Anthocyanin-based indicators design by polyelectrolyte complexation: A study on structural and thermodynamic properties, and application for milk freshness assessment

Non-pomace residue of grape juice powder (NPG) was used in the development of indicator films for food freshness assessment by polyelectrolyte complexation of alginate and gelatin, and alginate and chitosan. The surface topography of the films was evaluated qualitatively by the 3D plot of scanning electron microscopy images and quantitatively by mechanical profilometry, which demonstrated the formation of a heterogeneous surface. Higher NPG contents increased the average and the root mean square roughness of the films, with values of 0.450 and 0.634, respectively, for the alginate and chitosan complex with 1.0 g NPG 100 g complex-1. The diffractograms of the samples, as well as the FTIR spectra, demonstrated that NPG interacted differently with the two complexes. This behavior was evidenced in the evaluation of wettability, which was increased by the addition of NPG, as well as in the evaluation of thermodynamic properties. The water adsorption isotherms of the films were determined at 20, 30, and 40 degrees C. The GAB model presented the best fit for the adsorption data, which was an enthalpy-controlled process. The films demonstrated to be efficient in evaluating the freshness of whole milk samples acidified with lactic acid through the quantification of red chromatic shift as a function of pH, which proved to be a promising way to interpret the visual result of color change by digital image analysis.