3.1 Composition of PPEO
The extraction yield of PPEO was 6.7% on a wet basis. The PPEO presented 25 constituents, being myrcene (28.2%) the major compound, followed by α-pinene (20.2%), germacrene D (15.3%), and limonene (10.5%) (Table 1). Dannenberg et al. (2019) detected predominance of β-myrcene (41.0%), followed by β-cubebene (12.2%) and limonene (8.9%). Santos et al. (2009) also evaluated PPEO and obtained 20.4% of myrcene, 17% of limonene, and 10.8% of germacrene D. In contrast, Ennigrou, Hosni, Casabianca, Vulliet, & Smiti (2011) found predominance of germacrene (27.1%), α-felandrene (22.1%), and β-cubebene (10.0%). Such variations in data of studies regarding the composition and quantity of PPEO phenolic compounds are expected owing to differences in soil, rainfall periods, seasonality, plant age, and extraction methods (Sadeh et al., 2019).
According to Guimarães et al. (2019), studies that generally investigate the antimicrobial activity of essential oils are incomplete, as they cannot identify which compounds act with greater influence or whether it is a synergism between the components. Therefore, it is possible that compounds in smaller amounts also contribute to the activity.
3.2 Viscosity and conductivity of polymeric solutions
The apparent viscosity and the electrical conductivity of the PLA polymeric solutions with different concentrations of PPEO are shown in Table 3. The incorporation of PPEO reduced the viscosity of the polymeric solution, decreasing as the concentration of PPEO increased. Silva et al. (2018) also reported that the addition of ginger essential oil (Zingiber officinale) decreased the viscosity of the polymeric solution containing soy protein isolate, polyethylene oxide and zein. Similarly, Teilaghi, Movaffagh, & Bayat (2020) found the same behavior in zein solutions added with 5, 10 and 15% essential oil of cumin seed (Nigella sativa). Pelissari et al. (2012) reported that the viscosity of a solution is associated with the interactions between the component molecules and depends on the concentrations and nature of the solutes and reagents used. Thus, the presence of PPEO possibly interferes with the interactions and bonds inherent between the molecules of the PLA solution, weakening the bonds among its constituent components and resulting in a lower viscosity of the solution.
The presence of the essential oil also reduced the electrical conductivity of the solutions significantly, in all concentrations. Rafiq, Hussain, Abid, Nazir, & Masood (2018) evaluated the effect of incorporating lavender essential oil (Lavandula officinalis), cloves (Eugenia caryophyllus) and cinnamon (Cinnamomum cassia) on the formation of PVA nanofibers and sodium alginate, finding that their presence influenced the decrease in electrical conductivity. In another study, Mori et al. (2015) observed that the addition of candeia essential oil (Eremanthus erythropappus) to develop PLA nanofibers showed a performance similar to the aforementioned studies, also presenting reduction in conductivity with the increase in oil concentration.
As well as viscosity, electrical conductivity is affected by different factors, such as the ionic strength of the medium and the type of polymers and solvent (Ghorani & Tuker, 2015). Thus, for a constant solvent system in the electrospinning process, changes in the mass ratio of the polymer and bioactive mixtures present or in the type of polymers are the main factors for the changes in electrical conductivity.
3.3 Morphology of the ultrafine fiber membranes
The ultrafine fibers containing only PLA (control) exhibited bead-free morphology, with an average diameter of 426 nm. The addition of PPEO provided a reduction in the diameter of ultrafine fibers in treatments containing 10, 20 and 30% of PPEO in relation to the control, with values of 239, 226 and 167 nm, respectively, as shown in Figure 4. Similar behavior was found in a study by Unalan et al. (2019), who developed polycaprolactone nanofibers loaded with peppermint essential oil (Mentha piperita) by the electrospinning technique, showing that the addition of the oil led to a slight decrease in the diameter of the fibers.
The presence of PPEO in the formulation promoted the formation of beads and lumps, probably due to insufficient evaporation of the solvent used. Mori et al. (2015) reported that in mixtures of PLA and essential oil of candeia (Eremanthus erythropappus), the addition of the oil influenced the increase in the diameter of the nanofibers and a reduction in the amount of beads. Still, Scaffaro, Maio, & Lopresti (2018) observed that the presence of carvacrol in the functional PLA membranes affected the morphology of the nanofibers, leading to an increase in their diameters.
