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

Abstract

The invention discloses a preparation method of a blueberry fresh-keeping and freshness monitoring composite membrane, which comprises the following steps: weighing 28-32g of soybean protein isolate, 25-28g of casein, 24-26g of glycerol and 2-3g of 2, 5-furandimethanol, putting into a beaker, fixing the volume to 1000mL by using distilled water, magnetically stirring, and adjusting the pH value to 10 to obtain a mixed solution; the mixed solution is placed in an ultrasonic constant-temperature water bath to be subjected to heat preservation for 15-18 min and then cooled, then 5-6 g of carvacrol solution, 1-1.3 g of allicin and 0.8-1.2 g of anthocyanin are added, mixing and homogenizing are conducted, and a composite film solution is obtained; and pouring the composite film solution into a mold by adopting a tape casting method, and baking in an oven to obtain the composite film. The composite film prepared by the invention can solve the problems that blueberries are easy to soften, rot and deteriorate after being picked, and the like, can monitor the freshness of the blueberries, and has the dual functions of blueberry preservation and freshness monitoring and indication.

Description

Technical Field

[0001] The invention relates to the technical field of fruit preservation, and in particular to a method for preparing a composite film for blueberry preservation and freshness monitoring. Background Art

[0002] The scientific name of blueberry is Vaccinium spp., Ericaceae. Blueberry mainly contains anthocyanins, vitamins, phenols, pectin, tannic acid, folic acid, flavonoids and other nutrients, which have the functions of preventing cardiovascular diseases, protecting eyesight, anti-aging, enhancing human immunity, and moisturizing the intestines. As a berry with unique flavor and high nutritional value, blueberry is deeply loved by consumers. As the global blueberry planting scale continues to expand, consumers' understanding of the nutritional value of blueberries continues to deepen, and the market demand for blueberries is gradually increasing.

[0003] Blueberries are harvested in the hot and rainy summer when the air humidity is high and microbial activity is vigorous. Blueberries have a high respiratory metabolism. In addition, blueberries have thin skin and high juice, and lack nutrition and water supply, so their own metabolism increases, the flesh becomes soft, matures and ages, resulting in a decline in the quality of blueberries.

[0004] Blueberries have high nutritional value, but their main problems after harvest are softening and easy rotting, which make them difficult to store and transport, thus seriously restricting their commodity value. Therefore, research on blueberry preservation technology is of great significance to the development of the entire blueberry industry.

[0005] The post-harvest preservation of blueberries mainly includes physical preservation methods such as low-temperature refrigeration, controlled atmosphere preservation, and preservation with plastic wrap, as well as chemical preservation methods such as spraying and soaking with chemical preservatives. However, low-temperature refrigeration and controlled atmosphere preservation are suitable for large-scale use by enterprises, but are not suitable for the preservation of blueberries during the supermarket sales period. They also have high energy consumption and high costs. The preservation effect of plastic wrap is relatively poor. Spraying and soaking chemical preservatives for blueberries can easily cause chemical residues and affect the taste, flavor and color of blueberries. At the same time, the discharge of chemical preservatives can easily cause environmental pollution.

[0006] Currently, blueberries sold in supermarkets are mainly packaged in transparent polyethylene terephthalate (PET) boxes. When consumers purchase blueberries, due to force majeure factors such as storage temperature and transportation conditions, they cannot accurately judge the freshness of blueberries by simply looking at the production date and surface senses. Most consumers open the boxes to observe or even touch them with their hands, which can easily cause physical damage to the blueberries. Repeated opening and closing of blueberry boxes will affect the quality and shelf life of the blueberries.

[0007] Therefore, solving the problem of blueberries softening, rotting and spoiling easily after being harvested, as well as monitoring the freshness of blueberries during sales in supermarkets are technical issues that urgently need to be solved in the blueberry post-harvest preservation industry. Summary of the invention

[0008] The object of the present invention is to provide a method for preparing a composite film for preserving and monitoring the freshness of blueberries, so as to solve the problems of blueberries being easily softened, rotted and deteriorated after being harvested, and the problem of monitoring the freshness of blueberries during sale in supermarkets, as mentioned in the above background technology.

