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Scientific Reports volume 14, Article number: 28802 (2024 ) Cite this article packing unit
The aim of this research is to develop a composite antibacterial film for food packaging using low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), polyethylene-graft-maleic anhydride (PE-g-MA), and incorporating chitosan (CS) particles onto which ajwan essential oil (AEO) is adsorbed. The films were characterized using various techniques, including Fourier-transform infrared spectroscopy (FTIR), Gas chromatography/mass spectroscopy (GC-MS), X-ray diffraction (XRD), tensile testing, oxygen transmission rate (OTR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and antibacterial assays. FTIR results confirmed the presence of CS and/or AEO in the films. Mechanical testing indicated a decrease in tensile strength and an increase in elongation at break with the addition of AEO, while CS reduced elongation. In the sample containing only 7.5% chitosan (PE-7.5-0), the oxygen permeability was reduced to 910 cm2/m2·day·bar due to the presence of CS. However, the inclusion of AEO in the sample (PE-0-10) increased the oxygen permeability to 2200 cm2/m2·day·bar, which is higher than that of the control sample (PE-0-0) with an oxygen permeability of 1680 cm2/m2·day·bar. The antibacterial activity results demonstrated a synergistic inhibitory effect of CS and AEO. Data from GC-MS and inhibition zone (IZ) tests indicated that while chitosan alone does not exhibit significant antibacterial activity due to its incorporation in the bulk of the film, its combination with AEO enhances antibacterial efficacy. This enhancement occurs as the oil is adsorbed and protected from evaporation during the film formation process. Overall, the findings from this research suggest that the composite film PE-7.5-10, which possesses suitable mechanical properties and significant antibacterial activity, could be an effective candidate for food packaging applications.
Food products are primarily classified as perishable items, which have a short shelf life. Furthermore, they are susceptible to chemical reactions and bacterial contamination, and their quality may deteriorate during production, processing, transportation, and storage stages1. Alongside the growing demand for healthy, minimally processed, and high-quality foods, these challenges drive dynamic changes in the food packaging industry2.
Among the polymers used in food packaging, polyolefins such as polyethylene (PE) and polypropylene (PP) are preferred for several reasons, including low cost, flexibility, chemical inertness, recyclability, good processability, non-toxicity, and biocompatibility. Since these polymers have excellent sealing properties, they are also used as a bonding layer on aluminum foil when applying different polymers to the foil. Although low-density polyethylene (LDPE) is a good barrier against moisture, it is relatively permeable to oxygen and is considered a weak barrier against odors. Polyethylene also cannot limit the migration of microbes from the paper or mineral-coating pigments on the paper to the packaged food. These weak features make the use of this polymer challenging in food packaging applications and necessitate improvements. To enhance the mechanical properties of LDPE, it is blended with linear low-density polyethylene (LLDPE), enabling it to better withstand the strain of transportation3,4.
Passive traditional food packaging techniques involve the use of a covering material that serves merely as a barrier to delay harmful environmental effects, without guaranteeing food protection and quality5. Consequently, a shift from the conventional approach of minimizing interaction between food and packaging materials to a novel perspective that emphasizes the beneficial effects of certain food-package interactions appears inevitable.
A recent alternative with promising potential in this field is active packaging, which has the ability to alter the conditions of the wrapped products. This approach can enhance the preservation of properties, improve safety, prolong shelf life, and protect the quality of foods6. Among the various types of active packaging, antibacterial variants represent some of the most innovative concepts in the food industry. These systems combine the protective features of antibacterial agents with preservative characteristics for food7,8,9.
The direct application of antibacterial agents in foods can have a significant antibacterial effect due to the rapid diffusion of the antibacterial component into the product. However, adverse effects such as food denaturation and off-flavors may also arise. Therefore, incorporating antibacterial agents within the packaging film matrix could facilitate a gradual and sustained release over an extended period, thereby maintaining the antibacterial effect for a longer duration9,10.
Another solution for achieving the desired antibacterial activity involves indirect contact between food and antibacterial agents, specifically through the direct use of antibacterial compounds in film packaging4,11,12,13,14. Antibacterial agents such as thymol, potassium sorbate, nisin, hexamethylenetetramine, organic acids, and chitosan/essential oils have already been evaluated for use in polyethylene food packaging systems15,16,17,18,19,20,21.
Additionally, among the indirect methods, the use of oxidizing ions, such as copper or silver ions, as antibacterial agents in food packaging systems have been assessed. However, these ions must be used judiciously due to their potential toxicity. Concurrently, the significant rise in bacterial resistance to antibiotics has led to an increasing global trend towards the use of natural additives—such as essential oils (EOs)—in the food packaging industry22. Natural extracts, including EOs and their constituents, are recognized as safe (GRAS) by the U.S. Food and Drug Administration (FDA)23. They have also been classified as flavorings under European Decision 2002/113/EC24.
In this context, both packaging producers and consumers view the impregnation of polymeric plastic films with these agents as an effective method to prevent bacterial contamination of food.
