Damping antibacterial pullulan / gelatin / polyvinyl alcohol aerogel for fruit and vegetable packaging

Abstract

The invention discloses shock-absorbing antibacterial pullulan / gelatin / polyvinyl alcohol aerogel for fruit and vegetable packaging. The shock-absorbing antibacterial pullulan / gelatin / polyvinyl alcohol aerogel is prepared by the following steps: S1, embedding thyme essential oil into halloysite nanotubes to form TEO-coated HNTs; s2, mixing pullulan, gelatin and polyvinyl alcohol to form a pullulan / gelatin / polyvinyl alcohol mixed solution; and S3, mixing the TEO coated HNTs with the pullulan / gelatin / polyvinyl alcohol mixed solution, and performing freeze drying to form the shock-absorbing antibacterial pullulan / gelatin / polyvinyl alcohol aerogel. The degradable fruit and vegetable packaging bag has the characteristics of being green, safe and degradable, has good moisture resistance, high mechanical strength and good air permeability, and can be used for meeting the requirements of buffering, air permeability, bacteriostasis and the like in fruit and vegetable packaging.

Description

Technical Field

[0001] The invention relates to the technical field of fruit and vegetable packaging, and in particular to a shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel for fruit and vegetable packaging. Background Art

[0002] Active packaging is a new food packaging strategy that has emerged in recent years. Its purpose is to maintain or extend the shelf life of food while ensuring the flavor, nutrition and safety of food. Aerogel is an advanced material with low density, high porosity and high specific surface area. Its porosity is usually in the range of 80-99.8%, which can be used as an excellent carrier of active substances.

[0003] Pullulan (PUL) is a carbohydrate polymer, a linear glucan based on maltotriose, composed of two α-(1→4)-linked pyranose glucose rings connected to each other through α-(1→6). Pullulan is non-toxic, edible, biocompatible and biodegradable.

[0004] Gelatin (GA) is a single-chain protein obtained from the hydrolysis of collagen, which is one of the most abundant proteins present in animal skin, bones and sarcomas. Due to its low cost, biocompatibility, biodegradability and non-toxicity, it has been successfully used in pollutant adsorption, medical and food industries.

[0005] Polyvinyl alcohol (PVA) is a biodegradable synthetic polymer obtained by polymerizing vinyl acetate monomer into polyvinyl acetate and then hydrolyzing the vinyl acetate group. PVA has good compatibility, film-forming properties and safety, making it the preferred substrate for food packaging preparation.

[0006] Due to the hydrophilicity of gelatin, pullulan and polyvinyl alcohol-based products, products made with gelatin, pullulan and polyvinyl alcohol are easily soluble in water and have poor mechanical properties. The storage environment of conventional fruit and vegetable products will produce water vapor due to respiration and be highly humid. Conventional products made of gelatin, pullulan and polyvinyl alcohol used for fresh-keeping packaging of fruits and vegetables will cause softening and disintegration of the material due to high humidity, and the strength cannot meet the requirements of packaging materials.

[0007] Thyme essential oil (TEO) is a plant-derived essential oil extracted from Thymus vulgaris. Its main components are thymol, carvacrol, limonene and turpentine, and it has high antibacterial, antioxidant and anti-inflammatory activities.

[0008] Halloysite nanotubes (HNTs) are a natural layered mineral belonging to the kaolinite family of hydrous aluminosilicates with a molecular formula of Al2Si2O5(OH)4·nH2O, and have a unique nanotube morphology. The high specific surface area of ​​HNTs gives them excellent adsorption capacity, enabling them to effectively adsorb or load a variety of chemicals. By encapsulating TEO into HNTs, their application defects can be effectively addressed.

[0009] Since fruits and vegetables are extremely susceptible to mechanical damage and infection by pathogens and other microorganisms during transportation and storage after harvest, these damages affect the quality of fruit storage after harvest to varying degrees, accelerate the softening and aging of the fruit, and are not conducive to storage and sales. Traditional packaging has limitations in shock absorption and antibacterial properties. Although traditional synthetic antimicrobial agents are widely used, they are prone to residues and cause microbial resistance, threatening human health. Bio-based antimicrobial agents have the advantages of low toxicity, greenness, and versatility, and are gradually becoming the preferred antimicrobial agents for active food packaging. Bio-based or biodegradable materials are receiving more and more attention as a more environmentally friendly alternative. Summary of the invention

[0010] The purpose of the present invention is to provide a shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel for fruit and vegetable packaging, which has the characteristics of being green, safe and degradable, having good moisture resistance and high mechanical strength, and at the same time having good air permeability, and can be used to meet the requirements of cushioning, air permeability, antibacterial and the like in fruit and vegetable packaging.

