blueberries. The L value, a value and b value on the outside of the packaging box lid were measured by a colorimeter to simulate the color seen by consumers. The L value represents the lightness and darkness of the color, wherein a higher L value indicates a brighter color and a lower L value indicates a darker color. The a value and b value represent the intensity of red and green and yellow and blue respectively. A positive a value indicates a color bias towards red, and a negative value indicates a color bias towards green. A positive b value indicates a color bias towards yellow, and a negative value indicates a color bias towards blue. The △E value, also known as the total color difference value, is a comprehensive index for measuring the overall difference between two color samples. The larger the △E value, the greater the color difference, and the smaller the △E value, the closer the color. The calculation of the △E value is based on the difference between the L, a and b values, which is an important parameter for evaluating color consistency or change. The calculation formula of the △E value is:

[0073] Figure 10 The L value change of the composite film, Figure 11 The a value change of the composite film, Figure 12 The b value change of the composite film, Figure 13 The ΔE value change of the composite film, with the extension of the storage time, the L, a, b and ΔE values all show an upward trend, and the composite film treatment can monitor the freshness of blueberries during storage.

[0074] The same position, same time, same light source and same original camera are used to take pictures from the outside of the packaging box cover every day, simulating the color seen by consumers, Figure 14 The color change of the composite film of the square box packaging, Figure 15 The color change of the composite film of the round box packaging, Figure 16 、 Figure 17 The color change of the composite film of the honeycomb box packaging, Figure 16 And Figure 17 The pictures are taken at different shooting points of the same honeycomb box. The honeycomb box has uniform distribution of blueberries, and some places are the blueberry base color, and some places are the color of the film itself. Figure 16 The shooting point has blueberries at the bottom, Figure 17 The shooting point has no blueberries at the bottom. Figure 14 、 15 , 16, 17 are the pictures taken on the 1st day, the 2nd day, the 3rd day, the 4th day and the 5th day from top to bottom. From Figure 14 、 15 , 16 can be seen that the blueberries packaged in square boxes, round boxes and honeycomb boxes all show obvious changes in the composite film within 5 days of storage time. The blueberry base color initially lined by the composite film gradually turns red with the extension of the storage period, Figure 17 The shooting point of the blueberries packaged in the honeycomb box has no blueberries at the bottom. With the extension of the storage time of the blueberries, the color of the composite film gradually deepens and becomes light red. Therefore, the composite film has a color development effect on the common blueberry outer packaging on the market, can monitor the freshness of blueberries, and can help consumers more intuitively feel the freshness change of blueberries. Anthocyanins will show different colors in different acid-base environments. They show red in acidic conditions and blue in alkaline conditions. During postharvest storage, blueberries will produce CO2 due to respiration, which will produce carbonic acid with the moisture in the air, causing changes in the pH of the storage environment of blueberries.

[0075] In summary, the blueberry fresh-keeping and freshness monitoring composite film prepared by the application can solve the quality problems of postharvest blueberries, such as softening, rotting and deterioration, and can monitor the freshness of the blueberries, so that consumers can more intuitively feel the freshness change of the blueberries, and thus the composite film prepared by the application has the dual functions of blueberry fresh-keeping and freshness monitoring indication.

[0076] The above merely describes specific embodiments of the application, but the technical features of the application are not limited thereto. Any simple change, equivalent replacement or modification made on the basis of the application to solve the basically same technical problem and realize the basically same technical effect is included in the protection scope of the application.

Claims

1. A method for preparing a blueberry fresh-keeping and freshness monitoring composite film, characterized in that, It comprises the following steps: Step one, soybean protein isolate 28-32 g, casein 25-28 g, glycerol 24-26 g, 2, 5-furan dimethyl alcohol 2-3 g are weighed in a beaker, and distilled water is used to make up to 1000 mL, and then placed in a magnetic stirrer for stirring for 15-18 min, and then adjusted to pH 10 with NaOH to obtain a mixed solution; Step two, the mixed solution prepared in step one is placed in an ultrasonic constant temperature water bath for 15-18 min, then cooled, the temperature of the ultrasonic constant temperature water bath is set to 80℃, and the ultrasonic frequency is 75-80 Hz; Step three, 5-6 g of carvacol solution, 1-1.3 g of allicin and 0.8-1.2 g of anthocyanin are added to the cooled solution of step two, mixed and stirred, then placed in a homogenizer for 5-7 min to obtain a composite film solution; Step four, the composite film solution prepared in step three is poured into a mold by casting method, and then placed in an oven for baking to obtain a fresh-keeping and freshness monitoring composite film.

2. The method for preparing a composite film for blueberry preservation and freshness monitoring according to claim 1, characterized in that, The rotating speed of the magnetic stirrer is 600-650 rpm.

3. The method for preparing a composite film for blueberry preservation and freshness monitoring according to claim 1, characterized in that, The concentration of NaOH is 1-1.2 mol / L.

4. The method for preparing a composite film for blueberry preservation and freshness monitoring according to claim 1, characterized in that, The concentration of carvacol solution is 99%.

5. The method for preparing a composite film for blueberry preservation and freshness monitoring according to claim 1, characterized in that, The rotating speed of the homogenizer is 10000-12000 rpm.

6. The method for preparing a composite film for blueberry preservation and freshness monitoring according to claim 1, characterized in that, The temperature of the oven is set to 40-45℃, and the time is 20-25 min.

7. The method for preparing a blueberry fresh-keeping and freshness monitoring composite film according to any one of claims 1-6, characterized in that The thickness of the composite film is 0.3-0.5 mm.

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