1.7. Oxygen Permeability (OP)
OP is another key parameter of packaging materials that determines the oxygen transport across the film to enhance shelf life and quality of the packaged food. The presence of oxygen modulates the development of the different reactions like oxidation involving components responsible for the color and aroma of the food product
[21][16]. When packaging oxygen-sensitive products, the materials with the least amount of oxygen permeability are preferred. Biopolymer-based packaging materials have lower oxygen barrier properties when compared to conventional plastic packaging. Therefore, some modifications of biopolymers are necessary to restrict oxygen permeability to enhance its application in the food packaging industry.
1.8. Antimicrobial Properties
A novel cold plasma technique is applied to modify polymer surface attributes, the decontamination of packaging surfaces and food processing instruments, and to enhance the safety level of several food products
[4][23]. The active packaging films loaded with bioactive compounds like essential oils, peptides, or functional components when exposed to cold plasma show higher antimicrobial activity.
It is observed that plasma treatment facilitates the coating of functional components on the polymer layer and thereby increases antimicrobial efficacy. For instance, Wong, Hou
[24] prepared gallic acid (GA) coated polyethylene (PE) film applying plasma treatment (30 W for 60 s). The PE/GA active film reduced the growth of
E.coli and
S. aureus by 0.5–1.1 log reduction at a concentration above 1.0%. Such antimicrobial activity was also reported for plasma-treated polylactic acid (PLA) film coated with nisin where the active film showed a log reduction of 3.23 against
Listeria monocytogene, whereas pristine PLA film could not inhibit its growth. It was observed that with the increase in plasma treatment time (0–60 s) the microbial reduction increased and this could be ascribed to the content of nisin absorbed on the surface of the PLA. This indicates that cold plasma treatment influences the absorption capacity of PLA film and thus the content of nisin thereby affecting the antimicrobial activity of the film
[25].
Some studies demonstrated that plasma treatment had no significant effect on the inhibition of microbial growth. Chen, Ali
[26] showed that cold plasma had no significant effect on chitosan/ciprofloxacin hydrochloride antimicrobial film and further coating with zein had drastically reduced the release of ciprofloxacin hydrochloride resulting in reducing the antimicrobial effect. This could be attributed to the fact that plasma treatment may have enhanced intermolecular interaction (crosslinking) leading to firm adhesion between chitosan and zein, resulting in the increase in a barrier for the diffusion of ciprofloxacin hydrochloride
[26]. Such reduction in the release of the antimicrobial compound due to cross-linking was also reported for sodium tripolyphosphate crossed linked chitosan film loaded with ciprofloxacin
[27].
Overall, the impact of plasma treatment on the antimicrobial activity of the film containing functional compounds is critical. The strategic combination of polymers with antimicrobial substances treated with cold plasma can be a potential approach when preparing antimicrobial packaging materials suitable for food packaging applications.
1.9. Biodegradability
Biodegradable materials are degraded by the enzymatic action of living microorganisms such as bacteria, yeast, and fungi. Biodegradation is investigated in various environments such as soil burial, landfill and compost simulations, and microorganisms
[6][28][29]. Arolkar, Salgo
[6] investigated the degradation of corn starch/poly(ε-caprolactone) films following soil burial method and the extent of degradation was reported in terms of an alteration of TS and EAB. It was observed that the rate of degradation was higher for air plasma treated films as compared to untreated film and it increased with increase plasma treatment time. In another study, Chen, Chen
[18] reported that the biodegradability of the plasma-treated zein-PLA composite film was higher than that of untreated films. The higher degradation of the plasma-treated film was attributed to the increase in the surface area of the porous structure that enhanced the accessibility to the microbes present in the compost into the film resulting in the higher degradation. A similar result was reported for plasma-treated defatted soybean meal film, where the plasma treatment increased the surface area of the film by increasing the roughness of the film, facilitating microorganisms in the compost in faster degradation
[28]. In another study, Song, Oh
[8] investigated the biodegradation of PLA sachets for a period of 0 to 35 days and reported that at the end of 28 days, the extent of degradation of plasma-treated PLA sachets was higher than in untreated sachets.
