Simplified immersion route for water-kefir dextran film development and the role of glycerol as a plasticizer

Water kefir grains are a natural source of dextran-rich polysaccharides suitable for film formation. Dextran was characterized by intrinsic viscosity (suggesting a molecular mass above 3 × 10⁶ Da), NMR, and X-ray diffraction. Flow measurements of dextran–NaOH solutions guided the selection of NaOH concentrations for processing methods. Films were produced from dried, milled grains using two routes: a homogenization method and a simplified immersion method involving NaOH solubilization, ethanol–water precipitation, and glycerol diffusion, completely developed in this work. Thermogravimetric analysis of the films showed reduced stability with glycerol, while modulated-temperature DSC revealed a drop in glass transition temperature from 110–120 °C to below 10 °C. Water sorption isotherms indicated similar behavior for both film types, with glycerol increasing hydrophilicity. Tensile tests showed enhanced flexibility, with elongation at break up to 98% (homogenization method and 25 wt% glycerol content) and 363% (immersion method and plasticized by contact with 2 wt% glycerol solution). Overall, the developed immersion method provides a simple route to extensible glycerol-plasticized films.