**3. Emerging Innovative Approaches for Olive Oil Production**

Conventional techniques in olive oil extraction have not significantly changed in the last 25 years. Three main steps can be identified (Figure 2): crushing and malaxation, which mainly affect the oil quality and yield, and centrifugation [26,27].

**Figure 2.** Scheme of olive oil production.

At first, stems, twigs, and leaves are separated from olive fruits [28]. These latter are then washed in a proper plant to remove dust, dirt, etc. In some plants, the washing water is recycled for the process after solid sedimentation or filtration, whereas in other cases, olives are directly processed without the washing step [29]. The next step involves malaxation: olives are ground up, mixed with/without their stones, and put in tanks, where the paste is divided into vegetation waters, pomace, and oil. Pomace, a brown-colored residue, is obtained by centrifugation and sedimentation after pressing olives [30,31]. Pomace mainly consists of skin pulp and pit fragments. Its separation is carried out using a horizontal decanter centrifuge and an olive oil press. The centrifuge step can be performed in two- or three-phases (Figure 3).

**Figure 3.** Scheme of two-phases and three-phases centrifugation strategies.

In the former case, wet pomace (also known as two-phase olive mill waste, TPOMW) and olive oil are obtained by horizontal centrifugation. Then, the obtained oil is centrifuged with water, producing olive oil and a small stream of OMWW [32,33]. In the latter, the olive paste is divided into pomace and a liquid fraction (olive oil *plus* OMWW), which is centrifuged with water to obtain high-quality olive oil and OMWW [32,33].

However, given the ever more urgent market demand, interesting novel methods characterized by minimal processing are currently the object of study. These approaches aim to obtain a final product with the same nutritional qualities in less time. In this context, numerous solutions, including the use of microwave, high-pressure processing, pulsed light, radio frequency, Ohmic heating, ultrasound, and pulsed electric field (PEF), have been investigated thanks to their advantages (enhanced extraction efficiency in reduced time with increased yield, and low energy consumption) [26,34–37].

Among them, ultrasound emerged as a powerful technology widely used in several extraction processes [37,38] and food processing methods (i.e., emulsification, filtration, crystallization, enzymes' and microorganisms' inactivation, thawing) [39,40]. Ultrasound can be applied to the olive paste to induce oil release from vacuoles in lower malaxation time. It has been demonstrated that high oil quality and yield are obtained [36,41–45].

Pulsed electric field (PEF) technology, used mainly in food science since 1960, consists of exposing food products (solid or liquid) to an electric field, inducing pore formation in cell membranes [46]. Recently, it has demonstrated its efficiency in reversible or irreversible permeabilization of cell membranes in different plants without causing significant temperature increase [34]. The possibility of maintaining low operating temperatures during the oil extraction process represents a valuable goal, as it allows the preservation of the product's organoleptic and nutritional characteristics.

An alternative to the two previous processes is microwave-assisted extraction (MAE), which represents a more efficient and successful strategy than the conventional ones because microwaves provide rapid heating and biological cell structure destruction. As a result, it leads to high-quality products with shallow energy requirements, inducing reduced environmental impact and financial costs [47].

Recently, emphasis has been placed on obtaining an increased Extra Virgin Olive Oil (EVOO) quality, preserving its sensory characteristic and favorable health properties. The quality of the EVOO strongly depends on the presence of phenolic and volatile compounds [43,44]. So, the development of emerging technologies to increase the oil yield while protecting and improving the bioactive oil compounds and quality is of fundamental importance.

Table 1 summarizes some interesting studies related to innovative technologies applied to olive oil extraction, including the maximum extraction yield obtained (i.e., the percentage value given by the ratio between the weights of the extracted oil and olives).


**Table 1.** Emerging extraction methods for olive oil production. Adapted from Ref. [48].


**Table 1.** *Cont*.

<sup>a</sup> HPU: high-power ultrasound; PEF: pulsed electric field; MW: microwave; <sup>b</sup> expressed as percentage given by the ratio between the weights of the extracted oil and the olives; <sup>c</sup> not defined.
