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Home All issues Volume 31 (2024) OCL, 31 (2024) 15 Full HTML
Open Access
Issue
OCL
Volume 31, 2024
Article Number 15
Number of page(s) 18
Section Quality - Food safety
DOI https://doi.org/10.1051/ocl/2024013
Published online 15 July 2024

© M. Kıralan et al., Published by EDP Sciences, 2024

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Highlights

  • Ayvalik is an important Turkish olive cultivar.

  • Fruits were harvested at two periods, and oils extracted with/without olive leaves.

  • Leaf addition enriched phenolic levels and improved oils' physicochemical traits.

  • Breakdown of complex phenolics to simple ones (i.e., tyrosol and hydroxytyrosol) was less in samples stored at cold conditions.

1 Introduction

Extra virgin olive oil (EVOO) is a valuable part of the Mediterranean diet for its nutritional, therapeutic, and gastronomic benefits and sensory attributes (Di Stefano and Melilli, 2020; Dancausa-Millan et al., 2022). These beneficial properties are related to its high content of oleic acid (C18:1), phenolics, color pigments (carotenoids and chlorophylls), and aroma compounds (Pedan et al., 2019; Cecchi et al., 2021; Genovese et al., 2021; Gargouri et al., 2023; Kaur et al., 2023). Like other vegetable oils, olive oil’s chemical and sensory traits decrease due to natural oxidation during storage. Studies emphasized improving the oxidation stability of olive oils and, therefore, extending their shelf life during storage (Mousavi et al., 2021; Gargouri et al., 2023). A strategy to improve oxidative stability is using olive leaves, a by-product rich in phenolics and pigments (Difonzo et al., 2021; Najla et al., 2022; Delgado et al., 2023; Keskin et al., 2024). Adding olive leaves during olive oil processing could improve the olive oils’ phenolic profile, chlorophyll content, and sensory attributes (Pedan et al., 2019; Marx et al., 2022). In most of these studies, different amounts of dried or fresh olive leaves (1–10%) were used in laboratory or pilot scale extraction systems, especially the Abencor extraction system (Malheiro et al., 2013; Sevim et al., 2013; Baccouri et al., 2022). However, industrial-scale extraction systems were utilized in only a few studies by Di Giovacchino et al., (1996) and Marx et al., (2022). Di Giovacchino et al., (1996) demonstrated that olive leaf addition (1-3% concentration) to ripe Italian olive cultivars (Dritta and Leccino-Castiglionese mixture) increased trans-2-hexanal content, which is responsible for pleasant freshly cut grass sensory attribute. Marx et al., (2022) used olive leaves from cv. Arbequina and cv. Santulhana at 1% concentration in a famous Spanish olive cv. Arbequina. Leaf addition promoted a significant increment in total phenolics and oxidative stability as well as primary oxidation parameters such as peroxide value and K232 value. Besides, bitterness increased in olive oils with olive leaves while pungency decreased.

Storage conditions are as important as production in maintaining the quality of olive oils. Oxidation is expected to be the main quality deterioration mechanism during storage. Storage temperature is essential for evaluating olive oil’s oxidation stability and shelf life (Mousavi et al., 2021). Various storage temperatures ranging from 0 °C to 25 °C were studied to determine the oxidation stability of olive oils (Gómez-Alonso et al., 2007, Conte et al., 2020, Di Stefano and Melilli 2020). Several studies have demonstrated the effects of storage conditions on basic quality parameters such as free acidity, peroxide values, extinction coefficients (K232 and K270), fatty acids composition, chlorophyll and carotenoid content and as important quality parameters for olive oil such as phenolics, tocols and aroma compounds (Cecchi et al., 2019, Esposto et al., 2020, Delgado et al., 2020; Castillo-Luna et al., 2021). The storage period is generally applied up to 18 months, which is considered the maximum storage period from bottling to consumption (Kotsiou and Tasioula-Margari, 2016). Deiana et al., (2022) demonstrated that the peroxide, FFA, K232, and K270 values of Italian olive oils increased during 12 months of storage at 15 °C in the dark. Simple phenols such as tyrosol and hydroxytyrosol increased in olive oils stored in a basement without central heating for 24 months (Kotsiou and Tasioula-Margari 2016).

