1. Introduction
Camellia is a genus of flowering plants in the family
Theaceae, native to East Asia and widely distributed in China, India, Japan, and South-East Asian countries, whose seeds and leaves present high nutritional and medicinal values. This subtropical evergreen shrub or small tree arrived in Europe around the 16th century [
1], and was introduced into the gardens of the highest social classes of Galicia (NW of Spain) at the beginning of the 19th.
Nowadays, cultivars of
Camellia species are found worldwide in public and private gardens thanks to their excellent adaptation to climatic and edaphic conditions, easy spread, and resistance to pests and diseases. Particularly,
Camellia japonica L. is the best known internationally as a cultivated species for ornamental value. In the last decade, commercial interest was remarkable, and consequently, production in Spain reached about 2.5 million
Camellia plants per year, which are exported throughout Europe as ornamentals [
2,
3,
4].
Camellia oil is obtained from the seeds, known as one of the most popular edible vegetable oils that was utilized for more than 1000 years in China, and also abundantly used in southeast Asian countries (Japan, Korea, India, Sri Lanka, Indonesia, and Vietnam), where
Camellias are abundantly available [
5].
Camellia oil is also known as “Eastern Olive Oil” because it shares a similar chemical composition with olive oil [
6]. It contains several natural antioxidants, such as squalene, phytosterol, polyphenols, fat-soluble vitamins (vitamins A, B, E), sasanqua saponin, and other functional substances. It was recommended by the Food and Agriculture Organization of the United Nations as a high-quality, healthy vegetable oil because of its nutritional value and excellent storage qualities [
7]. For these reasons, it is commonly used as cooking oil (edible oil) [
8,
9]. In China, the main species used for oil production is
Camellia oleifera C. Abel [
10], while in Japan this is
C. japonica [
11], and
C. sasanqua in Vietnam [
12].
Camellia oil is an expensive product with a particular and characteristic aroma and taste, good storage stability, and high nutritional and medicinal values, with high value interest for trade [
13]. Thus, the economic interest in this crop increased exponentially in recent years for a variety of purposes [
14]. Specifically,
Camellia oil extracted from seeds of different species, including
C. reticulata Lindl.,
C. sinensis L.,
C. oleifera, and
C. japonica, was long processed as an industrial oil used for oligosaccharide production [
15], as a surfactant, in soaps, as a hair oil, and now it is generating interest as a biofuel source, lubricant, and biopolymer [
16,
17,
18,
19,
20]. Although, in cosmetics
C. japonica oil has a long history of traditional cosmetic usage in Japan as a protectant to maintain skin and hair health, where other species are nowadays commonly used for this purpose (e.g.,
C. oleifera, C. grijsii Hance, and
C. sasanqua) [
11,
21].
Camellia oil has fat-soluble natural compounds with health benefits, reducing cholesterol and triglycerides in the blood, lowering blood pressure, and promoting effects such as antioxidation, antipermeability, anti-inflammation, as an analgesic, and anticancer properties [
22,
23,
24], as well as antimicrobial and antiviral activities [
25]. In addition to this, they are used in traditional treatments in China to prevent cardiovascular diseases, arteriosclerosis, and burn injuries [
26,
27,
28].
Triacylglycerols are the principal components of
Camellia oils, with a high proportion of oleic and linoleic acids and low saturated acids. This general lipidic profile is associated with well-known health properties. The oil yield of seeds from this plant is high, being on average 30% oil per seed. However, the seed oil content varies according to species, cultivar, and environmental conditions [
29,
30]. The profile of fatty acids (FAs) allows correlation to be made with their botanical origin, which is a very important aspect from a commercial point of view, since the traceability of these oils is mandatory to avoid fraud by adulteration. The properties of the oils are also dependent on the FAs’ composition. The degree of unsaturation and chain length, and the presence of polyunsaturated FAs, appear to increase the potential beneficial properties of these oils [
31]. The unsaturated FAs content in
Camellia oil can reach as much as 90%, which is the highest amount so far reported for unsaturated FAs in edible oils [
22,
32,
33]. In recent years,
Camellia oil became one of the most popular and expensive edible vegetable oils on the market in China, being more susceptible to adulteration with other cheaper oils by unscrupulous traders for high profits. Another aspect of fraud, the mislabeling of oil extraction methods, and geographical or origin, also destabilize the local
Camellia oil market economies [
34]. The method for
Camellia oil authentication currently used officially, employing gas chromatography (GC) techniques, includes the FAs’ composition. The increased demand for
Camellia oil made the development of rapid and reliable methods for the unequivocal chemical plant species oil characterization associated with the quality of the edible oil a priority objective to avoid commercialization of adulterated
Camellia oils [
35,
36,
37,
38,
39].
