Sketching deep eutectic solvents for the development of halochromic smart packaging using blueberry residue anthocyanins and alginate-based films for monitoring food freshness

This study explores the valorization of blueberry processing waste for the development of halochromic smart packaging films, utilizing deep eutectic solvents (DES) for sustainable anthocyanin extraction and as plasticizing agents to improve film flexibility. Among the DES tested, choline chloride:formic acid (2:1, v/v) achieved the highest anthocyanin yield at 230.34 mg/L under optimized conditions (25 °C, 30 min, 1:3 w/v solid-liquid ratio). Compared to conventional acidified methanol (MeOH), DES-extracted anthocyanins displayed inferior thermal stability (retaining ~85 % at 25 °C after 72 h) and strong antioxidant activity (IC₅₀ = 61.07 mg/mL, vs. 58.65 mg/mL for MeOH). UV-Vis and CIELab colorimetric analyses confirmed pH-responsive color changes from red (pH 1, a* = +23.65) to green-blue (pH 14, a* = -2.53), enabling real-time pH monitoring. Films incorporating DES-anthocyanins and 1 % w/v sodium alginate formed biodegradable, flexible materials with mechanical strength of 0.24 MPa, elongation of 4.98 %, and water vapor permeability of 0.0026 g·mm/m<sup>2</sup>·h·kPa. The films effectively monitored spoilage in chicken, shrimp, and beef, showing visible color transitions within 24-48 h. This dual-purpose application of DES demonstrates a sustainable route for food waste upcycling, reduces reliance on hazardous solvents, and aligns with circular economy and smart packaging principles for enhanced food safety.