Abstract
Cold plasma, an emerging and versatile non-thermal technology, has gained substantial attention, particularly in the domain of surface modification, specifically within the context of packaging films. Recent developments in cold plasma technology have unveiled its potential to improve various aspects of packaged films, including chemical composition, physical attributes, structural characteristics, and overall functionality. These enhancements encompass surface roughness, contact angles, flexibility, thermal stability, barrier properties, and antimicrobial efficacy. The imperative for the advancement and expansion of decay-control technologies is crucial, not only for preserving the quality of fresh fruits and vegetables but also for mitigating biological risks throughout postharvest, processing, and storage. This, in turn, extends the shelf life of these products. This review aims to comprehensively outline the various systems utilized in in-package cold plasma (CP) treatments and their interactions with key parameters that significantly influence the efficacy of the process on fruits and vegetables. In this order, the review furnishes a comprehensive understanding of the mechanisms through which cold plasma impacts the quality characteristics of diverse fruits and vegetables. The review paper examines the potential of cold plasma technology in inhibiting spoilage and pathogenic microorganisms, deactivating enzymes, and altering the physical, mechanical, and chemical characteristics of fresh fruits and vegetables during packaging. Furthermore, It deals with the effect of cold plasma technology on increasing the quality and characteristics of edible films, alongside its utilization as an antimicrobial agent in food packaging.
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Introduction
In the contemporary era, a pressing global concern pertains to food waste, which exerts a detrimental influence on both the environment and food security. From an environmental perspective, the squandering of food resources leads to the depletion of natural assets and contributes to ecological degradation (Santeramo, 2021; Yin et al., 2023). Recognizing the gravity of this issue, the United Nations has set forth ambitious objectives to halve retail food waste by 2030 (Ardra and Barua, 2022). A reduction in food waste not only provides increased access to fresh produce but also bolsters food security.
Furthermore, the consumption of perishable, high-quality food items intensifies the challenges posed by food loss. Fruits and vegetables, integral to a well-balanced diet, offer essential nutrients. Furthermore, cases of foodborne outbreaks linked to fresh produce are frequently reported (Pan et al., 2019 ). As a result, the demand for these items, particularly in their freshest and highest quality state, remains substantial (Esua et al. 2022). This persistent demand has spurred the search for effective methods to extend the shelf-life and preserve the quality of these vital food products. While thermal processing methods are effective in microbial decontamination, it was marred by issues such as physicochemical degradation and taste alterations during the process (Zhou et al., 2022; Ahmad et al., 2019). In response to these challenges, researchers began seeking alternative and more efficient methods.
Non-thermal technologies, including high-pressure processing (Abera, 2019), pulsed electric fields (Raso et al., 2022), pulsed light (Mandal et al., 2020), ultrasound processing (Wang et al., 2019), and ozone processing (Pandiselvam et al., 2020), emerged as solutions to extend the shelf-life of food products while maintaining their quality. In contrast to thermal processing, these technologies demonstrated better conservation of nutrients and sensory properties (Gururani et al. 2021). However, their application has been constrained by high costs and limited efficacy against bacterial spores (Juneja et al., 2018; Li et al., 2023b; Ashtiani et al., 2023).
Addressing these limitations is crucial. Among the most recent non-thermal technologies is cold plasma (CP), which offers sterilization, enzyme deactivation, and functional modification of products (Sarangapani et al., 2020; Sruthi et al., 2022; Laika et al., 2023). Cold plasma technologies have gained prominence in the agri-food sector as nonthermal alternatives to traditional thermal food processing methods (Zhao et al., 2021). The increasing adoption of cold plasma (CP) in various food-related industries can be attributed to its low operating temperature, high efficiency, and minimal environmental impact (Chen et al. 2020b; Cheng et al. 2023b). This technology is finding applications in sterilization, toxin degradation, and pesticide residue elimination, offering numerous advantages in food processing and safety (Cheng et al., 2023a). Further investigations into CP have demonstrated that quality attributes can be retained within acceptable parameters or potentially enhanced when specific plasma treatment conditions are applied (Chen et al., 2020a).
CP occurs when ionized gas is generated at a relatively low energy level (1–10 eV) and electron density (up to 10^10 cm^-3), resulting in the production of electrons, ions, free atoms, molecules, reactive oxygen species (ROS), reactive nitrogen species (RNS or RONS), and electromagnetic radiation like UV. ROS encompass ozone, monoatomic oxygen, and superoxide anions, while RNS include excited nitrogen, monatomic nitrogen, and nitric oxide. These species induce beneficial modifications in food matrices, enhancing food quality (Sharma, 2020; Wang & Wu, 2022; Sruthi et al., 2022; Mutlu et al., 2023; Cheng et al., 2023a). Additionally, CP, as an ionized gas state composed of neutral molecules, electrons, positive and negative ions, exhibits dynamic properties capable of inducing physical and chemical alterations on package films (Ranjha et al., 2023; Harikrishna et al., 2023; Mao et al., 2021; Hoque et al., 2022; Gupta et al., 2022; Sani et al., 2023).
Food packaging plays a pivotal role within the supply chain, exerting a substantial impact on the quality and shelf-life of fruits and vegetables (Tang et al., 2023). One of the key areas of investigation in this domain centers on in-package cold plasma technology. Within this realm, two primary techniques have garnered attention. First, the product can be enclosed within a package, which is then hermetically sealed. These packaging materials encompass a range of options, including oil-based and renewable plastics. Recent research endeavors have been especially dedicated to addressing environmental concerns, leading to the ascendancy of biodegradable and renewable film materials such as zein, chitosan, polylactic acid (PLA), and starch-based films (Abdul Khalil et al., 2018; Perera et al., 2022; Zhao et al., 2023b; Zhang et al., 2023a, c, d; Chia et al., 2023). These packages can be filled with ambient air or a tailored mixture of gases. Subsequently, an electric field is applied to induce plasma reactive species formation within the sealed packages (Wang et al., 2016; Moutiq et al., 2020).
In an alternate approach, the film itself is subjected to cold plasma treatment before being employed for food product packaging. Recent investigations have underscored the benefits of this method, demonstrating enhancements in mechanical properties, structural integrity, reduced gas permeability, and effective decontamination when cold plasma is applied to the packaging film (Abdul Khalil et al., 2018; Chen et al., 2020a). In-package cold plasma (CP) technology stands as a robust method for ensuring adequate sterilization and preventing microbial contamination (Mir et al., 2016; Ebrahimi et al., 2023). A fundamental aspect of in-packaged CP is the introduction of gaseous disinfectants within the packaging environment. This novel approach effectively targets and eliminates microorganisms residing on the surfaces of fruits and vegetables, leveraging the presence of plasma reactive species.
Plasma reactive species have shown remarkable efficacy in the substantial reduction of bacteria, fungi, spores, viruses, and pesticides (Misra et al., 2019; Zhou et al., 2022). The versatile nature of in-packaged CP treatments becomes apparent when considering various factors such as product size (Peng et al., 2020), texture (Ziuzina et al., 2014), treatment time (Ali et al., 2021), voltage settings (Zhang et al., 2021), and the choice of packaging film materials. Ensuring the freshness and optimal delivery of fruits and vegetables to consumers is a paramount challenge in the agri-food supply chain (Thirumdas et al. 2015). The extended journey from farm to consumers introduces the risk of quality deterioration due to the inadequacy of proper maintenance equipment. In this context, the implementation of pre-treatment and post-treatment methods for packaging emerges as a viable solution to preserve freshness, extend shelf-life, and minimize product waste.
A particularly promising approach to address these challenges involves the utilization of in-packaged Cold Plasma (CP). While existing review papers provide a general overview of novel post-harvest technologies, encompassing topics such as cold plasma (Palumbo et al., 2022), the various effects of cold plasma on fresh produce (Chen et al., 2019), and assessments of different cold plasma methods on fruits and vegetables (Mao et al., 2021), there exists a research gap in the investigation of in-package cold plasma. Specifically, the deficiency lies in understanding the diverse systems employed and their interactions with crucial parameters that impact the efficacy of the in-package CP process on fruits and vegetables.
In this study, the impact of in-packaged CP on the physical, mechanical, and thermal properties of packaging films has been explored, which, in turn, influence the respiration rate of fruits and vegetables. Finally, a comprehensive evaluation of the influence of cold plasma under varying parameters on the physicochemical properties of fresh fruits and vegetables has been conducted, while also assessing the advantages and limitations of relevant studies.
The Concept of In-Packaged Plasma Systems (Systems and Equipment)
In the context of in-packaged cold plasma (CP), a gas mixture is introduced into the packaging to interact with the enclosed products. This gas mixture can comprise ambient air or a tailored mixture, commonly referred to as modified atmosphere packaging (MAP). MAP is favored for its superior efficacy in extending the shelf-life of food products (Robles-Flores et al., 2018; Wang et al., 2023a). Subsequently, the package is subjected to an electric field, facilitating uniform ionization of the gas due to the extensive diffusion coefficient of reactive species (Pankaj et al. 2018). The reactive species encompass a spectrum of elements, including reactive oxygen species (ROS), reactive nitrogen species (RNS), ultraviolet photons, hydroxyl or hydrogen peroxide ions, and radicals, all of which exhibit potent antimicrobial properties (Mandal et al., 2018; Ziuzina et al., 2020).
Throughout this process, the ionized gas, alongside other plasma-formed particles, precipitates microbial inactivation and imparts an antibacterial effect on the products enclosed within the package. This occurs through collisions with the molecular membrane, leading to rupture and, consequently, the preservation of product quality. Notably, electrons within the system tend to have higher temperatures than other particles, which prompts the recombination of reactive species to reconstitute the initial gas mixture. Consequently, the gases maintain their state over an extended period, effectively curbing the proliferation of contaminating microorganisms (Misra et al., 2019; Dong & Yang, 2019; Lee et al., 2023a).
Confining the reaction environment to the packaging enclosure facilitates direct exposure of bactericidal agents to the targeted microorganisms. This approach enables effective disinfection and bactericidal action within the product, thereby extending the activity time of reactive species and mitigating the risk of post-process contamination. Consequently, it leads to an enhancement in the product’s shelf-life and storage capability (Gao et al., 2021; Laroque et al., 2022).
