Enhancing the hydrophobicity and mechanical strength of purple sweet potato powder-based food freshness indicating film through heat laminating with polyethylene terephthalate film

• Anthocyanins in pre-sterilized purple sweet potato (PSP) powder were pH-sensitive. • Laminating with PET film elevated mechanical and hydrophobic properties of PSP film. • PSP/PET films showed pH-responsive and ammonia-responsive color change ability. • PSP/PET films were able to monitor shrimp freshness through color changes. Food freshness indicating films incorporated with anthocyanins have emerged as a prominent research focus in food intelligent packaging sector. However, the practical application of food freshness indicating films is normally limited by the low hydrophobicity and mechanical strength of the films. In this study, anthocyanin-rich purple sweet potato (PSP) roots were first sterilized and then powdered, and the obtained PSP powder was used to fabricate food freshness indicating film. In order to enhance the hydrophobicity and mechanical strength of PSP film, PSP film was complexed with polyethylene terephthalate (PET) film by heat laminating at 120, 140 and 160°C. The structure, physical properties and intelligent packaging potential of PSP/PET composite films were determined. Results showed that sterilization treatment elevated the extractable anthocyanin content in PSP roots from 1.60 to 1.82 mg cyanidin-3- O -glucoside equivalent/g. Meanwhile, the pH-responsive color-changing ability of anthocyanins was retained after PSP roots were sterilized. When PSP film was heat laminated with PET film, the obtained PSP/PET composite films displayed the same purple color as PSP film. Structural characterization revealed that PSP/PET composite films consisted of PSP and PET layers, and the two layers showed distinct morphologies and infrared spectra. PSP/PET composite films displayed no light transmittance at 200-360 nm and low light transmittance (< 15%) at 360-800 nm. Compared to PSP film, PSP/PET composite films exhibited higher tensile strength (21.67-22.82 MPa) but lower water vapor permeability (1.77-1.86 × 10 -11 g m -1 s -1 Pa -1 ), moisture sorption potential, surface wettability and water solubility. PSP/PET composite films possessed excellent pH-responsive and ammonia-responsive color-changing ability. In shrimp freshness indicating assay, purple PSP/PET composite films turned blue when shrimp was spoiled. This study demonstrated that heat laminating with PET film was an effective solution to elevate the hydrophobicity and mechanical strength of food freshness indicating films.