[0011] The technical solution adopted by the present invention to solve its technical problem is: A shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel for fruit and vegetable packaging is prepared by the following method: S1: Thyme essential oil was embedded into halloysite nanotubes to form TEO@HNTs; S2: mixing pullulan, gelatin and polyvinyl alcohol to form a pullulan / gelatin / polyvinyl alcohol mixed solution; S3: TEO@HNTs were mixed with a pullulan / gelatin / polyvinyl alcohol mixed solution and freeze-dried to form a shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel.

[0012] The present invention embeds thyme essential oil with antibacterial properties into halloysite nanotubes to prepare aerogel, which solves the shock absorption and antibacterial problems of traditional packaging. At the same time, TEO@HNTs can significantly improve the hydrophobicity, water resistance and mechanical strength of pullulan / gelatin / polyvinyl alcohol-based materials, so there is no need to modify the pullulan / gelatin / polyvinyl alcohol-based materials separately to overcome the problem that they are not suitable for fruit and vegetable packaging. The prepared aerogel has strong compressive resistance.

[0013] The specific steps of S1 are as follows: (1) 4-6 g of halloysite nanotubes, 8-12 g of thyme essential oil and 12-18 mL of ethanol solution are uniformly mixed, followed by ultrasonic treatment and vacuum degassing to obtain a mixture of halloysite nanotubes and thyme essential oil; (2) centrifuging the mixture of halloysite nanotubes and thyme essential oil, removing the supernatant, and collecting the precipitate; (3) adding an equal amount of ethanol solution as that in step (1) to the precipitate obtained in step (2), mixing, centrifuging, removing the supernatant, and collecting the precipitate; (4) Adding an equal amount of ethanol solution as that in step (1) to the precipitate obtained in step (3), mixing the mixture, centrifuging, removing the supernatant, collecting the precipitate, and freeze-drying the mixture to obtain TEO@HNTs.

[0014] The mass concentration of the ethanol solution is 60-70%.

[0015] S2 is specifically as follows: 4-5 grams of polyvinyl alcohol is added to 100 mL of deionized water, heated to 85-90° C. and stirred to obtain a polyvinyl alcohol solution, cooled to 40-45° C., 4-5 grams of pullulan and 4-5 grams of gelatin are added, stirred and mixed to obtain a pullulan / gelatin / polyvinyl alcohol mixed solution.

[0016] In S3, the mass percentage ratio of TEO@HNTs to pullulan / gelatin / polyvinyl alcohol mixed solution is: TEO@HNTs 3-12%, pullulan / gelatin / polyvinyl alcohol mixed solution 88-97%, and the total is 100%.

[0017] The fruit or vegetable is one of blueberry, strawberry, kiwi fruit, grape, tomato and cucumber.

[0018] Application of shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel as packaging material for preparing fruits and vegetables for preservation.

[0019] The beneficial effects of the present invention are: The aerogel prepared by the invention is a pure bio-based bio-aerogel, which has the characteristics of being green, safe and degradable, and is conducive to being used as food packaging.

[0020] The present invention prepares TEO@HNTs and loads them into a hybrid aerogel, thereby enhancing the crystallinity, thermal stability, compression performance, surface hydrophobicity and water resistance of the aerogel, and obtaining a multifunctional fruit and vegetable packaging material with good buffering performance and strong activity.

[0021] In the present invention, HNTs form hydrogen bonds with TEO and have a high encapsulation efficiency for TEO, and TEO@HNTs are released in the aerogel for up to 72 hours.

[0022] The aerogel of the present invention has a good inhibitory effect on the activity of Escherichia coli and Staphylococcus aureus, and also has a good reducing effect on the infection of gray mold during the storage of fruits and vegetables after harvest, and is expected to reduce the adverse effects of microorganisms and the like in the process of post-harvest fruits and vegetables.

[0023] The present invention is applied to fruits and vegetables, and can well maintain the weight and color of the fruits and vegetables, significantly reduce the adverse effects of collisions during the storage and transportation of fruits and vegetables, and extend the shelf life of fruits and vegetables. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 are the cross-sectional and surface scanning electron microscope (SEM) images of TEO@HNTs aerogels with different addition amounts, and the scale bar in the SEM image is 30 μm;

[0013] Figure 2 (A) FTIR spectra of TEO, HNTs and TEO@HNTs; (B) FTIR spectra of aerogels containing different proportions of TEO@HNTs.

[0025] Figure 3 (A) XRD pattern of TEO; (B) XRD patterns of HNTs, TEO@HNTs and aerogels containing different proportions of TEO@HNTs.

[0026] Figure 4 (A) Thermogravimetric curves of TEO, HNTs and TEO@HNTs; (B) Thermogravimetric curves of TEO, HNTs and TEO@HNTs; (C) Thermogravimetric differential curves of aerogels containing different proportions of TEO@HNTs; (D) Thermogravimetric differential curves of aerogels containing different proportions of TEO@HNTs.

[0027] Figure 5 (A) Aerogel compression performance test picture; (B) Compression-strain curve of aerogel.

[0028] Figure 6 Water contact angles of aerogels containing different ratios of TEO@HNTs.

