Thermal Characterization of the Gelatinization of Corn Starch Suspensions with Added Sodium Hydroxide or Urea as a Main Component of Corrugating Adhesives

Abstract

The effects of sodium hydroxide and urea on the gelatinization of corn starch suspensions, a main component of corrugating starch adhesives, were studied using differential scanning calorimetry (DSC). Sodium hydroxide and urea decreased the gelatinization starting temperature (T_s1), peak temperature, conclusion temperature, enthalpy of gelatinization (DSCΔH), apparent activation energy (ApEa), and van’t Hoff enthalpy change (ΔH_vHº). DSC analysis showed the gelatinization endotherm, the melting endotherm of starch−lipid complexes, and an exotherm attributable to the alkali-decomposition of starch. This exotherm was only observed in the first scan for sodium hydroxide additions of more than 3.8 mol%. The existence of the starch−lipid complexes was also suggested by the reappearance of the corresponding endotherm in the second DSC scan. The increases in the gelatinization rate constants caused by the two additives were deduced from the Arrhenius equation using ApEa. The relationship between the temperature corresponding to the half gelatinization transition (T_m) and DSCΔH, and that between T_m and ApEa quantitatively agreed for the two additives, suggesting that gelatinization might be stimulated by a similar mechanism involving breaking intermolecular hydrogen bonds in starch. Only sodium hydroxide caused a significant decrease of T_s1 and broadening of the gelatinization peak. Therefore, ΔH_vHº and the size of the gelatinizing cooperative unit estimated from the ΔH_vHº/DSCΔH ratio were decreased more by sodium hydroxide than by urea, indicating that sodium hydroxide facilitated the mobility of starch molecules more effectively during gelatinization.