Peanut, or Arachis hypogeae L., is currently a vital oilseed crop widely utilized in the confectionery, snack, and fat/oil manufacturing industries [1]. This plant belongs to the Fabaceae family and originates from South America. It is grown in areas with tropical, subtropical, and temperate climates [2] and has become well-known for being a high-protein source, containing between 22% and 30% protein. Additionally, it is a source of nutrients like niacin, which helps promote healthy blood flow and brain function, folate, antioxidants, vitamin E, magnesium, phosphorus, and dietary fibers [3]. Peanuts are commonly eaten as a snack or processed into peanut butter, while more than 70% of the harvest is used for oil extraction [4]. Indeed, peanuts contain between 45.9% and 55.4% of lipids that are specifically high in essential polyunsaturated fatty acids [5,6,7]. Among the unsaturated fatty acids, oleic acid and linoleic acid account for 33.3–61.3% and 18.5–47.5%, respectively. Despite being high in calories, many studies have highlighted the health-promoting properties of peanuts [8]. In fact, high consumption of nuts is associated with a beneficial impact on the cardiovascular system [9], due to their antioxidant and anti-inflammatory properties [10,11]. Research has also found that consuming peanuts and peanut butter five times per week can reduce the likelihood of developing type 2 diabetes [11,12] and chronic diseases such as cancer [11,13]. Peanuts and their by-products could potentially serve as natural chemo-preventive agents [14].

Removing lipids from food products is sought by health-conscious individuals who require diets that are low in fat and high in protein. While several methods are available for extracting oil from oilseeds, some of them might negatively affect the extracted oil and/or the remaining solid matrices. The mechanical methods are the most commonly used ones, including hydraulic pressing [1,2,3,4,8], extrusion [15,16], screw pressing [16,17,18], and cold pressing [6,19,20,21]. Hydraulic pressing has been significantly enhanced in certain instances to preserve the physical form of whole peanuts following defatting. This was achieved through a technique known as “Mechanical Expression Preserving Shape Integrity” (MEPSI) [8,22], combined with a reconstitution process called “Intensification of Vaporization by Decompression to the Vacuum” (IVDV) [1,8,23,24,25]. This method relies on the use of a separating agent during defatting to prevent irreversible physical damage and distortion of peanuts. In other scenarios, the reconstitution happened via soaking in water for a sufficient time and then drying to a water content of 7–10% dry basis (d.b.) before roasting [2,3]. Chemical methods are also frequently cited in the literature. They include organic solvent extraction, such as hexane [15,16,17], trichloroethylene [26], or utilizing supercritical CO₂ (SC-CO₂) adjusted through the addition of a co-solvent containing ethanol [19,20,21], as well as aqueous extraction processing (AEP) employing water [6,17]. Another technique is Soxhlet extraction, which uses various solvents such as ethanol [23]. Advanced, environmentally friendly technologies have been implemented, involving a synergistic combination of multiple treatments and extraction methods. This strategic approach aims to enhance oil yield while reducing expenses and energy consumption. Several methods of AEP are discussed, including enzyme-assisted aqueous extraction (EAAE) [27,28,29], aqueous and mechanical extraction by treatment with NaCl [30], and aqueous extraction combined with membrane separation by applying two-stage microfiltration and ultrafiltration (MF/UF) [31]. Additionally, aqueous enzymatic extraction was coupled to an ultrasonic pre-treatment [32,33], infrared radiation (IR) [34], or microwave radiation by treatment with CaCl₂ [35], which usually relies on heat transfer and varies depending on the type of product and the oven’s chamber design and operation [36].

Mechanical techniques for oil extraction from peanuts are grouped into three main sections: extrusion and screw pressing, cold pressing, and hydraulic pressing. Table 1 showcases all these mechanical methods employed, along with their experimental conditions, including pre-and post-treatments, evaluation of the nutritional value of the final products, and oil yield.

The process of food extrusion involves a series of thermal and mechanical steps that can result in various physicochemical changes in the raw materials. These changes include but are not limited to binding, cleavage, loss of native characteristics, fragmenting, and recombination. Extrusion processing is a more favorable option over conventional methods since it typically operates continuously at high temperatures for a short amount of time, resulting in greater nutrient retention [49]. In most cases, extrusion involves screw pressing, which makes oil extraction possible through the application of axial pressure generated through volumetric compression. The rotating worm shaft also contributes to the process by applying force, resulting in the squeezing of the oil from the kernels [50]. A screw press is made up of a horizontal screw that is secured in a perforated barrel, which is used to extract oil [46].