[0029] Figure 7 Images of the inhibition zones of Escherichia coli and Staphylococcus aureus containing different proportions of TEO@HNTs aerogels.

[0030] Figure 8 Picture of the appearance of blueberry fruit on the tenth day after being infected by gray mold.

[0031] Fig. 9 Changes in (A) firmness, (B) weight loss rate, (C) L* value, and (D) b* value during blueberry storage. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is further described in detail below through specific embodiments.

[0033] In the present invention, unless otherwise specified, the raw materials and equipment used can be purchased from the market or are commonly used in the art. The methods in the following embodiments, unless otherwise specified, are all conventional methods in the art.

[0034] Pullulan: Shanghai Aladdin Biochemical Technology Co., Ltd.; gelatin: Shanghai Sinopharm Chemical Reagent Co., Ltd.; polyvinyl alcohol: alcoholysis degree: 98.0-99.0 mol%, Shanghai Aladdin Biochemical Technology Co., Ltd.; halloysite nanotubes: Guangdong Jina New Materials Technology Co., Ltd.; thyme essential oil: thymol content ≥40%, Shanghai Yuanye Biotechnology Co., Ltd.; anhydrous ethanol: ≥99.7%, Shanghai McLean Biochemical Technology Co., Ltd.

[0035] Embodiment 1: (1) Weigh 5 g of HNTs and 10 g of TEO, add 15 mL of 60% ethanol solution, stir at room temperature for 2 hours to obtain a uniform mixture of HNTs and TEO, sonicate for 30 minutes, and vacuumize twice, 15 minutes each time; (2) centrifuging the mixture in (1) at 18,000 × g for 15 minutes and removing the supernatant to remove excess thyme essential oil; (3) adding 15 mL of 60% ethanol solution to the remaining precipitate in (2), centrifuging at 18,000 × g for 15 minutes, and removing the supernatant to remove excess thyme essential oil; (4) adding 15 mL of 60% ethanol solution to the remaining precipitate in (3), centrifuging at 18,000 × g for 15 min, removing the supernatant, precooling the obtained precipitate at -80°C, and then freeze-drying it for 72 h to obtain TEO@HNTs; (5) Weigh 4 g of pullulan, 4 g of gelatin, and 4 g of polyvinyl alcohol, add the polyvinyl alcohol to 100 mL of deionized water, and stir at 90° C. for 1 hour to obtain a completely dissolved polyvinyl alcohol solution; (6) After the polyvinyl alcohol solution is cooled to 40° C., pullulan and gelatin are added and stirred for 1.5 hours to obtain a pullulan / gelatin / polyvinyl alcohol mixed solution; (7) Add 3 g of TEO@HNTs to 97 g of pullulan / gelatin / polyvinyl alcohol mixed solution and mix well, then freeze-dry to obtain 3% TEO@HNTs hybrid aerogel (TEO@HNTsPGP).

[0036] Embodiment 2: (1) Weigh 5 g of HNTs and 10 g of TEO, add 15 mL of 60% ethanol solution, stir at room temperature for 2 hours to obtain a uniform mixture of HNTs and TEO, sonicate for 30 minutes, and vacuumize twice, 15 minutes each time; (2) centrifuging the mixture in (1) at 18,000 × g for 15 minutes and removing the supernatant to remove excess thyme essential oil; (3) adding 15 mL of 60% ethanol solution to the remaining precipitate in (2), centrifuging at 18,000 × g for 15 minutes, and removing the supernatant to remove excess thyme essential oil; (4) adding 15 mL of 60% ethanol solution to the remaining precipitate in (3), centrifuging at 18,000 × g for 15 min, removing the supernatant, precooling the obtained precipitate at -80°C, and then freeze-drying it for 72 h to obtain TEO@HNTs; (5) Weigh 4 g of pullulan, 4 g of gelatin, and 4 g of polyvinyl alcohol, add the polyvinyl alcohol to 100 mL of deionized water, and stir at 90° C. for 1 hour to obtain a completely dissolved polyvinyl alcohol solution; (6) After the polyvinyl alcohol solution is cooled to 40° C., pullulan and gelatin are added and stirred for 1.5 hours to obtain a pullulan / gelatin / polyvinyl alcohol mixed solution; (7) Add 6 g of TEO@HNTs to 94 g of pullulan / gelatin / polyvinyl alcohol mixed solution and mix well, then freeze-dry to obtain 6% TEO@HNTs mixed aerogel.