[0009] To achieve the above object, the present invention provides the following technical solution: a method for preparing a composite film for blueberry preservation and freshness monitoring, comprising the following steps:

[0010] Step 1: Weigh 28-32 g of soy protein isolate, 25-28 g of casein, 24-26 g of glycerol, and 2-3 g of 2,5-furan dimethanol into a beaker, dilute to 1000 mL with distilled water, place in a magnetic stirrer and stir for 15-18 min, then adjust the pH value to 10 with NaOH to obtain a mixed solution.

[0011] Step 2: Place the mixed solution obtained in step 1 in an ultrasonic constant temperature water bath and keep it warm for 15 to 18 minutes, then cool it down. The temperature of the ultrasonic constant temperature water bath is set to 80° C. and the ultrasonic frequency is 75 to 80 Hz.

[0012] Step 3: Add 5-6 g of carvacrol solution, 1-1.3 g of allicin, and 0.8-1.2 g of anthocyanin to the cooled solution in step 2, mix and stir, and then place in a homogenizer for 5-7 minutes to obtain a composite membrane solution.

[0013] Step 4: Pour the composite film solution obtained in step 3 into a mold by using a casting method, place the mold in an oven for baking, and obtain a composite film for preserving and monitoring freshness.

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

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

[0016] As a preferred technical solution of the present invention, the concentration of the carvacrol solution is 99%.

[0017] As a preferred technical solution of the present invention, the rotation speed of the homogenizer is 10000-12000 rpm.

[0018] As a preferred technical solution of the present invention, the oven temperature is set to 40-45° C., 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 invention has the following beneficial effects:

[0020] 1. The present invention uses soy protein isolate and casein as the film-forming substrate of the composite film. The protein purity in the soy protein isolate is high and has good film-forming property. Casein is a calcium-phosphorus binding protein, and contains hydrophilic groups and hydrophobic groups in its molecular structure. This structure can form hydrogen bonds or other interactions between casein and water molecules, thereby enhancing the binding force between casein and water molecules, thereby making carvacrol, allicin, anthocyanin and other substances more evenly distributed in the protein molecules, thereby improving the flexibility of the composite film, the preservation function and the sensitivity and accuracy of the color change indication. At the same time, casein contains a variety of bioactive peptides, which can capture free radicals, prevent the generation of free radicals, and remove the formed free radicals and active oxygen, thereby helping to maintain the color, flavor and nutritional value of blueberries and extend their shelf life.

[0021] 2. The present invention adds 2,5-furan dimethanol, which is a biomass-based furan derivative compound containing two alcohol hydroxyl functional groups. As a surfactant, it can interact with soy protein isolate and casein to form a denser film structure, thereby improving the flexibility, plasticity and barrier properties of the composite film.

[0022] 3. The present invention uses an ultrasonic constant temperature water bath to treat the mixed solution, which is conducive to more uniform mixing of soy protein isolate, casein, glycerol, and 2,5-furan dimethanol, ensuring the uniformity and consistency of film formation and improving the preservation and monitoring effects.

[0023] 4. Anthocyanin is a water-soluble natural pigment extracted from plants. It will show different colors in different acid and alkaline environments. Blueberries will produce CO2 due to respiration during storage after harvesting, and produce carbonic acid with water in the air, causing the pH of the blueberry storage environment to change. The composite film will change color during the storage of blueberry fruits due to the presence of anthocyanin, thereby helping consumers to more accurately judge the freshness of blueberries. The present invention compounds carvacrol, allicin, and anthocyanin in proportion, and prepares a composite film together with soy protein isolate and casein film-forming substrates, which solves the problems of blueberries being easily softened, rotted, and deteriorated after harvesting. At the same time, the freshness of blueberries is monitored during sales in supermarkets, achieving the two-in-one functions of blueberry preservation and freshness monitoring and indication. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is a flow chart for preparing the composite film for blueberry preservation and freshness monitoring;

