Effects of different pressing methods on physicochemical properties and volatile compounds of the apricot kernel oil

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s11694-024-02842-x ” Effects of different pressing methods on physicochemical properties and volatile compounds of the apricot kernel oil Peng-Hui Guo1, Fei-Fei Long1, Qing-An Zhang1*, Juan Francisco García Martín2 1 School of Food Engineering and Nutrition Science, Shaanxi Normal University, Xi'an 710119, China 2 Departamento de Ingeniería Química, Facultad de Química, Universidad de Sevilla, C/ Professor García González, s/n, 41012 Seville, Spain Abstract(150words) In order to efficiently utilize the apricot (Prunus armeniaca L.) kernel, the apricot kernel oil was extracted by two pressing methods, i.e. cold pressing method and hot pressing method. The apricot kernel oils obtained were firstly debitterized and refined, and then their physicochemical properties and oxidation activities were determined by Lovibond tintometer, Rancimat method, differential scanning calorimeter, gas chromatography-mass spectrometry (GC-MS) and gas chromatography-ion mobility mass spectrometry (GC-IMS), respectively. The results indicate that the baking pre-treatment to the apricot kernel in the hot pressing method changed the fatty acid profile, enriched the flavor characteristics, and prolonged the antioxidant properties of the oil. * Corresponding author: [email protected] (Q.-A. Zhang) Keywords: Apricot kernel, oil, pressing method, physicochemical properties, fatty acid profile, volatile compounds. 1. Introduction Apricot (Prunus armeniaca L.), which belongs to the family Rosaceae and originated in China, is widely distributed in Central Asia, West Asia, the Mediterranean basin and Western China. Globally, annual fresh apricot production was 4,260,466 t in 2018, whereas the production of dried apricot was 162,635 t in as per the reports of FAO (2019). Turkey is the world’s leading producer of apricots (127,328 t) (Stobdan et al., 2021). Apricot kernel includes sweet almond and bitter apricot kernel. As the homologous resources for drug and food, the bitter apricot kernels are mainly used as medicinal resources. Regarding the constituents of apricot kernels, they are an excellent oil source, which accounts for about 45-60% of the dry weight. Over 90% of fatty acids of the apricot kernel oil are polyunsaturated fatty acids such as linolenic acid and linoleic acid (Lesia, Liudas, Laurynas, & Victoriya, 2021; Shariatifar et al., 2017). These unsaturated fatty acids have an important physiological function and medicinal value for the human body, not only preventing hyperlipidemia and hypertension, but also being beneficial for the cerebrovascular and intellectual development (Pavlović et al., 2018a). Intake of apricot kernel oil also has anti-inflammatory, antioxidant, and antiapoptotic effects. Besides, it can also protect rat gastric mucosa from ethanol-induced damage, which may help to reduce the severity of gastric ulcers in these animals (Karaboğa, , 2018). In addition, the potential of apricot kernel oil as biodiesel feedstock has been highlighted. For example, the Taguchi method was employed for the optimization of biodiesel production from apricot kernel oil (Gurau & Sandhu, 2018). In order to enrich the variety of oil crops available in China and change the situation of vegetable oil shortage, different oils have been investigated from various plant sources (Yao & Xu, 2021). As a good raw material for oil production, the main drawback of apricot kernels is their amygdalin content, . It is therefore necessary