Edible pectin film with functions of monitoring and prolonging freshness of fruits and preparation method of edible pectin film

Abstract

The invention discloses a preparation method of an edible pectin film capable of monitoring and prolonging fruit freshness, which comprises the following steps: carrying out oxidation reaction on pectin and sodium periodate, then terminating the reaction and purifying to obtain oxidized pectin; the preparation method comprises the following steps: dissolving oxidized pectin in water, then adding histidine, and carrying out heating reaction at 60-120 DEG C for 1-3 hours to obtain a reaction product with pH-responsive fluorescence characteristics; adding a calcium chloride aqueous solution as a cross-linking agent into the reaction product, simultaneously adding glycerol as a plasticizer, stirring and mixing for 5-15 minutes at the temperature of 50-70 DEG C, and then stirring and mixing for 1.5-2.5 hours at the temperature of 20-30 DEG C to obtain a uniform film forming solution; pouring the film-forming liquid into a film, and drying to obtain an edible pectin film; the invention also provides the edible pectin film prepared by the preparation method. The prepared edible pectin film is mainly used for packaging fruits, and the freshness of the fruits can be visually displayed through fluorescence change while the fruits are kept fresh.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fruit preservation and intelligent packaging. More specifically, the present application relates to an edible pectin film with both monitoring and freshness prolonging functions and a preparation method thereof. BACKGROUND

[0002] Fresh-cut fruits are more prone to juice loss, microbial proliferation and physiological and biochemical deterioration due to the destruction of tissue structure and protective barriers, and the quality degradation rate is significantly faster than that of intact fruits. At present, in the preservation of fresh-cut fruits, the application of edible coating is one of the common technical means. Such coating is mainly composed of edible ingredients such as polysaccharides, and forms a protective film on the surface of fresh-cut fruits, which plays a physical barrier role in reducing water loss and blocking oxygen contact, thereby delaying the color deterioration and texture softening to a certain extent.

[0003] However, the existing edible preservation films for fresh-cut fruits usually have relatively single functions. They mainly focus on passively delaying the quality deterioration process, but cannot directly reflect the actual freshness change of fresh-cut fruits during storage. Since the early signs of spoilage of fresh-cut products may not be easily observed directly, this may lead to the fact that products with significantly deteriorated quality are not identified in time, affecting consumer safety, or causing unnecessary waste of still edible products.

[0004] In order to solve the problem of single function of fresh-cut fruit preservation film, researchers try to introduce intelligent packaging technology that can indicate quality change. Utilizing pH response mechanism is a possible way, because the microenvironment pH value of fresh-cut fruit surface will change dynamically due to microbial metabolism and tissue juice exudation. However, in this specific application scenario of fresh-cut fruits, it is difficult to develop edible films with both preservation and indication functions. On the one hand, many effective pH indicating ingredients are synthetic dyes or non-edible substances, and their safety indicators may not meet the regulatory requirements for direct and close contact with the surface of fresh-cut fruits. On the other hand, some natural pH-sensitive pigments are edible, but in the presence of a humid surface and exudate environment commonly found in fresh-cut fruits, they may lack stability and easily leak or change color, resulting in distorted indication signals, and their own color may also interfere with the true judgment of the appearance color of fresh-cut fruits. SUMMARY

[0005] The present application aims to provide an edible pectin film with the functions of monitoring and prolonging the freshness of fruits and a preparation method thereof, so as to solve the problems that the existing edible film for fruit preservation has a single function and lacks real-time and intuitive monitoring capability for the freshness of fruits, and that the pH value of the surface microenvironment of fruits changes during the decay process, but the existing technology is difficult to integrate a safe, stable and closely combined pH response indication mechanism with the film-forming matrix while providing the function of physical preservation, thereby achieving the dual purposes of preservation and monitoring.

