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. 2023 Mar 10;12(6):1174. doi: 10.3390/foods12061174

Effects of Three Extraction Methods on Avocado Oil Lipid Compounds Analyzed via UPLC-TOF-MS/MS with OPLS-DA

Yijun Liu 1,†, Qiuyu Xia 2,*, Yangyang Qian 3,†, Yu Kuang 4, Jiameng Liu 1,5, Lijing Lin 1,5,*
PMCID: PMC10048627  PMID: 36981101

Abstract

Avocado oil is excellent functional oil. Effects of three extraction methods (squeezing extraction, supercritical carbon dioxide extraction, and aqueous extraction) on the species, composition, and contents of lipids in avocado oil were analyzed via ultra-performance liquid chromatography–time-of-flight tandem mass spectrometry (UPLC-TOF-MS/MS), and the differential components of lipids were revealed by OrthogonalPartialLeast Squares-DiscriminantAnalysis (OPLS-DA), S-plot combined with variable importance in the projection (VIP). The results showed that the fatty acid composition of avocado oil mainly consisted of oleic acid (36–42%), palmitic acid (25–26%), linoleic acid (14–18%), and palmitoleic acid (10–12%). A total of 134 lipids were identified first from avocado oil, including 122 glycerides and 12 phospholipids, and the total number of carbon atoms contained in the fatty acid side chains of the lipids was 32–68, and the number of double bonds was 0–9. Forty-eight differential lipid compounds with significant effects of the three extraction methods on the lipid composition of avocado oil were excavated, among which the differences in triglycerides (TG), phosphatidylethanol (PEtOH), and phosphatidylmethanol (PMeOH) contents were highly significant, which provided basic data to support the subsequent guidance of avocado oil processing, quality evaluation, and functional studies.

Keywords: avocado oil, lipids, UPLC-TOF-MS/MS, extraction methods

1. Introduction

Avocado is typical subtropical fruit, and “Hass”, “Choquette”, “Gwen”, “Lula” and “Maluma” are the main cultivated varieties, among which “Hass” has the largest planting area. Avocado is rich in nutrients, containing a variety of vitamins, tocopherols, and trace metal elements such as calcium, magnesium, and zinc, and the oil in the avocado pulp mainly consists of various monounsaturated fatty acids and polyunsaturated fatty acids, of which oleic acid accounts for 34% to 81%, 7.2–38.9% for palmitic acid, 6–26.6% for linoleic acid, 2.1–5.8% for linolenic acid, etc. [1,2,3]. Avocado oil has been applied to the development of products that aid in lowering blood pressure, are anti-inflammatory, and promote wound healing, with promising applications [4].

Fatty acids are very important components of avocado oil, and their mechanism of action enhances vascular function, reduces the deterioration of nephropathy, and improves nonalcoholic fatty liver in hypertensive rats by improving mitochondrial dysfunction, reducing mitochondrial oxidative stress, decreasing reactive nitrogen species (RNS) production and normalizing NOx activity [5,6,7]. Cristian et al. [8] used avocado oil instillation in hypertensive rats and reduced diastolic and systolic blood pressure by 21.2% and 15.5%, respectively. In addition, avocado oil not only increases collagen synthesis, reduced the number of inflammatory cells, accelerated the coagulation process and the regeneration of epithelial cells, thus accelerating wound healing [9], but also regulated brain-derived neurotrophic factor (BDNF), oxidative stress and apoptotic molecules, and protected SH–SY5Y cells against cortisol-induced cytotoxicity [10]. Omar et al. [11] used rats as a model for type 2 diabetes and confirmed that lipid components such as oleic acid in avocado oil delayed the development of diabetic nephropathy. Pham et al. [12] isolated DKB122 from avocado oil extract, which effectively inhibited TNF-α or LPS-induced p65 nuclear migration in HEI-OC1 cells and THP-1 cells and reduced TNF-α-induced expression of inflammatory chemokines and interleukin genes.

The functionality of avocado oil was closely related to its nutritional composition, while the nutritional quality of avocado oil was influenced by factors such as fruit variety [3,13], extraction method [14], and fruit storage method [15]. Lozano et al. [13] confirmed that total sterols were higher in immature fruits (1.1–6.2%) than in mature fruits (0.8–2.0%) in four avocado varieties, “Zutano”, “Bacon”, “Fuerte”, and “Lula”. Ultrasonic-assisted water extraction [16], mechanical pressing [17], and supercritical CO2 extraction [18,19,20] methods were commonly used to extract avocado, and the results of a comparative study by Tan et al. [18,19,20] showed that different extraction methods had an effect on physicochemical properties such as iodine value in avocado oil, but did not have a large effect on fatty acid composition such as oleic acid, which varied in content. Fernanda et al. [17] showed that drying of avocado at 60 °C combined with mechanical pressing resulted in better retention of the biological activity of avocado oil. The drying method and storage conditions had a greater influence on the quality of avocado oil. Chaiyavat et al. [21] showed that drying conditions at 80 °C and above had a significant effect on the stability of avocado oil and that light-free conditions helped to extend the shelf life of avocado oil. The stability and quality of avocado oil were susceptible to temperature effects and were not suitable for continuous heating processes [22,23]. However, little research had been reported on the effects of extraction methods on avocado lipid composition. In this study, ultra-performance liquid chromatography–time-of-flight tandem mass spectrometry (UPLC-TOF-MS/MS) was used to investigate the effects of extraction methods on the components of avocado oil quality, to explore the differential components of avocado oil quality by extraction methods, and to provide basic data support for avocado oil extraction methods, product development, and functional studies.