[0037] Embodiment 3: (1) Weigh 5 g of HNTs and 10 g of TEO, add 15 mL of 60% ethanol solution, stir at room temperature for 2 hours to obtain a uniform mixture of HNTs and TEO, sonicate for 30 minutes, and vacuumize twice, 15 minutes each time; (2) centrifuging the mixture in (1) at 18,000 × g for 15 minutes and removing the supernatant to remove excess thyme essential oil; (3) adding 15 mL of 60% ethanol solution to the remaining precipitate in (2), centrifuging at 18,000 × g for 15 minutes, and removing the supernatant to remove excess thyme essential oil; (4) adding 15 mL of 60% ethanol solution to the remaining precipitate in (3), centrifuging at 18,000 × g for 15 min, removing the supernatant, precooling the obtained precipitate at -80°C, and then freeze-drying it for 72 h to obtain TEO@HNTs; (5) Weigh 4 g of pullulan, 4 g of gelatin, and 4 g of polyvinyl alcohol, add the polyvinyl alcohol to 100 mL of deionized water, and stir at 90° C. for 1 hour to obtain a completely dissolved polyvinyl alcohol solution; (6) After the polyvinyl alcohol solution is cooled to 40° C., pullulan and gelatin are added and stirred for 1.5 hours to obtain a pullulan / gelatin / polyvinyl alcohol mixed solution; (7) 9 g of TEO@HNTs was added to 91 g of pullulan / gelatin / polyvinyl alcohol mixed solution and mixed evenly, followed by freeze drying to obtain 9% TEO@HNTs mixed aerogel.

[0038] Embodiment 4: (1) Weigh 5 g of HNTs and 10 g of TEO, add 15 mL of 60% ethanol solution, stir at room temperature for 2 hours to obtain a uniform mixture of HNTs and TEO, sonicate for 30 minutes, and vacuumize twice, 15 minutes each time; (2) centrifuging the mixture in (1) at 18,000 × g for 15 minutes and removing the supernatant to remove excess thyme essential oil; (3) adding 15 mL of 60% ethanol solution to the remaining precipitate in (2), centrifuging at 18,000 × g for 15 minutes, and removing the supernatant to remove excess thyme essential oil; (4) adding 15 mL of 60% ethanol solution to the remaining precipitate in (3), centrifuging at 18,000 × g for 15 min, removing the supernatant, precooling the obtained precipitate at -80°C, and then freeze-drying it for 72 h to obtain TEO@HNTs; (5) Weigh 4 g of pullulan, 4 g of gelatin, and 4 g of polyvinyl alcohol, add the polyvinyl alcohol to 100 mL of deionized water, and stir at 90° C. for 1 hour to obtain a completely dissolved polyvinyl alcohol solution; (6) After the polyvinyl alcohol solution is cooled to 40° C., pullulan and gelatin are added and stirred for 1.5 hours to obtain a pullulan / gelatin / polyvinyl alcohol mixed solution; (7) Add 12 g of TEO@HNTs to 88 g of pullulan / gelatin / polyvinyl alcohol mixed solution and mix well, then freeze-dry to obtain 12% TEO@HNTs mixed aerogel.

[0039] Embodiment 5: (1) Weigh 4 g of HNTs and 8 g of TEO, add 12 mL of 70% ethanol solution, stir at room temperature for 2 hours to obtain a uniform mixture of HNTs and TEO, sonicate for 30 minutes, and vacuumize twice, 15 minutes each time; (2) centrifuging the mixture in (1) at 18,000 × g for 15 minutes and removing the supernatant to remove excess thyme essential oil; (3) adding 12 mL of 60% ethanol solution to the remaining precipitate in (2), centrifuging at 18,000 × g for 15 minutes, and removing the supernatant to remove excess thyme essential oil; (4) adding 12 mL of 60% ethanol solution to the remaining precipitate in (3), centrifuging at 18,000 × g for 15 min, removing the supernatant, precooling the obtained precipitate at -80°C, and then freeze-drying it for 72 h to obtain TEO@HNTs; (5) Weigh 5 g of pullulan, 5 g of gelatin, and 5 g of polyvinyl alcohol, add the polyvinyl alcohol to 100 mL of deionized water, and stir at 85° C. for 1 hour to obtain a completely dissolved polyvinyl alcohol solution; (6) After the polyvinyl alcohol solution is cooled to 45° C., pullulan and gelatin are added and stirred for 1.5 hours to obtain a pullulan / gelatin / polyvinyl alcohol mixed solution; (7) Add 6 g of TEO@HNTs to 94 g of pullulan / gelatin / polyvinyl alcohol mixed solution and mix well, then freeze-dry to obtain 6% TEO@HNTs mixed aerogel.

