3.1 Sensory analysis of deep-fried oils
To obtain the idea of the overall flavor of the deep-fried vegetable oils, a descriptive aroma analysis was carried out. The consequence can be used to compare the accuracy of the instrumental analyses and sensory evaluations. The analysis provided the flavor profile of the analyzed products, and the strongest intensity was noted for fried note, followed by salty, oily, burnt, cooked vegetables, green grass and pungent. Besides, they exhibited approximately flavor characteristics after frying at a relatively higher temperature. In the light of the overall score, the deep-fried soybean oil (S1) presented a better flavor profile.
PCA analysis (Figure. 1) was applied to the sensory scores, and the first two principal components (PC1 and PC2) explained 72.81% of the total variance, while PC1 (47.50%) got a much better explanation for the samples than PC2 (25.31%). The PCA results differentiated two groups. The first group was formed by soybean oil (S1), palm oil (S2) and corn oil (S4), and distributed in the positive side of PC1. These deep-fried oils were correlated to the sensory attributes of salty, fried, cooked vegetables, pungent and green grass. The second group was formed by olive oil (S3), sunflower oil (S5), camellia oil (S6) and colza oil (S7), and located in the negative side of PC1. They were related to the sensory attributes of oily and burnt. In general, the sensory evaluation of the deep-fried oil presented that a good flavor characteristic could be created by frying, and a significant role was played by the sensory evaluation in contrasting the differences of flavor.
3.2 Volatile compound analysis of deep-fried oils
A comparative analysis of volatile compounds was carried out among green onion deep-fried oils from the 7 kinds of vegetable oils by SAFE-GC-MS, and the corresponding results were showed in Table. 1. A total of 103 volatiles were identified by comparing the retention indices and mass spectrum information with previously reference, including the following 25 aldehydes, 20 alcohols, 4 nitrogen-containing compounds, 15 sulfur-containing compounds, 21 ketones, 3 furans, 5 acids, and 10 other compounds. This indicated that the frying process had a significant effect on the flavor characteristics of vegetable oils.
All the volatile flavor compounds of the 7 kinds of green onion deep-fried oils were quantitated in terms of the internal standards by a semi quantitative method, and presented in the heat map (Figure. 2). Clearly, the content of volatile flavor compounds in the colza deep-fried oil (S7) were higher than the others, and the flavor and fragrance was richer and better. The results showed that the most abundant aroma compounds in the deep-fried oils were (E)-2-heptenal (A30), dimethyl trisulfide (A34), furfural (A44), pyranone (A93), 5-hydroxymethylfurfural (A96), but the other specific compounds were different. Aldehydes and sulfur-containing compounds, such as (E)-2-hexenal (A18), (E,E)-2,4-heptadienal (A46), dimethyl disulfide (A1), and dimethyl trisulfide (A34) were the most prominent compounds in the extracts of the green onion soybean deep-fried oil (S1), and 5-methyl-2-furancarboxaldehyde (A61), nonanal (A35), methyl 1-propenyl disulfide (A26), (E)-1-methyl-3-(prop-1-en-1-yl) trisulfane (A63) were the most prominent compounds in the extracts of the green onion palm deep-fried oil (S2), and (E,E)-2,4-decadienal (A76), (E)-2-nonenal (A53), 2-undecenal (A71) were the most prominent compounds in the extracts of the green onion camellia deep-fried oil (S6). Besides, the major volatile compounds of the deep-fried soybean oil (S1) were 3,4-dimethyl-thiophene (A24), hexanal (A2), 2-pentyl-furan (A21), methyl propyl disulfide (A20), (E)-2-hexenal (A18), hexanoic acid (A81), limonene (A14), 1-allyl-2-isopropyldisulfane (A39), (E)-2-heptenal (A30), 2,4-dimethyl-cyclohexanol (A55), pentyl-oxirane (A7), β-myrcene (A9), 2-heptanone (A11), linalool (A56), linalyl acetate (A58), etc. In general, the volatile compounds of the 7 kinds of vegetable oils changed significantly after frying.
3.3 Electronic nose of green onion deep-fried oils
To screen the effective volatile compounds contributing to the aroma attributes of the 7 kinds of green onion deep-fried oils, the correlations between the chemical classes of volatile compounds and the intensities of the electronic nose were analyzed by PLS(Shiota et al. 2015; Xu et al. 2019). The data were standardized before the analysis and the PLS results showed in Figure. 3. The results indicate that most of the X variables (relative abundance of the chemical classes of volatile compounds) and Y variables (intensities of the electronic nose) are located within the ellipse (r2 = 100%, r2 represents the degree of interpretation)(Pu et al. 2019). A reliable model (Q2 = 0.89 ≥ 0.50) was devised for the intensities of electronic nose and the species of deep-fried oil using all the volatile compounds data(Mimura et al. 2014). The plot (explaining 74.0% of the total variance) suggested a correlation between the chemical classes of volatile compounds and the intensities of the electronic nose, and their species of the deep-fried oils analyzed. And the results (Figure. 3) showed that palm oil (S2), corn oil (S4) and sunflower oil (S5) were positioned in the first quadrant, camellia oil (S6) was placed in the second quadrant, olive oil (S3) and colza oil (S7) were distributed in the third quadrant while soybean oil (S1) was located in the fourth quadrant.