[0025] Figure 2 It is a bar graph of the elongation at break of the composite film of the present invention;

[0026] Figure 3 The bar graph of weight loss rate change during storage of blueberries of the present invention;

[0027] Figure 4 The bar graph of the decay rate change during the storage of blueberries of the present invention;

[0028] Figure 5 This is a line graph showing changes in hardness during storage of blueberries according to the present invention;

[0029] Figure 6 This is a line graph showing the change in color difference L value during the storage of blueberries of the present invention;

[0030] Figure 7 Box diagram for blueberry packaging;

[0031] Figure 8 This is a round box image for blueberry packaging;

[0032] Fig. 9 This is a picture of a honeycomb box used for packaging blueberries;

[0033] Fig.10 This is a bar graph showing the changes in L value of the composite film during blueberry storage;

[0034] Fig.11 This is a bar graph of the change of a value of composite film during blueberry storage;

[0035] Fig.12 This is a bar graph of the changes in the b value of the composite film during the storage of blueberries;

[0036] Fig.13 This is a bar graph showing the changes in △E value of composite films during blueberry storage;

[0037] Fig.14 This is the color change diagram of the composite film for square box packaging;

[0038] Fig.15 This is the color change diagram of the composite film for round box packaging;

[0039] Fig.16 This is a picture of the color change of the composite film for honeycomb box packaging (with blueberries in the bottom background);

[0040] Fig.17 This is a picture of the color change of the composite film for honeycomb box packaging (there are no blueberries in the bottom background). DETAILED DESCRIPTION

[0041] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0042] The present invention adopts soy protein isolate and casein as the film-forming substrate of the composite film. Soy protein isolate is a vegetable protein extracted from soybeans, has a high protein purity and good film-forming property. Casein is a calcium-phosphorus binding protein, and contains hydrophilic groups and hydrophobic groups in its molecular structure. This structure can form hydrogen bonds or other interactions between casein and water molecules, thereby enhancing the binding force between casein and water molecules, thereby making carvacrol, allicin, anthocyanins and other substances more evenly distributed in protein molecules, thereby improving the flexibility of the composite film, the preservation function and the sensitivity and accuracy of the color change indication. At the same time, casein contains a variety of bioactive peptides, which can capture free radicals, prevent free radical generation, and remove the functions of free radicals and active oxygen that have been formed, thereby helping to maintain the color, flavor and nutritional value of blueberries and extend their shelf life.

[0043] 2,5-Furan dimethanol is a biomass-based furan derivative compound containing two alcohol hydroxyl functional groups. As a surfactant, it can interact with soy protein isolate and casein to form a denser membrane structure and improve the flexibility, plasticity and barrier properties of the composite film. The mixed solution composed of soy protein isolate, casein, glycerol, 2,5-furan dimethanol and distilled water is treated with an ultrasonic constant temperature water bath, which can cause the microscopic particles in the mixed solution to collide and squeeze, destroy the bubbles and aggregates in the solution, increase the gas-liquid exchange surface area in the liquid, and thus enhance the reaction efficiency. The vibration and shearing action of the ultrasonic wave can break the interaction force between molecules, promote the uniform dispersion and rearrangement of the substance, and thus improve the physical and chemical properties of the mixed solution. At the same time, 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 soy protein isolate, casein, glycerol, and 2,5-furan dimethanol to mix more evenly in distilled water, ensure the uniformity and consistency of film formation, and improve the preservation and monitoring effects.