[0040] Embodiment 6: (1) Weigh 6 g of HNTs and 12 g of TEO, add 18 mL of 60% ethanol solution, stir at room temperature for 2 hours to obtain a uniform mixture of HNTs and TEO, sonicate for 30 minutes, and vacuumize twice, 15 minutes each time; (2) centrifuging the mixture in (1) at 18,000 × g for 15 minutes and removing the supernatant to remove excess thyme essential oil; (3) adding 18 mL of 60% ethanol solution to the remaining precipitate in (2), centrifuging at 18,000 × g for 15 minutes, and removing the supernatant to remove excess thyme essential oil; (4) adding 18 mL of 60% ethanol solution to the remaining precipitate in (3), centrifuging at 18,000 × g for 15 min, removing the supernatant, precooling the obtained precipitate at -80°C, and then freeze-drying it for 72 h to obtain TEO@HNTs; (5) Weigh 4 g of pullulan, 4 g of gelatin, and 4 g of polyvinyl alcohol, add the polyvinyl alcohol to 100 mL of deionized water, and stir at 90° C. for 1 hour to obtain a completely dissolved polyvinyl alcohol solution; (6) After the polyvinyl alcohol solution is cooled to 40° C., pullulan and gelatin are added and stirred for 1.5 hours to obtain a pullulan / gelatin / polyvinyl alcohol mixed solution; (7) Add 6 g of TEO@HNTs to 94 g of pullulan / gelatin / polyvinyl alcohol mixed solution and mix well, then freeze-dry to obtain 6% TEO@HNTs mixed aerogel.

[0041] Comparative Example 1: (1) Weigh 4 g of pullulan, 4 g of gelatin, and 4 g of polyvinyl alcohol, add the polyvinyl alcohol into 100 mL of deionized water, and stir at 90° C. for 1 hour to obtain a completely dissolved polyvinyl alcohol solution; (2) After the polyvinyl alcohol solution is cooled to 40° C., pullulan and gelatin are added and stirred for 1.5 hours to obtain a pullulan / gelatin / polyvinyl alcohol mixed solution; (3) The pullulan / gelatin / polyvinyl alcohol mixed solution was precooled at -80°C and then freeze-dried for 72 hours to obtain PGP aerogel.

[0042] The present invention evaluates the microscopic morphology, intermolecular interaction, thermal properties, mechanical properties, water contact angle (WCA), and porosity of TEO@HNTsPGP aerogels (Examples 1-4). In addition, the inhibitory effect of TEO@HNTsPGP on Staphylococcus aureus, Escherichia coli, and gray mold was verified, and the weight loss rate, color, and hardness of blueberries during storage were measured. The PGP group is comparative example 1.

[0043] 1. Apparent structure The internal micromorphology of the bioaerogel was observed by FE-SEM (GeminiSEM 300, ZEISS, Oberkochen, Germany) at a magnification of 100×.

[0044] 2. Fourier Transform Infrared (FTIR) Analysis Fourier transform infrared analysis was performed according to the KBr pellet method to determine the interactions between the components. Fourier transform infrared spectra were obtained using a NICOLET iS50 FT-IR instrument (Thermo Nicolet Ltd., Waltham, MA, USA). The wave number range was 4000–400 cm -1 Each sample was subjected to a 4 cm -1 An average of 32 scans.

[0045] 3. X-ray diffraction (XRD) analysis XRD diffraction was analyzed using an X'Pert Pro diffractometer (PANalytical BV, The Netherlands) at a voltage of 40 kV and a current of 30 mA. The diffraction angle (2θ) ranged from 5° to 90°.

[0046] 4. Thermogravimetric analysis Thermogravimetry was performed using a TA Q500 instrument (TA Instruments, Newcastle, USA) with a nitrogen flow rate of 50 mL min -1 The heating rate of TGA was 10 °C min -1 , the temperature range is 50℃ to 600℃.

[0047] 5. Compression performance test The compression properties were measured using a mechanical testing instrument (HDB609B–S, Haida Instrument, Xiamen, China). Before testing, the aerogels were cut into blocks (30 mm × 30 mm) and the compression rate was set to 10 mm min -1 , compression stops at 90% of the strain setting.

[0048] 6. Water contact angle (WCA) WCA was obtained by the sessile drop method using an OCA20 device (Data Physics GmbH, Germany). The aerogel was fixed on a glass slide and a drop of distilled water (3 μL) was dropped on its surface. The drop was allowed to equilibrate for 3 seconds and then the contact angle was measured to calculate the value of WCA. Each WCA value was calculated based on the average of five different locations on the surface of the same aerogel sample.

[0049] 7. Porosity Calculate the porosity of the aerogel using Archimedes' principle: Where Φ is the total porosity, mp is the total weight of the aerogel (g); Vp is the total volume of the aerogel (cm 3 ), ρa, ρb and ρc are the densities of gelatin, curdlan and PVA, respectively (g·cm -3 ).

[0050] 8. Determination of antibacterial activity The antibacterial activity of aerogels with different TEO@HNTs concentrations against Escherichia coli and Staphylococcus aureus was evaluated by measuring the diameter of the inhibition zone. 6 CFU mL -1) was dropped into a petri dish and spread evenly in the culture medium using a sterile mold. The aerogel sample (10 mm in diameter) was then placed in the center of the petri dish. After incubation at 37 °C for 24 h, the diameter of the inhibition zone was then measured with a vernier caliper and recorded in millimeters.

