Kou Y, Dickinson L C, Chinachoti P
Department of Food Science and Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, USA.
J Agric Food Chem. 2000 Nov;48(11):5489-95. doi: 10.1021/jf000633x.
The molecular mobility of waxy corn starch was studied by using wide-line (1)H nuclear magnetic resonance (NMR) spectroscopy. A suite of NMR techniques was used to measure relaxation times (i.e., T(2), T(2), and T(1)) and to characterize water and solid (starch) mobility of waxy corn starch. It was observed that the spectrum of each sample includes a complex broad proton component upon which is superimposed a narrow proton component over water activity (a(w)) ranges from 0.33 to 0.97 (i.e., 10.-25.6% water content) at 25 degrees C. Line shape analysis and relaxation times of both broad and narrow components show that T(2) and T(2) values decrease (i.e., decreasing mobility) with increasing solid concentration and show a "break point" in a concentration range between 19.8 and 21.9% water content. The T(1) shows a "T(1) minimum" in the same concentration range. Starch samples change from the glassy to viscous rubbery state in this same concentration range. This demonstrates that wide-line (1)H NMR relaxation times (i.e., T(2), T(2), and T(1)) may be useful as indicators of glass transition for starch samples in the solid state. The results demonstrate that wide-line (1)H NMR spectroscopy is able to separate modes and quantitate the magnitude of molecular mobility in complex systems.
采用宽线¹H核磁共振（NMR）光谱法研究了蜡质玉米淀粉的分子流动性。使用一系列NMR技术测量弛豫时间（即T₂、T₂和T₁），并表征蜡质玉米淀粉的水和固体（淀粉）流动性。观察到，在25℃下，水活度（aw）范围为0.33至0.97（即含水量10%-25.6%）时，每个样品的光谱都包含一个复杂的宽质子成分，其上叠加有一个窄质子成分。宽、窄成分的线形分析和弛豫时间表明，T₂和T₂值随固体浓度增加而降低（即流动性降低），并在含水量19.8%至21.9%的浓度范围内出现“断点”。T₁在相同浓度范围内出现“T₁最小值”。淀粉样品在相同浓度范围内从玻璃态转变为粘性橡胶态。这表明宽线¹H NMR弛豫时间（即T₂、T₂和T₁）可作为固态淀粉样品玻璃化转变的指标。结果表明，宽线¹H NMR光谱能够区分分子流动性模式并定量复杂体系中分子流动性的大小。
