Cold plasma is a non-thermal preservation technique with a wide range of applications such as microbial inactivation, control of biofilms, surface modification, pesticide dissipation, andmany more. Plasma treatment of water results in the generation of plasma activated water(PAW) . PAW has also been established for its antimicrobial activity and hence can be used as a chemical-free, non-thermal disinfecting method. Various reactive oxygen and nitrogen species (RONS) are produced, which are responsible for sterilization capability of PAW. This review provides a brief understanding of the chemistry of PAW, methods, and modes of generation and mechanisms involved for microbial inactivation. One of the potential uses of the cold plasma includes generation of plasma activated water as an alternative to chemical disinfecting agents. Some of the potential applications of PAW in the food processing sector is also discussed.
Recently, cold plasma has gained a lot of attention in the food sector due to its capability of inactivating microorganisms and decontaminating surfaces . Plasma is the fourth state of matter that can be defined as ionized gas comprising of positive and negative ions , electrons, free radicals, photons , and electromagnetic radiation, as shown in Figure 1. It can be classified into two categories: Thermal and Non- Thermal plasma depending upon the thermodynamic equilibrium among the constituents of plasma. One of the potential uses of the cold plasma includes generation of plasma activated water as an alternative to chemical disinfecting agents.
GENERATION MODES OF PAW
Plasma Activated Water (PAW) or Plasma Processed Water(PPW) or Plasma Treated Water(PTW) can be defined as the water obtained by exposing water to non-thermal plasma resulting in the formation of various reactive oxygen(ROS)and nitrogen species(RONS) which includes hydroxyl radical (OH●), singlet oxygen(O●) ,ozone(O₃), hydrogen peroxide(H₂O₂) , nitrous(HNO₂) , nitric(HNO₃) , and peroxynitric acid(ONOOH) .
There are two modes of generating PAW :
one is above the water surface (including the plasma stream over the water surface), and other is beneath the water surface(plasma stream directly inside the water) as shown in figure 2. ROS (Reactive Oxygen Species )production physicochemical properties, and microbial inactivation efficiency would vary in both modes. Plasma Activated Water is produced using water, air and electricity. Ambient air is brought into the plasma phase with electrical energy to produce plasma activated air. Plasma activated air is brought into contact with water. Reactive nitrogen and oxygen(RNOS) are dissolved into the water to produce Plasma Activated Water. The process of treating water with plasma is copied out of nature (When lightening strikes during a thunderstorm, rain and water are treated with plasma). It is one of nature’s ways to fixate nitrogen in the form of nitrate, the most important fertilizer for plants and crops. After the activation process, PAW possesses unique disinfecting properties that can be used for hygiene purposes. These properties are reduced over time and the reactive elements fall back to its original states leaving water with an elevated concentration of nitrate.
APPLICATION OF PAW IN FOOD PROCESSING
- Microbial inactivation
Microorganisms are the primary cause of food spoilage, thereby making the food unsuitable for human consumption. Hence, it is essential to kill these microorganisms with minimal loss of nutritional and organoleptic properties. Various researchers have highlighted the antimicrobial efficiency of PAW. The antimicrobial activity of PAW against various food pathogens such as Shewanella putrefaciens, Pseudomonas deceptionensis CM2, Escherichia coli, Leuconostoc mesenteroids has been widely investigated. Sterilization efficiency of PAW against staphylococcus aurues was investigated by and they concluded that PAW could be used as an eco-friendly disinfectant as it resulted in 7 log reduction due to damage DNA and cell integrity. Mechanisms responsible for microbial inactivation can be categorized into two broad categories viz. inactivation due to ROS and RONS and physical parameters. OH● radical attacks cell membrane lipid initiating peroxidation and results in the formation of malondialdehyde (MDA). Hydrogen peroxide diffuses inside the cells and starts reacting with intracellular metals(Fe²⁺/Cu⁺) resulting in the creation of OH● radicals. This results in oxidative stress inside the cell leading to cell death. Reactive nitrogen species such as HNO₂ , HNO₃ results in acidic pH, which is proven to be lethal for microorganisms. Peroxynitrite acid also causes lipid peroxidation of the cell membrane of microbes. The physical parameters such as the production of shockwaves, UV radiations, drop of pH, an increase in ORP, and electrical conductivity are also lethal for microorganisms. UV radiations cause photolytic changes in D NA and results in a disturbance of internal homeostasis of microorganisms. The important physical parameters responsible for microbial inactivation are pH, ORP, UV radiation, shock waves, photons and electric fields formed in PAW (Figure 3).
Figure 3: Schematic diagram of the PAW induced cell destruction through the different mechanisms caused by the reactive species (ROS and RNS), physical parameters (pH, ORP, UV radiations and shock waves), lipid peroxidation and electroporation (+).
Source : https://www.sciencedirect.com/science/article/pii/S0924224417305873
2. Seed Germination
PAW can also be used as a possible way of enhancing germination rate. Abscisic acid is a hormone responsible for the dormancy of seeds and delaying seed germination. Hence, the germination rate can be enhanced through the catabolism of abscisic acid. Reactive oxygen species produced in PAW act as the intracellular signals for seed dormancy release. Also, nitric oxide helps in the catabolism of abscisic acid and the synthesis of gibberellins. H₂O₂ present in PAW increases catalase enzyme, and this leads to the synthesis of new proteins, which helps in enhancing seed germination. 50% increases in rye seeds germinability when treated with plasma activated water for 5 minutes. Hence, it can be concluded that various reactive oxygen and nitrogen species present in PAW play a significant role in breaking seed dormancy and enhancing seed germination rate.
3. Meat, poultry, and seafood
Meat and seafood are the rich source of proteins, lipids, and other nutrients, making them an excellent medium for the growth spoilage microbes. This makes them highly perishable commodities limiting their shelf life and quality. To overcome these problems, several preservation methods such as freezing, curing, chilling, use of chemical preservatives, modified atmosphere, and active packaging have been adopted. Recently, the application of PAW ice for the preservation of shrimps. They found that PAW-ice resulted in rapid inhibition of microorganisms, retardation of quality loss, and longer shelf life of shrimps as compared to TW-ice (Tap Water). Curing is another preservation method adopted for meat and meat products which involve the addition of synthetic curing agents such as sodium nitrite. With the increasing awareness about natural foods, it has become fundamentally important to find an alternative to these chemical preservatives.
4. Fresh Produce
The greatest challenge faced by the fruit and vegetable industry is their high susceptibility to post harvest deterioration resulting in huge losses. Cleaning is one of the significant postharvest operation done to prevent fruit decay and to fetch a reasonable price from the market. However, this may lead to chlorine residues over the fruits and vegetable surface, which may pose serious health issues. In this context, PAW can be utilized as an alternative to chlorinated water due to its antimicrobial activity. PAW not only helps in the reduction of microorganisms but also results in no change of firmness. The effect of PAW on mushrooms and found a decrease in microbial load and browning, with no changes in firmness in PAW treated samples. The anti-browning effect was due to bleaching effect of ROS present in PAW.
In summary,It can be concluded that plasma activated water is a powerful non thermal technique which can be utilized for different remarkable applications such as inactivation of microorganisms, decontamination of surfaces, washing of fruits and vegetables ,enhancement of seed germination and plant growth. High sterilization capability and ease of generation of plasma activated water is useful for every food sector such as meat, dairy ,fruits and vegetables , and grain.