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Biodegradable starch-based packaging films incorporated with polyurethane-encapsulated essential-oil microcapsules for sustained food preservation Wei Wanga, Weiwei Zhanga, Lin Lia, Weijun Dengb, Ming Liua,*, Jing Hu, Professora,* aSchool of Perfume and Aroma Technology, Shanghai Institute of Technology, 201418 Shanghai, China bSchool of Chemical and Environmental Engineering, Shanghai Institute of Technology, 201418 Shanghai, China ARTICLE INFO Keywords: Polyurethane microcapsules Essential oils Starch-based packaging films Food preservation ABSTRACT Novel starch-based packaging films with sustained antibacterial activity were successfully made by incorporating polyurethane-encapsulated essential-oil microcapsules (EOs@PU) as an alternative synthetic preservative for food preservation. Herein, three essential oils (EOs) were blended to make composite essential oils with a more harmonious aroma and higher antibacterial ability and encapsulated into polyurethane (PU) to form EOs@PU microcapsules based on interfacial polymerization. The morphology of the constructed EOs@PU microcapsules was regular and uniform with an average size of approximately 3 μm, thus enabling high loading capacity (59.01 %). As such, we further integrated the obtained EOs@PU microcapsules into potato starch to prepare food packaging films for sustained food preservation. Consequently, the prepared starch-based packaging films incorporated with EOs@PU microcapsules had an excellent UV blocking rate (>90 %) and low cell toxicity. Notably, the long-term release of EOs@PU microcapsules gave the packaging films a sustained antibacterial ability, prolonging the shelf life of fresh blueberries and raspberries at 25 ◦C (>7 days). Furthermore, the biodegradation rate of food packaging films cultured with natural soil was 95 % after 8 days, clarifying the excellent biodegradability of the packaging films for environmental protection. As demonstrated, the biode-gradable packaging films provided a natural and safe strategy for food preservation. 1.Introduction Food spoilage during storage and transportation is exceptionally intractable, and can lead to tremendous economic losses and severe health hazards . Food packaging films could mitigate mechanical damage but fail to suppress bacterial growth. Therefore, it is necessary to add extra preservatives into packaging films to enhance their anti-bacterial properties for prolonged shelf life and improved food quality. However, the safety hazards of synthetic antibacterial preservatives pose a risk to the health and safety of consumers, thus leading re-searchers to focus on alternatives such as natural preservatives [2,3]. In particular, essential oils (EOs), possessing excellent antibacterial prop-erties and being naturally safe, have been utilized in packaging films, making them a potential alternate synthetic preservative [4–6]. How-ever, using free EOs directly weakened the preservation ability because the active ingredients of EOs suffered high volatility during a short period and extreme instability to ambient conditions. Furthermore, the heavy odor of free EOs could surpass the acceptable sensory quality of protected foods [7–9]. With these problems in mind, encapsulation technology was considered to eliminate the aforementioned shortcom-ings. This technology is capable of encapsulating EOs to form capsules separating the ambient conditions and even controlling their sustained release. Currently, EO microcapsules have been used for food preser-vation [10,11]. Hasheminejad reported the effects of chitosan-based EO nanocapsules on the preservation of pomegranate arils, which showed that the effectiveness of EO capsules is obviously better than that of free EO . Chang prepared EO microcapsules and tested them with lettuce at 20 ◦C to examine the preservative effect, which displayed no obvious color difference after 5 days, thus indicating an excellent antimicrobial effect . Therefore, it is important to integrate EO capsules into fresh foods to increase their shelf life. Notably, the methods for adding EO capsules to fresh foods are also critical for food quality and safety. Among them, packaging films are able to prevent food pollution and extend shelf life by limiting the *Corresponding authors. E-mail addresses: firstname.lastname@example.org (M. Liu), email@example.com (J. Hu). Contents lists available at ScienceDirect International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac https://doi.org/10.1016/j.ijbiomac.2023.123889 Received 1 December 2022; Received in revised form 24 February 2023; Accepted 26 February 2023