The presence of beads (Figures 4c, 4e and 4g) is generally negatively associated with the formation of the material. However, it is assumed that they can be not so negative, since their structures can hold some percentage of the bioactive compound present in the structure and be gradually released into the environment in which it is in contact. In nanofibers composed of polyethylene oxide and soy protein, Silva et al. (2018) found that the addition of ginger essential oil also increased their diameter. According to Bhardwaj & Kundu (2010), solutions that have low conductivities result in insufficient elongation of the jet to be electrified by electrical forces and lead to the production of nanofibers with larger diameters. However, it was observed that in the present study, the behavior proved to be opposite to the data from most of the literature. This fact can be explained by the inconstancy of the stretching of the solution, sometimes causing it to occur until the needle is clogged, influencing the heterogeneity of the formed material. Also, the solution was sometimes deposited in the collector in the form of fibers and sometimes in the form of beads. According to Haider, Haider, & Kang (2018), the presence of beads is attributed to the influence of gravitational force and another important factor that can cause these distortions in the fiber structure is the surface charge density, since any change in this parameter can also affect the morphology of the nanofiber.
3.4 Thermal properties of the ultrafine fiber membranes
The initial (TDi) and final (TDf) decomposition temperatures and the percentage of mass loss are shown in Table 4. The PPEO presented two stages of decomposition, one close to 82.3 ºC, indicating 58.4% mass loss and the other close to 151.4 ºC, showing 27.1% mass loss. These degradation peaks can be attributed to the evaporation of volatile compounds. The PLA showed a decomposition stage at 360.7 ºC and approximately 90% of mass loss. The degradation temperature of pure PLA around 300 ºC was reported by Thangaraju, Srinivasan, Kumar, Sehgal, & Rajiv (2012), being characteristic of this polymer. The incorporation of the oil provided less thermal stability in the treatments with 10, 20 and 30%, indicating mass losses from 106 ºC, in comparison with fibers produced with pure PLA (Figure 5).
Furthermore, it was observed that in ultrafine fibers, the PLA protected the PPEO because the TDis presented were from 131.8, 120.2 and 106.2 to 10, 20 and 30% of PPEO, compared to the TDi of 44.9 ºC of the pure PPEO. Thus, it is emphasized that this material can be applied in food packaging that will not be subjected to processes that require temperatures above 100 ºC.
3.5 FTIR of the ultrafine fiber membranes
The chemical interactions between the PLA and the PPEO were investigated by the FTIR, and the spectrum is shown in Figure 6. The characteristic absorptions of the PLA are three strong bands due to the vibrations of the C-CO-O-C group, that is, the band derived from the stretch of the C=O in 1747 cm-1, the band coming from the asymmetrical stretching of the CO in approximately 1195 cm-1 and, in 1110 cm-1, coming from the symmetrical stretching C-O-C. The lack of an intense band in the 3500-3000 cm-1 region (stretching of the O-H group) is indicative of the absence of PLA hydrolysis by-products (Palmieri, Pierpaoli, Riderelli, & Ruello, 2020).
For pure PPEO, the spectrum showed a characteristic band around 750 cm-1 related to the aromatic C-H bond. Also, bands between 1400 and 1500 cm-1 correspond to C=C bonds from aromatic rings characteristic of the oil (Mukherji & Prabhune, 2014). Bands that appear between 2750 and 3000 cm-1 are probably related to O-H bonds of terpenoid compounds (Boughendjioua & Djeddi, 2017).
The bands around 900 cm-1 are related to monoterpenic compounds in the oil, and those around 2943 cm-1 are attributed to C-H bonds of methyls and methylenes (Oréfice, Vasconcelos, & Moraes, 2004). The peaks were more accentuated in pure PPEO when compared to the lower intensities shown in the treatments with 10% (almost imperceptible), 20 and 30%. Thus, it can be inferred that a certain loss of PPEO probably occurred during the electrospinning process, through volatilization.
3.6 Wettability of the ultrafine fiber membranes
The wettability character of ultrafine fiber membranes was determined by the angles of contact with water that were measured, as shown in Figure 7. Regardless of the composition, all treatments had a contact angle greater than 90º, implying the hydrophobic character of the membranes of ultrafine fibers formed. This performance was expected due to the fact that the PLA has a hydrophobic character (Sun et al., 2020). As essential oils are composed of highly hydrophobic molecules (Dhifi, Bellili, Jazi, Bahloul, & Mnif, 2016), it was expected that the presence of PPEO would increase water repulsion. However, there was no significant increase in this aspect when adding the PPEO in the different concentrations.
