An RG-I structure apple young fruit pectin, pectin film, and their preparation methods and applications

Abstract

The present invention belongs to the technical field of pectin and pectin films, and specifically relates to an RG-I structured apple young fruit pectin, a pectin film, and their preparation methods and applications. The preparation method of the apple young fruit pectin includes: 1) Freeze-drying and crushing apple young fruits to obtain apple young fruit powder; 2) Mixing the apple young fruit powder with a citric acid solution, and performing heat-assisted ultrasonic extraction to obtain a pectin extract; 3) Filtering the extract by suction, mixing the filtrate with ethanol at a volume ratio of 1:4; collecting the mixed suspension, washing it with ethanol, and then performing re-dissolution, filtration, rotary evaporation, and freeze-drying to obtain the apple young fruit pectin. The present invention obtains apple young fruit pectin with an RG-I content of 79% through heat-assisted ultrasonic extraction; the pectin film made from the pectin has good mechanical properties and water permeability, and has a good fresh-keeping effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of pectin and pectin films, and particularly relates to an RG-I structure apple young fruit pectin, a pectin film, and a preparation method and application thereof. Background Art

[0002] Apples have a long planting history in China and are one of the important economic tree species, with the planting area and apple yield ranking first in the world. Flower and fruit thinning are essential means to ensure the growth of fruits and vegetables. Every year, about 1.9 million tons of apple young fruits are thinned in China. Apple young fruits are rich in nutritional value and have various functional activities such as antioxidant, antibacterial, anti-allergic, anti-cancer, and anti-radiation. If they are directly discarded in the orchard, it will not only cause a huge waste of resources but also increase the risk of microbial contamination. Therefore, the research on apple young fruits has very important research significance. In recent years, the research on apple young fruits includes the extraction of polyphenols and polysaccharides, antioxidant, functional research, and product development, etc. However, the research on the application of apple young fruit pectin in fruit and vegetable preservation is not common.

[0003] Pectin is a polysaccharide present in the cell walls and middle lamellae of many plants and fruits and is related to the structures of cellulose, hemicellulose, and lignin. The polysaccharides formed by pectin are the most complex, and the pectin backbone is composed of (1→4)-α-d-galacturonic acid molecules; the extraction method of pectin is a new trend in the resource utilization of agricultural residues; different pectin contents lead to different extraction methods, which depend on raw materials and extraction methods. Currently, the existing pectin extraction methods include: acid extraction method, microbial method, enzyme extraction method, microwave-assisted method, ultrasonic-assisted method, etc. The acid extraction method of pectin is the most widely used method in industrial production at present, and the technology is relatively mature. Its main principle is to convert the protopectin in the plant cell wall into water-soluble pectin by using an acidic solution at a certain temperature and pH, dissolve it in the extract, and then precipitate it with an organic solvent; the acid extraction method has the advantages of simple and rapid process, easy control, and high extraction yield. For example, it is disclosed in "Apple Pectin Preparation Process and Research Progress" (Zhang Xuedan, Deciduous Fruits, 2009) that the acid extraction method is one of the oldest industrial pectin production methods, and its basic principle is to extract pectin from plant tissues by using the solubility of pectin in acidic solutions. Usually, hot acidic solutions such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, sulfurous acid, etc. can be used, and organic acids such as citric acid, tartaric acid, acetic acid, lactic acid, and malic acid can also be used; however, in the traditional acid extraction method, the content of the RG-I structure in apple pectin is low, the molecular weight is small, and the degree of esterification is relatively high, which makes the pectin film have poor mechanical properties and poor water permeability when applied, affecting the use effect of the pectin film. Summary of the Invention

[0004] In view of the technical problems of low RG-I structure content, poor mechanical properties and poor water permeability in the above-mentioned existing technologies, the present invention provides an RG-I structure apple young fruit pectin, a pectin film and their preparation methods and applications. Through ultrasonic heating extraction, apple pectin with an RG-I content of 79% is obtained. The pectin film made of pectin has good mechanical properties and water permeability and has a good fresh-keeping effect.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] A preparation method of an RG-I structure apple young fruit pectin, comprising the following steps:

[0007] 1) The apple young fruits are freeze-dried and crushed to obtain apple young fruit powder;

[0008] 2) Mix the apple young fruit powder in step 1) with a citric acid solution and perform heating ultrasonic extraction to obtain a pectin extract; the dosage ratio of the apple young fruit powder to the citric acid solution is 1 g:(10 - 20) ml;

[0009] 3) Filter the extract obtained in step 2) respectively, mix the filtrate with ethanol according to a volume ratio of 1:(1 - 5); collect the mixed suspension, wash it with ethanol, and then perform re-dissolution, filtration, rotary evaporation and freeze-drying to obtain apple young fruit pectin.

