Application of electrical impedance spectroscopy and rheology to monitor changes in olive (Olea europaea L.) pulp during cold-pressed oil extraction

Abstract

Cellular changes during cold-pressed extraction of ‘J5’ olives (Olea europaea L.) were investigated to better understand factors that affect oil release and recovery. Electrical impedance, electrical conductivity, rheological flow properties and light microscopy were used to monitor flesh or pulp structure at defined steps during the extraction process (grinding, malaxing and decanting). Light microscopy revealed most parenchyma cells in the flesh were ruptured in the grinding step. A significant reduction in electrical impedance concurrent with an increase in conductivity of tissue that occurred when cells were ruptured during the grinding process. Malaxing assisted aggregation of oil into larger droplets, observed by microscopy. Increasing malaxing time resulted in a decrease in the storage modulus (G′) of olive pulp, which correlated with the oil droplets in the olive paste coalescing together and becoming larger. The extraction process can be monitored with different techniques to assist with improved extraction yields.