Generally, there are various natural chemical compounds capable of antibacterial activity25,26, including terpenoids and sesquiterpenes with diverse aliphatic hydrocarbons, aldehydes, acids, and alcohols. Thymol is the main constituent of Trachyspermum ammi, also known as Ajwain, and can be extracted at a concentration between 35 and 60% from the plant27. The non-thymol fraction includes para-cymene, gamma-terpinene, alpha-pinene, carvacrol, and other minor constituents28. Thymol is a predominant monoterpene phenolic compound that has been used as a food preservative for many years. With a wide range of antibacterial activity, thymol has great potential for enhancing the safety of perishable foods and prolonging their shelf life29. Thymol and carvacrol, which are isomers, are hydrophobic compounds that can easily dissolve in the hydrophobic domain of the bacterial cytoplasmic membrane and the lipid acyl chains, leading to a noticeable effect on the membrane’s functional and structural properties. Although many studies have demonstrated the antibacterial activity of essential oils (EOs) and their active components against a wide range of pathogenic bacteria30,31,32,33, only a few articles have focused on the use of EOs as additives in polymeric plastic films for food packaging34,35,36.
Min et al. used thymol as an antibacterial agent and introduced a composite of THY@PCN/PUL/PVA nanofibers, claiming that the composite has the ability to sustain the release of thymol, providing dual-antibacterial activity to prolong the shelf life of fruits. However, it is unclear whether the use of pullulan in packaging is economically viable37.
Among active biopolymers, chitosan (CS) is of great interest in the food industry due to its unique properties, such as antibacterial activity, biodegradability, biocompatibility, and non-toxicity. CS, the linear and partly acetylated (1–4)-2-amino-2-deoxy-β-d-glucan, can be easily derived from chitin, the second most abundant natural polymer found in crab, shrimp, lobster, coral, jellyfish, mushroom, and fungi38,39. It has been proven that CS can adsorb oils and hydrophobic compounds on its surface40. Bi, J., et al. claimed that CS-graft-PVA film has the potential to be applied as a carrier to bind large quantities of procyanidins, preparing an active food packaging with a procyanidin encapsulation efficiency over 95% and long-term release sustainability41.
The lack of interest in this field may stem from the fact that essential oils and their compounds are sensitive to heat and shear, leading to rapid decomposition or evaporation under high processing temperatures and pressures. To address this issue, some researchers have employed innovative methods. Nestro et al. developed active films using ethylene-vinyl acetate (EVA) incorporated with 3.5% and 7% carvacrol or cinnamaldehyde. They minimized mixing time and immediately placed the material in a liquid nitrogen bath after mixing to prevent further evaporation of the antibacterial additive42.
Solano et al. produced low-density polyethylene (LDPE) films containing 1% and 4% Origanum vulgare and Thymus vulgaris using a single-screw extruder. They gradually increased the temperature profile from the feed zone to the die to protect the antibacterial additives from evaporation43. A mixture of LDPE and EVA was selected to enhance solubility and ensure partial adhesion of the essential oils within the polymer matrix. The relatively higher retention of essential oils in the film is attributed to their higher concentration and lower polarity44,45.
Guarda et al. coated corona-treated, bi-axially oriented polypropylene (PP) films with microcapsules containing thymol and carvacrol, which serve as natural antibacterial agents46.
Esmaili and Asgari introduced a method for encapsulating Carum copticum essential oil (CEO) in chitosan nanoparticles. This encapsulation was achieved through an emulsion-ionic gelation process, utilizing crosslinking agents such as pentasodium tripolyphosphate (TPP) and sodium hexametaphosphate (HMP)47.
The aim of this research is to utilize Ajwain essential oil (AEO) in polyethylene-based food packaging films as an antibacterial agent. As noted, AEO is sensitive to environmental conditions and tends to evaporate at the elevated temperatures required for preparing polyethylene films; thus, the primary challenge of this research is to mitigate this issue. We propose using surface adsorption of AEO on chitosan particles to preserve it under the harsh conditions and high temperatures of the processing, which is an efficient and straightforward method. To the best of our knowledge, no research has yet been conducted on the adsorption of AEO molecules onto chitosan particles in antibacterial food packaging systems.
To enhance the distribution of chitosan chains within the LDPE-LLDPE matrix, polyethylene-graft-maleic anhydride (PEma) was employed as a compatibilizer48,49. The innovation of this research lies in the use of chitosan particles (2.5-7% w/w) as active carriers for Ajwain essential oil (AEO) to minimize its evaporation during melt processing and to improve the thermal stability of the adsorbed active oil.
The GC-MS results for AEO are presented in Table S1 (with GC-MS chromatograms shown in Figure S1 in the supporting information). Various constituents were identified based on their retention time indices and mass spectral fragmentation, referencing the NIST and Wiley libraries, and comparing fragmentation profiles with those described in the literature. The main constituents identified were thymol (47.34% w/w), p-cymene (28.59% w/w), and γ-terpinene (16.24% w/w), which together accounted for over 92% w/w of the AEO components, followed by carvacrol (1.27%). Basij et al. also identified thymol, p-cymene, and γ-terpinene as the major compounds in AEO, comprising 90–97% of the total components50. Notably, p-cymene and γ-terpinene are key precursors for the biosynthesis of thymol and carvacrol; thus, their high levels contribute to the elevated percentage of thymol in AEO.
As can be seen in Table 1, the value of \(\:\varDelta\:{{\updelta\:}}_{\text{i}.\text{j}}\) for the three constituent components of AEO is close to each other, suggesting that the affinity of these components for adsorption on chitosan is similar. According to the theory, the three components should be adsorbed almost equally, but due to the higher molecular weight, density, and boiling point of thymol compared to the other components, thymol is more adsorbed.
