Significant Research and Technology Landscape on Electrostatically Charged Sprays for Agro-Food and Health Industry

The trans-disciplinary aspects of the embryonic field of electrostatic spraying have provided a major motivation to researchers for the development of novel techniques for spraying of protective coatings to perishable food commodities for enhanced shelf-life, protective agents for health and hygiene in addition to other applications of sprays to manufacturing, transportation, agriculture, environment, medical facilities and devices. It has become the motives behind the renewed curiosity in the usage of the electrostatics in liquid based sprays. This provides an efficient and uniform transportation of liquid based agents which avoids the wastage of natural resource with enhanced performance. Edible coatings and incorporation of nutrition and active ingredients may improve fruit quality providing foodstuffs supplemented with extra flavour and a delicious, tantalizing and mouth-watering appearance. Electrostatic spray technology for coating of antioxidants and edible films for food products and fruits preservation is a sustainable development strategy in this revolutionary technology era that has gained a lot of attention owing to its importance in enhancing the shelf life of foods keeping its nutritional and sensory properties. Electrostatic disinfection and sanitization has gained a lot of attention and people became more alert about health and hygiene during the COVID-19 pandemic. Although few research initiatives have been taken over the past years, the work largely remains in the nascent stage as far as field-scale technology development and commercialization is concerned. These technological and life sustaining technologies will overall lead to improvement in the quality of life. A significant value of the charge-to-mass ratio (Q/M) of 4.83 mC/kg was achieved at applied air pressure of 4 bar and liquid flow rate of 130 ml/min at a target distance of 10 cm from the nozzle tip.


Introduction
Current world population of 7.2 billion is projected to reach approximately in the range of 9.0 to 12.0 billion by the end of the 21 st century.The demand of nutritional food and healthcare for a healthy lifestyle is increasing as the population is increasing.What steps can be taken to ensure that more than 9 billion people receive nutritious food by 2050 in a way that advances economic development and reduces environmental pressure?[1].Similarly, there is a huge demand of adequate healthcare facilities to fulfil the basic human needs.To address these concerns, electrostatic spraying technology seems to be one among the possible technological solutions for food safety, and healthcare.In healthcare, disinfection and sanitization is a very routine activity which has got more attention during a COVID-19 pandemic.Electrostatic spray technique is one among the most efficient methods to spray the liquid based protective and antioxidant solutions such as agrochemicals, disinfectants, and edible and biodegradable coating materials to the target surfaces.It has varied societal and industrial applications such as agricultural and farmer's welfare, dust mitigation and smog control, food safety and nutrition, pharmaceuticals and healthcare, painting, printing, sensors and thin film deposition [2][3][4][5][6].Such technologies advocate, while doing so, it must reduce the impact of spraying on climate, ecosystems, water, and ensure that electrostatic spraying supports inclusive economic and social development.It is found and expected that electrostatic spraying technique has industrial applicability in a larger scale for varied societal problems [7].
In this paper, two important aspects of electrostatic spraying i.e. electrostatic coating for perishable food commodities and electrostatic disinfection and sanitization have been discussed in details.These technological and life sustaining technologies will overall lead to improvement in the quality of life.Various technologies are developed in the field of advanced and efficient electrostatic spraying technologies for agriculture, environment, health and hygiene, food safety and nutrition which are directly linked to the society and the common man.The most prominent developed technologies are: electrostatic agrochemical sprayer, handheld electrostatic disinfection device, dust mitigation and smog control device, electrostatic coating system for fruits and vegetables, aerial electrostatic spraying, and thin film deposition system.

Fundamental of electrostatic spraying
In electrostatic spraying, the liquid is passed through a high electric field which is generated by a charging electrode placed at a calculated distance from the nozzle tip.As the liquid passes through the high electric field, the liquid droplet gets charged and comes out of the nozzle tip either negatively charged or positively charged droplets depending on the polarity of the applied high voltage supply applied to charging electrode.The aerodynamic conditions can be controlled in the desired manner and droplets can be uniformly deposited on the target surface by providing a significant level of charge to them.The droplet size and distribution play a significant role in a uniform deposition in electrostatic spraying [8].Charged droplets uniformly cover the surfaces that are directly exposed and obscured with increased efficiency, as demonstrated in Figure 1.In electrostatic spraying, the injection of charge to tiny droplets is one of the most important aspects which define the chargeability of liquid droplets.There are various methods such as induction, conduction and field induced charging depending upon the field of application.Once the droplets are charged, it must be acted upon by a high electric field either it is self-generated or externally applied.With the fulfilment of these two criteria, the electrostatic forces dominate over the other disadvantageous forces such as friction, drag, gravitational etc. Basically, by providing the charge to tiny droplets, the aerodynamics of the charged droplets can be controlled to achieve the uniform and targeted deposition.Electrostatic spraying provides a wraparound effective and reduces the consumption of natural resources with increased efficiency and efficacy [9].

