The effects of mixing and resting on the physicochemical properties of doughs prepared with strong and weak hard wheat flours were investigated, specifically concerning aspects related to their rheological behavior and molecular mobility. Small deformation dynamic tests showed that, during the initial resting period, the complex modulus G* decreased and phase angle decreased for undermixed dough, whereas overmixed dough showed opposite trends. G* values for optimally mixed dough did not vary during the resting period investigated. This was more obvious for the strong dough. Large deformation tests more clearly showed differences among optimal, under-, and overmixed dough, and also between doughs prepared with strong and weak flour. Optimally mixed dough exhibited the highest peak stress and strain for both samples. In addition, the peak stress of dough prepared with the strong flour was higher than that of dough prepared with weak flour. Inconsistent results between small and large deformation tests implied that small and large deformation tests reflected different structural aspects of dough. NMR measurements were performed to estimate the relaxation properties of the sample upon resting. Decreased water mobility during resting, indicated by decreasing T(1) relaxation time, was possibly attributed to increasing molecular interactions caused by continued hydration. Evidence of additional molecular interactions created by mixing was also observed.
Gelatinization of starch was studied by using modulated differential scanning calorimetry, nuclear magnetic resonance relaxometry, and confocal laser scanning microscopy. The effects of sugar or gum addition were analyzed in terms of changes in the gelatinization temperature and the effects on molecular mobility changes. Two states of water were found in both relaxation mechanisms (spin-spin and spin-lattice) and the extent of mobility depended on the concentration of the mixtures and the nature of the ingredients. With xanthan, dramatic changes of mobility were observed prior to gelatinization, indicating adsorption of xanthan on the starch granule surface. This result was confirmed by confocal scanning laser microscopy. The studies are contributing to the understanding of starch gelatinization in the presence of other substances. Potential industrial application is seen for the food industry (NMR as online method for gelatinization monitoring, synergistic thickening effects).
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