to investigate the physicochemical properties and functions of the apricot kernel oil as well as to provide a reference for its industrial extraction development. Hence, it is of great strategic significance to thoroughly develop industrial procedures to extract apricot kernel oil, so that the current situation in China, where domestic vegetable oils supply is far behind domestic vegetable oils consumption, can be alleviated. In recent years, the extraction of apricot kernel oil has been paid more and more attention, including different methods such as solvent extraction, supercritical carbon dioxide extraction (Özkal, Yener, & Bayındırlı, 2005) and ultrasound-assisted extraction (Azcan & Demirel, 2012). Despite not achieving high oil yields, the static pressure method is considered suitable for oil extraction in the oil and fat industry (Nadeem et al., 2015; Topare et al., 2011). In the meantime, Kiralan Mustafa et al. (2018) evaluated the oxidative and stability of apricot kernel oil under thermal-oxidation and photo-oxidation conditions. Fatty acids, tocopherol, and amygdalin concentrations were determined in apricot kernel oil from two techniques: cold pressing extraction and supercritical CO2 (SC‐CO2) extraction (Pavlović et al., 2018b). The results showed that a low amount of amygdalin content was still present in apricot kernel oil, i.e. the apricot kernel oil obtained from pressing method still entails safety risks. In this study, the apricot kernels were firstly debitterized, then the oil was extracted by two pressing methods including cold pressing method (with untreated apricot kernels) and hot pressing method (with apricot kernels subjected to baking pre-treatment). The physicochemical properties of the two oils were investigated, including the acid value, peroxide value, phospholipid content, color and odor. Furthermore, the fatty acid profile as well as oil flavor composition was analyzed by GC-MS and GC-IMS, respectively. The primary aim is to investigate the composition and the influence of the baking pre-treatment on the apricot kernel oil. Subsequently, the thermal stability of the oil was studied by a differential scanning calorimetry thermal analysis system, and the induction period was predicted by using an oil oxidation analyzer. Overall, the results would be beneficial for the potential application of apricot kernel oil and the use of debitterized apricot kernels. 2. Materials and methods 2.1 Materials and reagents Fresh apricot (Prunus armeniaca L.) kernels were purchased from the Northwest Medicinal Market (Xi’an City, Shaanxi Province, China). Sodium hydroxide, potassium iodide, and activated clay were supplied by Xi’an JingBo Biological Technology Co. Ltd. (Xi’an city, Shaanxi province, China). With regard to the reagents used throughout this research, methanol, ethanol, ether, glacial acetic acid, sodium carbonate, isonicotinic acid, barbituric acid, chloramine T, potassium hydroxide, and cyanide standard solution, all of analytical grade, were purchased from Xi’an JingBo Biological Technology Co. Ltd. Sodium thiosulfate standard titration solution, phenolphthalein indicator, and starch indicator were brought from Shanghai YuanYe Biological Technology Co. Ltd. (Shanghai, China). The compounds used as calibration standard (purity>98%), i.e, amygdalin and gallic acid, were purchased from Chengdu Munster Biotechnology Co. Ltd., (Chengdu city, Sichuan province, China). Distilled water was supplied by Hangzhou Wahaha Beverage Co. Ltd. (Hangzhou city, Zhejiang province, China). 