Colorimetric films based on pectin/sodium alginate/xanthan gum incorporated with raspberry pomace extract for monitoring protein-rich food freshness.

Raspberry pomace extracts (RPE) with different concentrations (0.5 g/L, 1.5 g/L and 3 g/L) were incorporated into pectin/sodium alginate/xanthan gum composite film (PAX) to prepare colorimetric raspberry films (PAXR5, PAXR15 and PAXR30). Fourier Transform Infrared and Scanning Electron Microscopy analysis showed RPE had good compatibility with PAX. Compared to PAX, the raspberry films had lower water vapor permeability and water swelling ratio, higher tensile strength, opacity and antioxidant capacity. The films presented a smoother surface and denser structure than PAX. Furthermore, PAXR15 had an excellent discoloration at pH 1-13, especially at pH 5-10, the color changes of PAXR15 from pink-red-brown-blue-dark green distinguished by the naked eyes. Therefore, it has the potential to become a pH-sensitive film used in monitoring protein-rich food freshness.