substance exchange between food and the environment . Wang et al.  prepared composite films loaded with ginger essential oil with long-lasting antioxidant capacity and antibacterial properties and revealed the adverse effect of these films on the environment. However, their actual ability to keep food fresh needs further verification. Therefore, there is still a great challenge to prepare safe, effective, and biodegradable packaging films incorporated with EO capsules possess-ing high stability with the ability to preserve food. Potato starch has been widely utilized as a film material due to its high biodegradability, high safety, and cost-effectiveness, making it promising and practical for food preservation . Furthermore, the polyurethane-based (PU) polymer synthesized by the TDI trimer (L75) possesses high mechanical properties and thermal stability due to all of its benzene rings, and a high degree of cross-linking due to all of its CNO groups. Hence, PU is an ideal polymer to encapsulate EOs with antimicrobial activity and improve their stability . In this study, to obtain a more harmonious aroma and higher anti-bacterial effect , we chose tea tree essential oil, lavender essential oil, and perilla leaf oil to make composite EOs, and then encapsulated them into PU microcapsules to ensure loading capacity [17–22] and UV blocking . Subsequently, biocompatible and cost-effective [24,25] potato starch was utilized to incorporate EOs@PU microcapsules (Scheme 1). Consequently, we successfully made biodegradable and effective food packaging films incorporated with EOs@PU microcap-sules with sustained slow-release, advanced UV blocking, and high stability for fresh food preservation. 2.Experimental section 2.1.Materials Tea tree essential oil, lavender essential oil, and perilla leaf oil were provided by Jiangxi Senhai Natural Plant Oil Co., Ltd. (Jiangxi, China). Arabic gum (pharmaceutical grade) and potato starch powder were obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Xylitol (purity >99 %), glycerol, and absolute ethanol were provided by Shanghai Titan Technology Co., Ltd. (Shanghai, China). In addition, 2,4-Toluene diisocyanate tripolymer (L75) was supplied by Bayer Co., Ltd. (Shanghai, China). Escherichia coli (ATCC 8739) and Staphylococcus aureus (ATCC 6538) strains were pur-chased from the China General Microbiological Culture Collection Center (Beijing, China). Agar powder was supplied by Shanghai Yuanye Bio-Technology Co., Ltd. (Shanghai, China). Lennox Broth (LB) was supplied by Qingdao Hope Bio-Technology Co., Ltd. (Qingdao, China). Mouse embryonic fibroblast cells (NIH-3 T3 cells) were purchased from Icell Bioscience Inc., Shanghai (Shanghai, China). Deionized water was made by Shanghai Institute of Technology. 2.2.Preparation of the EOs@PU microcapsules First, EOs (0.3 g, tea tree essential oil: lavender essential oil: perilla leaf oil =50:3:3 (w/w)), L75 (0.2 g), and Arabic gum aqueous solution (4 mL, 5 wt%) were mixed and stirred evenly in the oil phase. The mixture was stirred at 500 r min 1 for 10 min and then treated with an ultrasonic cell pulverizer for 10 min to obtain an oil/water emulsion. Afterward, the emulsion was transferred to a 25 mL flask, stirred at 50 ◦C, and then added to a xylitol aqueous solution (0.8 mL, 3.29 mol L 1). Then the temperature was raised to 70 ◦C for 2 h. Subsequently, the reaction solution was centrifuged, washed with water three times and lyophilized for 12 h to obtain white EOs@PU microcapsules. 2.3.Preparation of starch-based packaging films First, potato starch was dispersed in deionized water and stirred evenly at 70 ◦C for 30 min. Then, EOs@PU microcapsules were added and stirred evenly. Finally, the mixture was poured into a mold, dried at room temperature for 48 h, and then peeled to obtain the packaging films. 2.4.Characterization of materials 2.4.1.Optical morphology The optical morphology of the EOs@PU microcapsules was observed by an LW600LJT instrument (Shanghai Cewei Photoelectricity Tech-nology Co., Ltd., China) to collect photographs. 2.4.2.Dynamic light scattering The particle size and size distribution of the EOs@PU microcapsules were determined by a MASTERSIZER-3000 analyzer (Malvern Instru-ment Company, UK) to obtain the size data of the EOs@PU microcapsules. 2.4.3.Scanning electron microscopy The microscopic morphology of the microcapsules was investigated Scheme 1.EOs@PU microcapsules with high loading capacity were prepared to protect the essential oil as a natural antimicrobial agent, and subsequently, active packaging film containing EOs@PU microcapsules was made to have great UV blocking and sustained food preservation abilities. W. Wang et al.
