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°C, DMTA showed first a slight decrease in storage modulus (G′) to 45–60°C, then an increase of the shear modulus (predominant effect of swelling) to 68–74°C, followed by a decrease (predominant effect of melting‐softening) to 85°C. In this 25–85°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°C, followed by a pronounced decrease to 57–60°C. Then, a progressive increase in probe mobility was observed to 85°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.

The rheological behavior of concentrated starch preparations from two different origins (wheat and waxy corn) was studied in the presence of sucrose by dynamic mechanical thermal analysis (DMTA). Moisture contents ranged from 30 to 60% (w/w wsb), and samples contained 0, 10, or 20 g of sucrose for 100 g of the starch-water mixture. The storage modulus (G') changes during heating depended strongly on water content (in the moisture range studied), and the importance of these variations was dependent upon the starch type. Sucrose addition resulted in a shift to higher temperatures of the increase in G' during heating. Differential scanning calorimetry (DSC) and electron-spin resonance (ESR) analyses were performed in parallel in order to relate the viscoelastic changes to water migrations and to structural disorganization of starch. Sucrose was found to increase the gelatinization temperature and enthalpy of both starches, implying a stabilization of the granular structure during heating. The sugar-water interactions do not appear to be the only way by which sucrose delays starch gelatinization. The obtained results suggest that sugar-starch interactions in the amorphous and/or the crystalline regions of the starch granules should be envisaged.

Cereal Chem. 74(5):581-588The rheological behavior of concentrated starch preparations from various origins was studied by dynamic mechanical thermal analysis (DMTA). Four types of starch were used: wheat, potato, normal, and waxy corn adjusted to moisture contents in the 42-49% (w/w) range. The thermal treatments of the starch-water mixtures consisted of heating to 85°C and cooling to room temperature, both at a rate of 1°C/min. During heating, the storage modulus (E´) appearance was first characterized by an increase with a maximum at ≈70°C (or potato starch at 63°C) followed by a decrease to 85°C. During cooling, storage modulus increased steadily down to room temperature. The magnitude of these variations depended on the starch type. Despite some differences, all the loss tangent curves showed a decrease during heating from 60-70°C to 85°C, followed by a plateau during cooling. To propose an interpretation for the DMTA results, we measured, by laser-light diffraction, the influence of heating (up to the maximum E´ peak) on the distribution of the granule sizes of the different starches. Moreover, differential scanning calorimetry (DSC) was used to measure the temperature range where the melting of starches ordered regions occurred. Partial melting enthalpies were plotted against temperature. The hypothesis of a relationship between swelling and an increase in rigidity during heating seemed to be confirmed by laser-light diffraction, whereas DSC indicated the decrease in rigidity was caused predominantly by order-disorder transitions. During cooling, amylose gelation plays a major role in the rigidity increase, but a contribution of amylopectin is not excluded.

The small deformation rheological properties of wheat flour doughs in relation to their structure and hydration were studied by dynamic mechanical thermal analysis, differential scanning calorimetry, and electron spin resonance. The effect of salt and triglycerides was also examined and compared with results we obtained previously on starch dispersions. Moisture content was adjusted to 48 or 60% (w/w, wb). Samples contained 0–16% NaCl (g/100 g of flour‐water) and 0–18% triolein or lard (g/100 g of flour‐water). The obtained results suggested that starch has an active role in determining the evolution of dough rheological characteristics during heating. The main factors controlling rheological behavior during thermal treatment are the volume fraction and deformability of starch granules. Gluten changes the viscoelasticity of the continuous phase and competes with starch for water. The addition of sodium chloride to flour dispersions shifted the structural disorganization and rigidity increased during heating to higher temperatures. At >7% NaCl, the reverse effect was observed. The mechanism controlling the effect of salt on dough rheological behavior was explained in terms of effect on water properties and on starch structure and hydration. Triglycerides had a lubricant effect (i.e., lowering G′ modulus) on the wheat flour dough system. These effects are of great importance for production and quality of bakery products.

The rheological behavior of waxy cornstarch preparations at intermediate moisture contents (30 to 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. The results were compared to those obtained previously for wheat starch. DMTA results evidenced a critical moisture content (between 50 and 55%) for waxy cornstarch that delimited a radical change in the rheological behavior both at room temperature and during heating. This critical water content was around 45% for wheat starch.