[0051] The in vivo antibacterial activity of aerogels with different TEO@HNTs concentrations was evaluated by measuring the diameter of lesions on blueberry fruits. The fruits were divided into 6 groups: a blank control group (without aerogels) and treatment groups of Examples 1-4 and Comparative Example 1. In the Botrytis cinerea spore inoculation step, each fruit was inoculated with 10 μL of Botrytis cinerea spore suspension (10 6 CFU / mL) and inoculated 3 times. The fruit was then air-dried on a sterile table and stored in a sealed container. Gray mold was cultured at a constant temperature and stored for ten days. The changes in the diameter of the fruit lesions were observed and photographed.

[0052] 9. Blueberry storage experiment The cushioning performance of the aerogel was evaluated by a drop test. Blueberry fruits were divided into a treatment group and a control group. The treatment group was a blueberry fruit dropped from a height of 80 cm to the surface of Example 4, while the control group was a blueberry fruit dropped from a height of 80 cm to the ground. Subsequently, the blueberry fruits were stored at 25°C for 10 days, and their hardness, weight loss rate and color were measured every two days.

[0053] 10. Water resistance test Different aerogels were immersed in the same volume of room temperature distilled water and the changes in the aerogels were observed.

[0054] Results Analysis 1. The morphology and structure of aerogel Figure 1 A shows the image of the prepared aerogel and the SEM image of the microstructure, e.g. Figure 1 As shown in A, the surface of the picture placed on the rose and leaves shows that the prepared hybrid aerogel is light in weight. This is because the porous structure of the aerogel itself provides it with ultra-light weight.

[0055] Figure 1 B shows the microscopic morphology of the cross section and surface of Examples 1-4 and Comparative Example 1. The hybrid aerogel presents a porous structure. The addition of TEO@HNTs does not significantly change the microstructure of the aerogel, and the porosity of all samples is between 90% and 96%, which meets the porosity requirements of aerogel materials. The porous structure of the aerogel not only gives the material a higher specific surface area, but also allows the efficient release of active substances.

[0056] 2. Fourier infrared spectroscopy analysis FTIR spectra of TEO, HNTs, TEO@HNTs and aerogels containing different amounts of TEO@HNTs are shown in Figure 2. Figure 2 As shown. For TEO, its -1 The characteristic peaks at 3005-2808 cm-1 are attributed to the stretching vibration of -OH. -1 The characteristic peaks at 1587 and 1454 cm are attributed to CH stretching vibration. -1 The characteristic peaks at 3694cm are related to the stretching vibration of the aromatic ring. However, these characteristic peaks are not found in the spectrum of TEO@HNTs. This is because TEO is encapsulated in the cavity of HNTs and the characteristic peaks are covered by HNTs. After TEO is encapsulated by HNTs, the HNTs have a peak at 3694cm -1 The characteristic peak representing hydroxyl group shifted to 3697 cm -1 , indicating that hydrogen bonding interaction is formed between TEO and HNTs. After TEO@HNTs are loaded into aerogel, characteristic peaks of HNTs are observed in the aerogel spectrum, at 1030 cm -1 The Si-O stretching vibration is shown at 910 cm -1 The internal OH distortion at 3005-2808 cm-1 shows the successful loading of TEO@HNTs. After adding TEO@HNTs, the characteristic peaks of PGP aerogel remain unchanged, indicating that the addition of TEO@HNTs does not destroy the structural integrity of the composite aerogel. It is worth noting that in the spectrum of the aerogel, the peaks at 3005-2808 cm-1 show the internal OH distortion at 3005-2808 cm-1. -1 The characteristic peaks of TEO associated with CH stretching vibration were re-observed, indicating that part of TEO was released from HNTs during the aerogel preparation process.

[0057] X-ray diffraction analysis The crystal structures of TEO, HNTs, TEO@HNTs and Examples 1-4 and Comparative Example 1 are as follows Figure 3 shown. Figure 3 In A, the absorption peak at 14.6° in the diffraction spectrum of TEO is more obvious, but it disappears in the spectrum of TEO@HNTs. This result is consistent with the results of FTIR, which indicates that TEO is encapsulated in the cavity of HNTs. With the addition of TEO@HNTs, its characteristic peaks (2 θ =12.0°, 35.0°, 35.4°, 38.3°), which indicates that TEO@HNTs have been successfully loaded into the aerogel. In addition, the addition of TEO@HNTs improves the crystallinity of PGP aerogel.

[0058] Thermal Analysis Figure 4The TGA curves of TEO, HNTs, TEO@HNTs and aerogels containing different amounts of TEO@HNTs are shown. Detailed data are shown in Table 1. Due to the poor thermal stability of TEO, the thermal degradation rate reaches the maximum at 133.2°C, and there is no residual weight after the test. In contrast, HNTs have good thermal stability, losing only 5% of the total weight at 458.07°C, and the remaining weight is 83.97%. The thermal stability of TEO@HNTs and HNTs is comparable, which indicates that HNTs encapsulation of TEO can improve the thermal stability of TEO to a certain extent, while encapsulation does not affect the thermal stability of HNTs. As shown in the figure, the degradation curves of all aerogel samples are divided into two stages: the temperature of the first stage is below 120°C, which is the volatilization process of the water adsorbed in the aerogel; the temperature of the second stage is 200-550°C, which is the degradation process of pullulan, gelatin and PVA. In addition, with the increase of TEO@HNTs content, the temperature at which the aerogel sample reaches the maximum weight loss rate and the residual weight increase, which indicates that TEO@HNTs improve the thermal stability of PGP aerogel.