2. Materials and Methods

2.1. Preparation of Avocado Oil

The variety of avocado was “Hass”, purchased from Zhanjiang Chang-da-Chang Super Shopping Plaza Co., and the fruit was 80% mature (skin color changed from dark green to dark brown). Referring to the method of Liu et al. [1], three methods of squeezing extraction, supercritical carbon dioxide extraction, and aqueous extraction were used to extract the oil from avocado pulp, the crude oil was centrifuged in a centrifuge at 5000× g for 10 min, and the collected oil layer was stored at 4 °C. The parameters of that three methods were as follows:

Squeezing extraction: Avocado pulp dried at 55 °C for 24 h was squeezed by the sing screw expeller with normal temperature mode, and the crude oils were collected and centrifuged at 5000× g for 10 min, and the crude oil layer was collected.

Supercritical carbon dioxide extraction: Avocado pulp dried at 55 °C for 24 h was extracted in a supercritical carbon dioxide extractor. The extraction temperature grades I and II were 45 °C and 50 °C, respectively, and the extraction pressure grades I and II were 5 MPa and 6 MPa, respectively, and the crude oil was collected.

Aqueous extraction: A 1 kg sample of avocado oils and 2 kg distilled water were beaten and mixed evenly, and then colloid mill was used for 1 min to obtain slurry solution. Then, 2 kg distilled water was used to clean the machine, and cleaning solutions were collected. The slurry solution and cleaning solution, adjusted to 8.0 with a 1.00 mol/L sodium hydroxide solution, were combined and stirred for 1.5 h at 75 °C water bath, then the mixed solution was centrifuged at 25,000× g for 10 min, and the upper crude oil was collected.

2.2. Instrumentations

Squeezer (OP101, Shenzhen Yimeikang Electronic Commerce Co., Ltd., Shenzhen, China), supercritical carbon dioxide extractor (HSFE-5 + 1, Jiangsu Gaoke Pharmaceutical Equipment Co., Ltd., Nantong, China), high-speed freezing centrifuge (CR22GIII, Hitachi Limited, Tokyo, Japan), juicer (JYL-C020E, Jiuyang Co., Ltd., Jinan, China), pipeline high shear colloid mill (ZVF300-G5R5/P7R5T4MD, Shanghai Qike Machinery Equipment Co., Ltd., Shanghai, China), ultraviolet–visible spectrophotometer (UV-1780, Shimadzu Corporation, Kyoto, Japan), gas chromatography–mass spectrometry (AOC5000-GC/MS-QP2010plus, Shimadzu Corporation, Kyoto, Japan), ultra-high performance liquid chromatograph–time-of-flight tandem mass spectrometer (LC-30A liquid chromatography, Shimadzu Corporation, Kyoto, Japan), ultra-pure water system (Milli-Q-Synthesis, Milli-pore Company, Boston, MA, USA), multi-tube vortex mixer (MTV-100, Hangzhou Aosheng Instrument Co., Ltd., Hangzhou, China), nitrogen blower (DC-24, Shanghai Ampu Experimental Technology Co., Ltd., Shanghai, China).

2.3. Determination of Fatty Acid Composition

The fatty acid composition in avocado oil was determined by potassium hydroxide methylation method with reference to the method of Liu et al. [24]. A sample of 1.0 μL passed through the chromatographic column (DB-FastFA, 30 m × 0.25 mm × 0.25 μm, Agilent, California, USA) in gas chromatography–mass spectrometry with the inlet temperature of 260 °C, nitrogen as the carrier gas, and the split ratio of 20:1. The initial temperature of the column was 150 °C, then it was raised below the program, and the speed of 10 C/min was raised to 210 °C and kept for 8 min, and the speed of 20 °C/min was raised to 230 °C and kept for 6 min. Finally, the sample passed through a detector with a temperature of 280 °C.

2.4. Determination of Lipid Composition

The lipid composition in avocado oil was determined equipped with a Phenomenex Kinete C18 column (100 × 2.1 mm, 2.6 µm, Phenomenex, Torrance, CA, USA) with reference to the method of Liu et al. [24,25]. One microliter of sample was pumped onto the C18 column at a rate of 0.4 mL/min. The column temperature and chamber temperature were 60 °C and 4 °C, respectively. The mobile phases A and B consisted of H2O–methanol–acetonitrile = 1:1:1 (containing 5 mmol/L ammonium acetate) and isopropanol-acetonitrile = 5:1 (containing 5 mmol/L ammonium acetate). The elution program of mobile phase was performed as 20% B for 0.5 min, 40% B for 1.5 min, 60% B for 3 min, 98% B for 13 min, 20% B for 13 min, and 20%B for 17 min.

2.5. Data Processing and Analysis

All samples were measured 3 times in parallel. The qualitative analysis of shotgun-MS data was treated by the LipidView software (v2.0, ABSciex, Concord, ON, Canada). In the process of data analysis, the analysis parameters were set according to the following figures: the mass tolerance was 0.5, the minimum signal-to-noise ratio was 10, the minimum% intensity was 1, the average flow injection spectrum from the top was 30% TIC, and the total double bonds were ≤12. OriginPro (2021, OriginLab Corporation, Northampton, UK) was used for plotting, thermal map analysis, and statistical analysis, and the SIMCA (14.1, Sartorius Lab Instruments GmbH & Co., KG, Goettingen, Germany) was used for PCA, OPLS-DA, VIP, and S-plot analysis, etc.