While extensive information for different storage conditions is available on olive oils with added olive leaves using the Abencor extraction method, few works have been performed on some basic quality parameters and phenolic compounds of oils with added leaves that have been industrially extracted. Contrarily, and to the authors’ best knowledge, there was no information on the storage of olive oils incorporating olive leaves extracted on an industrial scale. Thus, the present study aimed to assess quality parameters, including FFA, peroxide value, extinction coefficients, the amount of chlorophylls, the amount of carotenoids, fatty acid composition and also, oxidation stability, antiradical potential and phenolics of Ayvalik olives with added leaves. For this purpose, olive fruits (cv. Ayvalik) were harvested at two different maturity stages during the 2021 crop year and mixed with fresh leaves of the same cultivar at 0, 2, 4, and 6% (w/w) before oil extraction. After oil extraction, the samples were stored at two conditions (room temperature and 12 °C in darkness) for 12 months and sampled at 3-month intervals for analysis.

2 Materials and methods

2.1 Samples and storage

Extra virgin olive oils used in this study were from the Ayvalik olive cultivar from the Ayvalik region in Turkey during the crop season 2021/2022. Olive fruits were harvested in two harvesting periods: the early-harvest date was set for the end of September, and the mid-harvest date was set for the end of December. The maturity levels of olive fruits were determined according to the IOC method (International Olive Council, 2011) based on the evaluation of the olive skin and pulp colors. The maturity indexes of early and middle harvested olives were between 1.00–1.50 and 5.00–5.50, respectively.

The early harvest sample was run on a total batch of 1970 kg. Before weighing, the sample was separated from the leaves and branches, and only the olive remained. The sample was divided into 4 equal parts of approximately 500 kg each. For each part, 0–2–4–6% (w/w) leaves were added before the crusher. The olive paste from early harvested olives was carried out for 30 min at 21–23 °C. A total of 250 kg of olive oil was obtained, averaging 60–65 kg for each 500 kg lot. The mid-harvest sample was run on a total lot of 1435 kg. Likewise, the leaves and branches were separated before the sample was weighed, and only olives remained. The sample was divided into 4 equal parts of approximately 360 kg each. For each part, 0–2–4–6% (w/w) leaves were added before the crusher. The olive paste from middle harvested oils was subjected to malaxation at 32–34 °C for 30 min. A total of 271 kg of olive oil was obtained, averaging 65–70 kg for each 360 kg lot.

The obtained olive oils were filtered through a thin layer of cotton and then placed in 200 mL clear glass bottles. The bottles were labeled, and two samples were prepared. The bottles were then hermetically sealed and divided into two batches for each harvest period. The first batch was placed in a cardboard box and stored in a dark room at room temperature. The second batch was placed in cardboard boxes the same way and stored in a dark room at 12 °C with special air conditioning. Samples stored at room temperature were stored at an average temperature of 25 °C for both winter and summer. The sampling times were at baseline (0 month) and 3, 6, 9 and 12 months after storage in the conditions described after bottling.

2.2 Analytical determinations

Free fatty acid content, given as % of oleic acid, peroxide value (PV) expressed as milliequivalents of active oxygen per kilogram of oil (meq O2/kg), and K232 and K270 extinction coefficients calculated from absorption at 232 and 270 nm, were measured, following the AOCS official method; Ca 5a-40, Cd 8-53 Cd 18-90, and Ch 5-91, respectively.

2.3 Fatty acid analysis

Fatty acid methyl esters (FAMEs) of olive oil samples were prepared (Anonymous, 2015). GC determined the fatty acid methyl ester composition with a Shimadzu GC-2025 (Kyoto, Japan) equipped with an Rtx-2330 capillary column(60 m × 0.25 mm × 0.20 µm, Restek, USA) and a flame ionization detector (FID). The column temperature was held at 190 °C for 50 min. Helium was used as a carrier gas with a constant flow rate of 0.67 mL/min, with the injector at 230 °C in split mode (split ratio 1:25) and detector at 250 °C. The identification of individual FAMEs was performed by comparison of retention times with those of a standard FAME mixture (Supelco 37 Component FAMEMix, Supelco Co.).

2.4 Oxidative stability

The oxidative stability of olive oil samples with/without leaves was measured using 892 Rancimat Oxidative Stability Instrument (Metrohm Ltda, Herisau, Switzerland), using a sample of 3 g oil heated at 120 °C under a constant air flow rate of 20 L/h.