In this context, both packaging producers and consumers view the impregnation of polymeric plastic films with these agents as an effective method to prevent bacterial contamination of food.
Table 1 further predicts a similar adsorption capacity for various constituents.
The dissolution between soluble component i and solvent j (\(\:\varDelta\:{\delta\:}_{i.j}\) ) is calculated using Eq. (1):
In this equation, dispersion forces (\(\:{\delta\:}_{D}\) ), intermolecular dipole forces (dipole interactions) (\(\:{\delta\:}_{P}\) ), and the energy of hydrogen bonds between molecules (\(\:{\delta\:}_{H}\) ) must be determined.
The smaller \(\:\varDelta\:{\delta\:}_{i.j}\) is, the greater the affinity between the solvent and solute. To estimate \(\:{\delta\:}_{D}\) , \(\:{\delta\:}_{P}\) , and \(\:{\delta\:}_{H}\) , a method based on the structural contributions of functional groups is employed. Thus, \(\:{\delta\:}_{D}\) is calculated using Eq. (2):
Here, \(\:{F}_{D}\) represents the dispersion component of the molar absorption constant. If the substance contains only one polar group, \(\:{\delta\:}_{P}\) can be calculated using the relation \(\:{\delta\:}_{P}=\:\frac{{F}_{P}}{{V}_{m}}\) . However, for substances with multiple polar groups, it is necessary to calculate the interactions of these polar groups using Eq. (5):
In this case, \(\:{F}_{P}\) denotes the polar component of the molar adsorption constant. Although the F method is not directly applicable for calculating \(\:{\delta\:}_{H}\) , Birbauer and Hansen proposed that hydrogen bonds are additive, leading to Eq. (6):
where \(\:{E}_{H}\) is the hydrogen bond energy for each structural group (\(\:{F}_{H}{V}_{im}\) ).
As shown in Table 1, which presents solubility parameters obtained according to the work of Van Krevelen et al.54, the total dissolution parameter (δTotal) for the three dominant components of Ajwain essential oil ranges from 32.39 for thymol to 10.32 for γ-terpinene. The partial dissolution parameter for dispersion (δD) contributes the most to the total dissolution parameter, indicating the lipophilic nature of the essential oil components. The lowest dispersion partial dissolution parameter is 32.08 for γ-terpinene, while the highest is 36.59 for thymol. In contrast, the polar (δP) and hydrogen (δH) partial dissolution parameters contribute the least to the overall dissolution parameter. The lowest partial polar dissolution parameter for γ-terpinene is 0.92, and the highest is 2.39 for thymol. The partial hydrogen dissolution parameter is zero for both ρ-cymene and γ-terpinene, with the highest value being 14.20 for thymol.
As indicated in Table 1, the value of \(\:\varDelta\:{\delta\:}_{i.j}\) for the three constituent components of Ajwain essential oil is similar, suggesting that their affinity for absorption onto chitosan is comparable, and similar adsorption behavior is anticipated for these components on chitosan. According to theory, these three components should be adsorbed almost equally; however, due to thymol’s higher molecular weight, density, and boiling point compared to the other components, it is adsorbed more readily.
Figure S5 shows that 6 h is the optimal time for AEO uptake by CS particles. The physical interaction between CS particles and AEO retains the AEO molecules in the film during processing.
The FTIR spectra of the individual components are shown in Fig. 1. The spectrum of CS powder displays a prominent absorption peak at 3444 cm−1, indicating the stretching vibration of –(NH₂) and –(OH) groups, as well as inter- and intramolecular hydrogen bonding. The peak at 2861 cm−1 is due to the asymmetric stretching vibration of –(CH) groups. The peak around 1650 cm−1 corresponds to the carbonyl group, which shows the acetylated amino groups of chitin, suggesting an incomplete deacetylation process of the CS55,56. Peaks at 1656 cm−1 and 1596 cm−1 for carbonyl stretching vibration (amide-I) and N-H stretching vibration (amide-II), respectively, correspond to the amide linkages57. The peaks at 1382 cm−1 and 1035 cm−1 are assigned to the saccharide and –(C-O-C)– stretching vibration of the glucosamine ring, respectively58.
The neat polyethylene spectrum shows a hydrocarbon stretching peak around 2848–2913 cm−1. The peaks at 1467 cm−1 and 717 cm−1 correspond to the methylene scissoring and methylene rocking vibrations, respectively48.
In the case of the PE-5-0 spectrum, widening in the 3700 –3000 cm−1 region is observed due to the addition of CS. The depicted peak at 1035 cm−1 is attributed to the vibration of –(C-O-C)– groups in CS.
FTIR spectra of CS powder, AEO, PE (PE-0-0), and samples PE-5-0 and PE-0-10.
As AEO is a mixture of different components, FTIR can only detect various functional groups but not the individual constituents. The spectrum exhibits a peak at 3421 cm−1 attributed to the hydroxyl –(OH) stretching vibration of phenolic compounds57. The absorption peaks at 2960 –2867 cm−1 are assigned to the symmetric and asymmetric stretching of (C-H) groups. The spectrum from 1400 to 1500 cm−1 shows the C-H sp2 bending and stretching vibration of the aromatic ring. The peak around 1149 cm−1 indicates meta-substitution for thymol. The peaks at 1087 and 1289 cm−1 are attributed to the thymol component. The absorption peaks at 1056 and 1513 cm−1 are assigned to the para-substitution and C-H(CH2) wagging of p-cymene. The peaks at 860, 1170, and 1250 cm−1 are attributed to carvacrol, while the peak at 1619 cm−1 is assigned to the aromatic ring59. The peak at 805–810 cm−1 is due to out-of-plane alkene C-H stretching7.
