Cold plasma modification of polysaccharide and protein edible films: substrate and state dependence

This study investigated the effects of cold plasma (CP) applied to chitosan (CS; polysaccharide-based) and soy protein isolate (SPI; protein-based) films in different physical states. CP treatment increased the viscosity of both film-forming solutions and reduced the free sulfhydryl content in SPI films owing to enhanced intermolecular interactions. The CP-treated films exhibited improved mechanical properties and thermal stability, even though their physical characteristics were affected contrarily: CP increased the hydrophilicity and water vapor permeability (WVP) of CS films, whereas SPI films became more hydrophobic and exhibited reduced WVP. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed that CP induced surface roughening in CS films via etching. In contrast, CP-treated SPI films exhibited smoother surfaces owing to reduced protein aggregation. Fourier transform infrared (FT-IR) and X-ray diffraction (XRD) analyses revealed that the improved performance could be mainly attributed to the formation of hydrogen bonds, as well as increased order and crystallinity. CP treatment had no significant effect on film transparency and antibacterial and antioxidant activities. Most importantly, the treatment of the film-forming solution was markedly more effective than that of pre-formed films, revealing a fundamental distinction between “bulk modification” (treatment of the film-forming solution) and “surface modification” (treatment of pre-formed films). This work elucidates the versatile role of CP in customizing edible films and the state-dependent nature of CP modification, thereby providing a foundation for targeted application of CP in food packaging.