2.5 Chlorophyll and carotenoid pigments

Following the procedures described by Minguez‐Mosquera et al., (1991), a sample of oil (7.5 g) was weighed exactly in a Falcon tube, dissolved in n-hexane, and taken to a final volume of 25 mL. The chlorophyll fraction was measured in a UV spectrophotometer (Shimadzu Co., Kyoto, Japan) at 670 nm and the carotenoids fraction at 470 nm, using the specific extinction values (E0 = 613 and 2000, respectively). Thus, pigment contents were calculated as follows:

where A is the absorbance, and L is the spectrophotometer cell thickness (1 cm).

2.6 Colorimetric determination of O-diphenol compounds and antiradical activity

Phenolic compounds are extracted using the method described by Kalantzakis et al., (2006) for colorimetric determination of O-diphenols and antiradical activity. Five grams of olive oil were dissolved in 10 mL n-hexane to remove oil, extracted with 10 mL of a methanol: water mixture (60:40, v/v) and then shaken vigorously using a vortex and centrifuged at 3,500 rpm for 10 min. The hydromethanolic phase was collected. These extracts were used to analyze o-diphenol and DPPH ·  according to the method of Kalantzakis et al., (2006). O-diphenols were measured colorimetrically at 370 nm after adding 5% (w/v) sodium molybdate in suitable aliquots of the hydromethanolic extract. Results are given as milligrams of caffeic acid per kilogram of oil. An aliquot of hydromethanolic extract (0.5 mL) was added to 3 mL of DPPH ·  solution (0.1 mM in methanol), mixed well, and the absorbance was measured at 515 nm after 30 min. Antiradical action toward DPPH ·  radical was estimated from the difference in absorbance with or without a sample (control). The percent of inhibition was calculated from the following equation:

2.7 Phenolic composition by LC-MS/MS

The extraction of phenolic compounds from olive oils was determined following a protocol described previously (International Olive Council 2009, Schneider 2013) with slight modifications. Briefly, 2 g of olive oil sample were weighed in a 15 mL screw-cap test tube and extracted with 5 mL of methanol/water (80:20, v/v). The mixture was vortexed for exactly 1 min. After vortexing, the extraction was sonicated with an ultrasound bath for 15 min at room temperature and centrifuged at 5000 rpm for 25 min. An aliquot of the supernatant phase was filtered through a 1-mL plastic syringe with Syringe Filters of 0.45 µm before injection into the LC-MS/MS.

LC-MS/MS analyses were performed using a Shimadzu 8050 triple quadrupole mass spectrometer (Kyoto, Japan), composed of LC-40D XS high-pressure pumps, an autosampler (model SIL-40C XS) and column oven (CTO-40S). Phenolic compounds were separated on a C18 column (Inertsustain Swift C18, 2.1 mm × 100 mm GL Sciences Inc.) maintained at 40 °C. Mobil phases were as follows: A (H2O/0.1% HCOOH) and B (CH3OH/0.1% HCOOH); the flow rate was 0.4 mL/min. The injection volume was held constant at 5 µL The elution gradient was: 0 min, 80% A and 20% B; 0 to 8 min, 50% A and 50% B; 8 to 12 min, 5% A and 95% B; 12 to 12.10 min, 80% A and 20% B. MS analyses were performed in negative and positive modes, the scan range was set at m/z 100-800, and the scan speed was set at 2500 u sec−1. The conditions of ESI were as follows: 0.3 s event time, 3.00 L min−1 nebulizing gas (N2) flow rate, 10.00 L min−1 drying gas (N2) flow rate, 300 °C interface temperature, 400 °C heat block temperature, 250 °C DL (desolvation line) temperature.

The following standards were used for the calibration curves: naringenin, p-coumaric acid, kaempferol, genistein and apigenin, oleuropein, hydroxytyrosol and tyrosol; regression coefficients (R2) above 0.995 were obtained. Stock solutions (2000 mg/L) were prepared from each phenolic standard in MeOH. Working solutions were prepared at 1 mg/L by taking appropriate portions from the stock solutions. The Linearity for phenolic compounds was assessed by three replicate injections per concentration of six calibration working standard solutions at concentration levels of 0.1, 0.2, 0.5, 0.75, 1 and 2 mg/kg.

2.8 Sensory assessment

The sensory assessment was performed by eight tasters (aged 25-50 years) experienced in the organoleptic evaluation of olive oil from a local olive mill (Güvenasa, Ayvalik-Turkey) according to the official method of International Olive Council (IOC, 2018). Positive and negative attributes were evaluated using a 10 cm scale, using the scale’s minimum (delicate) and maximum (robust) limits. The samples (approximately 15 g) were provided to the tasters coded with a three-digit number code, at 28 ± 2 °C in blue glass cups covered with a watch glass, presented randomly in different sessions with only five samples simultaneously. The panel leader compiled the notes given by each taster, and the statistical evaluation was carried out by the fruity median and the defects’ median. During 12-month storage, panelists evaluated olive oil samples’ positive (fruitiness, bitterness and pungency) and negative attributes (rancidity, musty and fusty).

