Oxidative Stability Analysis of Selected Oils from Unconventional Raw Materials Using Rancimat Apparatus

Abstract

This study aimed to evaluate the quality of selected oils from the seeds of herbs and vegetables (basil, fenugreek, coriander, tomato, garden cress, parsley, and dill), especially their oxidative stability. The oils were tested for oxidation degree (acid value, peroxide value, p-anisidine value, TOTOX indicator, and specific extinction under ultraviolet light), colours, content of carotenoid and chlorophyll pigments, fatty acid composition, indicators of lipid nutritional quality, oxidative stability, and oxidation kinetics parameters (Rancimat). Principal component analysis was applied to identify a correlation between the oils’ quality parameters. The results of the fatty acid compositions show that basil oil was a good source of omega-3 fatty acids. Coriander seed oil was found to be the most resistant to oxidation, containing mainly monounsaturated fatty acids. The highest value of activation energy was calculated for fenugreek oil (94.18 kJ/mol), and the lowest was for dill seed oil (72.61 kJ/mol). However, basil oil was characterised by the highest constant reaction rate at 120 °C—3.0679 h⁻¹. The colour determined by the L* parameter and the calculated oxidizability value had the most significant influence on the oxidation stability of the oils, and the correlation coefficients were r = −0.88 and 0.87, respectively.

1. Introduction

In recent years, there has been a growing demand for the consumption of minimally treated, enhanced, and functional foods. Such products include cold-pressed oils, which are identified as natural products. Their consumption may have a beneficial effect on health. This is because they provide unsaturated fatty acids, among which a large part are omega-3, omega-6, and omega-9 acids, which positively impact the cardiovascular system. Omega-3 polyunsaturated fatty acid (ω-3 PUFAs) intake improves vascular and cardiac hemodynamics, triglycerides, and possibly endothelial function, as well as the production of novel inflammation-resolving mediators for thrombosis and arrhythmia [1]. In addition, cold-pressed oils contain various bioactive components such as phenolic compounds, tocopherols, sterols, vitamins, and carotenoids, known for their antioxidant activity and their capacity for reducing levels [2].

The main challenge with cold-pressed oils is their susceptibility to oxidative changes, which is mainly related to the composition of fatty acids in vegetable oils, and more precisely, the content of polyunsaturated fatty acids (PUFAs), because these acids undergo oxidation processes faster than monounsaturated fatty acids (MUFAs). Apart from the fatty acid profile and the presence of pro- and antioxidant substances, the effect of external factors is extremely important. Cold-pressed oils are susceptible to change under the influence of light, oxygen, or increased temperature. The negative impact of oxidative changes is a deterioration of the oils’ nutritional value and sensory properties [3]. In addition, these processes are accompanied by secondary product formation, such as aldehydes and ketones, as well as free radicals, which, in turn, can lead to oxidative stress and disorders of the organism and even cancer. The quality of cold-pressed oils is influenced by a number of factors, ranging from the quality of the raw material itself to the pressing and storage conditions. There are more oils obtained from unconventional raw materials, which undoubtedly include oils from seeds of herbs and vegetables, such as oil from cress seeds, coriander seeds, or dill. They are gaining popularity among consumers because of their potential health-promoting qualities. As these oils are not produced on such a large scale as oils from typical raw materials, they are therefore not fully understood. It is important to test them for their overall quality, especially their oxidative stability, which is considered to be the main parameter of an oil’s quality [4]. Therefore, this research aims to explore the oxidative stability of seven cold-pressed, unrefined oils obtained from different unconventional raw materials. The kinetics of the oils’ oxidation parameters, quality parameters, fatty acid composition, and pigment contents were also studied.

2. Materials and Methods

2.1. Research Materials

The research material consisted of seven cold-pressed, unrefined oils, obtained from unconventional raw materials: basil—BO, fenugreek—FO, coriander—CO, tomato—TO, watercress—WCO, parsley—PO, and dill—DO. The tested oils were obtained from the manufacturer of cold-pressed oils. Analyses were performed between November 2020 and January 2021.

2.2. Chemicals

All chemicals and reagents used for the analysis were purchased from Merck Millipore (Darmstadt, Germany) and Chempur Company (Piekary Sląskie, Poland).

All chemicals and reagents used for the analysis and sample preparation for GC were of HPLC/GC purity. The Food Industry 37 Component Fatty Acid Methyl Esters (FAME) mix standard was supplied by Restek (Bellefonte, PA, USA). Distilled water (0.05 µS) was obtained with an HLP Smart 2000 Hydrolab apparatus (Straszyn, Poland).

2.3. Acid, Peroxide, p-Anisidine, and TOTOX Values

The physicochemical quality of oils was determined based on the lipid values. To evaluate the degree of the hydrolytic changes, the acid values (AV) were determined according to the EN ISO 660:2010 standard [5]. The peroxides content, expressed as the peroxide value (PV), was tested according to the EN ISO 3960:2012 standard [6]. The degree of secondary oxidation products was given in p-anisidine values (p-AnV) according to the EN ISO 6885:2008 standard method [7]. Based on the AnV and PV, the overall oil oxidation rates were calculated as TOTOX values (TOTOX = 2PV + p-AnV).

2.4. Specific Ultraviolet Extinction

The determination of ultraviolet absorbance, expressed as the specific UV extinction $E_{1\mathrm{cm}}^{1\%}$ at λ = 232 nm (determination of conjugated dienes) and λ = 268 nm (determination of conjugated trienes) in comparison to the pure solvent, was performed according to EN ISO 3656:2011 [8]. The determination was performed in four repetitions.

2.5. Determination of Carotenoid and Chlorophyll Pigments

The determination of the carotenoid pigments was performed according to BSI 684-2020 [9], and the measurements were made in a Genesys 180 spectrophotometer from ThermoScientific. An oil amount of 0.5 g was diluted in 10 mL of isooctane, and absorbance measurements were performed at a wavelength of λ = 445 nm. Due to the intense colour of fenugreek seed oil, a higher dilution was performed, which consisted of dissolving 0.5 g of oil in 25 mL of isooctane.

The chlorophyll pigment was determined according to AOCS Cc 13i-96 [10]. The determination consisted in measuring the absorbance of oil samples at different wavelengths: λ = 630 nm, λ = 670 nm, and λ = 710 nm, against air.

