How do you separate oil from groundnuts?

Separating oil from groundnuts, also known as peanuts, primarily involves a two-stage process that combines mechanical pressing with solvent extraction to maximize oil yield. This integrated approach ensures efficient oil recovery from the groundnut kernels.

Understanding the Primary Methods

The two main methods used for groundnut oil manufacturing are:

1. Mechanical Pressing (Expeller Pressing): This is often the first step, physically squeezing the oil out of the groundnuts.
2. Solvent Extraction: This method is typically used after mechanical pressing to recover the residual oil.

Let's explore each method in more detail.

1. Mechanical Pressing

Mechanical pressing is a widely used method for initial oil extraction. It's an effective way to obtain a significant portion of the oil, typically extracting around 85% of the available oil from the groundnuts.

The Process of Mechanical Pressing:

The steps involved in mechanical pressing ensure the groundnuts are prepared and processed efficiently to release their oil:

1. Cleaning: Raw groundnuts must first be thoroughly cleaned to remove impurities like stones, dirt, leaves, and metal particles. This prevents damage to the machinery and ensures a purer final product.

2. Dehulling (Optional but Recommended): While not always mandatory, removing the outer shells (hulls) from the groundnut kernels before pressing can increase oil yield, reduce wear on the machinery, and improve the quality of the oil and the press cake. Dehulling is achieved using specialized dehulling machines.

3. Crushing/Grinding: The cleaned and dehulled kernels are then crushed or ground into smaller particles or a coarse paste. This increases the surface area, making it easier to extract the oil.

4. Cooking/Conditioning: The crushed groundnut material is often heated in a cooker or conditioner. This step, known as "cooking" or "tempering," helps to:

   • Rupture oil cells, making oil release easier.
   • Coagulate protein, which helps in separation.
   • Reduce oil viscosity, allowing it to flow more freely.
   • Adjust moisture content for optimal pressing.

5. Pressing (Expelling): The conditioned material is fed into an oil press (expeller). These machines use continuous high pressure via a rotating screw or hydraulic plates to squeeze the oil out of the groundnut meal. The oil flows through small openings, while the solid residue (oil cake or press cake) is discharged separately.

   • Equipment Example: Screw presses are common, applying continuous pressure as the material moves through a confined barrel.

2. Solvent Extraction

After mechanical pressing, a significant amount of oil (around 15%) can still remain in the solid residue (press cake). Solvent extraction is employed to recover this remaining oil, ensuring a higher overall oil yield.

The Process of Solvent Extraction:

This method involves dissolving the oil out of the meal using a chemical solvent:

1. Preparation of Press Cake: The press cake from the mechanical pressing stage is further flaked or ground into smaller, uniform particles. This increases the surface area for efficient solvent penetration.
2. Extraction: The prepared press cake is then introduced into an extractor where it is thoroughly mixed or washed with a food-grade solvent, most commonly hexane. The solvent dissolves the oil, forming a mixture called "miscella" (oil + solvent).
   • Equipment Example: Continuous counter-current extractors are often used to maximize efficiency.
3. Desolventization of Meal: After extraction, the oil-free meal (now containing solvent) is passed through a desolventizer-toaster (DT) to remove the residual solvent. Steam is typically used to evaporate the hexane, and the meal is toasted to remove any remaining traces. This solvent-free meal is then dried and cooled, often used as animal feed due to its high protein content.
4. Distillation of Miscella: The miscella (oil and solvent mixture) is then subjected to a distillation process. This involves heating the mixture, causing the lower-boiling point solvent to vaporize, leaving the crude oil behind. The solvent vapor is condensed and recycled back into the extraction process, making it an economical method.
5. Oil Purification: The crude oil obtained from both mechanical pressing and solvent extraction often undergoes further refining steps. These steps may include degumming, neutralization, bleaching, and deodorization to remove impurities, free fatty acids, color, and odors, resulting in refined edible groundnut oil.

Summary of Methods

The table below summarizes the key aspects of both methods for separating oil from groundnuts:

For more detailed information on edible oil production, you can refer to resources from organizations like the American Oil Chemists' Society (AOCS).