Green Solvent Extraction of Pitaya (Stenocereus spp.) Seed Oil ^†

Abstract

Pitaya (Stenocereus spp.) is recognized for its nutritional properties, with its seeds and peel accounting for 22–29% of the fruit’s mass, often treated as agro-industrial waste. This study evaluates the potential of green solvents for oil extraction from pitaya seeds, aiming to enhance the valorization of these by-products. The efficiency of ethanol and supercritical CO₂ (SC-CO₂) was compared to hexane, a commonly used solvent. Soxhlet extractions were performed using ethanol and hexane, while SC-CO₂ extractions were conducted under two conditions: 180 bar at 50 °C and 250 bar at 35 °C. Oil yields ranged from 4.9% to 24.7%, with hexane achieving the highest yield. However, SC-CO₂ extraction demonstrated its potential as a sustainable alternative. Future studies will focus on characterizing the fatty acid profile of the extracted oil to further assess its nutritional and industrial applications.

1. Introduction

Pitayas are exotic fruits belonging to various species of the genus Stenocereus spp., which are part of the cactus family [1]. The fruit is characterized by numerous small seeds, typically pyriform in shape and black or dark brown in color [2]. In Mexico, pitaya cultivation holds significant cultural and social importance. Additionally, it represents an economic and ecological alternative for areas with arid and semi-arid climates that are unsuitable for other crops [3,4]. However, despite their considerable potential, pitayas remain underutilized, with limited research focusing on their physiological, nutraceutical, medicinal, and agro-industrial properties [5]. In this context, the valorization of underused biomass, such as pitaya seeds, alongside the utilization of agrowastes, emerges as a key strategy for achieving environmentally, socially, and economically sustainable development. This approach aligns with the principles of the circular economy, which aim to eliminate waste, keep products in use, and regenerate natural systems. This prospective seeks to safeguard and restore ecosystem health by avoiding non-essential products, prioritizing biomass flows to meet basic human needs and minimizing overall energy consumption [6]. Among the potential applications of pitaya seeds, oil extraction stands out as a promising strategy to increase the value of both the fruit and its by-products [7]. The oils extracted from these seeds are characterized by a high degree of unsaturation, which is beneficial for health. Linoleic acid, a polyunsaturated fatty acid, constitutes 50–53% of the oil, while oleic acid, a monounsaturated fatty acid, makes up 35–37%. These fatty acids are known for their positive effects on cardiovascular health [8].

Traditionally, oil extraction has relied on organic solvents such as hexane, which poses significant environmental challenges [9]. Traditional processes, such as Soxhlet extraction, have disadvantages such as longer extraction time and increased solvent consumption [10]. Emerging extraction techniques have been developed, including microwave-assisted extraction, aqueous enzymatic extraction, and supercritical fluid extraction [11]. Among these, SFE is highlighted as a green alternative to traditional methods such as Soxhlet extraction. It uses CO₂ under high pressure and temperature to extract oils, achieving high yields of unsaturated fatty acids, particularly linoleic acid, and tocopherols. This method is environmentally friendly and suitable for producing oils for functional foods and nutraceuticals [12]. Therefore, the objective of this study is to evaluate the effect of supercritical fluid and ethanol extraction, as green solvents, on the oil yield of pitaya seeds, compared to the conventional Soxhlet extraction with hexane.

2. Materials and Methods

2.1. Chemicals

Carbon dioxide (99.995%) was supplied by Grupo Infra (Veracruz, Mexico). Ethanol (95%) and hexane (99.9%) were provided by Golden Bell Reagents (Zapopan, Jalisco, Mexico).

2.2. Pitaya Seed Collection

Pitaya fruits (Stenocereus spp.) of second quality, at commercial maturity, were collected from Dolores Hidalgo, Puebla (18.7541, −97.8523). The fruits were depulped using a brush depulper equipped with a 1 mm mesh (Polinox S.A., Mexico City, Mexico). The recovered seeds were dehydrated at 50 °C using a tray dryer until their moisture content was below 3%. The dried seeds were then milled and sieved through a 0.105 mm mesh. The ground seeds were vacuum-packed in high-density polypropylene bags for storage.

2.3. Pitaya Seed Oil Extraction

2.3.1. Conventional Extraction

Conventional solvent extraction was conducted using 15.0 ± 0.5 g of dried, milled pitaya seeds and 250 mL of solvent (ethanol or hexane). The samples were enclosed in filter paper and placed in the Soxhlet extractor. The heating power was adjusted to achieve two extraction cycles per hour, completing a total of five cycles in 8 h. The obtained extract was concentrated under reduced pressure using a Büchi R-205 rotary evaporator (Büchi, Flawil, Switzerland) at 60 °C.