[0059] Table 1 sample <![CDATA[T 5wt％ (℃) a ]]> <![CDATA[T max (%) b ]]> <![CDATA[W red (%) c ]]> TEO 86.20 133.20 0.00 HNTs 458.07 481.07 83.97 TEO@HNTs 444.11 480.11 83.61 PGP 70.92 301.92 13.45 3%TEO@HNTsPGP 70.00 331.01 14.25 6%TEO@HNTsPGP 70.97 330.97 14.58 9%TEO@HNTsPGP 70.94 328.94 17.68 12%TEO@HNTsPGP 83.00 333.00 20.00 .

[0060] Compression performance analysis The compression performance results of Examples 1-4 and Comparative Example 1 are as follows: Figure 5 As shown, when the strain is 40%, the stress of Comparative Example 1 is 0.44MPa, while the stresses of Examples 2-4 are 0.68, 0.81, 0.84, and 0.70MPa, respectively. When the strain is 60%, the stress of Comparative Example 1 is 1.36MPa, while the stresses of Examples 2-4 are 1.85, 2.02, 1.93, and 1.83MPa, respectively. The above results show that the addition of TEO@HNTs improves the compressive strength of the aerogel. This is attributed to the fact that HNTs form a buffer area inside the aerogel, providing structural support for the porous structure. When the amount of TEO@HNTs added is too high (concentration is 12%), the compression performance of Example 4 is reduced.

[0061] Water Contact Angle (WCA) Analysis The water contact angle test results of PGP aerogels with different TEO@HNTs contents are shown in Figure 2. Figure 6As shown, the WCA of comparative example 1 is 17.91°, showing strong hydrophilic properties, which is related to the hydrophilic substrate material (such as pullulan and gelatin). When the addition amount of TEO@HNTs is 3%, 6% and 9%, the water contact angles of Examples 1-3 increase to 52.31°, 42.09° and 50.95°, respectively. When the addition amount of TEO@HNTs is 12%, the water contact angle of Example 4 is 23.87°, which may be due to the fact that TEO@HNTs increase the roughness of the aerogel surface, thereby reducing the surface potential energy and interfacial tension. And all samples show good hydrophilicity (WCA < 90°). This property can reduce the adhesion of bacteria and is beneficial to food packaging.

[0062] Antimicrobial activity analysis Food safety has always been a hot topic of global concern. According to the World Health Organization, about 6 million people are infected with foodborne diseases each year, of which about 420,000 die. Escherichia coli and Staphylococcus aureus are common pathogens of foodborne diseases. Figure 7 Table 2 shows the pictures and data related to the inhibition of aerogel on Escherichia coli and Staphylococcus aureus. No inhibition zone appeared in Comparative Example 1 and Example 1, indicating that they had no antibacterial activity. The average inhibition zone diameters of Example 2 for Escherichia coli and Staphylococcus aureus were 11.39 mm and 13.90 mm, respectively. When the addition amount of TEO@HNTs increased to 9% and 12%, respectively, in Examples 3 and 4, the inhibition diameters of aerogel on Escherichia coli and Staphylococcus aureus also increased, indicating that its antibacterial activity was enhanced. Therefore, PGP aerogel containing TEO@HNTs is expected to be used in the food packaging industry, thereby reducing food safety risks. Figure 8 As shown, the blueberry fruits in the control group were seriously infected by gray mold on the tenth day of storage. In comparative example 1, not only did the contamination of blueberry fruits not be reduced, but the invasion of gray mold was aggravated. This may be because the polysaccharides and proteins in comparative example 1 provide nutrition for the growth of Botrytis cinerea. After packaging blueberries with TEO@HNTs aerogels, the impact of blueberries on botrytis cinerea was significantly reduced. When aerogels with a TEO@HNTs concentration of 12% were used, the blueberries in Example 4 almost completely showed the presence of Staphylococcus mycelium in the inoculation wound. The above results show that the developed TEO@HNTs aerogel has broad application prospects in blueberry logistics and preservation.