2.6. Colour Measurement Using the CIE L*a*b* Method

The colour of the oils was measured using the CIE L*a*b* method with a Konica Minolta CR-5 stationary calorimeter. This method measures three colour components: L*, a*, and b*. L* evaluates the brightness of the sample on a scale from 0 to 100, where 0 means black and 100 means white. The a* parameter takes values from −100 to 100, where −100 is green, and 100 is red. The b* parameter takes values from −100 (blue) to 100 (yellow). Measurements were made in a 10 mm-wide plastic cuvette, light source D65, observer 10 °C. The colour of the oils was measured in four replicates.

2.7. Fatty Acid Composition Analysis

The determination of the composition of the fatty acids was carried out using gas chromatography. First, the methylesters of the fatty acids were prepared. A drop of oil was measured into a test tube, followed by 2 mL of 0.5 N methanolic KOH, and shaken. The samples thus prepared were placed in a water bath at 60 °C for 10 min. After cooling, 2 mL hexane and 2 mL distilled water were added. When the mixture had stratified, 1 mL was taken from the upper layer and placed in the vial. A TRACE TM 1300 gas chromatograph (Thermo Scientific, Waltham, MA, USA) equipped with an FID detector and a 60 m-long, 0.22 mm BPX 70 capillary column with a layer thickness of 0.25 µm was used to determine the fatty acid composition. The determination of the esterified fatty acids in the tested oils was performed according to the AOCS Official Method 966.06 [11], with minor modifications. The carrier gas was helium with a flow rate of 40 mL per minute; at a separation of 500:1, the injected volume was 0.8 µL. The oven temperature and analysis time was programmed as follows: 80 °C was maintained for 2 min, then heated at a rate of 2.5 °C per minute to 230 °C and maintained under these conditions for 6 min. The detector temperature was 250 °C.

The individual acids were identified by comparing the retention times of the fatty acid ester mixture of the standard. The determination was performed in duplicate.

2.8. Oxidizability Value—COX

To determine the effect of the fatty acid composition on the oils’ oxidative stability, the oxidizability value (COX) was calculated. The COX index was calculated according to the formula proposed by Fatemi and Hammond [12], applying Equation (1).

$$\mathrm{COX}=\frac{1\times \left(\mathrm{C}16 : 1+\mathrm{C}17:1+\mathrm{C}18:1+\mathrm{C}20:1\right)+10.3\times \left(\mathrm{C}18:2\right)+21.6\times \left(\mathrm{C}18:3+\mathrm{C}20:3\right) }{100}$$

(1)

2.9. Determination of Nutritional Quality Index of Oils

The lipid quality indicators represent the influence of the fatty acid composition of the oils on the risk of developing cardiovascular disease. For this purpose, it is necessary to know the fatty acid profile of the oils. The calculations were performed according to the formulas described by Ulbricht and Southgate [13] and Santos-Silva et al. [14]: the Atherogenicity index (AI) (Equation (2)), the Thrombogenicity index (TI) (Equation (3)), and the ratio of Hypocholesterolemic to hypercholesterolemic FA (HH) (Equation (4)).

$$\mathrm{AI}=\frac{\mathrm{C}12 : 0+4\times \mathrm{C}14 : 0+ \mathrm{C}16 : 0 }{{\displaystyle \sum } \mathrm{MUFA} +{\displaystyle \sum }\left(\mathsf{\omega }3\right)+{\displaystyle \sum }\left(\mathsf{\omega }6\right) }$$

(2)

$$\mathrm{TI}=\frac{\mathrm{C}14 : 0+ \mathrm{C}16 : 0+ \mathrm{C}18 : 0}{0.5\times \mathrm{MUFA} +\left(0.5\times {\displaystyle \sum }\left(\mathsf{\omega }6\right)\right)+\left(3\times {\displaystyle \sum }\left(\mathsf{\omega }3\right)\right)+\left(\mathsf{\omega }3/\mathsf{\omega }6\right)}$$

(3)

$$\mathrm{HH} =\frac{\mathrm{C}18 : 1+ \mathrm{C}18 : 2+ \mathrm{C}18 : 3+ \mathrm{C}20 : 4+ \mathrm{C}20 : 5+\mathrm{C}22:5+\mathrm{C}22:6 }{\mathrm{C}14 : 0+ \mathrm{C}16 : 0}$$

(4)

2.10. Oxidative Stability Determination

The oxidative stability of the tested oils was performed with the 892 Rancimat apparatus from Metrohm, according to the AOCS method Cd 12b-92 [15], using a sample of 2.5 g oil under a constant air flow at temperatures in the range of 90–150 °C (5 different temperatures for each oil). The air flow velocity was 20 L/h.

2.11. Determination of Oxidation Kinetics Parameters

The determination of the oxidation kinetics parameters was performed according to the methodology described by Kowalski et al. [16]. For this purpose, the results obtained from the analysed oils by Rancimat were used to develop graphs of the dependence of the logarithm of the induction time on temperature.

The regression lines were determined according to the following equation:

$$log\tau Rancimat=1/T\ast 1000$$

(5)

where

• τRancimat—induction time determined in the Rancimat,
• T—oxidation temperature [K].

Using the obtained results and the Arrhenius formula:

$$k=\mathrm{Aexp}\left(-\frac{\mathrm{Ea}}{\mathrm{RT}}\right)$$

(6)

The basic parameters of the oil oxidation kinetics were calculated: the activation energy (Ea), the pre-power factor (Z), and the rate constants of the oxidation reaction (k) at the measurement temperatures.

According to the activated complex theory, the enthalpy (ΔH) and entropy (ΔS) were calculated:

$$\ln \left(\frac{k}{T}\right)=\ln \left(\frac{k_b}{h}\right)+\left(\frac{\mathsf{\Delta }S}{R}\right)-\left(\frac{\mathsf{\Delta }H}{RT}\right)$$

(7)

where

• ΔH—enthalpy [kJ/mol],
• ΔS—entropy [J/mol*K],
• h—Planck’s constant [6.63 × 10⁻³⁴J*s],
• R—gas constant [8.314 J/mol*K],
• k_B—Boltzmann constant [1.38 × 10⁻²³ J/K].

2.12. Statistical Analysis

Statistica version 13.3. software (Tibco Software Inc., Palo Alto, CA, USA) was used for statistical analysis of the results. The homogeneous groups were divided using a one-way analysis of variance (ANOVA) and the Tukey HSD test at a significance level of p-value ≤ 0.05. Pearson’s linear correlation analysis was also performed. The relationships between the individual quality parameters and the grouping of the oils in terms of their similarity were determined using the principal component analysis (PCA).