2. Safety Concerns Relating to the Application of Cold Plasma for the Modification of Food Packaging Films
The application of plasma in food processing/packaging generates reactive species that come in direct or indirect contact with food surfaces. The interaction between plasma reactive species and the matrices of food or biopolymers is complicated and limited studies have been carried out investigating the potentially detrimental effects to human or animal health
[4][30]. Chen, Lin
[31] reported that He gas-plasma treatment of di-ionized water for 30 min generated ROS (reactive oxygen species) and RNS (reactive nitrogen species) caused significant apoptosis
[31]. In another study, Heslin, Boehm
[32] assessed the cytotoxic and mutagenic potential of DBD plasma-treated iceberg lettuce in an in vitro CHO-K1 (mammalian cell) model and short-term toxic effects in an in vivo
Galleria mellonella larva model. The results showed a low in vitro cytotoxic effect and spontaneous mutations, however reported a strong in vivo toxicity with less than a 10% larva survival when injected with lettuce broth treated for 5 min
[32]. Few studies have been found to evaluate the toxicology of plasm-treated food constituents. Some studies have reported that the plasma reactive species induces alterations in food constituents, like modification of amino acids in proteins, oxidation of high molecular weight compounds to organic acids, and peroxidation of lipids resulting in undesirable metabolites like short-chain aldehydes, keto-acids, hydroxyl acids, and short-chain fatty acids
[33][30][34]. Other effects of the application of cold plasma on food commodities include a reduction in firmness of fruits and vegetables, enhanced discolouration, and surges in acidity content
[4].
Some studies showed that the application of cold plasma in food products caused no acute toxicity, although the work conducted on this has been limited. For instance, Kim, Sung
[35] evaluated the mutagenicity and immune toxicity of sausage prepared with plasma-treated water as a nitrite source and reported that mutagenicity and inflammatory response was negative in mice fed with the plasma-treated sausage
[35]. Similarly, Jo, Lee
[36] reported that atmospheric plasma-treated winter mushroom powder caused no mutagenicity or acute toxicity in rats fed with 5000 mg/kg body weight
[36]. A few studies have been found to investigate the toxicity of biopolymer-based films treated with plasma. In one of these studies, Han, Suh
[37] investigated the safety of plasma-treated soya film by determining the acute and subacute oral toxicity in a rat model. It was reported that the rat fed with 5000 mg/kg body weight (Single-dose acute) or subacute 1000 mg/kg body weight/day for 14 days showed no acute toxicity resulting in the death of the rats. However, a change in blood components (like hematocrit, hemoglobin, bilirubin, creatinine, and aspartate aminotransferase) was observed. These changes were irrelevant to toxicity as their level were within acceptable physiological ranges
[37].
Overall, it can be seen that the application of plasma in food processing or in the modification of biopolymers is a novel technique that still requires approval. The Food and Drug Administration (FDA) has not approved any guidelines on the application of plasma in food or on food contact surfaces
[30][38]. Research on cold plasma optimization has to be performed specifically for each product. A safety and risk assessment should be comprehensively carried out in both an in vitro and in vivo environment to address the potential toxicity of food or food contact surfaces such as biopolymer films treated with cold plasma.
3. Conclusions
The commercial application of biopolymer-based films is limited due to their poor physical, structural, mechanical, thermal, and barrier properties, as well as poor ink printability and adhesion features. Herein was to investigate the impact of cold plasma used for the modification of the critical properties of packaging films prepared from protein, polysaccharides, or their combinations. Different factors that influence the effect of plasma treatment include the internal structure of biopolymers, types of plasma gas generating reactive species, and processing conditions (voltage, and treatment time). The application of cold plasma efficiently improved the physical, structural, and thermomechanical properties of the packaging films in most cases. The application of cold plasma modified surface properties enhancing the diffusion rate of functional components absorbed on the surface of the biopolymer. It also enabled adhesion between polymers facilitating the development of multilayer films and increased ink printability. The cold plasma treatment also enhanced the antimicrobial efficacy by increasing the diffusion rate, and retention of the volatile functional components. In addition, the plasma treatment augmented the biodegradability of the biopolymer-based films. Overall, the application of cold plasma treatment is a cost-effective approach to modifying the packaging properties of biopolymer-based films, as it is a simple inline process with an easy instrumental setup and no waste generation. However, comprehensive research is needed to understand the complex interaction between the plasma reactive species and components of the polymer, as well their migration into food concerning the safety of human and animal health, before cold plasma can be applied commercially.