with an S-3400 N microscope (HITACHI Company, Japan). The prepared EOs@PU microcapsules were dispersed in absolute ethanol. After ul-trasonic dispersion, the EOs@PU microcapsules were dropped onto sil-icon wafers. After drying, gold was sprayed on the EOs@PU microcapsules to be observed by using the instrument. 2.4.4.Fourier transform infrared spectroscopy Materials and films were measured by Fourier Transform Infrared (FT-IR) spectroscopy in a Nicoletttin10 spectrometer (NIKOLI Instru-ment Company, USA). All the above samples were mixed with potassium bromide for tableting, and then their FT-IR spectra were collected by the FT-IR spectrometer. Wavenumbers ranged from 4000 to 400 cm 1 with a resolution of 4 cm 1. 2.4.5.Thermogravimetric analysis The thermal stability of the corresponding samples was determined by a Q5000IR analyzer (TA Company, USA). TGA was performed by heating the specimens from 30 to 600 ◦C in nitrogen flow (20 mL min 1), and the heating rate was 10 ◦C min 1. 2.5.Loading capacity The loading capacity was determined according to the method described in the literature . The amount of EOs in the microcapsules were determined by extraction with absolute ethanol as the solvent. First, microcapsules (50 mg) were taken, and EOs were extracted by ultrasonic cell crushing treatment with 5 mL absolute ethanol every time for 10 min. The extraction process was repeated three times to ensure complete extraction. After centrifugation, the supernatant was sonicated for 10 min to ensure complete dissolution of the EOs. The supernatant was analyzed by a UV–vis spectrophotometer (UV-1900, Shimadzu In-struments (Suzhou) Co., Ltd., China), and the amount (g) of EOs in the microcapsules was obtained according to the standard curve of EOs. The extraction test was carried out 5 times to determine the average value. The maximum absorption wavelength of the ethanol solution of the EOs was 266 nm. Hence, ethanol solutions of EOs at concentrations of 0.04, 0.08, 0.12, 0.16, and 0.20 mg mL 1 were prepared and their absorbance at 266 nm was measured to fit the standard curve. The loading capacity of EOs can be calculated according to the following formula: LC(%)=W2W1×100(1) where LC(%) represents the content of the core material; W1 is the mass of the microcapsules; and W2 is the weight of the EOs extracted from broken microcapsules. 2.6.EOs release EOs@PU microcapsules (50 mg) and EOs (30 mg, with the same mass as loaded content) were placed at 25 ◦C and 50 ◦C, and samples were taken at different time intervals to test the release rate of EOs by UV–vis spectrophotometry. Furthermore, the packaging film with EOs@PU microcapsules was cut into 40 mm ×40 mm squares, placed in 50 mL of 50 % (v/v) ethanol solution in a beaker, and then stored at 25 ◦C for 11 h (samples were taken every 30 min). Finally, the EOs emission was measured at 266 nm by a UV–vis spectrophotometer. In addition, the EOs release profiles were plotted using four mathematical models [27–30]. 2.7.Antibacterial property The method of Gu et al.  was used to examine the antimicrobial properties of the EOs@PU microcapsules. The antibacterial activity of the control group, EOs, PU microcapsules, and EOs@PU microcapsules against E. coli and S. aureus was tested. According to the previous experimental method , the antibac-terial ability of the packaging films was determined by E. coli and S. aureus growth. Various kinds of active bacterial suspensions with a concentration of approximately 106 CFU mL 1 were prepared by using LB liquid medium. Then, the control samples and the packaging films were cut into small pieces of 20 ×20 ×2 mm3. Afterward, ultraviolet lamp irradiation for 30 min is used for sterilization. Then, 5 mL normal saline and 100 μL of the bacterial solution was added to the 10 mL sterilized centrifuge tube, and a membrane was added for coculture for 12 h. Sterilized LB agar medium was injected into a sterile Petri dish, and 100 μL bacterial suspension after coculture was inoculated after the culture solidified, and the bacterial liquid coated plate without coculture but with a membrane was used as a control. Plates coated with bacterial liquid were placed in an incubator and incubated at 37 ◦C for 12 h, and then the growth of colonies was observed. 2.8.Tensile testing The tensile strength and elongation at the breaking points of the packaging films were evaluated by using a universal testing machine (SUN500, CARDANO AL CAMP company, Italy) operated at a crosshead speed of 100 mm min 1. The samples were cut into an oblong shape (testing measure of 30 ×10 ×2 mm3) for tests. 