3. Results and Analysis

3.1. Analysis of Fatty Acid Composition and Lipid Composition in Avocado Oil

The fatty acid composition in avocado oil was determined by gas chromatography-mass spectrometer (GC-MS), and the retention time of each fatty acid standard was characterized with reference to the retention time of each fatty acid standard, and the relative percentage content was calculated according to the normalization method of chromatographic peak area. The fatty acids of avocado oil mainly consisted of oleic acid (36–42%), palmitic acid (25–26%), linoleic acid (14–18%), palmitoleic acid (10–12%), isoleic acid (6–7%), linolenic acid (0.5–0.8%) and stearic acid (0.5–0.6%). The content of saturated fatty acids and unsaturated fatty acids in avocado oil obtained by three extraction methods was about 26% and 73%, among which the content of monounsaturated fatty acids ranged from 54 to 60%.

UPLC-TOF-MS/MS combined with composite scanning mode was used to analyze the lipid composition in avocado oil, as well as the accurate relative molecular weight, isotope distribution, and secondary mass spectrometry fragmentation information. As shown in Figure 1, a total of 134 lipids were identified in avocado oil, including 122 glycerides and 12 phospholipids. Glycerides were composed of diacylglycerol (DG), ether-linked diacylglycerol (EtherDG), triglycerides (TG), oxidized triglycerides (OxTG), triglycerides (TG_EST), and ether-linked triglycerides (EtherTG), and among of which type numbers were 12, 3, 88, 14, 3, and 2, respectively. Phospholipids were composed of phosphatidylcholine (PC), phosphatidylethanol (PEtOH), phosphatidylglycerol (PG), ether-linked phosphatidylglycerol (EtherPG), and phosphatidylmethanol (PMeOH), and among of which, type numbers were 1, 5, 2, 2, and 2, respectively.

Figure 1.

Figure 1

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Lipids of composition of avocado oil.

As can be seen from Table 1, the total number of carbon atoms in the fatty acid side chains of lipids in avocado oil was 32–68, and the number of double bonds was 0–9. The carbon atoms and double bonds number of DG, EtherDG, TG, OxTG, TG_EST, and EtherTG in glycerides were (32–42, 0–5), (34–36, 2–4), (34–64, 0–9), (50–54, 2–5), (66–68, 3–4), and (53–55, 2–5), respectively. The side chain of glycerides was mainly composed of C15, C16, C17, C18, and C19. The carbon atoms and double bonds number of PC, PEtOH, PG, EtherPG, and PMeOH in glycerides were (34, 2), (34–36, 1–4), (32–34, 0–1), (34–37, 3–5), and (34, 0), respectively.

Table 1.

Composition of the 134 lipids in avocado oil.