To determine the FA composition, a wide variety of analytical methods are available. In this context, traditional methods are gas chromatography with flame ionization detectors (GC-FID) [
40] or gas chromatography-mass spectrometry (GC-MS) [
41]. In these methods, a pretreatment of the sample is necessary to convert the FA into the corresponding methyl esters (FAMEs). So, these methodologies are tedious, time-consuming, require the use of FAs standards, and involve complicated pretreatment of the samples prior to analysis, such as the triacylglycerol hydrolysis and esterification that could face problems of oxidation during the derivatization process [
42,
43,
44].
Currently, new, rapid, and nondestructive methods such as Near-InfraRed (NIR), Raman Spectroscopy, and Nuclear Magnetic Resonance (NMR) techniques were recognized as alternative analytical tools in combination with appropriate chemometrics in oil quality control [
45]. Specifically, recent studies confirmed that NMR is a powerful tool for qualitative and quantitative analysis of FAs composition in edible vegetable oils [
32,
40,
46,
47,
48,
49,
50].
Therefore, the aim of this study was to compare different analytical techniques, including chemical (quality parameters), chromatographic, and nuclear magnetic resonance methods, for the study of several species of Camellia seed oils harvested in Spain. The geographical traceability and species origin of Camellia oil was corroborated. Finally, the suitability of each of the analytical techniques applied in relation to its species grouping of oils according to their chemical profile was evaluated through the principal component analysis.
3. Materials and Methods
3.1. Materials and Reagents
Silica gel for column chromatography (0.063–0.22 mm), activated carbon (100 mesh particle size, powder), deuterated chloroform (CDCl3), and FAME Mix (C14-C22) certified reference material were purchased from Merck (Madrid, Spain). Wijs (iodine monochloride) solution (ICl, 0.1 M) was purchased from Scharlab (Barcelona, Spain). Deionized water was obtained in the laboratory by using the Millipore Q3 Ultrapure Water Distiller (Merck, Darmstadt, Germany). Reference material CRM-162 was obtained from the European Commission, Brussels, Belgium.
3.2. Plant Material
The selection was based on the abundance and availability of each species in the Galician landscape, where
C. japonica and
C. sasanqua are the major species used as ornamental plants, while the other species, such as
C. reticulata and
C. hiemalis, are less interesting because they present less availability and low seed oil in comparison with that of the aforementioned species [
5,
32,
75]. Samples from
Estación Fitopatolóxica Areeiro (EFA) of these species were harvested in different zones labelled 826 and 942. The harvesting was carried out when fruits began to split open and the seeds were visible, a phenological stage of fruit development that corresponds to the
Biologische Bundesantalt and
Chemische (BBCH scale) [
2]. Sampling was carried out in a stratified random fashion within the populations. More than 400 plants were sampled for the study. From each individual sample plant, at least 30 mature fruits were randomly selected for further analysis.
3.3. Camellia Seed Oil
The crude Camellia seed oil was extracted by the traditional mechanical pressing method that is still widely adopted for the commercial processing of Camellia seed oil. Camellia seeds were washed in water and dried at 22 °C for 48 h followed by the mechanical crushing process. Oil extraction was performed using a mild, cold-pressed method. Approximately, 2.5 kg of dried Camellia seeds were transferred to the automatic hydraulic press (Honmac 6YZ-260, Zhenfzhou City, China), and then pressed to 55 MPa for 5 min to obtain the oil. Subsequently, oils were filtered through cellulose, silica gel, and an activated carbon filter under a vacuum (p < 2 mbar) using a vacuum pump (ILMAC FB65454, Fisher Scientific, Madrid, Spain) to remove oil impurities and then were stored in amber bottles at room temperature. The oil samples were weighed, and the yield was expressed as mass of extracted oil per mass of dried seed in a percentage.
