Sea Fennel-Flavored Vegetable Oils: Chemistry and Stability During Storage

Abstract

Traditionally, various herbs and spices are used to enhance the flavor and aroma of vegetable oils, but also to improve their nutritional value and stability. The aim of this work was to investigate the effects of sea fennel, an aromatic edible Mediterranean halophyte plant, leaf infusion, on the chemical composition of four unrefined edible vegetable oils (olive, sunflower, sesame and flaxseed oil). During the 90-day storage period, the quality parameters of the oils (peroxide value, free fatty acids and fatty acid profile), as well as their volatiles, were monitored. Free fatty acids and peroxide values increased in all samples, with the greatest increase in the olive oil (11% and 45%, respectively), while the effect on the fatty acid profile was negligible. Gas chromatography-mass spectrometry analysis showed the effect of oil aromatization by sea fennel components and confirmed the differences between oil samples. The results suggest that the addition of sea fennel to vegetable oils leads to changes in their chemical composition, and the parameters tested varied between the oils used.

1. Introduction

Edible vegetable oils are complex food matrices primarily composed of fatty acid glycerides derived exclusively from vegetable sources. In addition, they may contain small amounts of other lipids, such as phospholipids, unsaponifiable constituents and free fatty acids. These oils are categorized by their production methods into virgin, cold-pressed, and refined varieties. While virgin (produced only by mechanical processes and by the application of heat) and cold-pressed (produced by mechanical processes) oils can only be purified by washing with water, settling, filtering and centrifuging [1,2], refined oils undergo extensive processing to remove undesirable compounds, including phospholipids, pigments, trace metals, and oxidation products [2,3].

Beyond their role as a dense energy source, vegetable oils are essential dietary carriers of omega-3 and omega-6 fatty acids, which play a very important role in the human diet. However, the commercial and culinary value of unrefined oils is largely driven by their sensory profiles, including their flavor (taste and aroma) [3,4,5]. The characteristic aroma of these oils is defined by the profile of volatile compounds present in very low concentrations. So far, hundreds of volatile compounds have been identified in vegetable oils, but only some of them, so-called key odorants, directly affect the overall odor quality and characteristics of oils [5,6]. The aromatic compound profile of vegetable oils is influenced by various factors, the most important being the characteristics of the raw material (e.g., variety and origin) [5,6,7,8] and raw material pretreatment (e.g., roasting and isolation conditions) [8,9,10]. Vegetable oils are also an important source of numerous nutritionally valuable and bioactive compounds, which, besides contributing to their overall quality, significantly affect their oxidative stability and shelf life. The most important among these are natural antioxidants, particularly tocopherols and phenolic compounds, as well as unsaturated fatty acids [2,11]. Traditionally, various herbs and spices are used to improve the taste and aroma of vegetable oils. Moreover, some herbs can enrich the oils with oil-soluble biologically active compounds, improving their nutritional value, biological activity, and oxidative stability [12,13]. While the chemical composition of the plants is widely studied, studies on the conversion of the plant metabolites into oils during infusion are scarce. Infusion of oils with herbs and spices can be done through fresh plants, but this type of infusion process is limited by the seasonality of the selected plants. Therefore, the possible solutions are the use of dried plant materials as well as the essential oils extracted from fresh materials [12,14].

Sea fennel is a perennial, edible halophyte from the Apiaceae (Umbelliferae) family found in large groups along the Mediterranean, Black Sea and European Atlantic coasts [15]. The plant is extremely rich in valuable phytochemicals such as carotenoids, phenolics, vitamin C, ω-3 and ω-6 fatty acids, iodine, organic acids and others, but it is also very aromatic due to its high content of essential oil with pleasant aromatic notes [16,17,18].

Although several studies investigated the effect of the addition of spices to oils [13,19,20], only a few have tested the effect and changes during a storage period [21,22,23,24]. Furthermore, these studies investigated the addition of different plant species during different oil processing steps or by employing different techniques (e.g., ultrasound, microwaves), used different vegetable oils and evaluated different final parameters [13,19,20,21,25,26], which makes result comparison additionally difficult. Consequently, the aim of the present work was to investigate the impact of sea fennel infusion via natural maceration on the aroma and chemical stability of four distinct unrefined oils: olive, sunflower, sesame, and flaxseed. Over a 90-day storage period, oils’ FFAs, peroxide values and volatile aroma components were monitored to evaluate the efficacy of these infused products.

2. Materials and Methods

2.1. Plant Material

Sea fennel (Crithmum maritimum L.) was harvested in Split, Croatia (geographic latitude 43°30′2.62” N, geographic longitude 16°29′21.63″ E) in August 2022 (plant flowering stage) [16]. Woody parts, dried leaves and other debris were removed from fresh plant material, after which the leaves and stems were cleaned with tap water to remove dirt. Clean plant material was frozen and then freeze-dried (FreeZone 2.5 L, −50 °C, Labconco, Kanzas City, MO, USA). Part of the dried plant material was ground using a commercial grinder (Model 980, Moulinex, Ecully, France) and sieved through a 1 mm diameter screen.