The PE-0-10 spectrum exhibits the characteristic bands of the polymer matrix and those of AEO simultaneously. The peaks representing the polymeric matrix can be seen at 2884–2913, 1467, and 717 cm−1. Since the observed peaks in the FTIR spectrum are the sum of the absorption of polymer matrix and AEO bonds, widening between 2848 and 2960 cm−1 is due to overlapping bands assigned to the stretching frequency of –(CH₃) groups in AEO and the polymer matrix. The characteristic peaks of AEO can be observed at 3421, 1289, 1149, and 1056 cm−1, which were already explained.
After optimizing the time required to adsorb AEO on CS, AEO-adsorbed CS particles were mixed with PE, and the films were prepared as mentioned before. The remaining AEO was then extracted from the films. The extracted solution was injected into the GC-Mass after the addition of decane. The results of the experiment are given in Table S2 as examples for two samples. The relevant chromatograms can be seen in the Supporting Information (Figures S6-S7).
Figure 2 shows the amounts of different components of AEO loaded in the samples. Figure 2a indicates the extracted AEO in milligrams per gram of each film. As seen in the figure, the AEO loading amount is significantly higher in CS-containing films, and by increasing the amount of CS, the percentage of adsorbed AEO increases. This could be attributed to the tendency of CS to adsorb oils and the high aspect ratio of the powder60,61.
The results shown in Fig. 2 reveal two important points. First, the presence of CS reduces oil evaporation during the film formation. Second, among the components of AEO, thymol evaporates less than p-cymene and γ-terpinene, indicating that CS adsorbs more thymol in comparison with the others. This test provided a similar result regarding AEO adsorption on CS. Additionally, the molecular weight of thymol is higher than p-cymene and γ-terpinene. The simultaneous effect of these two factors causes the high thymol content in the samples. It should be noted that the above reasons are also valid for carvacrol, but due to the low percentage of this component, it does not have a significant effect on the final properties of the film.
Amount and percentage composition of oil extracted from different samples: (a) content of AEO extracted, (b) content of different components of extracted AEO.
Considering that the samples containing 2.5 and 5% chitosan and 10% AEO (samples PE-2.5-10 and PE-5-10 in Table 1) have adsorbed a smaller amount of essential oil than the PE-7.5-10 sample, other tests were continued on the sample containing 7.5% w/w chitosan.
X-ray diffraction testing is commonly used to identify the crystalline or amorphous structure of materials based on their diffraction patterns and provide documentary evidence to describe polymorphic structures.
The XRD patterns of the polymer, CS, and the composite samples are shown in Fig. 3. As can be seen in Fig. 3a, the XRD pattern of CS illustrates two peaks at 2θ of 10.63° and 19.8°, which are assigned to the (110) and (200) lattice planes, typical fingerprints of semi-crystalline CS, which is in agreement with the results presented in the literature62,63,64. The high degree of crystallinity of CS is attributed to the large quantity of hydroxyl and amino groups, which are able to form strong intermolecular and intramolecular hydrogen bonds62.
As can be seen in the XRD pattern of PE-0-0, two peaks at 2θ of 21.41° and 23.70° were assigned to the (110) and (200) lattice planes. In Fig. 3a, the XRD patterns of PE/CS with different compositions show no obvious peaks for chitosan. This result confirms that CS was exfoliated in the polymer matrix. Furthermore, the CS content has no significant effect on the crystal size of PE (Table 2).
XRD pattern of the samples containing: (a) CS alone, (b) AEO alone, (c) both CS and AEO.
As illustrated in Fig. 3b and c, the addition of varying amounts of AEO did not significantly alter the crystalline structure of the films. Esfandiari et al. and Suppakul et al. produced linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) films containing rosemary essential oil and 1% linalool or methyl chavicol. They observed no significant differences in the crystalline structure of the oil-loaded films compared to pure LLDPE or LDPE65,66. Therefore, it can be concluded that the presence of CS and AEO, up to 7.5% and 10% respectively, does not significantly affect the crystal structure of the polyethylene films.
Packaging films require adequate mechanical strength and impact resistance during transportation and handling. To investigate the mechanical properties of the prepared films, a tensile test was conducted. The tensile strength and elongation at break for different samples are presented in Table 3. The elongation at break and tensile strength of samples PE-5-0 and PE-7.5-0 significantly decreased with the addition of CS compared to PE-0-0 (P < 0.05). The Young’s modulus of PE-5-0 and PE-7.5-0 improved with the introduction of CS particles compared to PE-0-0 (P < 0.05), due to the rigidity of CS molecules in the polymer matrix, which restricts the mobility of polymer chains67,68. Additionally, improper distribution of CS can contribute to reduced tensile strength, as CS aggregation may lead to film failure even under low stress45. Furthermore, thermodynamic immiscibility (weak interfacial adhesion) and intrinsic incompatibility between the polymer chains and CS particles can result in decreased elongation at break and tensile strength48,55,65,69.