A variety of systems have been employed for in-package cold plasma generation, with all of them relying on the dielectric barrier discharge (DBD) method for plasma production. The simplicity of DBD reactors for larger-scale operation at atmospheric pressure and the cost-effectiveness of utilizing ambient air or modified atmospheric compositions as the package’s internal gas are key factors favoring DBD deployment. The DBD discharge involves two electrodes, one high voltage, and one low voltage, often covered with a dielectric material. The gap between these electrodes is often filled with air. The dielectric material obstructs the discharge, providing limited current conduction and charge transfer, thereby preventing spark or arc formation (Zhou et al., 2022; Whitehead, 2016).
As noted by Peng et al. (2020), in-package cold plasma (CP) systems can be categorized into two primary types: direct and indirect. In the direct method, food products are situated between two electrodes, exposing them to the impact of the generated reactive species. This type is suitable for treating smaller items such as grape tomatoes (Lee et al., 2023a), spinach (Ziuzina et al., 2020), and grains (Los et al., 2018) since these products can be readily positioned between two electrodes and exposed to the influence of cold plasma (Peng et al., 2020).
Additionally, Min et al. (2017) employed this approach (42.6 kV, 10 min) to mitigate E. coli O157:H7 contamination in bulk Romaine lettuce. The treatment resulted in a reduction of E. coli O157:H7 in leaf samples across 1-, 3-, and 5-layer configurations by 0.4 to 0.8 log CFU/g lettuce consistently. While the 7-layer configuration exhibited higher decontamination (1.1 log CFU/g lettuce) on the top layer, this efficacy was diminished in the bottom layers. Notably, agitation of the container enhanced the uniformity of inhibition.
Treating large-sized food products directly with cold plasma can be challenging due to the considerable distance between electrodes required for such products as increased electrode distance may hinder the effective generation of cold plasma. Moreover, attempting to address this issue by raising the voltage, considering the high voltages involved, necessitates an elevation in frequency. However, such an approach is not advisable as it may result in gas heating, posing potential harm to the product (Misra et al., 2019). In these cases, segmentation into smaller pieces has been reported as an optimal option. Mahnot et al. (2020) demonstrated this by treating carrot discs with direct plasma at 100 kV (RMS) for 5 min, resulting in a 2.1 log10 CFU/g reduction in microflora population. Notably, this treatment had minimal effects on pH, color, texture, and total carotenoid content.
Furthermore, indirect in-package cold plasma serves as a viable solution for large-sized food products. In this scenario, the products are situated in proximity to the plasma discharge region. This method relies on the diffusion of plasma species within the package to effectively treat the food (Peng et al., 2020). Misra et al. (2019) categorized three different designs employing the dielectric barrier discharge (DBD) method for plasma generation. The first configuration, termed volumetric DBD plasma, places the package between a high-voltage electrode and a ground electrode, where plasma is responsible for decontaminating the products. In this approach, exceptionally high voltages can be applied to generate cold plasma, even at line frequencies, thereby eliminating the necessity for a frequency generator (Misra et al., 2019). Sarangapani et al. (2017) evaluated the efficiency of this method in reducing Boscalid and Imidacloprid pesticides on blueberries. A treatment of 80 kV for 5 min yielded an 80.18% reduction in Boscalid and a 75.62% reduction in Imidacloprid. This was due to the effect of ROS and RNS on decontamination.
Dielectric materials, such as polypropylene (Zhang et al., 2021) or quartz (Guo et al., 2018), can be applied as coatings on the electrodes. The packaging film itself can serve as the dielectric material (Rana et al., 2020). Additionally, it is feasible to employ ambient air in the atmospheric DBD technique (Feizollahi et al., 2021), which is a prevalent choice in this context.
In the second method, Surface Dielectric Barrier Discharge (SDBD) is applied outside the package, where the limited penetration depth of the intense electric field ionizes the air in the headspace, leading to the formation of reactive species. The package is positioned close enough to the generator to be affected. This design aligns with the indirect in-package CP method mentioned in Peng et al. (2020)’s review. Misra et al. (2014a) applied this design to strawberries, resulting in an average reduction of 3.0 log cycles from the initial levels of 5 log10 CFU/g after 300 s of treatment at 60 kV rms (50 Hz).
The third design, known as SDBD inside the package, involves positioning the electrodes within the package itself to generate surface plasma. This design is categorized as an indirect in-package cold plasma. Unlike other methods, this approach exerts a lesser impact on the packaging film and is suitable for continuous processing (Cullen et al., 2018). Due to this advantage, it can also be viewed as a special design of in-package CP, where the minimum effect on the package is considered. Wang et al. (2022) utilized a unique approach, employing metalized paper as both the packaging material and electrode, with the polymer coating on the paper serving as a dielectric. In their investigation, a 10-minute plasma treatment led to a significant reduction in E. coli and Listeria innocua in baby spinach leaves and tomatoes. Specifically, E. coli in spinach leaves and tomatoes decreased by 4.6 ± 0.6 log CFU, and Listeria innocua saw reductions of 4.8 ± 1.7 log CFU and 2.0 ± 0.4 log CFU, respectively.
Unlike the first method, the second and third approaches are capable of treating products of any size. Furthermore, SDBD has a less severe impact on packaging materials compared to volumetric DBD. However, due to the high frequency required for SDBD, there is a potential for gas heating during discharges. This gas heating phenomenon, if not controlled, may limit treatment times to levels insufficient for achieving effective log reduction of pathogenic microbes while maintaining nonthermal conditions. Despite the absence of a single parameter explaining their efficacy and the unexplained role of discharge regimes, volumetric DBD generally demonstrated higher microbial inactivation efficiencies (Misra et al., 2019). In summary, while volumetric DBD exhibits higher efficiency, it is constrained in treating food products with large sizes.
Another model known as the pin array utilizes pins installed on the high-voltage electrode for discharge, without the need for dielectric material (Zhou et al., 2022). Functionally, this method aligns with the direct in-package CP classification proposed by Peng et al. (2020). Min et al. (2018) employed this mechanism on packaged grape tomatoes, where the high-voltage electrode featured evenly spaced pins, while the ground electrode had a smooth surface and a dielectric layer. Two layers of packed grape tomatoes were treated with 35 kV voltage for three minutes. Uniform Salmonella inactivation was achieved across all tomato locations. In this configuration, a larger quantity of small-sized products can be effectively treated, in comparison with volumetric DBD, a greater headspace can be accommodated (Min et al., 2018). As a result, the impact of pin arrays on treating large-sized food products warrants further investigation in subsequent studies.
In general, in-packaged CP studies have commonly utilized ambient air or modified atmosphere as the dielectric material. Depending on the target products, different systems’ configurations have been employed to optimize their reduction. Additionally, observations point to the use of various electrode types (Misra et al., 2019). The schematic diagram of in-packaged CP performance, along with its diverse methods, is presented in Fig. 1.
Schematic of in-package CP function along with different in-packaged CP generation methods, (a) the volumetric in-package DBD plasma, (b) pin array (c) SDBD plasma on the package (d) SDBD plasma inside the package
All the discussed methods are employed for microbial inactivation and decontamination purposes. The impact of in-package CP on product decontamination has been consistently measured in each investigation. Furthermore, CP has the potential to enhance beneficial materials in food products. For instance, Rana et al. (2020) applied direct in-package CP to strawberries. In addition to a significant reduction of 2 log10 CFU/ml in microbial load after a 15-minute treatment at 260 kV and 50 Hz, an increase in the concentration of chlorogenic acid, hyprin, phloretin, vanillin, gallic acid, 4-hydroxybenzaldehyde, and rutin was observed. The study suggested that the scavenging ability of phenolic acids for free radicals generated by cold plasma, along with the release of flavonoids from cellular membranes in response to reactive species, contributed to the concentration increment of these beneficial materials.
Recent observations indicate that foodborne pathogens might exhibit resistance mechanisms when exposed to CP treatment within real food systems. Therefore, it is imperative to underscore the significance of additional research in various critical domains. Primarily, it is essential to gain comprehensive insights into the development of resistance mechanisms in foodborne pathogens concerning the active substances generated by CP. Moreover, there is a pressing need to strike an equilibrium between the objective of reducing food processing steps and ensuring food safety. Additionally, the pivotal step of translating laboratory findings, which substantiate the effectiveness of CP in bacterial sterilization, into viable industrial applications cannot be overstated (Cheng et al., 2020).
Effective Parameters on the Performance of In-Package Cold Plasma
Internal Parameters
The effectiveness of in-package cold plasma (CP) in decontamination and maintaining product quality is contingent on various parameters. Table 1 provides an overview of the impact of CP on different characteristics of various fruits and vegetables. These parameters can be categorized into two groups: internal and external factors. Internal factors are influenced by plasma generation systems, equipment, and their operational conditions. Voltage and treatment duration are critical internal factors influencing the effectiveness of cold plasma (Kumar et al., 2023). An increase in both parameters intensifies cold plasma, leading to the generation of more reactive species. However, it’s noteworthy that extended plasma exposure may not always be advantageous. In an investigation by Zhang et al. (2021) on the decontamination effect of volumetric DBD on fresh-cut pears with varying voltages (45 and 65 kV) and durations (1 and 5 min), pears treated at 65 kV for 5 min exhibited higher bacterial populations, increased oxidative stress, and accelerated texture deterioration compared to those treated at 65 kV for 1 min. This outcome was attributed to the presence of more intercellular reactive species such as N2+, NO−, and OH ¯. Similarly, Sarangapani et al. (2017) explored two voltage levels (60 kV and 80 kV) with durations of 1 and 5 min on blueberries. While the 80 kV treatment for 5 min effectively eliminated more boscalid and imidacloprid pesticides, it had detrimental effects on the nutritional qualities of blueberries, particularly impacting the fruit’s texture. Similar findings were reported by Hu et al. (2021), who compared the texture of blueberries treated at 4 kV for 5 min with those treated for 20 min. The extended treatment duration resulted in fruit softening and a loss of anthocyanins, reinforcing the notion that longer exposure to cold plasma may have adverse effects on the texture and quality of fruits.
This entails employing high voltage for short treatment durations or low voltage for longer durations. This is because, despite the significant role of increased plasma concentration, the presence of additional active species in the plasma, led to the potential negative impacts on product texture and bacterial populations. Air pressure was found to have no significant effect on the performance of CP. Rashvand and Abbaszadeh (2019) specifically investigated the influence of in-package cold plasma on olives and reported that there was no significant impact associated with pressure variations (atmospheric and vacuum pressure).