2.2 Oil preparation Fig. 1. Extraction and processing of apricot kernel oil: pressing and refining. Full apricot kernels were immersed in 100 ºC hot water for blanching . Skins were removed by rubbing to obtain the blanched/peeled apricot kernels. Subsequently, 5 kg apricot kernels were debitterized in an ultrasound bath (SB-500DTY, Ningbo Xinzhi Biotechnology Co. Ltd., Ningbo city, Zhejiang Province, China) at 55 ºC, 59 kHz, and 300 W for 2 h (Liu, Shi, & Zhang, 2021). The debitterized apricot kernels were equally divided into two groups: A and B. Kernels of Group A were not treated, while kernels of group B were baked in a SK21NUSE28T-R01A steam oven (Zhongshan YuHeng Electric Co. Ltd., Zhongshan city, Guangdong province, China) at 130 ºC for 30 min, and then put in the pressing equipment. A static press machine (Guangzhou Huamai Machinery Co., Ltd., Guangzhou city, Guangdong province, China) was used to obtain both the crude cold pressing apricot kernel oil (CPO) and the crude hot pressing apricot kernel oil (HPO) as shown in Fig. 1. After the oils were allowed to stand, the impurities were removed by filtration and the oils were cooled down to 4 ºC. 2.2.1 Apricot kernel oil refining In order to improve the quality of the apricot kernel oil, the crude oil must be refined. Oil refining methods applied to crude oil were hydration degumming, alkaline deacidification, and adsorption decolorization (Karim, Othman & Yusoff, 2022). The change of oil appearance during refining is shown in Fig.1. 2.2.1.1 Hydration degumming ydration degumming was performed according to the literature the crude oil was placed in a conical flask on a water bath with a magnetic stirrer, then 3 wt.% water was added to the oil and the mixture was stirred on the water bath for 30 min, thus obtaining the degummed apricot kernel oil. The oil degumming rate was calculated (Wu et al., 2022) as: 𝑋(%) = (𝑀1− 𝑀2)/𝑀1×100 where X is the oil degumming rate (%), and M1 and M2 are crude oil (g/100g) and degummed oil (g/100g), respectively. 2.2.1.2 Alkaline deacidification Free fatty acids are present in crude oil, which can form soaps by neutralization with NaOH solution. To remove them, the degummed apricot kernel oil was placed in a conical flask and heated to 60 ºC in a water bath. Then NaOH solution drops were inserted into the conical flask while stirring, until soap particles appeared in the oil. The acid value (Aν) was determined according to the method of García Martín, López Barrera, Torres García, Zhang, and Álvarez Mateos (2019). The percentage of Aν of the oil was calculated according to the following equation: Aν(mg/g) = (V − V0)× c × 40 𝑚 Where V is the spent volume of NaOH in mL, V0 is the volume of the NaOH standard titration solution consumed for blank determination (mL), c is the molar concentration of the standard titration solution (mol/L), 40 is the molar mass of sodium hydroxide (g/mol), and 𝓂 is degummed oil weight in grams. The oil deacidification rate (Y) was calculated as: 𝑌(%) = (𝐷1− 𝐷2)/𝐷1 ×100where D1 and D2 are the mass of degummed oil and the deacidified oil acid value, respectively. 2.2.2.3 Adsorption decolorization The adsorption decolorization method is the most widely used decolorization process for edible oils. White clay absorbs pigments and other substances to improve the color and quality of edible oils. Activated clay (4 wt.%) was added to the apricot kernel deacidified oil, which was placed in a water bath with stirring at 65 ºC for 15 min. Finally a Lovibond colorimeter (WSL-2, Shanghai Easy Test Equipment Co., Ltd. Shanghai, China) was used to observe the color change of oil before and after decolorization. 