The effect of water on the physicochemical and mechanical properties of gelatin

Strips of gelatin have been prepared by extrusion at different water contents varying from 20 to 50% H2O (dry weight basis, d.w.b.). The processes of subsequent hydration or dehydration of these strips were followed by dynamic mechanical thermal analysis (DMTA), wide-angle X-ray diffraction and NMR relaxation measurements. A comparison of the calculated dependence of the T-g of gelatin (T-g anhydrous, 200 degrees C) on water content (using the Ten Brinke and Karasz equation) with experimental results derived from DMTA showed that freshly extruded material followed the theoretical plot below 25% H2O (d.w.b.), but at higher water contents, the T-g deviated positively, probably due in part to the effect of delayed re-equilibration of water content after thawing of separated ice crystals. The experimental results determined after storage for one week fell on a different line, with a T-g of 145 degrees C for anhydrous gelatin Possibly, the T-g is elevated by crystallization - a view supported by the WAXS spectra. The NMR relaxation results also showed a profound mobilization of the gelatin protons at water contents greater than 25% d.w.b.