2.9 Statistical analysis

Statistical analysis was done using the SPSS 10 software (SPSS Inc., Chicago, USA). Data were analyzed by one-way analysis of variance (ANOVA). Duncan’s multiple range tests were used to determine if there were any statistical differences between the samples (P < 0.05).

3 Results and dıscussıon

3.1 Initial composition of fresh olive oils

The quality parameters of olive oil samples are presented in Table 1. The FFA of olive oils with/without olive leaves ranged from 0.28% to 0.54%. Meanwhile, The PVs of the studied olive oils were between 4.97 meq O2/kg and 5.70 meq O2/kg. Both parameters were within the legal limits (0.80%, 20 meq O2/kg) established by the International Oil Council (IOC) trade standards in the extra virgin olive oil category. The FFA values were lower in early-harvested oils than middle-harvested oils. However, The PVs did not change significantly in oil samples according to harvest time. These results align with those reported by other authors (Houshia et al., 2019; Navajas-Porras et al., 2020).

The specific extinctions of the studied oils, K232 and K270, remained within the limits established for extra virgin olive oils (K232 ≤ 2.5, K270 ≤ 0.22). The results for K232 and K270, in oils obtained at two harvest times, were 1.20–1.55 and 0.06–0.09, respectively. These values did not vary significantly according to harvest time. These results agree with the results of Piscopo et al., (2018).

Oleic acid (C18:1) is the most abundant of those analyzed, with percentages between 72.40% and 74.35%. Palmitic acid (C16:0, 11.62–12.78%), linoleic acid (C18:2, 8.78–11.06%) and stearic acid (C18:0, 2.31–3.04%) were the other major fatty acids. A slight decrease in the level of oleic and palmitic acids and a slight increase in linoleic and stearic acids were observed, and a similar trend was observed in the study of Baccouri et al., (2008). Considering the investigated fatty acid composition (Tab. 1), all the samples analyzed could be categorized as extra-virgin olive oil.

Table 1

Composition of Ayvalik olive oils with/without olive leaves at two different harvest times.

3.2 Oxidative stability

The changes in induction periods at 120 °C of all the samples with/without olive leaves are presented in Table 2. Considering the harvest periods (IPs), the induction period of middle harvested oils (4.32 h) was lower than that of early harvested oils (4.05 h). These results align with those of other authors (Baccouri et al., 2008; Nsir et al., 2017). In all cases, an increase in the IPs of olive oils was observed with the addition of olive leaves. These results aligned with published data on Arbequina oils (Marx et al., 2022). In early harvested oils, the leaf addition increased the IPs of olive oils (4.73–4.90 h). However, these enriched olive oils did not differ statistically significantly. A similar trend found in the study of Mezghani et al., (2023) noted the increase in IPs of Chemlali olive oils by adding 1%, 2% and 5% leaves, while the IP values of these enriched oils were similar. As for middle-harvested oils, although the induction periods of olive oils increased with the addition of leaves, the increase in IPs was greater when compared to early-harvest olive oils. Statistically, the highest increase in IPs was observed in olive oils with 4% and 6% leaf addition with an induction period of 6.48 h and 6.99 h, respectively. The difference between the induction periods of early and middle harvested olive oils could relate to the production conditions. While lower temperatures are used in processing early-harvested olives in the industry, the temperature slightly increases in middle-harvested olive oils. Due to the temperature, phenolic substance transfer was higher in middle-harvested olive oils than in early-harvested olive oils.

Table 2

Induction period values (h) of Ayvalik olive oils with/without olive leaves at two different harvest times.

3.3 Changes in basic quality parameters of oil samples during storage

Mean values for the free fatty acid of EVOO extracted from two harvested times stored during 12 months in glass bottles at room temperature in darkness and at 12 °C temperatures are summarized in Figure 1. The initial level of FFA in early and middle harvested oils with/without olive leaves was similar. These results agree with the findings of Malheiro et al., (2013) and Tarchoune et al., (2019). Besides, the FFA of middle-harvested oils had higher values than early-harvested oils. A similar increase in acidity during the maturation period was found in olive oils by Dag et al., (2011).

