Chitosan-Based Natural Colorant Intelligent Freshness Indicator: Recent Advances, Properties, Novel Techniques, and Applications

Intelligent indicators can display the quality status of food through the color change of dyes and have received increasing attention in recent years due to their convenience and accuracy in detection. Nowadays, intelligent indicators are mainly composed of a matrix and pH-responsive dyes. Anthocyanins are water-soluble natural pigments widely present in fruits and vegetables, showing different colors under different pH conditions. Chitosan is a product of the partial deacetylation of natural polysaccharide chitin. It has good biodegradability, biocompatibility, non-toxicity, and antibacterial properties. In terms of physiological functions, it has multiple functions such as anti-cancer, lipid-lowering, and enhancing immunity. Therefore, it has a wide range of applications in the field of food packaging. This article reviews the research of intelligent freshness indicators based on chitosan and anthocyanins in monitoring the freshness of packaged foods in recent years and compares the performance of indicators with different matrix types. Moreover, this article also focuses on the working mechanism of intelligent indicators, the extraction methods of anthocyanins, and a summary of their advantages and disadvantages. In addition, the preparation, properties, and applications of new indicators, including hydrogels, multilayer structure film, metal–organic framework (MOF), and inkjet printing technology, are reviewed. Finally, this study lists the applications of intelligent indicators in typical foods such as fruits and vegetables, meat, seafood, and dairy products; analyzes the current shortcomings; and proposes future research priorities.