Preparation of a shikonin-based pH-sensitive color indicator for monitoring the freshness of fish and pork

A novel intelligent pH-responsive color indicator was prepared by adsorbing a natural naphthoquinone pigment, shikonin, onto cellulose paper. FTIR results indicated that shikonin was crosslinked with the cellulose of the indicator paper. The addition of shikonin increased antioxidant activity, thermal stability, and water resistance properties of the paper. The indicator changed the color from red to dark blue, depending on the pH of buffer solutions. Also, the indicator showed high stability after 4 months of storage and maintained high sensitivity to pH changes. This indicator was used to monitor fish and pork freshness during storage at room temperature, and the results showed a high correlation between the color change of the indicator and the pH change of the sample. The shikonin-adsorbed indicator with stable and sensitive color change depending on pH can be used in the intelligent food packaging applications to monitor the quality of packaged food in real-time.