Food-protecting films based on soy protein isolate and natural deep eutectic solvents: Antimicrobial and antioxidant properties

Natural deep eutectic solvents (NADES) have just recently emerged as promising plasticizers in food coating applications owing to their excellent anti-frosting properties, low cost, and biosafety. Herein, we report for the first time the use of NADES as bioactive additives of soy protein isolate (SPI) to fabricate functional packaging materials. A neoteric eutectic solvent based on choline chloride/tannic acid (ChCl/TA) and the classical ChCl/citric acid mixture (ChCl/Cit) were explored to be integrated into the protein matrix at really high ratios (150 and 200 wt%). The resulting films were characterized by assessing morphological, physicochemical, and mechanical properties, as well as antioxidant and antimicrobial activities. The films exhibited a hydrophilic surface but limited water swelling (less than 10%), suggesting the formation of a crosslinked network between NADES and SPI. The ChCl/Cit films demonstrated potent antimicrobial activity in bacterial growth inhibition assay on chicken breast as well as in agar diffusion assays with inhibition zone diameters over 20 mm for gram+ and gram-bacteria, while ChCl/TA films showed the highest tensile strength (ranging from 744 to 2740 kPa), superior antioxidant activity (DPPH scavenging activity of ≈82%), and high light barrier properties, advantageous for storing light-sensitive food products. The incorporation of high concentrations of NADES in film production provides a novel and practical approach to improving the properties of protein materials for food packaging applications.