Boosting Food System Sustainability through Intelligent Packaging: Application of Biodegradable Freshness Indicators

World hunger is getting worse, while one-third of foods produced around the globe is wasted and never consumed. It is vital in reducing food waste to promote the sustainability of agri-food systems. Intelligent packaging embedded with freshness indicators can monitor food freshness in a real-time manner and be deployed for cutting food waste produced due to predetermined expiration date and informing consumers of food safety. Biodegradable halochromic films have been increasingly utilized as freshness indicators because of their low environmental impact. In this review, recent advances in biodegradable halochromic indicators for intelligent packaging are reported. The pH-responsive behaviors of natural pigments, the development of biodegradable solid supports for freshness indicators, the colorimetric response of freshness indicators to food products and simulated models, and future challenges in this field are discussed. Particularly, novel technologies coupled with halochromic indictors are highlighted, including sensor arrays, nanocomposites, smartphone-assisted detection, and ink-free printing.