2.2. Vegetable Oils and Infusion Preparation

Four types of edible vegetable oils were purchased from Bio&Bio store (Split, Croatia): olive oil (EKOZONA, Zagreb, Croatia), flaxseed oil (EKOZONA, Zagreb, Croatia), sesame oil (EKOPLAZA, Veghel, The Netherlands), and sunflower oil (EKOZONA, Zagreb, Croatia). Olive oil was classified as extra virgin, while the other three were cold-pressed oils. The oils were infused with either whole or ground dried plant material; 1 g per 100 mL of oil (1%, w/v) [23]. The infused oils were stored at room temperature (20–23 °C) in dark bottles and protected from direct sunlight for 90 days. The samples were prepared in three independent replicates, and each replicate was stored in a separate sealed bottle that was opened only at the predetermined sampling intervals for analysis. Before analysis, plant material was removed from oils by centrifugation.

2.3. Free Fatty Acids (FFAs) and Peroxide Value (PV)

FFAs and PVs in samples were determined by methods described by the International Olive Council (COI/T.20/Doc No 34/Rev 1., COI/T.20/Doc No 35/Rev 1.) [27,28].

For FFAs, oils (5 g) were weighed and dissolved in ethanol:diethyl ether (25 mL, 1:1, v:v) and then titrated with 0.1 M sodium hydroxide (0.1 M) using phenolphthalein as end-point indicator. FFAs are expressed as % oleic acid.

For PVs, oils (3 g) were dissolved in glacial acetic acid:chloroform (50 mL, 3:2, v:v), and after adding saturated potassium iodide solution (1 mL) and shaking, the mixture was diluted with water (100 mL) and titrated with sodium thiosulfate (0.01 M) using starch as an indicator. PVs are expressed as milliequivalents of active oxygen (meq O₂)/kg.

2.4. Volatile Aroma Components (VOCs)

The VOCs from oil samples were isolated using headspace solid-phase microextraction (HS-SPME) and detected via gas chromatography-mass spectrometry (GC-MS) using a divinylbenzene/carboxene/polydimethylsiloxane (DVB/CAR/PDMS) fiber (Agilent Technologies, Palo Alto, CA, USA).

Oil sample (1 g), placed in a glass vial and sealed, was equilibrated at 40 °C for 15 min. Extraction duration was 45 min, followed by thermal desorption of compounds at 250 °C for 6 min in the GC–MS inlet. The analysis was performed as previously reported [16]. Compounds were identified by comparing retention indices with n-hydrocarbon standards, matching mass spectra with commercial databases (Wiley 7 MS library, Wiley, NY, USA; NIST02, Gaithersburg, MD, USA), and comparing with published literature data [29].

2.5. Fatty Acids Analysis

The analysis of the fatty acid methyl esters (FAMEs) prepared by methylation was carried out using a Varian 3900 gas chromatograph (Varian Inc., Lake Forest, CA, USA). using an RTX 2330 column (30 m, 0.25 mm, 0.2 m; Restek, Bellefonte, PA, USA). Oil samples (0.1 g) were dissolved in heptane (2 mL) and 2 M methanolic KOH (0.2 mL). The mixture was shaken, and after a few min the upper FAME-containing layer was decanted. The sample (1 μL) was injected into an injector (type 1177) at a temperature of 250 °C, and the split ratio was set to 1:40. Helium (2.0 mL/min) was used as the carrier gas.

Separation was achieved as previously reported [16]. The individual FAMEs were identified by comparison with the retention times of a standard (Supelco 37 FAME Mix, Sigma-Aldrich, St. Louis, MO, USA). Only compounds with detectable and integrable peaks, identified through comparison with the standard, were reported numerically, while N.D. (not detected) indicates that the compound was not detected under the applied conditions. The results are expressed as % of each FAME.

2.6. Statistical Analysis

All measurements were carried out in technical replicates (n = 3), and the results are expressed as the mean value ± standard deviation. All collected data were subjected to one-way analysis of variance (ANOVA), and for comparisons among data, the Tukey–Kramer honest significant difference (HSD) test at a significance level of p < 0.05 was used (JMP software, v. 11.0.0, SAS Institute Inc., Cary, NC, USA).

3. Results and Discussion

3.1. FFAs and PVs

FFA content is a primary indicator of oil hydrolytic degradation due to hydrolysis of the glyceride moiety [22]. The results of FFAs determinations, expressed as percentages of oleic acid and PVs, are shown in

Table 1. Changes in free fatty acids (expressed as % of oleic acid) and peroxide number (expressed as meq O₂/kg) in infused vegetable oils prepared using whole (W) and ground (G) sea fennel samples during 90 days of storage.

