Oxidative stability of two commercial olive oils of different specificity (green type and black type) has been studied during thermal and photochemical accelerated processes through the evolution of quality indices. It might help to assure a good utilisation of olive oil. In most of works described in literature, they are measured individually. In this study, a Principal Component Analysis (PCA) has been performed to emphasize their variation and describe in concise way the quality and the safety of extra-virgin olive oil after two oxidative stresses. No difference had been detected between both type oils when they are heated. Peroxides, aldehydes and conjugated dienes and trienes were formed but rapidly degraded into final oxidation compounds, mainly acid compounds. During the photochemical process, similar changes occurred slower and the green type oil had shown better stability because of its higher phenolic content. The fatty acids had been more impacted (higher disappearance of monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA)) when the oils were heated than when irradiated. Saturated fatty acids (SFA), MUFA and PUFA were the most relevant indicators to characterize non-oxidized oils and PV characterized the early stage of oil oxidation.
Extra-virgin olive oil (EVOO) is considered to be the best olive oil for its organoleptic characteristics, its stability and its chemical composition because it contains important nutritional elements (fatty acids, vitamins, etc.). The other categories of olive oil have less taste and aroma. Thus, to estimate the chemical oil quality, the International Olive Council (IOC) and the International Organisation for Standardisation (ISO) recommend to measure quality indices such as free acidity (FA) [
Impact of light on olive oils has also aroused the interest of some researchers. Under an artificial light at high intensity, PV has quickly overstepped the European regulation limit for EVOO (≤20 meq O2∙kg−1) and the ortho-diphenols quantities have decreased [
Most of these studies have focused on the effect of either heat or light on the lipid oxidation in olive oils through measurements of some quality indices but results were difficult to compare because of the variability of the initial oil composition and the ageing processes. With this in mind, this work aimed to investigate the influence of two accelerated processes (heating at 180˚C―usual cooking temperature, or exposition at UV irradiation) on the main quality parameters and SFA, MUFA, PUFA, Tyr and HO-Tyr contents obtained every 30 min for 210 min for two commercial virgin olive oils from the same cultivar with different specificity: one “green type” EVOO and one “black type” VOO. The knowledge of the evolution of these parameters all together should help to assure a good utilisation of olive oil. As many parameters were measured and expressed with their own units, it is not easy to compare them. Thus a Principal Component Analysis (PCA) is performed to emphasize their variation and describe in concise way the quality and the safety of extra-virgin olive oil after two oxidative stresses. Furthermore, PCA plots allowed highlighting the quality parameters essential to characterize the oil type and the different oxidation stages of oil.
Two commercial virgin olive oils coming from the same mill (Auriol, France) were used throughout the oxidative studies to avoid the bias due to the parameters influencing the chemical composition other than the olive fruit maturity. These oils were obtained from olive fruits of the same cultivar (Aglandau, the most widespread in the Aix-en-Provence region) harvested during the crop season 2009-2010 at different degree of ripeness, resulting in oil composition differences. The first, an EVOO, named “green type” (G) oil, was obtained immediately after harvesting of olives before or during colour change and the second, a VOO, named “black type” (B) was obtained from olives harvested at maturity but crushed after a controlled fermentation of a few days. Oils of this type have lost the characteristics of fresh fruits but present, nowadays, a specific and well-identified taste and enjoy of a renewed interest from consumers wanting oils with constant gustative qualities, although they do not always meet the criteria of the EVOO [
Five grams of oils were placed in drastic conditions (with bubbling oxygen at 75 mL∙min−1) to accelerate oxidation. For every condition and for every specific period (30, 60, 90, 120, 150, 180 and 210 minutes), the process was performed 5 times, then, the oxidized oils were mixed together to obtain a sufficient volume for chemical analyses.
Oil sample was heated at 180˚C ± 2˚C under stirring in a glass tube, placed in a silicone oil bath. Every time period was counted down after 10 min, in order to take into account the required time so that the oil reaches 180˚C.