[0006] In order to achieve the purpose and other advantages of the present application, a preparation method of an edible pectin film with the functions of monitoring and prolonging the freshness of fruits is provided, comprising: step one, oxidizing pectin with sodium periodate, then terminating the reaction and purifying to obtain oxidized pectin; step two, dissolving the oxidized pectin in water, then adding histidine, heating the reaction at a temperature of 60-120℃ for 1-3 hours to obtain a reaction product with pH response fluorescence characteristics; step three, adding calcium chloride aqueous solution as a crosslinking agent and glycerol as a plasticizing agent to the reaction product, stirring and mixing at 50-70℃ for 5-15 minutes, and then stirring and mixing at 20-30℃ for 1.5-2.5 hours to obtain a uniform film-forming solution; and step four, pouring the film-forming solution into a film and drying to obtain the edible pectin film.

[0007] Preferably, in step one, the pectin with a degree of methylesterification of 30%-50% is prepared into a pectin solution with a concentration of 2%-5% (w / v); sodium periodate is added to the pectin solution for oxidation reaction, wherein 0.3-0.5 g of sodium periodate is added to 100 mL of the pectin solution; after stirring and reacting for 1-3 hours in the dark, ethylene glycol is added to terminate the reaction; the solution after termination of the reaction is subjected to dialysis purification to remove sodium periodate and ethylene glycol, and then freeze-dried to obtain the oxidized pectin.

[0008] Preferably, in step one, before adding sodium periodate to the pectin solution, the sodium periodate is first dissolved in water, and 3-5 ml of distilled water is used to dissolve each 0.3-0.5 g of sodium periodate.

[0009] Preferably, in step one, a 500 Da dialysis bag is used for dialysis purification, and the dialysis time is 3 days.

[0010] Preferably, in step two, the ratio of oxidized pectin to water is 2-5 g of oxidized pectin dissolved in 100 mL of water; and the final concentration of histidine is 20 mM to 80 mM.

[0011] Preferably, in step three, the concentration of the calcium chloride aqueous solution is 3%-7% (w / v), the volume of the calcium chloride aqueous solution added is 0.3 mL / g to 0.5 mL / g relative to the mass of the oxidized pectin, and the volume of the glycerol added is 1.0 mL / g to 1.2 mL / g relative to the mass of the oxidized pectin.

[0012] Preferably, in step four, the film-forming solution is poured in an amount of 1.5-2.5 g / cm 2 The pouring is followed by drying at 35-45°C.

[0013] The application also provides an edible pectin film with both monitoring and prolonging fruit freshness, which is prepared by the above preparation method.

[0014] The application also provides the use of the above edible pectin film in fresh-cut fruit preservation.

[0015] The application at least has the following beneficial effects: The application creatively introduces pH-responsive fluorescence characteristics into the edible film by chemically modifying the pectin to make it a carrier of the indicator functional group, so that the edible film is no longer a single preservation barrier, but becomes an intelligent packaging material. It can delay fruit spoilage while intuitively reflecting the pH change in the fresh-cut fruit storage microenvironment through the change of the fluorescence signal, achieving real-time and non-destructive monitoring of the freshness of fresh-cut fruits. In addition, since the pH-responsive characteristics come from the product of the reaction between pectin and histidine (both are edible and safe ingredients), the chromophore is covalently bonded to the pectin molecular chain, avoiding the use of exogenous synthetic dyes or easily migratory natural pigments, ensuring the edible safety of the film. At the same time, this chemical bonding effectively prevents the leakage or loss of the indicator ingredient during use, ensuring the reliability and durability of the indicator signal.

[0016] Other advantages, objects, and features of the application will be apparent from the following description, and will be understood by those skilled in the art. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 are scanning electron microscope results of different multifunctional edible pectin films; Figure 2 are coloration photos of different multifunctional edible pectin films under daylight and ultraviolet light; Figure 3 are antioxidant results of different multifunctional edible pectin films; Figure 4 are antibacterial results of different multifunctional pectin films; Figure 5is the fluorescence change result of different multifunctional pectin films applied in fresh-cut kiwifruit during storage process; Figure 6 is the hardness change result of different multifunctional pectin films applied in fresh-cut kiwifruit during storage process; Figure 7 is the pH response fluorescence experiment result of OLAP-H-50. DETAILED DESCRIPTION

[0018] The application will be further described in conjunction with the embodiments and the accompanying drawings, so that those skilled in the art can implement the application according to the description and the drawings.