[0063] Table 2

[0064] Blueberry storage experiment Blueberry fruits are more likely to have quality problems after harvest, and mechanical damage during the logistics process will also accelerate the decline of fruit quality. Therefore, the development of packaging with good shock absorption and cushioning effects is of great significance for blueberry post-harvest logistics. This study evaluated the cushioning performance of aerogels through drop tests. Fig. 9 As shown in Figure A, the hardness of blueberries subjected to drop impact decreased rapidly during storage. Compared with the control group, the aerogel as a cushioning material can effectively maintain the hardness of the fruit. In addition, Example 4 can effectively reduce the weight loss of blueberries during storage. Fig. 9 C and Fig. 9 As shown in D, during the entire storage period, when aerogel was used as a cushioning material, the L* value of blueberries was higher than that of the control group, while the b* value was lower than that of the control group, which respectively indicated that the brightness and blue color of blueberries were maintained. The above results show that the developed active aerogel material has good cushioning properties and can effectively reduce the negative impact on the quality of blueberries caused by mechanical impact during logistics.

[0065] Water resistance test The pure gel of comparative example 1 has strong hydrophilicity and is easily soluble in water. It will disintegrate in water for 3-5 minutes. When the addition amount of TEO@HNTs reaches 3%, 6% and 9%, the water resistance time of examples 1-3 is about 3 hours, more than 12 hours and more than 12 hours respectively. When the addition amount of TEO@HNTs reaches 12%, the water resistance time of example 4 is reduced to about 6 hours.

[0066] The present invention manufactures TEO@HNTsPGP composite bioaerogel by freeze-drying technology. Compared with pure PUL, gelatin and PVA bioaerogel, the PGP composite bioaerogel with TEO@HNTs added not only has a porous structure, but also improves the crystallinity, thermal stability, compression performance, surface hydrophobicity and water resistance of PGP aerogel. TEO@HNTsPGP aerogel not only has good antibacterial activity against Escherichia coli and Staphylococcus aureus, but also has a good inhibitory effect on the infection of gray mold during the storage of blueberries after harvest. At the same time, the aerogel is well buffered, which can well maintain the hardness, weight and color of blueberries, and significantly reduce the adverse effects of impact.

[0067] TEO@HNTsPGP aerogel has good biocompatibility. The prepared multifunctional aerogel has good antibacterial activity and buffering properties, and has certain potential in the research and development of packaging materials for fruits and vegetables such as blueberries. It can effectively reduce the adverse effects caused by physical and mechanical damage and microbial infection during the logistics transportation and storage of fruits and vegetables after harvest.

[0068] The above-described embodiment is only a preferred solution of the present invention and does not limit the present invention in any form. There are other variations and modifications without exceeding the technical solution described in the claims.

Claims

1. A shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel for fruit and vegetable packaging, characterized in that: Prepared by the following method: S1: Thyme essential oil was embedded into halloysite nanotubes to form TEO@HNTs; S2: mixing pullulan, gelatin and polyvinyl alcohol to form a pullulan / gelatin / polyvinyl alcohol mixed solution; S3: TEO@HNTs were mixed with a pullulan / gelatin / polyvinyl alcohol mixed solution and freeze-dried to form a shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel.

2. The shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel according to claim 1, characterized in that: The specific steps of S1 are as follows: (1) 4-6 g of halloysite nanotubes, 8-12 g of thyme essential oil and 12-18 mL of ethanol solution are uniformly mixed, followed by ultrasonic treatment and vacuum degassing to obtain a mixture of halloysite nanotubes and thyme essential oil; (2) centrifuging the mixture of halloysite nanotubes and thyme essential oil, removing the supernatant, and collecting the precipitate; (3) adding an equal amount of ethanol solution as that in step (1) to the precipitate obtained in step (2), mixing, centrifuging, removing the supernatant, and collecting the precipitate; (4) Adding an equal amount of ethanol solution as that in step (1) to the precipitate obtained in step (3), mixing the mixture, centrifuging, removing the supernatant, collecting the precipitate, and freeze-drying the mixture to obtain TEO@HNTs.

3. The shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel according to claim 2, characterized in that: The mass concentration of the ethanol solution is 60-70%.

4. The shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel according to claim 1, characterized in that: S2 is specifically as follows: 4-5 grams of polyvinyl alcohol is added to 100 mL of deionized water, heated to 85-90° C. and stirred to obtain a polyvinyl alcohol solution, cooled to 40-45° C., 4-5 grams of pullulan and 4-5 grams of gelatin are added, stirred and mixed to obtain a pullulan / gelatin / polyvinyl alcohol mixed solution.

5. The shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel according to claim 1, characterized in that: In S3, the mass percentage ratio of TEO@HNTs to pullulan / gelatin / polyvinyl alcohol mixed solution is: TEO@HNTs 3-12%, pullulan / gelatin / polyvinyl alcohol mixed solution 88-97%, and the total is 100%.

6. The shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel according to claim 1, characterized in that: The fruit or vegetable is one of blueberry, strawberry, kiwi fruit, grape, tomato and cucumber.

7. Use of the shock-absorbing and antibacterial pullulan / gelatin / polyvinyl alcohol aerogel as claimed in claim 1 as a packaging material for preserving fruits and vegetables.

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