In contrast to PE-0-0, the incorporation of AEO also reduced tensile strength and Young’s modulus but increased the elongation at break for PE-5-0 and PE-7.5-0 (P < 0.05). This decrease can be attributed to the discontinuities created by AEO in the polymer matrix, which acts as a plasticizer70. At low concentrations, AEO forms a discontinuous structure as single droplets. The presence of AEO enhances the slippage of polymer chains against one another, thereby improving the flexibility and elongation of the films44,71,72.
For samples containing both AEO and CS, a decrease in tensile strength and Young’s modulus was observed compared to PE-0-0 (P < 0.05). However, in the PE-7.5-10 sample, an increase in AEO concentration and improved wetting of chitosan facilitated better distribution in the film, resulting in enhanced tensile strength.
To ensure effective food protection, it is essential to minimize factors that contribute to spoilage. From this perspective, oxygen permeability is a critical factor influencing food shelf life.
OTR analysis was conducted to evaluate the barrier properties of different samples against oxygen permeation (Fig. 4). The OTR value for PE-0-0 was approximately 1680 cm2/m2·d·bar. The incorporation of CS reduced OTR values; at 2.5, 5, and 7.5% w/w CS, the OTR of the prepared samples decreased to 1390, 1100, and 910 cm2/m2·d·bar, respectively. Chitosan particles within the polymer matrix create a tortuous path that serves as a gas barrier, with higher tortuosity leading to enhanced barrier properties and reduced permeability73.
As shown in Fig. 4, the OTR values of AEO-loaded samples increased compared to PE-0-0. The incorporation of AEO raised the OTR value to 1880 cm2/m2·d·bar in PE-0-5 and 2064 cm2/m2·d·bar in PE-0-7.5 films. This increase is not surprising due to structural alterations in the film caused by AEO, resulting from two interrelated factors43,44,74. First, AEO can dissolve and migrate into the amorphous regions of the film. Once the amorphous regions are saturated, AEO interferes with polymer-polymer interactions, leading to increased OTR properties of the plastic films. Second, the plasticizing effect of the active oil enhances the mobility of polymer chains, thereby decreasing the samples’ resistance to oxygen diffusion. In CS/AEO-loaded samples, two opposing trends were observed: while the presence of CS molecules restricts oxygen flow, the increased free volume in the film structure due to chemical interactions between AEO and polymer chains enhances oxygen permeability75. Consequently, OTR values increased in all samples due to the predominant effect of AEO incorporation. (According to the statistical calculations performed by the origin software (based on ANOVA method and Tuley’s test), the difference of OTR of PE-7.5-7.5 sample with all samples except PE-7.5-10 and PE-0-2.5 is significant and the p-value is less than 0.05).
Effect of chitosan and/or Ajwain essential oil on the OTR properties of different samples.
Thermal stability is a crucial characteristic of packaging films, as they may undergo heat processes during production, distribution, and storage76,77. This study also aimed to determine the residual AEO in the films after processing. Figure 5a displays the thermogravimetric analysis (TGA) thermograms of AEO, CS, PE-0-10, and PE-7.5-10. The results of the thermal degradation test for CS indicated two distinct weight loss stages78. The initial weight loss of about 5% occurring below 150 °C is attributed to the evaporation of adsorbed water. The main degradation was observed between 250 and 400 °C, with a weight loss of approximately 50%. Thus, CS demonstrates resistance to thermal degradation up to processing temperatures. For subsequent steps in the compound preparation, CS was subjected to thermal treatment for 5 h at 70 °C to eliminate adsorbed water.
As shown in Fig. 5a, AEO completely evaporated by 160 °C. The thermogram for sample PE-0-10 reveals a two-step degradation process within the temperature ranges of 50–200 °C and 420–530 °C, corresponding to AEO evaporation and polymer matrix degradation, respectively. The results indicate that at 160 °C and 250 °C, more than 7% and nearly all of the AEO loaded in the film evaporated, respectively.
Three distinct thermal degradation steps are observable for sample PE-7.5-10. The initial weight loss at temperatures below 250 °C is attributed to AEO volatilization, while the second step corresponds to CS decomposition. Notably, the degradation temperature of CS increased from 250 °C to 290 °C due to composite fabrication. Importantly, the addition of chitosan enhanced the retention of AEO in the film, with only about 3% of the oil evaporating at 200 °C. This phenomenon can be attributed to the surface adsorption of AEO on CS, which allows for greater oil loading in the film. This finding aligns with the results of the gas chromatography-mass spectrometry (GC-MS) analysis.
Consequently, the method of adsorbing AEO onto CS particles is more effective for retaining essential oil than the direct addition of AEO mixed with polyethylene granules in the final film.
Electron microscopic images of the cross-section of the samples containing maximum amounts of AEO and CS, both individually and in combination, are shown in Fig. 5b1 to b3, respectively. Figure 5b1 illustrates the morphology of sample PE-0-10. As it can be seen in the Fig. 5b1 there is a fine compatibility between the polymer matrix and AEO. Figure 5b2 shows that in spite of good powdered CS distribution, the dispersion is not fine. However, a good distribution and dispersion of CS within the polymer matrix by addition of AEO. Figure 5b2 indicates poor distribution and aggregation of CS in the polymer matrix. Figure 5b3 shows a relatively good distribution of CS within the polymer matrix by addition of AEO. In contrast, However, with the assistance of wetting chitosan with the oil, the distribution of CS in the polymer matrix significantly improved, as confirmed by the mechanical properties results.