	Olive Oil		Flaxseed Oil		Sesame Oil		Sunflower Oil
	Free fatty acids (% oleic acid, w/w)
	W	G	W	G	W	G	W	G
Day 0 (control)	0.63 ± 0.03 c,A	0.63 ± 0.03 b,A	0.78 ± 0.03 b,A	0.78 ± 0.03 a,A	2.07 ± 0.03 d,A	2.07 ± 0.03 b,A	2.36 ± 0.00 c,*	2.36 ± 0.00 d,*
Day 15	0.65 ± 0.00 bc,B	0.69 ± 0.00 a,A	0.78 ± 0.02 b,A	0.78 ± 0.03 a,A	2.12 ± 0.02 c,B	2.22 ± 0.02 a,A	2.46 ± 0.00 b,A	2.40 ± 0.01 c,B
Day 45	0.68 ± 0.02 ab,A	0.70 ± 0.00 a,A	0.80 ± 0.00 b,A	0.78 ± 0.02 a,A	2.19 ± 0.00 b,A	2.23 ± 0.02 a,A	2.50 ± 0.01 a,A	2.45 ± 0.01 b,B
Day 90	0.70 ± 0.00 a,A	0.70 ± 0.00 a,A	0.85 ± 0.01 a,A	0.80 ± 0.00 a,B	2.26 ± 0.01 a,A	2.23 ± 0.02 a,A	2.53 ± 0.03 a,A	2.50 ± 0.00 a,A
	Peroxide number (meq O2/kg)
	W	G	W	G	W	G	W	G
Day 0 (control)	4.82 ± 0.02 d,A	4.82 ± 0.02 d,A	1.48 ± 0.01 d,A	1.48 ± 0.01 d,A	0.73 ± 0.00 d,A	0.73 ± 0.00 d,A	8.79 ± 0.04 d,A	8.79 ± 0.04 d,A
Day 15	5.49 ± 0.02 c,A	5.48 ± 0.05 c,A	2.07 ± 0.01 c,A	1.85 ± 0.03 c,B	1.45 ± 0.05 c,A	1.45 ± 0.03 c,A	11.12 ± 0.02 c,A	10.37 ± 0.01 c,B
Day 45	6.74 ± 0.01 b,B	7.19 ± 0.04 b,A	2.24 ± 0.00 b,B	3.26 ± 0.05 b,A	2.79 ± 0.04 b,A	2.43 ± 0.04 b,B	13.64 ± 0.02 b,B	17.78 ± 0.10 b,A
Day 90	9.25 ± 0.00 a,*	7.45 ± 0.00 a,*	4.61 ± 0.01 a,A	4.28 ± 0.03 a,B	4.52 ± 0.03 a,A	3.48 ± 0.01 a,B	23.55 ± 1.73 a,A	22.40 ± 0.04 a,A

Values are expressed as means ± standard deviations of oil samples (n = 3 technical replicates). Superscripts indicate statistical significance according to ANOVA and Tukey’s post hoc test. Statistical evaluations were performed independently for each parameter (free fatty acids and peroxide value). Different lowercase superscript letters indicate statistically significant differences for each column and oil type over storage time (p < 0.05). Different uppercase superscript letters indicate statistically significant differences for each row and oil type at the same sampling time, between W and G sea fennel samples (p < 0.05). (*) indicates values where statistical analysis was not performed due to zero variance among replicates.

. The concentrations of FFAs showed a significant increase in most oil samples after 90 days. Throughout the study, the infused sunflower oil with either ground or whole sea fennel maintained significantly higher FFA levels than all other samples. This contrasts with the findings of Rabiej-Kozioł et al. [30], who reported lower FFA levels for sunflower oil compared to flaxseed oil; in our study, the sunflower matrix exhibited the highest degree of hydrolytic acidification. Moreover, both forms of olive oil with either whole or ground sea fennel remained the most hydrolytically stable overall, showing significantly lower FFA levels than the sunflower, sesame, and flaxseed samples. The upper limit of FFAs for the “extra virgin” category is ≤0.8%, as prescribed by the European Community and the International Olive Oil Council (IOOC). Thus, our results confirm that infusion with both whole and ground sea fennel remained under this threshold, reaching a maximum of 0.7% at day 90. Arslan and Acar [19] reported that the addition of spice extracts (ginger and turmeric) can lower FFA levels compared to the control by inhibiting the lipases responsible for triglyceride hydrolysis. While other studies, such as Assami et al. [20], reported higher FFAs in oils aromatized with caraway seeds, our findings indicate that for olive, sesame, and sunflower oils, there was no significant difference between the whole and ground sea fennel forms in oil samples by the end of the study.

Therefore, this indicates that when compared across all oil matrices, the inclusion of sea fennel did not significantly affect the accumulation of FFAs, which were rather dominated by the base oil’s initial quality and fatty acid composition. This observation is consistent with Adal et al. [23], who suggested that FFA increases in herb-supplemented sunflower oils are often attributed to residual water in the plant material or the action of endogenous hydrolytic enzymes rather than the aromatization process itself.

Regarding oxidative stability, the PVs revealed a more complex interaction between the oil type and the infusion method. All oil samples exhibited a significant increase in primary oxidation products over time; however, the overall results highlight sunflower oil as being markedly more susceptible to oxidation, maintaining significantly higher PVs than all other oils and forms from day 15 through day 90. According to the Codex Alimentarius, the PV limit for cold-pressed and virgin oils is 15 meq O₂/kg, whereby the PV results for sunflower oil indicate significant primary oxidative degradation, particularly on days 45 and 90 of the study. This degradation may directly contribute to rancidity, negatively affect the oil’s sensory quality and acceptability, and potentially reduce its safety for consumption. Therefore, it is essential to conduct kinetic studies of this type in order to determine the optimal or safe duration of infusion/maceration that does not adversely affect product acceptability and, in particular, consumer safety. This time-dependent increase is consistent with Taleb et al. [21], who investigated the aromatization of olive oil by laurel and rosemary and noted that storage conditions, including light exposure, darkness and duration, significantly influence the stability of aromatized olive oils.