3. Results and Discussion

3.1. Analysed Oils’ Initial Qualities

The first step of the oil analysis was to evaluate the oils’ initial qualities. According to the Codex Alimentarius standard, the permitted value of the acid value (AV), which is used to determine the amount of free fatty acids in cold-pressed oils, should not exceed 4.0 mg KOH/g oil. The AV values of the tested oils ranged from 0.34 (dill seed oil) to 4.61 mg KOH/g oil (coriander seed oil) (

Table 1. Initial quality of analysed cold-pressed oils.

Oil Sample	AV [mg KOH/g]	PV [mEq O2/kg]	p-AnV	TOTOX	Chlorophylls [mg/kg]	Carotenoids [mg Pheophytin a/kg]	K232	K268
CO	4.61 g	9.43 b	12.01 e	30.69 c	23.43 e	23.03 c	4.49 a	1.21 a
FO	2.25 d	9.98 b	3.22 c	23.17 b	12.59 d	153.68 a	3.40 b	0.69 b
BO	1.34 b	3.41 a	2.83 b	9.65 a	2.60 a	5.40 g	0.56 c	0.43 c
PO	1.54 c	11.73 c	6.55 d	30.02 c	93.27 f	20.66 d	17.11 d	2.37 d
WCO	2.64 e	17.38 d	2.05 a	36.82 d	7.52 b	58.16 b	0.80 e	0.47 c
DO	0.34 a	26.10 e	2.83 b	55.09 e	102.20 g	20.26 e	1.17 f	0.67 b
TO	3.25 f	44.91 f	6.82 d	95.92 f	10.74 c	18.18 f	8.06 g	0.77 e

^a,b,c—mean values denoted in columns by the same letters are not statistically significantly different at p-value ≤ 0.05.

).

Ahmed and Alla [17] mentioned a lower value (2.84 mg KOH/g oil) in their study on the coriander seed oil, thus complying with the standard. The elevated AV of oils may be due to the quality of the raw material used. Damaged, too wet, or contaminated raw material determines intensive hydrolytic changes. Pressing conditions are also an important factor affecting the quality of the final product [18,19]. The peroxide values (PV) obtained in the oil analysis ranged from 3.41 to 44.91 mEq O₂/kg. Of the oils tested, only four met the requirements for the permitted content of primary oxidation products in oils (15 mEq O₂/kg): PO, FO, and CO with 11.73, 9.98, and 9.34 mEq O₂/kg, respectively. The highest PV was determined in TO with 44.91 mEq O2/kg. The values recorded by other researchers were much lower than the ones obtained in our study. Lazos et al. [20], studying this parameter in TO, obtained a PV equal to 9.3 mEq O₂/kg, and a lower peroxide value (3.93 mEq O₂/kg) was obtained by Yilma et al. [21]. The discrepancy between the results of these researchers may be due to the fact that they determined PV in fresh oil, i.e., immediately after pressing, which significantly affects the low degree of oil oxidation. A high PV value can be caused by a long period of oil storage and exposure to undesirable external factors: high temperature, oxygen, and light. An additional factor that accelerated the degradation transformations in TO is the high content of PUFAs (over 60%), mainly linoleic acid, which is oxidised much faster than MUFAs. The p-anisidine value (p-AnV), which determines the content of secondary oxidation products, took values from 2.05 to 12.01 in tested oils. In the case of cold-pressed oils, there are no designated p-AnV limits, but it is assumed that they should have lower values than their refined counterparts [22]. According to the PN-A-86908:2000 standard, the p-AnV of refined oils should not exceed the value of 8. The oil exceeding the p-AnV standard for refined oils was coriander seed oil (12.01), indicating an increased content of secondary oxidation products. Kalyna et al. [23], studying the p-AnV of coriander seed oil, obtained a value of 3.1; a lower value, amounting to 0.46, was achieved by Dedebas et al. [24]. Relatively high p-AnVs, of 6.82 and 6.55, respectively, were also achieved for TO and PO. For tomato seed oil, Giuffrè [25] obtained p-AnV values ranging from 7.48 to 17.32, depending on the solvent used for extraction. The p-AnV values presented in the literature differ, which may be due to different methods of oil extraction or an advanced degree of oxidation and the breakdown of peroxides (primary oxidation products) to carbonyl compounds (secondary oxidation products) under the influence of heating the raw material or oil in the technological process [26]. Lower values of p-AnV characterised the remaining oils. The lowest value, equal to 2.05, was characteristic of WCO. This value was similar to the result of Krajewska et al. [27], who tested the p-AnV of cress seed oil and obtained a value of 2.17. The total degree of the oils’ oxidation was indicated by the TOTOX fat oxidation index, which was calculated from the p-AnV and PV of the oils. The TOTOX index in this study took values in a wide range from 9.65 to 95.92, and only basil seed oil met the criteria of good quality, as the value of the tested parameter did not exceed 10 [28].

3.2. Carotenoid and Chlorophyll Pigments Content

Carotenoid and chlorophyll pigments are essential constituents of oil products as their metabolism is related to the oxidation processes of UFAs [29]. The tested oils were differentiated for both carotenoid and chlorophyll pigment content in oils (Table 1). FO had the highest content of carotenoid pigments with a value of 153.68 mg/kg. The other oils studied were characterised by much lower contents. WCO had a value of 58.16 mg/kg. CO, DO, PO, and TO had similar carotenoid pigment levels, respectively: 23.03, 20.26, 20.66, and 18.18 mg/kg. Uitterhaegen et al. [30] reported that the content of carotenoid pigments in CO was 10.1 mg/kg oil. The lowest content of carotenoid pigments was determined in BO (5.40 mg/kg). According to the literature, the difference in carotenoid content in plants depends on the cultivation system (in the greenhouse or field system), the degree of seed maturity, or the oil extraction method [31].

In the present work, DO was characterised by the highest content of chlorophyll pigments, for which a value of 102.20 mg pheophytin a/kg was determined. Another oil with a high chlorophyll content (93.27 mg pheophytin a/kg) was PO. CO had a chlorophyll content of 23.43 mg/kg. This value was higher than that obtained by Uitterhaegen et al. [30] for this type of oil (11.1 mg/kg oil). FO and WCO, which were characterised by high contents of carotenoid pigments, were simultaneously distinguished by low contents of chlorophyll pigments. TO and WCO had 10.74 and 7.52 mg/kg of chlorophyll pigments, respectively. The lowest amount of pheophytin a was found in BO (2.60 mg pheophytin a/kg). Mińkowski et al. [31] reported a similar value (2.8 mg/kg oil) for borage oil.

