1. Introduction
Caryocar brasiliense Cambess, commonly known as pequi, is a fruit of the pequizeiro tree that grows abundantly in Brazil’s Cerrado (savanna) [
1,
2]. Pequizeiro is a leafy tree that can reach a height of 10 m, with twisted medium-sized stems and wide leaves [
3]. Its fruits have a greenish epicarp (peel), yellowish mesocarp (pulp), brownish endocarp (thorny), and creamy chestnut hue [
3]. Pequi is an excellent source of nutrients because it contains proteins, lipids, dietary fibers, carbohydrates, vitamins, carotenoids, and phenolic compounds [
4,
5,
6]. Pequi almonds contain approximately 50% oil, which may be utilized to produce food, cosmetics, chemicals, and biofuels [
1,
2,
7,
8]. Rabbers et al. [
9] reported that pequi oil is high in unsaturated fatty acids and possesses anti-inflammatory, antibacterial, antifungal, curative, antioxidant, and antitumor properties [
10].
Studies have shown that pequi oil can be used in conjunction with physiotherapy to treat inflammation, lung infections, muscular discomfort, rheumatoid arthritis, and bruises [
11]. Thus, research on the extraction of pequi oil is important because of its significant health benefits. Pequi oil is also a good source of squalene for human consumption, as well as for use in medicines and therapy. In addition, pequi oil contains active chemicals, such as stigmasterol, γ-tocopherol, α-tocopherol, and β-sistosterol [
12].
According to the literature, the most frequently used methods for extracting oil from pequi almonds include solvent extraction [
13], mechanical processing [
14], and enzymatic extraction [
10]. Pressurized/supercritical gas technology can also be used to extract oil [
15,
16]. This technique yields more oil compared to other techniques because of the intrinsic characteristics of supercritical fluid extraction, such as enhanced transport and diffusion of supercritical or near-supercritical gases into the matrix during the extraction process. Temperature and pressure control can also be used to extract specific bioactive components from the oleaginous matrix. Apart from the substantially higher yield, another advantage of supercritical extraction is the absence of residual organic solvents.
The use of carbon dioxide (CO
2) eliminates the need for extensive heating while extracting thermolabile chemicals. Numerous studies have reported the extraction of oils and their active components using high-pressure gases (CO
2 and propane) [
17,
18].
To address the lack of comparative research in the literature, this study investigates the influence of temperature and pressure on oil extraction from pequi almonds. Pressurized CO2 is used, focusing on improving the total yield and fatty acid profile and increasing the bioactive compound content. A second-order quadratic model is used to compute the mass transfers and characterize the observed kinetic curves.
2. Results and Discussion
The experimental conditions and total yields of pequi almond oil extracted using supercritical CO
2 are listed in
Table 1. The highest extraction yield of 27.6 wt% was obtained at 318.15 K, 25 MPa, and 5.0 g.min
−1.
The yield in supercritical extraction may be associated with the duration of the experimental tests, as the kinetic curves did not present a maximum plateau.
Figure 1 shows that there was no change in the inclination of the extraction kinetic curves. The changes in inclination were attributed to variations in the convective mass transfer mechanism. The mass transfer velocity was significantly affected by the convection mechanism in the fluid phase and relatively less by the diffusive mechanism. The gradual removal of lipid materials resulted in a discontinuity in the surface layer. At the beginning of this discontinuity, the extraction rate, governed by the diffusive mechanism, decreased. However, for pequi oil extraction, the extraction rate did not decrease despite variations in the CO
2 flow rate. Thus, it appears that the duration of the experimental extraction was insufficient to reach the extractive equilibrium region. The characteristics of the extraction kinetics were similar for all the studied flows.
In general, increasing the CO2 flow in the extractor bed facilitated oil extraction. The highest extraction yields were obtained at a flow rate of 5 g.min−1. However, being sensitive to changes in pressure and temperature, these yields fluctuated significantly. In contrast, at a flow rate of 3.5 g.min−1, the extraction yields exhibited smaller fluctuations with variations in pressure and temperature. Given that pressure is a key factor in obtaining high yields, high CO2 densities help extract high oil yields.