The influence of initial gas composition on in-package cold plasma (CP) performance remains a debated topic. Misra et al. (2014a) found no significant difference in the decontamination rate of strawberries between different gas compositions (packages containing 65% O2, 16% N2, and 19% CO2 compared to those containing 90% N2 and 10% O2). In contrast, Subrahmanyam et al. (2023) observed superior quality in button mushrooms with higher oxygen concentration (the first included 80% O2 + 10% CO2 + 10% N2, 10% O2 + 10% CO2 + 80% N2, and the second one contained 10% O2 + 80% CO2 + 10% N2), suggesting a potential role of reactive oxygen species (ROS) in the decontamination process. The importance of initial gas composition in food preservation, similar to modified atmosphere packaging (MAP), calls for further research to explore its impact on cold plasma efficiency using controlled gas compositions in treated and control samples.
Perhaps the design of the electrodes constitutes a pivotal internal factor that significantly influences the efficiency of in-package CP treatment. Wang et al. (2022) introduced a novel approach for decontaminating fresh produce using flexible metallized paper-based cold plasma generating electrodes (CPPE). The study focused on spinach and tomatoes, employing a circular design for spinach and a kirigami and origami-based two-cone design for tomatoes to decontaminate E. coli and Listeria innocua. Activation of the circular CPPE for 10 min resulted in a reduction of E. coli by 4.6 ± 0.6 log CFU per spinach leaf and 4.6 ± 0.5 log CFU per tomato, while reductions for Listeria innocua were 4.8 ± 1.7 log CFU per spinach leaf and 2.0 ± 0.4 log CFU per tomato. The decontamination rate was notably higher than in previous studies, attributed to the optimal distribution of reactive species facilitated by the electrode shapes. However, the study did not provide an explanation for the specific influence of electrode structure on decontamination. Comparing these two designs with conventional electrodes may yield more reliable results in determining the optimal design for cold plasma (CP) generation. Additionally, further research could explore the impact of various parameters, including electrode diameter or width, electrode length, and dielectric material, on CP efficiency.
External Pararmeters
External factors also play a pivotal role in determining the efficacy of plasma application, and these factors are contingent upon the specific product and the intended purpose of the plasma treatment (Nasiru et al., 2023). The nature of the product and the contaminants present in it are significant external factors. The duration of plasma treatment can be tailored according to the product type, its surface characteristics, and the specific bacteria or contaminants targeted for elimination by plasma.
Ziuzina et al. (2014) investigated the properties of the targeted bacterial cells play a crucial role in the efficacy of in-package cold plasma (CP). The study revealed that Salmonella, E. coli, and L. monocytogenes populations on the surface of tomatoes were reduced to undetectable levels after treatment times of 10, 60, and 120 s, respectively. Gram-negative bacteria like Salmonella and E. coli exhibited higher susceptibility compared to the Gram-positive L. monocytogenes on strawberry surfaces. Moreover, Ziuzina et al. (2016) found that L. innocua demonstrated greater resistance to plasma treatment than E. coli when applied to the surface of cherry tomatoes within packages. This difference was attributed to the thicker membrane of gram-positive bacteria than gram-negative bacteria, rendering them more resistant to the effects of cold plasma.
The nature of the fruit or vegetable itself can impact CP effectiveness. Ziuzina et al. (2014) suggested that the rough surface of strawberries may act as a protective factor for bacteria, making them less susceptible to plasma treatment. Additionally, in pears, antioxidants were found to scavenge plasma-generated free radicals, influencing microbial inactivation efficiency (Zhang et al., 2021; Ziuzina et al., 2014).
Processes accompanying plasma treatment can also significantly influence its performance. For instance, Bang et al. (2020) employed a combination of mandarin orange washing with a detergent solution and atmospheric cold plasma treatment on their packaging. By using a calcium oxide solution at 0.2% solution weight and a fumaric acid solution at 0.5% weight, followed by plasma treatment at 27 kV for 2 min, a notable reduction in disease prevalence has been observed, reaching 64.3% ± 11.3%, compared to a control group with a disease prevalence of 97.3% ± 6.0%. The application of a calcium oxide solution in conjunction with plasma treatment was identified as the most effective method for preventing the growth of P. digitatum, likely owing to the high pH level associated with calcium oxide.
Furthermore, in Ziuzina et al.‘s (2016) study on cherry tomatoes, two processing modes, static and continuous, were employed for in-package CP treatment. After 150 s, the continuous mode resulted in lower populations of E. coli and L. innocua compared to the static mode. However, the static mode showed better reduction in mesophils and yeast/mold populations. The ozone concentration was higher in the continuous mode (900 ppm) than in the static mode (less than 450 ppm). While numerical values were reported, the study did not provide specific insights into how these operation modes impact decontamination and ozone generation. Further research is needed to explore the effects of these modes and other processing parameters on in-package CP efficiency.
Effect of Cold Plasma on the Package film
The application of cold plasma (CP) has a significant impact not only on the treated products but also on the packaging films (Jafarzadeh et al. 2021). The choice of an appropriate film material is essential for preserving food quality ( Múgica-Vidal et al., 2019; Chen et al., 2023). These effects manifest in alterations to mechanical properties, thermal characteristics, and surface structural properties, which are contingent on the film material and its composition (Hou et al., 2022; Wang et al., 2023a).
Preserving the integrity of the packaging film is crucial to maintain the desired gas composition within the package, prevent interactions with ambient air, and safeguard against product contamination. Consequently, the shelf-life of the products is prolonged, and their quality is enhanced as a result of proper packaging (Pasha et al., 2023).
In recent years, there has been a shift away from the use of oil-based and non-renewable plastic films in food packaging. Environmental concerns, coupled with considerations about the potential migration of contaminants from the film to the food, have led to the adoption of renewable biopolymer and biodegradable films. Materials such as polylactic acid (PLA), starch-based films, and gelatin have gained prominence as sustainable alternatives (Ulbin-Figlewicz et al., 2014; Abdul Khalil et al., 2018; Jeon et al., 2023).
Cold plasma exhibits the capability to enhance film properties and reduce water vapor permeation, particularly in biopolymer films like edible proteins and polysaccharides (Hoque et al., 2022). Plasma exerts a direct, rapid influence on film surfaces, bolstering adhesion strength, enhancing mechanical properties, directly disinfecting, and sealing surfaces through its physicochemical attributes in the context of packaging films (Chen et al., 2020a). These effects culminate in the creation of a confined insulating environment within the packaging, effectively retaining the active species (Zhao et al. 2020a). Furthermore, the non-damaging nature of plasma minimizes the presence of harmful substances, thereby amplifying the efficiency of in-package cold plasma. The findings presented in Fig. 2 support the conclusion that cold plasma (CP), as an innovative, highly efficient, and environmentally friendly physical technique, emerges as a viable alternative for the modification of cellulose, offering enhanced applicability in diverse fields, including the development of biological films for food packaging (Zhu et al., 2022).
Schematic of the changed structures of cellulose after CP treatment
CP can also be assisted by external materials. In order to produce cellulose nanofibrils (CNF), Zhu et al., 2023 pretreated pineapple peel cellulose using Fe2+ assisted cold plasma (CP) and investigates the underlying mechanisms behind this pretreatment. The research found that when cellulose was preabsorbed with Fe2 + and then subjected to 60 min of CP treatment, it could be effectively converted into CNF through ultrasonication. The Fe2+ assisted CP treatment significantly reduced the degree of polymerization of cellulose and enhanced electrostatic repulsion of cellulose fibrils. Moreover, it resulted in surface roughness and breakage on the pretreated cellulose. The study also observed a significant decrease in inter-molecular hydrogen bonds and the average crystallite size (Dhkl) of cellulose.
Biopolymer films, particularly those derived from pectin, exhibit high resistance to pressure, heat, and mechanical stress, as observed in studies by Li et al. (2023b); Perinban et al. (2023). When subjected to cold plasma treatment, these films undergo alterations, primarily in the film layers, with the degree of change influenced by applied voltage and plasma exposure duration. Higher voltages lead to more significant effects on mechanical and thermal properties. Notably, the surface properties are notably impacted, resulting in reduced contact angles and increased hydrophilicity, improving liquid spreadability on the films (Bahrami et al., 2016). Pectin film adhesion is enhanced through plasma treatment, causing the films to transition from a colorless, transparent state to a cloudy brown appearance. Protein films treated with atmospheric cold plasma exhibit reduced contact angles, a rougher surface, and diminished volume due to plasma etching, contributing to decreased permeability of water vapor, harmful gases, and microbes through packaging films. This reduction in contact angles plays a crucial role in ensuring packaging safety (Hoque et al., 2022).
Protein films encompass C = O and C = C bonds known for their elasticity and resistance. During CP application, mechanical properties are enhanced by molecular group interactions in the initial minutes, leading to increased film elasticity. As molecular groups draw closer, the contact angle decreases, and consequently, permeability remains nearly constant in this film after CP treatment (Moosavi et al., 2020). PLA films undergo substantial surface changes under the influence of CP, with surface roughness and contact angle increasing with prolonged plasma treatment. The wettability of PLA films remains constant compared to other films, and a 1.9% reduction in volume occurs, which can be restored by reducing the duration of plasma treatment (Song et al., 2015). When zein films are exposed to CP, some molecular structures composed of oxygen undergo oxidation within the initial seconds, leading to the formation of stronger S-S molecular bonds in place of the previous S-H bonds, thereby increasing the film’s resistance to mechanical stress (tensile strength and elongation at break) (Dong et al., 2018).
Applying plasma to biopolymer films enhances their impermeability, thereby preventing gases and microbes from infiltrating the package and enhancing the package’s biological safety. This contributes to maintaining the mechanical properties of the films and reducing physical damage to the package. Consequently, applying plasma to these films can extend the shelf life of food, fruits, and vegetable packaging.
Mechanical Properties
Cold plasma treatment can bring about changes in the mechanical properties of films by etching and crosslinking the film surface. The crosslinking reaction occurs due to the breaking of C-C and C-H bonds by ions released on the film’s surface, leading to improved mechanical properties (Perera et al., 2022; Hoque et al., 2022; Moradi et al., 2023). This phenomenon, along with the generation of free radicals through plasma bombardment on the film’s surface, results in the formation of bonds with surface radicals and participation in chain reactions, thereby modifying the film’s mechanical properties (Moosavi et al., 2020; Dong et al., 2020).
Among the polymers, PET, polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polystyrene are the most commonly used plastics in food industry packaging (Siracusa and Blanco, 2020; Wu et al., 2021; Pandey et al., 2022; Thakwani et al., 2023; Pulikkalparambil et al., 2023; Beji et al., 2023). Despite their numerous advantages, their hydrophobic properties and low surface energy have prompted researchers to modify their surfaces to achieve desirable packaging properties (Dong et al., 2018; Dufour et al., 2023).