2.9.Transparency and UV blocking Badges of the Shanghai Institute of Technology were placed on the packaging films with EOs@PU microcapsules, and the transparency of the films was observed. The absorbance of packaging films with EOs@PU microcapsules was determined by spectrophotometry (UV- 1800, Shimadzu Instruments (Suzhou) Co., Ltd., China) within the wavelength range of 200–900 nm. The UV blocking performance of packaging films with EOs@PU microcapsules was determined by investigating the wavelength band at 200–400 nm. 2.10.Sustained preservation The EOs@PU microcapsules with different contents incorporated into packaging films were used to preserve blueberries, raspberries, and grapes at room temperature. The change in perishable fruits was observed during storage to compare the antibacterial ability of the food packaging films with different contents of EOs@PU microcapsules. 2.11.Cytotoxicity analysis The Cell Counting Kit-8 (CCK-8) assay  was used to detect the changes in cell viability when NIH-3T3 cells were cocultured with packaging films with different contents of EOs@PU microcapsules for 24 h, to determine their cytotoxicity in vitro. First, the NIH-3T3 cells in the logarithmic growth phase were counted, and the cell concentration was adjusted. NIH-3T3 cells were inoculated into a 96-well plate at 4 ×104 well 1 and cultured overnight in a constant temperature incubator. Afterward, the samples were sterilized with ultraviolet light for 30 min and extracted with 1 mL complete medium at 37 ◦C for 24 h. A complete medium (100 μL well 1) was added to the control group. The sample group was added to 100 μL well 1 leaching solution with 1 cm2 mL 1 packaging films and cultured for 24 h. Finally, the medium was removed. The wells were washed with PBS three times, and a culture medium containing 10 % CCK-8, 5 % CO2 was added at 100 μL well 1 and kept at 37 ◦C in a constant temperature incubator for 2 h. The absorbance at 450 nm was detected by a microplate reader (TECAN, model: SPARK 10 M). 2.12.Degradability experiment Degradability was determined by measuring the weight loss of films W. Wang et al.
buried in the soil. The packaging films with EOs@PU microcapsules were cut into small pieces of 50 ×20 ×2 mm3, weighed, tied to a corner with thread, and buried approximately 15 cm below the soil surface. Soil humidity was adjusted by spraying tap water on the soil once a day, washed with distilled water several times to remove soil particles, and dried at room temperature until they reached a constant weight. The weight loss was then calculated using the following formula: Weightloss(%)=W0 WtW0×100(2) where W0 is the initial weight of the sample; and Wt is the dry weight of the sample after degradation in soil. 3.Results and discussion 3.1.Characterization of EOs@PU microcapsules Tea tree essential oil, lavender essential oil, and perilla leaf oil were blended in ratio, making the aroma harmonious, to prepare composite essential oils (Fig. S1). In addition, the composite essential oils had a synergistic antibacterial effect as shown in Fig. S2, which is consistent with literature reports. Then, the composite EOs, L75, and Arabic gum were mixed to form the oil phase, where L75 acted as the reactive monomer and Arabic gum acted as the emulsifier. Afterward, the aqueous solution of xylitol as the water phase was slowly added to the above oil phase, which allowed the CNO groups of L75 and the OH groups of xylitol to react at the oil/water interface. In particular, more xylitol was added to allow the CNO groups to react completely. After centrifugation, the obtained product was washed three times with water to remove excess xylitol. Finally, the EOs@PU microcapsules were ob-tained by lyophilization. First, to observe the overall morphology and size of the EOs@PU microcapsules, the suspension of EOs@PU microcapsules was dispersed in water and observed by optical microscopy and dynamic light scattering. As shown in Fig. 1a and b, the EOs@PU microcapsules appeared as round particles with good dispersion and the particle size was approximately 2.73 ±0.65 μm. Subsequently, to further observe the EOs@PU microcapsules morphology clear with more detail, the EOs@PU microcapsules were observed by SEM, in which they had a regular spherical shape and smooth surface, as presented in Fig. 1c. These results suggested that