Interestingly, the effect of the infusion method varied by oil matrix and time. In more detail, the sunflower oil with grounded sea fennel reached a significantly higher oxidative state than the whole infusion at day 45, though this difference was no longer significant by day 90, as both reached the highest levels of PVs. However, the ground infusion in olive oil was significantly more stable than the whole infusion at day 90. Similar matrix-dependent effects have been reported by Odeh et al. [22] regarding flaxseed oil with the addition of rosemary, oregano, basil, fennel, and chili, as well as by Adal et al. [23], who found that while rosemary provided significant protection in sunflower oil, other herbs like lavender reached values as high as 68 meq O₂/kg. The low PVs and FFAs observed in our flaxseed oil infusions are also in agreement with Krajewska and Kachel [31], who reported similar stability in flaxseed oil and noted the effectiveness of natural additives, such as garlic and chili pepper, in preventing oil degradation during a 6-week monitoring period. Furthermore, at the end of the 90-day period, both flaxseed and sesame oils with whole and ground infusions exhibited no significant difference. These findings confirm that while sea fennel acts as a functional antioxidant [15,32,33], its efficacy is highly matrix-dependent; using ground material provides a statistically significant advantage in preserving olive oil, whereas flaxseed and sesame oils appear to maintain high stability at the end of storage regardless of the sea fennel’s physical state. As suggested by Sousa et al. [34], the variability between such results confirms that the success of herb enrichment depends not only on the plant material but also on the specific lipid medium, a conclusion strongly supported by the results of the present study.

3.2. Fatty Acids Profiles

The FFA profiles indicate the quality of vegetable oils and are used for the identification of their authenticity [30]. This parameter also provides additional insight regarding oxidative changes in oils, FAs degradation or preservation. According to the results presented in

Table 2. Fatty acids profiles (%, w/w) of plant-based oils before (control) and after 90 days (flavored) infusion with sea fennel powder.

	Flaxseed Oil		Olive Oil		Sesame Oil		Sunflower Oil
Fatty Acid	Control	Flavored	Control	Flavored	Control	Flavored	Control	Flavored
16:0	5.56 ± 0.05 a	5.38 ± 0.33 a	11.02 ± 0.49 a	11.20 ± 0.47 a	8.01 ± 0.25 a	8.10 ± 0.14 a	6.27 ± 0.06 b	6.38 ± 0.01 a
16:1ω9	0.03 ± 0.00 a	0.02 ± 0.02 a	0.13 ± 0.01 a	0.13 ± 0.01 a	0.03 ± 0.00 a	0.03 ± 0.00 a	0.02 ± 0.00 b	0.11 ± 0.00 a
16:1ω7	0.07 ± 0.00 a	0.07 ± 0.01 a	0.71 ± 0.04 a	0.72 ± 0.04 a	0.11 ± 0.00 a	0.11 ± 0.00 a	0.10 ± 0.00 a	N.D.
17:0	0.05 ± 0.00 a	0.05 ± 0.00 a	0.02 ± 0.03 a	0.06 ± 0.00 a	0.04 ± 0.00 a	0.04 ± 0.00 a	0.03 ± 0.00 a	0.03 ± 0.00 a
17:1	0.03 ± 0.00 a	0.03 ± 0.00 a	0.13 ± 0.05 a	0.08 ± 0.01 a	0.02 ± 0.00 a	N.D.	0.02 ± 0.01 a	0.03 ± 0.00 a
18:0	3.91 ± 0.02 a	3.95 ± 0.06 a	3.23 ± 0.04 a	3.23 ± 0.06 a	5.55 ± 0.06 a	5.56 ± 0.03 a	3.36 ± 0.02 a	3.37 ± 0.02 a
18:1	19.25 ± 0.04 a	19.29 ± 0.06 a	75.51 ± 0.55 a	75.47 ± 0.36 a	40.71 ± 0.14 a	40.71 ± 0.09 a	31.54 ± 0.04 a	31.60 ± 0.07 a
18:2	15.64 ± 0.01 b	15.67 ± 0.01 a	7.87 ± 0.07 a	7.72 ± 0.01 b	44.38 ± 0.02 a	44.33 ± 0.01 b	57.35 ± 0.04 a	57.13 ± 0.18 a
20:0	0.02 ± 0.00 a	0.02 ± 0.00 a	0.01 ± 0.00 a	0.02 ± 0.01 a	0.01 ± 0.00 a	N.D.	0.01 ± 0.00 a	0.01 ± 0.00 a
18:3	55.24 ± 0.08 a	55.32 ± 0.20 a	1.21 ± 0.03 a	1.21 ± 0.05 a	0.93 ± 0.02 a	0.93 ± 0.02 a	0.41 ± 0.01 b	0.46 ± 0.01 a
20:1	0.02 ± 0.00 a	0.03 ± 0.01 a	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
22:0	0.10 ± 0.00 a	0.10 ± 0.01 a	0.10 ± 0.01 a	0.12 ± 0.02 a	0.12 ± 0.01 a	0.12 ± 0.00 a	0.68 ± 0.04 a	0.67 ± 0.05 a
24:0	0.08 ± 0.01 a	0.07 ± 0.01 a	0.05 ± 0.01 a	0.04 ± 0.03 a	0.08 ± 0.01 a	0.08 ± 0.01 a	0.21 ± 0.02 a	0.21 ± 0.02 a