In 2009, Rehrah et al. studied the use of PDPF, a protein-rich ingredient [51], to create a plant-based alternative to meat that would be attractive to health-conscious consumers. The flour is first processed into a PPC, which is then mixed under specific conditions (at 160–165 °C, screw speed of 80–90 rpm, and moisture content of 50–55%) through a process of extrusion to yield a final product with 60–65% protein. Riaz et al. studied the creation of PDPF, a new product with less than 10% oil that had reduced fat content, improved protein content, good flavor, and a long shelf life [16]. This was achieved by removing enough oil from raw peanuts through a combination of dry extrusion and screw pressing. An extruder was developed to aid in oil extraction, and the single-screw machine operated at a low moisture level of 6.13 ± 0.14%. Importantly, this process did not involve any chemical agents or produce waste streams. Using only extrusion, it was possible to remove 65.6% of the oil from raw peanuts, while extrusion combined with screw pressing removed 90.6% of the original oil. Optimal conditions for the process included a feeding rate of 142 kg/h, dry extrusion at 136–138 °C for 30 s, and coupling the extrusion to screw pressing at a temperature of 90 °C for 1 min [16]. These two studies provided complementary information on the process of producing PDPF from peanuts. They demonstrated a clear pathway from oil extraction, which has potential applications in various industries, notably the development of a meat alternative for vegans. Overall, these findings represent an exciting development in the utilization of peanuts that deviates from the concept of maintaining the peanut’s original structure.

A study carried out in 2019 by Suri et al. examined how peanut oil quality characteristics were affected by a combination of dry air roasting and mechanical extraction using screw pressing [17]. The researchers found that optimal conditions for air roasting were 180 °C for 10 min, followed by cooling at room temperature. Oil extraction using screw pressing at a temperature below 50 °C resulted in a yield of 41.18–46.28%, followed by centrifugation of the oil at 12,000 rpm for 10 min to remove impurities. The study demonstrated that this method led to a lower PV and a higher oxidative stability index (OSI) [17]. Mridula et al. conducted a study in 2020 that involved subjecting peanuts to hot air treatment at 105 °C and then mixing them with distilled water to increase moisture levels [46]. They achieved an extraction of 75.89% of the oil in peanuts by using a screw speed of 20 rpm and pressing the samples at a temperature of 70 ± 2 °C with a sample moisture content of 8%. RSM response parameters showed a desirability of 81.8%. It was observed that maintaining a lower pre-treatment temperature of 105 °C and a moisture content of 8% w.b. yielded a higher quantity and quality of oil, thus ensuring a relatively high level of desirability for consumers. However, it is important to note that the experimental conditions and methods employed in the two studies may have varied, leading to differences in the results. For instance, the optimal air roasting temperature and duration were different. In addition, factors such as the type of peanuts used, processing equipment, and the duration of storage after extraction could influence the quality of the oil obtained. Therefore, further research is required to determine the optimal pre-treatment conditions for extracting high-quality peanut oil.

The cold press extraction method has gained popularity in recent years, primarily due to its ability to obtain premium-quality oils without subjecting them to high temperatures or the use of solvents, thus aligning with environmentally friendly practices. This method can be classified into three types: expellers, expanders, and twin-cold systems [19]. The procedure generally involves the shelling, crushing, moisture content adjustment, and frying of peanuts before they are cold-pressed, with the resulting peanut oil then being filtered [20].

In 2017, Chen et al. examined the effects of pressing temperature and moisture content, which are associated with the cold-pressing technique, on the yield, acid value, moisture content, and volatile matter content of peanut oil [20]. When prioritizing acid value as a factor, the pressing temperature had the most significant impact, followed by moisture content. After experimentation, it was determined that the optimal cold-pressing conditions to ensure the production of high-quality peanut oil are an oil temperature of 65 °C and a moisture content of 7%. Under these conditions, the acid value of the oil was 0.133, the moisture and volatile matter content were 0.015%, and the oil yield was as high as 39.8% [20]. In 2020, Shin et al. extracted oil from peanuts using the cold pressing method and aimed to valorize the partially defatted peanut meal (PDPM) obtained [21]. In the process of producing cold-pressed peanut oil, mechanical shelling is commonly used, and the peanut kernels with red skin are then dehydrated to a moisture content of 4–8% to make them easier to peel. The peanuts are then placed in a conditioning tank, where the pressing temperature and moisture are adjusted to maximize the oil yield. To commercially extract cold-pressed peanut oil, a twin-screw press is used, and the pressing is performed at specific temperatures that do not exceed 60 °C. The oil yield was not evaluated in this study, but the researchers were very interested in utilizing the by-product of this extraction (i.e., the meal), knowing that 70 kg of PDPM was recovered out of 100 kg of peanuts [21]. In 2019, Konuskan et al. investigated the physicochemical properties of cold-pressed nuts, specifically peanuts in the Eastern Mediterranean region, with a focus on their fatty acid composition [52]. An oil screw expeller was used to extract the oil, which was then subjected to malaxation and centrifugation processes at 25 °C for 30 min and at 5000 rpm, respectively. Peanuts were found to have the highest free fatty acid content at 1.36%, which can result in poor-quality oils with significant losses during the refining process. Additionally, peanut oil had a high initial PV of 8.39 meq O₂/kg, indicating a short shelf life and limited suitability for human consumption [52]. On another note, a study performed by Ji et al. in 2020 aimed to investigate the presence of cancer-causing compounds in extracted peanut oil [6]. The amount of AF in oil extracted through hot pressing was much higher than that in oil extracted through cold pressing. This increase in concentration at high temperatures could be due to the breakdown of other food components and the release of bound AF. It is crucial to implement preventative measures against AF contamination from the outset because of its adverse health impacts [53]. This includes sound agricultural practices and effective chemical and bio-control strategies targeting AF-producing Aspergillus spp. [54]. It is worth noting that the cold press extraction method has a relatively low oil yield (<40%), which is considered mediocre. These findings suggest that appropriate pre-treatment/extraction methods can improve the quality and yield of cold-pressed peanut oil, making it more suitable for human consumption. Additional research may be needed to optimize the cold-pressing process to produce high-quality peanut oil with desirable physicochemical properties and minimal contaminants.