Oil sample placed in a glass tube was kept under stirring. A mercury arc lamp (ORIEL, model 6286, 250 watts), equipped with a water filter and an ozone trap, placed at 5 cm from the glass tube, was used to illuminate the sample horizontally in UV domain between 200 and 2500 nm. A concave mirror placed at 5 cm on the other side of the glass tube was used to refocus UV radiations on sample.
Oil samples thermally (T) and photochemically (P) oxidized under different time period were noted T0, T30, ∙∙∙, T210 and P30, ∙∙∙, P210. “FO” designed the fresh oil while “T0” designed the oil heated during 10 min, time to reach 180˚C. For the samples designation on PCA plots, the first letter indicates oil type (G or B), the second, the oxidative process (T or P) and the number the time.
Experiments were realized in triplicates to obtain the mean values ± standard deviation of their chemical indices (PV, AV, FA, K232 and K270), their fatty acid composition and their Tyr and HO-Tyr contents.
Peroxide value (PV), expressed as milliequivalents of active oxygen per kilogram of oil (meq O2∙kg−1), was determined according to ISO standard method 3960 [
Determination of fatty acids composition was based on the analysis by GC of fatty acid methyl esters (FAMEs) prepared according to IOC method [
where Ce is the concentration of the internal standard (2 mg∙mL−1), Ai, the peak area of the methyl ester and Ae, the peak area of the internal standard. The response factors ki are considered identical for all the compounds and so the ratio ki/ke is considered equal to 1. Final results, calculated on the basis of the analyzed oil weight, were expressed in mg∙eq C19∙g−1 oil.
Determination of phenolic content was based on the analysis by HPLC of phenolic extracts of samples. A solution of syringic acid (0.015 mg∙mL−1 methanol/water (80/20, v/v)) was used as internal standard. A solution of Tyr (0.030 mg∙mL−1 in methanol/water) was used to calculate the mean relative response factor of Tyr versus syringic acid (RRFSyr/Tyr) in order to express Tyr and HO-Tyr contents (the two most commonly studied compounds as indicators of polyphenols content) as mg∙eq Tyr∙kg−1 oil as described in the IOC testing methods [
where Ai is the peak area of the compound to quantify, ASyr is the peak area of syringic acid, Wsyr is the weight (mg) of syringic acid in 1ml of internal standard solution added to sample, Woil is the oil weight (g), 1000 a multiplier coefficient to express the final result in mg∙kg−1 oil. RRFSyr/Tyr was calculated according to Equation (3) form a calibration range injected before each series of samples.
where ASyr is the peak area of syringic acid, ATyr is the peak area of Tyr, CSyr is the concentration of acid syringic standard solution and CTyr is the concentration of Tyr standard solution.
All chemicals and reagents used were of analytical grade. Chloroform (≥99%), acetic acid glacial (≥99.5%), diethyl ether (≥99.8%) and cyclohexane (≥99.8%) were obtained from Carlo Erba Reactifs SDS (Val de Reuil, France), isooctane (≥99%) from Sigma-Aldrich (Steichein, Germany). Acetonitrile and methanol (HPLC grade, ≥99.9%) were purchased from Sigma Aldrich (Steichein, Germany). Milli-Q ultrapure water was purified in the laboratory by an ultrapure water purification system (Millipore-Merck KGaA, Darmstadt, Germany). Sodium thiosulfate (≥99%) and potassium iodide (≥99%) were purchased from Sigma-Aldrich, sodium carbonate (≥99.5%) from Carlo Erba Reactifs SDS, potassium hydrogenophtalate (≥99.8%) from Prolabo (Fontenay-sous-Bois, France) and p-anisidine (≥99%) from Alfa Aesar (Johnson Matthey Company, Karlsruhe, Germany). Nonadecanoate methyl ester (≥99%), syringic acid (≥95%) and tyrosol (≥98%) were purchased from Sigma-Aldrich.
