Fatty acid composition of commercial vegetable oils from the French market analysed using a long highly polar column

ARTICLE

Auteur(s) : Nathalie Vingering¹, Marine Oseredczuk², Laure du Chaffaut², Jayne Ireland², Martial Ledoux¹

¹Agence française de sécurité sanitaire des aliments, laboratoire d'études et de recherches sur la qualité des aliments et sur les procédés agroalimentaires, 23 avenue du Général-De-Gaulle, 94706 Maisons-Alfort cedex, France
²Agence française de sécurité sanitaire des aliments, centre d'information sur la qualité des aliments, 27-31 avenue du Général-Leclerc, 94701 Maisons-Alfort cedex, France

Article reçu le 28 Janvier 2010, accepté le 27 Avril 2010

Introduction

In this new context, the Information Centre on Food Quality of the French Food Safety Agency (Afssa-Ciqual) sought to update the French reference food composition tables regarding FA composition of edible oils. Afssa-Ciqual has published the French food composition tables on the Internet, available for consumers, food industries, health care professionals, and other scientists as reference data (Afssa-Ciqual). The aim of this study was to analyse the FA composition of edible oils using an efficient highly polar middle length GC column in order to provide accurate FA profiles of main oils commonly consumed in France at the present time, as well as FA composition of specific new oils.

Material and methods

Chemicals

Sampling

Fatty acid esterification

For the GC analysis, 50 μL of FAME solution was diluted in 1 mL hexane, and then 0.5 μL was injected in GC. Methylation and analysis were performed in duplicate for each sample at the same time.

Chromatographic conditions for FA analysis

A reference solution was made in the lab by spiking a CRM164 FAME solution with a fish oil FAME solution plus some commercial FAME standards (including Supelco #47885-U). This lab-made reference FAME solution was used to determine as many FAME peaks as possible using the BPX-70 column under working conditions. Chromatographic peaks were identified by comparison with several typical chromatograms such as: individual FAME reference chromatograms, CRM164 chromatograms (before and after spiking with reference FAME solutions), as well as chromatograms of both lab-made reference FAME and CRM164 FAME fractions from silver nitrate impregnated thin-layer chromatography [determination of saturated FA, mono-cis FA, trans-FA, and polyunsaturated fatty acid (PUFA) subclasses], and finally chromatograms reported in the literature under the same chromatographic conditions (Vingering and LeDoux, 2009). The lab-made reference FAME solution and Supelco #47885-U standard were injected twice each (at the beginning and at the end of a series sequence, after blanks) during each series of chromatographic runs as qualitative standards to check the FAME retention times and to identify peaks in samples by comparison of chromatograms. The FAME overlaps occurring when using the BPX-70 60-m column were described and discussed earlier (Vingering and LeDoux, 2009).

The correction factors were determined and calculated as previously reported (Vingering and LeDoux, 2009). A control oil with a simple FA profile was made in the lab by mixing non-refined olive and walnut oils. After homogenisation, small portions of the mixture were poured into amber vials and stored under nitrogen at – 28 °C, until use. Ten aliquots of this control oil were first methylated in duplicate following the working conditions and analysed using BPX-70 column to check repeatability and accuracy. A sample of this lab-made control oil mixture is systematically thawed and added as a quantitative control in each series of GC runs to regularly check the performance of the method. All FAMEs were quantified against C9:0 FAME 99% purity as internal standard (Vingering and LeDoux, 2009).

Results and discussion

Fatty acid analysis

All the oil FA composition analyses in the present study were made within the same series: samples were methylated the same day in duplicate and then analyzed on the BPX-70 60-m column within the same run. A typical chromatogram of a vegetable oil is shown in Figure 1.

The use of a 60-m highly polar GC capillary column allowed separation and identification of many fatty acids, including minor FAs, such as trans isomers. Only few authors (Ortiz Moreno et al., 2003; Noor Lida et al., 2002; Christopoulou et al., 2004) used such efficient middle length GC columns for studying FA composition of vegetable oils. Others worked with shorter columns (25-30 m) bonded with average or highly polar phases, the reason why poor information on specific minor FAs, such as TFA, were available in most publications (Juanéda et al., 2007).

Fatty acid composition of vegetable oils

Dubois et al., 2007, 2008 classified vegetable oils within different classes and subclasses according to their FA profiles. Results of this study are presented following this classification.

Oils from MUFA group

Our results regarding rapeseed oil FA composition are similar to the data from literature (Lee et al., 1998; Przybylski and Mag, 2002). Beside high contents of MUFAs with oleic acid as major FA, the rapeseed oil was found to be one of the richest oils in ALA (7.8 g/100 g oil) with a low LA n-6/ALA n-3 ratio (2.4), due to a low content in linoleic acid.

