HIGH-POWER ULTRASOUND IN OLIVE PASTE PRETREATMENT. EFFECT ON VIRGIN OLIVE OIL CHARACTERISTICS OF TWO APULIAN VARIETIES (CORATINA and PARANZANA)

A large increase in the demand for high-quality virgin olive oil continuously stimulating the search for new technologies. In olive oil extraction process, is known that to extract greater oil amount longer kneading time at optimal temperature is needed or, alternatively, higher process temperature. However, both high temperature and longtime of malaxation goes in damage of oil quality, oxidation processes are enhanced and losses in sensory characteristics take place. The malaxer is a heat exchanger characterized by a low overall heat transfer coefficient. At industry scale, extra malaxation time is required to reach the optimal paste temperature, usually around 15- 20 minutes for 30°C. The aim of this work is to show the results obtained from experiments carried out, at laboratory scale, applying high-power ultrasounds during the malaxation step and its effect on oil characteristics. High-power ultrasound application on olive paste has shown a positive effect on length of malaxation step. It provides a quick-heating of olive paste without alteration of olive oil composition. Two main mechanisms are involved in the ultrasonic treatment on olives and on olive paste: a thermal effect and a mechanical effect. The thermal effect was due to the attenuation characteristics of the medium. The mechanical effect is due to cavitation or particulate streaming which cause violent movement of the particles of the medium. Sound waves, which have frequencies higher than 20 kHz, are mechanical vibrations in a solid, liquid and gas. Unlike electromagnetic waves, sound waves must travel in a matter and they involve expansion and compression cycles during travel in the medium. Expansion pulls molecules apart and compression pushes them together. The expansion can create bubbles in a liquid and produce negative pressure. The bubbles form, grow and finally collapse. Close to a solid boundary, cavity collapse is asymmetric and produces high-speed jets of liquid that have strong impact on the solid surface and can disrupt biological cell walls. The mechanical effect of ultrasounds promotes the release of soluble compounds from the plant body by disrupting cell walls and improves mass transfer also in the olive tissues. The ultrasound technology provides a reduction of malaxing time improving extractability of oil and its antioxidant content both of Coratina and Paranzana variety.