Via the ANOVA approach, the experimental data obtained for both green and black type oils were analysed by the F-test to determine the variances equivalence of two data sets differing in the oil types or oxidative processes. The F-test generates a p-value. If p > 0.05 (at 95% confidence) then the null hypothesis that the variances are equivalent cannot be rejected, otherwise, the null hypothesis cannot be accepted. According to the measured parameter, when it can be safely accepted that the variances of the two sets are equivalent, the two oils are considered having the same behaviour.
Principal component analysis (PCA) is a method of data analysis which converts many correlated variables in many uncorrelated variables named principal components (PC) [
ANOVA and PCA were performed using the Unscrambler software version 10.3 from CAMO (Computer Aided Modelling, Trondheim, Norway).
G | B | European regulation limit (EEC, 2013) | ||
---|---|---|---|---|
EVOO | VOO | |||
PV (meq O2∙kg−1) | 15 ± 1a | 21 ± 2b | ≤20 | ≤20 |
AV | 9 ± 1a | 4 ± 0b | ||
TOTOX | 40 ± 3a | 48 ± 3a | ||
FA (% C18:1) | 0.3 ± 0.02a | 0.9 ± 0.1b | ≤0.8 | ≤2 |
K232 | 2.49 ± 0.16a | 2.80 ± 0.17b | ≤2.50 | ≤2.60 |
K270 | 0.17 ± 0.02a | 0.17 ± 0.02a | ≤0.22 | ≤0.25 |
SFA (mg∙eq C19∙g−1) | 148 ± 13a | 148 ± 13a | ||
MUFA(mg∙eq C19∙g−1) | 720 ± 36a | 693 ± 35a | ||
PUFA(mg∙eq C19∙g−1) | 96 ± 5a | 101 ± 6a | ||
Tyr (mg∙eq Tyr∙kg−1) | 80 ± 4a | 24 ± 1b | ||
HO-Tyr (mg∙eq Tyr∙kg−1) | 52 ± 2a | 11 ± 0.4b |
Values are the means ± standard deviation of 5 replicate analyses. EVOO, extra-virgin olive oil; PV, peroxide value; AV, p-anisidine value; TOTOX, total oxidation value; FA, free acidity; C18:1, oleic acid; K232, spectroscopic absorption at 232 nm; K270, spectroscopic absorption at 270 nm; SFA, saturated fatty acids; C19, nonadecanoic acid; MUFA, mono unsaturated fatty acids; PUFA, poly unsaturated fatty acids; Tyr, tyrosol; HO-Tyr, hydroxyl-tyrosol. Values with the same letter in the same line are not significantly different at 95% confidence.
results, given throughout the discussion, are transcribed by a letter in the table: values with the same letter in the same line are not significantly different (P-value > 0.05). The green type oil has values of PV and FA lower than the maximum values indicated for “EVOO” category by the EEC Regulations [
The samples are treated by thermal or photochemical process, and results obtained from chemical analyses are compared to evaluate the impact of the initial oil quality and aging process.
treatment is significantly different. To show the influence of the oil type, P-value was compared to 0.05 (test at 95% confidence). In the heating process, the starting point (noted T0) is when oil reached 180˚C and not the fresh oil at room temperature (noted FO). In beginning of oxidation, PV increases from 15 to 87 meq O2∙kg−1 for green type oil and from 20 to 85 meq O2∙kg−1 for black type oil before to decrease after less half hour of heating. After 210 min PV is 37 meq O2∙kg−1 for green type oil and 29 meq O2∙kg−1 for black type oil. The oil type seems to have any influence on the thermal process because both oils show similar evolution of PV according to the ANOVA test (P-value = 0.98). When the oils are submitted to a photochemical treatment, PV increases linearly and similarly for both oils from 15 to 412 meq O2∙kg−1 for green type and from 21 to 419 meq O2∙kg−1 for black type (P-value = 0.91). No decrease of this index occurs. For both treatments, after 30 min, PV already exceeds the European regulation limit for EVOO [
FA, K232 and K270 indices evolution are respectively represented in Figures 2(a)-(c) for thermal-oxidation and during the photochemical treatment (Figures 2(d)-(f)) for both oils. Their evolutions during thermal ageing are similar for the both oil types. Indeed, the F-test for the equivalence of variances provides p-values greater than 0.05 (respectively 0.74, 0.85 and 0.23). FA is an important quality index used as a criterion for classifying olive oil in various commercial grades [
FA has had the same linear increase for both oil types that reveals the accumulation of acids, the final oxidation products. In contrast, no evolution has been noticeable during the photochemical treatment. Indeed, the average deviation is smaller than the experimental error (respectively 4% and 3% versus 9%).