[0010] Further limited, in step 1), the conditions of freeze-drying are: temperature -40°C, time 24 h - 48 h; the particle size of the apple young fruit powder is 200 mesh - 240 mesh.

[0011] Further limited, in step 2), the pH value of the citric acid solution is 1 - 2.5; the conditions of the heating ultrasonic extraction are: temperature 45°C - 85°C, power 200 W - 600 W, extraction time 20 min - 60 min.

[0012] Further limited, in step 3), the temperatures of re-dissolution and rotary evaporation are both 45°C, and the conditions of freeze-drying are: temperature -40°C, time 24 - 48 h.

[0013] An apple young fruit pectin prepared by a preparation method of an RG-I structure apple young fruit pectin.

[0014] Further limited, in the apple young fruit pectin, the content of the RG-I structure is 79%, and the content of HG is 15.6%; the molecular weight of the apple young fruit pectin is 755.38, and the degree of esterification is 80.39%.

[0015] A preparation method of an apple pectin film, comprising the following steps: dissolving apple young fruit pectin in water, adding glycerol, and stirring to dissolve; then using the casting method to pour the film, and after drying, obtaining the apple young fruit pectin film.

[0016] Further defined, the mass ratio of the apple young fruit pectin to water is 1:(99 - 999); the mass of the glycerol is 20% - 40% of the mass of the apple young fruit pectin; the drying temperature is 40°C to 45°C, and the drying time is 10h to 12h.

[0017] An apple young fruit pectin film prepared by using the preparation method of the apple pectin film.

[0018] An application of the apple young fruit pectin film in fruit preservation.

[0019] The beneficial effects of the present invention are:

[0020] 1. Compared with the traditional acid - liquid extraction of pectin, the present invention uses the method of ultrasonic heating to extract pectin, which is a physical fragmentation extraction process. It mainly uses the cavitation effect and mechanical vibration effect to instantly break the cell wall structure, thereby accelerating the diffusion of plant active ingredients. The obtained apple young fruit pectin has the characteristics of high content of RG - Ⅰ structure, large molecular weight, and low degree of esterification.

[0021] 2. In the present invention, the content of the RG - I structure in the apple young fruit pectin reaches 79%, and the HG content is 15.6%; the molecular weight of the apple young fruit pectin is 755.38, and the degree of esterification is 80.39%. The pectin film prepared by using this pectin with a high RG - I structure has high mechanical strength and high hydrophobicity, and has potential application potential in wrapping moisture - containing foods.

[0022] 3. The present invention uses the apple young fruit pectin with a high content of RG - Ⅰ to make a pectin film, which has good thermal stability. When used for fruit preservation, the water loss rate is low, the good fruit rate is high, and the preservation performance is good.

[0023] 4. The preparation method of the apple young fruit pectin in the present invention has a high pectin extraction rate; compared with the traditional extraction method, the ultrasonic - assisted extraction method has a shorter extraction time, lower energy consumption, and less solvent used. Description of the Drawings

[0024] Figure 1 Infrared spectrograms of pectin (a) and pectin film (b) of the example and the control example;

[0025] Figure 2 XRD diagrams of pectin (a) and pectin film (b) of the example and the control example;

[0026] Figure 3 SEM diagrams of pectin and pectin film of the example and the control example;

[0027] Figure 4 Mechanical properties of pectin films of the examples and control examples;

[0028] Figure 5 Water contact angles of pectin films of the examples and control examples;

[0029] Figure 6 Thermogravimetric analysis diagrams of pectin (a) and pectin films (b) of the examples and control examples. Detailed implementation manners

[0030] The present invention will be described in detail below in conjunction with the accompanying drawings and examples, but the scope protected by the present invention is not limited thereto.

[0031] The present invention provides a method for preparing RG-I structured apple young fruit pectin, comprising the following steps.

