## 3 Impact of Cold Plasma on Packaging Properties
### Oxygen Permeability (OP)  
OP is another key parameter of packaging materials that determines the oxygen transport across the film to enhance shelf life and quality of the packaged food. The presence of oxygen modulates the development of the different reactions like oxidation involving components responsible for the color and aroma of the food product [78, 100]. When packaging oxygen-sensitive products, the materials with the least amount of oxygen permeability are preferred. Biopolymer-based packaging materials have lower oxygen barrier properties when compared to conventional plastic packaging. Therefore, some modifications of biopolymers are necessary to restrict oxygen permeability to enhance its application in the food packaging industry. Table 2 summarizes the effect of cold plasma on the OP of the biopolymer-based films.  
The literature reports that biopolymer films treated with cold plasma were observed to have increased oxygen barrier properties or that the treatment did not alter the inherent properties of biopolymers. [PERSON], [PERSON] [78] treated gelatin film with O\({}_{2}\), N\({}_{2}\), air, Ar, and ethanol-argon (EtOH-Ar) and found that OP decreased significantly. Similarly, whey protein film and gluten film showed a significant drop in the OP when treated with air and argon (Ar) plasma [100]. [PERSON], [PERSON] [99] treated starch film at low pressure with air and O\({}_{2}\) plasma and reported that OP decreased significantly.  
In general, the decrease could be attributed to the enhanced cross-linking between polymer chains resulting in a compactness that reduces free volume and prevents oxygen diffusion. Several other studies also suggest that plasma exposure induces physiochemical changes in the polymeric surface, like a breakdown of the C-C and C-H bond, creating free radicals. The interaction between the generated free radicals and activated plasma species like excited ions, electrons, and molecules leads to the development of intermolecular and interchain bonds and thereby enhances the oxygen barrier property. Moreover, OP depends upon polymer crosslinking and polarity. The application of plasma (like air and Ar) enhances the hydrophilic groups on the surface of the film increasing the polarity leading to the increase in cohesive energy density making it more difficult for permeants to open the polymer chains and permeate through its matrix [78, 99, 100].  
Some authors have reported that plasma treatment had no significant effect on the oxygen permeability (OP) of the film. [PERSON], [PERSON] [83] treated gelatin films with dielectric barrier discharge (DBD) atmospheric air cold plasma, resulting in no significant changes in the OP. Similarly, chitosan film treated with dielectric barrier discharge atmospheric air cold plasma showed no significant change in OP [86]. The authors reported that although the DBD atmospheric air cold plasma affects the surface properties of the film, bulk properties were unaltered. This may be the reason that no significant change in the OP of the DBD atmospheric air plasma treated films was observed. It is important to emphasise that gas permeability follows a combined mechanism of diffusion and solubility where the permeant (gas) passes through the void present between the polymer chains in the film network [83].