Physical stability and moisture sorption of aqueous chitosan-amylose starch films plasticized with polyols

The short-term stability and the water sorption of films prepared from binary mixtures of chitosan and native amylose maize starch (Hylon VII) were evaluated using free films. The aqueous polymer solutions of the free films contained 2% (w/w) film formers, glycerol, or erythritol as a plasticizer, as well as acetic acid (1%) and purified water. Characterization of the present fresh and conditioned film formers and free films was done using X-ray diffraction analysis, determination of moisture sorption isotherms, and near infrared spectroscopy. The results indicated that clear changes in the crystallinity of the films are evident within a 3-month period of storage, and the changes in the solid state are dependent on the plasticizer and storage conditions. When stored at ambient conditions for 3 months, the aqueous chitosan-amylose starch films plasticized with erythritol exhibited a partly crystalline structure. This was as a result of sugar recrystallisation due to the high hydrogen bonding. The respective films plasticized with glycerol and stored at 25 degrees C/60% relative humidity (RH) or at 40 degrees C/75% RH remained flexible and amorphous for at least 3 months. The water sorption of the free films greatly increased as a function of storage time at 75 and 95% RH. The second derivative spectra of starting material and free films were capable of distinguishing the internal water from the free water after storage at different relative humidities. Free water resulted in a separate band at a lower wavelength (1903 nm) in comparison to the structured absorbed water band at 1920 nm, in the case of films the free water resulted in a band around 1900 nm.