Ultrasonic bleaching of rapeseed oil: Effects of bleaching conditions and underlying mechanisms

Abstract

An ultrasonic bleaching process for rapeseed oil was studied by monitoring the removal of oil-containing pigments spectrophotometrically at 446nm in varied bleaching conditions. When low-power ultrasound was used, bleaching was resulted from pigment adsorption by adsorbents, among which bentonite decreased the A446 the most from 1.82 to 0.65. Ultrasound enhanced the absorption rate rather than the total absorption capacity. When high-power ultrasound was used, bleaching took place even in the absence of adsorbent as evidenced by the decease of A446 to 0.74, possibly due to the effects of combining heat and sonochemical degradation of pigments. The overall bleaching effects were enhanced with the increase of ultrasound power, sonication time and treatment temperature. The content of primary oxidation products in oils increased after sonication, whereas the content of secondary oxidation products remained constant. Due to its multiple mechanisms, ultrasonic bleaching may be a good alternative to the adsorption-based bleaching method.