the obtained EOs@PU microcapsules have regular and uniform morphology with an average size of approximately 3 μm. The structure of the prepared microcapsules was determined by FT- IR to clarify whether the EOs were successfully encapsulated into PU capsules. As shown in Fig. 1d, the absorption band at 3448 cm 1 cor-responds to O–H vibration, while the strong absorption bands at 2962 cm 1 and 2876 cm 1 come from C–H and -CH2- as well as asymmetric -CH(CH3) stretching vibration and -CH(CH2)- symmetric and asym-metric stretching vibration . The strong bands at 1447 cm 1 and 1377 cm 1 are caused by -CH2 and -CH3 vibrations , and the vi-bration of the C–C bond belonging to the hydrocarbon skeleton appears below 1300 cm 1. The strong absorption peak at 3389 cm 1 belongs to the stretching vibration of N–H, which means that there was a strong hydrogen bond on the capsule wall. The absorption peak at 2273 cm 1 is the characteristic peak of the CNO group. In addition, the content of the CNO group was determined by toluene-dibutylamine titration. The results showed that L75 was completely involved in the reaction and no harmful substance remained (Table S1). There was no CNO group absorption band at 2273 cm 1 in the EOs@PU microcapsules, which indicated that the polymerization of L75 and xylitol was completed and a polyurethane capsule wall was formed. The outcome of FI-IR confirmed that the EOs were successfully incorporated into PU microcapsules. To determine whether the thermal stability of the EOs was improved after encapsulation, the thermal stability of the EOs, PU microcapsules, and EOs@PU microcapsules was analyzed using thermogravimetric analysis (TGA). The results clearly indicate that the EOs decomposed before 92.5 ◦C (Fig. 1e), which indicates their exceedingly poor thermal Fig. 1.Morphology of the constructed EOs@PU microcapsules. (a) Optical morphology of aqueous EOs@PU microcapsules (The scale bar is 10 μm); (b) Particle size distribution of EOs@PU microcapsules; (c) SEM photograph of EOs@PU microcapsules; (d) FT-IR spectra of EOs, PU microcapsules, and EOs@PU microcapsules; (e, f) TGA and DTG curves of EOs, PU microcapsules, and EOs@PU microcapsules. Note: The PU microcapsules successfully encapsulate the EOs with regular and uniform morphology improved stability, and high loading capacity. W. Wang et al.
stability. The main weight loss of PU microcapsules was 220–350 ◦C (Fig. 1f), indicating the decomposition of the microcapsule shell. The weight loss of the EOs@PU microcapsules was 150–200 ◦C, which cor-responded to the loss of the compound EOs. Additionally, the loading capacity was calculated from the ratio of the mass of the EOs to the mass of the EOs@PU microcapsules as 59.01 %. TGA demonstrated that PU microcapsules are capable of improving the thermal stability of free EOs. To further quantify the loading capacity of the EOs@PU microcap-sules, a UV–vis spectrophotometer was used to determine the EOs con-tent. First, the absorbance of free EOs in the ethanol solution was obtained to establish the standard curve illustrated in Fig. S3. Subse-quently, the EOs in the PU microcapsules were extracted with ethanol, and the calculated LC by the standard curve was 63.40 %, which was similar to the core content calculated by TGA of 59.01 %, which might be due to the EOs@PU microcapsules requiring a freeze-drying process before TGA, resulting in the loss of some EOs. The value of the loading capacity of PU microcapsules was much higher than that reported in previous literature [36,37]. The results clearly showed that the PU mi-crocapsules successfully encapsulated the EOs with high loading ca-pacity (Fig. S4). 3.2.Sustained release and antibacterial properties of EOs@PU microcapsules The release rates of free EOs and EOs@PU microcapsules were investigated at 25 ◦C and 50 ◦C, respectively. Obviously, EOs volatilized Fig. 2.Sustained-release behaviors and long-term antibacterial properties of EOs@PU microcapsules. (a) Release of EOs and EOs@PU microcapsules at different temperatures; (b) Photographs and diameter (mm) of E. coli inhibition circles treated with EOs and EOs@PU microcapsules for 12 h, 24 h, 48 h; (c) Photographs and diameter (mm) of S. aureus inhibition circles treated with EOs and EOs@PU microcapsules for 12 h, 24 h, 48 h. Note: EOs@PU microcapsules have sustained release and excellent long- term antibacterial properties, making them potentially applicable in food pres-ervation techniques. W. Wang et al.
