EDIBLE OIL ANALYSIS
Introduction

A fast and non-destructive quality control was desired by edible oil producers and processors. Former researches described nuclear magnetic resonance (NMR) as a technique extending the analysis of edible oils [1-6].

On the one hand, in this research a proton nuclear magnetic resonance (1 NMR) method was developed and validated for target control of essential quality parameters within short time and in only one analytical run. Two completely new parameters were established during the research project: the aldehyde and the double bond value. The aldehyde value described the content of aldehydes independently of the aldehyde kind. The mean number of double bonds per fatty acid (FA) was described with the double bond value. These values could replace the conventional AV and IV.

Quality Control
On the other hand an authenticity control was developed identifying the kind of oil. With the use of multivariate models like Principle Component Analysis (PCA) the genuineness of edible oils was determined by visualizing the separation of the different oil types

Quality control of edible oils.

Fig. 1 1H NMR spectrum of olive oil, inclusively detailed schematic of quality parameters.

Types and number of collected samples were: Linseed Oil (8), Sunflower Oil (8), Walnut Oil (8), Rape Seed Oil (7), Almond Oil (5), Pumpkin Seed Oil (4), Peanut Oil (4), Hemp Oil (4), Black Seed Oil (4), Gold-of-Pleasure Oil (4), Sesame Oil (4), Mustard Seed Oil (4), Argan Oil (3), Chia Oil (3), Hazelnut Oil (3), Moringa Oil (3), Poppy Seed Oil (3), Evening Primrose Oil (2), Olive Oil (24), Pistachio Oil (2), Macadamia Nut Oil (2), Grape Oil (2), Safflower Oil (2), Apricot Kernal Oil (2), Avocado Oil (1), Borage Oil (1), Chili Oil (1), Dill Seed Oil (1), Germ Oil (1), Coconut Oil (1), Corn Oil (1), Carrot Seed Oil (1), Nangai Oil (1), Rapeseed – Chili – Oil (1), Rice – Germ – Oil (1), Castor Oil (1), Soy Oil (1), Tamanu Oil (1), Omega 3-6-9 Power Mix (1)
Statistical measurements were performed only on oils of which at least two samples were available.
The fatty acid (FA) profile was a very important quality parameter of edible oils. The relation of the different fatty a cids could determine the kind of oil, the nutritional value and physical properties. The results from this study concerning the saturated, monosaturated, C18:2 and C18:3 FAs [mol-%] of analyzed oils were presented in Fig. 2. The saturated FA of the analyzed edible was determined in a range of 6.98 to 21.45 %, monosaturated FA of 0.00 to 80.66 %, C18:2 FA of 0.68 to 72.22 % and C18:3 FA of 0.24 to 57.50 %. In Fig. 6 the FA profiles were compared. Some oils showed similar profile which could not be distinguished by eye. With the use of multivariate models like Principle Component Analysis (PCA) the genuineness of edible oils was determined by visualizing the separation of the different oil types.
In Fig. 6 the FA profiles were chemometrically compared.

Fig. 2 Saturated FA (blue), C18:1 FA (red), C18:2 FA (green) and C18:3 FA (purple) of different kinds of edible oils (mean ± deviation)

The 13C NMR spectroscopy is a versatile tool to identify and quantify the distribution of triacylglycerides (TAGs) with the respective isomers sn-1/3 and sn-2 (Fig. 7 to Fig. 9). TAGs like saturated (sat.) and unsaturated (unsat.) fatty acids can be distinguished. Between C16:0 and C18:0 a distinction is not possible. Unsaturated fatty acids can be distinguished into C18:1, C18:2, C18:3, γ-Linolenic acid (ALA), γ-Linolenic acid (GLA), C20:1, C22:1 and C22:1.
Gold-of-Pleasure oil is a representative oil, rich in γ-Linolenic acid. Borage oil is rich in γ-Linolenic acid.
Fatty acid profile analyzed by 13C NMR

Fig. 3 13C NMR spectra from Gold-of-Pleasure Oil (blue) and Borage Oil (red)

Iodine Value – an obsolete value? The iodine value (IV) in oils described the amount of iodine which is absorbed by a fat or oil determining the amount of unsaturation in fatty acids. Unsaturated fatty acids contained double bonds which reacted with iodine compounds. The higher the iodine value, the more double bonds were present in the fatty acid.