[0032] 1) The young apple fruits are freeze-dried and crushed to obtain young apple fruit powder.

[0033] Specifically, the conditions for freeze-drying are: temperature -40°C, time 24h - 48h; the particle size of the young apple fruit powder is 200 mesh to 240 mesh.

[0034] 2) The young apple fruit powder obtained in step 1) is mixed with a citric acid solution, and heated and ultrasonically extracted to obtain a pectin extract; the dosage ratio of the young apple fruit powder to the citric acid solution is 1 g:(10 - 20) ml; the pH value of the citric acid solution is 1 - 2.5.

[0035] Specifically, the conditions for heating and ultrasonic extraction are: temperature 45°C - 85°C, power 200W - 600W, extraction time 20min - 60min.

[0036] 3) The extract obtained in step 2) is respectively filtered by suction, and the filtrate is mixed with ethanol according to a volume ratio of 1:(1 - 5); the mixed suspension is collected, washed with ethanol, and then subjected to re-dissolution, filtration, rotary evaporation and freeze-drying to obtain young apple fruit pectin.

[0037] Specifically, the temperatures for re-dissolution and rotary evaporation are both 45°C, and the conditions for freeze-drying are: temperature -40°C, time 24 - 48h.

[0038] In the young apple fruit pectin prepared by the above method, the content of the RG-I structure is 79%, the HG content is 15.6%; the molecular weight of the young apple fruit pectin is 755.38, and the degree of esterification is 80.39%.

[0039] The present invention also provides a method for preparing an apple pectin film, comprising the following steps: dissolving the young apple fruit pectin in water, adding glycerol, and stirring to dissolve; then casting the film by the casting method, and drying to obtain a young apple fruit pectin film.

[0040] Specifically, the mass ratio of apple young fruit pectin to water is 1:(99 - 999); the mass of glycerol is 20% - 40% of the mass of apple young fruit pectin; the drying temperature is 40°C - 45°C, and the drying time is 10h - 12h.

[0041] The apple young fruit pectin film prepared by the above method of the present invention has good fresh-keeping performance for fruits.

[0042] Furthermore, several exemplary embodiments are selected from the above preparation method so that those skilled in the art can accurately understand the preparation method protected by the present invention and verify the performance of the apple young fruit pectin and the apple young fruit pectin film prepared by the present invention.

[0043] In the following examples, unless otherwise specified, all are conventional operations in the art, such as mixing, heating, filtering, and drying, etc.

[0044] In the following examples, the raw material drugs, reagents, etc. used are all analytical pure purchased from the market.

[0045] Example 1

[0046] The apple young fruit pectin provided in this example is prepared by the following steps.

[0047] (1) Cut the apple young fruit into thin slices about 8 mm thick, then freeze-dry and crush to 200 mesh for later use.

[0048] (2) Mix the apple young fruit powder obtained in step (1) with the citric acid solution whose pH has been adjusted. The dosage ratio of the apple young fruit powder to the citric acid solution is 1g:15ml; carry out ultrasonic heating extraction to obtain a pectin extraction solution.

[0049] The pH value of the citric acid solution is 1.5. The ultrasonic heating extraction is carried out with ultrasonic and heating simultaneously. The heating temperature is 85°C, the ultrasonic power is 240W, and extract while ultrasonic for 40 min.

[0050] (3) Filter the extraction solution obtained in step (2) separately, mix the filtrate with ethanol for precipitation. The volume ratio of the extraction solution to ethanol is 1:4; collect the suspension, wash it with ethanol, then redissolve and filter at 45°C; carry out rotary evaporation at 45°C, and freeze-dry at -40°C to obtain apple young fruit pectin.

[0051] Furthermore, in this example, step 4) is also included, which is to prepare an apple young fruit pectin film using the apple young fruit pectin in step 3).

[0052] Specifically, 1 g of the apple young fruit pectin obtained in step (3) is dissolved in 99 g of distilled water, and plasticizer glycerol is added. The concentration of the pectin solution is 1% (w / w); the mass concentration of glycerol is 30% of the mass of pectin. Then, it is dissolved by stirring in a water bath, and cast film is prepared by the casting method. The drying time is 12 h at 40 °C to obtain an apple young fruit pectin film.

[0053] Examples 2 to 5

[0054] The preparation method is the same as that of Example 1, but the preparation parameters used are different. For details, see Table 1.