No. Average Rt (min) Average Mz Lipid Name Adduct Type Formula Ontology
1 5.12 758.56958 PC 34:2 [M − H]− C39H75O8P PC
2 4.089 701.50745 PEtOH 34:1|PEtOH 16:0_18:1 [M − H]− C39H73O8P PEtOH
3 3.864 699.50714 PEtOH 34:2|PEtOH 16:0_18:2 [M − H]− C41H77O8P PEtOH
4 4.114 727.52087 PEtOH 36:2|PEtOH 18:1_18:1 [M − H]− C41H75O8P PEtOH
5 3.76 725.51746 PEtOH 36:3|PEtOH 18:1_18:2 [M − H]− C41H73O8P PEtOH
6 3.493 723.49786 PEtOH 36:4|PEtOH 18:2_18:2 [M − H]− C38H75O10P PEtOH
7 3.834 721.5014 PG 32:0|PG 16:0_16:0 [M − H]− C40H77O10P PG
8 3.939 747.51971 PG 34:1|PG 16:0_18:1 [M − H]− C40H75O9P PG
9 4.044 729.50385 PG O–34:3|PG O–18:3_16:0 [M − H]− C43H77O9P EtherPG
10 3.944 767.52594 PG O–37:5|PG O–16:2_21:3 [M − H]− C38H73O8P EtherPG
11 4.029 687.49158 PMeOH 34:1|PMeOH 16:0_18:1 [M − H]− C38H71O8P PMeOH
12 3.819 685.48138 PMeOH 34:2|PMeOH 16:0_18:2 [M − H]− C38H71O8P PMeOH
13 5.982 586.52954 DG 32:0|DG 16:0_16:0 [M + NH4]+ C35H68O5 DG
14 5.529 584.51715 DG 32:1|DG 16:0_16:1 [M + NH4]+ C35H66O5 DG
15 5.117 582.49945 DG 32:2|DG 16:1_16:1 [M + NH4]+ C35H64O5 DG
16 6.051 612.54742 DG 34:1|DG 16:0_18:1 [M + NH4]+ C37H70O5 DG
17 5.214 608.51532 DG 34:3|DG 16:1_18:2 [M + NH4]+ C37H66O5 DG
18 4.898 606.49695 DG 34:4|DG 16:1_18:3 [M + NH4]+ C37H64O5 DG
19 6.603 640.58044 DG 36:1|DG 18:0_18:1 [M + NH4]+ C39H74O5 DG
20 6.115 638.56628 DG 36:2|DG 18:1_18:1 [M + NH4]+ C39H72O5 DG
21 5.705 636.54767 DG 36:3|DG 18:1_18:2 [M + NH4]+ C39H70O5 DG
22 5.322 634.53503 DG 36:4|DG 18:2_18:2 [M + NH4]+ C39H68O5 DG
23 5.001 632.51526 DG 36:5|DG 18:2_18:3 [M + NH4]+ C39H66O5 DG
24 7.845 722.65991 DG 42:2|DG 24:0_18:2 [M + NH4]+ C45H84O5 DG
25 7.152 596.55109 DG O–34:2|DG O–17:0_17:2 [M + NH4]+ C37H70O4 EtherDG
26 7.219 622.56659 DG O–36:3|DG O–19:1_17:2 [M + NH4]+ C39H72O4 EtherDG
27 6.754 620.55212 DG O–36:4|DG O–19:2_17:2 [M + NH4]+ C39H70O4 EtherDG
28 6.414 628.53979 TG 34:0|TG 10:0_12:0_12:0 [M + NH4]+ C37H70O6 TG
29 6.93 656.57776 TG 36:0|TG 10:0_12:0_14:0 [M + NH4]+ C39H74O6 TG
30 7.455 684.60754 TG 38:0|TG 8:0_14:0_16:0 [M + NH4]+ C41H78O6 TG
31 7.044 682.59131 TG 38:1|TG 10:0_10:0_18:1 [M + NH4]+ C41H76O6 TG
32 7.985 712.63965 TG 40:0|TG 10:0_14:0_16:0 [M + NH4]+ C43H82O6 TG
33 7.559 710.62244 TG 40:1|TG 10:0_12:0_18:1 [M + NH4]+ C43H80O6 TG
34 8.504 740.67419 TG 42:0|TG 10:0_16:0_16:0 [M + NH4]+ C45H86O6 TG
35 8.066 738.65637 TG 42:1|TG 8:0_16:0_18:1 [M + NH4]+ C45H84O6 TG
36 9.019 768.7016 TG 44:0|TG 12:0_14:0_18:0 [M + NH4]+ C47H90O6 TG
37 8.561 766.68695 TG 44:1|TG 10:0_16:0_18:1 [M + NH4]+ C47H88O6 TG
38 8.122 764.67175 TG 44:2|TG 10:0_16:1_18:1 [M + NH4]+ C47H86O6 TG
39 9.469 796.7312 TG 46:0|TG 14:0_16:0_16:0 [M + NH4]+ C49H94O6 TG
40 9.052 794.71594 TG 46:1|TG 14:0_16:0_16:1/TG 12:0_16:0_18:1 [M + NH4]+ C49H92O6 TG
41 8.611 792.70563 TG 46:2|TG 14:0_16:1_16:1/TG 12:0_16:1_18:1/TG 14:0_14:1_18:1 [M + NH4]+ C49H90O6 TG
42 8.191 790.68896 TG 46:3|TG 12:0_16:1_18:2 [M + NH4]+ C49H88O6 TG
43 9.675 810.75385 TG 47:0|TG 15:0_16:0_16:0/TG 15:0_15:0_17:0 [M + NH4]+ C50H96O6 TG
44 9.351 808.73602 TG 47:1|TG 16:0_16:0_15:1 [M + NH4]+ C50H94O6 TG
45 8.953 806.71729 TG 47:2|TG 16:0_15:1_16:1 [M + NH4]+ C50H92O6 TG
46 8.504 804.70636 TG 47:3|TG 15:1_16:1_16:1 [M + NH4]+ C50H90O6 TG
47 9.884 824.7702 TG 48:0|TG 16:0_16:0_16:0 [M + NH4]+ C51H98O6 TG
48 9.483 822.75378 TG 48:1|TG 16:0_16:0_16:1 [M + NH4]+ C51H96O6 TG
49 9.076 820.74121 TG 48:2|TG 16:0_16:1_16:1 [M + NH4]+ C51H94O6 TG
50 8.642 818.7251 TG 48:3|TG 16:1_16:1_16:1 [M + NH4]+ C51H92O6 TG
51 8.279 816.70477 TG 48:4|TG 16:1_16:1_16:2/TG 14:1_16:1_18:2 [M + NH4]+ C51H90O6 TG
52 10.091 838.7887 TG 49:0|TG 16:0_16:0_17:0 [M + NH4]+ C52H100O6 TG
53 9.712 836.7666 TG 49:1|TG 16:0_16:0_17:1/TG 15:0_16:0_18:1 [M + NH4]+ C52H98O6 TG
54 9.019 832.73651 TG 49:3|TG 15:1_17:1_17:1/TG 16:0_16:1_17:2 [M + NH4]+ C52H94O6 TG
55 8.592 830.72235 TG 49:4|TG 15:1_16:1_18:2 [M + NH4]+ C52H92O6 TG
56 10.323 852.8009 TG 50:0|TG 16:0_16:0_18:0 [M + NH4]+ C53H102O6 TG
57 9.918 850.7876 TG 50:1|TG 16:0_16:0_18:1 [M + NH4]+ C53H100O6 TG
58 9.53 848.77069 TG 50:2|TG 16:0_16:1_18:1 [M + NH4]+ C53H98O6 TG
59 9.138 846.75555 TG 50:3|TG 16:0_16:1_18:2 [M + NH4]+ C53H96O6 TG
60 8.741 844.74176 TG 50:4|TG 16:1_16:1_18:2 [M + NH4]+ C53H94O6 TG
61 8.348 842.72314 TG 50:5|TG 16:1_16:1_18:3 [M + NH4]+ C53H92O6 TG
62 10.126 864.80042 TG 51:1|TG 16:0_17:0_18:1 [M + NH4]+ C54H102O6 TG
63 9.751 862.78601 TG 51:2|TG 16:0_17:1_18:1 [M + NH4]+ C54H100O6 TG
64 9.387 860.76953 TG 51:3|TG 16:0_17:1_18:2/TG 15:1_18:1_18:1 [M + NH4]+ C54H98O6 TG
65 9.061 858.75525 TG 51:4|TG 15:1_18:1_18:2 [M + NH4]+ C54H96O6 TG
66 10.709 880.83392 TG 52:0|TG 16:0_16:0_20:0/TG 16:0_18:0_18:0 [M + NH4]+ C55H106O6 TG
67 10.351 878.82001 TG 52:1|TG 16:0_18:0_18:1 [M + NH4]+ C55H104O6 TG
68 9.955 876.80646 TG 52:2|TG 16:0_18:1_18:1 [M + NH4]+ C55H102O6 TG
69 9.595 874.78833 TG 52:3|TG 16:0_18:1_18:2 [M + NH4]+ C55H100O6 TG
70 9.213 872.77362 TG 52:4|TG 16:1_18:1_18:2 [M + NH4]+ C55H98O6 TG