(a) TGA thermograms of the Chitosan, AEO, PE-0-10 and PE-7.5-10, (b) SEM micrographs of the cross section of the samples, (b1) PE-0-10, (b2) PE-7.5-0, and (b3) PE-7.5-10.
The aim of this research is to develop antibacterial polyethylene films for food packaging. Antibacterial tests were conducted on samples containing the highest amounts of AEO and CS, as well as a sample containing both components. Antibacterial studies were performed using the agar disc diffusion method, providing a qualitative assessment of the antibacterial properties of the samples. When polymeric films containing the antibacterial agent are placed on culture media, it is expected that the active agent will diffuse radially from the film into the agar, creating a clear zone of growth inhibition surrounding the sample74. Typical results of these tests for different samples are presented in Fig. 6. The ratio of the clear zone area to the sample area was used to demonstrate the antibacterial activity of the active films.
As anticipated, the control sample (PE-0-0) exhibited no antibacterial activity against any of the tested bacteria. Additionally, samples containing 5% and 7.5% CS did not show antibacterial activity, as they displayed neither inhibition nor retraction zones due to CS being encapsulated within the polymer matrix, preventing migration to the surface79,80.
Inhibition zones of the samples incubated at 37 ˚C for 24 h. (a) PE-5-0 and PE-7.5-0 against E. coli, (b) PE-5-0 and PE-7.5-0 against S. aureus, (c) PE-0-0, PE-0-10, PE-7.5-10 against E. coli, (d) PE-0-0, PE-0-10, PE-7.5-10 against S. aureus.
Further, Fig. 6 illustrates the inhibition zones of active films impregnated with AEO against the tested microorganisms. The results indicate that sample PE-7.5-10 exhibited greater antibacterial properties than PE-0-10, attributed to the presence of CS, which minimizes AEO evaporation during the film production process through adsorption. The constituents of AEO, thymol and carvacrol, possess polar functional groups81,82, including hydroxyl groups, which likely interact with CS chains through hydrogen bonding, enhancing the resistance of AEO against harsh processing conditions. These findings underscore the critical role of CS particles as active carriers of the volatile components of AEO.
The antibacterial properties of essential oils containing high levels of thymol and/or carvacrol have been documented by numerous researchers83, attributed to their ability to permeate and depolarize the cytoplasmic membrane. Thymol and carvacrol interact with the lipid bilayer of the bacterial cytoplasmic membrane, compromising its integrity, releasing lipopolysaccharides, and thus increasing the permeability of adenosine triphosphate within the cytoplasmic membrane. Consequently, this alters the passive permeability of the cell and leads to the leakage of cellular materials such as ions and nucleic acids42,84. Most studies have reported that Gram-negative bacteria exhibit greater resistance than Gram-positive bacteria to various essential oils85. The higher resistance of Gram-negative bacteria to the antibacterial activity of essential oils can be attributed to the external layer encasing the lipopolysaccharides in these bacterial species, which limits the diffusion of hydrophobic constituents46. However, our findings revealed a higher effectiveness of AEO against E. coli, classified as a Gram-negative bacterium. Other studies have corroborated these results43,45. Therefore, it can be concluded that the experimental conditions for antibacterial analysis are crucial in determining the sensitivity of specific bacterial species to active compounds.
This study aimed to develop antibacterial food packaging films based on AEO and enhance the performance of the antibacterial agent using chitosan as an adsorbent. Key parameters of the films, including mechanical properties, thermal stability, oxygen transmission rate (OTR), and antibacterial activity, were successfully evaluated, along with chitosan’s ability to adsorb AEO. Initially, the variation in the surface adsorption of the three main components in AEO (thymol, ρ-cymene, and γ-terpinene) on chitosan was investigated. The optimal time for effective physical interaction between AEO and CS particles, as well as the anchoring of AEO molecules on chitosan, was determined to be approximately 6 h. The results of TGA tests on films prepared with maximum amounts of AEO, CS, and both (PE-7.5-0, PE-0-10, PE-7.5-10) demonstrated that the presence of CS reduced oil evaporation (from 7% in PE-0-10 to 3% in PE-7.5-10) during the film formation process. Additionally, tensile test results indicated that after incorporating AEO and CS, the modulus and tensile strength of sample PE-7.5-10 were 224 and 14.5 MPa, respectively, while the elongation at break reached approximately 948%, indicating that the films maintained good mechanical properties for food packaging. The OTR value for the PE-7.5-10 sample was calculated to be 1500 cm2/m2·day·bar, a significant reduction from the 1680 cm2/m2·day·bar measured for PE-0-0. The results of antibacterial inhibition zone tests revealed that the control sample (PE-0-0) exhibited no antibacterial activity against either Gram-positive (S. aureus) or Gram-negative (E. coli) bacteria, nor did the PE-7.5-0 sample. However, the PE-0-10 sample showed relative resistance against both E. coli and S. aureus, while the PE-7.5-10 sample demonstrated high antibacterial resistance. These findings highlight the essential role of CS particles as active carriers of the volatile components of AEO. Consequently, it can be concluded that the sample containing the highest amounts of CS and AEO, due to its excellent antibacterial properties and acceptable mechanical attributes, represents a promising candidate for food packaging applications.
