Abstract The dielectric properties of starch based snack pellets have been measured in situ during microwave heating and expansion. The microwave setup consists of a single mode shorted waveguide, equipped with a six-port impedance analyser which measures the absorbed power and complex reflection coefficient during heating. The pellet is suspended in the electric field maximum, with an optic temperature sensor inserted in the centre. The dielectric properties of the pellet during heating and after expansion are obtained via an inverse mapping of the recorded reflection coefficient to dielectric properties, which are pre-computed via finite elements simulations. Experiments show that the dielectric properties of the starch pellets change significantly during heating, expansion and subsequent drying. The dielectric properties increase with increasing temperature up to the moment that the pellet starts expanding. Subsequently, the power absorption shows a sudden decline, which is mainly due to the sudden change in porosity. Addition of salt (2.5%) to the starch pellet composition results in a slight decrease of the dielectric constant and loss factor, as it apparently lowers the effective mobility of the dipoles. The dielectric properties as function of temperature and moisture content were fitted with a polynomial model. The strong effect of porosity for the dielectric properties of the expanded snack is well predicted with the effective medium mixing rule.

中文翻译：

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微波零食颗粒的原位单模介电测量

摘要 淀粉基零食小球的介电特性在微波加热和膨胀过程中进行了原位测量。微波装置由单模短路波导组成，配备六端口阻抗分析仪，可测量加热过程中的吸收功率和复反射系数。颗粒悬浮在电场最大值中，光学温度传感器插入中心。加热过程中和膨胀后颗粒的介电特性是通过记录的反射系数到介电特性的逆映射获得的，介电特性是通过有限元模拟预先计算的。实验表明，淀粉粒的介电特性在加热、膨胀和随后的干燥过程中发生显着变化。介电特性随温度升高而增加，直至颗粒开始膨胀。随后，功率吸收呈现突然下降，这主要是由于孔隙率的突然变化。将盐 (2.5%) 添加到淀粉颗粒组合物中会导致介电常数和损耗因子略微降低，因为它明显降低了偶极子的有效迁移率。作为温度和水分含量的函数的介电特性符合多项式模型。孔隙率对膨化零食介电性能的强烈影响可以用有效的介质混合规则很好地预测。将盐 (2.5%) 添加到淀粉颗粒组合物中会导致介电常数和损耗因子略微降低，因为它明显降低了偶极子的有效迁移率。作为温度和水分含量的函数的介电特性符合多项式模型。孔隙率对膨化零食介电性能的强烈影响可以用有效的介质混合规则很好地预测。将盐 (2.5%) 添加到淀粉颗粒组合物中会导致介电常数和损耗因子略微降低，因为它明显降低了偶极子的有效迁移率。作为温度和水分含量的函数的介电特性符合多项式模型。孔隙率对膨化零食介电性能的强烈影响可以用有效的介质混合规则很好地预测。