Chemical and Physical Properties of Meadowfoam Seed Oil and Extra Virgin Olive Oil: Focus on Vibrational Spectroscopy

Adam Mickiewicz University in Poznań, Faculty of Chemistry, Uniwersytetu Poznańskiego 8, 61-614 Poznań, Poland Department of Pharmaceutical Technology, Faculty of Pharmacy, University of Coimbra, Pólo Das Ciências da Saúde, Azinhaga de Santa Comba, 3000-548 Coimbra, Portugal Institute of Human Genetics, Polish Academy of Sciences, Strzeszyńska 32, 60-479 Poznań, Poland Poznań University of Economics and Business, Institute of Quality Science, Al. Niepodległości 10, 61-875 Poznań, Poland CREA-Research Centre for Food and Nutrition, Via Ardeatina 546, 00178 Rome, Italy Department of Pharmacy, University of Napoli Federico II, Via D. Montesano 49, 80131 Napoli, Italy Department of Agricultural Sciences, University of Napoli Federico II, Via Università 100, 80055 Portici, Napoli, Italy CEB-Centre of Biological Engineering, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal


Introduction
e oxidation of vegetable oils starts during their isolation from natural raw materials. e oxidation process reduces the stability of end-products containing vegetable oils significantly. Oils known to have a high Oxidative Stability Index (OSI) number often have longer shelf lives, making them desirable for applications in food, pharmaceutic and cosmetic industries [1][2][3]. Generally, extra virgin olive oil (EVO) is recognized as the most stable oil and can be obtained by cold pressing olives from the olive tree (Olea europaea L.). Olive oil represents a greatly interesting product from the nutraceutical perspective. Its composition is primarily based on a diversity of triglycerides, also possessing diacyl-and monoacylglycerols, as well as free fatty acids and an extra component of nonlipid substances. According to the type of olive oil obtained (extra virgin, virgin, refined, etc.), the amount of free fatty acids is different, providing the possibility of being a main property to distinguish the grade of the oil [4]. In the composition of olive oil, ingredients may be classified as derivatives of fatty acids, waxes and sterols, polyphenols, hydrocarbons, tocopherols, chlorophylls, and other compounds containing polarity. e oil is subject to several steps of refinement to remove phenol-derived compounds and phospholipids, but some fluctuations occur in other compounds. Olive oil is mainly composed of palmitic acid (7.5-20 wt.%), stearic acid (0.5-5 wt.%), palmitoleic acid (0.3-3.5 wt.%), oleic acid (55-85 wt.%), linoleic acid (7.5-20 wt.%), and linolenic acid (up to 1.5 wt.%). e oil also contains very small quantities of myristic, heptadecanoic, and eicosanoic acids. cis-Vaccenic and eicosenoic acids may also occur. e composition in fatty acids is dependent on the climate conditions, the variety of olive harvested, the latitude, and the stage of development of the fruit. Interestingly, the Italian, Greek, and Spanish olive oils are characterized by a lower profile of both linoleic and palmitic acids with a high amount of oleic acid. Contrariwise, Tunisian olive oil is characterized by a high profile of linoleic and palmitic acids with a reduced amount of oleic acid [5]. Interestingly, unfinished biosynthesis of triglycerides or reactions of hydrolysis can lead to the presence of mono-and diacylglycerols in olive oil. Age and storage environment also impact on the distribution of these molecules. Fresh olive oil has a predominance of 1,2-diacylglycerols, which are exposed to isomerization along the time to 1,3-diacylglycerols. is feature helps to infer the age of a given oil as well as tracing a history of the storage conditions [6]. Moreover, tocopherols contained in EVO are a crucial group of lipophilic vitamins with antioxidant properties able to quench reactive species and are present in a nonesterified form. An interesting finding is that α-tocopherol has its antioxidant capacity increased in diminished concentrations (100 mg/kg) more than in higher concentrations (500 mg/kg to 1 000 mg/kg). Although the overall quantity of tocopherol in a sample of olive oil is subjected to some large disparities, between 5 mg/kg to 300 mg/kg, typical values in trustable sources of olive oil range from 100 mg/kg to 300 mg/kg. e quantity of tocopherol is reduced if the oil is exposed to refinement processes [7]. e most representative hydrocarbon present in Olea europaea is squalene that has anticancer properties, and it is considered as an effective antioxidant. is steroid precursor may be quantified in a range of 0.7 g/kg to 12 g/kg and is responsible for more than half of the portion of unsaponifiable matter in EVO [8,9]. e oil contains pigments, such as carotenoids and chlorophylls. e former act as scavengers of reactive species avoiding oil degradation by oxidation and may vary in a concentration between 1 mg/kg and 20 mg/kg [10], while the latter provide green color to the oil. e concentration of chlorophylls in EVO usually ranges from 10 mg/kg to 30 mg/kg. e natural olive oil has also several other ingredients capable of quenching/antioxidizing the reactive species, and therefore the effect of singlet oxygen is not noticeable [11]. Additionally, four different types of sterols may be identified in EVO [12,13], namely, desmethylsterols, 4α-methylsterols, 4,4-dimethylsterols (triterpene alcohols), and triterpene dialcohols. Fatty alcohols are also of high importance in the classification of olive oils due to the several subtypes. ereupon, the overall content of aliphatic alcohol cannot surpass the limit of 350 mg/kg of olive oil [14,15]. e composition of olive oil in waxes cannot surpass 350 mg/kg of olive oil [16]. Phytol (in a range of concentrations between 120 mg/kg and 180 mg/kg) and geranylgeraniol are the two acyclic diterpenoids isolated in alcoholic fractions of olive oil [17].
