Tea Tree Oil Microemulsion-Gel-Strengthened Soy Protein Isolate Composite Films: A Multifunctional Active Packaging...

The development of stable and efficient essential oil delivery systems remains a persistent challenge in active food packaging applications. This research aimed to develop a multi-functional soy protein isolate (SPI)-based composite gel film integrating a tea tree oil micro emulsion (TME) via a microemulsion-in-gel approach, featuring sustained antioxidant release. The TME was first optimized using pseudo-ternary phase diagrams and exhibited excellent physicochemical stability. It maintained a droplet size ranging from 10 to 13 nm, with a polydispersity index (PDI) less than 0.2 under diverse stress situations (such as dilution, heat treatment, pH change, centrifugation, and 30-day storage). Afterward, TME-SPI composite gel films containing 1 to 3% TME were fabricated through solution casting and subsequent gelation of the protein matrix. The incorporation of TME markedly improved the properties of the gel film network. It raised the opacity by around 2.5 times, boosted the elongation at break to 144% (which is three times that of the control), and distinctively enhanced both water solubility and the water vapor barrier. Importantly, the 2% TME-SPI gel film exhibited sustained antioxidant activity from within the gel matrix, retaining more than 50% of its original 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging activity after 72 h, significantly outperforming films containing free TTO. The microemulsion-in-gel approach was shown to be effective in creating SPI-based gel films that possess combined light-barrier characteristics, adjustable moisture resistance, improved flexibility, and extended antioxidant release. This offers a promising framework for the next generation of active food packaging. Furthermore, the composite gel films exhibited concentration-dependent antibacterial activity against Staphylococcus aureus, with the 3% TME-SPI film achieving an 82% inhibition rate, thus experimentally validating its active packaging potential.