The polymers utilized in this study were LDPE and LLDPE, sourced from Amir Kabir Petrochemical Industry in Iran, identified as grades 209AA and 2420 F, respectively. PEma was obtained from Karangin, Iran. The antibacterial agent, AEO, was supplied by Barij Essence Co., Iran. Powdered CS was procured from Bio Basic, Canada (CAS 9012-76-4), characterized by a high molecular weight and a deacetylation degree of at least 90%. Prior to use, the chitosan was dried at 100 °C for 5 h. Ethanol, decane, and acetone were purchased from Merck, Germany, and were used as received.
A total of 960 g of LDPE and LLDPE, in a mass ratio of 70:30, was pre-mixed with 40 g of PEma and fed into a twin-screw extruder (Brabender, ZSK-25, Germany) with a length-to-diameter (L/D) ratio of 40 and a screw speed of 250 rpm, at a feeding rate of 2 kg/hr. The temperature was maintained at 215 °C across all zones. The materials were thoroughly mixed due to shear and pressure forces during extrusion. After exiting the extruder, the molten material was cooled in a basin of cold water, resulting in a string-like form that was subsequently pelletized using a pelletizer machine.
Initially, CS particles with a particle size of less than 108 microns were dried in an oven at 70 °C for 5 h. A solution of 100 µL of AEO dissolved in 3.5 mL of acetone was prepared in 10 mL test tubes. Gradually, 0.5 g of CS was added to each tube. The tubes were sealed and placed in a water bath to equilibrate at ambient temperature. Solid particles were separated from the mixture after 30, 60, 90, 180, 360, and 720 min by centrifugation at 5000 rpm (using a Sigma 2K15C refrigerated centrifuge, Sigma, England) for 5 min. The amount of AEO adsorbed onto the CS particles was characterized and quantified using the following method. A decane solution (2 mL, 120 ppm in acetone) served as the internal standard, which was added to 2 mL of the solution. The mixture was filtered through a syringe-head Teflon filter (Spartan 0.2 μm, Whatman, England) and injected into gas chromatography-mass spectrometry (GC-MS) equipment. The concentration of each component was determined according to the following equation (Eq. (1)).
In these equations, AUCIS and AUCEO’s represent the signal areas for the decane solution (internal standard) and any specific component of the AEO, respectively. CIS and CAEO’s denote the concentrations of decane and the specific AEO component, respectively. The amount of AEO adsorbed onto CS (nadsorbed) was calculated by subtracting the amount of AEO components in the supernatant (nsupernatant) from the initial AEO components (nblank) in the control sample (Eq. (2)).
Three samples, each containing 3 g of CS and varying amounts of AEO (2, 3, and 4 g), were prepared in separate beakers. The CS particles were allowed to adsorb the AEO for a contact time of 6 h at room temperature. The optimal contact time was determined using GC-MS analysis. The mixture was subsequently combined with PE granules in an internal mixer.
The prepared granules were blended with varying amounts of AEO and/or CS, as well as AEO-adsorbed CS particles, through melt mixing in a batch mixer (Brabender W-50, Germany). Initially, the granules were introduced into the mixer at a temperature of 115 °C and a rotor speed of 100 rpm. After melting for 3 min, the rotational speed was reduced to 60 rpm to protect the active oil compounds from oxidation and evaporation. AEO, CS, or a mixture of both was added, and to minimize evaporation, the blending process was limited to no more than 2 min. The resulting melt-mixed blends were hot-pressed for 1 min at 140 °C under 150 bars to produce films approximately 120 μm thick. The antibacterial films developed through this method were immediately wrapped in aluminum foil and stored in plastic bags at 0–3 °C for up to one week before their final use. The various formulations prepared in this research are detailed in Table 4. The samples were designated as Polyethylene-A-B, where A and B represent the percentages of chitosan and essential oil, respectively. It is important to note that all samples contained 4% w/w of PEma.
The structural characterization of the samples was performed using Fourier Transform Infrared Spectroscopy (FTIR). AEO and chitosan (CS) were separately mixed with potassium bromide (KBr) powder and then pressed into pellets for FTIR analysis. Films with different formulations were cut into 2 cm x 2 cm samples and affixed to a sample holder on a Bruker Equinox 55 spectrometer (Germany). We recorded 16 scans at a resolution of 4 cm−1 for each spectrum, measuring wavelengths from 400 to 4000 cm−1.
The amount of AEO in the samples was determined using Gas Chromatography-Mass Spectrometry (GC-MS). Five grams of the film samples were cut and extracted for approximately 24 h via Soxhlet extraction with 200 mL of ethanol. A 1 µL aliquot of the extracted solution was analyzed using an Agilent 6890 gas chromatograph (USA) and an Agilent 5973 mass spectrometer (USA), both equipped with a fused silica capillary column (HP-5 MS). The primary column temperature was set at 50 °C, with a heating rate of 5 °C/min until reaching 220 °C, where it was maintained for 4 min before sampling. The split ratio was 1:100, helium served as the carrier gas, and a decane solution (2 mL, 120 ppm in acetone) was used as an internal standard.
Additionally, AEO was analyzed using the GC-MS technique. A 10 µL sample of AEO was diluted in 2 mL of ethanol, and a 0.5 µL aliquot of the resulting solution was injected for the GC-MS test under the aforementioned conditions.
