Castor oil is made by pressing oil from the seeds of the castor plant, then refining the crude oil through several purification stages. The seeds contain roughly 33% to 37% oil by weight, and the finished product must be at least 90% ricinoleic acid (its key fatty acid) to meet pharmaceutical standards. The process ranges from relatively simple mechanical pressing to multi-stage industrial refining, depending on the intended use.
Starting With the Seeds
Castor oil production begins with harvesting and preparing the beans of the castor plant. The raw seeds arrive covered in a hard outer hull that needs to be removed before any oil can be extracted. First, the beans are cleaned to remove dirt, debris, and moisture, then sorted by size using industrial sieves. Size matters because the hulling step works best when beans are uniform.
Once cleaned, the beans go through a process called decortication, which strips the hull from the oil-rich kernel inside. Most commercial facilities use mechanical decorticators, typically impact hullers or disc hullers designed specifically for castor beans. One newer technique uses compressed air: clean beans are pressurized and then suddenly released into a vacuum chamber, causing the hulls to crack open. The degree of dehulling depends on the air pressure and the bean size. After hulling, the kernels are separated from the shell fragments and prepared for pressing.
Cold Pressing: The Primary Extraction
The most common way to extract castor oil is cold pressing, where cleaned and dehulled beans are fed into a hydraulic press that squeezes oil out under high mechanical pressure. No external heat is applied during true cold pressing, which helps preserve the oil’s natural properties and color. The basic steps are straightforward: load prepared beans into the press cylinder, apply pressure, and collect the oil that flows out.
Hydraulic presses used for castor oil come in various sizes, with cylinders rated by capacity (typically 8 to 15 kilograms per batch for smaller operations). Screw presses are another option and can run continuously. Research on Egyptian castor seeds found that a screw press operating at 200°C and 60 revolutions per minute achieved an oil yield of about 35.6% in under two minutes per batch. Higher temperatures during screw pressing increase yield but also change the oil’s characteristics, which is why cold-pressed castor oil is considered higher quality for cosmetic and pharmaceutical use.
After the first pressing, the leftover material (called press cake) still contains significant oil. Larger operations run this cake through a second pressing or move it to solvent extraction to recover more oil.
Solvent Extraction for Maximum Yield
To pull remaining oil from the press cake, industrial producers use solvent extraction. The cake is washed with a chemical solvent, most commonly hexane, which dissolves the trapped oil. The solvent-oil mixture is then heated to evaporate the solvent, leaving behind crude castor oil. This method recovers nearly all the oil the seeds contain, pushing total yields closer to the 37% to 39% range reported in some studies. The recovered solvent is recycled for the next batch.
Oil from solvent extraction is typically lower grade than cold-pressed oil and almost always requires full refining before it can be sold for cosmetic or pharmaceutical purposes.
Refining Crude Castor Oil
Crude castor oil straight from the press is dark, gummy, and contains impurities. Turning it into the clear, pale oil you find in stores requires four main refining stages: degumming, neutralization, bleaching, and deodorization.
Degumming
The first step removes gummy phospholipids from the crude oil. In water degumming, the oil is heated to about 60°C to 70°C, mixed with water, and left to sit for 30 minutes. The water causes the gums to swell and become insoluble in the oil, so they can be spun out in a centrifuge or simply drained off. For a more thorough result, producers use acid degumming, where a small amount of citric or phosphoric acid is added along with water at around 90°C.
Neutralization
Crude castor oil contains free fatty acids that affect shelf life and quality. To remove them, the degummed oil is mixed with a dilute alkali solution (typically 2% caustic soda) at 85°C to 95°C and stirred constantly for 45 to 60 minutes. The alkali reacts with the free fatty acids to form soap, which is heavier than the oil and settles out or gets separated by centrifuge. The oil is then washed with hot water to rinse away any remaining soap and alkali residue.
Bleaching
Even after neutralization, the oil retains color pigments and trace phospholipids. Bleaching removes these through adsorption rather than chemical reaction. The oil is mixed with activated clays and carbon under vacuum at around 100°C with constant stirring. The clay particles bind to the pigments and impurities, and the mixture is then filtered to produce a much lighter, cleaner oil.
Deodorization
The final stage strips out volatile compounds responsible for any remaining off-flavors or odors. This is essentially steam distillation under vacuum: steam passes through the hot oil at very low pressure, carrying away the unwanted volatile molecules. What remains is the pale, nearly odorless castor oil familiar from pharmacy shelves.
Some producers add a winterization step, where the oil is slowly cooled to crystallize and remove waxes, producing an even clearer product.
What About Ricin in the Seeds?
Castor beans famously contain ricin, a highly toxic protein. The good news is that ricin does not dissolve in oil, so it stays behind in the press cake rather than transferring into the extracted oil. The refining process provides an additional safety margin, as the high temperatures involved would denature any trace amounts. Research shows ricin breaks down significantly when dry-heated at 100°C for 30 minutes, and it can be effectively destroyed at air temperatures of 82°C to 88°C over longer periods. The multiple heating stages throughout refining ensure that finished castor oil contains no active ricin.
The press cake, however, does contain ricin and requires separate heat treatment before it can be safely used as animal feed or fertilizer.
Why You Cannot Easily Make It at Home
While the concept of pressing oil from seeds sounds simple, castor oil production involves serious safety and quality challenges that make home extraction impractical. The presence of ricin in raw castor beans is the most obvious concern: handling and processing the seeds without proper equipment creates real exposure risk. Beyond safety, achieving the pressure needed to extract meaningful oil volumes requires industrial or at least commercial-grade hydraulic equipment. And without the refining stages, homemade castor oil would be crude, dark, and full of impurities that could irritate skin or go rancid quickly.
Cold-pressed, minimally refined castor oil is widely available and inexpensive. For pharmaceutical-grade oil, the U.S. Pharmacopeia requires the finished product to consist of at least 90% ricinoleic acid triglycerides, a standard that’s only reliably met