3.3. Oil Colour in the CIE L*, a*, b* System

A product’s colour indicates its attractiveness to the consumer, who often associates it with the desired taste or smell [29]. The colour of vegetable oils is related to the presence of chlorophyll and carotenoid pigments, whose content is conditioned not only by the species of raw material used for production and its maturity, but also by the technology of extraction. These pigments are an important element due to their pro- and antioxidant activity; therefore, colour, apart from influencing consumer desirability, is also an important marker of oil quality [28]. Varied colours characterised the investigated oils. They differed significantly at p ≤ 0.05, and the analysis with Tukey’s test showed the existence of several homogeneous groups (

Table 2. Colour components in the CIE L*, a*, b* system of analysed oils.

Oil Sample	Colour Components
	L*	a*	b*
CO	73.03 d ± 0.07	15.04 c ± 0.02	104.06 c ± 0.06
FO	71.67 c ± 0.01	40.73 f ± 0.01	122.40 e ± 0.04
BO	96.69 g ± 0.01	–4.57 a ± 0.01	50.79 a ± 0.07
PO	65.45 b ± 0.01	12.71 bc ± 0.00	108.18 d ± 0.07
WCO	83.29 f ± 0.02	23.43 d ± 0.00	137.85 f ± 0.02
DO	58.70 a ± 0.10	6.99 b ± 0.03	97.59 b ± 0.14
TO	79.09 e ± 0.01	29.87 e ± 0.01	131.86 g ± 0.11

^a,b,c—mean values denoted in columns by the same letters are not statistically significantly different at p-value ≤ 0.05.

).

DO and PO were the oils with the darkest colour. They were characterised by the lowest values of the L* parameter of 58.70 and 65.45, respectively. At the same time, these oils had low values of the a* parameter (6.99 and 12.71), which means that they were characterised by a green colour. However, these values were not close to −100 (the extreme value for this parameter). The high content of chlorophyll pigments is responsible for the green colour of parsley seed and dill seed oils, as shown by the determination of the chlorophylls content. Sikorska et al. [32] highlight that the presence of chlorophylls is correlated with the L* and a* components. A lower value (−4.57) of the a* parameter was achieved by BO, which in terms of this colour component was similar to the value obtained by Wahidu et al. [33] for cumin oil (a* = −4.51). However, the highest value of the L* parameter was determined in the case of BO (96.69), which means that its colour was the brightest. In addition, for this oil, the obtained value of parameter b* (corresponding to blue colour (b = −100) and yellow colour (b = 100)) was 50.79, which is relatively high.

The oil from cress seeds achieved a high value of parameter L*, with the highest value of parameter b* among the tested oils, affecting the oil’s intense orange colour. TO, CO, and FO were also characterised by similar high values of the L* component, amounting to 79.09, 73.03, and 71.67, respectively. Furthermore, each of the oils mentioned was characterised by a value of the b* parameter above 100. The strong orange colour of FO may result from a very high content of carotenoid pigments in the oil (153.68 mg/kg), as their presence affects the value of the b* component [32]. Dedebas et al. [24], investigating the colour of CO in the CIE L* a* b* system for individual components, obtained different results from those presented in the present work. However, as far as their values are concerned, in a similar manner the parameter b* assumed the lowest and the parameter a* the highest value for this oil (L*—50.24, a*—24.12, b*—84.38).

3.4. Fatty Acid Composition of Analysed Oils

A large variability of the MUFA and PUFA contents between the analysed oils was observed. Chromatographic analysis showed that the highest proportion of PUFA acids, equal to 72.50%, was characteristic of basil seed oil (

Table 3. Fatty acids composition of analysed oils [%].

Fatty Acid	Oil Sample
	CO	FO	BO	PO	WCO	DO	TO
C4:0	0.85 ± 0.02	-	-	-	-	-	-
C6:0	0.26 ± 0.02	-	-	-	-	-	-
C8:0	4.42 ± 0.20	-		-	-	-	-
C10:0	0.34 ± 0.02	-	-	-	-	-	-
C12:0	0.21 ± 0.02	0.16 ± 0.04	-	-	-	-	-
C14:0	-	0.17 ± 0.00	-	<0.1 ± 0.00	<0.1 ± 0.01	-	<0.1 ± 0.01
C16:0	3.11 ± 0.03	9.19 ± 0.15	5.80 ± 0.11	5.24 ± 0.10	7.82 ± 0.04	5.55 ± 0.04	11.60 ± 0.05
C16:1	0.41 ± 0.01	-	<0.1 ± 0.01	0.69 ± 0.03	0.20 ± 0.00	0.38 ± 0.36	0.18 ± 0.01
C17:0	-	0.24 ± 0.05	<0.1 ± 0.02	0.40 ± 0.01	-	-	<0.1 ± 0.01
C17:1	-	0.13 ± 0.02	-	0.13 ± 0.02	-	-	0.32 ± 0.00
C18:0	0.81 ± 0.00	4.14 ± 0.04	3.28 ± 0.03	3.00 ± 0.02	2.81 ± 0.02	3.30 ± 0.02	4.49 ± 0.01
C18:1	73.01 ± 0.18	14.50 ± 0.01	17.77 ± 0.01	32.47 ± 0.08	24.17 ± 1.06	30.93 ± 0.52	17.64 ± 0.02
C18:2	14.68 ± 0.12	39.70 ± 0.07	20.16 ± 0.14	53.71 ± 0.32	11.48 ± 0.13	58.03 ± 0.93	61.07 ± 0.02
C18:3	-	0.16 ± 0.06	0.24 ± 0,01	2.35 ± 0.05	0.11 ± 0.01	0.52 ± 0.05	1.65 ± 0.00
C18:3	0.45 ± 0.01	28.50 ± 0.22	52.10 ± 0.20	0.29 ± 0.12	29.52 ± 0.42	-	1.96 ± 0.02
C20:0	-	1.31 ± 0.01	0.24 ± 0.00	0.25 ± 0.00	2.94 ± 0.02	0.23 ± 0.01	0.42 ± 0.00
C20:1	0.25 ± 0.01	0.31 ± 0.01	0.17 ± 0.00	0.27 ± 0.11	11.99 ± 0.01	0.14 ± 0.01	0.13 ± 0.00
C20:3	-	-	-	<0.1 ± 0.00	0.51 ± 0.01	-	-
C22:0	<0.1 ± 0.01	0.62 ± 0.00	0.06 ± 0.00	0.67 ± 0.02	0.84 ± 0.02	0.66 ± 0.04	0.11 ± 0.00
C22:1	-	0.19 ± 0.01	-	<0.1 ± 0.01	4.28 ± 0.06	-	-
C24:0	<0.1 ± 0.00	0.25 ± 0.00	-	0.25 ± 0.02	0.69 ± 0.02	0.24 ± 0.01	0.16 ± 0.00
C24:1	-	<0.1 ± 0.02	-	<0.1 ± 0.00	1.17 ± 0.11	-	-
∑SFA	8.89 ± 0.32	16.08 ± 0.29	9.32 ± 0.13	9.81 ± 0.17	15.10 ± 0.13	9.98 ± 0.12	16.78 ± 0.08
∑MUFA	73.67 ± 0.2	15.13 ± 0.07	17.94 ± 0.02	33.56 ± 0.25	41.81 ± 1.24	31.45 ± 0.89	18.27 ± 0.03
∑PUFA	14.93 ± 0.13	68.36 ± 0.35	72.50 ± 0.35	56.35 ± 0.49	41.62 ± 0.57	58.55 ± 0.98	64.68 ± 0.04
n-3	0.45 ± 0.01	28.50 ± 0.22	52.10 ± 0.20	0.29 ± 0.12	29.52 ± 0.42	0.00	1.96 ± 0.02
n-6	14.68 ± 0.12	39.86 ± 0.13	20.40 ± 0.15	56.06 ± 0.37	11.59 ± 0.14	58.55 ± 0.98	62.72 ± 0.02
n6/n3	33:1	1.5:1	1:2.5	193:1	1:2.5	1:0	32:1
n-3/n-6	0.03	0.72	2.55	0.01	2.55	0.00	0.03
COX	2.35	10.43	13.56	6.44	8.11	6.40	7.25
AI	0.04	0.12	0.06	0.06	0.10	0.06	0.14
TI	0.09	0.12	0.05	0.18	0.09	0.20	0.35
HH	28.34	8.84	15.52	16.19	8.23	16.03	6.89

