Salt Crystallization in Chitosan Films as Affected by Solvent pH and Moisture Content

Abstract

Chitosan (CH) is a cationic biopolymer used as a matrix to develop edible films and coatings for food applications. When developing chitosan-based film-forming dispersions and films, CH is usually dissolved in aqueous acetic acid solutions (pH = 3.8); sodium hydroxide is often added to increase the pH of the solvent, thus preventing film solubilization at high relative humidity conditions, which can cause changes in the solvent properties (pH and ionic strength) affecting the structure and properties of the obtained films. This is particularly significant for chitosan, since its conformation in solution greatly depends not only on structural parameters like the degree of acetylation and chain length, but also on solution parameters such as ionic strength, solvent, temperature, and pH (Sorlier et al. 2002). During film drying, changes in solvent properties occur in line with water evaporation: increase in ionic strength, change in pH, and conformational modifications in the polymer chain. Moreover, when sodium hydroxide is added to chitosan-based film-forming dispersions, sodium-acetate tri-hydrate salts can crystallize, since these are oversaturated when the film reaches determined water content, depending on molecular mobility, which in turn is greatly affected by film moisture content.