after being placed at 50 ◦C for 24 h and were completely volatilized after being placed at 25 ◦C for 96 h (Fig. 2a), because free EOs are mainly composed of volatile compounds. Comparably, sustained EOs release was achieved due to the powerful protective effect of the EOs@PU mi-crocapsules. EOs in the EOs@PU microcapsules gradually decreased over time. After 6.5 d, the EOs content of PU microcapsules calculated by TGA remained at 57.82 % and 33.91 % at 25 ◦C and 50 ◦C, respec-tively, which verified that the EOs@PU microcapsules are capable of sustained EOs release. Furthermore, the EOs release profiles were plotted using four mathematical models, where the first-order model had a better-fit degree than the others (Fig. S5), as shown in Table S2. The first-order release model is the basic release model for sustained- release materials with a constant half-life. Consequently, the dynamic release of EOs depended on their concentration in the solution. E. coli and S. aureus are highly insidious to contaminate fresh foods and endanger human health. Therefore, it is of great significance to investigate the antimicrobial properties of the microcapsules. Conse-quently, there were no antibacterial circles in the control and PU microcapsule groups, while the diameter of the antibacterial circle of the EOs@PU microcapsules decreased slightly with time. During the experiment, the average diameter of the E. coli inhibition circle of the EOs@PU microcapsules was at least 3.61 mm larger than that of the EOs, indicating that the EOs@PU microcapsules had a significant and long- lasting antibacterial effect on E. coli (Fig. 2b). In addition, the average S. aureus inhibition circle diameter of the EOs@PU microcapsules before 24 h was smaller than that of the EOs. However, after incubation for 48 h, the diameter of the S.aureus inhibition circle of the EOs@PU micro-capsules was 3.16 mm larger than that of the EOs (Fig. 2c). The above analysis indicated that the EOs@PU microcapsules had excellent long- term antibacterial properties and sustained-release behaviors, which made them promising to be used for food preservation. 3.3.Characterization of food packaging films with EOs@PU microcapsules After the potato starch solution was gelatinized, glycerin was added as a plasticizer, followed by incorporation of the EOs@PU microcap-sules. Then, the mixture was poured into the mold to prepare the packaging films. The packaging films with EOs@PU microcapsules are presented in Fig. 3a. All packaging films have different degrees of transparency, and with the increase in the content of EOs@PU micro-capsules, the transparency of the films decreases slightly . Overall, the packaging films with EOs@PU microcapsules have high trans-parency, making them have potential for commercial use. Certain mechanical properties can effectively maintain the integrity of packaging films to bear the pressure generated during storage and transportation. The film thickness is between 0.20 and 0.27 mm (Table S3). The results of the tensile strength of the packaging films showed packaging films with EOs@PU microcapsules had the lowest tensile strength of 1.67 MPa (Fig. 3b), which is far higher than the tensile strength of reported food packaging films (tensile strength of approxi-mately 36 kPa) . With the increase in the concentration of EOs@PU microcapsules in the packaging films, their strength gradually decreases, caused by the increase in additive content . Furthermore, the out-comes of the experiment showed that with the increase in EOs@PU microcapsules in packaging films, the values of elongation at the breaking point and fracture strength decreased (Table S3), which was due to the increase in pore size of the films, leading to possible rupture points . Test analysis revealed that the packaging films have good mechanical properties. It is obvious that the infrared spectra of packaging films and pack-aging films with EOs@PU microcapsules were basically the same (Fig. 3c). There was a strong absorption peak at approximately 3344 cm 1, which was the stretching vibration of -OH, and the antisymmetric stretching vibration peak of -CH2 was at approximately 2934 cm 1. Additionally, 1650 cm 1 represented the characteristic peak of tightly Fig. 3.(a) Appearance of packaging films with different contents of EOs@PU microcapsules; (c) Thickness of packaging films with different contents of EOs@PU microcapsules; (c) FT-IR spectra of EOs@PU microcapsules, packaging films, and packaging films with EOs@PU microcapsules; (d, e) TGA and DTG curves of EOs@PU microcapsules, packaging films and packaging films with EOs@PU microcapsules. Note: EOs@PU microcapsules were successfully incorporated into packaging films with high transparency and good mechanical properties. W. Wang et al.
