Castor oil is made by extracting oil from the seeds of the castor plant, then refining it through several purification stages. The seeds contain 35 to 55% oil by weight (once hulls are removed), and the extraction method, whether mechanical pressing or chemical solvent, determines the grade and quality of the final product. India produces roughly 90% of the world’s castor seeds, with Mozambique and Brazil as distant second and third.
Harvesting and Preparing the Seeds
Castor plants produce spiny seed pods that are harvested when mature and dried. Once the pods split open, the oblong, spotted seeds are separated and cleaned to remove foreign material, damaged seeds, and any seeds still attached to their hulls. This cleaning step matters because debris or broken seeds introduce impurities that complicate every stage that follows.
After cleaning, the outer hull is removed in a process called decortication. The exposed seed kernel is what actually goes into the press. Some operations also condition the seeds with gentle heat or moisture to soften them, which helps release more oil during extraction.
Pressing: Cold, Expeller, and Chemical
Most castor oil production starts with mechanical pressing, where seeds are fed into a machine that crushes them under heavy pressure. The oil flows out, and the remaining solid compresses into a dense cake. From there, the process splits depending on what kind of oil the manufacturer wants to produce.
Cold-pressed castor oil is extracted at temperatures below 122°F. Keeping temperatures low preserves the oil’s natural nutrients and antioxidants, which is why cold-pressed oil is the preferred type for skin and hair care products. The tradeoff is that cold pressing leaves a significant amount of oil behind in the seed cake, making it more expensive to produce.
Expeller-pressed castor oil uses mechanical pressure and friction that generates heat between 140 and 210°F. The higher temperatures pull out more oil per batch but can degrade some of the oil’s beneficial compounds. This method is common for industrial-grade castor oil.
Chemical solvent extraction picks up where pressing leaves off. After the seeds have been crushed into a cake, manufacturers can soak that cake in a solvent, typically hexane or heptane, to dissolve and capture the remaining oil. The solvent is then evaporated off, leaving behind additional oil. This step allows producers to get maximum yield from each batch of seeds. The resulting oil tends to be yellowish-brown and is used primarily in technical and industrial applications rather than cosmetics or medicine.
Many large-scale operations combine methods: they press the seeds first, then use solvent extraction on the leftover cake.
What About Ricin?
Castor seeds contain ricin, a highly toxic protein, which understandably raises questions about safety. The good news is that ricin is a protein, and proteins break down at relatively low temperatures. Research shows that ricin irreversibly unfolds and loses its toxicity at temperatures as low as 158°F (70°C). Hot pressing of castor seeds completely denatures the ricin during extraction. Even in cold-pressed operations where extraction temperatures stay below 122°F, the ricin stays behind in the seed cake rather than transferring into the oil. That leftover cake can then be boiled or autoclaved for as little as 10 minutes to neutralize any remaining ricin.
The finished oil itself contains no ricin. The protein is water-soluble, not oil-soluble, so it simply doesn’t make it into the final product.
Refining Crude Castor Oil
Freshly pressed castor oil is considered “crude” and contains impurities like phospholipids, free fatty acids, color pigments, and trace metals. Refining removes these in a specific sequence.
Degumming comes first. This step reduces phospholipids (gummy substances) and metals in the crude oil, typically by mixing the oil with water or a mild acid, which causes the gums to hydrate and separate out.
Neutralization follows, where an alkaline solution reacts with free fatty acids in the oil and converts them into soap, which is then washed away. This brings the oil’s acidity down to acceptable levels.
Bleaching uses absorbent materials, often activated clay or earth, to pull out color pigments, residual metals, and oxidation byproducts. The oil is mixed with the clay, which adsorbs impurities, and then filtered clean.
Deodorization strips out volatile compounds that give crude oil its natural smell. This is done by passing steam through the oil under vacuum at elevated temperatures. The result is a nearly odorless, pale oil.
Winterization is sometimes added as a final step. The oil is slowly cooled so that any waxes or saturated fats solidify and can be filtered out. This keeps the oil from becoming cloudy at cooler storage temperatures.
What’s in the Finished Oil
Castor oil’s unusual properties come from its fatty acid profile. About 88 to 90% of the oil is ricinoleic acid, a fatty acid rare in nature that gives castor oil its thick, sticky consistency and its ability to mix with alcohol (something most oils can’t do). The remaining fraction includes 4 to 5% linoleic acid, 2 to 3% oleic acid, and small amounts of palmitic, stearic, and dihydroxystearic acids.
That dominant ricinoleic acid content is what makes castor oil valuable across so many industries, from cosmetics to lubricants to pharmaceutical coatings.
Grades and Their Uses
Not all castor oil is the same product. Cold pressing produces a light, viscous oil that meets pharmaceutical standards (USP grade) and goes into medicines, skin care, and food-grade applications. The FDA permits castor oil as a food additive only when it meets United States Pharmacopeia specifications, which cap heavy metal contamination at no more than 10 mg/kg.
Oil from higher-temperature expeller pressing and solvent extraction produces a darker, lower-purity product sold as industrial grade. This oil goes into paints, coatings, biodiesel, nylon production, and lubricants. There’s also a “Number 3” and “Refined” grade that fall between pharmaceutical and industrial quality.
What Happens to the Leftover Cake
The pressed seed cake still contains protein and nutrients, but it also carries residual ricin and allergens that make it dangerous in its raw form. Currently, most castor cake is used as organic fertilizer, where the toxic compounds break down in soil.
Researchers have developed methods to detoxify the cake so it can be used as animal feed, which would significantly increase its value. One approach uses solid-state fermentation, growing a common fungus on the cake to break down the toxins. Another mixes the cake with inexpensive calcium compounds (calcium hydroxide, calcium oxide, or calcium carbonate) at concentrations of 4 to 8%. Both methods effectively inactivate both the ricin and the allergenic proteins, producing a cake safe enough for cattle feed. Calcium salts are already standard mineral additives in livestock nutrition, making this a practical solution for producers looking to get more value from every seed.