Bioactive Films: Cinnamon Oil Incorporation in Alginate/κ Carrageenan Films Enhanced by Limestone Sludge

This work aimed to develop bioactive films based on alginate and κ-carrageenan that were incorporated with different concentrations 0, 0.2, 0.4, 0.8, 1 and 2% (w/v) of cinnamon essential oil (CEO). The films were crosslinked with a solution of calcium chloride obtained from limestone sludge through acid dissolution. The films were characterised according to their physical, mechanical, optical, antioxidant and antimicrobial properties. The best film formulation consisted of 1.5% total carbohydrate concentration, 0.45% glycerol and 0.4% (w/v) of Tween 20. The Fourier transform infrared Spectroscopy analysis confirmed the crosslinking between the polysaccharides and the incorporation of the CEO into the polymer matrix. The addition of the CEO increased the film thickness, reduced moisture content and water vapour permeability, yet it increased solubility, due to matrix disruption invoked by the oil droplets. SEM analysis showed that CEO affected film microstructure, with moderate concentrations leading to more homogeneous structures. In terms of the mechanical properties, CEO incorporation reduced stiffness and yield strength whilst increasing film flexibility, showcasing a plasticising effect. The films were colourless and transparent; moreover, none of the samples exhibited absorbance in the visible region (400–800 nm); however, all films showed absorption in the UV region. The incorporation of the CEO into the films provided antioxidant activity. Particularly, the sample containing 2% CEO had the highest activity, with values of 97.5 ± 0.77% and 75.9 ± 1.82% in the ABTS and DPPH, respectively. Overall, these results suggest that the developed films have promising potential as sustainable food packaging materials with enhanced antioxidant functionality, although further optimisation is needed to improve antimicrobial performance and validate their effectiveness in real food packaging systems.