2.3 Physicochemical properties Transparency, odor, and taste of oils were determined according to Francesco, Stefania, Luigi & Stefania (2021), while the smoke point was measured following the procedure described by Laurence (2015). Finally, peroxide value determination according to ISO 660.2009, and saponification, moisture, and volatile compounds were determined according to Dong et al. (2022). 2.4 Determination of amygdalin and cyanide Amygdalin in apricot kernel oil was determined by HPLC (Dalian Elite Analytical Instruments Co., Ltd., Liaoning, China) equipped with a column TC-C18 (250 mm × 4.6 mm, 5 μm). According to the available literature (Zhang, Guo, Zhang, Wu & Zheng, 2021), the isocratic elution consisted of 28% (v/v) methanol and 72% (v/v) water solution with a flow rate of 1.0 mL/min. The detection wavelength and the temperature were set at 215 nm and 35 ºC, respectively. 10 mg/mL amygdalin (in methanol) standard solution was diluted with methanol to obtain amygdalin standards for the calibration curve in concentrations of 0.1, 0.3, 0.5, 1.0, and 2.0 mg/mL, respectively. Cyanide is the product of the decomposition of amygdalin. According Fig. 2. Oxidation induction curves of apricot kernel oil measured at different temperatures. As shown in Fig. 2, the temperature had a significant effect on the oxidation induction time of oil, as pointed out by other authors (Riciputi & Caboni, 2017). A temperature shelf life prediction model was established based on the Arrhenius equation, which predicts the shelf life of products through various quality decay factors such as the physicochemical properties or sensory indicators of oils, the equation being: 𝑘 = 𝑘0× 𝑒−𝐸𝑎 𝑅𝑇 Where k is the reaction rate constant; R is the gas molar constant, with a value equal to 8.31 J/(K·mol); T is the absolute temperature, Ea is the activation energy in J/mol, and k0 is the pre exponential factor. According to the Arrhenius model, the degradation rate at the required temperature can be inferred by predicting the degradation rate at three or more temperatures (Sytnik et al., 2019). a b Fig. 3. Linear regression of oxidation induction period ln(IP) against temperature,a: cold pressing oil, b: hot pressing oil. The the induced oxidation period ln(IP) measured by the OXITEST oil oxidation analyzer at temperatures of 90, 100, and 110 ºC were plotted against temperature as shown in Fig. 3. The plots ln (IP) against temperature at 90, 100 and 110 ºC had an excellent correlation, so it can be inferred that the IP at 25 ºC is about 1541 h for CPO and 1600 h for HPO, the time of oil deterioration is about 65 days in CPO and 67 days in HPO after the end of pressing. In addition, the rate of oil oxidation is also influenced by various factors such as light, air, temperature, and fatty acid composition (Nik, Ani, & Masjuki, 2004), which provides a reference for the preservation of apricot kernel oil. 3.3 Differential scanning calorimetry (DSC) and thermal analysis of apricot kernel oil Table 2. Kinetic parameters of thermal analysis of apricot kernel oil at different heating rates. Thermo-oxidative and oxidative stability are important properties of oils and fats for processing, storage, and consumption (Liangliang et al., 2023). In the DSC curves shown in Fig. 4, the oil releases heat during oxidation. The exothermic peak top temperatures obtained at various heating rates were different, and it can be considered that their conversion ratios were almost the same. DSC can assess the oxidative stability of oils by detecting this exothermic process. The Ozawa method and the Kissinger method were used to analyze the DSC