71 8.825 870.75818 TG 52:5|TG 16:1_18:2_18:2 [M + NH4]+ C55H96O6 TG
72 8.441 868.74011 TG 52:6|TG 16:1_18:2_18:3 [M + NH4]+ C55H94O6 TG
73 8.068 866.72589 TG 52:7|TG 16:1_18:3_18:3 [M + NH4]+ C55H92O6 TG
74 9.782 888.80188 TG 53:3|TG 17:1_18:1_18:1 [M + NH4]+ C56H102O6 TG
75 9.435 886.78467 TG 53:4|TG 17:1_18:1_18:2 [M + NH4]+ C56H100O6 TG
76 9.086 884.77094 TG 53:5|TG 17:1_18:2_18:2 [M + NH4]+ C56H98O6 TG
77 11.076 908.86023 TG 54:0|TG 16:0_16:0_22:0 [M + NH4]+ C57H110O6 TG
78 10.718 906.85022 TG 54:1|TG 16:0_20:0_18:1 [M + NH4]+ C57H108O6 TG
79 10.368 904.83752 TG 54:2|TG 18:0_18:1_18:1 [M + NH4]+ C57H106O6 TG
80 9.997 902.82233 TG 54:3|TG 18:1_18:1_18:1 [M + NH4]+ C57H104O6 TG
81 9.637 900.80609 TG 54:4|TG 18:1_18:1_18:2 [M + NH4]+ C57H102O6 TG
82 9.275 898.79083 TG 54:5|TG 18:1_18:2_18:2 [M + NH4]+ C57H100O6 TG
83 8.902 896.77527 TG 54:6|TG 18:2_18:2_18:2 [M + NH4]+ C57H98O6 TG
84 8.536 894.75952 TG 54:7|TG 18:2_18:2_18:3 [M + NH4]+ C57H96O6 TG
85 8.154 892.74481 TG 54:8|TG 18:2_18:3_18:3 [M + NH4]+ C57H94O6 TG
86 7.782 890.72479 TG 54:9|TG 18:3_18:3_18:3 [M + NH4]+ C57H92O6 TG
87 11.417 936.89081 TG 56:0|TG 16:0_16:0_24:0 [M + NH4]+ C59H114O6 TG
88 11.088 934.8833 TG 56:1|TG 16:0_22:0_18:1 [M + NH4]+ C59H112O6 TG
89 10.757 932.86841 TG 56:2|TG 20:0_18:1_18:1/TG 22:0_16:1_18:1 [M + NH4]+ C59H110O6 TG
90 10.391 930.84924 TG 56:3|TG 18:1_18:1_20:1 [M + NH4]+ C59H108O6 TG
91 10.058 928.83551 TG 56:4|TG 18:1_20:1_18:2 [M + NH4]+ C59H106O6 TG
92 9.693 926.82471 TG 56:5|TG 20:1_18:2_18:2 [M + NH4]+ C59H104O6 TG
93 11.417 962.91492 TG 58:1|TG 16:0_24:0_18:1 [M + NH4]+ C61H116O6 TG
94 11.114 960.90045 TG 58:2|TG 24:0_16:1_18:1/TG 22:0_18:1_18:1 [M + NH4]+ C61H114O6 TG
95 10.802 958.88574 TG 58:3|TG 24:0_16:1_18:2/TG 22:0_18:1_18:2 [M + NH4]+ C61H112O6 TG
96 10.483 956.87036 TG 58:4|TG 22:0_18:2_18:2 [M + NH4]+ C61H110O6 TG
97 11.592 976.92822 TG 59:1|TG 16:0_25:0_18:1 [M + NH4]+ C62H118O6 TG
98 11.283 974.91675 TG 59:2|TG 25:0_16:1_18:1 [M + NH4]+ C62H116O6 TG
99 10.991 972.8988 TG 59:3|TG 23:0_18:1_18:2/TG 25:0_16:1_18:2 [M + NH4]+ C62H114O6 TG
100 11.748 990.95013 TG 60:1|TG 16:0_26:0_18:1 [M + NH4]+ C63H120O6 TG
101 11.449 988.93219 TG 60:2|TG 24:0_18:1_18:1/TG 26:0_16:1_18:1 [M + NH4]+ C63H118O6 TG
102 11.153 986.9165 TG 60:3|TG 24:0_18:1_18:2/TG 26:0_16:1_18:2 [M + NH4]+ C63H116O6 TG
103 10.854 984.90588 TG 60:4|TG 24:0_18:2_18:2 [M + NH4]+ C63H114O6 TG
104 11.895 1004.96466 TG 61:1|TG 16:0_27:0_18:1 [M + NH4]+ C64H122O6 TG
105 11.605 1002.95099 TG 61:2|TG 25:0_18:1_18:1/TG 27:0_16:1_18:1 [M + NH4]+ C64H120O6 TG
106 11.322 1000.92993 TG 61:3|TG 25:0_18:1_18:2 [M + NH4]+ C64H118O6 TG
107 11.036 998.91034 TG 61:4|TG 25:0_18:2_18:2 [M + NH4]+ C64H116O6 TG
108 12.054 1018.98224 TG 62:1|TG 16:0_28:0_18:1 [M + NH4]+ C65H124O6 TG
109 11.761 1016.96429 TG 62:2|TG 26:0_18:1_18:1 [M + NH4]+ C65H122O6 TG
110 11.488 1014.94641 TG 62:3|TG 26:0_18:1_18:2 [M + NH4]+ C65H120O6 TG
111 11.205 1012.92883 TG 62:4|TG 26:0_18:2_18:2 [M + NH4]+ C65H118O6 TG
112 11.914 1030.98181 TG 63:2|TG 27:0_18:1_18:1 [M + NH4]+ C66H124O6 TG
113 11.65 1028.96179 TG 63:3|TG 27:0_18:1_18:2 [M + NH4]+ C66H122O6 TG
114 12.073 1045.00183 TG 64:2|TG 28:0_18:1_18:1 [M + NH4]+ C67H126O6 TG
115 11.802 1042.98071 TG 64:3|TG 28:0_18:1_18:2 [M + NH4]+ C67H124O6 TG
116 8.217 864.76392 TG 50:2;1O|TG 16:0_18:1_16:1;1O [M + NH4]+ C53H98O7 OxTG
117 7.791 862.74622 TG 50:3;1O|TG 16:0_18:2_16:1;1O [M + NH4]+ C53H96O7 OxTG
118 8.726 892.79755 TG 52:2;1O|TG 16:0_18:1_18:1;1O [M + NH4]+ C55H102O7 OxTG
119 8.282 890.77795 TG 52:3;1O|TG 16:0_18:1_18:2;1O [M + NH4]+ C55H100O7 OxTG
120 7.479 880.76093 TG 50:2;2O|TG 17:1_17:1_16:0;2O [M + NH4]+ C53H98O8 OxTG
121 7.086 878.7486 TG 50:3;2O|TG 16:0_19:2_15:1;2O [M + NH4]+ C53H96O8 OxTG
122 6.661 876.73419 TG 50:4;2O|TG 16:1_16:1_18:2;2O [M + NH4]+ C53H94O8 OxTG
123 7.923 908.79285 TG 52:2;2O|TG 16:0_19:1_17:1;2O [M + NH4]+ C55H102O8 OxTG
124 7.571 906.77588 TG 52:3;2O|TG 18:1_19:2_15:0;2O [M + NH4]+ C55H100O8 OxTG
125 7.17 904.76459 TG 52:4;2O|TG 16:0_18:2_18:2;2O [M + NH4]+ C55H98O8 OxTG
126 6.794 902.74146 TG 52:5;2O|TG 18:2_19:2_15:1;2O [M + NH4]+ C55H96O8 OxTG
127 7.973 934.80988 TG 54:3;2O|TG 18:1_18:1_18:1;2O [M + NH4]+ C57H104O8 OxTG
128 7.633 932.79602 TG 54:4;2O|TG 18:1_18:1_18:2;2O [M + NH4]+ C57H102O8 OxTG
129 7.278 930.77985 TG 54:5;2O|TG 18:2_19:2_17:1;2O [M + NH4]+ C57H100O8 OxTG
130 10.873 1103.00757 TG 66:3;O2|TG 16:1_18:1_16:0;O(FA 16:0) [M + NH4]+ C69H128O8 TG_EST
131 11.174 1131.03638 TG 68:3;O2|TG 18:1_18:1_16:0;O(FA 16:0) [M + NH4]+ C71H132O8 TG_EST
132 10.912 1129.01904 TG 68:4;O2|TG 18:1_18:1_16:0;O(FA 16:1) [M + NH4]+ C71H130O8 TG_EST
133 9.976 876.83447 TG O–53:2|TG O–17:0_18:1_18:1/TG O–19:1_16:0_18:1 [M + NH4]+ C56H106O5 EtherTG
134 9.249 898.82886 TG O–55:5|TG O–19:1_18:2_18:2/TG O–19:2_18:1_18:2 [M + NH4]+ C58H104O5 EtherTG
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3.2. Analysis of Lipid Content in Avocado Oil