3.4. Determination of Acid Value
The acid value (AV) was determined according to the standard method ISO 660:2009 [
76]. The method is based on the titration of a solution of 10 g of
Camellia oil dissolved in ethanol/diethyl ether (1:1,
v/
v) with a KOH solution (0.1 M in ethanol), using phenolphthalein as indicator. Results were expressed as mg of KOH per 1 g of oil. All determinations were carried out in triplicate.
3.5. Determination of Iodine Value
The iodine value (IV) was calculated according to the standard method ISO 3961:2018 [
77]. About 0.20 g of sample oil was dissolved in a mixture of cyclohexane and glacial acetic acid (50:50,
v/
v). Then, 25 mL of Wijs solution were added and the mixture was maintained during 1 h in the dark. Finally, the excess of iodine generated was titrated with sodium thiosulfate with the previous addition of 20 mL of potassium iodide and 150 mL of deionized water. Results were expressed as grams of iodine per 100 g of oil. All determinations were carried out in triplicate.
3.6. FAMEs Preparation and Analysis by GC-FID
The preparation and analysis of FA methyl esters (FAMEs) were based on the method proposed by Alonso et al., (2000) [
78]. About 100 mg of
Camellia oil was weighed and dissolved in 1 ml of hexane. Then, 0.1 ml of methanolic potassium hydroxide (2 M) was added and the mixture was stirred for 1 min and left to rest for 15 min. Next, the hexane layer was separated, and 0.1μL of the hexane fraction was injected into the GC.
The GC analysis of FAME was performed on an Agilent Technologies GC Agilent Technology 5975 B (Palo Alto, CA, USA) equipped with a flame-ionization detector (FID). Analyses were performed with a CP Sil 88 column (100 m × 0.25 mm i.d.) containing 100% cyanopropyl siloxane, stationary phase, with 0.20μm film thickness (Chrompack, Middelburg, The Netherlands). The initial temperature of 175 °C was maintained for 28 min, then raised to 210 °C at a rate of 1.3 °C/min for 10 min. The split ratio was 1:50, and the carrier gas was helium with a flow rate of 1 ml/min. The injector and detector temperatures were 250 °C. For quantitative determinations of total FAMEs, anhydrous soy, corn oil blend with a certified FAs composition (reference material CRM-162) was used. All determinations were performed in duplicate.
3.7. FAMEs Preparation and Analysis by GC-MS
FAMEs were prepared according to International Olive Council IOC/T.20/Doc 24 protocol [
79], with some modifications. A solution of 0.1 g of the sample oil in 2 ml of heptane was vortexed for 1 min. Then 0.2 ml of methanolic potassium hydroxide solution (2 M) was added. The solution was vortexed vigorously for 30 s. When the solution was stratified, the upper layer with methyl esters was separated. An aliquot was filtered through a 0.45μm Polyvinylidene Fluoride (PVDF) filter. The analyses were performed in triplicate.
According to the International Olive Oil Council COI/T.20/Doc. 33 protocol [
80], FAMEs were separated and quantified using an Agilent GC-7890B coupled to MSD-5977A detector instrument (Agilent Technologies, Santa Clara, CA, USA) with an HP-5MS (5%-phenyl) methylpolysiloxane, length 30 m × 0.25 mm i.d. (0.25 μm film thickness) capillary column (Agilent Technologies, Santa Clara, CA, USA). The analysis conditions were as follows: the temperature of the injector was set at 250 °C, and the injection volume was 1 μL with 1:100 split ratio. Helium was used as the carrier gas at a flow rate of 1 ml/min. A temperature program with injection at 165 °C was held for 8 min, and then raised at a gradient of 2 °C/min to 280 °C with 2 min hold, with 37 min total run time. The electron ionization source was 70 eV in negative voltage at 230 °C, and the range of the mass detector was set from 40 up to 850
m/z. They were quantified according to their percentage area, obtained by the integration of the peaks. The results were expressed as the percentages of individual FAs. FAMEs were identified through a comparison of retention times of pure standards analyzed under the same conditions. MassHunter Software Version B.07.00 was used to control and process the obtained data. The identification of compounds was achieved by comparing the retention index with the spectral data obtained from Mass Spectral Library Version 2.0 g (NIST-MS, 2012, Agilent Technologies, Santa Clara, CA, USA).
