Food products mainly classified as perishable products, suffering from short shelf life. Furthermore, they are vulnerable to chemical reactions and bacterial contamination, and their quality may deteriorate during production, processing, transportation, and storage stages [1]. Along with the growing demand for healthy, less processed, and high-quality foods, these challenges are the driving force behind some dynamic changes in the food packaging industry [2].
Among the polymers used in food packaging, polyolefins such as polyethylene (PE) and polypropylene (PP) are preferred for several reasons such as low cost, flexibility, chemical inertness, recyclability, good processability, non-toxicity, and biocompatibility. Since these polymers have very good sealing properties, they are also used as a bonding layer on aluminum foil when applying different polymers on the foil. Although LDPE is a good barrier against moisture, it is relatively permeable against 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 need to be improved. To improve the mechanical properties of LDPE, it is blended with linear low-density polyethylene (LLDPE), so it can better withstand the strain of transportation [3, 4].
Passive traditional food packaging techniques, involves the use of a covering material, which only act as a barrier to postpone harmful environmental effects, and cannot guarantee food protection and quality [5]. As a result, a switch from the standard conception of packaging to minimize the interaction between food and packaging material to a novel view emphasizing the favorable effects of some food/package interactions seems inevitable. A most recent alternative with promising potentials in this field is active packaging, as a system with ability of changing conditions of the wrapped products, which can enhance the preservation of properties, provides better safety, prolongs shelf-life, and protect the quality of foods [6]. Among different sorts of active packaging, the antibacterial types are the most innovative concepts in the food industry. These systems intermingle the protective features of antibacterial agents with preservative characteristics for foods[7–9]. In direct use of antibacterial agents in foods have a reasonably large antibacterial effect, due to quick diffusion of antibacterial component into the bulk. however, adverse effects of antibacterial agents such as food denaturation and off-flavoring may also occur. Therefore, the use of antibacterial agents within the package film matrix could be useful for a gradual and steady liberation over a long time. thus, the antibacterial effect could have been sustained over a long period of time[9, 10].
Another solution to achieve the desired antibacterial activity is provided through indirect contact between food and antibacterial agent, by direct use of antibacterial compounds in the film package [4, 11–14]. Using antibacterial agents in polyethylene food packaging systems have already been evaluated such as Thymol, Potassium sorbate, Nisin, Hexamethylenetetramine, organic acid and chitosan/essential oil [15–21].
In addition, among indirect methods, using oxidizing ions such as copper or silver ions as antibacterial agents in food packaging systems, have already been evaluated. But they should be consumed optimally because of their potential for toxicity. Meanwhile, a tremendous rise in bacterial resistance to antibiotics has increased the global trend towards using natural additives - such as essential oils (EOs) – in the food packaging industry. Natural extracts such as EOs and their constituents possess the GRAS (Generally recognized as safe) status by the US Food and Drug Administration (FDA) [22]. They also have been classified by the European Decision 2002/113/EC (Commission Decision 2002/113/EC) as flavorings [23]. In this sense, packaging producers and consumers consider the impregnation of polymeric plastic films by these agents as an attractive method to avoid bacterial food contaminations.
Generally, there are different natural chemical compounds capable of antibacterial activity [24, 25], including terpenoids and sesquiterpenes with various aliphatic hydrocarbons, aldehydes, acids, alcohols. Thymol is the main constituent of Trachyspermum ammi, also known as Ajwain, and can be extracted between 35 to 60% from the plant [26]. The non-thymol fraction includes para-cymene, Gamma-terpinene, Alpha-pinene, carvacrol, and other minor constituents [27]. Thymol is a predominant monoterpene phenolic compound which has been used as a food preservative for many years. With a wide range of antibacterial activity, thymol is capable of a high potential for perishable foods' safety and prolongs shelf life [28]. 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 and its functional and structural properties. although many studies have so far demonstrated the antibacterial activity of EOs and their active components against a wide range of pathogenic bacteria [29–32], only a few articles have focused on the use of EOs as additives in polymeric plastic films used for food packaging [33–35].
Min et al. used thymol as antibacterial agent and introduced a composite of THY@PCN/PUL/PVA nanofibers and claimed that the composite has the ability of sustained release of thymol as dual-antibacterial activity to prolonged the shelf life of fruits. Although it is not clear whether the use of Pullulan in packaging is economical or not [36].
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-<beta>-d-glusan, can be easily derived from chitin, the second most abundant natural polymer, found in crab, shrimp, lobster, coral, jellyfish, mushroom, and fungi [37, 38]. It had been proven that CS is capable of absorbing oils and hydrophobic compounds on its surface [39]. Bi, J., et al. claimed that CS-graft-PVA film had a potential ability to be applied as a carrier to bind quantities of procyanidins to prepare an active food packaging, that the procyanidin encapsulation efficiency is over 95% and long-term release sustainability [40].
The lack of interest in this field may be because essential oils and their compounds are heat and shear-sensitive components, so, can quickly decompose or evaporate under high processing temperature and pressure. To overcome this problem, some researchers have used innovative methods. Nestro et al. fabricated active films based on ethylene-vinyl acetate (EVA) incorporated with 3.5 and 7% carvacrol or cinnamaldehyde by minimizing the mixing time and placing the material in the liquid nitrogen bath immediately after mixing to prevent any further evaporation of the antibacterial additive [41]. Solano et al. produced LDPE films with 1 and 4% Origanum vulgare and Thymus vulgaris using a single screw extruder. From the feed zone to die, the temperature profile was increased gradually to preserve the antibacterial additive from evaporation[42]. A mixture of LDPE and EVA was chosen to enhance the solubility and to ensure partial adhering of the EOs in the polymer matrix. The relatively higher retention of EOs in the film with is capable by higher concentration and lower polarity [43, 44]. Guarda et al. coated corona-treated bi-axially oriented PP films with microcapsules containing thymol and carvacrol as natural antibacterial agents [45].
Esmaili and Asgari presented a method in which Carum copticum essential oil (CEO) encapsulated in CS nanoparticles. In the method, encapsulation of CEO in the particles was performed by an emulsion-ionic gelation with creating of crosslinking using pantasodium tripolyphosphate (TPP) and sodium hexametaphosphte (HMP), separately [46].
The aim of this research is to use Ajwain essential oil (AEO) in polyethylene-based food packaging films as an antibacterial agent. As explained AEO is sensitive to the environmental conditions and evaporates at the elevated temperature required to prepare polyethylene films, so, the main challenge of this research is to solve this problem. We decided to use the surface adsorption of AEO on chitosan particles to preserve it in the harsh conditions and high temperature of the process, which is a very efficient and simple method. To the best of our knowledge no research yet has been accomplished the adsorption of AEO molecules on CS particles in antibacterial food packaging systems. To improve the distribution of CS chains in the LDPE-LLDPE matrix, PE-graft-maleic anhydride (PEma) was used as a compatibilizer [47, 48]. The innovation of this research is the use of CS particles (2.5-7% w/w) as an active carrier for Ajwain essential oil (AEO) to minimize its evaporation during the melt processing and enhance the thermal stability of the absorbed active oil.