3.1. Fatty Acid Profile
The fatty acid profile composition of oils provides information on processing conditions and could be applied for quality control purposes, to identify the purity or the mixture of oils [
41].
Table 1 shows the summary of the fatty acid profile for the three analyzed oils. Regarding unheated samples (T0), great differences can be observed among the three oils; coconut oil is the most saturated (89.9% SFA), rapeseed oil is the most monounsaturated (65.3% MUFA) and grape seed oil is the most polyunsaturated one (63.8% PUFA). These data agreed with the nature of these oils. Martin et al. showed the composition of different oils, finding values of 91.1% SFA for coconut oil, 72.8% MUFA for rapeseed oil and 74.3% PUFA for grape seed oil [
14]. Although certain variability within the same type of oils can be noticed in the fatty acid profiles of oils depending on the variety analyzed, the geographical location and the cultivar conditions, among other factors, the obtained profiles indicate that the samples could be considered as standard for these types of oils.
Regarding healthy aspects, it has to be noted that the presence of trans fatty acid fraction was very low in the three oils (0.01–0.43%). Concerning the unsaturated/saturated and ω-6/ω-3 ratios, relevant differences among the oils were observed. Rapeseed oil showed the best ratio for PUFA + MUFA/SFA ratio, reaching values of 14, and also the best values for ω-6/ω-3 ratio (approximately 2). This oil is poor in ω-3 fatty acids. However, in this case, its higher amount of oleic acid in detriment of linoleic acid, leads to a good equilibrium between ω-6 and ω-3 amounts. Marventano et al. in a review of evidence in human studies of ω-3 and ω-6 PUFA intake on cardiovascular disease, cancer and depressive disorders, concluded that ω-3 PUFAs have been proved to be beneficial, but the role of ω-6 PUFAs needs to be better assessed. These authors pointed out that only a limited number of clinical studies considered the ω-3:ω-6 PUFAs ratio, rather than reporting contrasting results [
42].
Analyzing the effects of cooking and accelerated oxidation conditions (Schaal at 65 °C during 20 days) on the fatty acid composition in the three oils (
Table 1, and
Supplementary Material Tables S1–S3 for the detailed profiles), only slight quantitative modifications were observed, although in some cases they were statistically significant. In the case of SFA, coconut oil maintained values of 89.4 and 90.4% with accelerated storage and cooking, respectively, rapeseed oil values were 6.7 and 6.8% and grape seed oil values were 11.9 and 12.0%. They were all very similar values to those shown for raw samples (89.9%, 6.6% and 11.5%, for coconut, rapeseed and grapeseed oil, respectively). In the case of MUFA, coconut oil showed values of 8.3 and 7.9% (for 20 days storage and cooking, respectively), rapeseed oil 66.0 and 66.7% and grape seed oil 24.6 and 24.7%. Again, these values were very similar to the MUFA amounts found in raw samples (8.1% for coconut, 65.3% for rapeseed and 24.1% for grape seed oil). Finally, in the case of PUFA, values were 2.1–1.5% in coconut, 26.9–26.2% in rapeseed and 62.6–62.3% in grape seed in storage and cooking conditions, respectively. This PUFA fraction was the one that showed the greatest modifications as compared to the values found in unheated samples (2.0%, 27.8% and 63.8% PUFAs in raw samples of coconut, rapeseed and grape, respectively). Coconut oil showed the highest PUFA relative decrease with cooking, around 0.5 g (27%). However, as their presence in the oil is low, this loss does not affect in a relevant manner the general profile of the oil. Pazzoti et al. found that PUFA was the only fraction showing significant modifications in coconut oil after accelerated storage during 20 days [
23], showing a greater decrease as compared to that found in our study (from 3.18% to 1.95%). Decreases of PUFAs in the case of rapeseed oil were 1–1.6 g (3.5–5.8%) and in the case of grape seed oil 1.2–1.4 g (1.8–2%). The analysis of the fatty acid profile of a sunflower and soybean mix oil (50%) heated for 30 h at 180 °C showed decreases in the unsaturated fraction of 2.5% (approx. 2 g) mainly due to the decrease in the amount of linoleic acid [
43].
Indeed, the significant differences found between the raw samples and those cooked or stored at 65 °C were not substantially relevant from a nutritional point of view considering the dietary intake of the different oils and their different fatty acid proportions. However, these differences could explain some of the changes found in the parameters related to the oxidation status.
3.2. Oxidation Status
The intensity of the oxidation process during accelerated storage and cooking treatment was followed through the determination of different chemical markers.