3.8. FA Analysis by 1H-NMR
The
1H-NMR analysis was carried out according to the method described by Barison et al., (2010) [
71]. The determinations of FAs composition by
1H-NMR spectroscopy (
Figure 2) were performed on a Bruker AVANCE ARX400 NMR spectrometer operating at 9.4 T observing the
1H nuclei at 400 MHz. About 200 μL of each oil sample was directly transferred into 5 mm OD Lab Class Precision NMR sample tube (Wilmad Labglass Inc. USA), and the volume was completed to 600 μL with CDCl
3 and shaken in a vortex mixer for 30 s. The temperature of the sample in the probe was maintained at 30 °C. The relaxation delay (14 s) and pulse (20°) were parameters equally fixed in both instruments. The chemical shifts are reported in ppm, calibrated by setting the peak of tetramethylsilane as an internal reference (δ = 0.00 ppm). Phase and baseline corrections were performed automatically to ensure a better quantitative comparison of the spectra. The spectra were integrated by Mestrenova software (ver. 12, Mestrelab Research SL, Santiago de Compostela, Spain). All analyses were performed in triplicate.
3.9. Statistical Analysis
Statistical analysis was performed using IBM SPSS Statistics 24.0 for Windows (SPSS Inc., 2016, Chicago, Illinois, USA). Significant differences between the values of all parameters were determined at p ≤ 0.05 according to the one-way ANOVA with the posthoc Turkey HSD Test. The results were expressed as mean ± standard deviation.
Data from acid and iodine values, yield, and FA profiles attained from different chromatographic (GC-FID and GC/MS) and proton nuclear magnetic resonance (1H-NMR) techniques were subjected to principal component analysis (PCA) using XLSTAT Software (Addinsoft, NY, USA) to examine the differences amongst Camellia species (C. japonica, C. sasanqua, C. reticulata, and C. hiemalis).
4. Conclusions
In recent years, the commercial interest in high-quality vegetable oils such as those obtained from Camellia seeds increased, which is associated with their healthy properties. Economic aspects and beneficial effects of this kind of vegetable oil have provoked interest into the study of them, both by researchers and industry. In this sense, the studies are focused on the characterization of the fatty acid profile and other quality parameters such as acid and iodine values of different species of Camellia because they are critical factors involved in oil quality. Four species of Camellia grown in the northwest of Spain were studied, with C. japonica and C. sasanqua being the most abundant, and C. reticulata and C. hiemalis the least abundant species. In general, results showed quite similar profiles of FAs in the four species, with higher contents of unsaturated FAs (UFAs > 85%), especially highlighting the contents in C. japonica species (87–89%), and a low concentration of saturated FAs (10.1–13.6%). Furthermore, the fatty acid profile obtained showed analogous characteristics with other edible Camellia oils commercially available in other regions of the world. In addition to the mentioned FAs profile, the extraction yield (16–32%) and the acid (0.4–5.6 mg KOH/g oil) and iodine (70–92 g I2/100 g–oil) values of these Camellia oils could indicate that this vegetable oil could be used as a high-quality edible oil and be commercially viable, in addition to the preferential use of the species as ornamental plants.
On the other hand, a large number of analytical tests are currently necessary to ensure the quality of oils. The officially recommended chromatographic methods that are used for the identification and the quantification of FAs from Camellia oil are tedious, destructive, and time- and resource-consuming. In the present study, the results of a set of analytical techniques (the chromatographic GC-FID and GC-MS and the spectroscopic NMR) were compared for the characterization of FAs. The combination of analytical results from these techniques with multivariate statistics (chemometrics) was an excellent tool to group the Camellia oils according to the different species studied. In other words, 1H-NMR combined with PCA showed the best grouping of oils by Camellia species and discrimination by location, compared to that of traditional chromatographic techniques. This alternative, nondestructive technique is fast, accurate, and simple to perform, avoiding the problems associated with sample handling and pretreatment of alternative conventional techniques. Thus, the combination of methodology based on 1H-NMR and PCA could be a suitable tool for quality control of Camellia oils and authentication of Camellia species used in oil production.