Fatty acid composition and oxidative characteristics of novel edible oils in Poland

ABSTRACT Plant oils are sources of unsaturated fatty acids (FA) and antioxidant compounds. Herbs, spices, and fruit seeds are potential source of oils which may be used as functional food and dietary supplements. FA profile and oxidative quality of oils (n = 28) were investigated by the chromatographic and titration methods. Peroxidability index (PI) was calculated and cluster analysis was performed. Differences in FA content show which of them is responsible for clusters allotment and also for similarities within each cluster. Oils from cluster S1 were characterized by high content of C18:2 n-6, whereas oils from cluster S2 by higher content of C16:1 and C18:1 n-9. Oils from S3 cluster were distinguished by higher amount of C18:3 n-3. A total of 4 of 17 FAs detected in all samples significantly differed among clusters. Novel oils may be an interesting functional food component, but detailed analysis is needed to confirm their applicability and human health influence.


Introduction
Dietary fat is one of the most important macronutrient in human diet. It is the most concentrated source of energy, acts as the medium of fat-soluble vitamins and essential fatty acids (EFA), and plays other important physiological functions in organism. According to the Codex Alimentarius, the amount of energy from fats in well-balanced diet should be 25-30%. Biological significance of fat is determined mainly by the type and proportion of fatty acids (FA). Gas chromatography seems to be the most popular method to investigate the FA profile of different fats and oils. The presence of monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA), of which vegetable oils are good sources, appears particularly desirable. Edible oils are a group of important food ingredients, not only as a source of unsaturated FAs but also as a source of natural antioxidants (e.g. tocopherols, carotenoids, and phenolic compounds). Those compounds are of consumers' awareness due to their proven beneficial influence on human health (Bendini et al., 2007;Nikokavouraa, Christodouleas, Yannakopouloua, Papadopoulos, & Calokerinos, 2011;Ramadan & Moersel, 2006). However, fats with high content of unsaturated FAs are labile food ingredients, which may undergo chemical changes during storage and thermal treatment, e.g. oxidation. During this process, the amount of unsaturated FAs decreases and simultaneously a number of off-flavor and toxic compounds arises. This leads to deterioration of both nutritional value and sensory attributes of oils and food products containing them. Taking into account that the offer of edible oils from herbs, spices, and fruit seeds has grown and many of them are used as functional food, dietary supplements, and personal care products, as well as that the nutritional information concerning these food products is still limited, the investigation of the FA profile of commercially available edible oils is of great demand.
The main aim of this study was to evaluate the FAs' profile of selected edible oils commercially available in polish market in order to improve understanding of their nutritional and oxidative quality as well as applicability for human nutrition.

Materials
Samples of different vegetable oils (n = 28) were purchased from local grocery shops in Warsaw during the period between June 2014 and September 2014. Detailed characteristic of examined oils is presented in Table 1. Examined oils were cold-pressed and extra virgin oils with expiration date ≥6 months, which were stored in original containers at 4°C or in room temperature. Before analyzes, they were stored according to the manufacturer's recommendation in dark. All analyzes were performed immediately after opening from fresh oil.

FAs analysis
The FA profile of examined oils was determined by gas chromatography. FA were transformed and analyzed as FA methyl esters (FAME). From each single oil, three parallel samples were prepared. To one drop of examined oil (c.a. 200 μL), 2.5 mL of 0.5 mol/L natrium hydroxide solution in methanol was added. The mixture was vigorously shaken mechanically. After that, the mixture was heated at 80°C for 10 min and afterwards it was cooled to the room temperature. Subsequently, 1.0 mL of 14% BF 3 methanol solution was added. It was vigorously shaken mechanically and heated at 80°C for 15 min. To stop the reaction after cooling to the room temperature, 1.0 mL of water was added. The extraction of FAME was performed with hexane (2 × 0.5 mL). The combined organic extracts were dried with anhydrous sodium sulfate and 100 μL of purified extract was dissolved in 900 μL of hexane. Until further analysis obtained, samples were stored in −20°C. Separation of FAME was performed on gas chromatograph (Shimadzu GC-17, Kyoto, Japan) with BPX 70 capillary column (30 m × 0.25 mm ID × 0.2 μm film; SGE, Ringwood, Australia) and flame-ionization detector. Helium was used as carrier gas. Flame ionization detector temperature was 270°C and injector temperature was set at 250°C. The column temperature program started from 140°C for 5 min and then increased to 240°C at 4°C per min. Individual FA methyl esters were identified by comparing to the retention times of FAME standards (Supelco 37 FAME Mix No. 47885-U standard (Sigma Aldrich)). Results are expressed as percentage of total FA [%FA].