Table 2 show Peroxide values and TBARs values.
Peroxide values, indicating the intensity of the primary oxidation products formation, were 0 in raw coconut oil and also after the accelerated storage, reaching high values only after cooking (20.35 meq O
2/kg of product). A notable increase in PV during cooking coconut oil at 170 °C for 120 min has also been reported [
44]. These results indicate higher stability of the lowest unsaturated oil during accelerated storage. However, in the case of the more unsaturated oils (rapeseed and grape seed), the behavior was very different, with medium values in raw samples (14.57 and 12.23 meq O
2/kg of product for rapeseed and grape seed, respectively), which increased with storage time (18.14–21.86 meq O
2/kg) and decreased for cooking conditions. Additionally, it has to be pointed out that the primary oxidation products would be degraded to secondary products explaining the lowest values of cooked samples found in rapeseed and grape seeds. Kiralan et al. found a PV of 12.2 meq O
2/kg of product in raw cold press grape seed oils, with very significant increases during storage (60 °C 6 days) reaching a PV of 80 meq O
2/kg [
45]. Dedebas et al. obtained PV of 7.25 meq O
2/kg for raw grape seed oils, reaching 21.68 meq O
2/kg after 12 months at 4 °C and 58.24 meq O
2/kg after 12 days at 35 °C [
46]. Maszewska et al., analyzing the oxidative stability of selected edible refined oils (peanut, corn, rice bran, grapeseed, and rapeseed) during storage found a 30% decrease in oxidative stability in all oils after 12 months of storage [
19].
The most widely used method for the determination of malondialdehyde, which is considered the most representative marker of the secondary products of lipid oxidation, is the TBARs method [
3]. Again, coconut oil did not seem to suffer from lipid oxidation during the 20 days of accelerated storage, as no amounts of MDA were detected at times 0, 10 and 20 days. Only cooked samples showed a certain level of TBARs, which was lower than the values shown in the more unsaturated oils.
Regarding the TBARs results obtained for rapeseed and grape seed oils, both in unheated and samples stored at Schaal oven conditions, there were some results that were difficult to explain. Very low values were found for both oils after 20 days of accelerated storage, being lower than those obtained at 10 days of storage and even when unheated, in the case of rapeseed oil. This fact was also found in refined olive oil [
47] and in hemp seed oil [
20] and it was explained as a consequence of the formation of yellow chromophores from the aldehydes (which were formed in high amounts during intense oxidation conditions) which do not absorb at 532 nm. Papastergiadis et al. measuring TBARs in different oils (corn, sunflower, colza and olive) after heating for 6 days at 75 °C, concluded that it is a reliable test to measure the MDA formation, and found the highest values for colza oils (11.75 microg/g) [
48]. Berasategi et al. found TBARs around 2–3 mg MDA/kg for avocado and olive oils (also monounsaturated oils) heated for 9 h at 180 °C [
49]. These results seemed to indicate that the TBARs test, although measuring the amount of MDA adequately, could show other problems depending on the nature of the oil or conditions of the treatment, so the evaluation of the lipid oxidation in food matrices has to be completed with other oxidation parameters.
3.3. Volatile Compounds
The amount and type of volatile compounds are also indicators for the degradation suffered by lipid compounds during oxidation processes. As with the rest of the secondary products, the type of volatile compound formed depends on the fatty acid substrate [
2,
6].
Our study identified a total of 35 compounds including hydrocarbons, aldehydes, ketones, acids, alcohols, furans and terpenes in the different oil samples (
Table 3,
Table 4 and
Table 5 for coconut, rapeseed and grape seed oil, respectively).
Unheated samples showed very few volatile compounds compared with samples subjected to accelerated storage and, especially cooking conditions. Cooking at 180 °C for 90 min increased the total area reported for unheated samples 18-fold (for rapeseed oil), 30-fold (for grape seed oil) and 35-fold (for coconut). However, for accelerated storage conditions, relative increments were lower than for cooking, and no relevant quantitative differences were found between the two times tested (T1 and T2). Particularly, coconut oil increased, with the storage at 65 °C, 3–4 fold the area detected in unheated samples, whereas the two unsaturated oils showed a 10–11 fold increase after 10 or 20 days of storage, respectively.