[0055] Table 1 Preparation parameters of Examples 2 to 5

[0056]

[0057]

[0058] Comparative Example 1

[0059] The difference from Example 1 is that only heating is used.

[0060] The apple young fruit pectin provided in this comparative example is prepared by the following steps.

[0061] (1) The apple young fruit is cut into slices about 8 mm thick and then freeze-dried and crushed to 200 meshes for standby.

[0062] (2) The apple young fruit powder prepared in step (1) is mixed with the citric acid solution with the pH adjusted. The dosage ratio of the apple young fruit powder to the citric acid solution is 1 g:15 ml; heating extraction is carried out to obtain a pectin extract.

[0063] The pH value of the citric acid solution is 1.5, the heating temperature is 85 °C, and the extraction time is 2 h.

[0064] (3) The extract obtained in step (2) is filtered by suction respectively, and the filtrate is mixed with ethanol for precipitation. The volume ratio of the extract to ethanol is 1:4; the suspended matter is collected, washed with ethanol, redissolved and filtered at 45 °C; rotary evaporation is carried out at 45 °C, and after freeze-drying, apple young fruit pectin is obtained.

[0065] Furthermore, in this comparative example, the apple young fruit pectin obtained in step (3) is dissolved in distilled water, and plasticizer glycerol is added. The concentration of the pectin solution is 1% (w / w); the mass concentration of glycerol is 30% of the mass of pectin. Then, it is dissolved by stirring in a water bath, and cast film is prepared by the casting method. The drying time is 12 h at 40 °C to obtain an apple young fruit pectin film.

[0066] Comparative Example 2

[0067] The difference from Example 1 is that ultrasonic treatment is carried out first and then heating.

[0068] The apple young fruit pectin provided in this comparative example is prepared by the following steps.

[0069] (1) Cut the apple young fruits into slices about 8 mm thick, then freeze-dry and crush them to 200 mesh for standby.

[0070] (2) Mix the apple young fruit powder obtained in step (1) with the citric acid solution whose pH has been adjusted. The dosage ratio of the apple young fruit powder to the citric acid solution is 1 g:15 ml, and the pH value of the citric acid solution is 1.5; After ultrasonic treatment at 45°C with a power of 400 W for 10 min; Then heat extract at 85°C for 2 h to obtain a pectin extract.

[0071] (3) Filter the pectin extract obtained in step (2) separately, mix the filtrate with ethanol for precipitation, and the volume ratio of the extract to ethanol is 1:4; Collect the suspension, wash it with ethanol, redissolve and filter it at 45°C; Rotate and evaporate at 45°C, and after freeze-drying, obtain apple young fruit pectin.

[0072] Furthermore, in this comparative example, dissolve the apple young fruit pectin obtained in step (3) in distilled water, add the plasticizer glycerol, and the concentration of the pectin solution is 1% (w / w); The mass concentration of glycerol is 30% of the mass of pectin, then stir and dissolve it in a water bath, cast a film by the casting method, and dry it at 40°C for 12 h to obtain an apple young fruit pectin film.

[0073] Comparative Example 3

[0074] A commercial apple pectin film is prepared using the commercial apple pectin of Sigma Company, USA.

[0075] In this comparative example, dissolve the purchased commercial apple pectin in distilled water, add the plasticizer glycerol, and the concentration of the pectin solution is 1% (w / w); The mass concentration of glycerol is 30% of the mass of pectin, then stir and dissolve it in a water bath, cast a film by the casting method, and dry it at 40°C for 12 h to obtain a commercial apple pectin film.

[0076] Comparative Example 4

[0077] In this comparative example, a pectin film is prepared using the commercial citrus pectin of Shanghai Diyi Biotechnology Co., Ltd.

[0078] In this comparative example, dissolve the purchased commercial citrus pectin in distilled water, add the plasticizer glycerol, and the concentration of the pectin solution is 1% (w / w); The mass concentration of glycerol is 30% of the mass of pectin, then stir and dissolve it in a water bath, cast a film by the casting method, and dry it at 40°C for 12 h to obtain a commercial apple pectin film.

[0079] The performance of the pectin prepared in the above examples and comparative examples was verified by the following tests.

