Introduction
Perishable foods like fruits and vegetables are susceptible to easy spoilage due to the presence of spoilage causing microbes. These spoilage microorganisms can have a negative impact on food’s nutritional content, colour, texture and palatability, thereby leading to a significant economic loss. In addition, foodborne disease outbreaks can be brought on by consuming foods or beverages polluted with bacteria or other pathogens.
According to the Centre for Disease Control and Prevention (CDC), Campylobacter spp., Clostridium perfringens, Salmonella spp., Listeria monocytogenes and norovirus are the main causes of foodborne pathogen infections, which are believed to be responsible for 48 million illnesses, 128,000 hospitalizations and 3000 fatalities annually. Other most significant global safety issues are chemical residues in food which offer a serious threat to human health globally. For instance, chemical residues from industrial and agricultural activities, such as organic dyes, fertilizers and pesticides can contaminate agricultural products, such as crops, livestock, vegetables and fruits which further increase the risk for humans. These contaminants may be toxic, mutagenic and carcinogenic to humans, which are linked to a number of diseases, including cancer, disorders of the reproductive system, neurological disorders, skin disorders, eye disorders and immune system-related diseases. Also, to prevent the microbial spoilage, few manufacturers adopt the chemical treatment for pre and post-harvest operation like cleaning, washing which also cause problems due to the presence of chemical residues. To decrease chemical pollutants in agricultural produce, a number of decontamination techniques have been investigated. The most popular traditional methods for removing chemical residues are washing with chlorine-based solutions and thermal treatments.
These technologies possess a number of drawbacks. For example, the usage of chlorinated water for washing is severely discouraged, as it can produce byproducts that are carcinogenic, which have a harmful impact on both the environment and human health. Additionally, the thermal processing procedures such as boiling might result in considerable chemical degradations. However, it can also have a detrimental impact on the sensory qualities and intrinsic qualities of treated foods.
Therefore, Non-thermal food processing technologies, including High Hydrostatic Pressure, Pulsed Electric Field, Ultrasound and Cold Plasma have been thoroughly investigated in recent years, due to increase in demand for fresher, safer and more nutritious foods from consumers. Due to its non-thermal treatment characteristics, cold plasma (CP) is an excellent alternative for preserving the quality of fresh-cut produce, while avoiding many of the drawbacks of traditional processes. NTP is an ionized gas consisting of electrons, positive and negative ions, gas atoms, molecules in the ground or excited state, free radicals and quanta of electromagnetic radiation (UV photons and visible light). As a result, Cold plasma has received a lot of interest in the food and agricultural sector in recent years, primarily for its applications in food sterilization and preservation.
The mechanism through which NTP inactivates microorganisms is caused by a number of synergistic processes, including the breakdown of cell membranes by charged particles and reactive species, UV damage to cell components and membranes and DNA chain breaking by UV and reactive species. Recent research indicates that CP treatments have a good effect on the functional qualities of a number of fresh fruits and vegetables, including cherry tomatoes, kiwifruit, blueberries and wheat germ.
However, the negative effects of CP treatment on produce are rarely mentioned. There is a direct discharge using this approach, which produces etching of the food surface morphology, as evidenced by the finding of black and rough edges in foods like banana slices treated with CP. Meanwhile, CP treatment of strawberries resulted in surface degradation due to colour loss and bioactive compound depletion. Highly reactive species, specifically reactive oxygen (ROS) and nitrogen species (RNS) are known to play important roles in microbial inactivation using cold plasma. However, the lifetime of some species generated in the gas phase is relatively short (<1 ns), as they are rapidly quenched during their frequent collisions with molecules or other species. The finding suggests that the efficacy of gas plasma in the inactivation of microorganisms is possibly hindered when it is surrounded by a wet environment or bulk liquids, (Zhou et al., 2016).
Also, food products has extremely uneven surface topography provides multiple hiding spots for bacteria which raising their resistance to cold plasma treatment. To solve this problem, plasma-activated water (PAW) has been developed. PAW or plasma-exposed water has been extensively accepted as an alternate approach for microbial disinfection of food products (Thirumdas et al. Citation 2018). PAW demonstrates remarkable and wide anti-bacterial action as an environmentally friendly and cost-effective disinfectant, bringing up new application opportunities in the food, agricultural and biomedical subjects. Therefore, this article has focused on the use of PAW for improving the quality and shelf life of fresh products.
Plasma-activated Water
Plasma-activated Water (PAW) is a liquid containing reactive species that might potentially be utilized to overcome such challenges, since it has the capacity to treat the whole surface of the foods on a wider scale. These plasma-generated active particles react with water molecules to initiate a cascade of chemical reactions and create a unique mixture of highly biochemically reactive chemistries, referred to as PAW.