The lipid content of avocado oil obtained by three extraction methods was shown in Figure 2. As shown in Figure 2, the highest TG content in glycerides of avocado oil was (830–960) mg/g, followed by DG at (25–30) mg/g, and the highest PEtOH content in phospholipids was (180–1200) ng/g, followed by PMeOH at (40–545) ng/g. The significant difference results showed that the three extraction methods had the highest effect on the TG, PEtOH and PMeOH contents were highly significant, and the differences for EtherDG and PG contents were not significant.

Figure 2.

Figure 2

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The composed of phospholipids and glycerides in avocado oil from three extraction methods. Note: Different letters a, b and c represented significant difference.

3.3. Modeling and Evaluation of Differential Metabolites of Lipids in Avocado Oil

From Figure 3A, it can be seen that the avocado oil samples obtained by the three extraction methods could be better distinguished in the OPLS–DA model, and the three oil samples were distributed in the first, third, and fourth quadrants, indicating that they differed from each other. From Figure 3B, it can be seen that the lipid composition data obtained by the three extraction methods were subjected to permutation test and cross–validation analysis (CV–ANOVA), the intercepts of R2 and Q2 curves with vertical coordinates were less than one, and the intercept of Q2 in vertical coordinates was less than zero, indicating that the established OPLS–DA model did not show any overfitting phenomenon. In addition, the significance probability value p < 0.05 in CV–ANOVA analysis indicated that the established OPLS–DA model was stable, reliable, and statistically significant [26]. As shown in Figure 3C, the avocado oil obtained from the three extraction methods was well clustered.

Figure 3.