[0080] Test 1 Chemical composition of pectin

[0081] Samples: The pectins in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 were denoted as UAE, CAE, UBE, CP, and AP in sequence.

[0082] The extraction rates of the pectins prepared in Example 1, Comparative Example 1, and Comparative Example 2 were measured respectively, and the results are shown in Table 2.

[0083] The physicochemical properties and molecular weights of all the above samples were measured respectively, and the results are shown in Table 2. Mw represents the weight-average molecular weight, and Mn represents the number-average molecular weight.

[0084] Table 2 Chemical composition of pectin

[0085]

[0086] It can be seen from the results in Table 2 that with the increase of ultrasonic treatment, the physicochemical properties of pectin have changed. This is because the cavitation effect of ultrasound breaks the structure of plant cells, facilitating the chelating and hydrolysis abilities of citric acid. In terms of the degree of esterification, the esterified branches are destroyed by ultrasound, reducing the degree of esterification. The content of RG-I type pectin in Comparative Example 1 is the lowest. After ultrasonic treatment, the content of RG-I type pectin increases, but the content of RG-I type pectin is the highest when heating and ultrasound are carried out simultaneously. The content of rhamnose and arabinose in the pectin treated by ultrasound is high, while the content of glucose and xylose is low. The molar ratio of (Gal+Ara) / Rha is used to prove the contribution of the RG-I domain to the structure and the branch length connected to RG-I. The molar ratio of (Gal+Ara) / Rha in Comparative Example 1 is the highest at 19.38, followed by Comparative Example 2 at 18.09, and Example 1 at 15.56, the lowest. The protein concentrations in Example 1, Control Example 1, and Control Example 2 are between 2.66% and 4.83%, significantly higher than those in Control Example 3 and Control Example 4. And there is a certain positive correlation between the protein concentration of pectin from young apple fruits and the degree of esterification. The acetyl degrees of the pectin obtained in the present invention are not very different. Except for Comparative Example 4 (5.61), the acetyl degrees of the other four pectins are between 9.28 and 10.07.

[0087] The molecular weight of pectin increases significantly after ultrasound treatment. The relatively large molecular weight is because pectin contains rich neutral sugar side chains.

[0088] Verification 2 Infrared spectrogram

[0089] Samples: The pectins in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 were denoted as UAE, CAE, UBE, CP, and AP in sequence; the pectin films in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 were denoted as UAE-F, CAE-F, UBE-F, CP-F, and AP-F in sequence.

[0090] The infrared spectra of the above pectins and pectin films were measured respectively under the following conditions: scanning range: 4000 - 400 cm -1 , scanning times: 16 times, resolution: 4 cm -1 ; The measurement results are as Figure 1 shown. Figure 1 a is the infrared spectrum of the pectin; Figure 1 b is the infrared spectrum of the pectin film.

[0091] It can be seen from Figure 1 that UAE, CAE, UBE, CP, and AP have similar structures and all belong to high-methoxyl pectins. UAE-F, CAE-F, UBE-F, CP-F, and AP-F made from them also have similar structures, indicating that ultrasonic and heat treatments did not change the pectin structure.

[0092] Verification of 3XRD patterns

[0093] Samples: The pectins in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 were denoted as UAE, CAE, UBE, CP, and AP in sequence; the pectin films in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 were denoted as UAE-F, CAE-F, UBE-F, CP-F, and AP-F in sequence.

[0094] The XRD patterns of the above pectins and pectin films were measured respectively under the following conditions: scanning range 4° - 60°, step size 0.02°; the measurement results are as Figure 2 shown. Figure 2 a is the XRD pattern of the pectin; Figure 2 b is the XRD pattern of the pectin film.

[0095] It can be seen from Figure 2 that UAE, CAE, UBE, and AP are all amorphous polymers, while CP has high crystallinity; and UAE-F, CAE-F, UBE-F, CP-F, and AP-F are typical amorphous polymers.

[0096] Verification of 4SEM images

[0097] Samples: The pectins in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4 were sequentially denoted as UAE, CAE, UBE, CP and AP; the pectin films in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4 were sequentially denoted as UAE-F, CAE-F, UBE-F, CP-F and AP-F.

[0098] The SEM images of the above-mentioned pectins and pectin films were measured respectively under the following conditions: acceleration voltage of 10 kV; the measurement results are as Figure 3 shown.