Problematically was that other olefinic double bonds in by-products like sterols were also quantified leading to an increased, falsified IV. Fig. 4 described the comparison of the conventional IV values and the 1H NMR values of double bond integrals.
Conventional Quality Parameter

Fig. 4 Comparison of specific NMR integrals with the conventional iodine value.

Due to the falsified values by simultaneous quantification of other double bond containing substances, the double bond value (DBV) was provided. This value described the mean amount of double bonds per fatty acid in the analyzed oil.
Conventionally, molecular weights of TAGs were determined with liquid or gas chromatography. But, the molecular weight could be determined also with 1H NMR spectroscopy. The three TAGs Glyceryl trioleate, Glyceryl tristearate and Glyceryl trinonadecanoate with known molecular weights were analyzed and quantified with 1H NMR (Tab. 1). The from literature known molecular weights were compared with the quantified molecular weights (Fig. 5). The comparison proved that the NMR spectroscopy is a versatile tool for the determination of molecular weights of edible oils. Using NMR spectroscopy for molecular weight analysis was that all parameters except of the fatty acid distribution were determined in only ONE analysis run leading to VERY FAST RESULTS.

Molecular weight

Tab. 1 Molecular weights of TAGs.

Fig. 5 Comparison of quantified and known molecular weights of TAGs.

Authenticity control
Edible oils were analyzed for their FA profile being measured without an internal standard. The FA profile of 10 different edible oil types (peanut, hazelnut, pumpkin seed, gold-of-pleasure, linseed, almond, olive, rapeseed, sunflower, walnut oil), mostly used in households, were measured with 1H NMR and evaluated with Principle Component Analysis (PCA) due to the high complexity and signal overlapping. For the statistical evaluation the signals at d 0.85; 2.00; 2.75 and 2.80 ppm were used. The samples could be separated into clusters identifying and confirming the authenticity of edible oils. The first three components described 99.89% of the total variance (Fig. 6).

Fig. 6 PCA scores plot on the first and the second principle components for 1H NMR integrals of the fatty acid profile of edible oils, most frequently used in the household.

Another PCA was performed for oils with identical or very similar FA profile, divided in Tab. 2. The first two components described 88.28% of the total variance Fig. 7 illustrated the PCA score plot of edible oils with the four similar FA profile groups. The clusters showed a wide internal dispersion. Group 1 (argan and peanut oil) and group 3 (pumpkin and black seed oil) respectively could not be distinguished by the PCA. In group 2 (apricot, hazelnut and sunflower oil), the sunflower oil could be separated by the clusters of apricot and hazelnut oil. Almond and rape seed oil, group 4, could be distinguished by their separated clusters.

The 1H NMR results supported the capability of NMR combined with chemometric techniques to be used in quality assessment and authenticity control of edible oils and in detection procedures of adulteration and incorrect labelling.

Fig. 7 PCA scores plot on the first and the second principle components for 1H NMR integrals of the fatty acid profile of edible oils with similar profile.

Tab. 2 Classification of oils with similar fatty acid profiles.

Adulteration Control

Fig. 8 PLS scores on the first and the second PLS components for cross validation for olive-sunflower blends. Ellipsoids show 95%
probability for each “spiking group” (defined as an average sunflower oil content in each group).

Fig. 9 Predicted-reference plot and 95% prediction bands (solid lines) obtained from spiking experiments.

References

[1] Diehl, B., Skiera, C.; Nuclear Magnetic Resonance (NMR)-Spektrokopie. [book auth.] B. Matthäus and H.-J. Fiebig. Speiseöle und -fette; Recht, Sensorik, Analytik. EU : Erling, 2013, pp. 169-180.
[2] Diehl, B.; Multinuclear high-resolution nuclear magnetic resonance spectroscopy. [book auth.] R. J. Hamilton. Lipid Analysis in Oils and Fats. : Blackie Academic & Professional, 1998, pp. 87-135.
[3] Skiera, C., et al.; 1H NMR approach as an alternative to the classical p-anisidine value method. Eur Food Res Technol. 2012, 235, pp. 1101–1105.
[4] Skiera, C., et al.; 1H NMR spectroscopy as a new tool in the assessment of the oxidative state in edible oils. J Am Oil Chem Soc. 2012, 89, pp. 1383-1391.
[5] Skiera, C., et al.; Determination of free fatty acids in edible oils by 1H NMR spectroscopy. Lipid Technol. 2012, 24, pp. 279-281.
[6] Guillén, M., Ruiz, A.; Edible oils: dicrimination by 1H nuclear magnetic resonance. J. Sci. Food Agric. 2003, 83, pp. 338-346.