curves of apricot kernel oil. As shown in Table 2, the Ozawa method used 1/𝑇𝑝 and lg 𝛽 to make a (x-y) curve to obtain y = - 2.9726x + 6.0526 (R2 = 0.9997) for CPO and y = -3.0512x + 6.1254 (R2 = 0.9964) for HPO, the activation energy of CPO and HPO being 52.257 and 53.638 kJ/mol, respectively, as showed in Table 3. When applying the Kissinger's method, y = -5.6678x - 0.8208 (R2 = 0.9996) for CPO and y = - 5.83x - 0.6846 (R2 = 0.9942) for HPO were achieved, leading to activation energies of 46.087 and 47.421 kJ/mol for CPO and HPO, respectively. CPO HPO Β (K/min) lg 𝛽 𝑇𝑝/K 1/𝑇𝑝 （×103） ln 𝛽 𝑇𝑝2 𝑇𝑝/K 1/𝑇𝑝 （×103） ln 𝛽 𝑇𝑝2 5 0.699 554.93 1.802 -11.028 563.48 1.775 -11.059 10 1.000 589.09 1.698 -10.455 592.88 1.687 -10.467 15 1.301 625.17 1.600 -9.880 633.68 1.578 -9.907 Fig. 4. DSC curves of A (CPO) and B (HPO) apricot kernel oils for heating rates of 5 K·min-1, 10 K·min-1, and 20 K/min. Fig. 5. TG curves of A (CPO) and B (HPO). As shown in Fig. 5, the starting temperatures for decomposition of CPO and HPO were 207 ºC and 216 ºC, respectively. The result was almost consistent with the smoke point illustrated in Table 1. The smoke points of other common edible oils such as refined peanut oil, refined soybean oil, palm oil, refined sunflower oil, and tea seed oil are 232, 238, 230, 227 and 252 ºC, respectively. The lower the smoke point, the poorer the oxidative stability of the oil, the higher the concentration of unsaturated fatty acids in the oil, and the easier the oxidation and decomposition of low-molecular volatile components when heated under air. The heating rate has a significant impact on the size, position, and shape of the TG curve of apricot kernel oil, and the decomposition rate decreases with the increase of heating rate. The mass loss rate of oil can be calculated by the weight loss mass fraction of oil at different temperatures. There is a transition state from the original reaction system to the intermediate stage of the product, and the energy difference between this transition state and the original system is the activation energy E, the level of activation energy reflects the level of thermo-oxidative stability. The larger the activation energy, the more difficult the oil to be oxidized during storage (Jadhav Harsh et al., 2022). From the comparison with data in Table 3, it can be found that the activation energy of apricot kernel oil obtained by DSC thermal analysis is significantly lower than those of peanut oil (Wang et al. 2001), palm oil (Kuang et al. 2019), and arowana oil (Leonardis & Macciola, 2012). Results showed that apricot kernel oil may not be suitable as a commonly used frying oil. The reason may be that the main fatty acids in apricot kernel oil are linoleic acid and oleic acid, so that the unsaturated fatty acid content is over 92 wt.% (Tab. 4), which can well explain the results of DSC analysis. Table 3. Analysis of activation energy of the oxidation reaction of different oils. *From Wang et al. (2001); **From Kuang et al. (2019); ***From Leonardis & Macciola (2012). 3.4 Fatty acid composition of apricot kernel oil The fatty acid composition depends on the physicochemical properties, stability, and usage field of the edible oil. As seen in Table 4, the percentages of unsaturated fatty acids (UFA) in CPO and HPO were approximately 90%, which verified that apricot kernel oil was indeed a good quality edible oil. The most representative UFA in CPO and HPO were oleic acid (C18:1) and linoleic acid (C18:2n6c) while palmitic acid (C16:0) and stearic acid (C18:0) were the main saturated fatty acids (SFA) (Table 4). These results are