Plasma-activated Water (PAW) is a result of non-thermal atmospheric plasma interacting with water and contains a diverse range of highly Reactive Oxygen Species (ROS) and Reactive Nitrogen Species (RNS). Both ROS and RNS are important characteristics in food decontamination. When plasma is discharged in water or above the water’s surface to form PAW, the reactive species O3, H2O2 and NO3 are relatively stable and long-lived and they are transported from the plasma into the liquid. Initially, Plasma-activated Water (PAW) was used in agriculture to accelerate seed growth, disinfect medical equipment and as a medicinal component. Recent investigations have focused on PAW plasma systems, reactive chemistries, in-vitro biological activity and applications in agriculture and medicine. PAW applications in food safety have grown rapidly in the recent decade after the first successful investigation on the anti-bacterial activity of PAW in fresh strawberries.
Methods to produce PAW
Plasma-activated Water (PAW) is produced as a result of interactions between NTP active particles and water samples. Several NTP-water reaction systems, including atmospheric pressure plasma jet (APPJ), dielectric barrier discharge (DBD), gliding arc discharge, pulsed corona discharge and plasma steamer discharge are purportedly employed to create PAW to allow these NTP-water interactions. However, APPJ and DBD are the most commonly used, because of their ease of use and ability to produce a diverse range of reactive species in PAW. In general, Plasma-activated Water (PAW) is generated primarily through atmospheric cold plasma (ACP) discharge in three categories like direct discharges within liquids (Figure 1a); discharges in the gas phase over the liquid surface (Figure 1b); and discharges in bubbles inside liquids (Figure 1c). To produce PAW, APPJ uses discharge plasma that extends beyond the plasma generation area into the surrounding ambience and stimulates a strong direct interaction with the water sample, whereas DBD uses discharges generated between two electrodes separated by a dielectric barrier material (glass or ceramic). Several critical parameters are also involved in PAW generation, such as the water source, power, activation time, working gas and position or distance of the plasma electrode toward the water.
Characteristics of PAW
Several physicochemical properties, such as pH, electrical conductivity (EC) and oxidation-reduction potential (ORP) are usually tested after successful PAW generation.
Concentration of Hydrogen Ions (pH)
The concentration of hydrogen ions (H+) in certain solutions is frequently referred to as pH. The interaction of NTP and water causes water acidification (low pH solution), which results in the formation of different reactive species. A reduction in pH is also caused by the formation of NO2 and NO3 from the breakdown of NOx, which produces H+. As a result, pH can be utilized as one of the primary indications of PAW. During the NTP-water interaction, the pH is also controlled by activation time and power. A low pH of PAW is caused by a lengthy activation period and a high power. Following a 20-minute PAW treatment, the pH of PAW was 4.11, compared to the pH of Distilled water, which was 6.92. The pH of the treated water or solution is inversely related to the plasma activation time in most circumstances. The pH of deionized water (pH = 6.80) fell to 2.00 after 3 minutes of DBD plasma treatment. Furthermore, the production of acidic hydronium ions (H3O+) as a result of the reaction of water molecules with H2O2 generated in air or liquid may contribute to the pH decrease. As a result, acidic pH is thought to play a crucial role in the reduction of microbial inactivation by PAW.
Oxidation Reduction Potential (ORP)
ORP measures a solution’s capacity to oxidize or reduce and it is thought to be a main factor influencing microbial inactivation by compromising cell membrane integrity and cellular defence systems. The higher ORP values of PAW are not only associated with the generation of ROS, such as H2O2 but are also highly related to nitric acid and HNO2. ORP values of PAW increased significantly, owing mostly to plasma activation time. PAW created under the water surface had a greater ORP value than that produced above the water surface. The generation of reactive chemical species such as H2O2, O3, NO3, NO2 and ONOOH is primarily responsible for PAW’s high ORP.
Electrical Conductivity (EC)
The EC value indicates the active ions present in electrolytic solutions. In PAW, these active ions included different ROS, RNS and other reactive species. The concentration of EC rises linearly with the activation time of PAW. As a result of these findings, the EC value increased considerably as a function of PAW production, providing evidence for the formation of active ions during NTP-water interactions.
Storage stability of PAW
The lowest storage temperature (−80°C) was proposed since the least loss of active substances and the maximal anti-bacterial activity of PAW was revealed at this temperature. PAW stored for 7 days under these storage conditions exhibited considerable inhibitory activity against S. aureus. Storage at 4°C could maintain PAW quality (including pH, NO2−, and NO3−) for 2 weeks. Moreover, Liao, Su, et al. (2018) revealed that PAW frozen and kept at −20°C for 24 hours still possessed significant inhibitory activity against microbial growth. From these studies, it is clear that PAW stability depended on the storage temperature.