[0019] It should be understood that the terms such as "have", "contain" and "include" used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0020] It should be noted that the experimental methods in the following embodiments are all conventional methods unless otherwise specified, and the reagents and materials can be obtained from commercial channels unless otherwise specified.

[0021] Example 1: Preparation of edible pectin film, which includes the following steps: Step one, using pectin with a degree of methylation of 40%, prepare a pectin solution with a concentration of 3.5% (w / v); take 100 mL of pectin solution, add 0.32 g of sodium periodate dissolved in 3 mL of distilled water, stir under lightless conditions for 2 hours, then add 5 mL of ethylene glycol to terminate the reaction; collect the reaction solution, use a 500 Da dialysis bag, dialyze for 3 days to remove sodium periodate and ethylene glycol, then freeze-dry to obtain oxidized pectin, named OLAP; Step two, dissolve 2 g of OLAP in 80 mL of distilled water, stir to dissolve, then add histidine His to make the final concentration 50 mM, mix well, then put the mixture in a 60 ° C for 3 h, collect the reaction solution; Step three, add 0.6 mL of 5% (w / v) CaCl2 solution and 2 mL of glycerol to the reaction solution, stir at 60 ° C for 10 minutes, mix well. The above solution is further stirred at room temperature (20-30°C) for 2 h to obtain a film-forming solution; Step four, divide the film-forming solution into 20 g each in disposable petri dishes, dry at 40°C for 14 h to obtain multifunctional edible pectin film OLAP-H-50.

[0022] Example 2: Preparation of edible pectin film with the same steps as in Example 1, except that in step two, the final concentration of histidine added is 20 mM, obtaining multifunctional edible pectin film OLAP-H-20.

[0023] Example 3: Preparation of edible pectin film with the same steps as in Example 1, except that in step two, the final concentration of histidine added is 80 mM, obtaining multifunctional edible pectin film OLAP-H-80.

[0024] Comparative Example 1: Preparation of edible pectin film with the same steps as in Example 1, except that in step two, no histidine is added, obtaining multifunctional edible pectin film OLAP-H-0.

[0025] Comparative Example 2: Preparation of edible pectin film with the same steps as in Example 1, except that in step two, 1.5 g of OLAP is dissolved in 100 mL of distilled water with thorough stirring and dissolution, then histidine is added to make the final concentration 20 mM, and after mixing, the mixture is placed in a 160 ° C, heat reaction for 1 h, and collect the reaction liquid. This method cannot form a pectin film after drying, which may be due to the fact that the reaction temperature of OLAP and histidine is too high, which destroys the structure of pectin, resulting in a decrease in film-forming property.

[0026] Experimental Example 1: The multifunctional edible pectin films prepared in Examples 1-3 and Comparative Example 1 were scanned by electron microscopy, and the results are shown in Figure 1 . Figure 1 a, c, e and g are respectively the results of observation at 0.8k magnification, and b, d, f and h are respectively the results of observation at 1.5k magnification. As can be seen from Figure 1 , when the concentration of histidine is higher than 50 mM, cracks appear on the surface of the edible pectin film.

[0027] Experimental Example 2: The multifunctional edible pectin films prepared in Examples 1-3 and Comparative Example 1 were placed under a daylight lamp and an ultraviolet lamp, respectively, and the results of coloration are shown in Figure 2 . As can be seen from Figure 2 , as the concentration of His increases from 0 mM to 80 mM, the film under the daylight lamp shows a gradually deepening yellow appearance, which is due to the fact that as the concentration of His continues to increase, more coloration products are formed. In addition, OLAP-H-0, OLAP-H-20, OLAP-H-50 and OLAP-H-80 all show blue fluorescence under ultraviolet light.

[0028] Experimental Example 3: ABTS and DPPH antioxidant capacity tests were performed on the multifunctional edible pectin films prepared in Examples 1-3 and Comparative Example 1.

