Effect of moisture content on thermomechanical behavior of concentrated wheat starch-water preparations

The rheological behavior of wheat starch preparations at intermediate moisture contents (25-60%, w/w) was studied by dynamic mechanical thermal analysis (DMTA). Differential scanning calorimetry (DSC) and electron spin resonance (ESR) experiments were also performed in parallel. Upon heating wheat starch preparations from 25 to 85 degrees C, DMTA showed first a slight decrease in storage modulus (G') to 45-60 degrees C, then an increase of the shear modulus (predominant effect of swelling) to 68-74 degrees C, followed by a decrease (predominant effect of melting-softening) to 85 degrees C. In this 25-85 degrees C temperature range, the initial swelling and subsequent softening were less pronounced with decreasing moisture content. The 45% moisture content level appeared critical, since there was a radical change in the thermomechanical behavior below this concentration. DSC showed that gelatinization did not appear as a single endotherm but as two endotherms. Whatever the moisture content, the:melting started within a quite narrow temperature range, while the end of melting shifted progressively to higher temperatures as moisture content was decreased. ESR showed first a slight decrease in the water-soluble probe (Tempol) mobility as temperature was increased to 47-50 degrees C, followed by a pronounced decrease to 57-60 degrees C. Then, a progressive increase in probe mobility was observed to 85 degrees C. These changes in probe mobility suggest some modifications of the kinetic and thermodynamic properties of the aqueous phase associated with changes in the starch physical state. For the lowest moisture contents, the probe mobility was quite stable during heating.