Effect of NaCl on the thermal behaviour of wheat starch in excess and limited water

Abstract

The effect of NaCl on the thermal behaviour of wheat starch was investigated with particular focus on starch at low moisture contents (25–45wt%). Increasing the level of NaCl reduced the starch peak viscosity (in 90% water) as measured by RVA and shifted all of the thermal peaks (up to 120°C) to higher temperatures as observed by DSC. Above a moisture content of 45%, the temperature difference of the first thermal transition of starch in the presence of 2% NaCl and in the absence of NaCl was found to be constant. In the absence of NaCl, the peak temperature of gelatinisation (Tp) increased by 12°C (from 62 to 74°C) as the water content was reduced from 35% to 25%. In the presence of 2% NaCl, the variation in Tp due to changes in water content was significantly reduced. At NaCl concentrations greater than 2% (w/w total), the Tp of the starch remained constant irrespective of water content. Evidence of this effect was observed in situ using confocal microscopy. In the presence of 2% NaCl, images taken at elevated temperatures show little difference in the extent of starch swelling at 25% compared to 45% water content. However, in the absence of NaCl, significantly more swelling was observed at 45% than at 25% water content. With increasing NaCl concentration, the interaction of starch and NaCl became dominate. Thus the on-set of the thermal transitions of starch granules is primarily controlled by the amount of NaCl present, and secondarily by the water content which becomes dominant when the NaCl concentration is low.