2.3.2. Supercritical CO₂ Extraction

Supercritical fluid extraction was performed using the Helix™ Speed SF System (Applied Separations, Allentown, PA, USA). A detailed description of the equipment is provided in a previous study [13]. The extraction process was conducted with 10 g of pitaya seeds, and the extracted oil was collected and weighed using an output glass tube. Two extraction conditions were evaluated: 180 bar at 50 °C and 250 bar at 35 °C, with a CO₂ flow rate of 50 g/min. The extraction kinetics were determined by measuring the weight of the extracted oil at 15 min intervals until a constant weight was achieved.

2.4. Extraction Yield and Efficiency

Yield extraction was determined as the difference between the initial sample weight ($S$) and oil recovery ($O$) for each process condition as shown in Equation (1).

$$y(\%)={\displaystyle \frac{O}{S}}*100$$

(1)

The extraction efficiency ($\eta$) was determined based on the calculated yield, which is the ratio of the amount of oil recovery ($O$) and the oil content in the sample $(OS$), using Equation (2).

$$\eta (\%)={\displaystyle \frac{O}{OS}}*100$$

(2)

The amount of oil recovery (O) was determined by evaporating the extract obtained by using a vacuum rotary evaporator model R-205 (Büchi, Flawil, Switzerland) at 60 °C and 7.2 × 10³ Pa. After that, the samples were put in a vacuum oven (Lab Line Instrument, Mod. 3818-1, Tripunithura Kochi, India) at 60 °C and 6 × 10⁴ Pa until reaching constant weight.

2.5. Statistical Analysis

Experimental data are reported as mean ± standard deviation. Statistical analyses were performed using one-way ANOVA followed by Tukey’s pairwise comparison test, with a significance level set at p < 0.05. Analyses were conducted using Minitab 16 software (State College, PA, USA).

3. Results and Discussion

3.1. Oil Extraction Kinetics Using Supercritical CO₂

The extraction of oil from seeds using supercritical CO₂ (SC-CO₂) is influenced by both pressure and temperature. These parameters significantly affect the yield, composition, and quality of the extracted oil [12]. Figure 1 shows the extraction kinetics of oil from pitaya seeds under different SC-CO₂ conditions. The highest oil recovery was achieved at 250 bar and 35 °C. According to various authors, the oil extraction yield increases with higher pressures and lower temperatures, due to their combined effect on oil solubility. In general, higher pressures enhance the extraction yield by increasing the solvent density, which improves the solubility of oils in SC-CO₂ [14,15]. Additionally, extraction time is an important factor influencing the yield and efficiency of the process. Under the evaluated conditions, the highest oil recovery was achieved after 450 min.

3.2. Oil Yield Process

The highest oil recovery from pitaya seeds was 24.70 ± 0.55%. These results are comparable to those reported for other Stenocereus varieties, such as S. thurberi (26.50 ± 0.66%) and S. gummosus (28.4 ± 0.1%) [8,16]. When comparing different extraction methods (

Table 1. Pitaya seed oil yield obtained by different extraction conditions.

Extraction Conditions	Yield (%)
Hexane	24.70 ± 0.55 a
Ethanol	6.77 ± 1.16 c
CO2-SC, 180 bar, 50 °C, 450 min.	4.98 ± 0.06 c
CO2-SC, 250 bar, 35 °C, 450 min.	15.25 ± 0.07 b

The values are the means of triplicates ± SD. Different letters in the same column indicate that the means differ significantly by Tukey’s test (p < 0.05).

), Soxhlet extraction using hexane as a solvent yielded the highest oil recovery, followed by supercritical CO₂ extraction (250 bar, 35 °C). Similar findings were reported by Al-Naqeb et al. [12] for the extraction of Opuntia ficus seed oil. Ethanol resulted in lower oil recovery due to its limited solubility for oils and its tendency to co-extract polar non-oil components, which ultimately reduces the overall yield [17]. In contrast, hexane produced higher oil yields thanks to its superior selectivity and efficiency in dissolving lipids, allowing for the extraction of oils with fewer impurities.

In terms of supercritical CO₂ extraction, conditions of 250 bar and 35 °C resulted in higher oil yields compared to 180 bar and 50 °C. Similar pressure conditions have been reported for other cactus seeds, such as Opuntia dillenii Haw [18] and Selenicereus undatus (commonly known as pitahaya or dragon fruit) [19]. Supercritical CO₂ extraction is a promising method for obtaining pitaya seed oil, but the extraction parameters need to be further optimized. This method offers significant potential for developing healthy oils from pitaya seeds.

4. Conclusions

Supercritical fluid extraction represents a viable alternative for extracting pitaya seed oil, with higher pressures yielding greater oil recovery. However, further optimization of the extraction conditions is necessary to maximize efficiency. This method supports the sustainable utilization of agricultural by-products, aligning with current trends in environmentally friendly practices and contributing to the principles of the circular bioeconomy.