Oxidative quality determination
The content of hydroperoxides was determined as peroxide value (PV) according to ISO 3960:2012. Results are expressed as meq O/kg oil. The content of free FAs was determined as acidic value (AV) according to ISO 660:2010. Results are expressed as mg KOH/g oil.
Peroxidability index (PI) of investigated oils was calculated on the basis of their FA composition, according to the following formula (Yun & Surh, 2012):
(COR), pumpkin seed (PUMP), PARS, fennel seed (FEN), sunflower (SNF), BOR, RIC, soybean (SOY), sesame (SES). Cluster S2 includes MACA, APR, peanut (PEAN), Carotino (Canola and red palm fruit) (CAR), HAZ and OL oils, in which higher content of C16:1 and C18:1 n-9 was detected. As far as Cluster S3 is concerned, all oils from this cluster (CAM, BHOR and LN) contained more C18:3 n-3 than other examined oils. Detailed examination of Cluster S1 showed that this cluster consists of two sub-clusters. Oils from sub-cluster A (WAL, HEM, WAT, SAF, SBUC, GRAP, EPRIM, POP) were distinguished by significantly higher (p = 0.0020) content of C18:2 n-6 and lower content of C14:0 (p = 0.0431), C17:0 (p = 0.0093), and C18:1 n-9 (p = 0.0006) in comparison to oils from sub-cluster B (BCUM, MTH, COR, PUMP, PARS, FEN, SNF, BOR, RIC, SOY, SES). Only 4 of 17 FA detected in all samples significantly differed among clusters (Table 6). Significant differences in FA content show which of them are responsible for clusters' allotment and also for similarities within each cluster. Moreover, FA profiles of Clusters S2 and S3 are more similar to each other than each of them to Cluster S1 (Table 6). Analyzed oils were characterized by different oxidative quality (Table 1). PI calculated based on the FA composition ranged from 9.3 in OL to 140.2 in BHOR. BHOR oil has the lowest PV (3.07 ± 1.76 meqO/kg oil), whereas the highest PV was measured for PARS (42.8 ± 1.68 meqO/kg oil). Content of free FA measured as AV ranged in examined oils from 0.13 ± 0.03 mg KOH/g oil in PEAN to 5.91 ± 0.08 mg KOH/g oil in BCUM (Table 1).