The adsorption of AEO constituents on CS particles after various contact times (1, 1.5, 3, 6, and 12 h) was also assessed using GC-MS analysis. The concentration of AEO constituents decreased after specific intervals due to their absorption onto CS particles. The quantification of the GC-MS results was based on comparing the area under the peak corresponding to decane with the peaks of the AEO components. Normalized data were obtained by dividing the adsorbed amount for each constituent by its percentage.
The crystal structure of the samples and CS powder was analyzed using X-ray Diffraction (XRD). XRD patterns were obtained using a SIEMENS D5000 (Germany) with a detector operating at a voltage of 40 kV and a current of 40 mA, utilizing Cu Kα radiation. The scanning speed was set at 1.2 °/min, and the scanning range for 2θ was from 5° to 80°.
To study the effect of AEO and/or CS on the mechanical properties of the polymer, tensile strength and elongation at break were evaluated. Samples were cut lengthwise and crosswise to dimensions of 115 mm x 19 mm according to the ASTM D-638 method. The strips were mounted and clamped with pneumatic grips on a SANTAM STM-20 universal testing machine, with an initial grip distance of 30 mm. The grip separation rate was set at 50 mm/min, using a load cell with a capacity of 1000 N. Measurements were conducted on five replicates for each sample.
The gas barrier properties were represented by the Oxygen Transmission Rate (OTR). Measurements were conducted using a GDP-C gas permeability tester (Coesfeld Materialtest, Germany) according to ASTM D 1434. Each film sample had a diameter of 14 cm, and measurements were performed at 23 °C and 50% relative humidity.
The thermal stability of the films was monitored using a thermogravimetric analyzer (TOLEDO METTLER, Switzerland). Thermogravimetric data were recorded to obtain mass-loss curves versus temperature for the film samples. All experiments were conducted at a heating rate of 10 °C/min in an inert nitrogen atmosphere over a temperature range of 25 °C to 600 °C.
Scanning Electron Microscopy (SEM) was utilized to study the morphology of fracture surfaces. The cross-sectional morphology of the samples was examined using a TESCAN VEGA scanning electron microscope (Czech Republic) at an accelerating voltage of 5–30 kV. The samples were immersed in liquid nitrogen for 1 min and then fractured for cross-sectional analysis. Prior to analysis, the samples were sputter-coated with a gold layer to prevent electrostatic charging under the electron beam and to enhance electrical conductivity.
The antibacterial activity of polymeric films containing AEO and/or CS was quantitatively evaluated against two food spoilage bacteria: the Gram-negative Escherichia coli (E. coli) and the Gram-positive Staphylococcus aureus (S. aureus). A virgin film without AEO and CS served as a control. The selected bacterial strains represent typical spoilage microorganisms commonly found in various food products.
The antibacterial activity of the active films was assessed using the agar disc diffusion assay. Polymeric films were cut into discs with a diameter of 1 cm and placed in the center of Petri dishes containing nutrient agar (purchased from Merck, Germany). From each inoculum (106 CFU/mL), 0.1 mL was spread onto the agar medium. The Petri dishes were incubated under appropriate conditions (37 °C for 24 h). The antibacterial activity of each sample was determined by observing the clear zone around the active films where the targeted microorganisms were inhibited. The diameter of the inhibition zone was measured using a digital micrometer and recorded as the ratio of the zone of inhibition area to the sample area. Mueller Hinton Agar medium was used for determining antibacterial activity, and the test was conducted in duplicate for each formulation. The inhibitory activity against E. coli and S. aureus was evaluated by measuring the diameter of the transparent inhibition zone, and the antibacterial effect was assessed by comparing the diameters of the inhibition zones. The average and standard deviation of the two measurements were calculated.
Data points were presented as the mean of the measured values. The data were subjected to analysis of variance (ANOVA) at a significance level of p < 0.05 using the OriginLab 2022 software package. Statistical procedures were resolved using confidence intervals based on Tukey’s test.
The datasets generated during and/or analyzed during the current study are available from the corresponding author (F. Farahmand ghavi) on reasonable request.
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This work was supported by Iran Polymer and Petrochemical Institute (Grant No.23794109).
Department of Polymer Processing, Faculty of Processing, Iran Polymer and Petrochemical Institute, P.O. Box: 14965/115, Tehran, Iran
Kasra Shiva, Adel Soleimani & Jalil Morshedian
Department of Novel Drug Delivery Systems, Faculty of Science, Iran Polymer and Petrochemical Institute, P.O. Box: 14965/115, Tehran, Iran
Kasra Shiva, Adel Soleimani & Farhid Farahmandghavi
Department of Biomaterials, Faculty of Science, Iran Polymer and Petrochemical Institute, P.O. Box: 14965/115, Tehran, Iran
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F. Farahmandghavi and J. Morshedian and A. Soleimani conceived of the presented idea. K. Shiva and A. Soleimani developed the theory and performed the computations. F. Farahmandghavi and F. Shokrolahi verified the analytical methods. F. Farahmandghavi supervised K. Shiva and A. Soleimani the findings of this work. All authors discussed the results and contributed to the final manuscript.
The authors declare no competing interests.
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Shiva, K., Soleimani, A., Morshedian, J. et al. Improving the antibacterial properties of polyethylene food packaging films with Ajwain essential oil adsorbed on chitosan particles. Sci Rep 14, 28802 (2024). https://doi.org/10.1038/s41598-024-80349-7
DOI: https://doi.org/10.1038/s41598-024-80349-7
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