Peanut Oil – How to Extract Oil from Peanut

Peanut oil can be extracted with a variety of methods. Mechanical pressing uses the power of gravity to squeeze out the oil from peanuts.

Solvent Extraction uses hexane as the solvent, but this is highly toxic and poses health hazards. The hexane residue left behind can also degrade proteins, reducing the functional properties of peanut protein and its nutritional value.

1. Preparation

Peanut oil is a popular edible oil that can be made at home. However, the production process is a delicate one, as it requires careful handling to ensure optimal yields and quality. From harvesting the nuts to pressing and refining, each step is important in unlocking the potential of this versatile crop.

To maximize the peanut oil yield, efficient dry grinding is essential in destroying the cellular structures that contain the oil, thus making it easier for it to diffuse. This can be achieved by using a high-energy mill or a roller mill. Moreover, an effective dewatering method can also contribute to the extraction process. The water content of the peanuts can be reduced by soaking in a tank or by using an expeller.

Enzymatic aqueous extraction has shown promising results in improving the free oil yield. It is a low-cost and environmentally friendly alternative to solvent extraction. However, the high vapor pressure required for this method limits its application.

Furthermore, the use of a solvent such as hexane can cause irreversible physical damage to peanuts. To overcome these challenges, a mechanical extraction technique that preserves the shape integrity of the peanut is needed.

Despite the fact that numerous mechanical techniques have been developed, they still require further research and validation for their industrial applications. Nevertheless, some of these methods have shown promising results, including MEPSI, enzyme-assisted aqueous extraction, and microwave and ultrasonic-assisted aqueous extraction. These new methods are expected to reduce the energy consumption and improve the yield of crude peanut oil, thereby contributing to the sustainable development of this vital crop. They are also believed to offer opportunities for the valorization of protein-rich by-products.

2. Soak

Peanuts are a very important source of protein and oil. Moreover, their low fat and caloric content help to reduce the incidence of obesity and cardiovascular diseases. Despite their nutritional benefits, peanuts contain anti-nutritional factors that need to be dealt with properly before incorporating them in the diet. Therefore, obtaining high-quality peanut oil requires specific pretreatment steps such as defatting.

To ensure a successful defatting process, the peanuts should be soaked with water until their moisture content is about 5-10% dry basis (d.b.). This soaking is necessary to limit irreversible physical damage and distortion of the seeds during the mechanical expression.

The soaking also increases the surface area of the peanuts, which allows for a more efficient extraction of oil. This is especially true if the peanuts are ground into a paste before the extraction process begins. The soaking step also helps to reduce the amount of water and steam present in the oil, which can cause evaporation during the heating stage of the extraction process.

Traditional methods for peanut oil extraction include cold pressing (CP) and solvent extraction (SE). However, the CP process has a low production rate and is not suitable for industrial applications. SE is an environmentally friendly method, but it can leave residual solvent in the product and is susceptible to contamination from heavy metals. Moreover, the peanut meal obtained after high-temperature desolvation can only be used as feed, resulting in wasted protein resources [3,4].

To avoid this problem, researchers have developed an innovative mechanical oil extraction process for peanuts called MEPSI (Mechanical Extraction Preserving Shape Integrity). The MEPSI technique uses a screw press with specially designed spacers that separate the peanut kernels during the defatting process, which prevents irreversible damage to the grains and their organoleptic quality. This is accomplished by adjusting the pressure and water content of the peanuts to achieve a high oil yield without affecting the grain shape.

3. Extraction

Peanuts (technically, legumes) are a healthy source of unsaturated fat. However, extracting the oil from these snackable seeds isn’t without challenges. The process involves a high level of skill and specialized equipment. Without proper care, peanut oil extraction can result in a costly mess and even pose a safety risk. The good news is that processors can unlock a valuable resource from these peanuts by transforming the oil into premium cooking oils and protein-rich byproducts for animal feed.