[0029] ABTS+antioxidant activity test: (1) Solution preparation: 7.0 mmol / L ABTS solution: weigh 38.4 mg ABTS in a 5 mL beaker, dissolve with distilled water to a 10 mL volumetric flask; 2.45 mmol / L potassium persulfate solution: weigh 33.1 mg potassium persulfate in a 10 mL beaker, dissolve with distilled water to a 50 mL volumetric flask; 150 μmol / L Trolox solution: weigh 14.8 mg Trolox in a 5 mL beaker, dissolve with 80% methanol to a 250 mL volumetric flask; ABTS working solution preparation: mix 7.0 mmol / L ABTS solution and 2.45 mmol / L potassium persulfate solution at a volume ratio of 1:1, after standing at room temperature 30°C for 14 h in the dark, dilute with 80% methanol to an absorbance of 0.70 ± 0.02 at 734 nm (usually 50 times), to prepare the ABTS·+stock solution. (2) Standard curve preparation: use a pipette to take 0.1 mL of Trolox solution with concentrations of 0, 25, 50, 75, 100, 125, 150 μmol / L, add 3.6 mL of ABTS·+solution and mix well, stand at room temperature for 1 min, measure the absorbance at 734 nm. Plot the standard curve with Trolox concentration (μmol / L) as the vertical coordinate (y) and the absorbance value as the horizontal coordinate (x). (3) Sample testing: use a pipette to take 0.1 mL of 1 mg / mL sample solution, add 3.6 mL of ABTS·+solution and mix well, stand at room temperature for 1 min, measure the absorbance at 734 nm. Use 0.1 mL of deionized water as a blank control.

[0030] DPPH antioxidant activity detection: (1) Solution preparation: 100 μmol / L DPPH preparation: weigh 9.8 mg DPPH in a 5 mL beaker, dissolve with 80% methanol to 250 mL brown volumetric flask. 100 μmoL / L Trolox preparation: weigh 6.2 mg Trolox in a 5 mL beaker, dissolve with 80% methanol to 250 mL brown volumetric flask. (2) Standard curve drawing: use a pipette to take 2 mL of Trolox solution with concentrations of 0, 20, 30, 40, 60, 80, 100 μM Trolox μmol / L, add 4 mL DPPH solution and mix completely, stand at room temperature for 30 min, measure the absorbance at 517 nm. With Trolox concentration (μmol / L) as the ordinate (y), the absorbance value as the abscissa (x), draw the standard curve. (3) Sample test: use a pipette to take 2 mL of sample solution, add 4 mL of DPPH solution and mix completely, stand at room temperature for 30 min, measure the absorbance at 517 nm. 2 mL of deionized water as blank control.

[0031] The experimental results are shown in Figure 3 As shown, the ABTS free radical scavenging rates of OLAP-H-20, OLAP-H-50 and OLAP-H-80 were 77.81%, 83.28% and 85.05% respectively, which were significantly higher than that of OLAP-H-0 (30.57%), indicating that the film formed by OLAP and His had excellent antioxidant activity. Consistent with the trend of ABTS free radical scavenging rate, the DPPH free radical scavenging rate increased with the increase of His concentration, and the antioxidant capacity showed a ladder-like enhancement, from 20.89% (OLAP-H-20) to 53.58% (OLAP-H-80). These results showed that the antioxidant activity of the film formed by OLAP and His was positively correlated with the concentration of His. The mechanism is that on the one hand His has the inherent antioxidant properties of the electron-donating imidazole ring, on the other hand the Schiff base product formed by His and OLAP enhances the free radical scavenging capacity.

[0032] Experimental Example 4: The multifunctional edible pectin film prepared in Examples 1-3 and Comparative Example 1 was subjected to antibacterial capacity detection.