Five identified factors can affect hydraulic pressing. They include the moisture content of the peanuts before pressing, the level of pressure applied, the speed at which the piston moves, the thickness of the cake after the number of seeds per unit area, and how long the pressing duration lasts [2].

In 2007, Olajide et al. assessed the predictive precision of a recently devised neural network concerning oil yield. The extraction process involved hydraulic pressing, and the researchers identified the optimal operating conditions as follows: pressure of 25 MPa, pressing time of 7 min, and moisture content of 1.76% d.b. Under these specified conditions, the maximum oil yield was 33.36% [47]. Another related study explored the influence of operating parameters on the mechanical extraction of oil from groundnut kernels through the utilization of a hydraulic press [48]. The oil yield recorded was 32.36% under a pressure of 15.77 MPa for 6.69 min, and with a moisture content set at 8.13% d.b. The low oil yield raises concerns about the efficiency of the extraction process, knowing that higher oil yields are often desired in industrial applications to maximize the utilization of raw materials and increase overall production efficiency. Additionally, the obtained results can be compared with alternative extraction methods to ascertain the competitiveness and practicality of the hydraulic press approach.

A recent improvement has led to a more efficient and economical defatting process for whole peanuts, resulting in less waste of misshapen seeds, a reduced risk of grain breakage under high pressure, and streamlined oil extraction for diverse applications. The unique aspect of this defatting method is the use of a special separation material placed between the grains within the press chamber [3,46], which helps to retain the structural and sensory qualities of the final product [2]. Consequently, the objective of this technique is to maximize oil extraction while generating defatted whole peanuts with high levels of protein and fiber and reduced oil content. The details are clearly explained in the patents LB-10,492 [55] and LB-10,493 [56], in which MEPSI involved air-roasting peanuts as a pre-treatment. The purpose of this step was to improve the taste and appearance of the nuts, enhance the oil extraction, reduce the deformation of the kernels, and decrease the moisture content to 2–3% d.b. The peanuts were then rehydrated and pressed at room temperature to achieve different water content levels ranging from 5% to 15%. This was necessary to improve the compressibility of the grains and their resistance to disintegration, which in turn reduced the occurrence of permanent deformation during the pressing process [1,4,8,46,57]. A supplementary post-treatment using IVDV was applied to improve the properties of the final product. It involved subjecting the distorted seeds to a steam pressure of up to 1.5 MPa, reached within one second, and treating them under such high pressure and temperature for a certain period of time. The pressure is then suddenly released into the vacuum, causing some of the water content to auto-vaporize, thus leading to an expanded structure. The resulting products were subsequently dried in a ventilated oven at 50 °C until the relative humidity reached 7–10% w.b. This thermo-mechanical process was aimed at texturizing the product by regaining its original shape and size, as well as enhancing its textural, physiochemical, and sensorial properties [1].

A coupling between MEPSI and IVDV was achieved by Nader et al., reaching around 56% of the oil extraction rate from whole peanuts when the optimal conditions were applied [1,8]. When lower pressure and processing times were used, a slight decrease in the oil yield was noted [4]. The physical characteristics of peanuts were examined after oil extraction. The consumer color evaluation and the textural sensory analysis demonstrated a significant advantage over previously produced partially defatted whole peanuts [2,3]. Further enhancements were introduced by the same team, and a significantly higher defatting ratio of 70.62% was reached upon applying greater pressure [22]. An evaluation of the response factors was performed, providing insights into the impact of the treatment on the textural qualities of the peanuts. The treated whole peanuts exhibited reduced malleability, lighter coloration, and enhanced crunchiness, indicating desirable textural characteristics [45]. Following an RSM optimization, the desirability of the defatted peanuts increased by up to 80%. The physical constraint induced by pressing caused a greater exposure of the remaining oil to oxygen during defatting and final roasting, resulting in a reduction in the oxidative stability of peanuts. Furthermore, the optimal conditions obtained through multiple optimizations using RSM resulted in a significant reduction of lipid oxidation. The texture of partially defatted whole peanuts appears to be preserved better when MEPSI is combined with IVDV, owing to the lower risk of damaging the product. With that being stated, the integration of MEPSI-IVDV enables manufacturers to provide a variety of product lines to cater to diverse consumer demands with varying levels of fat and protein content. Processing conditions can therefore be optimized to maximize oil extraction while keeping desirable partially defatted whole peanuts. It is also pertinent to note that in a product that underwent expansion, an increase in porosity leads to heightened product aeration, decreased hardness, an elevated occurrence of mechanical fractures, and a greater number of acoustical events [58].