3.7 Antimicrobial activity by disk-diffusion, MIC, MBC and in micro-atmosphere
The results referring to the inhibition halos, MIC and MBC of the PPEO are shown in Table 2. For the membrane, only the inhibition halo was used (Table 2). The lowest MIC value observed was for S. aureus, with 256.9 mg/mL. For E. coli, the PPEO did not indicate an antimicrobial effect. As for the inhibition halos, it was observed that the effect of the PPEO did not show any significant difference between L. monocytogenes and S. aureus, with halos of 11.5 ± 1.1 and 13.2 ± 1.7 mm, respectively. In agreement with the MIC and MBC assay, E. coli showed resistance to the PPEO.
As for the ultrafine fiber membrane, the diameters of the halos for L. monocytogenes and S. aureus were smaller compared to pure PPEO, and for S. enteritidis there was no inhibition. It is noteworthy that this behavior is probably due to the lower concentration of PPEO (30%) used in the manufacture of fiber membrane.
Dannenberg et al. (2019) developed investigations about the essential oil of pink pepper and found that the MIC values for S. aureus (ATCC 6538) and L. monocytogenes (ATCC 7644) were 0.68 and 1.36 mg/mL, respectively, whereas the MBC was 2.72 mg/mL for both. On the other hand, Santos et al. (2020) tested different concentrations of the essential oil of pink pepper fruits to inhibit strains of E. coli (ATCC 25922), S. enteritidis (ATCC 13076), L. monocytogenes (ATCC 19117) and S. aureus (ATCC 25923), verifying inhibition only in the last, with an MIC of 5 μg/mL. In comparison to our study, these values are well below, a fact that can be justified by the time of harvest of the fruits, climate, soil situation, precipitations and different types of strain used.
Gram-positive and Gram-negative bacteria have distinct cytological structures, a fact that corroborates the greater resistance of Gram-negative bacteria and greater sensitivity of Gram-positive bacteria in relation to the action of essential oils. The Gram-positive cell wall is composed of approximately 90 to 95% peptidoglycan, which is bound to proteins and teioic acid (Nazzaro, Fratianni, De Martino, Coppola, & De Feo, 2013). In addition, it allows hydrophobic molecules to easily cross and act on both the cell wall and the cytoplasm. The phenolic compounds present in oils, for example, are considered one of the most responsible for the antimicrobial action against Gram-positive bacteria, but their effect depends on the amount of the compound: at low concentrations, they can interfere with enzymes involved in energy production, while at high concentrations they can denature proteins (Tiwari et al., 2009). However, there are also studies that prove the antimicrobial activity of essential oils acting against gram-negative bacteria, such as Cinnamomum camphora essential oil, reducing the development of E. coli (Wu et al., 2019) and essential oil of oregano and lemongrass acting against Salmonella enteritidis present in refrigerated steaks (Oliveira, Soares & Piccoli, 2013).
The micro-atmosphere test is based on the action of volatile compounds in the essential oil, which can significantly inhibit the growth of some bacteria. The reductions in microbial load in this assay are shown in Figure 1, only for Gram-positive bacteria, since Gram-negative bacteria did not show positive results for the antimicrobial action in the disk-diffusion assay of ultrafine fibers. It was possible to observe that the pure oil (100%) indicated reductions of around 90% for both bacteria. On the other hand, the concentration of 30% of PPEO showed a reduction of around 40% for L. monocytogenes and 50% for S. aureus.
In a similar study using pink pepper essential oil, Dannenberg et al. (2017) found that in the micro-atmosphere, the reduction was 100% in the development of S. aureus and L. monocytogenes, and 16 and 15% for E. coli and S. typhimurium. Antunes et al. (2017) developed nanofibers with eucalyptus essential oil and observed that at concentrations of 0.25, 0.38 and 0.63 µL/cm³ there was total inhibition of the growth of viable cells of S. aureus and L. monocytogenes. Silva et al. (2018) evaluated the application of nanofibers with polyethylene oxide, isolated soy protein and ginger essential oil, noting that the last influenced the reduction of approximately 43% in the count of L. monocytogenes, using concentrations of 0.2 and 0.3 µL/cm³.