N.D. = not determined, no clear chromatographic peak could be observed or identified under the applied analytical conditions. Values are expressed as mean ± standard deviation (n = 3). Statistical evaluations were performed independently for each oil type (flaxseed, olive, sesame, and sunflower oil). Different lowercase superscript letters within the same row and under the same oil type indicate a statistically significant difference between the control and flavored oil samples (p < 0.05). No statistical analysis was performed for compounds designated as N.D. due to the absence of variance.

, all studied oils have typical fatty acid compositions recommended by the Codex Alimentarius standard [1] and described in previous studies [23,31,35,36]. Statistical analysis revealed that the addition of sea fennel for flavoring did not significantly affect the primary FA structures, with the vast majority of compounds showing no significant differences (p > 0.05) between the control and flavored conditions within each oil type. For instance, the main monounsaturated fatty acid in olive oil (C18:1) remained statistically identical between the control and flavored samples for all oil types, while the dominant polyunsaturated fatty acid in flaxseed oil (C18:3) similarly showed no significant modification except for sunflower oil, where the flavored oil exhibited a significantly higher value than the control oil. Moreover, a significant reduction in C18:2 content occurred after flavoring both olive oil (7.87% to 7.72%) and sesame oil (44.38% to 44.33%). In sunflower oil, minor structural shifts were restricted to the low-abundance components, where flavoring induced a significant increase in C16:0, C16:1ω9, and C18:3 (p < 0.05).

Despite these minor fluctuations, the overall results for the sunflower oil FA profile remain consistent with those of Adal et al. [23], who reported high C18:2 (48–63%) and C18:1 (28–33%) contents and minimal saturated fat content (C16:0 and C18:0), confirming that the lipid stability of the oils was successfully preserved during processing.

It should be noted again that vegetable oils are an important source of various valuable bioactive compounds (e.g., fatty acids, phenolics, and tocopherols), which affect oil oxidative stability [11], and their presence and amount in the tested oils are crucial to interpret their effects on changes in these parameters in control and flavored oils.

3.3. Oil Aromatization

Our previous studies on sea fennel essential oils (EOs) from Croatian samples collected in Central Dalmatia confirmed that the most abundant constituents are limonene, sabinene and γ-terpinene [16,37,38], which categorized them into the monoterpene hydrocarbon EO chemotype [39]. The EO of the used plant material was investigated in our previous study [16] and confirmed a strong dominance of limonene (79%) in the leaf sample. Therefore, this compound was the main one for the assessment of aromatization.

Table 3. Volatile organic compounds (VOCs) of sea fennel-flavored plant-based oils (%).