The existing method of peanut oil extraction relies on a combination of pressing and solvent methods. However, this process denatures the proteins in the peanut seeds and severely reduces their functional properties, thereby limiting their use as food ingredients. In addition, the solvent residue is toxic and poses a health hazard to human beings and the environment.

In order to overcome the limitations of current methods, researchers have explored various chemical methods for oil extraction and protein valorization from peanuts. The chemical methods can be categorized as organic solvent extraction, aqueous extraction processing, and supercritical fluid extraction (SC-CO2). A comprehensive review of these methods is presented in Table 2.

The first step in the extraction process involves cracking the peanut kernels using roller mills to increase the surface area for conditioning and pressing. The conditioned peanuts are then passed through a dryer to remove moisture and air and then pressed in a roller press. The resulting oil is then separated from the nutmegs and shells by centrifugation.

In recent studies, researchers have investigated the potential of replacing hexane with an enzyme-aided aqueous extraction process to improve the quality of peanut oil. The optimum conditions for this method were determined to include a roasting temperature of 190 °C, seed-to-water ratio of 1:5 with an enzyme concentration of 2%, an incubation time of 20 min, and centrifugation to separate the phases.

4. Filtration

The final step is to filter the oil through a cloth or paper to remove any remaining solids. This is necessary to maintain the quality of the peanut oil and protect against any potential contamination from other sources. The filtered peanut oil is then ready for use in a variety of applications.

Various patented defatting and oil extraction methods are available that can improve the quality of the finished product by removing fatty compounds from the peanuts prior to pressing. These techniques involve mechanical and chemical processes that are supplemented with biological or physical pre-treatment approaches. They aim to enhance the physicochemical properties of the finished peanut oil and unlock its untapped potential.

Mechanical methods of oil extraction can be categorized into three groups: extrusion and screw pressing, cold pressing, and hydraulic presses. They all aim to achieve the highest possible oil yields in a shorter period of time. However, they may negatively impact the physical properties of the final peanuts.

One of the most promising new technologies for oil extraction is supercritical carbon dioxide (SC-CO2). It is a safer and environmentally friendly alternative to traditional Soxhlet solvents such as hexane or petroleum ether. It also requires significantly less energy than conventional oil pressing and can be done at lower temperatures.

However, the main challenge with SC-CO2 is the lack of knowledge on how to optimally design and operate the extraction system. This is important because the optimal conditions can affect both the oil yield and the extraction rate. For example, a high temperature can result in increased degradation of the extracted oil. In addition, a low modifier flow rate can lead to reduced extraction rates.

5. Refinement

During this step, the crude peanut oil is refined to improve its quality. The first step is to remove any remaining water and fatty acids from the peanut oil. This is done by boiling the peanut oil at a high temperature for a period of time. This also sterilises the oil. Once the fatty acid and water have been removed, the peanut oil is ready for storage or for further processing.

Mechanical techniques are commonly used in the extraction of peanuts for oil, and these methods are categorized into three main groups: mechanical extrusion and screw pressing, cold-pressed and hydraulic pressing. The first two methods involve the shelling, crushing, moisture content adjustment and frying of peanuts before they are extracted using a mechanical press or an expeller. The resulting oil is then filtered.

A recent study evaluated the ability of supercritical carbon dioxide (SCCO2) to replace hexane in the extraction of oil from peanuts. The study found that SCCO2 extraction resulted in a higher oil yield than hexane. The study also found that the oil yield was dependent on the extraction vessel orientation and the temperature of the peanuts. In addition, the extraction time increased as the pressure and temperature decreased.

Peanuts are a vital crop for human health and offer an abundant supply of nutrients. Their oil has been utilized in various ways, from cooking to cosmetics. However, the majority of the available peanut oil is derived from the use of hexane in solvent extraction. This is a concern because hexane has been shown to be harmful to the environment and human health. Therefore, it is important to develop a safe and effective alternative to hexane for peanut oil extraction.

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