Using tannins as active compounds to develop antioxidant and antimicrobial chitosan and cellulose based films

Three tannin extracts from different sources (white peel grape (W), red peel grape (R) and oak bark (O)) have been used to obtain active films based on different polysaccharides: chitosan (CH), methylcellulose (MC) and hydroxypropylmethylcellulose (HPMC). Tannin-rich films were obtained by casting as monolayers (by blending all the components) and as bilayers (one polysaccharide layer plus one tannin layer applied as a coating). Microstructural and physicochemical properties (water solubility, water vapor permeability, mechanical and optical properties), as well as the antioxidant and antimicrobial activities of the films, were analysed. Tannins, regardless of their origin, acted as crosslinking agents when incorporated into CH or MC films due to the establishment of specific associations, mainly hydrogen bonds and hydrophobic interactions. Thus, these tanninrich films turned less water soluble, mechanically stiffer and less stretchable. In contrast, tannins provoke precipitation in the HPMC film-forming solutions and the application of tannins onto the HPMC film surface gave rise to a weakened network due to the local precipitation of HPMC-tannin complexes. The greatest antioxidant capacity was found in both mono and bilayer tannin-rich MC based films in different food simulants, in line with a tannin release from the polymer matrix. The MC bilayer films also exhibited a bacteriostatic activity against both E.Coli and L. innocua. So, MC films with tannins could represent a good alternative as active coatings or packaging material so as to prevent the occurrence of oxidative reactions in sensitive food products or to control bacterial growth.