). Among these types of acids, the largest amount was α-linolenic acid C18:3 (cis-9, 12, 15), constituting 52.10%. Thus, this oil was characterised by the highest content of rapidly oxidising α-linolenic acid among all the oils studied. The other acids found in the composition of this oil were linoleic, oleic, palmitic and, stearic acids. Idris et al. [34] also obtained the highest content of α-linolenic acid in basil seed oil, which was 43.92%. Considering the other acids, the researchers obtained higher proportions of saturated fatty acids: palmitic and stearic, amounting to 13.38% and 6.55%, respectively. In turn, results similar to those obtained in this paper were reported by Nour et al. [35], who identified in several basil seed oils 49–75% α-linolenic acid, 12–32% linoleic acid, 6–10% oleic acid, 5–13% palmitic acid, and 2–3% stearic acid. Fenugreek seed and tomato seed oils were also characterised by a high PUFA acid content, with percentages of 68.36 and 64.68%, respectively. In the case of FO, linoleic acid LA (C18:2) (39.70%) was dominant. The second position was occupied by α-linolenic acid (28.5%). A slightly higher content of LA acid, amounting to 43.2%, in FO was determined by Sulieman et al. [36]. On the other hand, acid profiles similar to those obtained in this study were obtained by Al-Jasass and Al-Jasser [37]. They reported that FO contains 34.85% linoleic acid and 30.80% α-linolenic acid. The other acids identified in FO were oleic, palmitic, stearic, and arachidic acids at levels equal to 14.50%, 9.19%, 4.14%, and 1.31%, respectively.

Munshi et al. [38], investigating the fatty acid composition of FO for individual acids (listed above), obtained similar values falling within the following ranges, respectively: 12.77–15.06%, 9.89–10.87%, 4.04–4.36%, and 1.05–1.26%, depending on the oil extraction solvent used.

In the tomato seed oil, linoleic acid (C18:2) was also the predominant acid, with a content of 61.07%. Other identified acids, such as oleic, palmitic, and α-linolenic acids, were present in much smaller amounts. In the TO of the Rebelion F1 variety, the researchers obtained a linoleic acid percentage of 61%. The other varieties studied were characterised by a much lower content of this acid (47.11–51.90%). The other acids identified in TO were oleic (17.16–27.98%), palmitic (12.97–13.92%), stearic (4.61–8.17%), and α-linolenic (1.57–1.99%) [39].

The oils with a lower but still above 50% PUFA acid content were the parsley seed and dill seed oils. These oils did not differ significantly in terms of fatty acid composition as they are from the same family, i.e., celery plants [40]. Both PO and DO were dominated by linoleic acid (C18:2), with 53.71% and 58.03%, respectively. The similarity was also shown in the content of the second most abundant oleic acid and saturated acids: palmitic and stearic. PO contained 32.47% oleic acid, 5.24% palmitic acid, and 3% stearic acid. On the other hand, the values for DO were 30.93%, 5.55%, and 3.30%, respectively. The results obtained in DO are similar to the data reported in the literature. Badar et al. [41], studying this type of oil, report the following fatty acid contents: 54.12% linoleic acid, 37.05% oleic acid, 4.66% palmitic acid, and 3.26% stearic acid.

The results presented in the literature regarding the fatty acid composition of PO differ. Drăghici et al. [42] report that the dominant one is the atypical petroselinic acid- (Z)-6-octadecenoic acid, C18:1. Thus, it can be assumed that the obtained oleic acid content (32.47%) actually refers to petroselinic acid. Parry et al. [43] presented a different composition of PO; they found the presence of oleic acid in the range of 80.9–81.00%, linoleic acid at 11.00–15.20%, stearic acid at 4.20%, and palmitic acid at 3.10%. In contrast, the results presented in the present study were similar to those obtained by Ying et al. [44], who obtained a total proportion of SFAs in the range of 8.7–9.1%, MUFAs in the range of 30.5–31.5%, and PUFAs in the range of 53.8–56.8%.

The content of polyunsaturated fatty acids in watercress seed oil was 41.62%, and the acids with the highest proportion were α-linolenic acid (29.52%) and oleic acid (24.17%) (Table 3). The other acids identified in this oil were eicosenoic (C20:1), linoleic (C18:2), and palmitic (C16:0), which were 11.99%, 11.48%, and 7.82%, respectively. The proportion of eicosenoic acid was similar to that reported by Diwakar et al. [45]. Different results were obtained by Al-Jasass and Al-Jasser [37], who reported that the WCO contained 48.43% α-linolenic acid and only 15.35% oleic acid. However, the content of the linoleic and palmitic acids obtained by the researchers was similar to the results of the tested oil. It is noteworthy that in WCO several other acids were also identified which were not present in other oils or present in trace amounts. Such acids included erucic acid (C22:1) in the amount of 4.28%, nervonic acid (C24:1) constituting 1.17%, or lignoceric acid (C24:0) at the level of 0.69%, as well as dihomo-γ-linolenic acid (C18:3 cis-6,9,12), constituting 0.11% of all the acids. Diwakar et al. [45] reported that cress seed oil contains 3.6% erucic acid (C22:1), but no other unusual acids were identified in the oil studied by the authors.

