TA Instruments-Waters LLC, New Castle, Delaware, USA.
J Texture Stud. 2020 Aug;51(4):575-584. doi: 10.1111/jtxs.12517. Epub 2020 Feb 28.
In the food industry, many food products experience extreme processing conditions of high temperature and high shear stresses. The measurements of sample behavior for water-based formulations above 100°C is extremely challenging due to changes in material composition from the boiling of volatile ingredients. We have developed a high-sensitivity, pressurized starch pasting cell (up to 5 bar) which utilizes a design free of mechanical bearings and seals, resulting in an order-of-magnitude improvement in torque sensitivity (1 μN.m in oscillatory and 10 μN.m in shear flows) compared to traditional pressure cells. A pressurized atmosphere in the cell suppresses boiling of the volatile components, allowing the characterization of the structure-property relationships of the sample over a range of testing conditions (-5 to 150°C) which simulate industrial processing and storage conditions. This cell is employed to investigate the pasting properties of a commercial starch dispersed in water. In situ gelatinization of starch dispersions of varying starch particle weight fractions (ϕ) subjected to a high temperature (120°C) at elevated pressure and at a fixed shear rate is studied. A phase transition, from an initial flowable starch slurry to a paste, takes place during which the viscosity evolves by several orders of magnitude. Typical parameters associated with the viscosity evolution during gelatinization such as onset temperature, peak temperature, and peak viscosity are analyzed to probe the impact of high temperature on the gelation process and the rheological properties of the final starch paste. Furthermore, yield stresses of the final paste, measured at 120°C, are examined for varying ϕ through traditional rheological methods such as flow ramps, oscillatory shear, and stress growth, demonstrating the capabilities of this cell for studies of steady shear and nonlinear viscoelastic behavior of the starch pastes. The yield stress values are found to be in good agreement when comparing various testing methods. Yield stresses range from 0.25 to 6.5 Pa for ϕ between 0.05 and 0.15, with 0.05 being the minimum starch weight fraction for which there is any measurable yield stress. The yield stress and the paste viscosity both scale with starch particle weight fraction as (ϕ - ϕ ) , where ϕ = 0.04 as no yield stress is observed for ϕ ≤ 0.04. The exponent, m, for yield stress is found to be in the range of 1.15-1.4 depending on the analytical method used and the definition of yield stress while for peak and breakdown viscosities it is noted to be 1.6 and 1.1, respectively. The Herschel-Bulkley model is found to fit the flow curves well. The starch pastes are found to exhibit shear-thinning and significant thixotropic behavior.
在食品工业中，许多食品经历了高温和高剪切应力的极端加工条件。由于挥发性成分沸腾导致材料成分发生变化，因此测量 100°C 以上水基配方的样品行为极具挑战性。我们开发了一种高灵敏度、加压淀粉糊化单元（最高可达 5 巴），该单元采用无机械轴承和密封件的设计，与传统压力单元相比，扭矩灵敏度提高了一个数量级（振荡时为 1 μN.m，剪切流时为 10 μN.m）。单元内的加压气氛抑制了挥发性成分的沸腾，从而可以在模拟工业加工和储存条件的测试条件范围内（-5 至 150°C）对样品的结构-性能关系进行表征。该单元用于研究商业淀粉在水中分散体的糊化性能。研究了在高温（120°C）和升高的压力下，不同淀粉颗粒重量分数（ϕ）的淀粉分散体的原位胶化。在粘度通过几个数量级变化的过程中，发生了从初始可流动淀粉浆料到糊的相变。分析与胶化过程中粘度演变相关的典型参数，如起始温度、峰值温度和峰值粘度，以探究高温对凝胶过程和最终淀粉糊流变性能的影响。此外，还通过传统流变学方法（如流量斜坡、振荡剪切和应力增长）测量了最终糊的 120°C 下的屈服应力，以研究不同ϕ的情况，证明了该单元在淀粉糊稳态剪切和非线性粘弹性行为研究方面的能力。比较各种测试方法时，发现屈服应力值吻合良好。对于 0.05 至 0.15 之间的ϕ，屈服应力值范围为 0.25 至 6.5 Pa，其中 0.05 是可测量屈服应力的最小淀粉重量分数。屈服应力和糊的粘度均与淀粉颗粒重量分数呈ϕ关系，其中ϕ = 0.04，因为对于ϕ ≤ 0.04，没有观察到屈服应力。屈服应力的指数 m 取决于所使用的分析方法和屈服应力的定义，范围为 1.15-1.4，而对于峰值和破坏粘度，分别为 1.6 和 1.1。发现 Herschel-Bulkley 模型很好地拟合了流动曲线。淀粉糊表现出剪切变稀和显著的触变行为。