[0044] Carvacrol has strong antibacterial and antioxidant properties, which can inhibit the growth of bacteria in food, delay the oxidation process of ingredients in food, reduce food spoilage, and help maintain the freshness and taste of food. Allicin is the main active ingredient of garlic, which can prevent the invasion of external harmful microorganisms into fruits and vegetables and reduce the decay rate of fruits and vegetables. Anthocyanin is a water-soluble natural pigment extracted from plants. It will show different colors under different acid and alkaline environments, red under acidic conditions and blue under alkaline conditions. Blueberries will produce CO2 due to respiration during storage after harvesting, and produce carbonic acid with water in the air, causing the pH of the blueberry storage environment to change. The composite film will change color during the storage of blueberry fruits due to the presence of anthocyanins, thereby helping consumers to more accurately judge the freshness of blueberries. The present invention mixes carvacrol, allicin and anthocyanidin in proportion, and prepares a composite film together with soy protein isolate and casein film-forming substrate, thereby solving the problems of easy softening, decay and deterioration of blueberries after being picked, and at the same time monitoring the freshness of blueberries during sales in supermarkets, thus achieving the two-in-one functions of blueberry preservation and freshness monitoring and indication.

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

[0046] Embodiment 1:

[0047] A method for preparing a composite film for blueberry preservation and freshness monitoring comprises the following steps:

[0048] Step 1. Weigh 28 g of soy protein isolate, 25 g of casein, 24 g of glycerol, and 2 g of 2,5-furan dimethanol into a beaker, dilute to 1000 mL with distilled water, place in a magnetic stirrer and stir for 15 min, then adjust the pH value to 10 with NaOH to obtain a mixed solution. The speed of the magnetic stirrer is 600 rpm and the NaOH concentration is 1 mol / L.

[0049] Step 2: Place the mixed solution obtained in step 1 in an ultrasonic constant temperature water bath and keep it warm for 15 minutes, then cool it down. The temperature of the ultrasonic constant temperature water bath is set to 80° C. and the ultrasonic frequency is 75 Hz.

[0050] Step 3: Add 5 g of carvacrol solution, 1 g of allicin and 0.8 g of anthocyanin to the cooled solution in step 2, mix and stir, and then place in a homogenizer for homogenization for 5 minutes to obtain a composite membrane solution. The concentration of the carvacrol solution is 99%, and the speed of the homogenizer is 10,000 rpm.

[0051] Step 4: Pour the composite film solution obtained in step 3 into a mold by using a casting method, place it in an oven for baking, and obtain a composite film for preservation and freshness monitoring. The oven temperature is set to 40°C, the time is 20 minutes, and the thickness of the composite film is 0.3 mm.

[0052] Embodiment 2:

[0053] A method for preparing a composite film for blueberry preservation and freshness monitoring comprises the following steps:

[0054] Step 1. Weigh 32 g of soy protein isolate, 28 g of casein, 26 g of glycerol, and 3 g of 2,5-furan dimethanol into a beaker, dilute to 1000 mL with distilled water, place in a magnetic stirrer and stir for 18 min, then adjust the pH value to 10 with NaOH to obtain a mixed solution. The speed of the magnetic stirrer is 650 rpm and the NaOH concentration is 1.2 mol / L.

[0055] Step 2: Place the mixed solution obtained in step 1 in an ultrasonic constant temperature water bath, keep warm for 18 minutes, and then cool. The temperature of the ultrasonic constant temperature water bath is set to 80° C. and the ultrasonic frequency is 80 Hz.

[0056] Step 3: Add 6 g of carvacrol solution, 1.3 g of allicin and 1.2 g of anthocyanin to the cooled solution in step 2, mix and stir, and then place in a homogenizer for homogenization for 7 minutes to obtain a composite membrane solution. The concentration of the carvacrol solution is 99%, and the speed of the homogenizer is 12000 rpm.

[0057] Step 4: Pour the composite film solution obtained in step 3 into a mold by using a casting method, place it in an oven for baking, and obtain a composite film for preservation and freshness monitoring. The oven temperature is set to 45°C, the time is 25 minutes, and the thickness of the composite film is 0.5 mm.

[0058] Comparative Example 1:

[0059] The casein in Example 1 is removed, and the other method steps are the same as in Example 1.

[0060] Comparative Example 2:

[0061] The 2,5-furan dimethanol in Example 1 is removed, and the other method steps are the same as in Example 1.

[0062] Comparative Example 3:

[0063] The step 2 in Example 1 is changed to: the mixed solution obtained in step 1 is placed in a constant temperature water bath at 80° C. for 15 minutes and then cooled. The other method steps are the same as those in Example 1.