The lowest content of PUFAs was found in coriander seed oil, which contained only 14.93%. On the other hand, it was the richest source of MUFA acids among all the analysed ones. The monounsaturated fatty acid (mainly oleic) content in this oil was 73.67% (

Table 4. Induction time of analysed oils at 90–150 °C determined using the Rancimat apparatus.

Oil Sample	Induction Time [h]
	90 °C	100 °C	105 °C	110 °C	120 °C
FO	16.26 b ± 0.11	8.02 b ± 0.11	5.83 b ± 0.57	4.23 b ± 0.01	2.24 a ± 0.01
BO	8.55 a ± 0.21	4.10 a ± 0.14	2.94 a ± 0.13	2.11 a ± 0.01	1.07 c ± 0.07
PO	28.59 d ± 0.13	12.77 d ± 0.13	8.63 c ± 0.16	5.87 c ± 0.16	2.91 d ± 0.04
WCO	15.59 b ± 0.00	7.70 b ± 0.38	5.42 b ± 0.28	4.14 b ± 0.04	2.26 b ± 0.01
DO	33.33 e ± 0.32	14.42 e ± 0.30	10.06 d ± 0.15	7.12 d ± 0.34	3.60 e ± 0.01
TO	20.71 c ± 0.24	9.27 c ± 0.15	6.24 b ± 0.25	4.12 b ± 0.06	2.10 a ± 0.14
Oil Sample	Induction Time [h]
	120 °C	130 °C	140 °C	145 °C	150 °C
CO	28.92 f ± 0.16	13.49 a ± 0.58	6.51 a ± 0.06	4.52 a ± 0.04	3.48 a ± 0.20

^a,b,c—mean values marked in the columns with the same letters do not differ statistically significantly with a p-value ≤ 0.05.

). Similar results for CO were obtained by Ramadan et al. [46], who determined the content of MUFAs at 78.74% and PUFAs at 15.03%. The relatively high content of MUFAs was characterised by WCO, which contained as much as 41.81% (Table 4). Chatoui et al. [47] report a slightly lower value of 37.37%. PO and DO were characterised by more than thirty percent MUFA content, while for FO, BO, and TO much lower values ranging from 15.13% to 18.27% were obtained (Table 3).

Taking into account the saturated fatty acid SFA, it was found that their highest content was characterised by TO—16.78% (Table 3). A similar high content of SFA acids was characterised by TO, as analysed by Zuorro et al. [48], for which a value of 20.61% was obtained. A similar content of saturated fatty acids, equal to 16.08%, was obtained in FO. The remaining oils were characterised by a lower proportion of SFAs ranging from 8.89 to 15.10% (Table 4). CO was noteworthy because only in it were saturated short-chain acids such as butyric, caproic, and caprylic acids identified, and the total proportion of these acids was 5.53%. However, the tested coriander seed oil had the lowest total content of SFAs, i.e., at 8.89%. Uitterhaegen et al. [30] reported an even lower proportion of saturated acids—3.2%, of which only 0.1% was (the only short-chain acid identified) caproic acid (C6:0).

3.5. Calculated Oxidizability Value of Oils

A COX value is a beneficial element usually taken to evaluate the oil’s tendency to undergo autoxidation. Based on the calculated COX values (Table 3), it could be concluded that the oils had different values of this parameter. The oxidative stabilities of the tested oils were in the following order: BO > FO > WCO > TO > PO > DO > CO. Among all the analysed oils, BO was the most susceptible to oxidation, where the COX value reached 13.56. It should be noted that CO had a COX value six times better than that of BO (2.35). Most of the analysed oils had high oxidizability values, higher than those investigated by Xu et al. [49]: palm (1.62), peanut (4.63), and camelina oil (1.77). Abril et al. [50] reported that the COX value of extra virgin olive oil equals 2.38. It follows that the tested oils should undergo rapid oxidation. It is noteworthy to mention that the rapidity of oxidation depends on the degree of unsaturation, the presence of antioxidants, and the prior storage conditions [51].

3.6. Nutritional Quality Index of Oils

The content of fatty acids significantly impacted the dietary factors of the fats and plant oils (Table 3). Dietary fat quality is determined by the amount of SFA, MUFA, and PUFA, as well as the ratio of n-6 to n-3 fatty acids. Three nutritional quality indexes were created to assess the nutritional value of oils: the atherogenicity index (AI), the thrombogenicity index (TI), and the hypocholesterolemic: hypercholesterolemic FA ratio (HH); these were calculated based on the fatty acid compositions of the studied oils. The concentrations of lauric acid (C12:0), myristic acid (C14:0), arachidonic acid (C20:4 n-6), docosapentaenoic acid (C22:5 n-3), and docosahexaenoic acid (C22:6 n-3) were considered as zero for the calculations of AI and HH because they were not identified in the total lipids of the oils.

The atherogenicity (AI) and thrombogenicity (TI) indices indicate whether or not the studied oil has the potential to stimulate platelet aggregation. The lower the AI and TI values, the more the anti-atherogenic fatty acids present in a particular oil/fat, and thus the greater the potential for preventing the development of coronary heart disease. The HH index considers the specific effects of fatty acids on cholesterol metabolism, and high HH values are desired from a nutritional standpoint [52]. In the present work, the highest HH ratio value (28.34) was found in coriander seed oil among the tested oils. Ratusz et al. [53] reported that the HH ratio values ranged from 11.2 to 15.0 in camelina oils. The AI values of the oils ranged between 0.04 and 0.14 for the analysed oils (Table 3). Ulbricht and Southgate [13] showed that the AI of oils from coconut, palm, and olive are 13.63, 0.88, and 0.14, respectively. The studied oils were also characterised by different TI values, which ranged from 0.05 (BO) to 0.35 (TO). Significantly higher TI values for coconut (6.81) and palm (2.07) oils were demonstrated by Ulbricht and Southgate. These indices suggest that incorporating coriander, basil, parsley, and dill seed oils into the human diet may help prevent coronary heart disease development.

3.7. Oxidative Stability of Analysed Oils

Oxidative stability is an important quality characteristic of edible oils that determines their resistance to oxidation. Due to the fact that it depends mainly on the composition of fatty acids, oils with a high content of polyunsaturated fatty acids are the least stable. Moreover, it is significantly affected by the presence of pro- and antioxidant substances. The oxidation induction time of the tested oils varied in terms of the type of oil and the temperature used in the Rancimat test, ranging from 90 to 150 °C (Table 4).