Discussion
There are convincing evidences that consumption of vegetable oils have beneficial effects on human health (Barceló-Coblijn et al., 2008;Foster, Hardy, & Alany, 2010;Schwab et al., 2006). It is mainly due to their FAs composition, with unsaturated FA as predominant. The main dietary sources of these nutritionally favorable compounds are oils obtained from raw materials typical for particular country or region, e. g. Turkey, Saudi Arabia, Sudan, Romania, etc. (Dulf, 2012;Matthäus & Özcan, 2015;Rezig, Chouaibi, Msaada, & Hamdi, 2012;Sabahelkhier, Ishag, & Sabir Ali, 2011;Salmanizadeh & Moazzezi, 2013). FA profile of oil may significantly vary regarding the region of origin, plant variety, etc., and because of that, determination of FA composition can provide a good indicator of product authenticity (Aparicio & Aparicio-Ruiz, 2000). The assortment of edible vegetable oils is constantly growing, because they are derived from novel sources, such as nuts, spices, fruit, and vegetable seeds (Koyuncu, Islam, & Küçük, 2005;Matthäus & Özcan, 2015;Sabahelkhier et al., 2011). Nowadays, the availability of these oils is not restricted only to the country of origin, but it spreads around the world. That is why we made  an attempt to assess the FA composition of selected oils commercially available in Poland. The most pronounced differences between analyzed oils were found for unsaturated FA, with proven beneficial impact on humans' health. Content of linoleic, oleic, and linolenic FA distinguished oils into three clusters, while content of C14:0 and C17:0 revealed allotment of sub-cluster A and B in S1.
FA profile of oil from black cumin seeds, with content of eicosadienoic acid (C20:2) over 2%, was similar to those reported by other authors (Mankowska & Bylka, 2009). Nigella sativa oil traditionally used as medicine (Khan, Chen, Tania, & Zhang, 2011) may inhibit the inflammation by the decrease of eicosanoids formation and lipids peroxidation in cell membranes (Mankowska & Bylka, 2009;Ramadan, 2013). Moreover, it can be useful in treatment of allergic reaction and can decrease the levels of glucose, total cholesterol, and triglycerides, as well as exerts anticarcinogenic effect (Mankowska & Bylka, 2009).
As far as the oils of high GLA content are concerned, FA profile of Borago officinalis oil obtained in our study corresponds with results obtained by Foster et al. (2010). High level of eicosenoic FA (C20:1) (3.9%) determined by us in BOR confirmed earlier report of Tso, Caldwell, Lee, Boivin, and De Michele (2012), who also found slightly higher share of GLA. Supplementation of diet with Borage oil enhances the levels of n-6 PUFA (dihomogamma-linolenic and arachidonic acid) in the plasma, liver, and vascular tissue of Wistar Kyoto and spontaneously hypertensive rats, and may be responsible for the observed anti-hypertensive effect (Engler & Engler, 1998). It may be also useful in some individual patients with atopic dermatitis, as it is associated with essential FAs metabolism abnormality (Bialek & Rutkowska, 2015;Foster et al., 2010). Also, application of Evening primrose oil is clinically efficient in atopic dermatitis, as well as in mastalgia, menopause, and premenstrual syndrome (Bayles & Usatine, 2009). These potential benefits are mainly due to considerable amounts of γ-linolenic acid. In present study, about 10% of GLA was detected in Evening primrose oil ( Table 2).
As far as oils obtained from fruit seeds are concerned, grape and watermelon seed oils were classified into the subcluster A of Cluster S1. Content of C18:2 n-6 as predominant FA in WAT and GRAP was very similar (66.8 ± 0.1% and 67.5 ± 0.9%, respectively). Results obtained in this study were in accordance with those reported by Lutterodt, Slavin, Whent, Turner, and Yu (2011), who detected LA between 66% and 75%, depending on the origin of the grape seeds. Moreover, an antiproliferative activity against HT-29 colon cancer cells of examined grape oils was observed in his study.
content of oleic acid and lower content of linoleic acid in both refined and cold-pressed pumpkin oil. Such FA profile together with high content of tocopherols, phenolic acids, and sterols may exert positive impact on oxidative stability and increase the range of PUMP potential use not only in food but also in pharmaceuticals (Nyam, Tan, Lai, Long, & Che Man, 2009). Our results are in accordance with earlier observation that in sesame oil the content of two predominant FA (oleic and linoleic acids) was nearly equal (Haiyan et al. (2007), which is also confirmed by the ratio of these FA which in our study was 0.8. We observed similar ratio of C18:1 n-9 and C18:2 n-6 in CAM and BHOR oils, respectively, assigned into S3 cluster, while the predominant FA in those oils was C18:3 n-3. ALA plays an important role in the regulation of biological functions, prevention, and treatment of number of disorders (Kostik, Memeti, & Bauer, 2013). We detected the highest amount of C18:3 n-3 in Black horehound oil (BHOR, about 62%), while other authors considered linseed oil as the best source of α-linolenic acid (Kostik et al., 2013;Matthäus & Özcan, 2015). In analyzed LN this FA consisted only slightly more than 50% of total share of FA (Table 5).
Hydrolytic fragmentation of fat triacylglycerols (TAG) occurs due to their contact with water, and resulted with free fatty acids (FFAs), mono-and di-glycerides and glycerol arise (Dobarganes, Marquez-Ruiz, & Velasco, 2000;Warner, 1999). More susceptible to such changes are fats with a high content of short and unsaturated FA (Choe & Min, 2007). According to Codex Alimentarius, the recommended AV for both virgin and cold-pressed oils should not exceed 4.0 mg KOH/g oil (Codex Alimentarius, 2001). For most of the examined oils, AV was below this value. Only for POP, BCUM, and PARS content of FFA exceed recommended value (Table 1).
Summing up, edible oils, especially those obtained from novel, unconventional sources, should be carefully examined to check whether they meet the safety requirements. It is very important for the consumers, who require their health benefits, as well as for the manufacturers, who want to ensure the proper quality of the offered food products.

Conclusions
In conclusion, consumption of the edible oils derived from unconventional sources such as herbs, spices, and fruit seeds may be an interesting way to improve humans' diet quality, due to its beneficial FAs composition. However, detailed analyzes of their oxidative stability during storage or processing are needed to be conducted.

Disclosure statement
No potential conflict of interest was reported by the authors.