Thermal properties of frozen water in the native and amorphous starches with various hydration degrees

The melting process of frozen water (FW) in various starches (rice, wheat, potato) that have crystal lattices of A- and B-type has been studied by means of differential scanning calorimetry (DSC). The dependences of the melting temperature (Tm) and heat (Qm) of FW on the size of the water clusters forming in starches with various humidity (the so-called size effect) and on conformational changes of starch molecules upon the destruction of native starch crystals (starch amorphization) have been considered. It has been found that in the native starches with low humidity (<35%) the Tm of FW lies below 0 °C and decreases with the decrease of their humidity. The sizes of the water clusters formed under these conditions have been estimated. In contrast, such a size effect in amorphous starches has not been observed and possible reasons for its masking have been discussed. Still, all the values of Tm for FW obtained in the amorphous starches are also negative thereby indicating the dispersed state of water in these systems. Conversely, the size effect for Qm of FW in starches with low humidity, very pronounced in the amorphous state and barely visible in the native one, has been found. It has been established that the starch amorphization leads to a redistribution of the relative amounts of FW and unfrozen water (UFW) resulting in the decrease of Qm for FW in both A- and B- starch types. Finally, it has been concluded that in all the investigated starches there exist water clusters with Tm and Qm lower than that of bulk water.