Depending on the desired effect of olive oil, owing to the constitution in polyphenols, the extraction process can be adapted. When dealing with olive oils containing a high fraction of these compounds, the extraction process adopted shall be the stone mill to prevent bitter taste and strong flavor of the final product.
e system consisting in a hammer shall be preferred for final olive oils with a low content in polyphenols. Among the diverse phenomena that are prone to affect the content in polyphenol compounds are the surface area of the pieces used to crush the olives and the released enzymes that hydrolyze pectin. According to multiple studies carried out along the past years, these hold plenty of benefits to human well-being [18]. Polyphenol compounds are not the most abundant elements in olive oil but account for its good flavor and its pleasant effect [19]. Garcia et al. classified phenolic compounds with biological activity typical of olive oil into four groups: simple phenols, secoiridoid derivatives, lignans, and flavones [20]. Examples of specific compounds usually present in olive oil are oleuropein, tyrosol, hydroxytyrosol, and ligstroside; in addition, characteristic components are also caffeic acid, vanillic acid, and syringic acid [21,22]. e recent review of Garcia-Martinez et al. is worth mentioning [23]; they have summarized the benefits of olive oil phenolic compounds in disease prevention. e content of virgin olive oil in phospholipids ranges from 40 mg/kg to 135 mg/kg when recently extracted. Crude olive residue oil has, however, a higher ratio of this class of molecules which are usually linked with proteins [24].
Meadowfoam (Limnanthes alba) is a herbaceous plant typical of northern California, Oregon, Vancouver Island, and British Columbia. e base of Limnanthes alba is branched with a system of thin fibrous roots, which allows simple transplantation during any phase of its development. Its physiology permits an adaptation to almost every type of soil although soils with less water-retaining capacity are less prone to withstand the growing of such species [25,26]. Meadowfoam seed oil (MSO), obtained from the seeds of Limnanthes alba, is not as wellknown as olive oil, although it is characterized by a high content of long chain fatty acids (C20-22). ese fatty acids help the MSO to resist oxidation, making it a stable oil [27,28]. Moreover, it consists of more than 98 wt.% of fatty acids, mainly monounsaturated fatty acids, which include gondola acid (C20: 1, ω-9) and erucic acid (C22:1, ω-9), but also vitamins A and E, both having antioxidant properties [29,30]. e seeds of MSO are composed, in average, of 20 wt.% to 30 wt.% of oil, which is a combination of three fatty acid molecules. e stability of MSO is described as 20-fold higher than soybean oil, a fact inherent to the capacity of an oil to support oxidative processes. Once the seed is cracked and the oil removed through a special process of extraction, using a solvent, the remnants are available to be used as a foundation in the nourishment of cattle but with a daily limitation of consumption up to 25 wt.% of the overall food intake without negative outcomes to the weight gain in these animals. If it is vital for nourishing other animals, this food supplement must be cooked or subjected to a reduction in its proportion in the food, given the presence of glucosinolate residues, which are poisonous. e world market of oilseeds currently holds two large competitors-meadowfoam oil and rapeseed oil. To thrive in such markets, it is required from the newest competitor, meadowfoam oil, to present two main features: a competitive price and reliable suppliers [31,32]. e uses for MSO are wide since it can be transformed into a solid wax composed of a polymeric web of sulfur residues that has plenty of applications in rubber industry or may even undergo chemical modifications to be converted into ester of liquid wax, a replacement for oils of jojoba and sperm whale. According to Athar and Nasir, the Oxidative Stability Index (OSI), being a measure of the relative resistance of an oil to oxidation, amount to 50.6 h for EVO and 67.3 h for MSO [33]. In food and pharmaceutical industries, chemical methods are used for quality analysis on a regular basis. ese analyses are usually costly and time-consuming. ey often employ toxic solvents and reagents. erefore, there is a growing demand for replacing these traditionally used analytical methods with instrumental methods, especially spectroscopic techniques. e infrared spectroscopy includes a part of the spectrum of electromagnetic radiation in the range between visible and microwave radiation. It is a method based on the absorption of infrared radiation by oscillating molecules. e infrared region is divided into three ranges. Near infrared (NIR) covers a range of 12500-4000 cm −1 , in which mainly bands corresponding to overtones and combination vibrations occur. e medium infrared (MIR) is a range of 4000-400 cm −1 , in which most of the vibrations of stretching and bending organic molecules take place. Far infrared (FIR) is a range of 400-10 cm −1 providing information on rotational transitions, crystal lattice vibrations, and skeletal vibrations of large molecules. e analysis of different parts of the infrared region provides more information about the analyzed product. e aim of this work was to compare EVO and MSO using the vibrational spectroscopy (NIR and MIR region) to determine their chemical and physical properties.