Various studies have focused on enhancing the mechanical properties of polyethylene (PE) films, commonly employed in food packaging, through cold plasma (CP) treatment. Specifically, investigations into surface modifications of low-density polyethylene film (LDPE) revealed a gradual decrease in tensile strength (TS) within the power range of 15–75 W. However, at 90 W power, along with the emergence of polar groups and increased surface hardness, the TS was consistently higher for all CP treatment times compared to lower power levels. It is noteworthy that films treated with powers exceeding 45 W exhibited unsuitable appearances for packaging. Consequently, based on the experimental findings, a 30 W power level for 60 s was chosen as it resulted in fewer aging effects on the films (Wong et al., 2020).
Proteins play a vital role in the structure and function of plant and animal cells and can be used to create packaging films for fruits and vegetables. Common vegetable proteins used in fruit and vegetable packaging films include soy protein, wheat gluten, and zein (Abdul Khalil et al., 2018; Zhang et al., 2023a, c, d).
Soy protein, due to its affordability, easy accessibility, high protein content, and desirable film characteristics, is a suitable alternative to non-renewable films in food packaging. However, it has some disadvantages such as low flexibility, susceptibility to bacterial growth, and poor stability, which have limited its use (Li et al., 2022; Lee et al. 2023a, b). Li et al. (2022) studied the modifications in the physicochemical and structural properties of soybean protein films following cold plasma (CP) treatment. They found that applying CP to films with a concentration of 10% soy protein and 2.8% glycerol resulted in the highest elongation at break (EAB), which increased by 227.37% after CP treatment. This research aimed to make edible films suitable for the packaging industry with optimal performance.
In the context of sustainable food packaging, cellulose-based films are esteemed for their biodegradability and potential to substitute non-renewable materials. However, the inherent hydrogen bonding in cellulose poses challenges to its dissolution, limiting its versatility. To address this, Zhu et al. (2022) investigated the use of cold plasma (CP) technology for modifying cellulose from sugarcane bagasse pulp. CP treatment significantly improved cellulose dissolution rates under varied conditions. Visual analysis of treated cellulose surfaces revealed roughness, hole formation, and occasional breakage. Crystallinity index reduction and changes in hydrogen bonding forces were observed, indicating CP-modified cellulose’s potential for enhanced properties, making it promising for future applications in sustainable biopolymer films for food packaging.
Zein is a fraction of corn protein prolamin and has a transparent appearance, low permeability, and resistance to grease (Perera et al., 2022). CP treatment can be utilized to enhance the cell compatibility of Zein films. Dong et al. (2021) applied CP treatment to graft zein films with polycaprolactone (PCL). They observed an increase in tensile strength (TS) and EAB parameters after a 15-second treatment. This was attributed to a stronger chemical bonding between zein and PCL, which was achieved by a larger area for grafting and greater reactivity of the zein film surface due to plasma application. However, voltages higher than 100 V led to a decrease in TS, suggesting that the film bulk structure is more related to the mechanical properties than the surface layer.
CP can also be employed to enhance the properties of zein films for effective grafting. Li et al. (2023a) demonstrated that subjecting zein films to a 60-second CP treatment before coating them with PLA resulted in a 50.47% increase in TS and a 29.67% increase in EAB. The observed improvements may be attributed to potential cross-links and the additional deposition of a layer of polar groups on zein following CP treatment. However, extending the treatment time to 120 s resulted in a decrease in TS and an increase in EAB, possibly due to the degradation of the zein network induced by the high-energy properties of CP treatment.
Contact Angle
The wettability or hydrophobicity of packaging films is an important property that determines how liquids interact with the film’s surface. The contact angle is a measure of this interaction and represents the angle formed between the solid surface of the film and the tangent to the droplet of liquid placed on it. A lower contact angle indicates greater wettability, meaning that the liquid spreads more easily on the surface and adheres to it.
By applying cold plasma (CP) treatment on the surface of films, it is expected that the contact angle will decrease, resulting in increased wettability (Hoque et al., 2022; Perera et al., 2022). This improved wettability can be advantageous for several reasons, such as increasing the ability to print on the film.
Polypropylene (PP) and polyethylene terephthalate (PET) are commonly used polymers in the food packaging industry. PP is valued for its low density, low cost, high melting point, heat-sealing properties, and chemical neutrality. PET is appreciated for its high strength, rigidity, transparency, thermal stability, gas barrier properties, chemical resistance, and plasticity.
However, both PP and PET can exhibit weak surface adhesion properties. Applying CP treatment can enhance the surface properties of these materials with minimal destructive effects on the film (Lei et al., 2014). Lei et al. (2014) demonstrated that DBD plasma discharge for 3 min reduced the contact angle of PP and PET composite films from 98° to 68.1°, indicating improved hydrophilicity of the films. The presence of polar groups, such as oxygen functional groups, is a possible factor contributing to this reduction.
Exposing LDPE to CP before applying extracts to the film surface resulted in a significant reduction in the contact angle, decreasing from 89° to 42°. This was achieved using a plasma reactor with a power of 350 W and a treatment time of 45 s. The CP treatment induced modifications in carbonyl and oxygen molecular bonds within the film, thereby influencing its strain and tensile properties (Moradi et al., 2023).
Polylactic acid (PLA) and polycaprolactone (PCL) are both biodegradable polymers with unique properties that make them suitable for various applications, including in the field of packaging.
PLA is a biodegradable polymer that is typically derived from the ring-opening polymerization of lactic acid. It is known for its biodegradability and environmentally friendly nature. PLA exhibits low brittleness at high temperatures, and some of its physical properties, such as elongation at break, can be improved with increased temperature. The degradation characteristics of PLA can also be influenced by the addition of lactide rings in the polymer structure (Rafique et al., 2023).
PCL, on the other hand, is another biodegradable polyester, primarily produced by the polymerization of ε-caprolactone. It is valued in packaging due to its ability to integrate well with other materials, good viscoelastic and rheological properties, and slow degradation (Butnaru et al., 2021). Dong et al., 2020 mentioned the hydrophilicity improvement of zein films after cold plasma treatment was attributed to several factors, including surface morphology modifications through etching and bombardment during discharge, leading to an uneven surface that provided a larger contact area for water. Additionally, the increase in polar groups, particularly oxygen- and nitrogen-containing groups, contributed to the improved hydrophilicity of the surface. The voltage level during ACP discharge played a crucial role in enhancing wettability, with higher voltages leading to more pronounced effects.
A study by Heidemenn et al. (2019) explored the use of PLA and PCL films treated with an atmospheric cold plasma (CP) reactor to enhance their adhesion properties with starch layers, which is relevant to food packaging applications. The films were subjected to different CP treatment times, ranging from 1 to 15 min. The initial contact angle of PCL was 70.86°, and that of PLA was 50.89°.
The results of the study showed a significant reduction in the contact angle for both PCL and PLA films after CP treatment. The maximum decrease in the contact angle was 70% for PCL after 15 min and 35% for PLA after 10 min. This indicates that the wettability of the films improved due to the plasma treatment. The enhanced adhesion and wetting properties of the films are likely associated with increased surface roughness and the introduction of polar groups as a result of the plasma treatment. The higher sensitivity of PCL films to plasma treatment suggests that PCL films could be effectively modified to improve their adhesion properties for various packaging applications.
Water Vapor Permeability
Water vapor permeability (WVP) is a critical factor in the design and manufacturing of food packaging materials as it directly affects the shelf-life of products. Packaging films must provide an effective barrier against the permeation of moisture, oxygen, and organic vapors to prevent changes in the product’s quality and safety. Historically, chemical additives were used to modify film permeability, but this approach raised concerns about the impact on food safety and nutritional value (Li et al., 2022).
The impact of ACP treatment on the physicochemical and structural properties of soy protein films revealed that the application of ACP with a 30 kV voltage for 3 min minimized WVP. However, extending the treatment time beyond this point resulted in an increase in WVP. This observation suggests that prolonged exposure to ACP induces substantial alterations in the structure and composition of soy protein films, adversely affecting their barrier properties.
It’s important to note that the impact of ACP treatment on WVP may not follow the same pattern in all biopolymers.
Cui et al. (2022) conducted a study in which lily polysaccharide (LP) based edible films were enhanced with sodium alginate (SA) and cold plasma (CP) treatment. It is observed that significant decreases in WVP occur with the incorporation of SA into the films. This decrease in WVP is associated with the increase in hydrophobicity induced by the addition of SA, potentially due to hydrogen bonding between film components, leading to reduced water attraction.
WVP does not always change after cold plasma treatment. Edible films were developed using varying ratios of wild almond protein isolate and Persian gum by Tahsiri et al., 2023. After the optimal blend of these materials, it was subjected to cold plasma treatment for 5, 10, and 15 min. No statistically significant differences were observed in the WVP values between films treated with cold plasma treatment and untreated films. These findings suggest that the application of cold plasma treatment does not significantly impact the WVP, thus maintaining the shelf life characteristics related to water vapor permeability in packaged foods. While cold plasma treatment may influence the film’s solubility, it does not affect the diffusivity of water molecules. Consequently, the WVP value remains relatively unaffected.
In conclusion, the impact of ACP treatment on WVP can vary depending on the specific biopolymer and its characteristics. Factors such as treatment time, film composition, and the type of biopolymer may all influence the direction and magnitude of the changes in WVP. Therefore, it’s crucial to assess the effects of ACP treatment on different biopolymer films and their suitability for specific packaging applications, taking into consideration the potential influence of emitted gases from various food products during packaging. This individualized approach can help optimize packaging materials and maintain product quality and safety.
Thermal Properties
The thermal properties of packaging films play a pivotal role in determining their suitability for various applications, notably in the food industry. Understanding how cold plasma (CP) treatment influences these properties is essential for optimizing film performance in food packaging and preservation. This section provides a concise overview of the influence of CP treatment on the thermal properties of different packaging films, supported by relevant references.
Zein Films
Thermal stability in Zein films subjected to CP was assessed using thermogravimetric analysis (TGA). TGA analysis revealed that Zein films undergo thermal degradation at approximately 300 °C. Notably, CP treatment primarily impacts the surface layer of Zein films, leading to cross-linking and enhanced thermal stability (Dong et al., 2018). The choice of gas composition during plasma formation, such as air, can affect Zein film thermal properties due to alterations in polymer chain mobility and interaction ).
