Analysis of water binding in starch plasticized films

The water-binding properties of wheat starch films were studied through the determination of water vapour adsorption isotherms at 20 degrees C. Starch films were obtained by casting after mixing native starch at 95 degrees C for 15 min with different ratios of plasticizer (glycerol). Unplasticized starch film was found to adsorb less water than native starch granules. The effect of glycerol on water vapour adsorption by plasticized films depends on equilibrium relative humidity (ERH) value and glycerol content. Below ERH = 44%, plasticized film is less hygroscopic than native starch and above this value, adsorbed water increases with glycerol content. An amount of 20% glycerol seems to be the maximum that can act as plasticizer. Above this percentage, phase separation occurs and the amount of adsorbed water increases as it binds to starch film as well as to "free" glycerol. The interactions between water, glycerol and starch determined in a wide range of ERH (0-80%) and of plasticizer ratios (0-52.6%) were analysed on a stoichiometric basis. A phase diagram is reported which contains characteristic points distinguished by their composition, stoichiometric ratios and type of phase observed. The phase separation and quantification of free glycerol is particularly discussed. A schematic representation of the different types of binding between starch water and glycerol is given to illustrate our understanding of the molecular interactions in the different phases. The hydration of plasticized starch films depends on the nature and amount of plasticizer. Water vapour adsorption is proportional to the number of hydrophilic sites (hydroxyl groups) in the plasticizer. Modified starch films adsorb less water than native or plasticized films. The higher the hydrophobicity of substituent, the lower the amount of adsorbed water. Binding of water to starch films influences their thermo-mechanical properties. (c) 2005 Elsevier Ltd. All rights reserved.