[0064] Comparative Example 4:

[0065] The carvacrol solution, allicin and anthocyanin in Example 1 are removed, and the other method steps are the same as those in Example 1.

[0066] The composite films prepared in Examples 1 and 2 and Comparative Examples 1, 2, 3 and 4 were tested for elongation at break. Elongation at break = (length when stretched to break - original length) × 100% / original length. The results are shown in Figure 2 As shown, the elongation at break of the composite films of Examples 1 and 2 and Comparative Examples 3 and 4 are 20.17%, 19.55%, 19.23%, and 19.79%, respectively, and the elongation at break of the composite films of Comparative Examples 1 and 2 are 15.81% and 15.63%, respectively, which are 78.4% and 77.5% of that of Example 1. The elongation at break refers to the ratio of the elongation length of the composite film after being stretched to the length before being stretched when the composite film is broken by an external force, and is an important indicator for measuring the flexibility of the composite film. The composite membranes prepared in Comparative Examples 1 and 2 do not contain casein and 2,5-furan dimethanol. Casein is a calcium-phosphorus binding protein with hydrophilic and hydrophobic groups in its molecular structure. This structure enables casein to form hydrogen bonds or other interactions with water molecules, thereby enhancing the binding force between casein and water molecules and improving the flexibility of the composite membrane. 2,5-furan dimethanol is a biomass-based furan derivative compound containing two alcohol hydroxyl functional groups. As a surfactant, it can interact with soy protein isolate and casein to form a denser membrane structure, thereby improving the flexibility and plasticity of the composite membrane.

[0067] The composite films prepared in Examples 1 and 2 and Comparative Examples 1, 2, 3 and 4 were affixed to the inside of a transparent polyethylene terephthalate (PET) packaging box lid. After blueberries were picked, they were precooled at 0°C for 3 hours to dissipate field heat, and the blueberry fruits that had dissipated field heat were placed on the inside of the PET packaging box lid affixed with the composite film. A CK group was a PET packaging box without a composite film, and each box of blueberry fruits weighed (125±10) g. The treatment process was carried out under natural light conditions, and after treatment, they were stored at a temperature of (20±0.5)°C and a humidity of (95-99)%RH. Samples were taken every 2 days, and the 0-day index was measured using the fruits that had dissipated field heat on the day of picking.

[0068] Figure 3 The weight loss rate of blueberries was calculated by weighing method according to the following formula: weight loss rate (%) = (weight before storage - weight after storage) × 100% / weight before storage. Figure 3As shown in the figure, the weight loss rate of blueberry fruit increases with the extension of storage time. Within the first 2 days, there is no significant difference between the results of CK group and the embodiment and comparative example. From the 4th day, the weight loss rate of blueberry fruit in CK group is significantly lower than that in the embodiment and comparative example (P<0.05). When stored for 8 days, the weight loss rate of blueberry in CK group reaches 3.53%, which is 72.4% higher than that in embodiment 1, and the weight loss rate difference reaches the maximum, indicating that the composite membrane treatment can effectively inhibit the weight loss of blueberry fruit. The weight loss rate of blueberry treated with the composite membrane prepared in embodiments 1 and 2 and comparative examples 3 and 4 is not much different. When stored for 10 days, the weight loss rate of blueberry in embodiment 1 is 2.7%, and the weight loss rates of comparative examples 1 and 2 are 3.01% and 3.47%, which are 11.6% and 28.7% higher than that in embodiment 1, respectively. The addition of casein and 2,5-furan dimethanol in the composite membrane preparation process helps to form a denser membrane structure, thereby delaying the loss of water in blueberries.