FFA content increased slightly at early harvested oils and did not exceed 0.5%. There is a slight increase in the FFA content of EVOOs with the addition of olive leaves under storage. The EVOOs stored at two conditions showed similar FFA values under the storage of 12 months. Regarding middle-harvested oils, there was a slight decrease in the FFA of oils with olive leaves compared with the control sample throughout the storage period. The level of FFA in all analyzed samples was below the value of 0.7%. The decrease in FFA of oils with olive leaves could be related to the presence of antioxidant compounds transferred from olive leaves to oil. The FFA content in oils stored at 12 °C was lower than those stored at room temperature. A similar trend was observed in the study of Mousavi et al., (2021), which evaluated that oils stored at ambient conditions had higher values than those stored at low temperatures (4 °C).

Figure 2 shows the changes in PVs in olive oils stored under dark and cold storage conditions. The initial PV in early harvested oils was either slightly lower or similar to middle harvest olive oils. This changing trend in PV of oils with olive leaves in the present study is quite similar to that observed in Cobrançosa olive oil extracted with different percentages of olive leaves in a previous study by Malheiro et al., (2013). The PV in olive oils stored at 12 °C increased gradually throughout the storage period, while the fluctuations in PV were observed in oil samples during room temperature conditions. These differences could be related to temperature changes in room temperature. During storage, the PV remained below the 8 meq O2/kg value in all analyzed oils and was within the legal limits (20 meq O2/kg) established by the International Oil Council (IOC) trade standards in the extra virgin olive oil category.

The K232 values determined in the oil samples during two different storage conditions are shown in Figure 3. In initial samples, the fluctuations in K232 value were observed in the enriched olive oils extracted with olive leaves. Similar changes in the K232 values were recorded in olive oils extracted with olive leaves (Mezghani et al., 2023). At early harvested oils, slight increases were observed in the olive oils with olive leaves during two different storage conditions, while the maximum value of K232 (2.03) during storage did not exceed the limits established for extra virgin olive oils (K232 ≤ 2.5). In the middle harvested oils, the K232 value of oils with olive leaves was lower than that of control samples throughout the storage period. Besides, the K232 of oil samples had lower values at the end of 12 months of cold storage than those stored in the dark at room temperature. At 12 months of storage in the dark, the maximum value for K232 was 2.27 in the control sample, while this value ranged between 1.63 and 1.81. All values were below 2.5, the upper limit established for extra virgin olive oil.

Changes in the K270 of oils during two different storage conditions are given in Figure 4. The addition of leaves slightly increased the K270 value compared to the control with statistical differences. The increasing trend observed in the study of Malheiro et al., (2013) found an increase in the K270 value of Cobrançosa olive oil with the addition of leaves. Comparing the results from the olive oils from early and middle-harvested olives, higher values were detected in the middle-harvested oils. Besides, at the end of the 12th month of storage under the dark condition, the K270 values increased to 0.15, while the maximum K270 value reached up to 0.11 in olive oils stored at 12 °C. However, all the samples reported a K270 lower than the limit value of 0.22 for the EVOO quality parameter established by the International Oil Council (IOOC) trade standards.

thumbnail Fig. 1

The FFA content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0 and 6 next to the letters (AK, AD, BK, and BD) are the number of leaves added.
thumbnail Fig. 2

Peroxide value of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
thumbnail Fig. 3

K232 value of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early harvested olive oil at room temperature in the dark, AD: Early harvested olive oil at 12 °C, BK: Middle harvested olive oil at room temperature in the dark, BD: Middle harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
thumbnail Fig. 4

K270 value of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early harvested olive oil at room temperature in the dark, AD: Early harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.

3.4 Changes in color parameters of oil samples during storage

The chlorophyll contents of oils are shown in Figure 5. An increase in chlorophyll content was determined due to the addition of leaves in both early-harvest and mid-harvest olive oils. On the other hand, the chlorophyll content of the mid-harvest olive oils (3.155–9.390 mg/kg) was higher than the early-harvest olive oils (1.450–4.020 mg/kg). This is because the processing steps applied in mid-harvest olive oils were carried out at slightly higher temperatures than in early-harvest oil production. During storage, chlorophyll content in all olive oil samples decreased. Several authors have reported similar results (Morelló et al., 2004; El Yamani et al., 2022). While the highest loss in chlorophyll amount was observed in the control sample, the least loss was determined in the samples with 6% leaf addition. A similar trend was observed in the study of Sevim et al., (2013), which reported that the decrease in chlorophyll content of Memecik olive oils enriched with olive leaves was lower than that of control samples under dark ambient conditions. Besides, the chlorophyll content was lower in the oil samples stored in the cold than in the oils stored in the dark. These results are in agreement with Gözüpek and Otağ (2022).