Materials.
Cold pressed Meadowfoam Seed Oil ™ was obtained from Natural Plant Products, Inc. (Oregon, USA) and extra virgin olive oil was sourced from Costa d'Oro (Spoleto, Umbria, Italy). n-Hexane (HPLC grade) and sodium methoxide, used for gas chromatography analysis, were purchased from Sigma-Aldrich, whereas 37-component FAME mix were obtained from Supelco (Bellefonte, PA, USA).

Gas Chromatography.
Fatty acids methyl esters were obtained by a direct methylation with 14 wt.% BF3-MeOH procedure. Fatty acids methyl esters were separated on a BPX-70 capillary column (60 m × 0.25 mm x 0.25 μm; SGE Analytical Science) installed in an Agilent Technologies 7820 gas chromatograph equipped with an autosampler. Helium was used as a carrier gas at a flow rate of 0.8 mL/min. Column temperature was programmed from 140 to 240°C at 6°C/min. Initial and final temperature were held for 5 and 20 min, respectively. Detector temperature was set at 270°C. Fatty acids were identified by comparison of the retention times with those of authentic standards. e relative content of individual components was calculated by area normalization method [34].

Near-Infrared Spectroscopy.
MPAFT-NIR spectrophotometer (Bruker Company) was used to measure the near-infrared (NIR) spectrum range using transmitted light. e oils tested in this experiment were placed into glass vials, having an optical path length of 8 mm. e spectra were measured between 12500 and 4000 cm −1 . For each sample, ten spectra were recorded.

Mid-Infrared Spectroscopy.
A Spectrum spectrophotometer 100 FT-IR with a technique of attenuated total reflection (ATR) was used to measure the of mid-infrared (MIR) spectrum. e spectra were measured (32 scans per sample or background) between 4000 and 400 cm −1 at a resolution of 4 cm −1 . e spectra were corrected using the background spectrum of air. e analysis was carried out at room temperature. Before recording the spectrum, the ATR crystal (ATR Pro One, produced by Jasco) was carefully cleaned with ethanol and acetone. e cleaned crystal was checked spectrally to ensure that no residue was retained from the previous sample. For a measurement, one droplet (20 μl) of the oil was placed on the surface of the ATR crystal and covered with a glass lid in order to avoid contamination with ambient moisture. For each sample, ten spectra were recorded.

