Abstract:Sweep frequency ultrasound is an important physical processing technique for food that can provide an acoustic field, which improves the cavitation effect and enhances the extraction yield of oils. However, oxidation may occur, and the viscosity is affected as a result of the cavitation effect. Therefore, it is necessary to study the oil oxidation and the rheological properties of this technique. In this study, a piezoelectric polyvinylidene fluoride (PVDF) sensor was used to monitor the sweep frequency ultrasonic fields with the eigenfrequencies of 20 kHz, 40 kHz, and 60 kHz during the entire process of oil extraction. The oxidative properties of the extracted oils were measured according to the national standards. The rheological properties of the samples were determined using a rotational rheometer. The results showed that the resonance frequency of the oil extraction was 40 kHz, and its cavitation effect was the most significant. When the ultrasonic eigenfrequency was less than 40 kHz, the peak voltage amplitude, frequency spectrum coverage range, and oxidative properties increased with the increment of eigenfrequency, and the viscosity decreased. The opposite effect was observed when the ultrasonic eigenfrequency was greater than 40 kHz, such that the voltage amplitude, frequency spectrum coverage range, and oxidative properties decreased with the increment of eigenfrequency, and the viscosity increased. The viscosity of the oils was the lowest at the resonance frequency. The extracted oils were plastic fluid with the flow characteristic of Bingham flow, and demonstrated a shear-thinning trend.