K232 and K270, rise rapidly in the beginning of thermal oxidation from about 3 to 12.5 for K232 and from 0.5 to 1.7 for K270 in one half hour. Then, these parameters remain almost constant at about 15.5 and 2.5 and widely exceed the European regulation limits, respectively 2.5 and 0.2 for EVOO and 2.60 and 0.25 for VOO [
An important parameter to estimate the oxidative stability is the fatty acid composition of triglycerides and their unsaturation degree in particular. Green type oil and black type oil have initially similar composition because these oils come from olives of the same cultivar. Evolution of the contents of fatty acid of triglycerides over time is represented by unsaturation classes in
Thermal | Photochemical | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
SFA | MUFA | PUFA | SFA | MUFA | PUFA | |||||||
Ga | Ba | Ga | Ba | Ga | Ba | Gb | Bb | Gb | Bb | Gb | Bb | |
FO | 154 ± 13 | 156 ± 13 | 724 ± 36 | 687 ± 58 | 96 ± 8 | 100 ± 9 | 154 ± 13 | 156 ± 13 | 724 ± 62 | 687 ± 58 | 96 ± 8 | 100 ± 9 |
0 | 155 ± 13 | 158 ± 13 | 714 ± 36 | 687 ± 58 | 94 ± 8 | 98 ± 8 | − | − | − | − | − | − |
30 | 140 ± 12 | 141 ± 12 | 600 ± 30 | 584 ± 50 | 57 ± 5 | 61 ± 5 | 143 ± 12 | 160 ± 14 | 641 ± 54 | 694 ± 59 | 84 ± 7 | 101 ± 9 |
60 | 134 ± 11 | 156 ± 13 | 570 ± 28 | 608 ± 52 | 45 ± 4 | 52 ± 4 | 135 ± 11 | 143 ± 12 | 638 ± 54 | 619 ± 53 | 84 ± 7 | 88 ± 7 |
90 | 134 ± 11 | 140 ± 12 | 520 ± 26 | 505 ± 43 | 32 ± 3 | 35 ± 3 | 142 ± 12 | 146 ± 12 | 631 ± 54 | 616 ± 52 | 82 ± 7 | 84 ± 7 |
120 | 127 ± 11 | 129 ± 11 | 466 ± 23 | 454 ± 39 | 25 ± 2 | 26 ± 2 | 141 ± 12 | 141 ± 12 | 630 ± 54 | 595 ± 51 | 80 ± 7 | 69 ± 6c,b |
150 | 136 ± 12 | 129 ± 11 | 450 ± 22 | 456 ± 39 | 24 ± 2 | 22 ± 2 | 136 ± 12 | 140 ± 12 | 611 ± 52 | 590 ± 50 | 76 ± 6 | 78 ± 7 |
180 | 130 ± 11 | 149 ± 13 | 426 ± 21 | 449 ± 38 | 16 ± 1 | 17 ± 1 | 144 ± 12 | 142 ± 12 | 606 ± 52 | 587 ± 50 | 66 ± 6 | 60 ± 5 |
210 | 121 ± 10 | 129 ± 11 | 379 ± 19 | 386 ± 33 | 12 ± 1 | 15 ± 1 | 135 ± 12 | 148 ± 13 | 602 ± 51 | 610 ± 52 | 72 ± 6 | 76 ± 6 |
% | 21 | 17 | 48 | 44 | 87 | 85 | 12 | 5 | 17 | 11 | 26 | 24 |
Values are the means ± standard deviation of 3 replicate analyses. FO, fresh oil; %, overall percentage of decrease. The letter (a or b) characterizes the influence of oil type and treatment. If it is identical for the same treatment, the behaviour of both oils is not significantly different. If it is identical for the same oil for the two treatments, the influence of treatment is not significantly different.
