Effect of Water Vapor Sorption on Complex Formation in Amylose-lauric Acid Blend Powder

The purpose of this study was to understand the effect of relative humidity (RH) on amylose-lipid complex (ALC) formation in amylose-lauric acid blend powder held at 50 °C (temperature slightly higher than the melting point of lauric acid) using differential scanning calorimetry (DSC) and X-ray diffraction. From DSC curves, the melting of crystalized lauric acid and two melting peaks of ALC were observed depending on RH. ALC formation was confirmed by X-ray diffraction pattern. The melting enthalpy (∆Hm) of lauric acid in the sample held at RH 0 % was lower than that of lauric acid only though there was no ALC formation. This suggests that crystallization of lauric acid was prevented by amylose. The ∆Hm of lauric acid increased with an increase in RH up to 79.0 % because liquid lauric acid would have fused as the result of enhanced repulsive force between liquid lauric acid and hydrated amylose. The ∆Hm of ALC increased with an increase in RH between 79.0 and 95.0 %. For ALC formation, amylose has to be mobile in the system, but dehydrated amylose is in a glassy (immobilize) state. According to the glass to rubber transition behavior of amorphous polymer, amylose held at 50 °C is suggested to become rubbery (mobile) state at RH 76.0 %. This interpretation will explain the reason why ALC formation began to be observed at the RH range between 72.4 and 79.0 %.