Pectin Films as a Base for Incorporating Spent Coffee Grounds Extracts Obtained by Microwave-Assisted Extraction

This study examines the development and characterisation of sustainable, bio-functional films based on a pectin matrix enriched with spent coffee grounds (SGs) extract. The physicochemical and mechanical properties of the resulting films were evaluated as a function of pectin, plasticiser (glycerol), cross-linker (calcium chloride), and solvents (water, water/ethanol mixture), and different proportions of SG extract (5%, 10% and 20%). Combining 20% SG extract with 50% glycerol yielded the highest total phenol content, 11.4 mg/g, in the pectin-based film. After dissolving pectin in SG extracts, neither their fraction nor the glycerol mass fraction (50% and 70%) significantly altered the water solubility index (WSI) or the water absorption index (WAI) compared with the extract-free control films. However, the WSI and WAI values for the pectin-based films without SG extracts confirmed that the combination of 1.5% pectin, 1% calcium chloride and 50% glycerol provided the optimal results among the different extract-free formulations. This stability was further enhanced by the water/ethanol solvent system and the competitive binding of coffee phenolics, which limited moisture retention. This reduced hydration state and the formation of strong intermolecular hydrogen bonds within the functionalised matrix was confirmed by FTIR analysis. Colorimetric analysis showed that the solvent type, i.e., water and water/ethanol mixture, governed all colour parameters, and the addition of SG extract affected only the lightness value (L* = 31.98–32.61). Furthermore, mechanical strength was primarily determined by the structural density of the pectin network, with higher pectin contents significantly enhancing deformation and fracture resistance. Overall, the results showed that SG can be successfully used to develop a functional pectin-based film, providing a sustainable alternative to synthetic food packaging.