bound water in starch, 1459 cm 1 represented the bending vibration of -CH2 in -CH2OH and 1352 cm 1 represented the bending vibration of -CH. Moreover, 1200–700 cm 1 was the absorption peak of poly-saccharides, and 1150 cm 1 was the stretching vibration of C–O and C–C bonds. Together, 1019 cm 1 was mainly the stretching vibration of the C–O bond and the bending vibration of C-OH. The packaging films contained EOs@PU microcapsules, so there were characteristic peaks of PU microcapsules. The absorption band at 2877 cm 1 came from asymmetric -CH(CH3) stretching vibration and -CH(CH2)- symmetric and asymmetric stretching vibration (Gallart-Mateu, Largo-Arango, Larkman, Garrigues, & de la Guardia, 2018). The characteristic peak near 1714 cm 1 was related to the C––O stretching vibration in the EOs and PU microcapsules, and the characteristic peaks near 1540 cm 1 and 1225 cm 1 were related to the urethane bond. The vibration of the C–C bond belonging to the hydrocarbon skeleton appeared below 1300 cm 1. From the above analysis, it can be seen that packaging films with EOs@PU microcapsules were successfully prepared and that the EOs@PU microcapsules were not destroyed in the packaging film. It was obvious that packaging films and packaging films with EOs@PU microcapsules had the same initial decomposition tempera-ture, and there were mainly three weight loss stages (Fig. 3d). In the first stage from 30 ◦C to 89 ◦C, packaging films lost 6.47 % of their weight, while packaging films with EOs@PU microcapsules lost 5.64 %. The decomposition temperatures of the two films were almost the same during the transition from the first stage to the second stage because the quality change in this stage was due to the evaporation of bound water in the films. In the second stage, the weight loss of the packaging films was significant from 89 ◦C to 240 ◦C, with a 53.75 % decrease in residual mass from 93.53 % to 39.78 %. Meanwhile, packaging films with EOs@PU microcapsules decreased in residual mass by 51.11 % from 94.36 % to 43.25 % from 89 ◦C to 210 ◦C. The weight loss of packaging films with EOs@PU microcapsules at this stage was mainly due to the weight loss of partial starch and the loss of essential oils in microcap-sules. In the third stage, packaging films lost weight from 240 ◦C to 339 ◦C, and continued to lose 24.23 %, while the weight of packaging films with EOs@PU microcapsules decreased by 36.26 % from 210 ◦C to 350 ◦C. The mass reduction rate and decomposition temperature of the packaging films were less than those of the packaging films with EOs@PU microcapsules, which was due to the high weight loss tem-perature of the PU microcapsules. It can be observed from Fig. 3e that the first decomposition rate peak of the two films was small, the second stage rate peak was wide and short, and the third decomposition rate peak was narrow and high, indicating that although the weight loss ratio of the films was large in the second stage, the rate was slow, while the maximum weight loss rate occurred in the third stage. The reason was that the decomposition rate of the starch films was more thorough in the third stage after it was decomposed into many short chains in the second stage, while the microcapsules in the packaging films made its decomposition rate lower. 3.4.UV blocking ability and antibacterial properties of packaging films with EOs@PU microcapsules When the content of EOs@PU microcapsules in packaging films was ≥2 %, the UV absorption rate was ≥90 % (Fig. 4a), which indicated that the packaging films with EOs@PU microcapsules had excellent UV blocking ability. The addition of EOs@PU microcapsules into the packaging films has a blocking effect on ultraviolet light (200–400 nm), which is consistent with the results reported by Arrieta et al.  Packaging films have an excellent UV blocking ability, which prevents the oxidative breakdown of the active compounds in EOs . The inhibition of E. coli and S. aureus by packaging films with EOs@PU microcapsules is illustrated in Fig. 4b. Obviously, it can be seen that the higher the content of EOs@PU microcapsules, the stronger the antibacterial ability of packaging films. When the content of EOs@PU microcapsules in packaging films was 2 %, the colony number of the two bacteria was obviously reduced. Meanwhile, when the content of EOs@PU microcapsules in packaging films was ≥8 %, there was almost no inhibition. The main reason was that the higher content of the EOs@PU microcapsules, the more EOs were released from the films, which significantly changed the permeability of the cell membrane and killed the bacteria , thus making packaging films with EOs@PU microcapsules have an antibacterial effect. Similar to the release of EOs in PU microcapsules, four mathematical models were used to draw the release curve of EOs in the packaging film, of which the first-order model had a better fit than other models (Fig. S6). 3.5.Sustained preservation of perishable fruit Blueberries and raspberries were used for freshness experiments as perishable fruits. With the extension of storage time, the water content of the fruits gradually decreased, the surface state gradually deterio-rated, and more antibacterial agents were released from packaging films . It may be that microcapsule packaging films can effectively delay the ripening process of fruits . The fruits remained fresher with increasing EO@PU microcapsule content in the films. The packaging films with 8 % content of EOs@PU microcapsules could prolong the shelf life of blueberries and raspberries for 7 d (Fig. 5) at room tem-perature, therefore, packaging films incorporated with EOs@PU mi-crocapsules have a huge positive effect on delaying perishable fruit decay. Simultaneously, the grapes did not decay after 10 d storage (Fig. S7) at room temperature. Moreover, the weight loss test (Fig. S8a), sensory evaluation (Fig. S8b), and biological evaluation (Fig. S8c and Fig. S8d) during grape storage proved that 8 % content of EOs@PU microcapsules in the packaging film had the best effect on maintaining the freshness of the fruit. Using packaging films to preserve fruit at room Fig. 4.Excellent UV blocking ability and antibacterial properties of packaging films with EOs@PU microcapsules is shown. (a) UV–vis transmittance spectra of packaging films with different contents of EOs@PU microcapsules; (b) Antibacterial properties of packaging films with different contents of EOs@PU microcapsules. Note: Packaging films with EOs@PU microcapsules have excellent UV blocking ability and good antibacterial properties. W. Wang et al.