Figure 3

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Scores plot of OPLS-DA model (A), cross-validation of OPLS-DA model (B), HCA (C), and S-plot of lipids with different extraction methods. Sq, Su, Aq in Figure 3A represent squeezing extraction, supercritical carbon dioxide extraction, and aqueous extraction, respectively. (D) Represents S–plot of squeezing extracted and aqueous extracted, (E) represents S–plot of supercritical carbon dioxide extracted and aqueous extracted, and (F) represents S–plot of supercritical carbon dioxide extracted and squeezing extracted. The red dots and green dots in Figure 3D–F indicate metabolites with VIP values > 1, and VIP values < 1, respectively.

The S–plot was used to identify significant differential metabolites between the two samples, and metabolites with large contributions were concentrated at two ends of the S–plot, while those with small contributions were concentrated around the origin [27]. The abscissa and ordinate represented the co-correlation coefficient and correlation coefficient of the principal component and metabolite, respectively. The red dots in Figure 3D–F indicate metabolites with VIP values >1. From Figure 3D, seventeen significantly different components were analyzed between the squeezing extraction and aqueous extraction methods, including eight metabolites with VIP values >2, namely TG 52:2|TG 16:0_18:1_18:1 (68), TG 54:3|TG 18:1_18:1_18:1 (80), TG 52:4|TG 16:1_18:1_18:2 (70), TG 50:2|TG 16:0_16:1_18:1 (58), TG 54:5|TG 18:1_18:2_18:2 (82), TG 50:3|TG 16:0_16:1_18:2 (59), TG 50:1|TG 16:0_16:0_18:1 (57), TG 52:5|TG 16:1_18:2_18:2 (71). From Figure 3 (E), eighteen significantly different components were analyzed between the supercritical carbon dioxide extraction and aqueous extraction methods, including eight metabolites with VIP values >2, namely TG 52:2|TG 16:0_18:1_18:1 (68), TG 54:3|TG 18:1_18:1_18:1 (80), TG 52:3|TG 16:0_18:1_18:2 (69), TG 54:4|TG 18:1_18:1_18:2 (81), TG 50:2|TG 16:0_16:1_18:1 (58), TG 50:1|TG 16:0_16:0_18:1 (57), TG 50:3|TG 16:0_16:1_18:2 (59), TG 52:4|TG 16:1_18:1_18:2 (70). From Figure 3F, seventeen significantly different components were analyzed between the supercritical carbon dioxide extraction and squeezing extraction methods, including ten metabolites with VIP values >2, namely TG 52:3|TG 16:0_18:1_18:2 (69), TG 52:4|TG 16:1_18:1_18:2 (70), TG 54:4|TG 18:1_18:1_18:2 (81), TG 54:5|TG 18:1_18:2_18:2 (82), TG 50:3|TG 16:0_16:1_18:2 (59), TG 50:1|TG 16:0_16:0_18:1 (57), TG 48:0|TG 16:0_16:0_16:0 (47), TG 50:2|TG 16:0_16:1_18:1 (58), TG 52:2|TG 16:0_18:1_18:1 (68), TG 52:5|TG 16:1_18:2_18:2 (71).

3.4. Differential Metabolite Differential Analysis and Mass Spectrometry of Lipids in Avocado Oil

VIP analysis of lipid components in avocado oil obtained by the three extraction methods was performed on the OPLS–DA model, 77 differential metabolites with VIP value > 1 were obtained, and Krural–Walli’s significance test was performed on the 77 metabolites, and 48 significantly different metabolites were obtained. The differential metabolites were subjected to Z–score transformation to standardize the data, Z–score = (original data – mean)/standard deviation, and the standardized data were produced as a heat map, as shown in Figure 4A.

Figure 4.

Figure 4

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(A) Represented the thermogram of lipid components with significant differences among the three extraction methods, and (B) represented the mass spectrogram of PETOH 34:1|PETOH 16:0 _18:1 in negativity mode.

From Figure 4A, it can be seen that the metabolites of the three extraction methods can be categorized into three groups. Groups I, II, and III were the groups with significant upregulation of differential lipid components obtained by the squeezing extraction method, supercritical carbon dioxide extraction method, and aqueous extraction method, respectively, in which 23 lipid components including PC 34:2 (1), PEtOH 34:1|PEtOH 16:0_18:1 (2), PEtOH 34:2|PEtOH 16:0_18:2 (3), PEtOH 36:2|PEtOH 18:1_18:1 (4), TG 60:3|TG 24:0_18:1_18:2/TG 26:0_16:1_18:2 (102), and TG 61:4|TG 25:0_18:2_18:2 (107) were upregulated in group I. Seven lipid components including PEtOH 36:4|PEtOH 18:2_18:2 (6), PG O–37:5|PG O–16:2_21:3 (10), TG 38:0|TG 8:0_14:0_16:0 (30), TG 40:0|TG 10:0_14:0_16:0 (32), TG 40:1|TG 10:0_12:0_18:1 (33), TG 42:1|TG 8:0_16:0_18:1 (35) were upregulated in group II. Twenty–three lipid components including TG 54:3|TG 18:1_18:1_18:1 (80), TG 59:1|TG 16:0_25:0_18:1 (97), TG 64:2|TG 28:0_18:1_18:1 (114), TG 50:3;1O|TG 16:0_18:2_16:1;1O (117), TG 52:2;1O|TG 16:0_18:1_18:1;1O (118), TG 52:3;1O|TG 16:0_18:1_18:2;1O (119), TG 52:2;2O|TG 16:0_19:1_17:1;2O (123), TG 52:4;2O|TG 16:0_18:2_18:2;2O (125), TG 54:3;2O|TG 18:1_18:1_18:1;2O (127), TG 54:5;2O|TG 18:2_19:2_17:1;2O (129) were upregulated in group III.