Results and Discussion
e value of MSO was apparent immediately because of its unique composition of fatty acids, over 95 wt.% of which have chain lengths of 20 carbon atoms or longer [36]. e MSO is enriched in the unusual fatty acid Δ 5 -eicosenoic acid (20:1Δ 5 ). is fatty acid has physical and chemical properties that make MSO useful for many industrial applications [25]. e high stability of MSO is due to the presence of double bonds which are not conjugated and the absence of oxidative susceptible polyunsaturated fatty acids common in other vegetable oils [37]. A total of twelve components were identified by GC, which varied from 0.13 to 63.16 wt.%, as shown in Table 1.
As pointed out by Mahesar et al. [42], the application of NIR and MIR techniques to olive oils is increasing, from the perspective of a multidisciplinary study approach for food research [43]. Generally, infrared spectroscopy represents a rapid, less destructive, and high-throughput method for the analysis of food products. e MIR spectra of the analyzed oils are shown in Figure 1(a). Spectrum bands of triglycerides, the main components of oils, dominate the typical MIR spectra of the vegetable oil, although other components in the oil also contribute to the spectra [44]. e spectral profile of MSO is similar to the spectral profile of EVO. e only differences in the shape and intensity of some bands appear due to different unsaturated fatty acid contents (higher in MSO). e very intense band at 1743 cm −1 of the stretching vibrations of carbonyl group (C�O) is attributed to the presence of glycerol-fatty acids ester bonds (COOR) of triglycerides [44]. is band is more intense in EVO sample than in MSO. e characteristic band at 1653 cm −1 represents stretching vibrations of -C�C-of cis-olefins (of disubstituted olefins, RHC � CHR) [45]. In the fingerprint region (between 1500 and 900 cm −1 ), several absorption bands appear, as seen in Figure 1(b). Generally, the fingerprint region represents a region rich in information, but, on the other hand, difficult to analyze due to its complexity [46]. e bands at approximately 1460 cm −1 and 1375 cm −1 are characteristic for CH 2 and CH 3 scissoring vibrations. e bands at 1238, 1160, 1117, and 1097 cm −1 arise from C-O stretching vibrations [47,48]. ese bands are more intensive for EVO than for MSO, which confirms that EVO contains more oleic and α-linolenic acid than MSO. e higher value of α-linolenic acid, which is more prone to oxidation, may indicate that the olive oil will be less stable. In contrast to EVO spectrum, a peak in the wave at number 914 cm −1 is observed for MSO. is band is assigned to the bending vibration of cis-olefin group and vinyl groups [49,50]. e band at approximately 720 cm −1 is assigned to the overlapping peaks of CH 2 rocking vibration and the outof-plane vibration of cis-disubstituted olefins (CH 2 rocking vibration) and is also more intense for the olive oil. e intensive bands with maxima at 2924 and 2852 cm −1 arise, respectively, to asymmetric and symmetric stretching vibrations of aliphatic CH 2 methylene and terminal methyl groups of the fatty acid chains in triglycerides [47,51]. ese bands are more intense for MSO than for EVO, as seen in Figure 1(c). e NIR spectra of oil exhibit broad overlapping bands, which are typical for overtones and combination tones of the fundamental vibrational modes [44]. e NIR spectra of the analyzed oils are depicted in Figure 2(a). Similar to the MIR spectra, bands originating from triglycerides also dominate the NIR spectra. e differences in the shape and intensity of some bands are also due to the differing contents of unsaturated fatty acids. e spectra of EVO and MSO are rather similar, having few differences in the intensity of some bands. In the range 9000-6500 cm −1 (Figure 2(b)), the MSO has more intensive bands than EVO. e band at approximately 8264 cm −1 originates from the second overtones of the C-H stretching vibrations. e band with maxima at 7180 and 7074 cm −1 is characteristic for C-H combination vibrations [44,52]. e first overtones of the C-H stretching vibrations in the methyl, methylene, and ethylene groups correspond to a band with the two maxima at about 5791 cm −1 and 5676 cm −1 . It was found that trans-unsaturated triglycerides absorbed light waves at 5797 cm −1 and 5681 cm −1 [52]. is band is more intense for MSO, meaning that this oil will be more stable than EVO (melting time is higher for trans-unsaturated triglycerides than for cis-unsaturated). e low-intensity bands at 4662 cm −1 and 4595 cm −1 correspond to the -HC�CH-stretching vibrations [53], Figure 2(d). ere is only a little difference in the intensity of the oils analyzed (more intensive band is noticed for MSO). e final composition of oils, particularly compounds present in the oil aromas, is dependent on the applied mechanical processes. Crushing process, for example, leads to an intensification from 20 wt.% to 50 wt.% of trans-2hexenal in the final olive oil after 70 min of mechanical crushing. Another ingredient also affected was hexanal. Although its quantity was increased, it was still far distant from the higher level of trans-2-hexenal. Despite this, after finishing the processing methodology, the circumstances were inverted with an upsurge in hexanal and diminution in trans-2-hexenal, probably due to enzymatic action and reduction of the antioxidants present in the oil. Among the volatile and nonvolatile compounds responsible for the aroma and flavor of olive oil are those derived from polyunsaturated fatty acids/alcohols and 6-carbon aldehydes. e literature has stated that carotenoids and chlorophylls are likewise modified depending on the extraction methodology used. ese components are present in higher levels when the oil is submitted to centrifugation using metallic grinders, because they can extract more pigments. e presence of aliphatic alcohols and waxes may also be upgraded if the temperature of extraction increases.
During drying, olive oil shows an astonishing stability when compared to other oils such as soybean, corn, or cotton seed oil. Such fact is due to the capacity of olive oil to resist oxidant environments given its composition in abundant polar and nonpolar antioxidant molecules such as tocopherols and squalene and the complex constitution in fatty acids, allowing the oil to be reutilized. e reuse of the same olive oil for some occasions may also modify the diet lipid intake due to changes induced by temperatures during the frying process.

