Study Of Steamed Bread During Processing And Storage Using NMR And MRI Techniques

Water plays an important role in dough and steamed bread. Dough making is in fact a hydration process whose characteristics or quality have profound impact on the quality of the final products. However, there is a serious lack of understanding of molecular mechanisms involved in flour hydration and quality deterioration of flour-based products. The overall goal of this study was to adapt NMR and MRI techniques to study the role of water in dough making and steamed bread storage.NMR and MRI are non-destructive method that can be used to study the physical and chemical properties of materials including foods and related processes such as freezing, drying, soaking, and storage. MRI allows direct and uninterrupted visualization of structures and spatial distribution of water and mobility of water in foods. Chinese traditional food -steamed bread was studied using low field NMR and MRI techniques in this thesis. The first specific research aim was to study the water absorption of flour and the influence of sucrose and moisture on the molecule mobility of dough of steamed bread. The second specific research aim was to study two key steps of dough making process: mixing and fermentation. The third specific research aim was to research hardness and water mobility of steamed bread stored in room temperature(about 24℃) . The last specific research aim was to study water mobility in frozen steamed bread during microwave reheating using MRI technique. The mainly objectives were to discuss the influences of dough making, storage and freezing-reheated processes on the quality of steamed bread products.Flour hydration is mainly a function of the chemical composition of the flour, additives, and mixing. NMR technique was used to study the water absorption of flour with different gluten content from the angle of water mobility and combined ability. The result showed that 40%⁵5% water contents were the best to allow enough water absorption for the three flours with 25.4% 30.4%å’Œ35.8%gluten content(wet base), respectively. During water absorption process, free water content increaced rapidly, while bound water content remained relatively steady. We found sucrose increased the T₂ constant of dough, further more, it did not change the exponential tread of the T₂ constant of dough,which increased as the water content increased. The results indicated that sucrose increased the mobility of water in dough, decreased the hydrophilic effects on some macromolecules (e.g. protein and starch), enhanced the water availability in dough, and improved the rheological properties of dough.According to the curve of the spin—spin relaxation time (T2) as a function of mixing time, we divided the mixing process of dough into three stages: the initial mixture stage, the network formation and perfection stage of wheat gluten, and the network breakage of wheat gluten. The result of the NMR and MRI experiment indicated it was feasible to study the mixing process of dough by NMR and MRI techniques. The appropriate dough mixing time for the flour with 25.4% gluten content is...