Olive oil is mainly a source of oleic acid (> 70% of total FAs) and shows the lowest content in linoleic acid compared to the other oils analysed in this study. Overall, our results regarding this oil are within the range of French and Greek olive cultivars FA composition (Ollivier et al., 2006, 2003; Boskou, 2002), but slightly lower than Italian, Korean, and Japanese olive oils regarding 16:0 and 18:2 n-6 contents and slightly higher for 18:3 n-3 content (Lee et al., 1998; Mannina et al., 2003; Ranalli et al., 2003). Our results are in accordance with specifications of trade standards for olive oils as defined by the International Olive Oil Council (IOOC). FA composition of various olive cultivars showed significant differences (Ollivier et al., 2003; Mannina et al., 2003), so olive oil FA profiles have been considered as a parameter to characterise a registered designation of origin (RDO) (Ollivier et al., 2006). Moreover, significant differences were observed between olive pulp and seed FA compositions (Ranalli et al., 2003).

Hazelnut oil shows the highest oleic acid contents (72.7 g/100 g oil). Our results are close to the FA profile reported for Turkish hazelnut oil with slight differences: OA content is slightly lower, and both LA and ALA contents are higher in the present study than in the Turkish one (Alasalvar et al., 2003).

In addition to a high oleic acid level, avocado oil shows high contents in two other MUFAs, palmitoleic (16:1 n-7) and cis-vaccenic (18:1 n-7) acid (respectively, 7.7 and 7.1 g/100 g oil), compared to other vegetable oils (from 0.1 to 0.8 g/100 g oil), confirming previous reports (Ortiz Moreno et al., 2003; Bora et al., 2001). Nevertheless, this oil shows the highest palmitic acid (16:0) content (16.9 g/100 g compared to 0.1 - 15.9 for other oils). In our study, the oleic (OA, 18:1 n-9), linoleic (LA, 18:2 n-6), and α-linolenic (ALA, 18:3 n-3) acid contents of avocado oil (respectively, 53.1%, 11.1%, and 0.6% of total FAs) are slightly lower than the average values reported for Mexican avocado (respectively, 60.3%, 13.7, and 1.4%) (Ortiz Moreno et al., 2003). These authors extracted oil from avocado pulp in their lab using four different methods including solvent and/or microwaves and observed a significant influence of extraction procedure on the FA profile. In our study, the analysed avocado oil, labelled as cold pressed oil, was bought in a specialised store and FAME was directly made from the oil sample. Considering individual results by Ortiz-Moreno et al., 2003, our results were close to their “microwaves/squeezing extracted oil” except for ALA which remained higher in Mexican avocado (1.8% of total FAs). A different cultivar from Brazil showed ALA content of avocado pulp oil as low as in our study (0.5% of total FAs) (Bora et al., 2001). These authors also reported that avocado pulp and seed oils showed significant differences in FA composition, the seed oil being much richer in PUFA than the pulp oil, especially both LA and ALA (Bora et al., 2001). Last point, the avocado oil studied here also contained trans-FAs as high as 0.5% of total FAs. Such a relatively high content is at the upper limit for TFA contents in cold pressed oils (Brühl, 1995). Such TFA contents were previously reported while testing various extraction procedures in laboratory, ranging from 0.3 to 0.9% of total FAs depending on the method of oil extraction from avocado pulp (Ortiz-Moreno et al., 2003). As we used a moderate methylation procedure with sodium methoxide as catalyst in small amounts of hexane in order to reduce the risk of generating TFAs during analysis, TFAs from avocado oil were probably present either in avocado pulp or/and generated during process. On the other hand, we did not find any TFA, even as traces, in virgin wheat germ, walnut oils, or olive oil mixtures in the present study (table 2), so the methodology used here did not seem to generate TFAs.

MUFA/SFA + LA subclass

Argan oil, an exotic oil from Morocco, with putative biological properties interesting for human health (Charrouf and Guillaume, 2008), is more or less balanced in OA and LA as major FAs, with respectively, 40 and 33 g/100 g of oil and is also found to be rich in palmitic acid (16:0). Those findings are in agreement with data from literature (Rezanka and Rezankovà, 1999; Hilali et al., 2005). ALA content of argan oil is very low (0.1 g/100 g oil). Such low ALA contents have been observed in some argan oils from different origins (Hilali et al., 2005), but ALA contents as high as 3.8% have also been reported (Rezanka and Rezankovà, 1999).
Table 2 FA composition of analysed vegetable oils (in g FA/ 100 g oil).