Dobson et al. [
Evolution of Tyr and HO-Tyr contents over thermal ageing time is represented in
content does not evolve over time. Indeed, the average deviation is close to the experimental error (about 5%), and HO-Tyr has been slowly degraded than during thermal oxidation. The persistence of HO-Tyr for longer time (two hours), may explain the significant persistence of hydroperoxides (high PV value) which are not degraded into aldehydes or acids (low value of AV and FA). Indeed, antioxidants as HO-Tyr or Tyr act by reacting with lipid radicals [
To compare the oils behaviour according the ageing processes, the oils oxidative stability and to determine the significant indices of oils quality, a principal component analyse (PCA) has been performed on all aged samples with all indices as variables (
Oxidative stability of two virgin olive oils (green type and black type) has been studied during thermal and photochemical treatments through the evolution of legal quality indices. No differences have been detected for green and black type oils when samples have been heated: black type oil is not worse than green type oil. Nevertheless, green type oil has shown better oxidative stability upon the UV treatment because of its higher phenolic content. Moreover, the heating has more impacted the oil composition than the UV irradiations. The thermal treatment has induced a rapid formation of peroxides and aldehydes degradation of which has lead to great linear increase of FA. An almost complete disappearance of PUFA and a great loss of MUFA were recorded after
210 min. During the photochemical treatment, a greater quantity of peroxides and few aldehydic compounds have been formed. The evolution of PUFA and MUFA were less pronounced. Only FA value of the green type oil remained below the European regulation limit after the UV irradiation. PCA analysis, has demonstrated that the best indicators for characterizing non-oxidized oils are MUFA, PUFA and SFA, FA and AV for control of thermal oxidation and PV and TOTOX for photo-oxidation control. Therefore, from a practical standpoint, these results support the fact that precautions must be taken when cooking to avoid high temperatures for a long time, because different components from degradation can change the taste of some food [
Authors want to thank Pr Jacques Artaud for his advice and knowledge and Noelly Thomas and Sabrina Abdi for their technical support.
Plard, J., Le Dréau, Y., Rébufa, C. and Dupuy, N. (2016) Comparative Study of the Effects of Thermal and Photochemical Accelerated Oxidations on Quality of “Green Type” and “Black Type” French Olive Oils. American Journal of Analytical Chemistry, 7, 890-907. http://dx.doi.org/10.4236/ajac.2016.712076
ANOVA, analysis of variance; C16:0, palmitic acid (hexadecanoic acid); C16:1ω9, hypogeic acid (7-hexadecenoic acid); C16:1ω7, palmitoleic acid (9-hexadecenoic acid); C17:0, margaric acid (heptadecanoic acid); C17:1ω8, margaroleic acid (9-heptadecenoic acid); C18:0, stearic acid (octadecanoic acid); C18:1ω9, oleic acid (9-octadecenoic acid); C18:1ω7, cisvaccenic acid (11-octadecenoic acid); C18:2ω6, linoleic acid (9,12-octadeca- dienoic acid); C18:3ω3, linolenic acid (9,12,15-octadecatrienoic acid); C19, nonadecanoate methyl ester; C20:0, arachidic acid (eicosanoic acid); C20:1ω9, gondoic acid (11-eicosenoic acid); C22:0, behenic acid (docosanoic acid); C24:0, lignoceric acid (tetracosanoic acid); AV, p-anisidine value; FA, free acidity; FAME, fatty acid methyl esters; HO-Tyr, hydroxyl-tyrosol; IOC, International olive council; ISO, international standardisation organisation; K232, spectroscopic absorption at 232 nm; K270, spectroscopic absorption at 270 nm; MUFA, monounsaturated fatty acids; PCA, Principal component analysis; PUFA, polyunsaturated fatty acids; PV, peroxide value; SFA, saturated fatty acids; TOTOX, total oxidation value; Tyr, tyrosol; UV, ultra violet.