[0099] From Figure 3 it can be seen that UAE, CAE and UBE have similar layered structures, while CP and AP are granular. The surface images of UAE-F, CAE-F, UBE-F, CP-F and AP-F are relatively smooth and uniform, with some protrusions in part, which may be related to the tiny air bubbles formed with proteins, or may be the tiny air bubbles that did not detach during the drying process of the film. There are no obvious cracks, pores and delamination in the cross-section of the pectin film, indicating that the microstructure of the young fruit pectin film is uniform and has good application prospects.

[0100] Verification 5 Mechanical properties

[0101] Samples: The pectin films in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4 were sequentially denoted as UAE-F, CAE-F, UBE-F, CP-F and AP-F.

[0102] The mechanical properties of the above-mentioned pectin films were measured respectively, including tensile strength and elongation at break. The measurement conditions were as follows: the TA.XT.Plus texture analyzer was used to measure its mechanical properties, the initial gap was 30 mm, and the tensile speed was 50 mm / min for testing; the measurement results are as Figure 4 shown.

[0103] From Figure 4 it can be seen that the tensile strengths of the pectin films from high to low are UAE-F (33.76 Mpa) > UBE-F (26.63 Mpa) > CAE-F (21.91 Mpa) > AP-F (16.02 Mpa) > CP-F (12.24 Mpa). Therefore, the young apple fruit pectin film is more resistant to elastic deformation than the commercial pectin film, which is reflected in its stiffness and strength. CP-F has the highest elongation at break, which is 14.36%, followed by UAE-F, which is 7.65%, and UBE-F is the smallest, which is 3.51%. However, there is no significant difference in the elongation at break between AP-F and UAE-F, UBE-F, CAE-F. Generally speaking, under the same length, width and test parameters, the mechanical properties of the young fruit pectin film are higher than those of the commercial pectin film.

[0104] Verification of 6 water contact angle samples: The pectin films in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4 were denoted as UAE-F, CAE-F, UBE-F, CP-F and AP-F in sequence.

[0105] The water contact angles of the above pectin films were measured respectively. The measurement conditions were as follows: A video optical contact angle measuring instrument was used, 2 μL of distilled water was gently dropped onto the film surface, and the contact angle was measured at 25 °C to obtain an image. For each treated film, 5 repeated tests were carried out, and the average value was finally taken. The measurement results are as Figure 5 shown.

[0106] From Figure 5 it can be seen that the order of the water contact angles of the five films is UAE-F (108.7°) > UBE-F (107.4°) > CAE-F (104.5°) > AP-F (92.62°) > CP-F (83.2°). The water contact angles of the films made from pectin of young apple fruits are all greater than 90°, while the commercial pectin film is around 90°. This indicates that the pectin film of young apple fruits is a hydrophobic film and its hydrophobicity is significantly higher than that of the commercial pectin film. This shows that the pectin film of young apple fruits has potential application potential in wrapping moisture-containing foods.

[0107] Verification 7 Thermogravimetric analysis

[0108] Samples: The pectins in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4 were denoted as UAE, CAE, UBE, CP and AP in sequence; the pectin films in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4 were denoted as UAE-F, CAE-F, UBE-F, CP-F and AP-F in sequence.

[0109] The thermogravimetric analyses of the above pectins and pectin films were measured respectively. The measurement conditions were as follows: The test temperature was 30 - 600 °C, and the rate was 10 °C / min; the measurement results are as Figure 6 shown. Figure 6 a is the thermogravimetric analysis diagram of pectin; Figure 6 b is the thermogravimetric analysis diagram of pectin film.

[0110] From Figure 6It can be known that the temperature at the maximum thermal decomposition rate (Tmax) can effectively reflect the thermal stability of substances, and with the increase of Tmax, the thermal stability is enhanced. The Tmax values of CAE, UBE, and UAE are 281.25, 280.48, and 278.12 respectively; after ultrasonic treatment, Tmax decreases, and the higher the DE value of pectin, the better its thermal performance. Moreover, the Tmax values of CAE, UBE, and UAE are all higher than those of CP and AP, indicating that apple young fruit pectin has good application potential in terms of thermal stability. Comparing all pectins, the decomposition stage of the corresponding pectin films remains unchanged, but they all start to release heat earlier. Among them, UAE-F has the highest Tmax, which is 269.17 °C, followed by UBE-F (268.09 °C), CAE-F (267.61 °C), and CP-F (249.48), and the Tmax of AP-F is 257.65 °C.

