Effect of Moisture Content on Thermomechanical Behavior of Concentrated Wheat Starch‐Water Preparations

Abstract

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.

A Review of Methods, Data and Applications of State Diagrams of Food Systems

“…The use of ESR for observation of state and phase transitions in food systems is limited. Rolee and LeMeste [156] analyzed the rotational diffusion coefficient for starch-water systems of intermediate-moisture contents at selected temperatures using ESR. One disadvantage of ESR is that the rotational relaxation time and the molecular mobility of the external probe introduced into the matrix are approximated using the component signals rather than the relaxation times of matrix itself, unlike NMR that approximates the relaxation time and molecular mobility of the food [32].…”

“…In a mechanical experiment, the response is an oscillating stress or strain, while in dielectric studies, an oscillating electric field is determined as a function of the frequency of the applied field. The use of advanced spectroscopic methods, such as nuclear magnetic resonance (NMR) and electron spin resonance (EPR), have also been explored [2,32,90,91,156,174]. The relaxation methods are introduced in which the decay in response to an applied pulse instead of a continuous sinusoidal wave is monitored.…”

Influence of water on potato starch-lipid interactions. An electron spin resonance (ESR) probe study

“…Upon heating, a complicated dependence of D rot was observed. For the least concentrated dispersion the values of rotational diffusion coef®cient (D rot ) ®rst decreased, and then, above 60°C, which is the temperature corresponding approximately to the onset of starch gelatinisation as observed by DSC, 37 D rot increased with temperature. This behaviour might be explained by the structural transformations of potato starch granules associated with the irreversible process of gelatinisation, ie crystal melting, increase in starch± water interactions and subsequent granule swelling.…”

Water and temperature contribution to the structuration of starch matrices in the presence of flavour

“…A single endotherm with trailing was found for all starch-water ratios and flavour additions. Cooking starch at limiting water has been described to exhibit a multiphase endotherm (Baks, Ngene, van Soest, Janssen, & Boom, 2007;Biliaderis, 1992;Jang & Pyun, 1996;Jouquand et al, 2006;Rolée & Le Meste, 1999). However, the water threshold to observe two separate endotherms for wheat starch has been described to be below 55% moisture content, which was the lowest water content (55.5% for C samples) (Rolée & Le Meste, 1999) which probably explained the trailing shape rather than a clear double endotherm.…”

“…At 50-55°C, the enthalpy changes attributed to dissociation of crystalline clusters dramatically decrease, while those attributed to double helices dissociation are observed at 55-60°C (Tester & Morrison, 1990). The gelatinisation temperature is sensitive to the dry matter content of the starch suspensions: the higher the starch content, the higher the gelatinisation temperature (Rolée & Le Meste, 1999).…”