similar to those found in a previous research (Chowdhury, Banu, Khan & Latif, 2008). The contents of linoleic and oleic acid were higher than in a large number of edible oils, such as rice bran oil, olive oil, soy oil, and sunflower oil (Li et al., 2015). Oleic acid was the MUFA with the highest content in apricot kernel oil. Its content affects the quality of the apricot kernel oil. It comes from saturated fatty acid during the growth and development of the oilseed fruit. Linolenic acid has obvious effects of lowering blood pressure, anti atherosclerosis and improving intelligence, and is considered as an important indicator of the value of vegetable oil. Generally speaking, the higher the oleic acid content in vegetable oil, the lower the linoleic and linolenic acids Activation energy (kJ/mol) CPO HPO Peanut Oil* Palm Oil** Arowana oil*** Ozawa method 52.257 53.638 73.742 149.73 144.70 Kissinger method 46.087 47.421 70.375 145.82 139.19 content, and the better its oxidation stability. Vegetable oils with high oleic acid usually have better oxidation stability (Hunter, Crosbie, Weir, Miller, & Dutta-Roy, 2000). By comparing the contents of oleic acid and linoleic acid in CPO and HPO, it can be concluded that the oxidation stability of HPO is better than that of CPO. This can be explained by the fact that heat pretreatment barely caused UFA (mainly MUFA) oxidation and decomposition, hence having little effect on overall nutritional value. Table 4. Fatty acid (relative content, %) composition of CPO and HPO. Fatty acids CPO HPO Caprylic acid (C8:0) 0.01±0.002b 0.01±0.003b Capric acid(C10:0) 0.01±0.00b 0.01±0.004b Undecanoic acid(C11:0) 0.08±0.04b 0.17±0.09b Lauric acid(C12:0) 0.01±0.00b 0.01±0.004b Tridecanoic acid(C13:0) ND ND Myristic acid(C14:0) 0.01±0.00b 0.04±0.01b Pentadecanoic acid(C15:0) 0.01±0.00b 0.05±0.01b cis-10-Pentadecenoic acid(C15:1) ND 0.004±0.00b Palmitic acid (C16:0) 7.72±0.03b 7.15±0.10b Palmitoleic acid(C16:1) 0.01±0.01b 0.03±0.01b Heptadecanoic acid(C17:0) 0.03±0.01b 0.04±0.004b Cis-10-Heptadecenoic acid(C17:1) 0.09±0.04b 0.19±0.01b Stearic acid (C18:0) 1.88±0.64b 1.72±0.03b Oleic acid (C18:1n9c) 41.93±0.83a 46.74±0.30a Linoleic acid(C18:2n6c) 47.87±0.16a 43.45±0.23a γ-Linolenic acid(C18:3n6) 0.03±0.01b 0.07±0.01b Arachidonic acid(C20:1n9c) 0.18±0.05b 0.16±0.01b Different letters in the same row indicate significant differences (p < 0.05) 3.5 Volatile composition of apricot kernel oil By using the pattern difference comparison mode, as shown in Fig. 6, the spectrum of CPO is used for comparison. The spectrum of HPO is subtracted from the reference. Fig. 6. GC-IMS spectrum of volatile organic compounds in (A) CPO and (B) HPO. α-Linolenic acid(C18:3n3) 0.02±0.01b 0.02±0.001b Henicosanoic acid(C21:0) 0.01±0.00b 0.02±0.002b Behenic acid(C22:0) 0.01±0.00b 0.01±0.00b Tricosanoic acid(23:0) 0.03±0.02b 0.06±0.004b Lignoceric acid(24:0) 0.02±0.01b 0.05±0.01b ∑SFA 9.82 9.34 ∑MUFA 42.21 47.10 ∑PUFA 47.92 43.54 ∑UFA 90.16 90.64 (Difference comparison) After deduction, the white part of the spectrum is the component with the same volatile component. The red part represents the part in the HPO where the concentration of volatile compounds is higher than that in the CPO, and the blue part represents the part in the HPO where the concentration of volatile compounds is lower than that in the CPO. As shown in Fig. 7a, a total of 45 volatile compounds were identified in HPO, including 36 compounds found in the database and other 8 unknown compounds. These 36 compounds include 12 alcohols, 10 aldehydes, 8 ketones, 3 pyrazines, 2 esters, and 1 acid. Many peaks and peak