The induction time for oils at 120 °C ranged from 1.07 h to 28.92 h. Coriander seed oil proved to be the most stable oil, with an induction time of 28.92 h. An equally high value of 14.6 h in the Rancimat test at 110 °C was also obtained by Moser and Vaughm [54]. The study of Dedebas et al. [24] also confirms the low susceptibility to oxidation of coriander seed oil, as the induction time they determined at 110 °C was equal to 27.3 h. The high oxidative stability of this oil may be determined by a high content (>70%) of monounsaturated petroselinic acid, which is oxidised more easily than polyenoic acids. It is known that linoleic acid oxidises from 10 to 40 times faster than oleic acid, whereas α-linolenic acid oxidises 2–4 times faster than linoleic acid [55]. In addition, coriander seed oil was characterised by a high content of antioxidant components, such as tocopherols, sterols, or carotenoids, which affects the delay of the oxidation process [56]. Moreover, the main component (81.6%) of coriander essential oil—linalool—inhibits lipid peroxidation [57].

Basil seed oil had the shortest induction time at 120 °C of 1.07 h. When measuring the oxidative stability at 110 °C, Ghaleshahi et al. [58] recorded 1.39 h. The low oxidation resistance of basil seed oil is the high content of unsaturated fatty acids, especially polyunsaturated acids. The fatty acid profile is dominated by α-linolenic acid (52.10%) and linoleic acid (20.16%), with the total content reaching up to 70%. The remaining oils (dill seed, fenugreek seed, parsley seed, watercress seed, and tomato seed) were characterised by oxidative stability at a similar level, with induction times ranging from 2.1 to 3.6 h (Table 4).

Of the above, dill seed oil had the longest oxidation induction time at 120 °C—3.6 h. A 10-degree lower temperature increased this time to 7.12 h. According to van ‘t Hoff’s rule, a 10 K increase in temperature results in a 2–4 fold increase in reaction rate; so, the formation time of oxidation products at 110 °C was much longer. Ying et al. [44] also obtained 7 h at 110 °C. Parsley seed oil had lower oxidative stability than dill seed oil. The oxidation induction time at 120 °C was 2.91 h. However, at 100 °C, the time increased to almost 13 h. The relatively good oxidative stability of parsley seed oil may be determined by the presence of a large amount of monounsaturated petroselinic acid, the content of which reaches over 70% [40]. The oxidation induction time obtained for fenugreek seed oil and tomato seed oil at 120 °C was short and was 2.24 and 2.10 h, respectively. The fatty acid profile of fenugreek seed oil is dominated by linoleic acid (C18:2), with more than half of the total acids, which may cause low stability [59]. Analysing the oxidative stability at 120 °C for FO, Gu et al. [60] reported an induction time of 2.85 h, which they also explained by the high content of PUFA acids. Hassanein et al. [61], investigating the stability of oils from different non-conventional feedstocks at 110 °C, obtained 25.4 h for tomato seed oil.

3.8. Kinetics Parameters of Analysed Oils Oxidation under Rancimat Conditions

The parameters of the oxidation kinetics were determined based on the results obtained in the Rancimat apparatus (

Table 5. Kinetics parameters of the analysed oils’ oxidation reactions.

Oil Sample	Z	k90 °C	k100 °C	k105 °C	k110 °C	k120 °C	Ea [KJ/mol]	ΔH [kJ/mol]	ΔS [J/mol*K]
FO	9.7 × 109	0.2144	0.4138	0.5674	0.7716	1.3941	94.18	77.2	−118.2
BO	6.5 × 1010	0.4303	0.8578	1.1946	1.6494	3.0679	74.08	80.9	−102.3
PO	4.6 × 1011	0.1486	0.3212	0.4650	0.6668	1.3340	77.72	90.0	−86.4
WCO	6.1 × 109	0.2193	0.4178	0.5694	0.7697	1.3743	86.84	75.8	−121.8
DO	1.1 × 1011	0.1277	0.2667	0.3798	0.5360	1.0397	72.61	86.1	−98.3
TO	7.6 × 1011	0.2077	0.4510	0.6545	0.9404	1.8889	82.97	90.5	−82.4
CO	Z	k120 °C	k130 °C	k140 °C	k145 °C	k150 °C	Ea [KJ/mol]	ΔH [kJ/mol]	ΔS [J/mol*K]
	4.9 × 1011	0.1504	0.3073	0.6066	0.8420	1.1596	87.34	97.6	−85.2

). Graphs of the dependence of the log of the induction time on the analysis temperature, as well as the dependence of the log of the induction time on the reciprocal of the temperature, were prepared. Due to the obtained diagrams, the corresponding regression equations and the correlation coefficient values were determined.

The applied regression analysis methods made it possible to determine the directional coefficients. In turn, these coefficients, combined with the Arrhenius equation, made it possible to determine the activation energy (Ea), i.e., the smallest amount of energy needed to initiate the reaction, in this case the oxidation reaction of the analysed oils. The value of this parameter allows for the determination of which oil starts to oxidise faster.

The lowest value of Ea among all the analysed oils was characteristic for dill seed oil, for which this value was 72.61 kJ/mol. The activation energy determined for fenugreek seed oil was 94.18 kJ/mol. Basil seed oil, 74.08 kJ/mol, needed a slightly more significant amount of energy to initiate the oxidation reaction. These results were similar to those reported by Symoniuk et al. [4] for cold-pressed rapeseed oils—the value of the Ea parameter ranged from 75.73 to 77.64 kJ/mol. The oils from coriander seeds, parsley seeds, and tomato seeds were characterised by energy activation ranging from 80 to 90 kJ/mol, and successively, the results were equal to 87.34, 86.84, and 82.97 kJ/mol. Parsley seed oil, studied by Nogańska [62], was characterised by an activation energy of 82.03 kJ/mol. This result was lower than that obtained for this type of oil analysed in this study but similar to that obtained for the dill seed oil.

Tan et al. [63] investigated the Ea of ten different vegetable oils (including grape seed, safflower, and nut) and achieved values ranging from 79.9 kJ/mol to 104.3 kJ/mol. Thus, the results obtained in the study do not differ significantly from the Ea results obtained by other researchers for oils from various plant materials. Coriander seed oil (94.18 kJ/mol) needed the most energy to initiate oxidation. A slightly higher result (99.6 kJ/mol) was obtained by Aktar and Adal [64] for avocado oil, as well as by Nogańska [62] for avocado oil. Both oils are a good source of oleic acid, approx. 70%, which is resistant to oxidative changes.