				Olive Oil		Flaxseed Oil		Sesame Oil		Sunflower Oil
RT (min)	Compound	RI	RI (Library)	Control	Flavored	Control	Flavored	Control	Flavored	Control	Flavored
2.022	Pent-1-en-3-ol	<800	673	N.D.	N.D.	6.72 ± 0.57	N.D.	N.D.	N.D.	N.D.	N.D.
2.288	3-Methylbutan-1-ol	<800	737	2.58 ± 0.42	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
2.510	Pentan-1-ol	<800	779	N.D.	N.D.	11.80 ± 0.65 b	13.85 ± 0.69 a	N.D.	N.D.	N.D.	N.D.
2.521	(Z)-Pent-2-en-1-ol	<800	769	2.71 ± 0.39	3.20 ± 0.03	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
2.821	Hexanal	817	811	8.65 ± 3.15 a	10.31 ± 0.85 a	16.52 ± 2.58 a	17.77 ± 1.27 a	30.00 ± 3.12 a	30.39 ± 0.13 a	3.76 ± 0.08	4.30 ± 0.59
3.145	1-Methoxyhexane	845	832	N.D.	3.53 ± 0.28	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
3.479	(E)-Hex-2-en-1-al	868	857	25.92 ± 1.57 a	23.96 ± 1.15 a	2.53 ± 0.15	2.07 ± 0.11	N.D.	N.D.	N.D.	N.D.
3.650	Hexan-1-ol	879	874	11.73 ± 0.11 a	9.50 ± 0.21 b	40.02 ± 0.17 a	26.67 ± 0.94 b	26.27 ± 1.46 a	16.47 ± 0.22 b	0.83 ± 0.02	0.56 ± 0.05
4.236	Heptanal	910	901	0.59 ± 0.02	0.98 ± 0.04	N.D.	1.37 ± 0.32	0.78 ± 0.11	2.90 ± 1.09	0.40 ± 0.16	0.18 ± 0.01
4.494	Butyrolactone	924	920	N.D.	N.D.	N.D.	N.D.	3.13 ± 0.15	1.85 ± 0.40	0.68 ± 0.07	0.50 ± 0.00
4.810	α-Thujene	940	930	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	0.68 ± 0.02	0.69 ± 0.04
4.977	α-Pinene	947	939	N.D.	1.71 ± 0.12	2.40 ± 0.00	2.17 ± 0.34	3.53 ± 0.15	3.65 ± 0.19	50.26 ± 1.82 b	56.99 ± 1.80 a
5.076	6-Methylheptan-2-one	952	957	N.D.	0.84 ± 0.00	1.45 ± 0.08	3.80 ± 0.19	N.D.	N.D.	N.D.	N.D.
5.164	(5Z)-3-ethylocta-1,5-diene	955	958	2.24 ± 1.67	1.43 ± 0.01	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
5.338	Camphene	962	961	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	1.18 ± 0.00	1.28 ± 0.04
5.422	(E)-Hept-2-enal	965	956	2.12 ± 0.18	1.85 ± 0.12	1.11 ± 0.03	1.82 ± 0.00	1.64 ± 0.31	4.89 ± 0.33	N.D.	N.D.
5.452	Thuja-2,4(10)-diene	967	958	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	3.42 ± 0.09	3.90 ± 0.39
5.574	Benzaldehyde	971	961	N.D.	0.56 ± 0.02	0.96 ± 0.19	0.77 ± 0.11	1.12 ± 0.09	1.86 ± 0.13	N.D.	N.D.
5.700	Heptane-1-ol	976	975	N.D.	N.D.	1.26 ± 0.03	0.90 ± 0.07	N.D.	N.D.	N.D.	N.D.
5.909	Sabinene	983	977	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	5.65 ± 0.02	5.35 ± 0.15
5.958	Oct-1-en-3-ol	985	986	N.D.	N.D.	1.05 ± 0.04	0.83 ± 0.00	N.D.	N.D.	N.D.	N.D.
6.011	β-Pinene	987	987	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	2.48 ± 0.06	2.70 ± 0.05
6.183	6-Methylhept-5-en-2-one	992	987	N.D.	0.34 ± 0.04	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
6.319	2-Pentylfuran	997	996	0.33 ± 0.03	0.34 ± 0.01	1.55 ± 0.13	1.06 ± 0.09	7.89 ± 0.38 a	3.02 ± 0.07 b	0.49 ± 0.02	0.30 ± 0.01
6.460	(2E,4Z)-hepta-2,4-dienal	1001	999	N.D.	5.53 ± 0.05	N.D.	4.07 ± 0.04	N.D.	N.D.	0.96 ± 0.08	0.81 ± 0.12
6.627	Octanal	1008	1005	N.D.	0.70 ± 0.04	0.95 ± 0.11	1.10 ± 0.06	0.76 ± 1.08	2.09 ± 0.20	N.D.	N.D.
6.734	[(Z)-hex-3-enyl] acetate	1012	1006	16.97 ± 1.10 a	12.95 ± 0.54 b	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
6.916	Hexyl acetate	1019	1011	2.90 ± 0.06	2.28 ± 0.22	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
6.874	(E,E)-Hepta-2,4-dienal	1017	1012	N.D.	N.D.	N.D.	1.70 ± 0.07	N.D.	N.D.	N.D.	N.D.
6.912	δ-3-Carene	1018	1018	N.D.	N.D.	1.11 ± 0.05	N.D.	3.16 ± 0.02	2.42 ± 0.04	N.D.	N.D.
7.091	α-Terpinene	1025	1020	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	0.15 ± 0.01	0.19 ± 0.00
7.334	p-Cymene	1033	1030	N.D.	N.D.	N.D.	N.D.	3.35 ± 0.01	2.87 ± 0.35	0.98 ± 0.05	0.85 ± 0.04
7.452	Limonene	1037	1035	N.D.	6.16 ± 0.18	0.29 ± 0.02 b	7.02 ± 0.04 a	1.91 ± 0.09 b	14.00 ± 0.10 a	1.91 ± 0.14	2.42 ± 0.20
7.578	Benzyl alcohol	1041	1061	1.44 ± 1.75	0.25 ± 0.03	0.26 ± 0.25	0.55 ± 0.18	1.86 ± 0.24	2.29 ± 0.27	N.D.	N.D.
7.726	(E)-β-Ocimene	1046	1040	N.D.	N.D.	N.D.	N.D.	N.D.	2.06 ± 0.11	N.D.	N.D.
8.038	(Z)-β-Ocimene	1055	1051	1.83 ± 0.21	1.35 ± 0.12	N.D.	N.D.	6.54 ± 0.10 a	5.67 ± 0.52 b	N.D.	N.D.
8.338	(E)-Oct-2-enal	1064	1063	N.D.	0.19 ± 0.01	0.89 ± 0.01	1.08 ± 0.09	N.D.	1.71 ± 0.09	N.D.	N.D.
8.418	γ-Terpinene	1066	1064	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	0.69 ± 0.26	0.99 ± 0.07
8.775	(E,E)-Octa-3,5-dien-2-one	1076	1083	N.D.	0.28 ± 0.04	2.84 ± 0.55 b	7.41 ± 1.04 a	N.D.	N.D.	N.D.	N.D.
9.494	p-Cymenene	1094	1095	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	0.81 ± 0.08	0.72 ± 0.07
9.574	(E,Z)-Octa-3,5-dien-2-one	1096	1098	N.D.	N.D.	N.D.	1.08 ± 0.11	N.D.	N.D.	N.D.	N.D.
9.794	Undecane	1102	1100	N.D.	N.D.	0.80 ± 0.02	0.63 ± 0.00	N.D.	N.D.	N.D.	N.D.
9.810	Linalool	1102	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	1.04 ± 0.06	0.85 ± 0.08
9.965	Nonanal	1107	1102	2.47 ± 0.58	1.53 ± 0.21	1.98 ± 0.16	0.76 ± 0.01	1.50 ± 0.14	1.00 ± 0.02	0.28 ± 0.04	N.D.
10.095	p-Mentha-1,3,8-triene	1111	1119	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	2.88 ± 0.18	1.99 ± 0.23
10.433	(3E)-4,8-Dimethyl-1,3,7-nonatriene	1121	1117	1.55 ± 0.50	0.85 ± 0.20	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
10.817	α-Campholenal	1132	1130	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	2.69 ± 0.23	2.25 ± 0.24
11.308	(E)-Pinocarveol	1145	1147	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	2.04 ± 0.22	1.62 ± 0.19
11.524	β-Terpineol	1151	1159	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	3.56 ± 0.27	2.83 ± 0.41
12.148	(E)-Pinocamphone	1166	1160	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	0.38 ± 0.02	0.26 ± 0.03
12.231	Pinocarvone	1168	1164	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	0.67 ± 0.05	0.43 ± 0.07
13.566	Myrtenal	1198	1196	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	0.67 ± 0.02	0.46 ± 0.04
13.676	Dodecane	1201	1200	N.D.	N.D.	N.D.	0.64 ± 0.14	N.D.	0.86 ± 0.38	N.D.	N.D.
13.943	Decanal	1208	1200	13.19 ± 0.67	8.66 ± 1.33	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
14.087	Verbenone	1212	1204	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	3.23 ± 0.33	2.67 ± 0.41
14.471	(E)-Carveol	1222	1222	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	0.31 ± 0.04	0.26 ± 0.03
17.243	Bornyl acetate	1288	1286	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	1.32 ± 0.13	1.10 ± 0.15
17.801	Tridecane	1300	1300	N.D.	N.D.	0.45 ± 0.04	0.44 ± 0.07	N.D.	N.D.	N.D.	N.D.
20.942	α-Copaene	1378	1379	0.57 ± 0.12	0.41 ± 0.02	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
21.946	Tetradecane	1400	1400	N.D.	N.D.	N.D.	0.46 ± 0.03	N.D.	N.D.	N.D.	N.D.
23.200	α-Gurjunene	1434	1434	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	2.84 ± 0.24	2.58 ± 0.34
26.307	(E,E)-α-Farnesene	1510	1510	0.50 ± 0.19	0.33 ± 0.02	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.