signal distributions were similar in double samples, while most of the peak signal intensities in HPO were significantly higher than those in CPO. The compounds labeled in Table S1 were identified using the built-in NIST and IMS databases in the GC-IMS database, and each point represents one particular volatile organic compound. The results indicated that the alcohols and ketones were mostly perceived in apricot kernel flavor, but benzaldehyde and its dimer play the major role in apricot kernel oil, which was also the source of the most important special flavor of apricot kernel oil (Pawar & Nema, 2023). The same fact was also verified in CPO. 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Compound Formula MW RI Rt [sec] Dt [a.u.] Peak intensity A B (E)-3-hexen-1-ol C6H12O 100.2 852.5 328.255 1.25963 161.59±2.96 169.05±7.93 (Z)-3-Hexen-1-ol C6H12O 100.2 867.2 344.468 1.22023 20.93±0.44 183.19±0.88 1-butanol C4H10O 74.1 658.4 163.525 1.18147 862.15±17.65 673.87±42.20 1-pentanol C5H12O 88.1 766.1 238.583 1.25227 296.27±10.21 595.72±15.10 2,3-diethyl-5-methylpyrazine C9H14N2 150.2 1174.8 904.512 1.27721 240.34±1.58 237.92±15.47 2-Butanone monomer C4H8O 72.1 585.3 131.585 1.23891 991.05±19.09 7691.31±56.69 2-Butanone dimer C4H8O 72.1 582.4 130.308 1.06125 1794.82±7.28 1919.21±12.51 2-heptanone monomer C7H14O 114.2 888.7 368.078 1.62237 811.18±29.13 1419.29±16.77 2-heptanone dimer C7H14O 114.2 890.4 370.054 1.2575 1683.22±11.22 1993.27±9.09 2-hexanol monomer C6H14O 102.2 788.8 258.066 1.55684 4747.11±24.49 6864.85±27.32 2-hexanol dimer C6H14O 102.2 789.7 259.025 1.28166 2907.02±19.47 3067.49±42.19 2-hexanone monomer C6H12O 100.2 777.3 247.526 1.49406 123.32±3.11 199.72±2.22 2-hexanone dimer C6H12O 100.2 777.3 247.526 1.19082 349.93±11.72 347.19±13.03 2-methyl-1-propanol monomer C4H10O 74.1 623.1 148.091 1.37028 1146.77±7.46 1035.04±103.89 2-methyl-1-propanol dimer C4H10O 74.1 620.5 146.955 1.17092 2061.64±3.17 1085.93±6.16 2-methylbutanal C5H10O 86.1 653.3 161.289 1.39788 550.15±2.77 5410.43±15.74 2-pentanone C5H10O 86.1 678.4 172.239 1.12172 286.06±7.92 171.69±21.50 3-methylbutanal C5H10O 86.1 637.2 154.262 1.40055 758.58±31.49 5288.24±54.66 acetic acid C2H4O2 60.1 558 119.682 1.14373 199.83±25.28 718.54±26.04 acetone C3H6O 58.1 526.6 105.955 1.11061 1674.73±84.94 3196.42±155.32 benzaldehyde monomer C7H6O 106.1 967.4 505.426 1.46306 97279.23±586.47 96990.45±258.34 benzaldehyde dimer C7H6O 106.1 967.9 506.414 1.14273 33761.51±109.37 33180.90±153.70 ethanol C2H6O 46.1 526.6 105.955 1.04497 5424.27±78.17 3464.48±26.92 ethyl 2-methyl propanoate C6H12O2 116.2 750.1 225.807 1.19617 151.14±2.59 167.57±15.97 furfurol C5H4O2 96.1 852.1 327.832 1.08127 156.47±5.25 686.78±7.16 heptanal monomer C7H14O 114.2 897 380.579 1.68774 139.95±3.93 347.11±8.20 heptanal dimer C7H14O 114.2 898.3 382.79 1.35455 855.62±6.27 1246.25±4.91 isopentyl alcohol monomer C5H12O 88.1 721.1 202.638 1.49197 28.75±1.43 138.03±4.87 Isopentyl alcohol dimer C5H12O 88.1 720.4 202.069 1.23306 46.24±2.15 261.04±23.15 methyl acetate C3H6O2 74.1 537.8 110.842 1.17742 429.85±7.98 437.31±35.80 methyl pyrazine C5H6N2 94.1 820.4 292.881 1.08515 59.61±5.39 104.30±40.46 n-nonanal monomer C9H18O 142.2 1105 768.627 1.93187 95.67±4.64 172.76±23.40 n-nonanal dimer C9H18O 142.2 1105 768.627 1.49051 1172.94±58.46 1731.73±34.90 pentanal monomer C5H10O 86.1 692.3 179.626 1.41171 951.05±0.99 2020.80±71.52 pentanal dimer C5H10O 86.1 692.3 179.626 1.20717 1449.98±38.37 1908.60±18.86 propanal C3H6O 58.1 491.8 90.752 1.15398 332.55±25.15 446.86±19.92 trimethylpyrazine C7H10N2 122.2 994.2 553.03 1.61915 725.45±25.17 737.75±6.63