[0069] Figure 4 The rot rate of blueberries was determined by sensory evaluation. Fruits with rotten spots, mold, or soft rot on the surface were considered rotten. 40 fruits were randomly tested in each group, and the average value was taken and calculated according to the following formula: rot rate (%) = (rotten fruit / total number of fruits surveyed) × 100%. Figure 4 As shown, under normal temperature conditions, as the storage time increases, the fruit decay rate gradually increases. During the storage period, the decay rate of blueberry fruits treated with the composite films prepared in Examples 1 and 2 is lower than that of Comparative Examples 1, 2, 3, 4 and CK group. From the 8th day of storage, the decay rate of blueberry fruits treated with the composite films prepared in Examples 1, 2 and Comparative Examples 1, 2 and 3 is significantly lower than that of the CK group and Comparative Example 4 (P<0.05). On the 10th day of storage, the decay rate of blueberry fruits in the CK group reached 24.15%, while the decay rate of blueberries in the examples was below 15%. The decay rates of blueberries treated with 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 in the examples, indicating that the addition of casein, 2,5-furan dimethanol, carvacrol solution, allicin and anthocyanidin in the preparation process of the composite film can reduce the decay rate of blueberry fruits. Treating the mixed solution in an ultrasonic constant temperature water bath can make the reactions and mixing between the substances more uniform, thus playing a role in better preserving blueberries and reducing the decay rate of blueberries.

[0070] Figure 5 The hardness of blueberries was measured using a texture analyzer at two equatorial points of the blueberry fruit. The measurement depth was 7 mm and the measurement speed was 0.5 mm / s. The hardness of the blueberry fruit was expressed in Newton (N). Figure 5As shown in the figure, the hardness of blueberries decreases with the extension of storage time under normal temperature conditions. When the blueberries are stored for 8 days, the hardness of blueberries treated with the composite films prepared in Example 1 and Example 2 is relatively high, which are 1.56N and 1.60N respectively, while the hardness of blueberries in the CK group is only 1.31N. The hardness of blueberries treated with the composite films prepared in Comparative Examples 1, 2, 3 and 4 is 0.16N, 0.2N, 0.09N and 0.17N lower than that of Comparative Example 1 respectively. The addition of casein and 2,5-furan dimethanol helps to form a denser membrane structure, reduce the loss of water in blueberries, and thus reduce the hardness of blueberries. Carvacrol solution, allicin and anthocyanin can reduce the decay rate of blueberry fruits. Therefore, the composite film treatment can effectively prevent the decrease in the hardness of blueberries and ensure the freshness of blueberries.

[0071] Figure 6 The L value is the change of the color difference L value of the blueberry fruit skin. The L value represents the brightness of the color. 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 the blueberry, which is of great significance for evaluating the quality of blueberries. Use a colorimeter to measure the L value. Randomly select 15 blueberries of the same size and shape. First calibrate the colorimeter with a white board, and then use the lens of the colorimeter to measure at the equator of the blueberry. Figure 6 As shown, with the extension of storage time, the skin brightness L value of blueberry fruit shows an overall downward trend. When the blueberries are stored for 10 days, the L value of blueberries in Example 1 is 4.7%, 6.3%, 3.4%, 7.4% and 11.1% higher than that in Comparative Examples 1, 2, 3, 4 and CK group, respectively. The composite film treated blueberries prepared by the present invention can effectively delay the decline of the skin brightness L of blueberry fruit, thereby ensuring the freshness of the blueberries.

[0072] Use PET square box (such as Figure 7 ), round box (such as Figure 8 ), honeycomb box (such as Fig. 9 ) Three kinds of packaging boxes were used to package blueberries, and the composite film produced in Example 1 was attached to the inner lids of three kinds of PET packaging boxes to observe the effect of the composite film on monitoring the freshness of blueberries. The L value, a value, and b value were measured on the outer side of the packaging box lid with a colorimeter to simulate the color seen by consumers. The L value represents the brightness of the color, where 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-green and yellow-blue, respectively. A positive a value indicates that the color tends to be red, while a negative value indicates that the color tends to be green; a positive b value indicates that the color tends to be yellow, while a negative value indicates that the color tends to be blue. The △E value, also known as the total color difference value, is a comprehensive indicator used to measure 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 three values ​​of L, a, and b. It is an important parameter for evaluating color consistency or change. The calculation formula of △E is:

[0073] Fig.10 is the change of L value of composite film, Fig.11 is the change of a value of composite film, Fig.12 is the change of b value of composite film, Fig.13 The △E value of the composite film changes. With the extension of storage time, the L, a, b, and △E values ​​all show an upward trend. The use of composite film treatment during the storage of blueberries can monitor the freshness of blueberries.

