Visible freshness: Natural pH-sensitive indicator films for intelligent food packaging

Natural pH-responsive indicator film has become a research hotspot in the field of intelligent food packaging because of its environmental friendliness, sensitive pH-responsive ability and good biocompatibility. This kind of indicator film is mainly composed of natural colorants and natural polymer matrix. In this review, the research progress of natural pH sensitive indicator film in material composition, performance optimization and application potential is systematically reviewed, especially 8 natural colorants and 17 main natural polymer matrices covering polysaccharide, protein and lipid. The color development mechanism of natural colorants were analyzed, and the regulation of biopolymer matrix on various properties of the film was discussed. This work also delves into the intrinsic bioactive properties (e.g., antioxidant, antimicrobial) of both colorants and matrices, which enable the design of dual-function (smart and active) packaging systems.Although the natural pH indicator membrane has made important achievements in the laboratory stage, it still faces many challenges in industrial production and commercial application, including insufficient colorant stability, water vapor barrier problem and high production cost. Future research should focus on the development of multifunctional composite film, the application of green stabilization technology and scalable production technology, so as to promote the practical application of natural pH-responsive indicator film in smart packaging. • Natural pH-sensitive biopolymer-pigment films provide real-time freshness monitoring, sustainability, safety. • Natural colorants show pH-responsive color change, antioxidant/antibacterial activity, and multi-role smart packaging uses. • Biopolymer matrices boost film strength, barrier, stability, and colorant compatibility. • Poor pigment stability, weak water vapor resistance, and high costs are key limits reviewed with optimization strategies. • Future directions include multifunctional films, digital sensors, green stabilization, and sustainable industrial regulation.