Pressure also reduces the distance between molecules and enhances the interactions between CO
2 and the sample, which promotes convective mass transfer [
19]. Shi et al. [
20] and Wang et al. [
19] found that increasing the temperature and pressure helped remove carotenoids from pumpkin seeds via supercritical CO
2 extraction.
These yield variations were verified by statistical analysis (
Table 2). The results showed a linear relationship between temperature and pressure (Equation (1)), where T, P, and F correspond to temperature, pressure, and solvent flow rate, respectively.
The coefficients of determination (R
2) and adjusted R
2 (R
2adj) for the model were 0.994 and 0.989, respectively. The F-value = 218 and
p-value < 0.05 indicate the significance of the model. Thus, the temperature, pressure, and flow rate had synergistic effects, i.e., an increase in these parameters led to the highest oil yields (
Figure 2).
According to
Table 3, the experimental parameters for the application of the mathematical model are as follows: bed density (ρ
bed), porosity (
ε), and apparent solubility (
Ceq). The model had only one adjustable parameter,
k = 19.536 cm
3.g
−1.min
−1. An AARD of 7.3% indicates excellent agreement between the experimental data and model values. Klein et al. (2020) [
21] employed a second-order kinetic model to describe the supercritical extraction of
Eugenia pyriformis Cambess leaves. They obtained
k values in the range of 4–16 cm
3.g
−1.min
−1, which are of the same order of magnitude as the values estimated in this study. Parameter
k was used to generate the calculated extraction kinetics curves, as shown in
Figure 1.
Analysis of fatty acids in pequi almond oil showed the following composition: oleic (~54%), palmitic (~37%), linoleic (~5%), and stearic (~1.5%) acids. Small amounts of myristic, palmitoleic, arachidic, linolenic, and gadoleic fatty acids were also observed (
Table 4). The fatty acid profile obtained in this study was similar to that obtained by Soxhlet extraction methods reported in the literature [
22,
23]. The predominance of oleic acid, with no significant differences across extractions, indicates that the supercritical CO
2 extraction process does not damage the primary fatty acids present in the oil.
The bioactive compounds detected in the extracts of pequi almonds were squalene, stigmasterol, β-sistosterol, octacosanol, triacontanol, γ-tocopherol, and α-tocopherol. Among these, squalene had the highest concentrations of 3069–14,220 mg per 100 g of oil. The squalene content extracted with pressurized CO
2 is listed in
Table 5.
The main fatty acids observed were oleic and palmitic acids. Under the optimal conditions, oleic acid in test 10 had the best extraction response, with a 6% statistical difference from that of the other tests. Palmitic acid exhibited the highest extraction in test 6 with 40% extraction. The differences between the experimental and quantitative extraction and identification results are attributed to the molecular mass of the fatty acids and to the temperature and pressure used in the supercritical extraction system [
24,
25].
Different extraction methods lead to variations in the fatty acid content. Although the supercritical extraction method is the most suitable, it is important to evaluate the extraction conditions, large-scale industrial processes, and industrial plants, which are influenced by the raw material and economic factors. In comparison, the Soxhlet extraction method remains the most widely applied process, although quantitative variations and degradation of molecules during heating limit further studies [
26,
27,
28].
The highest solubility between CO2 and squalene was obtained at approximately 15 MPa, 303.15 K, and 3.5 g.min−1. However, despite the mass of squalene/mass of oil obtained under these conditions, the amount of oil extracted under the same experimental conditions was significantly low (1.1 wt%). Thus, these conditions were not optimal when the mass of the oil and squalene binary system was considered.
With regard to the quantitative variations between the tests, it should be noted that squalene, γ-tocopherol, α-tocopherol, stigmasterol, and β-sitosterol molecules promote numerous biological activities, such as hormone repositories, vitamin complexes, and cytotoxic agents in specific groups of cancer cell lines [
29,
30]. Quantitative analysis showed differences between the tests applied; however, in the test with high significance (run 2), there was a greater extraction per partition.
Among all extraction methods (e.g., hot or cold solvent extraction and crushing), the supercritical CO
2 extraction method has the highest yield. However, the extraction plants still have to deal with problems such as low laboratory-scale production [
31,
32]. Although the supercritical system remains a small-scale system, the use of residues, mainly those from the epicarp and mesocarp of pequi fruit, has gained increasing attention because of the economic potential of the bioactive compounds contained in this fruit [
33,
34].