The effect of sodium chloride on the glass transition of potato and cassava starches at low moisture contents

The effect of NaCl on the glass transition of cassava and potato starches at low water levels (<20%dwb) was investigated. Sodium chloride (up to 6% of the starch dry weight) was mixed thoroughly with cassava and potato starches using a twin-screw extruder. The samples were equilibrated over saturated salt solutions (LiCl, CH3COOK, MgCl2, NaBr, CuCl2 and NaCl) to give a range of moisture contents. The addition of sodium chloride caused a considerable reduction in the DSC measured glass transition temperature for both starches. For example, the Tg of cassava starch without and with 6% NaCl equilibrated at relative humidity of 11% was 166 and 136°C, respectively. Similar reductions were found for potato starch. Although the starch sorption isotherms are affected by the addition of salt when Tg is plotted against water content as opposed to relative humidity a Tg reduction on salt addition is still observed. The possible reasons for the plasticization of starch by salt are discussed.