Modelling the Rheology of Olive Paste for Oil Extraction Plant Automation: Effects of the Crushing Process on the Rheology of Olive Pastes

Abstract

In extra virgin olive oil production, it is essential to obtain a well-prepared olive paste which allows not only the extraction of the oil drops from the olives, but also the achievement of a high-quality oil while maintaining high yields. This work addresses the problem of determining the effect of three crushing machines on the viscosity of the olive paste: a hammer crusher, a disk crusher and a de-stoner were tested. The tests were repeated on both the paste leaving each machine and the paste to which water was added; this was done with the main aim of considering the different dilutions of the paste while entering the decanter. A power law and the Zhang and Evans model were used to analyse the rheological behaviour of the paste. The experimental results allow validation of the two models with a high (more than 0.9) coefficient of determination between experimental and numerical data. The results also show that the pastes obtained with the two classic crushing methods (hammers and disks) are almost identical, with a packing factor of about 17.9% and 18.6%, respectively. Conversely, the paste obtained with the de-stoner entails higher viscosity values and a smaller solid packing factor, of about 2.8%. At 30% dilution with water, the volume of the solid concentration dropped to about 11.6% for the hammer and disc crushers, while for the de-stoner it only reached 1.8%. This behaviour is also reflected in the evaluation of yields, which were 6% lower with the de-stoner. No significant differences regarding the legal parameters of oil quality were found using the three different crushing systems. Finally, this paper establishes some fundamental pillars in the research for an optimal model for identifying the rheological behaviour of the paste as a function of the crusher used. Indeed, since there is an increasing need for automation in the oil extraction process, these models can be of great help in optimizing this process.

Exploring the relevance of the type of horizontal separator to optimize the extraction efficacy for the Arbequina variety

“…The Pareto chart shows the absolute values of the standardized effects from the largest effect to the smallest effect. The results showed that the greatest variability depends on the fruit moisture content, in agreement with previous studies that relate the oil yield with fruit characteristics, such as ripening, pectic substances, and moisture and oil content ( Kalogianni et al., 2019 ; Perone et al., 2023 ). In addition, it can also be seen in the Pareto chart that the machinery, characterized by specific technological characteristics, and malaxation temperature are factors of great relative impact that must be taken into account when setting up the processing conditions ( Altieri et al., 2013 ; Esposto et al., 2013 ).…”

“…The results of pomace fat content showed average values ranging from 9% to 14% on a dry basis (db), reaching an extraction efficiency within the range of 78%–91%, in agreement with the results of oil extractability reported in other studies for the ‘Arbequina’ cultivar ( Boudebouz et al., 2021 ; Hermoso et al., 2021 ; Khaleghi et al., 2023 ; Perone et al., 2023 ). It must be pointed out that, during virgin olive oil production, oil losses of up to 10% through pomace are traditionally accepted for this variety.…”

“…An explanation for this behavior could be the fact that the high moisture content generates the so-called “difficult paste,” causing emulsions and instability of the paste’s viscosity, making it difficult to handle ( Tamborrino et al., 2014 ). A reduction of these extraction problems can be obtained with careful management of the olive paste preparation steps (crushing and malaxation) and the right setting of technological parameters of the separation phase ( Squeo et al., 2017 ; Caponio et al., 2018 ; Vallone et al., 2022 ; Nardella et al., 2023 ; Perone et al., 2023 ).…”

“…The processing of these “difficult pastes” generates significant oil losses, which requires special attention during the preparation of the olive paste and along the oil separation steps. In fact, the right management and control of the different phases of the VOO mechanical extraction process, such as the crushing step and conditioning of olive paste, can have a significant impact on the rheology of the olive paste with a high effect on the oil yield ( Perone et al., 2023 ). The treatment of olive paste before the extraction phase was recently the subject of several studies concerning the introduction of emerging technologies in olive oil mechanical extraction plants such as the application of heat exchangers, ultrasounds, microwaves, and vacuum systems able to modify the physicochemical structure of the olive paste ( Puertolas and de Maranon, 2015 ; Bejaoui et al., 2016 ; Tamborrino et al., 2021 ; Taticchi et al., 2019 ; Veneziani et al., 2022 ; Juliano et al., 2023 ).…”