**1. Introduction**

Olive oil plays an important role in the diet in Mediterranean countries [1]. Extra virgin olive oil (EVOO) is the only vegetable oil that must be extracted only by mechanical means without any adjuvants [2]. EVOO is therefore, in effect, a fruit juice, hence the phytosanitary state of drupes is the main factor determining the quality of the extracted olive oil [3]. To best preserve the raw material before processing, post-harvest management is strategic to obtain extra virgin olive oils, since during this period oxidation of fat matrix and fermentation can occur [4].

However, in olive-producing regions such as Italy, Spain and Greece, because of the difficulty in synchronizing fruit harvesting and extraction of its oil, the olive sector is often forced to store the fruits piled up, in poor conditions and for periods of up to several weeks. During this period, the fruits suffer mechanical, physicochemical and physiological alterations that may eventually cause the breakdown of their cell structures [5,6]. During prolonged olive storage, anaerobiosis processes can occur in the lower portion of the olives kept inside the containers, and heat production from the respiratory activity may also accelerate fruit deterioration and eventually cause the breakdown of the cell structure [6]. Olive oils obtained from damaged olives present a characteristic high acidity, low oxidative stability and high level of oxidation, due to the increased peroxide value, and specific extinction coefficients at 232 and 270; they can also develop a high content of volatile acids (acetic or butyric) that cause a typical musty smell [7]. These processes will deteriorate the chemical and sensory quality of the resulting EVOO, so in order to better manage the postharvest period, several technological solutions have been proposed such as cold storage of olives [7], storage in a modified atmosphere [8], and other preservation conditions such as storage in sea water, brine or drinking water have also been investigated [9].

**Citation:** Rotondi, A.; Morrone, L.; Bertazza, G.; Neri, L. Effect of Duration of Olive Storage on Chemical and Sensory Quality of Extra Virgin Olive Oils. *Foods* **2021**, *10*, 2296. https://doi.org/10.3390/ foods10102296

Academic Editor: Theodoros Varzakas

Received: 16 September 2021 Accepted: 23 September 2021 Published: 28 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The importance of processing olives a short time after harvesting is also linked to the fact that most fruit is harvested mechanically and could, therefore, be internally damaged, more so than in the case of manual harvesting; however, allowing for proper storage conditions the fruits can be stored for several days maintaining the appropriate chemical and sensory quality standards of the final oil. Yousfi et al. [10], in fact, studied the quality of EVOO from mechanically harvested Arbequina olives under different storage conditions, and found that storage at 3 ◦C for a period of up to 10 days allowed the highest commercial level of oil quality to be maintained.

The problem of synchronization of harvest and transformation phases has not been widely considered in Italy, where this study was carried out in the past due to the production fragmentation, the structure of olive mills (small and widespread) and the presence of different olive cultivars, a factor broadening the collection window. However, the presence of numerous different cultivars on the Italian territory is a characteristic feature of Italian olive growing that increases its sustainability as the loss of biodiversity is an environmental threat. The production of monovarietal olive oils has increased to a great extent lately since the quality of olive oil depends on the olive variety from which it originates [11]. Nowadays, however, the structure of production is changing in Italy, due to the presence of an increasing number of intensive orchards that can exceed the processing capacity of the mills, and therefore synchronization between harvest and transformation should be considered.

The aim of this study was to assess the effect of the duration of olive storage on the chemical and sensory quality of the EVOO, identifying which parameters were most affected by olive storage; in particular, we focused on product parameters that are more easily illustrated to actors in the supply chain (mills, producers and consumers), thus making it easier to understand and assimilate the results.

#### **2. Materials and Methods**

#### *2.1. Olive Fruit Analysis and Oil Sampling*

Olive fruits and the corresponding oil samples (*n* = 228) were collected during 3 crop seasons, from seven industrial oil mills located in the Emilia-Romagna region in northern Italy, all equipped with hammer crusher, two-phase decanter, and centrifugation and filtration facilities. Data characterising olive oil samples, such as olive cultivar, harvesting method, and olive storage duration, were collected by interviewing olive growers. Only samples of healthy olives without signs of infection were considered after visual inspection.

Oil samples were poured into dark glass bottles, keeping headspace to a minimum, and stored in the dark in a temperature-controlled cupboard set at 15 ± 1 ◦C, until chemical and sensory analyses were carried out.

#### *2.2. Chemical Analysis of Olive Oils*

Free acidity, peroxide value, and UV-spectrophotometric indices (K232, K270) were evaluated in triplicate in line with official methods described in Regulation EC 2568/91 and subsequent amendments [12].

Analysis of fatty acids was carried out according to Regulation EC 2568/91 and subsequent amendments [12] using a Chrompack CP 9000 gas chromatograph with a flame ionization detector (FID), equipped with a capillary column (Stabilwax, Restek Corporation, Bellefonte, PA, USA) and helium as the carrier gas (flow rate = 1 mL min<sup>−</sup>1; split ratio of 1:20, v:v). Chromatographic parameters were as follows: injection and detection temperature 250 ◦C; 230 ◦C; column oven temperature, 240 ◦C. All parameters were determined in triplicate for each sample.

The phenolic fraction was extracted in triplicate from 30 g of oil using 30 mL of methanol. The combined extract was brought to dryness through a rotary evaporator and then suspended in 2 mL 50% methanolic solution. Total phenol content was determined by the Folin–Ciocalteau spectrophotometric method at 750 nm [13] using a spectrophometer (Jasco V-500, Jasco Corporation, Tokyo, Japan).

Quantitative analysis of tocopherols, lutein and β-carotene was carried through olive oil filtration on PTFE (Polytetrafluoroethylene) membrane filter of 25 mm, 0.2 μm pore size (GyroDisc, Orange Scientific, Waterloo, Belgium) and direct injection of 20 μL in HPLC (high-performance liquid chromatography) [14] equipment (LC-10ADvp, Shimadzu, Kyoto, Japan) with a degasser (Flow 154, Gastorr Flom, Tokyo, Japan), a low-pressure gradient unit (FCV-10ALvp, Shimadzu, Kyoto, Japan) and a column oven (CTO-10ASvp, Shimadzu, Kyoto, Japan). Analytes were separated on a C18 column 150 mm × 4.6 mm (Inertsil ODS-2 5U, Alltech, Deerfield, IL, USA); the flow rate was 1 mL min<sup>−</sup>1, the injection volume was 20 μL and the column temperature was 25 ◦C. The eluent used was: A methanol: water 80:20 (*v*/*v*) and B methanol: tetrahydrofuran 20:80 (*v*/*v*). Quantification of analytes was carried out using their relative analytical standard's calibration curves all purchased from Merk (Deisenhofen, Germany). Tocopherol quantification was carried out at 295 nm, β-carotene and lutein at 450 nm using a photodiode array detector (UV6000, ThermoQuest, San Jose, CA, USA).

### *2.3. Oil Stability Determination*

For determination of oil stability, an eight-channel Oxidative Stability Instrument (OSI) (Omnion, Decatur, IL, USA) was used; the instrument was set at 110 ◦C and at 120 mL min−<sup>1</sup> (airflow) [15]. The OSI index was expressed as time (hours and hundredth of hours) and was reported as "OSI time".
