Moisture Sorption of Sodium Carboxymethyl Starch

Abstract

Sodium carboxymethyl starches (Na-CMSs) with different degrees of substitution (DS) were prepared from potato starch (PS) and their moisture sorption properties were investigated. Little or no difference in moisture content was found between Na-CMSs (DSO≤0.3) and PS. The moisture-sorption isotherms of Na-CMSs (DS≥0.6) rose appreciably at a water activity of 0.7. Appearance of new sorption sites due to sorption and swelling required a certain number of residual hydroxyl groups (DS≤0.6). It was increasingly difficult for hydroxyl groups to appear at the surface of Na-CMSs with higher DS, and their new sorption sites had weaker activity. The entropy change of the moisture sorption on PS was considerably lower than that on the Na-CMSs. Moisture sorption on Na-CMSs (1) and (Z), and that on Na-CMSs (3)-(5) seemed to be composed of three and two energetic states, respectively. Moisture sorption on Na-CMSs (3)-(5) apparently holds a constant, energetically stable state after a drop in entropy change, while sorbed moisture affects the Na-CMSs because of their unstable porous structure. The Henderson, Chung and Pfost and Smith equations all fit the moisture-sorption isotherms of Na-CMSs. The constants of their equations indicated that the stability of the Na-CMS structure to moisture sorption gradually decreased with increasing DS, and that the bound fraction of sorbed moisture decreased while the condensed fraction increased.