RI = retention index; RI (Library) = linear retention indices; N.D.—not determined. Statistical comparisons were performed independently within each individual oil type and on the same row to compare control vs. flavored oils, for exclusively major volatile compounds exhibiting a relative share above 5%. Superscripts indicate statistical significance according to ANOVA and Tukey’s post hoc test. Different lowercase superscript letters within the same row and under the same oil type indicate a statistically significant difference (p < 0.05).

presents results for VOCs of control oils used in this study, as well as for flavored oils at the end of the study. According to Kowalski et al. [13], oil is a good carrier for VOCs, and it slows down their release into the atmosphere, binding mainly to components with good affinity to the lipophilic medium, such as oil. As previously reported, the transfer of VOCs to the oils depends on the type of aroma profile of the used spice [25,40].

Flaxseed oil sample contains the highest amount of hexan-1-ol (40.02%), as well as lower concentrations of hexanal (16.52%) and pentan-1-ol (11.80%), while all other constituents were detected in significantly lower amounts. However, in flavored oil, the content of hexan-1-ol significantly decreased (26.67%), while the other two compounds recorded a slight increase in content. Also, limonene content significantly increased from 0.29% in control oil to 7.02% in flavored oil, which confirmed efficient oil aromatization by the addition of sea fennel. According to Wei et al. [6], VOCs are essential for flaxseed oil quality, and their presence varies depending on seed variety, degree of ripeness, growing region, environmental factors, applied processing techniques and storage conditions. The authors investigated three oil samples from different Chinese flax cultivars and reported high amounts on 2,4-penta-dienal which was not detected in this study, but also high content of (E,E)-2,4-heptadienal (sweet, hazelnut and woody aromatic notes), hexanol (herbaceous, woody, green characteristics) and (E,E)-3,5-octadien-2-one (bitter, almond, sour characteristics) which were also dominant constituents of the tested oils. Kiralan et al. [36] investigated changes in flaxseed oil during thermal oxidation and reported that hexanal was the main VOC in fresh oil and that its content increased during storage. The authors also reported an increase in concentration of 2,4-heptadienal, 2-heptadienal and (E,E)-2,4-heptadienal, which were formed upon oil oxidation. The presented results also confirmed an increase of (2E,4Z)-2,4-heptadienal, which was not detected in fresh oil, and its amount in flavored oil was 4.07%, probably due to oxidation reactions that occurred in the oil during storage.