PLA Films
In certain cases, CP treatment exhibited minimal changes in the thermal properties of polylactic acid (PLA) films. The evaluation of thermal properties in PLA films involved both thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. Interestingly, these studies indicated that PLA films generally maintained their thermal stability after exposure to CP, with limited impact on parameters like glass transition temperature (Tg) and the primary polymer structure (Pankaj et al., 2014; Romani et al., 2019).
Fish Protein Films
CP treatment effects on fish protein films included reductions in Tg and enthalpy (ΔH). The observed decrease in ΔH aligns with the increased rigidity and reduced flexibility of fish protein films post-CP treatment. Changes in Tg serve as indicators of structural alterations within the polymer matrix influenced by CP (Romani et al., 2019).
Plant Based Films
Thermal stability of plant-based films have been increased after CP treatment. In the study of Cui et al., 2022, the incorporation of Sodium Alginate (SA) led to the formation of a dense filamentous structure, resulting in a significant enhancement of tensile properties. the CP treatment in the study had several effects on the thermal properties of the film. The CP-modified film showed an increase in thermal stability as evidenced by a higher Tmax (temperature at the highest decomposition rate) in thermogravimetric analysis. This improved thermal stability could be attributed to the introduction of nitrogen-containing functionalities and the crosslinking of functional groups on the film surface due to CP treatment.
When the potential of mung bean starch (MBS) for food packaging applications was explored by Chen et al. (2023). The challenge of preparing robust and uniform MBS films via industrial casting was addressed by modifying MBS using DBD-CP, that effectively broadens the temperature range within which the packages can be applied. Throughout the film treatment process, water infiltration into the granules triggers interactions between water molecules and the hydrophilic groups present in the modified starch, leading to heightened thermal stability.
It is essential to acknowledge that the impact of CP treatment on thermal properties is subject to variations based on factors such as film material, CP effective factors, and the intended application. Tailoring CP treatment parameters to specific film materials is crucial for achieving desired properties, as different films respond variably, with some showing enhanced thermal stability, while others may experience minimal or adverse effects.
Furthermore, the proposed avenue of further research entails exploring the implications of distinct thermal properties, subsequent to CP application, on the shelf-life extension of diverse fruits and vegetables. Such investigations would provide valuable insights into optimizing CP treatment methods for enhanced food preservation.
Antimicrobial Characterization
Within the domain of food packaging, the role of antimicrobial features has gained paramount importance in preserving the safety and integrity of food products. Antimicrobial properties offer a critical defense against microbial contaminants that can compromise product quality and safety, particularly during storage and transportation (Abdul Khalil et al., 2018). CP treatment enhances antimicrobial properties in packaging films containing bioactive compounds, such as oils or peptides. This comprehensive exploration underscores the effectiveness of CP-treated films in mitigating contamination risks and prolonging the shelf life of food products, offering valuable insights into enhancing food safety (Zhao et al. 2023a).
The incorporation of effective disinfectants into packaging films has emerged as a promising avenue to enhance antimicrobial properties. Honarvar et al. (2017) undertook an investigation into the development of polypropylene (PP) packaging films imbued with antimicrobial attributes. In a similar method, Lei et al. (2014) explored the application of CP as a means to incorporate chitosan and various preservatives into films composed of a polyethylene terephthalate (PET) and polypropylene (PP) blend. Subsequently, the modified films were rigorously assessed for their antimicrobial properties against three distinct microorganisms, namely S. aureus, B. subtilis, and E. coli. The results of this investigation unveiled a notable inhibition ratio of approximately 100% against B. subtilis and E. coli, highlighting the significant antimicrobial potential of CP-modified packaging films. Although effective, the inhibition ratio against S. aureus was found to be slightly less, standing at approximately 85%.
Packaging films can also be disinfected through Plasma-Activated Liquids (PALs). PALs are identified as having an efficient impact on microorganisms and their biofilms, with a range of reactive species and physiochemical properties playing a vital role in mitigating biofilms. The effectiveness of plasma-activated water (PAW) in biofilm inactivation is influenced by several key factors, including treatment time, exposure time, and physicochemical changes of plasma-activated liquids (PALs). Extended treatment times result in enhanced inactivation rates of bacterial biofilms due to increased physicochemical alterations in PALs, such as heightened acidity and a higher concentration of reactive species. Moreover, exposure time of biofilms to PAW is another vital factor affecting the antimicrobial effectiveness of PAW. Prolonged exposure time leads to higher biofilm removal rates due to the continuous generation of long-living reactive species in PAW. However, there is a point where the increase in exposure time no longer contributes to higher inactivation and bacterial counts begin to gradually increase. This phenomenon suggests that biofilm resistance to reactive species plays a crucial role, with the optimal inactivation occurring within a specific (short) exposure time (Zhao et al., 2023b).
The kinetics of antimicrobial compound release from CP-treated films, along with their release rate, constitute pivotal determinants of the films’ antimicrobial efficacy (Abdul Khalil et al., 2018; Kongboonkird et al., 2023). It is noteworthy that the judicious integration of CP with bioactive compounds and protective coatings can afford an effective food preservation strategy, forestalling product spoilage and impeding the proliferation of contaminating microorganisms (Bahrami et al., 2016). Table 2 provides an informative summary of the diverse modifications observed in the properties of various films following CP treatment.
Influence of cold Plasma on the Shelf life of the Fruit and Vegetables
To examine the impact of in-package cold plasma (CP) on the preservation of quality and extension of the shelf-life of fruits and vegetables, a comprehensive analysis of critical quality attributes is imperative. These attributes can be categorized into two primary groups: “chemical” and “physical.” The interplay of these characteristics plays a pivotal role in shaping the overall quality and longevity of fruits and vegetables. Variations, whether in terms of enhancement, deterioration, or stability, within these attributes can exert a discernible influence on factors such as taste, appearance, and consumer preference. Given the fundamental objective of in-package CP, its implementation is anticipated to uphold product quality and protract the shelf-life of an assorted array of fruits and vegetables.
Chemical Attributes
Active Species
CP treatment generates a multitude of active and transient species within the packaging environment (Zhou et al., 2022). These species encompass ROS, RNS, ultraviolet (UV) photons, positively and negatively charged particles, and free radicals (Misra et al., 2019; Zhang & Jiang, 2023). ROS encompasses a spectrum of compounds such as ozone, superoxide anions, singlet oxygen, atomic oxygen, and hydroxyl radicals. In parallel, RNS comprises nitrogen oxide, peroxynitrite, and alkyl peroxynitrite (Kang et al., 2021; Zhang et al., 2023a, c, d; Yang et al., 2023b, c, d). Some studies employ the umbrella term “reactive oxygen and nitrogen species” (RONS) to collectively refer to both groups of these chemically active species. Within this array, hydroxyl, alkoxyl, and ozone emerge as the most highly reactive, whereas superoxide, nitric oxide, and lipid hydroperoxides exhibit relatively lower reactivity and controllability. For instance, ozone’s prolonged action generates hydroxyl species, aiding in pesticide elimination within fruits. The involvement of nitric oxide leads to nitric acid production, and superoxide interacts with lipids in fresh produce, triggering hydrogen removal. These processes contribute to the effective reduction of pesticide residues in fruits and vegetables (Deng et al., 2019; Sarangapani et al., 2017; Sruthi et al., 2022; Zhao et al., 2021; Jeon et al., 2022).
Under oxidative stress, the antioxidant system within cells plays a critical role in maintaining intracellular redox balance and combating the harmful effects of reactive oxygen species (ROS). In a study conducted by Wu et al., 2022, the activities of various antioxidant enzymes were affected by the presence of ROS generated during plasma exposure. Plasma treatment initially increased catalase (CAT) and superoxide dismutase (SOD) activities in spores, peaking at 7 min. CAT was 4.54 times and SOD 2.59 times higher than untreated spores. However, activities declined after 9 min due to elevated intracellular ROS, damaging CAT and SOD enzymes, causing their leakage through the compromised membrane. GSH-Px activity increased consistently with prolonged plasma treatment, peaking at 9 min, showing a 7.18-fold rise compared to untreated spores. This demonstrated a robust antioxidant defense mechanism against oxidative stress. Investigating the impact of CP treatment on fresh-cut mangoes, CP significantly reduced the accumulation of O2 and H2O2 throughout storage, indicating its role in suppressing ROS generation. Despite initial ROS elevation during early stages, CP exposure later inhibited ROS production during storage (Yi et al., 2022).
Evidently, these chemically active species stand as the principal determinants influencing the impact of in-package CP on the quality and storage longevity of fruits and vegetables. While active species have a relatively short lifespan, their lingering antibacterial effect on the packaging endures for several days.
Microbes
A diverse range of microbial contaminants, including bacteria, yeast, spores, and toxins, pose a significant challenge to the preservation of fruits and vegetables (Dasan et al., 2017; Sharma et al., 2023). Cold plasma (CP) technology, acting through its active species, has emerged as a promising sterilization method (Gan et al., 2022; Lee et al., 2023a; Khodabandeh et al., 2023). Among these species, ozone stands out as a key bactericidal agent, while nitrogen oxide and nitrogen dioxide species, in conjunction with ozone, enhance the disinfection process (Yarabbi et al. 2023). Additionally, the combined use of methods such as the incorporation of citric acid has demonstrated promising results (Zhou et al., 2022; Wu et al., 2023).
Efforts to inactivate harmful bacteria such as Salmonella, Campylobacter, Escherichia coli, and Listeria monocytogenes have been a focal point of extensive research (Misra et al., 2019; Chen et al., 2019). Treatment of strawberries with ACP using a voltage of 260 V at 50 Hz for 10 min, followed by storage for nine days at 25 °C, resulted in a microbial load reduction of 1 log CFU/sample (Rana et al., 2020).
Zhang et al. (2020) investigated the effect of voltages (45 kV and 65 kV) and treatment durations (1 to 5 min) on pear fruit, followed by storage at 10 °C for a week. Results indicated that the use of a 65 kV voltage led to a greater reduction in bacterial counts, with yeast and mold levels also showing a decrease compared to the control (Zhang et al., 2021). In the study by Bang et al. (2020), ACP treatment was combined with calcium oxide (CaO) and fumaric acid (FS) washing to decontaminate mandarins within commercial PET packaging. In this setup, electrodes were placed 6.5 cm apart. The findings revealed that applying CP below 26 kV had no substantial impact on reducing P. digitatum levels, but a 27 kV voltage for 2 min proved effective, resulting in a 20% reduction compared to the control (Bang et al., 2020). The reduction of L. innocua and E. coli on strawberries and spinach leaves and Salmonella on tomatoes was significantly observed by implementing in-package CP on them (Ziuzina et al., 2020; Min et al., 2018).
