Currently, food packaging is focused on substituting conventional plastics for bio-based materials, with polysaccharides and proteins being commonly chosen. Polysaccharides impact the hardness and adhesiveness of films, while proteins contribute to their gas barrier properties. This study aimed to develop edible films containing carboxymethyl cellulose (CMC) and whey protein isolate (WPI) and evaluate the effect of processing temperature on their properties.

Mixtures of CMC (2% w/v) and WPI (4% w/v) were heated between 50 ºC and 85 ºC for 15 min, and films were formed via the solvent casting method (dried at 37 °C for 16 h). The physicochemical properties of the films were analysed through Scanning Electron Microscopy (SEM), Attenuated Total ReflectanceFourier Transform Infrared Spectroscopy (ATR-FTIR), and mechanical testing by measuring the tensile strength (TS) and elongation at break (EB).

The CMC-WPI films obtained were homogeneous and compact, suggesting good incorporation of WPI into the CMC. FTIR showed alterations in the absorption bands with increasing temperature: the disappearance of peaks at 1633 and 1542 cm^-1, associated with protein denaturation; the appearance of a peak at 1594 cm^-1, attributed to the Maillard reaction; and alteration at 3280 cm^-1, attributed to a secondary amine from the reaction between polymers. These results were corroborated by the SEM images, which show a smoother surface and cross-section when T ≤ 70ºC and different structures and increased roughness with T ≥ 75ºC. Additionally, the films exhibited a tendency to increase in their TS and decrease in their EB with increasing temperature. For example, the TS increased from 1.86 N mm^-2 to 2.48 N mm^-2 and the EB decreased from 98.81% to 79.38% from 50 ºC to 70 ºC.

This study demonstrates that temperature affects the structural and functional attributes of the films. Higher temperatures enhanced the crosslinking and triggered Maillard reactions, as evidenced by FTIR and SEM, and improved the mechanical strength and robustness. Overall, the results highlight the potential of thermally modified CMC-WPI films for bio-based packaging.