Electrostatic spray coating for perishable food commodities
3.2.Processed food with edible coatings and aesthetic look has got a lot of attention in recent times.In conventional methods, various strategies and technologies have been developed, however, they are accompanied with bottlenecks such as non-uniformity, uneven thickness and side effects of coating materials.To address these challenges, an electrostatic spray coating system has been designed and developed for fruits and vegetables to enhance the shelf-life of perishable food commodities.It also helps to maintain and control the phytonutrients (antioxidants, phenolic, pigments) and the physiochemical qualities (inhalation and exhalation rate, weight loss, total dissolved matters, pH) for longer time duration.In Figure 2(a), 35 patents have been analysed to understand the technology landscape in the area of electrostatic spray coatings.Electrostatic spray coating is one of the most promising methods to apply the nutritional ingredients, antioxidants and edible films to food and fruits to enhance the shelf life.Figure 2(a) shows that there are various methods of coating including electrostatic spray coating, however, it has not been explored much as far as technological development is considered.It reduces the material consumption, loss of natural resource, environmental pollution and human health hazards.This provides an efficient and uniform coating, which avoids the wastage of natural resource with enhanced performance.The quality of food can be improved by using electrostatics in both powder and liquid coating, including its appearance, aroma, taste, and shelf life.The three main biopolymers used to create edible coatings are polysaccharides, proteins, and lipids, or their combinations.Material properties of edible-based coating such as conductivity, viscosity, surface tension, Charge-to-mass and density have an impact on electrostatic spray system.In addition, these properties also influence the modes of electrostatic spray coating.Conductivity of spraying material determines the amount of charge a liquid can acquire while passing through the spray nozzle.An intense electric field applied to the surface of a liquid may induce an electrostatic force of a defined intensity that can overcome the surface tension, which exists between the droplets, causing disruption of the liquid droplet surface and leading to the dispersion as uniform spray of charged particles [10].Additionally, In the case of electrostatic coatings to perishable commodities, the chargeability of coating materials is one of the most important factors, which has been measured in terms of the charge-to-mass ratio.The charge-to-mass ratio defines the performance of electrostatic coating system.In prior arts, it is stated that there should be significant charge present on the droplets so that electrostatic forces dominate over other disadvantageous forces.The measured spray current by electrometer was divided by the mass flow rate of the liquid to determine the charge-to-mass ratio.The charge-to-mass ratio was calculated by using the equation ( 1): Where   is the measured spray current (A) and   is the mass flow rate of liquid (kg⁄Sec).
Selection of suitable and appropriate coating material/solution that works under electrostatic conditions is an essential step during coating.Therefore, material characterization is a requisite step for coating process.The obtained formulations were evaluated based on various parameters to assess their chargeability.To design optimal coating systems for the food processing, it is necessary to have a good understanding of these factors.The chargeability of laboratory formulated coating materials i.e. as Starch, Polysaccharide, Protein, Gums, and Cellulose has been examined and a significant charge-tomass ratio of 2.76 mC/kg, 1.269 mC/kg, 2.593 mC/kg, 2.454 mC/kg, and 1.269 mC/kg has been achieved respectively at an applied voltage of 2.0 kV and applied air pressure of 4 bar.The chargeability of coating materials depends on the viscosity, conductivity, density, applied air pressure and applied high voltage to charging electrode.The minimum of 1.0 mC/kg charge-to-mass ratio is desired to achieve the wraparound effect [11].
Apart from the technological aspects of the development of electrostatic spray coating system as shown in Figure 2(b), the coating materials are of utmost importance.The work has been carried out based on edible and biodegradable coating materials so that side effect of existing coating material can be minimized.The combination of developed electrostatic coating system and edible coating materials provides an optimal solution of food safety and nutrition.To validate the developed technology, the experiments were carried on apples, plums, strawberries, oranges, and tomatoes which have enhanced the shelf life by 1.5 to 2 folds with uniform deposition [12].

