Influence of Moisture Content on Starch Esterification by Solvent‐Free Method

Abstract Moisture is of vital importance in starch modification; however, the molecular interaction between water and starch during the process is far from being fully understood. In this study, two fundamental issues of starch esterification by the solvent‐free method are focused on: 1) the effect of moisture content of starch on the modification and 2) the effect of the physical state, solid or liquid, of chemical agents. The esterification of corn starch with varying contents of moisture (2.69–23.48 wt%) is investigated by the solvent‐free method, using both solid (maleic anhydride and maleic acid) and liquid (acetic anhydride and acetic acid) esterifying agents for comparison. The degree of substitution (DS) and reaction efficiency (RE) is measured by titration. FTIR, 1 H NMR, DSC, XRD, and SEM are used to evaluate the properties of the modified starches. It is found that when the moisture content is below 5.55 wt%, it is hard for esterification to happen because of strong interactions between H‐bonds and the low activity of hydroxyl groups. Both DS and RE reached the maximum points when the moisture is ≈14.89 wt%, then decreased with further increasing moisture content, mainly due to side reaction and dilution of the esterifying agents.