Conformational contribution to the heat capacity of the starch and water system

The heat capacities of starch and starch-water have been measured with adia batic calorimetry and standard differential scanning calorimetry and are re ported from 8 to 490 K. The amorphous starch containing 11-26 wt % (53-76 m ol %) water shows a partial glass transition decreasing from 372 to 270 K, respectively. Even the dry amorphous starch gradually increases in heat cap acity above 270 K beyond that set by the vibrational density of states. Thi s gradual increase in the heat capacity is identified as part of the glass transition of dry starch that is, however, not completed at the decompositi on temperature. The heat capacities of the glassy, dry starch are linked to an approximate group vibrational spectrum with 44 degrees of freedom. The Tarasov equation is used to estimate the heat capacity contribution due to skeletal vibrations with the parameters Theta (1) = 795.5 K, Theta (2) = 15 9 K, and Theta (3) = 58 K for 19 degrees of freedom. The calculated and exp erimental heat capacities agree better than +/-3% between 8 and 250 K. Simi larly, the vibrational heat capacity has been estimated for glassy water by being linked to an approximate group vibrational spectrum and the Tarasov equation (Theta (1) = 1105.5 K and Theta (3) = 72.4 K, with 6 degrees of fr eedom). Below the glass transition, the heat capacity of the solid starch-w ater system has been estimated from the appropriate sum of its components a nd also from a direct fitting to skeletal vibrations. Above the glass trans ition, the differences are interpreted as contributions of different confor mational heat capacities from chains of the carbohydrates interacting with water. The conformational parts are estimated from the experimental heat ca pacities of dry starch and starch-water, decreased by the vibrational and e xternal contributions to the heat capacity. (C) 2001 John Wiley & Sons, Inc .*.