Application of PAW for Fresh Produce
PAW has been extensively studied in the food industry, which includes preservation of fruits and vegetables, decontamination of meat and shell eggs, pesticide reduction and the curing of meat products.
PAW for preservation of fruits and vegetables
Plasma Activated Water (PAW) is a cutting-edge technique that has been proved to improve fruit quality and colour retention by altering the activity of the enzymes. According to prior work from (Feizollahi et al., 2021), the discharge voltage was set to 40 V, the discharge current was 0.8 0.1 A and the discharge distance was adjusted to 6 mm at normal atmospheric pressure with air as the working gas. After pouring deionized water into the reaction dish for 15, 30, 60 and 90 seconds respectively, the CP treatment produced the PAW. After 10 minutes of ultrasonic impact, the banana slices were submerged in untreated water and PAW and the parameters were observed. The ROS level in the cells of the banana slices was seen to rise after PAW treatment as compared to control samples and increased with the length of CP treatment, showing that the ROS in PAW penetrate the intracellular region during the treatment. After PAW treatment, POD and PPO activity in banana slices changed and this change was associated with leaking from banana cells and disruption of protein structures. Through the generation of ROS and the ensuing alteration of POD and PPO activities, we were able to show through these measurements how PAW treatment improves banana storage. According to the findings, PAW treatments raised intracellular ROS levels, increased the activity of polyphenol oxidase (PPO) in banana slices by 22% and decreased the activity of peroxidase (POD) by 21%. PAW activates PPO activity while inhibiting POD activity of banana slices. PAW maintains banana slices’ textural properties and improves their ester content. PAW preserves the original colour properties of banana slices as shown in Fig 2.
Similarly, the Fresh cut ‘Fuji’ apples were submerged for 5 minutes in plasma-activated water (PAW) produced by plasma created with sinusoidal voltages of 8 kV at 7.0 kHz amplitudes. Instead of PAW, distilled water was used to immerse the control group for 5 minutes. The findings showed that PAW treatments upon storage at 4°C prevented the development of bacteria, moulds as well as yeasts and they acted as most effective in microbial inactivation. Additionally, the fresh-cut apple bacteria counts after PAW treatment were <5 log10CFU g-1 up to 12 days of storage which is safe to consume. Without influencing their firmness or titratable acidity, PAW treatments decreased the surface browning of freshly cut apples. Furthermore, there was no discernible difference between the control and PAW-treated groups in terms of antioxidant content or radical scavenging activity. The suggestion is that PAW is a potential strategy for protecting freshly cut fruits and vegetables, which is typically advantageous to the preservation of fresh-cut fruits and vegetables’ quality during storage.
PAW for Pesticide reduction
The study on the impact of microbubble-assisted PAW on lowering pesticides in food has been initiated by Phan et al. (2018). After 5 minutes of treatment, their research showed that PAW generated from gliding arc discharge at 5 L/min. Air flow rate significantly reduced the amounts of chlorpyrifos residues in mangoes by 74% (Phan et al., 2018). The amount of chlorpyrifos in tomatoes was reduced the most (51.97%) when treated for a longer period of time (15 minutes) with PAW generated by the DBD system at an airflow rate of 10 L/h, (Gracy et al., 2019). After 10 minutes of PAW treatment, which was produced by a pin-to-plate atmospheric plasma discharge, Sarangapani et al. (2020) showed a 79% and 69% decrease of chlorpyrifos in grapes and strawberries respectively.
Conclusion
PAW is a very promising technology to control the microbial growth on fresh-cut apples, which is beneficial to inhibiting browning without inducing other quality changes. PAW treatment has great potential to protect banana slice quality and enhance banana slice storage resistance, while also being a novel and effective non-thermal pre-treatment method. In recent times, PAW has also been suggested as a possible chemical cleansing tool in the food system due to its extensive applicability as an anti-bacterial agent.
Since PAW has no discernible toxic effects in itself, it can be used as a safe chemical decontamination agent of foods without compromising human health, based on both in-vitro and in-vivo toxicological analyses. According to recent research, it may have a role in the safe breakdown of chemical pollutants (such as pesticides and mycotoxins) on the surfaces of a variety of foods and fruits, including wheat, tomato, grape and strawberry. The feasibility of using this technology on a larger scale and its commercialization needs further investigation.
References:
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About the Authors:
1. Sangeetha Karunanithi
Department of Agricultural and Food Engineering,
Indian Institute of Technology – Kharagpur,
West Bengal 721302, India.
E-mail ID: sangeetvp1996@gmail.com
2. Proshanta Guha
Department of Agricultural and Food Engineering,
Indian Institute of Technology – Kharagpur,
West Bengal – 721302, India.
3. Prem Prakash Srivastav
Department of Agricultural and Food Engineering,
Indian Institute of Technology – Kharagpur,
West Bengal – 721302, India.
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