temperature is better than that reported in the literature [46,47] and even comparable to the effectiveness of preserving fruit by refrigeration. 3.6.Biosecurity and biodegradation of packaging films with EOs@PU microcapsules To verify that the packaging films prepared in this work are harmless to humans, cytotoxicity experiments were conducted by the CCK-8 method to test the effect of packaging films on the cell activity and proliferation of NIH-3 T3 cells. With varying contents of EOs@PU mi-crocapsules in packaging films, cell viability was ≥60 % as presented in Fig. 6a. Therefore, packaging films with EOs@PU microcapsules are considered to be biosecure. Furthermore, to verify the environmental friendliness of the packaging films, degradation experiments were con-ducted. The outdoor natural soil embedding of packaging films was measured by weight loss of the films over 8 d as illustrated in Fig. 6b. The mass loss of all films was approximately 50 % on the second day of degradation, but not >75 % on the fourth day of degradation, therefore the degradation rate of packaging films slowed down overtime. This may be due to the interaction of amylose and amylopectin in the gelatini-zation process of starch films to form a three-dimensional network structure. In the early stage of degradation in soil, the network structure was destroyed, and in the later stage, each starch segment was degraded. Although the degradation rate of packaging films with EOs@PU mi-crocapsules slows down with time, the mass loss of all kinds of pack-aging films exceed 95 % on the 8th day of degradation, which is much higher than that of biodegradable food packaging materials previously reported.25 This shows that packaging films with EOs@PU microcap-sules have good soil degradability, making them a potential sustainable material. 4.Conclusions In this work, EOs@PU microcapsules were successfully prepared by interfacial polymerization and subsequently incorporated into pack-aging films. The obtained EOs@PU microcapsules were morphologically uniform and had regular spherical shape with an average size of approximately 3 μm, thus giving the capsules a high loading capacity and sustained release behavior. In particular, both the EOs@PU micro-capsules and the packaging films with EOs@PU microcapsules had excellent and prolonged antibacterial activity against E. coli and S. aureus, thus giving them the ability to preserve perishable fruit. Moreover, the packaging films with EOs@PU microcapsules have good mechanical properties, advanced UV blocking, favorable biosecurity, and excellent biodegradability for preservation of fresh fruit. As demonstrated, it is highly applicable to incorporate natural antibacterial essential oil delivery systems with sustainable packaging films as a sustainable food preservation technique commercially. In addition, it provides a general approach to fabricating other active ingredient de-livery systems with sustained release performance by interfacial poly-merization. However, there is no current regulation of polyurethane as a food packaging material. Fig. 5.Sustained preservation of blueberries and raspberries treated with packaging films with no treatment, treatment with 0 % EOs@PU microcapsules and packaging films with 8 % EOs@PU microcapsules for 0 d, 1 d, 2 d, 3 d, 5 d, 7 d. Note: Packaging films with EOs@PU microcapsules have low cytotoxicity and sustained fruit preservation. Fig. 6.Biosecurity and biodegradation of packaging films incorporated with EOs@PU microcapsules. (a) Effect of EOs@PU microcapsule content in packaging films on NIH-3 T3 cell viability (**p ≤0.01); (b) Soil biodegradability of packaging films with different contents of EOs@PU microcapsules. Note: Packaging films with EOs@PU microcapsules have excellent biodegradability as well as biosafety in the environment. W. Wang et al.
CRediT authorship contribution statement Wei Wang: Conceptualization, Methodology, Investigation, Soft-ware, Formal analysis, Writing – original draft, Writing – review & editing. Weiwei Zhang: Methodology, Investigation, Validation. Lin Li: Resources, Supervision. Weijun Deng: Resources, Supervision. Ming Liu: Supervision, Investigation, Visualization. Jing Hu: Resources, Su-pervision, Funding acquisition, Conceptualization, Data curation, Writing – review & editing. Declaration of competing interest All authors have seen and approved the final version of the manu-script being submitted. We warrant that the article is the authors’ orig-inal work, hasn’t received prior publication and isn’t under consideration for publication elsewhere. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare no competing financial interest. 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