The comparison of the VOCs from olive oils is also very hard and depends on raw material characteristics, among which the main factor is fruit variety, processing parameters and production/storage conditions, among others. However, there are a large number of studies reporting similar observations about main VOCs in olive oil samples [40,41,42,43].

C6 aldehydes were mainly represented by (E)-2-hexenal, which is responsible for green and fruity sensory notes [40]. The content of this compound in the study sample was 25.92% in control oil and 23.96% in flavored oil samples. The second most dominant compound is usually hexanal (4.1%), the content of which in our sample was 8.65% in the control oil, while it increased in the flavored sample to 10.31%. Among esters, the authors reported high content of (Z)-3-hexenyl acetate and 1-hexyl acetate, which is in accordance with this study, where these two compounds were found in amounts of 16.97 and 2.90%, respectively. The presence of these two compounds usually correlates with oil freshness and has a positive effect on consumer preference. As in the case of flaxseed oil, a high content of (2E,4Z)-2,4-heptadienal, which was not detected in fresh oil, in olive oil, was also recorded, as well as limonene (6.16% in flavored oil) as a result of aromatization.

Similar observations can be noted for sesame oil, which was also characterized by the high amount of hexanal (30%) and hexan-1-ol (26.27%). Similar to flaxseed oil, hexan-1-ol content significantly decreased in flavored oil (16.47%), while limonene content significantly increased, reaching 14%. Among other constituents, high amounts of 2-pentylfuran and (Z)-β-ocimene were also detected. Individual aroma compounds of sesame oil were studied by Ivanova-Petropulos et al. [44], and they also reported a domination of alcohols (1-hexanol) and aldehydes (hexanal), while β-ocimene was the dominant terpene. Among furans, the dominant one was 2-pentyl furan, as well as in our study, where its content was 7.89% in the control oil sample. Volatiles in sesame oil produced under various roasting conditions were studied by Zheng et al. [45], and the authors reported a high amount of O-heterocyclic compounds in samples, such as 2-pentyl-furan, which has been detected in this study. They concluded that the content of this component is related to the raw material processing conditions (roasting temperature and time), but also that it has a great impact on the sweet notes of the oil.

Cold-pressed sunflower oil aromatic substances were investigated by Yin et al. [46]. The predominant compounds were α-pinene (piney and woody) and hexanal (green and fruity), which were also detected in samples from this study, especially α-pinen (50.26%). The authors also detected other terpenes which were found in our study, including sabinene (5.65 and 5.35%), β-pinene (2.48 and 2.70%), γ-terpinene (0.69 and 0.99%), linalool (1.04 and 0.85%). This sample showed the lowest effect of aromatization by the sea fennel addition, particularly regarding limonene content, which exhibited the lowest concentration of 2.42% at the end of the study. Sousa et al. [34] investigated the addition of sea fennel to sunflower oil and concluded that it resulted in positive characteristics of the oil, described as a pleasant aroma of the sea fennel. However, these observations were only confirmed by sensory evaluation by trained panelists.

Finally, defining and evaluating the optimal maceration time is highly relevant for industrial application, as it directly influences extraction efficiency and the overall quality of flavored oils. Such a study would require kinetic monitoring of all relevant oxidation parameters (parallel analysis of control and flavored oils). In this context, more pronounced negative changes in the oil chemical composition would enable the determination of the optimal maceration time. Although this aspect was not covered in the present study, its importance is evident and should be considered in future research.

4. Conclusions

The addition of natural herbs and spices represents one of the more recent approaches used to prevent undesirable changes and degradation processes in vegetable oils during storage. The primary objective of this approach is to inhibit oxidative reactions that lead to quality deterioration, adverse organoleptic properties, and a reduction in nutritional value. Furthermore, various bioactive compounds derived from plant materials (such as vitamins, carotenoids, phenolic compounds, and essential oil constituents) may migrate into the lipid medium, where they can act as natural antioxidants, thereby preventing degradation reactions and potentially improving the sensory characteristics of the oil, particularly its aroma. The results of this study indicated the occurrence of oxidative processes in the tested oil samples during storage, but due to the lack of control oils stored under the same conditions without sea fennel addition, these changes may also reflect natural oxidative and hydrolytic processes occurring during storage. Nevertheless, the aromatization effect was favorable, especially in the case of sesame oil. This study only gives insight into VOCs of the oils before and after flavoring, although it is well known that the impact of individual compounds on the overall oil aromatic characteristics and detection of key aroma-active compounds is not based only on the qualitative and quantitative results of VOCs profiles. Finally, these results should be combined with consumers’ sensory acceptance evaluation performed by a trained panel, which would represent an essential step in assessing the real overall acceptability of flavored oils. Since sensory analysis was not included in the present study, the interpretation of the aromatic contribution of sea fennel to oil aroma remains partially limited.