Irreversible structural transitions, heating

In starch, disruption of the ordered granular structure when heated in a solvent (usually water) is called destructuration or gelatinization. The temperature at which this occurs increases as the amount of solvent is decreased and is seen as an endothermic peak during the first scan in DSC. Destructuration is an irreversible transition and, after cooling, an amorphous entanglement of amylose and amylopectin is formed. 

When the solvent-isolated losartan was subjected to DSC characterization, the DSC curve (Fig. 3-5, curve A) showed a minor endotherm at an extrapolation onset temperature of about 229°C (10°C/min) and a major melting endotherm at an extrapolation onset temperature of about 273°C (10°C/min). When a sample of Form I was heated to 255°C and then cooled back to room temperature, the subsequent DSC curve showed only the high-temperature endotherm (Fig. 3-5, curve B). Chemical analysis (HPLC) and solution NMR showed no change in the material heated to 255°C and cooled back to room temperature. However, XRPD indicated a change in the crystal structure. Therefore, it was concluded that the minor endotherm corresponded to a kinetically irreversible enantiotropic polymorphic transition and that the losartan system was not under complete thermodynamic control. Form I is the low-temperature stable form, up to the transition point, and the high-temperature stable form was... 

---