Looking into the detailed profiles, raw coconut oil showed the lowest number of compounds with just five volatiles identified, 1,2,3-propanotriol triacetate the most predominant (accounting for approximately 50% of total area), followed by terpenes (30% of total area). With cooking, the amount of total volatiles increased significantly, 24 new compounds were detected, highlighting the significant increase in the saturated aldehydes hexanal, octanal, and nonanal, the unsaturated E-2-heptenal, as well as the relevant amount of 2-pentyl furan, all of them markers of oxidation [
20]. Thus, aldehydes accounted for 60% of the total area reported in coconut cooked samples. This sharp increment in volatile aldehydes agreed with that reported by Katragadda et al. [
26] in this oil when heated above its smoking point (175°), suggesting that this oil should not be used for deep-fat frying. Moreover, this increment in aldehydes is related to the values observed for PV and TBARs, which showed significant changes in cooked samples for this oil. However, results found during accelerated storage pointed out a slight degree of oxidation, with increments of mostly aldehydes, that accounted for 30–45% of the total area in T1 and T2 samples, respectively. It was interesting to note, contrary to what was observed in the other two oils, the high amounts of octanoic and hexanoic acids, which are characterized by their “fatty” flavor. These fatty acids have also been previously reported in virgin coconut oil treated by different non-thermal processing methods [
50].
Grape seed oil, and especially rapeseed oil, showed much more appreciable amounts of volatile compounds in unheated samples as compared to coconut oil. In both cases, the aldehydes family was the predominant one, accounting for approximately 65% (grape seed) 75% (rapeseed) of the total area, with hexanal as the major compound, followed by (E,E)-2,4-heptadienal in rapeseed oil and (E)-2-heptenal in grape seed oil. The higher amount of aldehydes in rapeseed oil was in agreement with the highest values found for TBARs in these samples. High values of aldehydes (hexanal and 2-octenal, mainly) were found during accelerated storage of rapeseed oil (16 days at 60 °C) [
51]. Additionally, in accelerated storage, Mildner-Szkudlarz et al. found that secondary oxidation products accounted for approximately 60% of total volatile compounds [
21]. In our work, total aldehydes showed the highest increase during storage at 60 °C and especially as a consequence of cooking at 180 °C, reaching in this case, values of around 80% of the total area in the case of rapeseed and 90% for grape seed. Consequently, ketones and furans were more abundant in stored samples than in those submitted to cooking. The most abundant volatiles found in cooked rapeseed samples were 2,4-Heptadienal, (E,E) 2,4-heptadienal, nonanal, (E,E) 2,4-decadienal. 2,4-hepadienal has been described as a typical linolenic acid oxidation product, and it was reported as highly abundant in rapeseed oil treated at 160 °C for more than 30 min [
52]. However, in the case of grape seed oil, the two decadienal isomers (2,4-decadienal and (E,E)-2,4-decadienal) were by far the most abundant compounds. These two odor-active compounds are associated with fried fat aroma [
53] and oil rancidity [
54]. They form chromophores that absorb at 390 nm in the TBARs reaction, and not at 532 nm [
47], so this method underestimates oxidation detection in those oils that give rise to these compounds. Regarding nonanal, it is related to the decomposition of hydroperoxides formed by the autoxidation of oleic acid [
55] and reached the highest value in cooked samples of the highest monounsaturated oil (rapeseed oil).
Jeleń et al. analyzing volatile compounds in refined and cold press rapeseed oils during 10 days of storage at 60 °C found a significant increase in volatiles, especially of aldehydes, that were higher in the case of refined samples [
22]. They also found that in refined oils, 2-hexenal was the most abundant compound with higher increases than the rest of the aldehydes and that 2-heptenal was correlated with the worst samples of refined rapeseed oil stored during 10 days.
Terpenes are usually present in fresh oils. Their amounts in unheated oil were lower in rape seed oil than in coconut and grape seed oils. In all cases, they significantly decreased during the storage, but mostly with cooking (even disappearing in the case of rapeseed oil). These results agree with other papers in which this family of compounds was analyzed during different treatments of oils. A significant decrease in terpene content was also noticed in sunflower oil when being subjected to deep-frying [
17] and in hempseed oil during 18 days of storage at 60 °C [
20].
In relation to some toxic volatiles formed as a consequence of the degradation basically of ω 3 fatty acids, such as EE-2,4-heptadienal, EE-3,5 octadien-2-one and EE-2,4-decadienal, they were scarcely detected in coconut samples, except for the certain amount of EE-2,4-heptadienal in cooked samples. Regarding the rest of the oils, the highest amounts were found after 20 days of storage and cooked rapeseed samples for EE-3,5-octadien-2-one (which was not detected in grape seed samples) and EE-2,4-heptadienal. EE-2,4-decadienal, as previously stated, reached very high amounts in grape seed samples.