According to Trombetta et al. (2005), Gram-positive bacteria are more susceptible to the vapor phase that contains terpenes. This fact can be observed in the present study, since the PPEO presented a greater amount of myrcene, which is considered a monoterpene. However, some exceptions have also been reported in the literature, indicating that there is no apparent association or positive correlation between the nature of the bacterial wall and the degree of inhibition of microbial strains (Saida et al., 2020).
The components of PPEO may have acted in synergism to affect the activity. However, the mechanisms of action are complex, requiring further investigation of the raw material and substrate on which they will act. According to Saad, Muller, & Lobstein (2013), the mechanisms of action of the oils will depend on their chemical composition. The location of one or more functional groups can influence its antimicrobial activity. As an example, thymol and carvacrol have similar antimicrobial effects, but have different mechanisms of action against Gram-positive and Gram-negative bacteria. Reyes-Jurado et al. (2020) reported that in the vapor phase, the oil disperses freely: it has a particular impact against microorganisms due to its surface action, making them more susceptible to volatiles.
In this way, the volatile antimicrobial capacity of PPEO, without requiring direct contact with food, promotes investigations for the development of packaging systems that can control the spread of pathogenic and deteriorating bacteria.
3.7.1 Antimicrobial action of the ultrafine fiber membranes on cream cheese
For the evaluation of the effect of the developed ultrafine fiber membrane, the treatment with the concentration containing 30% PPEO was chosen because it showed better results in the antimicrobial evaluations against Gram-positive bacteria, although it also indicated inhibition against S. enteritidis. The analysis for the verification and quantification of colony forming units was carried out one day after the beginning of the experiment. However, the results were not expressed because there was not enough growth of both bacteria.
For L. monocytogenes, it was observed that the presence of the ultrafine fiber membrane in the period of 7 days did not indicate growth inhibition, in relation to the positive control. The same behavior was observed in the 14-day period. However, in 21 days a significant reduction in colony count was noticed, around 26%, as shown in Figure 2. For S. aureus, the presence of the ultrafine fiber membrane in the period of 7 days indicated a reduction in cell content, but it was not significant. On the other hand, in 14 days there was a significant reduction of approximately 30%. Analogous behavior was identified after 21 days, with an even greater significant reduction, around 62%, as shown in Figure 3.
Considering that the expiration date indicated on the evaluated food is 5 days after opening the package, the results obtained for both bacteria showed that until the end of the period, the presence of fibers in the package was not relevant. However, if the fibers were inserted into the packaging lid at the time of filling, soon after the product was manufactured, there would probably be a positive effect, as the volatile compounds would be trapped in the hermetically sealed packaging, as there was a relevant result for a longer period, 21 days. The results also served to show the behavior profile of the product during a longer storage period, suggesting that PPEO has been gradually released.
Dannenberg et al. (2017) studied the effect of the presence of pink pepper essential oil in cellulose acetate films produced by the casting technique and applied to cheeses. It was observed that the release of the oil is related to the affinity between the nonpolar compounds of the oil and the evaluated food. Silva et al. (2018) produced nanofibers containing ginger essential oil and applied it to slices of Minas cheese, verifying that the presence of the material indicated a significant reduction in L. monocytogenes colonies on days 3 and 9 of storage. The latter presented about 17% reduction in relation to the positive control.
Therefore, it is assumed that at first, the volatiles of the PPEO came into contact at least with the surface of the cream cheese layer. As the storage time passed, the retention of PPEO inside the package was prolonged, causing these compounds to act more actively. As a result, the data shown in this study stimulates further investigation on foods that have a longer shelf life, as the PPEO has been shown to be effective in reducing cell counts on the 21st day.
The antimicrobial activity of essential oils is commonly assessed using methods of direct contact between pathogen and microbial agent, through diffusion and dilution methods. However, the role of essential oils in the vapor phase as antimicrobial agents is increasing in importance. Tyagi & Malik (2010) suggested that essential oils in the vapor phase have a greater degree of antimicrobial activity, since the active compounds are highly volatile and can quickly disperse in the environment. According to Kloucek et al. (2012), each constituent present in the oil has a different volatility, therefore, when the oil is introduced into a closed microenvironment, the volatiles begin to disperse at different rates in the vapor phase within the space in question, according to the degree of volatility, until they reach equilibrium.
Thus, it was observed that the ultrafine fiber membrane showed a good result, contributing to microbial reduction when compared to the positive control. In addition, the release of compounds from essential oils to the food through volatilization did not require direct contact, allowing the reduction of undesirable sensory characteristics that may occur in the food.