Cold plasma treatment resulted in the extensive downregulation of virulence gene groups. This included genes associated with the intracellular infection lifecycle, biofilm formation, quorum sensing, and flagellar biosynthesis. Expression of key virulence genes, such as hly, iap, and plcB, which are crucial for the intracellular infection lifecycle, was significantly inhibited after plasma treatment. Cold plasma treatment inhibited the biofilm-forming capacity of L. monocytogenes. Plasma treatment resulted in a significant downregulation of the argA gene, linked to biofilm formation, and quorum sensing-related genes such as hly, agrA, plcB, luxS, lacD, and oppB. The AgrA–AgrC two-component system involved in biofilm formation and virulence factors control exhibited remarkable downregulation, indicating disruption in intercellular communication. Additionally, the dltABCD operon associated with cell wall synthesis and biofilm formation was significantly downregulated, suggesting reduced cell wall synthesis and diminished biofilm-forming capacity. Overall, cold plasma treatment exhibited inhibitory effects on quorum sensing-related genes, highlighting its potential in disrupting bacterial behavior and coordination. (Pan et al., 2023b). Further studies, including the use of mutant strains and investigation within blood or infected animal models, are recommended to explore the detailed mechanisms of cold plasma’s effect on L. monocytogenes pathogenicity.
Enzymes
Active species generated during in-package cold plasma (CP) treatment have been observed to cause the degradation of enzymes (Feizollahi et al., 2020). Enzymes, which encompass proteins, polymers, and amino acids, have been a subject of increasing interest regarding their interaction with CP (Sruthi et al., 2022; Akaber et al., 2023). The formation of reactive oxygen and nitrogen species (RONS) in in-package CP, along with alterations in enzyme structure and bonds, plays a pivotal role in enzyme deactivation (Zhou et al., 2022, Mandal et al., 2018). Since the method does not achieve the requisite temperature for enzyme deactivation through thermal processing (Pankaj et al., 2023), it is reasonable to attribute enzyme deactivation to the plasma generation process. The reduction in enzyme activity not only impacts the texture of products, reducing their softness (Kaavya et al., 2022), but also exerts broader effects on various parameters in fruits and vegetables (Zhang et al., 2021; Pan et al., 2023b).
Treatment duration is a crucial factor affecting enzyme reduction, and the impact of treatment on enzymes becomes evident over the storage period. Additionally, different enzymes exhibit varying reactions to in-package CP, influenced by their distinct protein structures (Sruthi et al., 2022). This variation can be attributed to the multitude of active species generated, each potentially reacting with a different enzyme structure, thus linking protein structure to treatment effectiveness. Moreover, the voltage level employed also plays a role in reducing enzyme activity; however, this effect is contingent upon the specific enzyme due to its influence on the generation rate of active species (Pan et al., 2021).
During in-package CP treatment on peroxidase (POD), higher voltage levels resulted in greater deactivation, and this voltage effect became more pronounced during storage. Over time, the activity increase in treated samples remained significant versus control, indicating the minor effect of the process and applied voltage on POD (Zhang et al., 2021). Denoya et al. (2023) explored the impact of in-package CP on the activity of the polyphenol oxidase (PPO) enzyme in apple fruit using a 30 kV voltage. A 3-minute treatment led to a significant decrease in enzyme activity. The difference between the control and the 3-minute treated sample was reported as 20%.
In the study by Han et al. (2023), the effect of in-package CP on the activity of horseradish peroxidase (hrp) enzyme was investigated. The gas mixture consisted of 98% Ar and 2% O2, and a 7 kV voltage was applied. A noticeable decrease in enzyme activity was observed, ultimately reaching a reduction of 17% after 10 min. The observed significant effect of voltage on various films suggests its importance in enzyme deactivation. However, it is crucial to consider treatment time for achieving an optimal film structure. Future studies should explore a range of voltages and treatment times tailored to specific films to enhance understanding and optimize the process.
Antioxidants
Antioxidants play a vital role in influencing the quality of treated food products, with significant implications for human health (Yang et al., 2023b, c, d). Recent research has explored how antioxidants respond to various factors associated with in-package cold plasma (CP) processes (Sruthi et al., 2022). The assessment of antioxidant activity, often determined using methods such as DPPH (2,2-Diphenyl-1-picrylhydrazyl), can provide insights into the impact of CP treatments on these compounds. Notably, a decrease in DPPH activity generally correlates with a decrease in antioxidant activity (Rana et al., 2020). In several studies (Chen et al., 2019; Yang et al., 2023b, c, d), it has been observed that appropriate concentrations of antioxidant compounds can enhance antioxidant activity, thus positively impacting the overall quality of treated products.
The effect of CP treatment in maintaining blueberry quality during cold storage, encompassing its dual role in inhibiting microbial growth and ameliorating the decline in firmness, soluble sugars, and ascorbic acid levels has been investigated by Zhou et al. (2023). CP treatment positively impacted attributes such as firmness, sugar content, and levels of ascorbic acid, while concurrently enhancing concentrations of total phenols, flavonoids, and anthocyanins in blueberries. Moreover, CP treatment significantly influenced the phenolic compound metabolism in blueberries, resulting in elevated contents of total phenols, flavonoids, and anthocyanins. Metabonomic analyses revealed that CP treatment contributed to the conservation of various phenolic acids, chalcones, flavanols, flavones, and anthocyanins, concurrently reducing the levels of L-phenylalanine.
However, the interplay of active species, oxidants, and enzymes can influence antioxidant levels during and after CP treatment. While active species can initially enhance antioxidant activity, they may lead to subsequent reductions over time (Fernandes et al., 2021). Several studies have reported on the complex relationship between in-package CP and antioxidants, yielding varying results. Subrahmanyam et al. (2023) investigated the impact of in-package CP on button mushrooms under different modified atmospheres. Their findings suggested that the CP treatment did not exert a significant influence on antioxidant activity.
The impact of cold plasma (CP) on antioxidant activity appears variable across different studies. For blueberries, enzymes like ascorbate peroxidase (APX), peroxidase (POD), superoxide dismutases (SOD), and catalase (CAT) reached peak levels two days post-treatment, followed by a decline. In contrast, fresh-cut pears experienced an initial POD activity decrease with a sustained low level during storage. In-packaged strawberries exhibited a 15% increase in antioxidant activity after a 10-minute CP treatment, diminishing with longer durations. Conversely, mandarin peel treatment with 26–27 kV for 1–4 min showed no significant effect on antioxidant capacity. These findings highlight the variable effects of CP treatment on different antioxidants, products, and potentially, voltage and treatment duration combinations.
While these studies reveal varied outcomes regarding the effect of in-package CP on antioxidants, they suggest that the voltage employed in the treatment process may be a crucial determinant of the impact on antioxidant levels. Additionally, factors such as the type of antioxidant and treatment duration can play pivotal roles in shaping the overall effect of CP treatment on antioxidant activity.
TSS and pH
The active species generated by plasma treatment can exert an influence on the pH of food products. Reactive oxygen species (ROS) can decrease the pH by forming carbonyl groups, while reactive nitrogen species (RNS) can contribute to the formation of nitric acid (Fernandes et al., 2021; Chou et al., 2023; Da Pinto et al., 2023). The presence of ascorbic acid, a significant component of fruit and vegetables, can also play a role in these changes.
Total soluble solids (TSS) is a critical parameter in assessing fruit quality, as it characterizes the amount of dissolved solids in a liquid and impacts the fruit’s sweetness and taste. In the context of in-package cold plasma (CP) treatment, it is generally expected to lead to a decrease in pH, resulting in increased acidity. However, previous research has not shown a consistent decrease in ascorbic acid or vitamin C, an important component of many fruits (Fernandes et al., 2021). Several studies have explored the impact of in-package CP treatment on pH, ascorbic acid, and TSS levels in different food products.
Mahnot et al. (2020) observed a maximum pH reduction of 0.5 units, primarily influenced by the treatment duration through a treatment of 100 kV for 5 min in carrot discs. No significant changes in pH and TSS were observed in the CP treatment of cherry tomatoes using a voltage of 100 kV for 150 s by (Ziuzina et al., 2016). It was reported by Ziuzina et al., (2019) and Rana et al. (2020) that TSS and pH of strawberries were not significantly impacted by CP treatment, and strawberries remained relatively stable in terms of acidity. Min et al. (2018) also observed no significant effect of CP treatment on grape tomatoes. The influence of CP treatment on the pH and TSS levels of fruits seems negligible based on current observations. However, it is suggested that additional studies explore the impact of CP treatment on various fruits and vegetables to comprehensively understand its effects on different types of produce.
The study conducted by Obajemihi et al. (2023) delved into the impact of diverse pretreatments, namely plasma functionalized water (PW), osmodehydration (OD), and the combined use of plasma functionalized water and osmodehydration (PO), on the drying characteristics and quality attributes of tomato slices during hot air drying at 55 °C with an air velocity of 1.5 m/s. According to this, Cold plasma pretreatment, in conjunction with hot air drying (HAD), demonstrated variable influences on the pH levels of tomato slices. An increase in ascorbic acid levels was reported by Sarangapani et al. (2017), applying CP treatment of 80 kV for 1 min on bluueberries. As cold plasma generates various active species, these active species can engage in chemical reactions with the components of the fruit. It is possible that the specific conditions applied in the study, such as the voltage and treatment duration, resulted in the production of active species that promoted the synthesis or preservation of ascorbic acid.
In summary, the effect of in-package CP treatment on pH and other parameters like ascorbic acid and TSS appears to vary across different studies. While there is a potential initial impact on pH due to the active species generated, no consistent pattern of change during storage has been observed. The effect on TSS is generally minimal, with no significant trends, although the potential influence of treatment duration and voltage on these parameters may warrant further investigation.
Weight loss
In the context of in-package cold plasma (CP) treatment and its impact on weight loss, it is essential to consider factors such as respiration, transpiration, and the role of packaging. The findings from various studies shed light on these aspects.
Respiration
Respiration is a vital biological process in fruits and vegetables, as it plays a significant role in determining their shelf life. Zhang et al. (2021) reported an increase in respiration due to CP treatment. This increase in respiration can be related to the metabolic activity of the treated produce. Respiration rates are influenced by factors like temperature, oxygen (O2) levels, and carbon dioxide (CO2) levels within the packaging environment. Bremenkamp et al. (2021) highlighted the importance of respiration in affecting the shelf life of products.
Effect of Storage Conditions and Bacterial Growth
Storage temperature can significantly affect respiration rates. Lower temperatures tend to slow down respiration, extending the shelf life of the products. The presence of O2 and CO2 in the packaging environment also plays a role in regulating respiration.
