Ultrasonic shear‐wave reflectometry applied to monitor the dynamic viscosity of reheated edible oil

Effects of frying cycles of two types of edible oil on dynamic viscosity were studied by ultrasonic shear‐wave reflectance method. Factors affecting the reflection coefficient were analyzed theoretically. The experimental methods and parameters are carefully analyzed and optimized in the low‐viscosity case. The viscosity of the reheated oil samples was characterized by the reflection amplitudes at (25 ± 0.2)°. Three methodologies for data post‐processing were used to calculate the reflection magnitudes. The results show that viscosity increases with repeat times of heating. The viscosity obtained by the integral ratio in a frequency band is closer to those obtained by rheometers than those from central frequency and multiple reflection. It indicates that the shear‐wave reflectometry can be extended further used to measure low‐viscosity liquids by appropriate signal processing. In addition, the heating times of a given procedure is highly correlated with the viscosity of edible oil (R² > .93). We also attempted to evaluate the oil deterioration by predicting the amounts of carbonyl and fatty acids based on the relationship between viscosity and indicator content. PRACTICAL APPLICATIONS: Viscosity is an important physical property and a quality index of edible oil. Not only viscosity changes, aging, and decay phenomena will even occur in the process of repeated heating of edible oils at high temperature. As a convenient and nondestructive method, the ultrasonic shear‐wave reflectance method can realize the rapid detection of the viscosity of reheated oils in situ. Therefore, this method has the potential to monitor frying oil quality online, which is expected to evaluate the degree of aging and deterioration of reheated edible oil.