Novel pH-sensitive films containing curcumin and anthocyanins to monitor fish freshness

Visual pH-sensing films containing curcumin (CR) and anthocyanins (ATH) were prepared as on-package indicator labels for nondestructively detecting fish freshness in real time. Fourier transform infrared spectroscopy analysis indicated that CR and ATH were successfully immobilized into the film-forming substrate based on starch, polyvinyl alcohol (SPVA) and glycerol. The incorporation of different natural dyes had no significant effect on moisture content, water solubility, water vapor permeability and thickness of colorimetric films. Results of color stability test illustrated that the composite films incorporated with CR were the most stable and the composite film incorporated with ATH was least stable at room temperature for 180 days. The colorimetric film without glycerol was found less sensitive to volatile ammonia. Based on the volatile amines released by fish, an application trial of five colorimetric films containing natural dyes was conducted in bighead carp (Hypophthalmichthys nobilis) at 4 °C. Results showed that the SPVA/glycerol film incorporated with CR and ATH at a ratio of 2:8 (v/v) could provide three different colors, which were assigned to the sign of freshness, medium freshness, and spoilage for packaged fish. Hence, this colorimetric film can be applied as a promising material of intelligent packaging for non-destructively monitoring the real-time freshness of fish products.