Respiration can promote bacterial growth, which in turn can impact the structural integrity of the produce. Bacterial growth may contribute to weight loss in treated products, especially when respiration rates are elevated. Pan et al. (2021) emphasized the potential connection between respiration and bacterial growth. It appears that adjusting the storage duration and temperature is a common practice to improve the decontamination process. Min et al. (2018) observed a decrease in Salmonella from 5.7 ± 0.7 log CFU/tomato to 4.1 ± 0.2 log CFU/tomato during storage at 10 °C. The conclusion drawn was that the sublethally injured Salmonella, as a result of CP treatment, could not withstand the cold stress during storage. This explanation is supported by the fact that the bacterial count did not significantly change when tomatoes were stored at 25 °C. It is recommended to optimize the treatment duration considering the time required for the maximum production of the desired reactive species. Beyond this point, there is a risk of decomposition through interactions with the packaging or other species. (Misra et al., 2019).
Role of Packaging
Subrahmanyam et al. (2023) investigated the effect of in-package CP on button mushrooms under various modified atmosphere packaging (MAP) conditions. Weight loss was compared between untreated samples, treated samples, and samples placed in MAP packaging. The study revealed that untreated and unpackaged samples experienced a weight loss of approximately 22%, while treated samples and those in MAP packaging had a weight loss of less than 5%. The MAP packaging appears to mitigate weight loss, possibly due to reduced transpiration and moisture escape. The closed environment of the packaging helps maintain the weight of the materials.
CP treatment, as applied in the mentioned studies, does not seem to cause significant weight loss in the treated products. Min et al. (2018) observed no significant change in weight following CP treatment of grape tomatoes. This led to the conclusion that weight loss in CP-treated products is primarily influenced by factors associated with respiration, transpiration, and the packaging environment. Further research is recommended to explore methods or strategies that can enhance the impact of CP treatment on weight loss, considering the potential for improving this aspect. These findings collectively suggest that while CP treatment may influence factors like respiration and bacterial growth, its effect on weight loss in treated products is relatively minimal, and the packaging itself plays a critical role in maintaining product weight. Future research can focus on optimizing treatment parameters to maximize the benefits of CP without causing significant weight loss.
Softening and Firmness
Over time, fresh fruits and vegetables experience alterations in their cellular structures, leading to a gradual loss of their initial moisture content (Zhou et al. 2022a). These transformations are primarily responsible for the development of softness (Pan et al., 2020). As the softening of produce can compromise its overall quality during storage, it becomes imperative to employ methods that effectively mitigate this softening process to extend the shelf-life of these products.
The softening of fruit is intricately linked to the disassembly of the cell wall, driven by various enzymes such as polygalacturonase (PG), pectin methylesterase (PME), cellulase (Cx), xyloglucan-endotransglycosylase (XET), xylanase (Xyl), and glycosidases. PG is responsible for lysing pectic acid and hydrolyzing a-1,4-galacturonosyl linkages in unesterified pectin, generating galacturonic acid. PME catalyzes the demethylation of esterified pectin, leading to homogalacturonan production catalyzed by PG. Cx acts on b-1,4-glucan linkages, promoting the degradation of cellulose and hemicellulose, thereby contributing to cell wall degradation. XET exhibits dual effects, cutting and connecting b-glucan linkages, likely associated with cell wall loosening, cell amplification, growth, and fruit softening. Lastly, Xyl and glycosidase negatively impact fruit firmness, leading to fruit softening (Fig. 3) (Pan et al., 2020).
In summary, fruit softening is orchestrated by the collaborative action of enzymes like PG, PME, Cx, XET, Xyl, and glycosidases, each playing a distinct role in cell wall modification and degradation, ultimately influencing fruit texture.
Interconnections between Fruit Softening, Influential Factors, and Cold Plasma. Cell wall-modifying enzymes contribute to the degradation of cell wall structure, resulting in fruit softening. Pathogens amplify the respiration rate, expediting fruit softening through the action of cell wall-modifying enzymes. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) generated by cold plasma exert effects on enzymes, pathogens, and respiration rate. PG: polygalacturonase; PME: pectin methylesterase; XET: xyloglucan-endotransglycosylase; Cx: cellulase (Pan et al. 2020)
Firmness Evaluation
Denoya et al. (2023) used a Texture Analyzer to assess the firmness of apples treated with CP. The study applied a voltage of 30 kV for 1 and 3 min with a 2 cm electrode distance. During the initial 7 days of storage, no significant differences in firmness were observed between the CP-treated samples and the control group. However, after the seventh day, all samples began to soften uniformly.
Effect over Time
It has been observed that CP treatment does not have an immediate, rapid impact on the firmness of fruits and vegetables. Instead, it appears to have a more gradual effect on preventing softening. The reasons for this effect could be attributed to several factors, including the reduction of the microbial load, the influence on enzymes, or the preservation of antioxidants. This effect also appears to be influenced by treatment duration. Cheng et al. (2023a) investigated the impact of plasma-activated water (PAW) treatment on the shelf life of blueberries stored at room temperature. Blueberries immersed in PAW for 10 min, generated with varying plasma exposure times (1 and 2 min), exhibited slower firmness reduction compared to the control group over 10 days. PAW treatment, especially for 2 min, effectively delayed softening by inhibiting cell wall degradation, contributing to improved fruit texture.
Enzyme Involvement
Enzymes, such as pectin methylesterase (PME), are known to contribute to the degradation of cell walls and, consequently, the softening of fruits and vegetables. Further research is recommended to explore the effects of CP treatment on other enzymes involved in softening, such as peroxidase (POD), superoxide dismutases (SOD), and catalase (CAT). Various enzymes, including polygalacturonase (PG), pectin methylesterase (PME), cellulase (Cx), xyloglucan-endotransglycosylase (XET), xylanase (Xyl), and glycosidases, play crucial roles in fruit softening by promoting cell wall degradation. PG and PME target pectic acid and esterified pectin, respectively, leading to the breakdown of cell wall components. Cx acts on b-1,4-glucan linkages, contributing to cellulose and hemicellulose degradation (Yang et al. 2023a). XET is involved in cell wall loosening and fruit softening by cutting and connecting b-glucan linkages. Xyl and glycosidases negatively impact fruit firmness.
Recent research indicates a correlation between fruit softening and polysaccharide solubilization, involving changes in pectin fractions (Wang et al. 2023b). Studies on different strawberry cultivars showed that PG and endoglucanase (EGase) are associated with the reduction of specific polymer content. Activities of b-galactosidase (bGal) and b-xylosidase (bXyl) were linked to neutral sugar removal and hemicellulose degradation, contributing to reduced fruit firmness. Similar findings in blueberries underscore the significant role of enzymes in cell wall depolymerization affecting fruit texture (Pan et al., 2021).
In summary, the study and references indicate that CP treatment may not immediately alter the physical state or firmness of fruits and vegetables. The effect of CP treatment on softening appears to be gradual and influenced by various factors, including treatment duration and its impact on enzymes and microbial load. Further research into the role of enzymes and other parameters in softening could provide a deeper understanding of the mechanisms involved.
Color
The color of fruits and vegetables serves as a vital indicator of their quality and can also reflect underlying chemical changes (Sruthi et al., 2022). This coloration is influenced by various factors, including the presence of antioxidants within these produce items. Active species, such as ozone and hydroxyl radicals, have the potential to induce color changes by affecting the oxidation of chromophores (Fernandes et al., 2021). Among the enzymes responsible for browning in fruits and vegetables, peroxidase (POD) and polyphenol oxidase (PPO) play prominent roles (Han et al., 2019). The activity of these enzymes becomes particularly pronounced when fruits and vegetables are subjected to injury or damage, and PPO, in particular, has been noted for its resistance to heat (Khoshkalam Pour et al., 2022). Research has indicated that reactive oxygen and nitrogen species (ROS and RONS), notably produced by plasma treatment, can impact enzyme activity, including browning enzymes (Zhou. R et al., 2022). As a result, it is anticipated that plasma treatment may mitigate the occurrence of undesirable color changes.
Button mushrooms exhibited better color retention during 4 and 7 days of storage after being treated at 28 kV for 15 min (Subrahmanyam et al., 2023). yellowness parameter of in-package CP-treated radish paocai was observed after 7 days of storage (Zhao et al., 2021) However, no significant differences in color parameters of grape tomatoes were observed after CP treatment. This lack of significant change was attributed to the absence of thermal treatment (Min et al., 2018). Also, the anticipated decolorization effect of ozone was not observed after in-package CP treatment of mandarines by employing a 27 kV voltage for 1 to 4 min (Bang et al., 2020).
In-package cold plasma (CP) treatment has shown significant effectiveness in preventing darkening and browning of treated products, with a clear correlation observed between reduced browning and decreased activity of enzymes, PPO, and POD. Min et al. (2018) suggested the crucial role of voltage level in determining treatment effectiveness on product color. Their study, involving samples without visible scars or injuries, found no significant changes in ascorbic acid content over time, indicating a potential limited effect on browning enzymes. Future research could expand on these findings by examining the treatment’s impact on fruits with natural scars for a more comprehensive understanding.
Bang et al., 2020, reported that CP treatment did not produce a discernible effect on brightness during storage at temperatures of 4 and 25 °C. Moreover, there was no significant alteration in the number of antioxidants present in the fruit peel. In contrast, a partial treatment effect with a lower voltage was observed by Zhao et al., 202. This difference in outcomes may be attributed to the use of MAP, which appears to enhance the formation of more effective active species targeting browning enzymes. Further research in this area may explore the specific mechanisms underlying these variations in treatment outcomes.
Respiration Rate
Respiration plays a significant role in the reduction of product weight and is a key factor influencing the storage duration of fruits and vegetables (Kumar & Pipliya 2023). Misra et al. (2014a) conducted an experiment applying in-package CP to strawberries with a 60 kV voltage and a 4 cm electrode distance. The examination of the respiration rate revealed that the treatment initially led to an increase in respiration during the first 10 h. However, from the 100th hour onwards, it became evident that the respiration rate was lower for the treated samples. This indicates that in-package CP treatment can effectively reduce the respiration rate of strawberries, contributing to their prolonged shelf life.
In a separate study, Misra et al. (2014b) investigated the impact of in-package CP on cherry tomatoes using a 60 kV voltage and a 2.2 cm electrode distance for various durations of 30, 60, 180, and 300 s. Interestingly, no distinctive correlation was observed between the treatment duration and its effect on the respiration rate. After 200–300 h of storage, both treated and control samples exhibited similar respiration rates.