[0033] Antibacterial capacity detection method: Gram-negative bacteria (Escherichia coli, E. coli ) and Gram-positive bacteria (Staphylococcus aureus, S. aureus ) were used as models to evaluate the antibacterial activity of the film. The activated bacterial suspension was diluted 10 4The working bacterial solution was prepared by adding 0.25 g of the film to the working bacterial solution after UV sterilization for 1 h. The working bacterial solution without the added coating material served as a blank control group. 100 µL of the working bacterial solution with / without the added film was spread onto a solid culture medium and incubated at 37°C. º The bacteria were incubated upside down at C for 24 hours, and their growth changes were recorded by taking pictures.

[0034] Experimental results are as follows Figure 4 As shown, Figure 4 The first row contains E. coli. E. coli The second row contains Staphylococcus aureus. S. aureus It can be seen that, compared with OLAP-H-0, OLAP-H-20, OLAP-H-50, and OLAP-H-80 films all significantly reduced colony density, while OLAP-H-0 had no significant effect. This is especially true for... E. coli The inhibitory effect of His on Gram-negative bacteria increases with increasing His concentration in the membrane. This phenomenon is attributed to the protonation of the imine bond (C=N) under physiological conditions. At this point, the positively charged group disrupts the negatively charged lipopolysaccharide membrane of Gram-negative bacteria, thereby achieving the inhibitory effect. Experimental Example 5: The multifunctional edible pectin films prepared in Examples 1-3 and Comparative Example 1 were respectively affixed to the inner top surface of a food storage box containing fresh-cut kiwifruit. Samples were taken at 0d, 0.5d, 1d, and 6d to detect the fluorescence intensity of the edible pectin film under ultraviolet light and the firmness of the fresh-cut kiwifruit. The results are as follows: Figure 5 and Figure 6 As shown, Figure 5 The area within the white circle is the region covered with pectin film.

[0035] Depend on Figure 5 It was found that the OLAP-H-0 film showed no fluorescence as the storage time of fresh-cut kiwifruit increased, making it unsuitable for monitoring. However, the OLAP-H-20, OLAP-H-50, and OLAP-H-80 films all showed a decrease in fluorescence intensity with prolonged storage. During kiwifruit storage, the respiration and metabolism of the fruit produces carbon dioxide, causing changes in the acidity of the system and resulting in pH-responsive changes in the film's fluorescence. This indicates that these films have the ability to monitor changes in the freshness of fresh-cut kiwifruit. More importantly, the OLAP-H-20, OLAP-H-50, and OLAP-H-80 films showed significant fluorescence attenuation within 12 hours (0.5 days), and this short-term response is more conducive to their application in monitoring the freshness of fresh-cut fruit.

[0036] Softening of kiwifruit is a significant indicator of its quality decline; therefore, monitoring changes in firmness to reflect its freshness is feasible. Figure 6It can be seen that OLAP-H-20, OLAP-H-50 and OLAP-H-80 films effectively delayed the softening process of kiwifruit. In particular, OLAP-H-80 film can significantly delay the decrease of hardness of fresh-cut kiwifruit, and can increase the hardness of the sample by 1.41-2.26 times. Thus, it indicates that the pectin film has the function of preserving kiwifruit.

[0037] Experimental Example 6: pH-responsive fluorescence experiment of OLAP-H-50 prepared in Example 1.

[0038] Experimental method: 10 mg / mL sample solution was prepared using deionized water. 0.2 M HC1 and 0.2 M NaOH were used to prepare solutions with different pH values (pH 1-13). 0.2 mL of the prepared sample solution was mixed with 1.8 mL of the solution with different pH values, and then the fluorescence spectrum was measured using a fluorescence spectrophotometer under an excitation wavelength of 370 nm. The slit width of the excitation and emission monochromators was 2.5 nm, and the voltage of the photomultiplier tube was set to 700 V.

[0039] The experimental results are shown in Figure 7 It can be seen that the peak intensity of OLAP-H-50 at 400-500 nm decreases in steps as the pH value gradually decreases. It shows that the fluorescence intensity (I 400-500 nm ) of AP+H at 400-500 nm has a good linear relationship with the pH value (X) (R²=0.9819).