[0074] Every day, take photos from the outside of the box lid at the same location, time, light source and original camera in the photo box to simulate the color seen by consumers. Fig.14 The color change of the composite film for square box packaging, Fig.15 The color of the composite film for round box packaging changes. Fig.16 , Fig.17 The color of the honeycomb box packaging composite film changes. Fig.16 and Fig.17 The pictures are taken from different points of the same honeycomb box. Since the blueberries are evenly distributed in the honeycomb box, some places are blueberry background color, and some places are the color of the film itself. Fig.16 There are blueberries in the background at the bottom of the shooting point. Fig.17 There are no blueberries at the bottom of the shooting point. Fig.14 , 15 Photos 1, 2, 3, 4, and 5 were taken on the 1st, 2nd, 3rd, 4th, and 5th days, respectively. Fig.14 , 15 As shown in Figure 16, during the 5-day storage period, the blueberries packaged in square boxes, round boxes, and honeycomb boxes all showed obvious changes in the composite films. The composite films gradually changed from the blueberry base color at the beginning to red as the storage period prolonged. Fig.17 There are no blueberries in the bottom background of the honeycomb box packaging. As the storage time of blueberries increases, the color of the composite film gradually deepens and turns light red. Therefore, the composite film has a coloring effect on the common blueberry outer packaging on the market, which can monitor the freshness of blueberries and help consumers feel the freshness changes of blueberries more intuitively. Anthocyanins will show different colors in different acid and alkaline environments, showing red under acidic conditions and blue under alkaline conditions. During the post-harvest storage period, blueberries will produce CO2 due to respiration, which will produce carbonic acid with moisture in the air, causing the pH of the blueberry storage environment to change.

[0075] In summary, the blueberry preservation and freshness monitoring composite film prepared by the present invention can solve the blueberry quality problems such as easy softening, rotting and deterioration after blueberries are harvested, and at the same time can monitor the freshness of blueberries, helping consumers to feel the freshness changes of blueberries more intuitively. Therefore, the composite film prepared by the present invention has the dual functions of blueberry preservation and freshness monitoring and indication.

[0076] The above are only specific embodiments of the present invention, but the technical features of the present invention are not limited thereto. Any simple changes, equivalent replacements or modifications made based on the present invention to solve basically the same technical problems and achieve basically the same technical effects are all included in the protection scope of the present invention.

Claims

1. A method for preparing a composite film for blueberry preservation and freshness monitoring, characterized in that: The following steps are involved: Step 1, weigh 28-32 g of soy protein isolate, 25-28 g of casein, 24-26 g of glycerol, and 2-3 g of 2,5-furan dimethanol, put them in a beaker, dilute to 1000 mL with distilled water, stir in a magnetic stirrer for 15-18 min, and adjust the pH value to 10 with NaOH to obtain a mixed solution; Step 2: Place the mixed solution obtained in step 1 in an ultrasonic constant temperature water bath for 15 to 18 minutes and then cool it down. The temperature of the ultrasonic constant temperature water bath is set to 80° C. and the ultrasonic frequency is 75 to 80 Hz. Step 3: add 5-6 g of carvacrol solution, 1-1.3 g of allicin, and 0.8-1.2 g of anthocyanin to the cooled solution in step 2, mix and stir, and then place in a homogenizer for 5-7 minutes to obtain a composite membrane solution; Step 4: Pour the composite film solution obtained in step 3 into a mold by using a casting method, place the mold in an oven for baking, and obtain a composite film for preserving and monitoring freshness.

2. The method for preparing a composite film for blueberry preservation and freshness monitoring according to claim 1, characterized in that: The rotation 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 NaOH concentration 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 carvacrol solution was 99%.

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

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

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

Images