Figure 6 presents the change of carotenoid contents in olive oils from two harvesting times during two different storage conditions. Carotenoid content followed a similar trend to that of chlorophylls. At early harvested oils, the lower decrease was observed in olive oils with 6% olive leaves, while the lower losses were in olive oils with 4% and 6% olive oils from mid-harvest time. Regarding storage conditions, the carotenoid decreased slowly in early harvested oil samples with 6% olive leaves at the end of 12 months with losses of 1.75% of the initial value. This loss value was 13% in dark stored oils. On the other hand, in the middle of harvested olive oils enriched with 6% leaf, the loss rates did not change according to the storage conditions after 12 months of storage.

thumbnail Fig. 5

The chlorophyll content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
thumbnail Fig. 6

Carotenoid content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.

3.5 Changes in phenolic composition, O-diphenols and antiradical activity during storage

Changes in the content of individual phenolic compounds of oils are shown in Figures 7 – 9  and Table 3. The eight phenolic compounds were determined in all analyzed samples. In all studied fresh olive oil samples, these phenolics increased in enriched oil samples with olive leaves, which is in agreement with the findings of Ammar et al., (2017) for various Tunisian olive oils, adding 0% and 3% w/w olive leaves.

Simple phenolics, such as hydroxytyrosol and tyrosol (Figs. 7 and 8), generated from the hydrolysis of oleuropein aglycon and ligstroside aglycon, respectively, were present in all studied olive oil samples. Hydroxytyrosol exhibits strong antioxidant activity and is found at high concentration in initial olive oils with the addition of leaf at the 6% level, increased to 1.31 mg/kg compared to the control sample (max value 0.50 mg/kg). These results are similar to those reported by several authors (Marx et al., 2022) for Arbequina olive oil. A similar increment trend in hydroxytyrosol of olive oils with olive leaves was observed for tyrosol. Tyrosol content increased to 1.26 mg/kg in olive oils, adding 6% olive leaves. The increasing trend in tyrosol with olive leaves is by the previous study of Ammar et al., (2017) found a slight increase in Chemlali and Chétoui olive oils with the addition of olive leaves (3%). The content of hydroxytyrosol and tyrosol increased gradually, especially enriched olive oils with olive leaves throughout storage. This increment for control samples agrees with the results of previous studies (Kotsiou and Tasioula-Margari 2016; Mousavi et al., 2021). In addition, the amount of tyrosol and hydroxytyrosol in cold-stored samples was lower than those stored under room conditions. This increasing trend is probably due to phenolic aglycons’ hydrolytic and oxidative degradation reactions (Losito et al., 2021). Furthermore, total phenolics and the antioxidant potential are influenced by numerous intrinsic and extrinsic parameters such as light exposure, sample pretreatment, temperature used, storage period, and oxygen (Mohdaly et al., 2022).

Oleuropein is a secoiridoid glycoside, which is increased by adding olive leaves. The concentration of oleuropein increased in olive oils with the amount of olive leaves added (Figure 9). The highest concentration was observed in olive oil samples with the addition of olive leaves at 6% (18.03 mg/kg (early harvest)–19.54 mg/kg (middle harvest). Oleuropein content decreased in all analyzed samples with storage time due to enzymatic and chemical reactions. At the end of 12 months of storage, the higher concentration of oleuropein was up to 0.44 mg/kg in enriched oils with olive leaves, while the value for this compound was determined at the maximum level of 0.11 mg/kg in the control sample.

The other determined phenolic compounds in all analyzed oils are naringenin, p-coumaric acid, kaempferol, genistein, and apigenin (Tab. 3). The concentration of these phenolic compounds increased considerably with the quantity of leaves added to initial samples. For these components, a general decrease was observed during different storage conditions. At the end of storage, their amounts were higher in olive oils enriched with olive leaves compared to the control sample.