[0040] Example 7: The preparation of edible pectin film is substantially the same as that of Example 1, except that in step two, 1.5 g of OLAP is dissolved in 100 mL of distilled water, and then tyrosine is added to make its final concentration 80 mM. This method forms a pectin film with fluorescence after drying, but the pectin film does not have pH-responsive fluorescence characteristics, and the antioxidant capacity and antibacterial property are weak, which may be due to the fact that tyrosine itself does not have antioxidant capacity compared with histidine.

[0041] The number of devices and the scale of processing described herein are used to simplify the description of the present application. The application, modification and change of the edible pectin film and its preparation method for monitoring and prolonging the freshness of fruits are obvious to those skilled in the art.

[0042] While embodiments of the application have been disclosed in connection with the above specification and drawings this description is not intended to limit the scope of the application and many modifications, enhancements, alternatives, and variations will become apparent to those skilled in the art from this disclosure. Accordingly, it is intended that the application not be limited to the described embodiments, but that it include all variations falling within the scope of the claims, and their equivalents.

Claims

1. A method for preparing an edible pectin film that can both monitor and prolong the freshness of fruits, characterized in that, include: Step 1: The pectin is oxidized with sodium periodate, and then the reaction is terminated and purified to obtain oxidized pectin. Step 2: Dissolve oxidized pectin in water, then add histidine, and heat the mixture at 60℃-120℃ for 1-3 hours to obtain a reaction product with pH-responsive fluorescence properties. Step 3: Add calcium chloride aqueous solution as a crosslinking agent to the reaction product, and simultaneously add glycerol as a plasticizer. First, stir and mix at 50℃-70℃ for 5-15 minutes, and then stir and mix at 20℃-30℃ for 1.5-2.5 hours to obtain a uniform film-forming solution. Step 4: Pour the film-forming solution into a film and dry it to obtain an edible pectin film.

2. The method for preparing an edible pectin film that can both monitor and prolong fruit freshness as described in claim 1, characterized in that, In step one, pectin with a degree of methyl esterification of 30%-50% is used to prepare a pectin solution with a concentration of 2%-5% (w / v); sodium periodate is added to the pectin solution to carry out an oxidation reaction, wherein 0.3-0.5g of sodium periodate is added per 100mL of pectin solution; after stirring the reaction under light-protected conditions for 1-3 hours, ethylene glycol is added to terminate the reaction; the solution after the reaction is terminated is purified by dialysis to remove sodium periodate and ethylene glycol, and then freeze-dried to obtain oxidized pectin.

3. The method for preparing an edible pectin film that can both monitor and prolong fruit freshness as described in claim 2, characterized in that, In step one, sodium periodate is dissolved in water before being added to the pectin solution. 0.3-0.5g of sodium periodate is dissolved in 3-5ml of distilled water.

4. The method for preparing an edible pectin film that can both monitor and prolong fruit freshness as described in claim 2, characterized in that, In step one, dialysis purification is performed using a 500 Da dialysis bag, and the dialysis time is 3 days.

5. The method for preparing an edible pectin film that can both monitor and prolong fruit freshness as described in claim 1, characterized in that, In step two, the ratio of oxidized pectin to water is 2-5g of oxidized pectin dissolved in 100mL of water; the final concentration of histidine is 20mM to 80mM.

6. The method for preparing an edible pectin film that can both monitor and prolong fruit freshness as described in claim 1, characterized in that, In step three, the concentration of the calcium chloride aqueous solution is 3%-7% (w / v), the volume of calcium chloride aqueous solution added is 0.3 mL / g to 0.5 mL / g of oxidized pectin, and the volume of glycerol added is 1.0 mL / g to 1.2 mL / g of oxidized pectin.

7. The method for preparing an edible pectin film that can both monitor and prolong fruit freshness as described in claim 1, characterized in that, In step four, the film-forming solution is prepared at a concentration of 1.5-2.5 g / cm³. 2 The material is poured and then dried at 35℃-45℃.

8. An edible pectin film that can both monitor and prolong the freshness of fruit, characterized in that, It is prepared by any one of the preparation methods described in claims 1-7.

9. The application of the edible pectin film as described in claim 8 in the preservation of fresh-cut fruits.

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