[0111] Verification of the fresh-keeping performance

[0112] Samples: Pectin films in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4.

[0113] The experimental method takes cherry tomatoes as an example and is divided into 6 groups. One group is the blank control group without any treatment, and they are respectively packaged and subjected to antibacterial fresh-keeping treatment with water and the pectin films prepared in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4, and stored at room temperature (temperature: 26.5 ± 1 °C, humidity: 44 ± 3%). The results are shown in Tables 3 and 4.

[0114] Table 3 Water loss performance of pectin films for the fresh-keeping of cherry tomatoes

[0115] Day 1 Day 2 Day 3 Not processed 2.09％ 4.24％ 6.58％ Example 1 1.66％ 3.52％ 5.65％ Comparative Example 1 1.97％ 3.93％ 6.53％ Comparative Example 2 1.78％ 3.78％ 6.16％ Comparative Example 3 1.92％ 4.01％ 6.24％ Comparative Example 4 2.01％ 4.12％ 6.38％

[0116] Table 4 Good fruit rate of pectin films for the fresh-keeping of cherry tomatoes

[0117]

[0118] As can be seen from Tables 3 and 4, the water loss rate and good fruit rate of fresh-cut fruits before and after packaging with pectin films are lower than those of untreated cherry tomatoes, and the water loss rate after treatment with Example 1 is the lowest and the good fruit rate is the highest, indicating that Example 1 has the best fresh-keeping performance for cherries and tomatoes.

[0119] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features, and all should fall within the protection scope of the present invention.

Claims

1. A preparation method of RG-I structure apple young fruit pectin, characterized in that, It includes the following steps: 1) The young apple fruits are freeze-dried and crushed to obtain young apple fruit powder; 2) The young apple fruit powder obtained in step 1) is mixed with a citric acid solution, and heated and ultrasonically extracted to obtain a pectin extract; the dosage ratio of the young apple fruit powder to the citric acid solution is 1 g:(10 - 20) ml; In step 2), the pH value of the citric acid solution is 1 - 2.5; the conditions for the heated ultrasonic extraction are: temperature 45°C - 85°C, power 200 W - 600 W, extraction time 20 min - 60 min; 3) The extract obtained in step 2) is filtered by suction, and the filtrate is mixed with ethanol according to a volume ratio of 1:(1 - 5); the mixed suspension is collected, washed with ethanol, and then subjected to re-dissolution, filtration, rotary evaporation and freeze-drying to obtain young apple fruit pectin.

2. The preparation method of RG-I structured apple young fruit pectin according to claim 1, characterized in that, In step 1), the conditions for freeze-drying are: temperature -40°C, time 24 h - 48 h; the particle size of the young apple fruit powder is 200 mesh - 240 mesh.

3. The preparation method of RG-I structured apple young fruit pectin according to claim 1, characterized in that In step 3), the temperatures for re-dissolution and rotary evaporation are both 45°C, and the conditions for freeze-drying are: temperature -40°C, time 24 h - 48 h.

4. A young apple fruit pectin prepared by using the preparation method of the RG-I structure young apple fruit pectin described in claim 1.

5. The apple young fruit pectin according to claim 4, characterized in that, In the young apple fruit pectin, the content of the RG-I structure is 79%, and the content of HG is 15.6%; the molecular weight of the young apple fruit pectin is 755.38, and the degree of esterification is 80.39%.

6. A preparation method of an apple pectin film, characterized in that, It includes the following steps: The young apple fruit pectin described in claim 4 is dissolved in water, glycerol is added, and stirred until dissolved; then a casting film is formed by the casting method, and after drying, a young apple fruit pectin film is obtained.

7. The preparation method of the apple pectin film according to claim 6, wherein, The mass ratio of the young apple fruit pectin to water is 1:(99 - 999); the mass of the glycerol is 20% - 40% of the mass of the young apple fruit pectin; the drying temperature is 40°C - 45°C, and the drying time is 10 h - 12 h.

8. A young apple fruit pectin film prepared by using the preparation method of the apple pectin film described in claim 6.

9. An application of the young apple fruit pectin film as described in claim 8 in fruit preservation.

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