After the precursor ions of selected lipid molecules enter the mass spectrometry Q2, collision–induced dissociation (CID) occurs at a certain collision energy (CE), resulting in fragment ions, and the neutral loss of specific fragment ions or specific functional groups from lipid molecules lead to diagnostic ions. In this study, the differential metabolite PEtOH 34:1|PEtOH 16:0_18:1 in phosphatidylethanol (PEtOH) was used as an example to analyze its mass spectrometric behavior and fracture mechanism in detail, as shown in Figure 4B. From Figure 4B, m/z 701.5220 corresponded to the mass spectrum information of [M − H]− parent ion of PEtOH 34:1, m/z 125.0009 was phosphoethanol, and m/z 255.2327 and m/z 281.2475 represented the mass spectrum information of [FA 16:0–H]− and [FA 18:1–H]−, respectively.

4. Discussion

There were few studies on the lipid composition in avocado oil, but there were more studies on pitaya seed oil, coffee bean oil, canola oil, and soybean oil. The present study showed that avocado oil was mainly composed of oleic acid (36–42%), palmitic acid (25–26%), linoleic acid (14–18%), and palmitoleic acid (10–12%), similar to the fatty acid composition in avocado reported by Fernandes et al. [28], but there was variability in the fatty acid content, such as low oleic acid content of 10–20%, palmitic acid content was 10–15% higher, and linoleic acid was about 5% higher, with differences in the variety and origin of avocado leading to differences between the two.

In this study, a total of 134 lipid molecules were identified from different extraction methods, which was less than that of pitaya seed oil (152) [24] and cycad oil (169) [29]. Avocado oil was similar to pitaya seed oil in that it consists mainly of glycerides and phospholipids and had the highest content of TG in glycerides and PEtOH in phospholipids, but some variability exists in that avocado oil contained phosphatidylcholine PC, which was lacking in dragon fruit seed oil [24]. Additionally, it was based on the variability of glycerides and phospholipid species in oils and fats that much research work had been completed to identify the source, quality, and variety of oils and fats. Tian et al. [30] analyzed and identified 24 triglycerides, mainly OOO (triglyceride of trioleic acid), OOL (triglyceride of 1,2–dioleic acid–3–linoleic acid), OOP (triglyceride of 1,2–dioleic acid–3–palmitic acid), and other unsaturated triglycerides from six different oil tea species and nine different common oil tea varieties and constructed a fingerprint profile of triglycerides in oil tea seeds. The fingerprint profiles of triglycerides in oil tea seeds were also constructed to identify different varieties of oil tea seed oil. The results of Zhao et al. [31] showed that LL and OO in DAGs and OLLn and LLL in TAGs were important indicators for the grade identification of olive oil, and these indicators could be used for the quality identification of different grades of olive oil. Therefore, the information on the type and content of microscopic lipid components in oils and fats by profiling could provide new ideas and more accurate analysis for the source, type, and quality identification of oils and fats.

The fatty acid content and composition of avocado oil varied depending on the variety, origin [32], and extraction method [1,19,33], with the differences existing in extraction methods being particularly pronounced, yet there were few comparative studies from a microscopic perspective. In this study, 48 differential metabolites were identified from 134 lipid components using OPLS–DA combined with VIP and other methods, among which 23, 7, and 23 differential metabolites were upregulated by the squeezing extraction, supercritical carbon dioxide extraction, and aqueous extraction, respectively, while phospholipids were more abundant in avocado oil obtained by supercritical carbon dioxide extraction, which was in accordance with the principle of similar compatibility. The long extraction process by the aqueous extraction method and the long air contact time resulted in higher OxTG content. In addition, the principles of the pressing method and extraction method were different, resulting in differences in both PMeOH and glycerol ester compounds. Therefore, revealing the differences among the oils and fats obtained by the three extraction methods from the perspective of lipid molecules could provide basic data to support the study of the transformation mechanism of lipid molecules during processing.

5. Conclusions

In this study, the UPLC–TOF–MS/MS was used to profile the lipid profile of avocado oil first, and 134 lipid components were identified, including 122 glycerides and 12 phospholipids. The total number of carbon atoms contained in the fatty acid side chains of the lipids ranged from 32 to 68, and the number of double bonds ranged from 0 to 9. The differences between the three extraction methods were highly significant for the contents of TG, PEtOH, and PMeOH, and not significant for the contents of EtherDG and PG. The analysis by OPLS–DA, S–plot, and VIP identified 44 differential metabolic components, which provided theoretical data for guiding the avocado oil’s processing, quality evaluation, and in–depth functional research.

Author Contributions

Conceptualization, Y.L. and Q.X.; methodology, Y.L., Y.Q. and Y.K.; software, J.L. and Y.K.; formation analysis, Y.L., L.L. and Y.Q.; investigation, Y.L., Y.Q. and Q.X.; resources, L.L. and Q.X.; writing—original draft preparation, Y.L., Y.Q. and Q.X.; writing—review and editing, L.L.; supervision, Y.K. and J.L.; project administration, Y.L. and Q.X.; funding acquisition, Y.L. and Q.X. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding Statement

This research was funded by the Hainan Provincial Natural Science Foundation of China (No. 320QN326), the 2021 Guangdong Science and Technology Innovation Strategy Special Fund (No. 2021A101 and 2021A05217), and the Basic and Applied Basic Research Foundation of Guangdong Province of China (No. 2021A1515010538), and the Key Laboratory of Food Nutrition and Functional Food of Hainan Province (KF202007).

Footnotes

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