Oils from PUFA group

PUFA/LA subclass

PUFA/LA+MUFA subclass

The main fatty acids in “conventional” sunflower oil are linoleic and oleic acids, along with palmitic acid (Noor Lida et al., 2002; Guinda et al., 2003; Gupta, 2002; Wang, 2002), but field-grown mutant sunflower seeds can show much higher contents of oleic or oleic plus palmitic acids (Guinda et al., 2003). Sunflower oils analysed in the present study show an FA composition similar to the conventional sunflower oil profile (Codex Alimentarius).

Like argan oil, sesame oil is balanced in both OA and LA as major FAs, but contrary to argan, LA content was found to be higher than OA content. Such results are in accordance with previously reported data (Kochbar, 2002), even if LA content from the present study (42% of total FAs) was at the lowest limit of results from literature (Lee, et al., 1998; Abou-Gharbia et al., 2000; Alpaslan et al., 2001; Jahaniaval et al., 2000). Nevertheless, our results are within the range of values established by the Codex Alimentarius for sesame oil (Codex Alimentarius). Such low LA contents were also reported from sesame seeds in Turkey depending on the interaction between row space and irrigation effects (Alpaslan et al., 2001). Dubois et al., 2007 classified sesame oil in PUFA group and argan in MUFA group, but average LA content of sesame oil calculated from published data was higher than LA content measured in the present study. This is the reason why both argan and sesame oils, which have close FA compositions in our study, are presented in different groups in table 2.

Contrary to most of the oils analysed in the present study, the tested wheat germ oil is not a mixture, but a unique sample from a traditional local factory producing a virgin cold-pressed oil. However, our results are quite similar to the results previously reported for FA compositions of wheat germ oil extracted either by solvents at cold or hot temperatures (Dunford and Zhang, 2003), or supercritical CO₂ (Panfili et al., 2003). Only small differences are observed, for instance OA content is slightly lower, and both LA and ALA contents are higher in the present study than in the reported data (Dunford and Zhang, 2003; Panfili et al., 2003). Moreover, LA and ALA contents of wheat germ oils extracted by solvents decreased with increasing temperatures during solvent extraction (Dunford and Zhang, 2003). No trans-PUFA is detected in the virgin wheat germ oil analysed in the present study, even as traces.

PUFA/LA+SFA subclass

In addition to the planed work, we analysed two walnut oils individually (walnut I & II, table 1) and an extra walnut oil (organic walnut III, table 1) labelled as virgin cold-pressed oil. Overall, the FA compositions of these three oils conform to our previous findings, including good LA n-6/ALA n-3 ratios (table 2). However, both the walnut I & II oils contained TFAs (respectively, 0.8% and 1.8% of total FAs) and both of these oils showed trans-18:2 as well as trans-18:3, while the FAME chromatograms for organic walnut oil did not show any TFA, even as traces.

Variation factors of oil FA composition

Significant differences in FA profiles were reported for the same kind of oil in relation with several parameters. Among influencing factors, the cultivar and the origin of the oleaginous plant have been quoted (Lee et al., 1998; Ollivier et al., 2006; Ollivier et al., 2003; Mannina et al., 2003; Guinda et al., 2003). In contrast, numerous varieties of a same oleaginous fruit from different geographical origins have been reported to have very close FA compositions (Hilali et al., 2005). Changes in total FA amounts as well as in FA proportions were also observed during development of oilseeds (Chung et al., 1995) and, for a same vegetable, pulp and seed can shows drastic differences in their FA compositions (Ranalli et al., 2003; Bora et al., 2001).

Overview

As a particular attention is now being paid to trans-fatty acids (Mozaffarian et al., 2006; Baylin et al., 2003), we examined the TFA contents of the oils. Olive and argan oil mixtures, as well as organic wheat germ and walnut oils did not present any trans fatty acids even as traces. All other oil mixtures and avocado oil showed low trans-18:2 and trans-18:3 isomer contents (from 0.1 to 0.9 g total TFA/100 g oil) representing about 0.1 to 0.9% of total FAs. Both the individually analysed walnut oils showed TFA contents, with walnut II sample as high as 1.7 g total TFA/ 100 g oil. These trans-PUFA are mainly produced during the deodorisation step of oil refining process (Wolff, 1993a, 1993b). Avocado oil was the only oil to contain trans-18:1 isomers as low as 0.1% of total FAs. Crude vegetable oils with such low TFA contents are not considered to be significant contributors to TFA intakes (Laloux et al., 2007).

Conclusion

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