Electrostatic disinfection and sanitization
The sudden outbreak of novel coronavirus SARS-CoV-2E, also known as COVID-19, has caused a tremendous crisis for human beings worldwide.To prevent the spread of the novel Corona virus, disinfection and sanitization are essential tasks.Disinfection is the process of destroying or inhibiting the growth of disease, which causes microorganisms to thrive on both living and non-living surfaces.Electrostatic disinfection device has been designed and developed efficient and effective sanitization and disinfection of indoor and outdoor surfaces to stop the spread of the novel Corona virus [13,14].The chargeability of sodium hypochlorite (NaOCl) has been studied in terms of charge-to-mass ratio (Q/M).The chargeability defines the performance of the designed electrostatic spaying system.The significant charge-to-mass (Q/M) ratio of 0.814, 2.04, 2.73, 4.83 and 3.91 mC/kg was achieved for 3% NaOCl disinfectant at an applied air pressure of 1-5 bar respectively.Initially, the liquid flow rate increases with the increase in applied air pressure and achieved the highest flow rate at 3 bar and then started decreasing with the increase in applied air pressure.Here, it is worth noticing that the charge-tomass ratio is optimum at 4 bar for the designed nozzle.The variation of the charge-to-mass with the change in applied high voltage to charging electrode and applied air pressure has been shown in Figure 3.The charge-to-mass increases with the applied high voltage, however, after a certain threshold voltage, the charge-to-mass starts decreasing at a particular applied air pressure.Once, the applied air pressure has been increased to a new value, the threshold voltage increases, however, the charge-to-mass will decrease after a threshold value.The charge-to-mass ratio increases with the increase in applied air pressure, however, it increases only up to a particular value of applied air pressure depending on the parameters of the nozzle design.For a particular design of the nozzle, an optimized value of applied air pressure and applied high voltage needs to be optimized for optimized charge-to-mass ratio and hence, the performance of the designed system.The liquid flow rate of the nozzle was 86, 154, 176, 130 and 75 ml/min at an applied air pressure of 1-5 bar.The liquid slow rate was maximum at an applied air pressure of 3 bar not at 4 or 5 bar.It means the for the designed nozzle, the optimum applied pressure is 3 bar, however, the charge-to-mass ratio is optimum at 4 bar not at 3 bar.This concludes that in the design and development of a prototype, there is a trade-off among the parameters such as applied air pressure, applied high voltage, liquid flow rate, conductivity of the liquid etc. to achieve the optimum performance of the designed system.Two fundamental principles are used in the design and development of the electrostatic disinfection device, namely an air-induced air-assisted based electrostatic disinfection device and a hand-pressure based or liquid pump based electrostatic disinfection device.In the case of an air-induced air-assisted electrostatic spraying device (Figure 4(a)), the liquid stream is converted into fine droplets when the applied air pressure and liquid stream meets at the atomization zone of the nozzle [15].
In the case of a liquid pump based electrostatic disinfection device (Figure 4(b)), the pressurized liquid is supplied to the nozzle by the liquid pump.The liquid pump creates enough pressure to transform the liquid stream into the desired sized-droplets.The design and performance parameters are optimized for the motorized electrostatic disinfection device in order to achieve required charge-to-mass ratio for effective deposition over the target surface.Upon optimizing the design parameters, the charging capability of various chemical disinfectant solutions and correlation with their physical properties were studied [16].

Conclusion
In today's scenario, the food processing and healthcare industry are the exciting areas to be explored and there is an endless requirement of the growing population and the billions of hungry tummies.In every country, these fields of work impact society and societal life a lot.Electrostatic spraying has proved to be a very satisfactory technological intervention to solve the numerous societal problems.A significant charge-to-mass ratio of 4.83 mC/kg for the 3% NaOCl disinfectant has been achieved at an applied high voltage of 2.0 kV, applied air pressure of 4 bar and liquid flow rate of 130 ml/kg.The achieved value is much more than desired value of 1.0 mC/kg to achieve the wraparound effect for uniform and backside deposition.Electrostatic spray technologies have a great socio-economic impact and becoming visible and will speed up in coming few years.The significance of the work as a whole is very much for society since the demand of safe and adequate supply of the healthy food is increasing day by day.People are more concern about health and hygiene to live a healthy lifestyle.The developed products i.e. electrostatic spray coating system and electrostatic disinfection device have a great socioeconomic impact and huge market potential.The products based on electrostatic spraying technology are highly cost effective, efficient and efficacious with reduced carbon footprint on the environment.

Figure 1 :
Figure 1: Fundamental of electrostatic spraying and wraparound effect for backside deposition.

Figure 2 .
Figure 2. (a) Technology landscape of method of coating, and (b) Electrostatic spray coating system for perishable food commodities for enhanced shelf life.

Figure 3 :
Figure 3: Chargeability study of NaOCl (3%) for the effective disinfection and sanitization of indoor and outdoor surfeces.

Figure 4 :
Figure 4: (a) Electrostatic disinfection device for large scale, and (b) Handheld electrostatic disinfection device for small scale applications.