Preparation and characterization of sodium caseinate films reinforced with cellulose derivatives

Edible composite films, based on glycerol-plasticized sodium caseinate (SC) and either carboxymethyl cellulose (CMC) or cellulose acetate (CA) fibers, were respectively prepared by suspension casting. The effects of mixing SC with very low amounts (up to 3 wt%) of CMC or CA were systematically investigated through changes in morphology, surface hydrophilicity, moisture sorption, water vapor permeability, opacity, dynamic mechanical response and mechanical properties of the films. Incorporation of 3 wt% cellulose derivatives into the protein matrix led to slight but measurable decreases of equilibrium moisture contents, achieving reductions of 7.5 and 14.4% for CMC and CA reinforced films, respectively. Besides, the addition of CA contributed to the decrease of water vapor permeability, leading to a 38% falloff at 3 wt% filler. Contact angle measurements using a polar solvent ranged from 40° to 29–30° as filler concentration increased, indicating that reinforced films had higher superficial hydrophilicity than neat caseinate ones. Transparency decreased as filler concentration increased: the opacities of the most concentrated samples were 1.9 (CMC) and 2.3 times (CA) higher than that of the SC matrix. Moreover, tensile tests revealed that the addition of cellulose derivatives enhanced the tensile modulus (i.e. an 80% increase with 3 wt% CMC) and strength (i.e. a 70% increase with 3 wt% CA) although the elongation at break decreased. The differences found in the performance of both cellulose derivatives were explained in terms of water-solubility, hydrophobic character, rigidity of the fibers and quality of the filler dispersion in the protein matrix.