Additionally, chemical groups of volatile compounds and electronic nose sensors were highly correlated and associated to the deep-fried oils. In terms of the electronic nose sensors, Figure. 3 showed positive correlations between deep-fried oil samples and the sensors. In general, Sensor 1 (W1C, aromatic compounds), sensor 3 (W3C, ammonia compounds) and sensor 5 (W5C, aromatic-aliphatic) were related to palm oil (S2) and corn oil (S4), sensor 10 (W3S, methane-aliphatic), sensor 4 (W6S, hydrogen), sensor 8 (W2S, broad-alcohol compounds) and sensor 6 (W1S, methane compounds) were correlated to olive oil (S3) and colza oil (S7). Besides, sensor 2 (W5S, broad-range compounds), sensor 7 (W1W, sulfur compounds) and sensor 9 (W2W, sulfur-chlorine) were closely associated to soybean oil (S1), which agreed with the volatile compound analysis and sensory evaluation results.
In addition, the flavor fingerprints could be established to differentiate the different kinds of green onion deep-fried oils by the electronic nose and GC-MS analysis results. For example, the most important attributes to the flavor of soybean oil (S1) were the sulfur-containing components and aldehydes, included 3,4-dimethyl-thiophene (A24), dimethyl trisulfide (A34), methyl propyl disulfide (A20), and (E)-2-heptenal (A30), etc. The noteworthy components of the flavor of colza oil (S7) were aldehydes, furan and furanones, included (E,E)-2,4-heptadienal (A46), 5-hydroxymethylfurfural (A96), furyl hydroxymethyl ketone (A87), and 3,5-dihydroxy-2-methyl-4H-pyran-4-one (A94), etc. Generally, the electronic nose was effective for executant to differentiate the different kinds of deep-fried oil or the true and false deep-fried oil.
3.4 fatty acids of green onion deep-fried oils
To go a further insight into the constitutes of the deep-fried oils, variations of saturated fatty acids (SFA), unsaturated fatty acids (UFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) were tested and analyzed in the 7 kinds of green onion deep-fried oils (Figure. 4).
According to Figure. 4, the deep-fried oils whose contents of SFA increased after frying included soybean oil (S1), palm oil (S2), olive oil (S3), corn oil (S4), and camellia oil (S6). Among them, the deep-fried palm oil (S2) increased the most. On the contrary, the oils whose contents of SFA decreased after frying included sunflower oil (S5) and colza oil (S7). For the UFA, the contents of soybean oil (S1), olive oil (S3), corn oil (S4), and camellia oil (S6) were increased after frying and the palm oil (S2), sunflower oil (S5), and colza oil (S7) were decreased clearly. Specifically, the deep-fried oilve oil (S3) whose contents of UFA increased the most. For the MUFA and PUFA, the deep-fried oils whose contents of MUFA increased after frying, included soybean oil (S1), olive oil (S3), corn oil (S4), and camellia oil (S6), and the deep-fried olive oil (S3) increased the most. And the oils with incremental contents of PUFA slightly consisted of soybean oil (S1), palm oil (S2), olive oil (S3), corn oil (S4) and camellia oil (S6). Conversely, the oils with descending contents of MUFA and PUFA were sunflower oil (S5) and colza oil (S7). Significantly, the reason of the total amount of UFA of the deep-fried palm oil (S2) decreased was that the decrease of MUFA was significantly higher than the increase of PUFA after frying.
Contrast Analysis of the results of volatile flavor compounds and fatty acids, the noteworthy composition of fatty acids in frying oil impacted the flavor of deep-fried oil products. Among them, UFA (such as oleic acid, linoleic acid, arachidonic acid, etc.) were prone to oxidation due to their double bonds, which resulting in the formation of peroxides(Dobarganes and Márquez-Ruiz 2007). These peroxides are further decomposed to form volatile carbonyl compounds such as ketones, aldehydes, acids, and so on, and resulting in the characteristic flavor profile(Perkins 2007). Fatty acids containing hydroxyl groups are dehydrated and cycled to form lactones with pleasant aroma(Chen et al. 2004). Besides, products from thermal degradation continue to react with proteins and amino acids in the matrix to obtain heterocyclic compounds with special aroma(Mahanta et al. 2021). Above all, linoleic acid and oleic acid were the two unsaturated fatty acids with the highest contents in this research, and which had an important influence in forming note of fried and note of oily in the flavor profile of the deep-fried oils.