The constant of the oxidation rate shows the influence of temperature on the rate of fatty acid oxidation. Therefore, it is the most important parameter related to the kinetics of oil oxidation. The obtained results show the dependence in the fact that with the increase of the temperature used in the Rancimat test, the value of the constant rate of oxidation increases. The fatty acid composition may also influence the value of this constant [63]. Due to the high content of polyene fatty acids, the oils dominated by these acids have higher k values for the same temperature than the other oils. A particularly high value of the parameter k at 120 °C (compared to the others) was observed for basil seed oil—it amounted to 3.0679 h⁻¹. In the case of the remaining oils, the course of the oxidation reaction was slower, and it ranged from 0.1504 to 1.8889 h⁻¹. For linseed oils, Symoniuk et al. [4] obtained k parameter values in the range of 0.6537–0.7056 h⁻¹. In turn, the results obtained by Nogańska [62] were more diversified—0.4934–9.8196 h⁻¹. The author reported a value of 1.0162 for parsley seed oil. This result was lower than that obtained for this type of oil analysed in this study (1.3340 h⁻¹) but similar to that obtained for dill seed oil (1.0397 h⁻¹). The obtained values of the constant rate of oxidation at the temperature of 120 °C for the oils of fenugreek seeds, dill seeds, parsley seeds, and cress seeds were in the range of 1.0391–1.5873 h⁻¹, as obtained by Ratusz et al. [65] for linseed oils tested at the temperature of 110 °C. The dill and coriander seed oils were characterised by the lowest k values at 120 °C. The coriander seed oil has a large proportion of unsaturated acids, but they are mainly MUFA acids (Table 4), and additionally, for this oil, a long oxidation induction time was determined in the Rancimat test (28.92 h) (

Table 6. Correlation coefficients between the oxidation parameters and the quality parameters of the tested oils.

Quality Parameter	IT at 120 °C	Ea	k at 120 °C
AV	0.10	0.66	−0.16
PV	0.14	−0.08	−0.19
p-AnV	0.46	0.20	−0.26
TOTOX	0.20	−0.06	−0.22
K232	0.22	−0.07	−0.21
K268	0.43	−0.11	−0.35
Chlorophylls	0.69 *	−0.56	−0.52
Carotenoids	−0.08	0.78 *	−0.28
L*	−0.88 *	0.02	0.85 *
a*	−0.04	0.86 *	−0.35
b*	0.30	0.64	−0.65
SFA	−0.33	0.61	−0.07
MUFA	0.63	0.17	−0.49
PUFA	−0.61	−0.30	0.53
COX	−0.87 *	−0.14	0.77 *
AI	−0.41	0.49	0.06
TI	0.17	−0.12	−0.19
HH	0.53	−0.18	−0.18
Ea	−0.01	-	-
k at 120 °C	−0.89 *	-	-
IT at 120 °C	-	-	-

* statistically significant correlation.

). These factors may be the reason for a relatively low value of the k parameter for coriander seed oil.

As the next part of the kinetics parameters, the oxidation reaction was the calculation of the analysed oils’ enthalpy (ΔH) and entropy (ΔS). In the case of both enthalpy and entropy, CO had the highest values—97.6 kJ/mol and −85.2 J/mol K, respectively. On the other side, GCO had the lowest values of those parameters—75.8 kJ/mol and −121.8 J/mol K, respectively. According to Farhoosh et al. [66], negative entropy values indicate a more significant ordering of active complexes than reactant molecules. Higher entropy values mean a low probability of active complex formation and a slower oxidation reaction.

3.9. Principal Component Analysis

In order to differentiate the oils based on their quality parameters, the principal component analysis was applied. The scores for the first two factors of the seven cold-pressed oils are presented in Table 6 and Figure 1. The first two factors amounted to 60.45% of the total variation. According to the scatter plots constructed by Factor 1 and Factor 2 (Figure 1), the oil quality parameters group the oils into three groups according to the similarities of the fatty acid composition, oxidation parameters, and colour. The first group includes CO, PO, and DO, which are similar in terms of monounsaturated acid content and induction time at 120 °C. The next group includes WCO, FO, and TO; a separate group is BO, which had the shortest induction time.

PCA analysis also shows a correlation between the analysed quality parameters. Table 6 presents the value of the correlation coefficients between the individual quality factors and the discriminators of the oil stability. The value of the L* parameter and the oxidation coefficient calculated on the basis of fatty acid composition had a great impact on the oil stability (r = 0.88 and r = −0.87, respectively. This means that the lighter the oil—that is, the smaller amounts of dyes that it contains, the more stable it is, which works well in the case of refined oils. However, the opposite is the case with the COX. The higher value of this parameter, the lower the oil stability.

The highest correlation was found between the value of the oxidation rate constant and the induction time, but it is related to the calculation of this parameter on the basis of the induction time. The activation energy is the least correlated with the a* parameter of the oil colour and the content of the carotenoid pigments related to it. Along with their content, the activation energy of the oxidation reaction increases, i.e., the initiation of the reaction requires more energy.

4. Conclusions

The oils from unconventional raw materials such as herb or vegetable seeds are characterised by intense colour, specific taste, and smell but are also distinguished by unique chemical composition. The investigated selected oils (basil, fenugreek, coriander, tomato, watercress, parsley, and dill) were characterised by low free fatty acid content and a high degree of oxidation. The specific colour of the oils results from the varied content of the carotenoid and chlorophyll pigments. Consuming cold-pressed oils can positively affect human health, as evidenced by the designated nutritional quality indicators. These oils show a low pro-atherosclerotic and pro-thrombotic potential. Cold-pressed oils vary in their oxidation stability. One of the factors influencing the stability of the oils is undoubtedly the composition of the fatty acids. The oils’ induction times reduced with the increasing temperature of the Rancimat analysis, which confirms that the temperature significantly influences the course of the oxidation reaction. The determined reaction rate constants (k) increased with the increasing temperature oxidation in the Rancimat apparatus. Additionally, the oils rich in polyene fatty acids had higher k values than the oils containing more saturated and monounsaturated fatty acids. The calculated oxidation kinetics parameters correlated with varying strength with the individual oil quality discriminants. The oxidation rate constant was strongly correlated with the induction times of the oils, the colour of the oil, and the calculated COX coefficient. On the other hand, the activation energy of the oxidation reaction correlated strongly with the oil’s colour and the carotenoid pigment content. The oxidation reaction rate does not correlate with the energy needed to start the reaction.