Conclusions
e composition of fatty acids of MSO and EVO was determined by gas chromatography. e high stability of MSO is due to the presence of double bonds which are not conjugated and the absence of oxidatively susceptible polyunsaturated fatty acids common in other vegetable oils.
e results showed that the four major acids in MSO are 5eicosenoic (C20:1), 5,13-docosadienoic (C22:2), 13-docosenoic (C22:1), and 5-docosenoic (C22:1). Collectively, these four acids constitute about 96 wt.% of total fatty acids in MSO. e determined value of α-linolenic acid, which was higher in EVO, may indicate that the olive oil will be less stable. e Cox value for MSO was found to be 0.032. If compared to the Cox values of olive oil (1.780), crude soybean oil (7.690), sunflower oil (6.600), and canola oil (4.140), MSO is more stable according to our results. e spectral results proved that methods of vibrational spectroscopy can be successfully used for analyzing the chemical composition of meadowfoam seed oil. Distinct regions, which pertain to unsaturated fatty acids, can be used as markers for differentiating between oils (MSO and EVO) and for monitoring their quality. e differences in the shape and intensity of some bands are due to the differing contents of unsaturated fatty acids. e NIR spectra of EVO and MSO are rather similar, having few differences in the intensity of some bands due to the differing contents of unsaturated fatty acids. e MIR spectra differ in the intensity and shape of the spectrum. Meadowfoam seed oil has a peak in the wave at number 914 cm −1 which is not present in the olive oil spectrum. is band is assigned to the bending vibration of cis-olefin group and vinyl groups. e bands at 1238, 1160, 1117, and 1097 cm −1 arise from C-O stretching vibrations being more intensive for EVO than for MSO, which confirms that EVO contains more oleic and α-linolenic acid than MSO. Based on the obtained results and studied literature, it could be concluded that MSO ensures higher stability compared to EVO.
Data Availability e authors declare that the data supporting the findings of this study are available within the article. All the other data are available on reasonable request from the corresponding authors.

Conflicts of Interest
e authors declare no conflicts of interest regarding the publication of this paper.