O-Diphenols showed antioxidant activity, and changes in these substances in olive oils during storage are presented in Figure 10. Although O-diphenols fluctuated during storage, they were generally higher in leaf-added oils than in the control group. Similar fluctuations in behavior in O-diphenols observed in the study of Cinquanta et al., (1997) noted the rise and fall trend for hydroxytyrosol, an O-diphenol in olive oils, during 18 months of storage. This fluctuation trend could be related to the decomposition of hydroxytyrosol (Krichene et al., 2010). We can also suppose that these fluctuations in the samples during storage are because of the other O-diphenols such as hydroxytyrosol acetate, oleuropein aglycon, and the dialdehydic form of elenolic acid linked to hydroxytyrosol undergo hydrolysis reactions which were not analyzed in our study.

As regards antioxidant activity, a significant decrease was observed during storage. The changes in antioxidant activity of the samples were determined using DPPH ·  free radicals, and the inhibition rate is summarised in Figure 11. During storage, samples with olive leaves showed higher antioxidant activity than the control sample. Similar results were obtained by Köseoğlu et al., (2019).

Table 3

Phenolic compound compositions of the studied enriched olive oils.

thumbnail Fig. 7

Hydroxytyrosol content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early harvested olive oil at room temperature in the dark, AD: Early harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
thumbnail Fig. 8

Tyrosol content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early harvested olive oil at room temperature in the dark, AD: Early harvested olive oil at 12 °C, BK: Middle harvested olive oil at room temperature in the dark, BD: Middle harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
thumbnail Fig. 9

Oleuropein content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early harvested olive oil at room temperature in the dark, AD: Early harvested olive oil at 12 °C, BK: Middle harvested olive oil at room temperature in the dark, BD: Middle harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
thumbnail Fig. 10

O-Diphenol content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
thumbnail Fig. 11

The radical scavenging capacity of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.

3.6 Changes in sensory parameters of oil samples during storage

In addition to chemical analyses of olive oil, sensory analysis of olive oils is one of the important parameters for evaluating the quality of virgin olive oils, which directly affects consumer approval (Fernandes et al., 2018). The intensity of sensory attributes was evaluated during two storage conditions of olive oils from two harvesting periods and presented in Figures 12 and 13. No defects were evaluated in the fresh olive oils, while three positive attributes (bitter, pungent and fruity) were perceived in all samples, and these results are in agreement with the International Olive Oil Council (IOC) standard (2018) and also classified the oil samples as extra virgin.

During storage of Ayvalik olive oils from two different harvest times in two different conditions, no defects were observed, and there was a slight decrease in the intensity of positive attributes (fruity, bitter, and pungent) (Figs. 12 and 13). In fresh samples from early and middle harvested olives, fruity attributes had slightly higher values than pungency and bitterness, while enriched olive oils with olive leaves exhibited an improved trend for these positive attributes. These results follow those of previous reports by Sonda et al., (2014) and Malheiro et al., (2017), who emphasized that the addition of olive leaves to İtalian and Tunisian olive cultivars, improved the sensory characteristics of olive oils, especially enriched oils with 3% olive leaves addition. Besides, early-harvested oils had higher positive sensory characteristics than middle-harvested oils.

As for storage conditions, the decrease rate was higher in olive oils stored at room temperature than those stored at 12 °C. In cold stored samples, the median of the fruity in enriched oils decreased up to 3.3–4.2 in early harvested oils from 5.6 up to 2.8–3.6 in middle harvested oils from 3.8, while this value descended to 2.8 and 2.5 in control sample from early and middle harvested oils, respectively, after 12 months storage. In stored samples at room temperature, the fruity attributes in enriched samples with olive leaves were reduced from 2.2–2.8 in early harvested oils to 1.8–2.4 in middle harvested oils, while the control sample was decreased to 2 and 1.6 in the control sample from early and middle harvested oils, respectively, after 12 months storage. The other positive attributes, such as pungency and bitterness, were observed to have a similar trend to the fruity attribute. After the cold storage of 12 months, the control sample had a lower median value for pungency and bitterness attributes (2.0, 2.3 and 2.6–2.2, respectively) compared with enrichment oils (2.3–3.4 and 2.5–4.0, respectively). During storage, the median values in olive oils decreased dramatically compared with those stored at cold temperatures. The median value for pungency and bitterness in control samples obtained from early and middle harvested oils were 1.6, 1.2 and 1.9, 1.5, respectively, while the median value of these attributes in early and middle harvested oils decreased to 1.8–2.5, 1.4–2.2 and 2.0–2.8, 1.9–2.5, respectively.

thumbnail Fig. 12

Sensory properties of olive oil extracted from early harvested time and kept at two different storage conditions for 12 months. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C; the numbers between 0 and 6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
thumbnail Fig. 13

Sensory properties of olive oil extracted from the middle harvested time and kept at two different storage conditions for 12 months. BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C; the numbers between 0 and 6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.