Zhang et al. (2023a, c, d) conducted research involving in-package CP treatment on fresh-cut pears using voltages of 45 and 65 kV for durations of 1 and 5 min. In this study, both treated and control samples experienced a decrease in the respiration rate over the storage period. Notably, the 5-minute treatment with a voltage of 65 kV resulted in a higher rate of respiration, which can be attributed to the physical stress exerted on the pears during treatment. These findings emphasize the dynamic interplay between in-package CP treatment, respiration rates, and the preservation of product freshness.
Morphological Properties
Field emission scanning electron microscopy (FE-SEM) has played a crucial role in research on in-package cold plasma (CP) treatment to assess the morphology of treated fruits and vegetables. It has been observed that direct treatment with high voltage can lead to the disruption of the morphological structure (Pan et al., 2020; Hemmati et al., 2021). Additionally, in-package CP treatment is known to roughen the surface of food products (Sruthi et al., 2022).
In a study conducted by Bang et al. (2020) on mandarins, FE-SEM was employed to examine the mandarin surface before and after in-package CP treatment. Two treatment methods, static and continuous, were applied. One method involved direct atmospheric treatment using the Dielectric Barrier Discharge (DBD) technique, while in the other, mandarins were first washed with CaO and then subjected to CP treatment. The samples were treated with voltages of 26–27 kV for durations of 1–4 min and were stored at temperatures of 4 and 25 °C. The FE-SEM analysis revealed no significant difference in the structural morphology between the control and treated samples. However, a layer of calcium was observed on the peel of the washed samples. This calcium layer appeared to prevent evaporation by entering the pores of the peel, with more pronounced effects at lower temperatures (4 °C).
Exploring the impact of varying plasma durations on the structural and rheological characteristics of water-soluble pectin (WSP), chelator-soluble pectin (CSP), and diluted alkali-soluble pectin (DASP) fractions extracted from okra pods by Zielinska et al. (2022), led to efficient extraction at prolonged CP treatments (15 and 30 s) by inducing microcrack formation on okra surfaces, enhancing uronic acid, rhamnose, and galactose richness in pectin fractions without significantly impacting the degree of methylation. Conversely, short CP treatment (5 s) resulted in debranching and reduced molecule dimensions in the WSP fraction, while longer treatments extracted disrupted arborization-like structures from the CSP fraction and impeded the formation of a network by pectin molecules in the DASP fraction. Furthermore, CP treatment altered fluid behavior from dilatant to Newtonian (CSP and DASP) or pseudoplastic (WSP). In another study by Bang et al. (2021) involving in-package CP treatment on mandarins, SEM was used to investigate surface morphology modifications. It was concluded that the calcium layer created a barrier that prevented evaporation by entering the pores of the peel.
In research by Min et al. (2017) on romaine lettuce, in-package CP treatment was applied using a voltage of 42.6 kV for 10 min with a 5 cm electrode distance. The samples had varying numbers of layers of lettuce in the package, and SEM was used to assess morphological changes. The results indicated that there was no significant alteration in the morphology of the samples after the treatment. Therefore, it appears that in-package CP treatment does not significantly affect the surface morphology.
It is worth noting that washing with CaO has demonstrated pore-related effects that have outperformed in-package CP treatment. Given that treatment voltage has shown effectiveness in other cases, it may be advisable to repeat experiments with higher voltages to explore potential improvements. Additionally, investigating whether the treatment has an impact on the morphological properties obtained with detergents would be a valuable avenue for future research. In Table 3, an overview of investigations exploring the effects of in-package CP on the quality and shelf-life of fruits and vegetables is presented.
Challenges of Applying In- Packaged Cold Plasma in Food Industry
While in-package cold plasma has shown potential for many applications in the food industry, there are some challenges associated with this technology which need to be taken into account. Generally, Cold plasma technology is still in its early stages of development and there are limitations on scalability. It may be difficult to achieve the consistent and controlled plasma generation in food packages. The compatibility of packaging materials with plasma requires to be carefully considered. The safety and quality of foodstuffs may be impaired by some packaging materials not complying with the plasma treatment process, or it may become damaged while being treated. Regulatory approval is necessary for the use of in-package cold plasma as a food processing technology. Food manufacturers will have to comply with food safety rules and demonstrate that the cold plasma treatment is efficient and safe for regulatory authorities, which can be a lengthy and complicated process. In some regions, although cold plasma technology appears promising, full regulatory approval may not yet be granted for specific applications. This is likely to impact on its adoption and commercialization in the food sector. The production of reactive species and free radicals, which can be harmful to human health if not properly controlled, is involved in cold plasma utilization. It is imperative to ensure the safety of processed food products and minimize possible risks related to chemical residuals or byproducts. Another problem is the acceptance and perception of plasma processed foods by consumers.
The safety, taste and texture of treated products may be subject to concern. In order to make the use of in-package plasma technology widely available in food processing, it is essential that consumers be educated and informed about its benefits and safety. The sensory properties of the food, for example colour or flavour, may be adversely affected by cold plasma treatment. For instance, it can result in discoloration or alteration of the texture of sensitive food products. Investment in equipment, process optimization and training of personnel is necessary to implement cold plasma technology. For small and medium sized food manufacturers, the costs of adopting this technology could be a barrier to its widespread use in the sector. Due to limitations in the design and scale of equipment, it may be difficult to implement cold plasma treatment for largescale food production. Given the high costs of plasma production equipment and the need for special infrastructure, wide adoption achievement may not be easy. Meanwhile, In the articles, 20 s to 30 min have been reported for treatment, but for industrial use, less durations are acceptable. Also, factories, when implementing a new technology, prefer not to make big changes in the existing production line, so this should be considered in the system design. The efficacy of in-package cold plasma treatment can vary depending on the composition and characteristics of the food product. The effectiveness of cold plasma treatment may be affected by factors such as surface topography, moisture content and fat content. In the case of individual food types, optimization may therefore be necessary. Cold plasma treatment may reduce the microbial load and improve food safety, but it may not always lead to significant prolongation of shelf life. To attain the desired prolongation of shelf life, other factors such as temperature control, good packaging and storage conditions may also have to be taken into account. Approaches combining two or more non-thermal technologies such as in-package plasma, based on the Hurdle concept, are also worth investigating.
Future Trend
The future for the use of CP in the food industry seems to be very promising. However, a lot of research has been done for applying CP in food technologies, there are many issues need to be considered before introducing industrial-scale plasma equipment for treatment of foods. As with all emerging technologies, there is a need to standardize the main parameters such as frequency, power density, working temperature, etc., to converge efforts for CP industrialization and accelerate developments in plasma applications for the food industry. On the other hand, environmental and safety considerations should also be studied in the design of scaled-up CP systems, as well as the health of operators. CP sources having the capability of ionizing air in large gaps would be ideally suited for the food industry. Plasmas generating in gas mixtures other than air, is critical for MAP application, while the behavior of the CP system in different ratios of gas mixtures needs to be investigated and modeled. The main expectation in the food industry is the ability of such systems to continuously work at high speed for months, while needing the least maintenance.
There is a knowledge gap in the reaction chemistry of CPs and their treatment effects on the food and packaging materials, therefore, efforts should focus on understanding plasma kinetics and controlling the plasma chemistry. This is critical for the application of in-package CP for treatment. It is crucial to quantify all possible changes in the CP treated packaging including migration of monomers, oligomers and low molecular weight volatile compounds from the packaging material into the food. Atmospheric CP has a unique potential to be applied for food decontamination, removing toxins, modifying bio-based packaging films, as well as generating reactive gas from air and electricity. To operationalize these potentials at industrial levels, it is needed to enhance corresponding manufacturing technologies, which requires dedicated research and development in new technology. This requires that research should be conducted sequentially at laboratory scale, prototype development, pilot scale manufacturing, and then commercialization. It is also suggested that to meet the best performances of safety, efficiency and functionality, it is needed to optimize operational conditions of CP. The CP applicability in the field of food is in laboratory scale, yet. The low penetration of CP is also a major issue for its decontamination applications because microorganisms that exist below the effective penetration depth of plasma may be spared. Effects of CP processing on sensory and physicochemical properties of foods should be determined. The development of CP systems for liquid food processing may be a major and attractive area of future research. Furthermore, combining CP with other novel and traditional technologies, such as power ultrasound, microwave, high pressure processing, etc., may be another approach for intensifying benefits of CP-based systems. However, we mentioned several applications of CP in the food industry, but we encourage researchers to explore and develop new applications of this technology in food processing.
Conclusion
This review article offers a comprehensive overview of in-package plasma technology, highlighting its conceptual foundations and recent progress. The technology holds significant promise in enhancing the quality, safety, and durability of packaged products. Several applications, benefits, and challenges associated with in-package plasma have been explored, shedding light on its potential to revolutionize the food industry. In-package plasma technology represents an innovative solution with the capability to effectively eliminate pathogens, extend product shelf life, and provide environmentally friendly alternatives to conventional chemical treatments. However, there are inherent challenges in implementing this technology at an industrial scale. These limitations can be addressed through ongoing research and advancements in cold plasma treatment techniques.
To fully harness the potential of in-package plasma systems, further studies are essential. These investigations will contribute to optimizing the design, performance, and cost-effectiveness of in-package plasma technology while also uncovering any potential side effects. As research efforts continue, it is expected that in-package cold plasma will become more widespread and commercially viable. This advancement holds the potential to significantly transform the food sector, contributing to a more sustainable and healthy future.
Data Availability
No new data were created or analysed in this study.
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This project received support from the Food engineering department, University of Basilicata as well as University of Tehran.
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Mehrad Nikzadfar: Data curation; visualization;writing–original draft. Amirali Kazemi: visualization;writing–original draft. Reyhane Aboyi: visualization;writing–original draft. Rouzbeh Abbaszadeh: Supervision; validation; writing–original draft. Mahmoud Soltani Firouz: Supervision; validation; writing–original draft. Abbas Akbarnia: visualization; writing–review and editing. Mahdi Rashvand: writing–original draft; Investigation; methodology; project administration; Supervision.
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Nikzadfar, M., Kazemi, A., Abooei, R. et al. Application of Cold Plasma Technology on the Postharvest Preservation of In-Packaged Fresh Fruit and Vegetables: Recent Challenges and Development. Food Bioprocess Technol (2024). https://doi.org/10.1007/s11947-024-03380-6
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DOI: https://doi.org/10.1007/s11947-024-03380-6