WATER SORPTION ISOTHERMS AND GLASS TRANSITION PROPERTIES OF GELATIN

Abstract

ABSTRACT The water sorption isotherms of gelatin of different molecular weights (317,700, 228,900, and 197,400) were determined at 50°C using an isopiestic method. The sorption isotherms were modeled using the Brunauer–Emmett–Teller (BET) and Guggenheim–Anderson–deBoer (GAB) equations. The BET and GAB equations were able to predict the equilibrium moisture content (EMC) with a mean relative error of 5.2 and 5.0%, respectively. The BET monolayer moisture content varied from 4.81 to 5.70% (d.b.) while modeling with the GAB equation predicted monolayer moisture content of 6.14–7.58% (d.b.) depending upon molecular weight. The monolayer moisture content increased with increasing molecular weight. Studies on the effect of moisture content on the “rheological glass transition temperature” (T g ) showed a smooth increase in the value of T g as a function of increasing concentration of gelatin solids. This varied from 7 to 35°C at 75% and 97% solids, respectively for the protein sample with MW = 317,700. Pinpointing of the T g was implemented with the technique of small deformation dynamic oscillation. It was proposed that the “rheological” T g is the point between the glass transition region and the glassy state. It acquires physical significance by identifying the transition from free volume phenomena of the polymeric backbone in the glass transition region to an energetic barrier to motions in the glassy state involving stretching and bending of chemical bonds.