4 Conclusion

In the extraction of Ayvalik olive oils, adding olive leaves caused changes in the chemical composition of the oils, significantly increasing phenolic compounds, oxidative stability, color compounds, and favorable sensory properties. Adding leaves in middle harvested olive oils extended the induction period more than those harvested early. This is because a more delicate production method is preferred during processing in early-harvested olive oils compared to middle-harvested olive oils. In middle-harvested olive oils, the transfer of antioxidant compounds such as chlorophyll and carotenoid from olive leaves increased compared to early-harvested oils.

During storage, the essential quality parameters such as free fatty acids, peroxide value and absorbance values (K232 and K270) exhibited an increasing trend, while the values for these parameters did not exceed the limit value for virgin olive oil by EU. However, the decreased behavior in phenolic compounds and positive sensory attributes were observed during storage, mainly in the samples with control samples and also with smaller amounts of leaves added. Regarding storage conditions, a significant decrease was noted in simple phenolics in oils stored in the cold compared with dark storage at room temperature. The tendency of simple phenolics in cold storage conditions could be related to the low hydrolytic and oxidative degradation rate of complex phenolics to simple phenolics. Therefore, complex phenolics, which are beneficial for health, remain more in cold stored oils. Besides, positive sensory attributes decreased less during storage in cold stored oils than in room conditions.

Conflicts of interest

The authors declare no competing interests

Author contribution statement

MK, HÇ, İT: conceptualization, methodology, formal analysis, investigation, data curation, writing-original draft preparation, visualization. MFR: writing-review and editing. All authors have read and agreed to the published version of the manuscript.

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Cite this article as: Kıralan M, Çengel H, Toptancı İ, Ramadan MF. 2024. Effect of olive leaf on physicochemical parameters, antioxidant potential and phenolics of Ayvalik olive oils at two maturity stages OCL 31: 15.

All Tables

Table 1

Composition of Ayvalik olive oils with/without olive leaves at two different harvest times.

In the text
Table 2

Induction period values (h) of Ayvalik olive oils with/without olive leaves at two different harvest times.

In the text
Table 3

Phenolic compound compositions of the studied enriched olive oils.

In the text

All Figures

thumbnail Fig. 1

The FFA content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0 and 6 next to the letters (AK, AD, BK, and BD) are the number of leaves added.
In the text
thumbnail Fig. 2

Peroxide value of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 3

K232 value of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early harvested olive oil at room temperature in the dark, AD: Early harvested olive oil at 12 °C, BK: Middle harvested olive oil at room temperature in the dark, BD: Middle harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 4

K270 value of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early harvested olive oil at room temperature in the dark, AD: Early harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 5

The chlorophyll content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 6

Carotenoid content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 7

Hydroxytyrosol content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early harvested olive oil at room temperature in the dark, AD: Early harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 8

Tyrosol content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early harvested olive oil at room temperature in the dark, AD: Early harvested olive oil at 12 °C, BK: Middle harvested olive oil at room temperature in the dark, BD: Middle harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 9

Oleuropein content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early harvested olive oil at room temperature in the dark, AD: Early harvested olive oil at 12 °C, BK: Middle harvested olive oil at room temperature in the dark, BD: Middle harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 10

O-Diphenol content of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 11

The radical scavenging capacity of olive oil extracted from two harvested times and kept at two different storage conditions for 12 months; means followed by the same letter, lowercase letters (a-c) indicate the difference for each sample during storage, and uppercase letters (A-C) indicate the difference between samples at the same storage time, do not differ by ANOVA analysis at 5% of error probability. Error bars represent standard deviation. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C, BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C. The numbers between 0-6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 12

Sensory properties of olive oil extracted from early harvested time and kept at two different storage conditions for 12 months. AK: Early-harvested olive oil at room temperature in the dark, AD: Early-harvested olive oil at 12 °C; the numbers between 0 and 6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text
thumbnail Fig. 13

Sensory properties of olive oil extracted from the middle harvested time and kept at two different storage conditions for 12 months. BK: Middle-harvested olive oil at room temperature in the dark, BD: Middle-harvested olive oil at 12 °C; the numbers between 0 and 6 next to the letters (AK, AD, BK and BD) are the amount of leaves added.
In the text

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