Phase transitions of pigskin gelatin

Edible films are flexible thin materials based on biopolymers. Tt is theref ore necessary to know the physicochemical properties of those macromolecule s in order to obtain films with desirable characteristics. Dried gelatin is a partially crystalline polymer that exhibits glass and helix-coil transit ions. The knowledge of phase properties is important for the choice of the type and concentration of the plasticizer to be utilized to obtain a flexib le and easy-to-handle film. The objective of this work is to deter-mine the phase transitions of pigskin gelatin as a function of moisture content in the hygroscopic domain. Pure gelatin was conditioned over different saturat ed salt solutions at 25 degreesC to allows samples with various moisture co ntent. After equilibrium was established, the samples and an empty pan, as reference, were heated twice between - 100 and 250 degreesC at the rate of 5 degreesC/min, in a DSC (TA 2010) after quench-cooling with liquid nitroge n. Gelatin was placed in a pan with a perforated cover, and maintained at 1 05 degreesC for 24 h in the DSC cell before thermal analyses, to obtain com pletely dried samples. The glass transition temperature of these samples wa s found to be 220.2 degreesC. The DSC traces obtained in the first scan, ex cept those conditioned at 11% relative humidity, showed a glass transition followed by two endothermic peaks due to two sol-gel transitions in the gel atin. The plasticizing effect of moisture on Tg was observed in all the sam ples conditioned by absorption and in the second scan with the samples cond itioned by desorption. This behavior was well represented by the Gordon and Taylor model, with kappa = 5.26 and R-2 = 0.96. Also, a plasticizing effec t of moisture over the sol-gel transitions was observed. The Flory-Huggins model was applied to experimental data with: chi (1) = 1.94 and R-2 = 0.999 , for the first peak Tm, and chi (1) = 1.90 and R-2 = 0.989, for the second peak. (C) 2001 Elsevier Science Ltd. All rights reserved.