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Cold-plasma technology: the new pioneer of novel processing

The applications of cold plasma in the food industry extend to microbial decontamination of food products, packaging material processing, modification of food components, seed germination performance and degradation of agrochemical residues. It can also be used for applications like hydrogenation of oils, mitigation of food allergens and anti-nutritional factors and effluent management.

Cold plasma technology is a novel non-thermal technology that uses high energy, reactive gases to inactivate microbes. It is an emerging, environmentally-friendly technology, that poses many potential applications, in food processing and packaging sector.

The physical and chemical properties of polymers are similar to that of with conventional packaging materials, in terms of functionality. In addition, polymeric packaging materials have greater flexibility, transparency, chemical inertness, have low specific weights and are typically low weight.

Plasma technology is considered as modern non-conventional technique which is used for the preparation of modified starches, altering its physical and chemical properties.

All about cold plasma technology

Going back to history in 1960’s for the first time sterilization property of plasma was introduced, and a patent was filed in the year 1968. It was reported that the destruction of 106 spores in inner surface of vials occurred in less than second using argon plasma by pulsed RF field atmospheric plasma.

For years cold plasma processing has been viewed as useful for microbial inactivation while maintaining quality of fresh produce. However, this process is not effective for in vitro model food systems for inactivation of microbes or enzymes which are present in intact tissues, as it is a surface phenomenon.

Cold plasma technology is also used to inactivate endogenous enzymes which are responsible for browning reactions particularly polyphenol oxidase and peroxidases. Several research investigations showed reduced growth of microorganism via a different mode of actions by etching phenomenon, cell disruption by electrophoration etc.

The main advantage of cold plasma technology is in volatile organic compounds (VOC) removal from the food industries which are toxic and harmful are their relatively low energy consumption and generally moderate cost, compared to conventional air treatment methods, and more importantly, their ability to treat air containing low concentrations of VOCs at relatively low operating temperatures

Chemistry Behind it

The term “plasma” refers to a quasi-neutral ionised gas, primarily composed of photons, ions and free electrons as well as atoms in their fundamental or excited states with a net neutral charge. Plasma discharges are widely used for processing and are indispensable for many technological applications

In plasma processing ionization is always considered as first important element followed by other factors like reaction rate, rate constants, the mean free path, the electron energy distribution. Plasma chemical process can be divided into two categories based on reactions:

  1. homogeneous gas-phase reaction (for example generation of N3 from N2)
  2. heterogeneous reaction where plasma comes in contact with the solid or liquid medium.

The heterogeneous reaction is further classified into three sub categories.

  1. In the first sub category, material is removed from the surface termed as etching or ablation
  2. In the second sub category, material is added on the solid surface in the form of thin film observed during plasma polymerization by a process called plasma enhanced chemical vapour deposition
  3. In third sub category there is no material added or removed but the substrate surface is modified physically and chemically during exposure to plasma.

Plasma can be produced by subjecting a gas to an electric field (between two electrodes), either of constant (direct current field) or alternating amplitude (usually high frequency field). Plasma state can be attained by the application of energy in several forms including; thermal, electric or magnetic fields and radio or microwave frequencies, which increase the kinetic energy of the electrons resulting in increased number of collisions in the gas forming plasma products like electrons, ions, radicals and radiation of varying wavelengths including that in the UV ranges.

Plasma is an effective, economical, environmentally safe method for critical cleaning. The vacuum ultraviolet (VUV) energy is very effective in the breaking most organic bonds (i.e., CeH, CeC, C]C, CeO, and CeN) of surface contaminants. This helps to break apart high molecular weight contaminants. The second cleaning action is carried out by the oxygen species created in the plasma (O2þ, O2, O3, O, Oþ, O, ionised ozone, metastable-excited oxygen, and free electrons). These species react with organic contaminants to form H2O, CO, CO2, and lower molecular weight hydrocarbons.

The resulting surface is ultra-clean/sterilised.

Other applications

  1. Plasma modification of polymeric surfaces has evolved as an alternative to wet chemical surface modifying treatments due to its many important advantages such as uniformity, reproducibility, short reaction time and environmental safety. The technique can also be used for suitably modifying the bioresponsive properties of food contact surfaces, including metals. For example, when stainless steel is deposited with ethylenediamine (EDA) it decreases the microbial attachment and creates bacterial anti-fouling surfaces.
  2. Food packaging materials are responsible for the protection of food materials from the outside environment during handling, transportation and distribution. Cold plasma is used in the decontamination of packing material externally where the chance of shadow effect is negligible as plasma flows all-round the surface.

Plasma processing is well known to change or make surface modifications in the case of packing materials. It serves purposes for surface treatment such as cleaning, coating, printing, painting, and adhesive bonding. Low temperature gas plasma sterilization allows fast and safe sterilization of packaging materials such as plastic bottles, lids and films without adversely affecting the properties of the material or leaving any residues.

Cold plasma can be used for sterilization of heat sensitive packing materials like polythene ethylene and polycarbonate as the temperature is low. Surfaces of polymers particularly for edible packaging films nature of surface should be more hydrophobic with lower surface energies

Disadvantages

There are some limitations of plasma processing like increase in oxidation of lipids, reduction in colour, decrease in firmness of fruits, and increase in acidity etc. were reported.

Conclusion

In the past decade, CP treatment of food products has experienced increased popularity due to its potential contribution to non-thermal food processing. Several experimental investigations have been conducted thus far on known applications while others explored new pathways for exploitation of non-thermal plasma for the food industry.

The applications of cold plasma in the food industry extend to microbial decontamination of food products, packaging material processing, modification of food components, seed germination performance and degradation of agrochemical residues.

It can also be used for applications like hydrogenation of oils, mitigation of food allergens and anti-nutritional factors and effluent management. In consideration of the foregoing, it should be safe to conclude that CP technology holds potential significance for the food industry.

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